Published bi-annually by the Report on MARGINS Workshop ...rjstern/egypt/PDFs... · Report on MARGINS Workshop: Interpreting Upper-Mantle Images James B. Gaherty1, Greg Hirth2, and

In This Issue:Workshop Report ....................1-5

From the Chair ........................ ... 6

Steering Ctte. Highlights ............. 7

Red Sea Special Section

Overview .............................. 8-9

Stockli, et al. .................... 10-12

Nyblade, et al. .................. 13-15

Reilinger, et al. ................. 16-19

MARGINS at AGU .....................20

MARGINS Reception ..............21

AGU Sessions ................... 22-25

Call for Inter-disciplinary

MARGINS Workshops...............26

Contact Information .................27

Published bi-annually by the

MARGINS OfficeWashington University in St. Louis

1 Brookings Drive, CB 1169

St. Louis, MO 63130 USA

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No. 17, F

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2006

Report on MARGINS Workshop: Interpreting

Upper-Mantle ImagesJames B. Gaherty1, Greg Hirth2, and Geoffrey A. Abers3

1Lamont Doherty Earth Observatory of Columbia University, 2Dept. of Marine Geology and Geophysics, WoodsHole Oceanographic Institution, 3Dept. of Earth Sciences, Boston University

Introduction

From May 19 to 21, 2006, approximately70 scientists gathered at the Woods HoleOceanographic Institution (WHOI) inWoods Hole, Massachusetts to discussthe role that seismic imaging can play inconstraining the physical parameters ofthe upper mantle, specifically in the con-text of MARGINS Subduction Factory(SubFac) and Rifting Continental Litho-sphere (RCL) Initiatives. Both SubFacand RCL include large seismic experi-ments with a main goal to establish thetemperature and flow fields, as well asthe distribution and extent of melting andH

2O, in the mantle. Volcanism occurs

both in arcs and rifts, so a natural tie ex-ists between igneous geochemistry, seis-mology, and rheology in these settings.The goal of the workshop was to bringnew experimental and theoretical resultsto allow the analysis of these data sets interms of the interpretation of physicallymeaningful parameters. Such analyseswill be critical for integrating seismicdata with other observations in any fo-cus area synthesis.

The Interpreting Upper-Mantle Im-ages (IMI) workshop was sponsored bythe U.S. National Science Foundationthrough the NSF MARGINS Program.

Woods Hole Oceanographic Institutionprovided the meeting venue and logisti-cal support. Workshop conveners wereGeoffrey Abers, James Gaherty, and GregHirth. Julie Morris (as MARGINS Chair)was instrumental in obtaining workshopfunding, and Paul Wyer and MeredithBerwick from the MARGINS Office pro-vided invaluable organization and sup-port prior to the workshop, on site, andfollowing the meeting. Additional infor-mation about the meeting, including par-ticipant list, technical program, anddownloadable oral and poster presenta-tions, is available at:

http://www.nsf-margins.org/IMI06/

Workshop philosophy andorganization

To make progress in SubFac or RCL, itis necessary to constrain upper mantleproperties in terms of bulk composition,temperature, water content, and melt dis-tribution. Some key questions include:How much melt is present in situ withinthe mantle? What is the water content ofthe lithosphere and asthenosphere? Howdoes subduction deplete or otherwise al-ter the residual mantle? How does melt-ing occur beneath incipient rifts? Can weconstrain the geometry and mechanism

Figure 1. Participants of the Interpreting Upper Mantle Images worksop, on the grounds ofthe National Academy of Sciences, Woods Hole, MA. Photo courtesy of M. Berwick.

Page 2 MARGINS Newsletter No. 17, Fall 2006

of melt focusing at spreading centers orarcs? Do low velocities below the Mohoin both settings require melt to be present,or do they indicate compositional hetero-geneity or something else? What definesthe lithosphere-asthenosphere boundary,and where is it?

The seismological database neededfor this task is rapidly building owing tonew MARGINS-funded experiments(Table 1). At the same time, the labora-tory community is making significantadvances towards interpreting the influ-ence of these variables and the fundamen-tal physics behind their variation.However, much of this work has beendone without significant interaction be-tween the different communities. Evenwithin communities, a lack of discussionhas led to several first-order differencesin opinion for what should be establishedphysical relationships. There has beenmuch need for a focused meeting of theminds on this subject, in order to cross-educate communities and to hash out dif-ferences.

To this end, the IMI workshop wasdesigned specifically with the goal ofencouraging cross-disciplinary discus-sion and interaction. The 2.5-day sched-ule focused on keynote speakers andextensive open discussion time (includ-ing poster presentations). The workshopbegan with two keynote presentationsthat introduced the range of science prob-lems that can be addressed with mantleimaging in both SubFac and RCL focussites (Wiens; Lizarralde). There werethen two presentations that emphasizedstate-of-the-art imaging experiments fo-cused on the presence of partial melt and

compositional heterogeneity in themantle (Forsyth; Nettles). These werefollowed by two presentations compar-ing theoretical and experimental resultson anelasticity and seismic attenuation(Jackson; Cooper). Open discussion withthe speakers and around posters roundedout the first day. The second day beganwith two keynote presentations on theconstraints that petrology and electro-magnetic imaging place on mantle prop-erties (Hirschmann; Evans), followed bya pair of presentations on theoretical andexperimental constraints on the influenceof water and melt on seismic and electri-cal properties (Karato; Takei). The lasttwo keynote talks of the day discussedthe application of experimental resultsand geodynamic modeling (Faul; Billen),followed by another round of open dis-cussion and posters. On day three, theworkshop closed with two final presen-tations keying on the integration of seis-mology, experimental constraints, andgeodynamic modeling in MARGINS set-tings (van Keken; Fischer), and an opendiscussion of critical experiments andfuture directions.

Summary of workshopdiscussion

Over the last few years, the explosion ofdatasets from dense deployments ofportable broadband instruments,recorded on quiet high-dynamic rangeseismographs, has revolutionized ourability to extract information about thedeep subsurface, with a vast toolkit ofresources now available. At a minimum,seismic imaging provides estimates ofspatial variations in Vp, Vs, and Qs.

Recent studies have also extractedvariations in dlnVp/dlnVs, Qp/Qs, thegeometry of discontinuities and theirimpedances, anisotropy parameters, andfrequency dependence of severalobservables. The importance of this widespectrum of observations is that they aresensitive to physically meaningfulvariables in different ways. For example,Qs represents anelasticity in shearmodulus, which is dominantly a functionof temperature, somewhat a function ofpore geometry and water content, andnegligibly a function of major-elementcomposition. On the other hand, the ratioVp/Vs (essentially Poisson’s ratio) hasweaker temperature sensitivity but oftenis used to indicate pore geometry andpresence of minerals with unusualphysical properties in comparison withtypical mantle minerals, such as quartzor serpentine.

Workshop Report

Simultaneously, there have been sev-eral noteworthy advancements in theo-retical and experimental constraints onseismic properties of mantle materials.First, improved thermodynamic analysesand petrological databases can be usedto constrain both the phase proportionsand mineral compositions in the uppermantle as a function of pressure and tem-perature. Combined with experimentallymeasured elastic properties, these analy-ses provide a critical baseline to use inevaluating the roles of temperature andpressure on seismic velocity. In particu-lar, the temperature dependence of seis-mic velocities is controlled strongly byanelastic dissipation, and these effectshave been recently quantified in the labo-ratory at geologically relevant tempera-

Table 1. MARGINS geophysical projects for mantle imagingPI Project Focus AreaAbers & Fischer TUCAN broadband array CentAmSchwartz SEIZE passive seismic array & followup CentAmWiens & others IBM broadband/OBS imaging IBMWiens & Condor IBM broadband analysis and modeling IBMFouch Anisotropy, geodynamic modeling IBMChave Magnetotelluric transect IBMClayton NARS-Baja broadband array GoCGaherty & Collins SCOOBA broadband OBS array GoCNyblade & others Saudi broadband network analysis Red Sea(Steckler & others) (CAT/SCAN broadband/OBS array) (Red Sea; relocated)

MARGINS Newsletter No. 17, Fall 2006 Page 3

tures, pressures and seismic frequencies.These experimental studies also demon-strate the importance of other variableson seismic velocity, such as grain size andmelt content. While experimental mea-surements on the role of water on anelas-tic properties have not yet beenconducted, theoretical analyses moti-vated by experimental observations onthe influence of water on high strain creepindicate that seismic velocities in themantle may be significantly lower underhydrous conditions. Experiments havebegun to quantify the effect of water con-tent on the dominant slip systems in oli-vine and the role of melt fraction on strainpartitioning during deformation underpartially molten conditions; these resultsprovide new insight into the interpreta-tion of seismic anisotropy.

Over the course of the workshop, pre-sentations and discussions continuallymigrated to two major geodynamicsquestions that require a comprehensiveintegrated approach. The first waswhether the range of seismic velocitiesobserved in the upper mantle is effec-tively modeled using simple temperaturedependence of solid-state peridotite, withpartial-melt, composition, and/or waterplaying a role only in very localized re-gions beneath spreading centers, rifts,and/or arcs. The second was whetherwater-induced changes to the dominantolivine slip systems can explain seismicanisotropy observations within arcs. Wesummarize these discussions here; withinthis summary, bold citations by lastname refer to keynote or poster presen-tations that are available on the meetingwebsite:

http://www.nsf-margins.org/IMI06/At what scale are melt, water, and/orcomposition important? There have beenseveral recent papers that have arguedthat much of the velocity variation in theupper mantle is consistent with expectedtemperature variations in the uppermantle, with no need to call upon the ef-fects of melt, composition, or water [Fauland Jackson, 2005; Stixrude andLithgow-Bertollini, 2005; Priestley andMcKenzie, 2006]. These arguments com-bine well-calibrated thermal models of

the upper mantle with experimental andtheoretical estimates of pressure and tem-perature variation of seismic velocity andattenuation. The resulting predicted pro-files of seismic velocity can be explic-itly compared to observed velocity pro-files. An example is shown in Figure 2,in which Faul argues that the observeddeepening and increasing velocity of theseismic low-velocity zone with increas-ing seafloor age in the Pacific can bemodeled using dry, solid-state peridotite.This notion was countered with examplesfrom a variety of settings using a diverserange of observations: seismic velocityestimates suggesting the presence of meltbeneath and substantially west of the EastPacific Rise (EPR) (Forsyth); electricalconductivity profiles indicating the pres-ence of water in the asthenosphere be-neath the EPR (Evans); evidence for ve-locity variations in back-arc and near-ridge settings that are too large to bepurely thermal (Wiens; Gaherty); andevidence from seismic tomography andgravity that the upper-mantle composi-tion is heterogeneous at the scale of con-tinents and ocean basins (Nettles).

In detail there are several aspects ofthe predicted velocity/temperature rela-tionships that require further consider-ation. All such predictions requireextrapolation of experimental results torealistic grain sizes, but the kinetic basisand appropriate grain sizes for this ex-trapolation are uncertain. Also, the pre-dicted velocity profiles have steepnegative gradients in both Vs and Vp thatare not generally observed in seismicstudies of the uppermost mantle (e.g.,Figure 2). Whether this is a shortcom-ing with the seismic models or the ex-perimental predictions is unclear. Whatis clear is that the new experimental re-sults, coupled with robust thermal mod-els of the upper mantle, provide anexcellent baseline for evaluating the un-derlying cause of velocity variations ob-served in MARGINS settings.

An interesting component of the dis-cussion surrounding the relationship be-tween temperature and seismic velocityis the increased recognition of the domi-nant role that seismic attenuation (de-

noted by its inverse, Q) plays in control-ling observed velocities. In particular, attemperatures appropriate for the upper-mantle, much of the expected velocityvariation is due to physical dispersion,i.e., the wavespeed variation produced byanelasticity. Anelasticity is strongly tem-perature dependent, but it is also sensi-tive to grain size (Jackson; Faul), water(Karato; Cooper) and melt (Cooper;Takei), and these effects are beingmapped out experimentally. The remain-ing anharmonic effects are well known.The coupling suggests that in the idealcase, laboratory experiments can pre-cisely describe the co-variation betweenvelocity and attenuation for a given com-position and physical state. These co-variations can then be compared to in situestimates of velocity and attenuation inthe mantle. Faul and Jackson [2005] takethis analysis part way by utilizing labo-ratory estimates of Q and Vs, along withfield estimates of Vs, to argue for a grain-

Figure 2. Comparison of observed andpredicted shear velocity as a function of agefor the Pacific. Symbols represent upper-mantle velocity models of Nishimura andForsyth, while lines show velocitiescalculated for conductively coolinglithosphere from the fit to experimentallymeasured shear moduli. Figure courtesy ofU. Faul.

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Page 4 MARGINS Newsletter No. 17, Fall 2006 Workshop Report

size increase with depth in the Pacificasthenosphere. These constraints couldbe made stronger by incorporating geo-physical measurements of upper-mantleQs from the same region. Wiens pre-sented a nice example of such an analy-sis. A major hurdle is that robustlyquantifying upper-mantle Q remains asignificant challenge (Forsyth; Nettles;Dalton); however, if velocity and attenu-ation can be robustly determined, theexperimental constraints provide a pow-erful tool for unraveling the underlyingcause of observed variations.What can explain arc-parallel shear-wave splitting? Shear-wave splitting ob-servations from subduction zones oftenexhibit a characteristic pattern of fast-wave polarizations, with a transition fromroughly arc-parallel polarizations belowthe arc or forearc, to convergence-paral-lel polarizations further into the back-arc,although some arcs show different pat-terns (e.g., Wiens; Fischer; van Keken).Interpreted through the typical relation-ship between flow and fabric in dry oliv-

ine (i.e., fast seismic direction corre-sponds to the flow direction), these ob-servations suggest arc-parallel upper-mantle flow that is difficult to explainusing geodynamic models of subduction.An alternative has been suggested byKarato and co-workers, who found ex-perimental evidence for a change in thedominant slip system in olivine underwet, high-stress conditions. Samplesdeformed under these conditions gener-ate a “B-type” fabric that produces seis-mic fast directions roughly orthogonal tothe dominant flow direction. This fabricthus can explain arc-parallel splittingobservations with simple arc-normal con-vergent flow due to subduction.

Distinguishing between the slipmechanism active within SubFac andRCL regions provides a potentially pow-erful tool for evaluating underlying dy-namics, as the B-type fabric exists in avery narrow range of water and stressconditions. Numerical models of subduc-tion zones (van Keken) suggest that thereis a small “nose” in the fore-arc corner

between the slab and the overlying platewithin which B-type fabric will be stable(Figure 3). The question is then whetherthe observations of arc-parallel splittingare consistent with the expected locationof this wet, high-stress zone. If so, thenthe stress and water content within thearc can be tightly constrained by theanisotropy observations. If not, then al-ternative mechanisms are implied. Arc-parallel flow is one possibility, butapparent anisotropy due to shear-inducedmelt segregation has also been proposed(Holtzman). This latter mechanism maybe particularly relevant for RCL (includ-ing Gulf of California, Lizarralde), butit might prove useful in interpreting split-ting patterns from arcs as well.

At the workshop, observationsbroadly consistent with B-type fabricwere presented from the Japan andRyukyu arc (Long). New results fromrecent broadband deployments in theSubFac focus zones (Izu-Bonin-Marianas and Central America) are lessconsistent with this hypothesis. In par-

Figure 3. (a) Individual SKS-splitting observations from the BEARR array across the Alaska range, projected to 100-km depth. Approximatedepth of the subducting slab is indicated by black contours. Sharp transition from arc-parallel to arc-perpendicular fast directions is observedat a slab depth of approximately 75 km. (b) Two-dimensional numerical model of temperature and fabric-type in the SubFac environment.Primary image shows temperature contours, with slab interface marked with a bold black line. Inset shows the region in dashed box,indicating fabric type in the nose just above the slab. In this model, stress and fluid content consistent with B-type fabric is restricted to adepth interval of 50-100 km, at a distance of 200-300 km from the trench. Location of B-type fabric is consistent with the transition observedin (a), but the 1-2 s split times in (a) appear too large to be consistent with this model. Additional models incorporating arc-parallel flowabove and arc-perpendicular below the slab are being considered. Figures courtesy of D. Christensen and E. Kneller.

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ticular, arc-parallel fast directions ob-served in Costa Rica and Nicaragua arenot located in the expected region of thewet, high-stress nose (Fischer; Abt), andalternative explanations based on arc-parallel flow are being sought (Behn;Kneller). A similar conclusion was pro-posed for the Marianas, where a transi-tion from arc-parallel to arc-perpendicular fast directions is observed,but the location of the transition appearsto be well into the back arc (Wiens;Pozgay). Both of the latter results arepreliminary, but it appears that themechanisms controlling seismic anisot-ropy observations in arcs remain contro-versial.

Concluding Remarks

The MARGINS IMI workshop provided2.5 days of spirited discussion on the useof seismic images to constrain criticalphysical and geodynamical processes inMARGINS focus areas. This report sum-

marizes the goals and underlying issuesaddressed at the workshop. We cannotpossibly address all of the topics dis-cussed, however, and we encourage in-terested parties to access the meeting pre-sentations for additional information. Weemphasize that the confluence of new,high-quality observations from severalMARGINS experiments, along with anew generation of experimental and theo-retical results and modeling tools, makesthis a very opportune time to addressthese problems. We hope that the dis-cussions from the workshop will filterdown through many communities, andform a basis for substantially improvingour understanding of the deep earth.

ReferencesFaul, U. H., and I. Jackson, 2005, The seis-

mological signature of temperature andgrain size variations in the upper mantle,Earth Planet. Sci. Lett., 234, 119-134.

Priestley, K. and D. McKenzie, 2006, Thethermal structure of the lithosphere from

shear wave velocities, Earth Planet. Sci.Lett., 244, 285-301.

Stixrude, L., and C. Lithgow-Bertelloni,2005, Mineralogy and elasticity of theoceanic upper mantle: Origin of the low-velocity zone, J. Geophys. Res., 110,B03204, doi:10.1029/2004JB002965.

MARGINS Interdisciplinary Mini-Workshop on the

Izu-Bonin-Marianas Subduction Factory Focus SiteAGU Fall Meeting, 2006, Mon., 11 Dec., 6-8 pm, Salon A3, San Francisco Marriott

Conveners: R.J. Stern (U Texas at Dallas), Y. Tatsumi (IFREE/JAMSTEC), R.W.Embley (PMEL/NOAA), Y. Kaneda (Japan Continental Shelf Project)

Efforts to reach InterMARGINS and MARGINS-Subduction Factory science objectives in theIzu-Bonin-Mariana focus site have been enhanced by recent NOAA “Submarine Ring of Fire”investigations and the Japan Continental Shelf Project. Geoscientific studies in the region arebeing further stimulated by a set of IODP preproposals for drilling in the IBM arc system. Thesecomplementary efforts can be stimulated in turn by involving the MARGINS Subduction Fac-tory community. This mini-workshop will inform the three communities of these efforts, solicitfeedback, and explore possible synergies. The conveners also hope to present the status of aproposal for a future ~3 day MARGINS/IFREE Workshop to Integrate Subduction Factory andIODP Studies in the Izu-Bonin-Marianas Arc System.

This mini-workshop shares its location with the MAR-GINS Middle America Subduction Zone mini-workshopfollowing immediately after. Food and drink will be pro-vided at both events.

A MARGINS Interdisciplinary Mini-Workshop

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From the MARGINS Chair ~ Fall 2006Doug Wiens, Interim MARGINS Chair, Washington University in St. Louis

E-mail: [email protected]

September 28, 2006

As I write this I have three days left in myterm as interim MARGINS Chair, and I ampleased to report that the program is in goodshape. The MARGINS Chair rotation fromJulie Morris, who has become head of NSF-Ocean Sciences, temporarily to myself, andnow to Geoff Abers of Boston Universityis going smoothly. This is largely due tothe hard work of MARGINS Office Direc-tor Paul Wyer and Office AdministratorMeredith Berwick, who have provided thecontinuity and institutional memory nec-essary for a smooth transition. We were for-tunate to have the opportunity to visit withJulie during the September Steering Com-mittee meeting in Washington DC; sheseemed to be flourishing in her new posi-tion, although she said she misses the closeinteraction she developed with the MAR-GINS community over the past three years.

The two issues that have dominatedmuch of the MARGINS Program discus-sion over the past few months are interac-tions with the ORION Program and thediscussion about future directions for theRifting Continental Lithosphere (RCL) Ini-tiative. In both cases we opened web fo-rums on the MARGINS website to allowinput from the community. All the inputwas passed on to the Steering Committeeduring the September meeting; thanks toall of you who contributed input on theseissues.

Much of the ORION discussion grewout of a request from the ORION STACcommittee in April for the MARGINS Pro-gram to help provide guidance for twohighly-rated responses to the Ocean Ob-servatory Initiative (OOI) request for as-sistance (RFA) that involved MARGINSscience at MARGINS focus sites. Thesewere the buoyed observatory proposals forthe Costa Rica seismogenic zone, coordi-nated by Kevin Brown, and the Marianaforearc, lead by Patty Fryer. Short descrip-tions of these programs were included inthe Spring 2006 MARGINS Newsletter.Unfortunately, detailed budgeting and longterm operations and maintenance cost es-timates developed during the summer in-

dicate that it is unlikely that OOI will beable to include either of these buoys withinthe initial program. Therefore the MAR-GINS Steering Committee decided therewas little reason to prioritize these twoprojects at this time. However, we feel thatthe MARGINS community has learned alot about the capabilities of seafloor obser-vatories for advancing our science, and thatseafloor observatories will be too impor-tant to ignore during the next decade.

There has also been a lot of discussionon the web forum and at the Steering Com-mittee meeting about the future of the RCLinitiative, after NSF made the decision tochange the status of the Red Sea region toan “ancillary site,” as detailed in the letterfrom Bilal Haq in the last newsletter. Thecommunity input and committee discus-sions indicate that it is not the right time inthe program to be considering alternativeRCL focus sites, and that the main MAR-GINS RCL studies will be undertaken inthe Gulf of California – Salton Trough.There is some continued discussion that,although NSF will not fund seagoing workin the Red Sea, there may be some impor-tant questions that can be addressed withinthese limitations. Geoff Abers will discussthis subject in more detail at the MARGINSAGU reception and in a future newsletter.

The last half-year has also seen twohighly successful MARGINS-sponsoredworkshops. The “Interpreting UpperMantle Images” workshop was held atWoods Hole on May 17-19, organized byGreg Hirth, Geoff Abers and Jim Gaherty.The goal of the workshop was to bring to-gether experimental, observational, andmodeling communities to discuss howphysical properties can be determined fromupper mantle seismic images. Applicationsfor the meeting ran far ahead of the con-veners expectations, so the meeting sizewas increased somewhat from the initialproposal to accommodate all the interest.In the end, around 70 scientists participatedin 2.5 days of talks and animated discus-sions. See the report in this newsletter formore details.

The MARGINS “Teleconnections Be-

tween Source and Sink in Sediment Dis-persal Systems” Source-to-Sink Theoreti-cal and Experimental Institute (S2S TEI)was just last week. Rudy Slingerland, JohnMilliman, Bill Dietrich and Lincoln Pratsonorganized the workshop around two suc-cessive venues separated by a field tripthrough the Eel River system in northernCalifornia. A little over 80 scientists fromdiverse disciplines gathered together to dis-cuss ways to better understand the com-plex feedbacks that occur throughoutsource-to-sink systems. Keynote talks,panel discussions, poster sessions andbreakout groups all contributed to thistheme. Some participants stayed on for anoptional extra half-day to review progressand goals in relation to the MARGINS S2SScience Plan. Geoff Abers and the Wash-ington University MARGINS Office staffwere joined at the TEI by the new BostonUniversity MARGINS Office Administra-tor, Cary Kandel, enjoying her first inter-action with the S2S community. Expectmore about the meeting on the MARGINSwebsite and in a future newsletter.

Well, it is almost time to “turn out thelights” on the Washington University of-fice. We have really enjoyed the MAR-GINS presence here and have beendelighted to help in this effort. In caseanyone’s still using the old MARGINS webaddress, now is the time to update towww.nsf-margins.org, which will carrythrough the move. Once again, I thank Pauland Meredith in particular and also our ad-ministrative and computer support staff forall their hard work to make the programrun smoothly. Also, speaking of goodthings coming to an end, John Milliman hasrotated off the Steering Committee as ofthis last meeting, and I thank him for hisexcellent service.

Geoff Abers is well underway in get-ting the Boston University office up tospeed; Cary started her position early inSeptember, and Geoff expects a new Of-fice Coordinator to start soon. I wish themwell, and look forward to seeing furtherprogress as the MARGINS Program movesforward during the next three years.

From the Chair


The MARGINS Steering Committee(MSC) met with visitors, 7-8 September,2006, at NSF Headquarters, Arlington, VA.1. Doug Wiens, Interim MARGINS

Chair, welcomed new MSC membersSteve Holbrook, University ofWyoming and Jim Gill (in absentia),University of California - SantaCruz.

2. Bilal Haq, Program Director forMarine Geology and Geophysics andCoordinator for the MARGINSProgram at NSF, welcomed andbriefed the MSC on relevant NSFtopics:• NSF-Ocean Sciences (NSF-OCE)

rotations recently filled includedAdam Schultz as a Marine Geol-ogy and Geophysics ProgramDirector and Kevin Johnson as anOcean Drilling Associate ProgramDirector.

• NSF-OCE continues to welcomecruise proposals and is maintain-ing a good award rate whileworking to alleviate ship time andcost pressures.

• Although the Administrationmight increase NSF’s budget for2007, the main draws on anyadditional funds would likely befacilities and possibly ship time.Hence the 2007 MARGINSbudget may not increase from2006.

MSC discussion with NSF Program Direc-tors considered opportunities for MAR-GINS to increase its appeal to NSF-EarthSciences (EAR), especially mutually ben-eficial overlaps with the EarthScope Pro-gram. This theme continued in conjunctionwith EarthScope discussions later in themeeting. Similar advantages could con-tinue to come from interaction with othermajor geoscience initiatives, such asORION and IODP.3. The MSC and NSF Program Direc-

tors discussed amendments to a draftMARGINS Program Event Responsepolicy. This policy will be publiclyreleased once a final version hasbeen approved.

4. Kevin Brown, Scripps Oceano-

MARGINS Steering Committee Highlights, Fall 2006graphic Institute and Patty Fryer,University of Hawaii, briefed theMSC on ORION Global OceanObservatories and their potentialapplications in the MARGINSCentral America and Izu-Bonin-Mariana Focus Sites. Kendra Daly,ORION Program Director, later gaveher own briefing to the MSC regard-ing MARGINS-related opportunitiesin the ORION Program.

5. Lina Patino, NSF-EAR AssistantProgram Director for EarthScope,briefed the MSC on the structure andprogress of the EarthScope Program.She encouraged close interaction ofthe MARGINS Office and SteeringCommittee with the futureEarthScope National Office. Thisoffice will be established for up tofour years under an NSF ProgramSolicitation that closes on January12, 2007:

http://www.nsf.gov/funding/pgm_summ.jsp?pims_id=5618

6. Initiative subgroups updated theMSC on scientific progress inMARGINS Focus Sites.

7. Andrew Goodwillie of the MarineGeoscience Data System(www.marine-geo.org) at Lamontupdated the MSC on developmentsin MARGINS database submission,content and access. A key step is toincrease community awareness of therange of data now available throughthe MARGINS database. Commu-nity input on essential datasets forthe database to host is welcome.

8. In the context of rising publicationcosts, the MSC discussed prioritiesfor future MARGINS PublicationSeries volumes, including issuessuch as online availability, time topress and citation impact.

9. The MSC reviewed contributionsfrom the MARGINS RupturingContinental Lithosphere (RCL)online discussion forum. It wasagreed that a new RCL focus siteshould not be added at this time.Until further notice, RCL will focuson the Gulf of California - Salton

Trough, and completion of existingRed Sea projects. This topic will bediscussed at the 2006 AGU FallMeeting, and the MSC continues toconsider how to proceed.

10. Workshops:• A report on the September 2006,

MARGINS “TeleconnectionsBetween Source and Sink inSediment Dispersal Systems”Theoretical and ExperimentalInstitute will appear in a futureMARGINS Newsletter.

• A report on the May 2006, MAR-GINS “Interpreting Upper MantleImages” Workshop begins on thefront page of this newsletter.

• Proposals are in preparation forMARGINS co-sponsored CentralAmerica and Izu-Bonin-Marianasynthesis workshops.

• MARGINS will co-sponsor aplanned international data-sharingworkshop in 2007.

11. Education and Public Outreach:• The MARGINS Education

Advisory Committee continues towork on concepts outlined at theSpring 2006 MARGINS SteeringCommittee Meeting (MSC High-lights, MARGINS Newsletter #16,Spring 2006).

• The second annual MARGINSDistinguished Lectureship Pro-gram received over 60 applica-tions competing for an expectedtotal of 12 tour stops in 2006-07.At time of writing, the selectionprocess is ongoing. DVDs andstreaming video of selectedlectures from the 2005-06 serieswill be available soon through theMARGINS Office and website.

• The MARGINS AGU StudentPrize has been extended this yearto highlight student participationat the MARGINS AGU reception(see p. 21).

12. The MSC considered proposals forMARGINS mini-workshops (seesolicitation on p. 26, and MARGINS

See “Highlights” cont. on pg. 25


MARGINS, Rupturing Continental Lithosphere, Northern and

Central Red SeaRobert Reilinger1, Simon McClusky1, Daniel Stockli,2 and Andrew Nyblade3

1Massachusetts Institute of Technology, 2University of Kansas, 3Penn State University

Introduction

The NSF MARGINS Program is de-signed to encourage the development ofa systematic framework for integratingmarine- and land-based geological andgeophysical studies to further our under-standing of the physical processes thatcharacterize the boundaries between theEarth’s lithospheric plates. The MAR-GINS Rupturing Continental Lithosphere(RCL) Initiative’s main aim is a betterunderstanding of the initiation of conti-nental extension and its transition toocean spreading, because many of thefundamental processes that govern con-tinental rifting and that ultimately leadto rupturing of continental lithosphereand the birth of an ocean remain poorlyunderstood. During the January 2000MARGINS Theoretical and Experimen-tal Institute and workshop on “Ruptur-ing of the Continental Lithosphere” inSnowbird, Utah, the community formu-lated a science plan for the focused in-vestigation of the geodynamic, mechani-cal, kinematic, thermal, and temporalevolution of two sites of active continen-tal rifting to study the transition fromcontinental extension to initial seafloorspreading, the Gulf of California and thenorthern and central Red Sea/Gulf ofSuez rift system. It was anticipated thatthis integrated research effort at the fo-cus sites will enhance understanding ofthe evolution of ocean basins and pas-sive continental margins, with implica-tions for lithosphere rheology, the dy-namics of lithosphere – mantle interac-tions, and the geologic evolution of as-sociated sedimentary basins that are com-monly major source areas for oil and gasreserves.

At an organizational workshop andfield trip for the RCL Red Sea focus sitein Sharm el-Sheikh, Egypt, in March2001, members of the scientific commu-nity met to formulate detailed scientific

questions and strategies. One of the ma-jor objectives of this workshop was toprovide a forum where the logistical re-quirements of working in the variouscountries bordering the Red Sea regioncould be discussed and researchers fromthe United States could meet and explorepossible scientific collaborations withscientists from Egypt, Sudan, Jordan,Eritrea, and Saudi Arabia. Unfortunately,due to the current security situation andpolitical climate in the Middle East, NSFdecided that they could no longer con-sider U.S.-led marine geophysical experi-ments in the Red Sea at this tiime. TheRed Sea has been re-designated as an “an-cillary” focus area, indicating that whileMARGINS will continue to encouragerelevant research in this area, funding forfuture efforts must come from core NSFprograms (see Comments from the Pro-gram Director, MARGINS Newsletter#16, Spring 2006). Discussion of this is-sue continues, see the Chairman’s Reportin this issue. In spite of this issue, a num-ber of Red Sea projects supported underthe initial phase of the Red Sea RCL Ini-tiative have produced results that providenew constraints on rifting processes andlithospheric rheology and dynamics.

This series of articles presents the ini-tial results of three MARGINS-supportedgeophysical studies of the Red Sea Rift.One study uses quantitative dating tech-niques to determine the geologic evolu-tion of rifting and the subsidence historyof the sedimentary basins that developduring the rifting process. A second studyreports the initial results on crust andmantle structure beneath the eastern mar-gin of the Red Sea and the Arabian shieldfrom an analysis of seismic data from theSaudi Arabia National Digital SeismicNetwork. The third study uses geodetictechniques (predominantly GPS) to mea-sure directly present-day deformations asa function of the stage of rifting (i.e.,

variations in style along strike). Thesestudies are complementary in that theseismically constrained structure is theresult of deformational processes ob-served with the GPS and inferred frombasin analysis and rift flank exhumationand uplift. In addition, the geodetic re-sults define present-day deformationwhile the seismic and quantitative geo-logic studies reflect deformation overgeologic times – the relation between thedeformations observed from these stud-ies may provide another means of con-straining the temporal evolution of riftingprocesses.

Tectonic Setting

The Red Sea Rift forms one arm of theAfar Rift/Rift/Rift Triple Junction, sepa-rating the Nubian and Arabian shields,the others being the Gulf of Aden sepa-rating Somalia and Arabia, and the moreslowly spreading East African Rift thatforms the Nubia–Somalia boundary(Reilinger et al. 2006, Figure 1, thisnewsletter). The separation of Arabiafrom Nubia initiated in the Miocene (~30Ma; Stockli et al., 2006, this newsletter;Reilinger et al. 2006, Figure 1 inset, thisnewsletter), and involved counterclock-wise rotation of Arabia relative to Nubia.This rotation, which continues to thepresent, results in increasing spreadingrates and total extension from north tosouth, with the northernmost Red Seabeing characterized by the early stagesof continental breakup and the central andsouthern Red Sea having a well devel-oped mid-ocean ridge and associatedmagnetic anomalies (e.g., Chu and Gor-don, 1998). It is the variation in the stageof rifting along the strike of the Red Seathat makes it an ideal location to studythe transition from rupturing continentallithosphere to full ocean spreading.

The Tertiary Red Sea-Gulf of Suez riftsystem is one of the best-exposed and



studied examples of a continental rift. Itprovides both along-strike and across-strike views of the rifting process fromdistributed continental extension to thefinal rupturing of the lithosphere betweenthe Arabian and African plates. Thoughmuch progress has been made in under-standing the plate tectonic frameworkand modern strain field of this region, thetransition from distributed continentalextension to sea-floor spreading remainspoorly understood. Specifically, our lim-ited knowledge of how extensional strainis spatially and temporally distributed onthe adjacent continental margins hasmade it difficult to adequately evaluateand test models for the dynamic evolu-tion of this rift.

Several recent geodynamic models ofrift geometries and evolution have reliedheavily on data and concepts derivedfrom the Red Sea rift system [e.g.,Bosworth et al., 2005 for summary]. TheGulf of Suez was the first rift system inwhich large-scale, long-axis segmenta-tion into sub-basins and tilt domains byaccommodation zones was clearly rec-ognized, and has served as one of the pre-mier examples demonstrating theinterplay between extensional tectonism

and sedimentation. Intracontinental rift-ing in the Red Sea began in the Oligoceneand led to sea-floor spreading in thesouthern and central parts of the Red Seain the Early Pliocene at ~4.5 Ma [e.g.,Bosworth et al., 2005]. The margins ofboth the Gulf of Suez and Red Sea arebordered by large uplifted rift flanks thatare actively undergoing erosion, exhum-ing basement rocks from >5 km depth[e.g., Omar and Steckler, 1995]. Theflanks are asymmetrical, forming a steepescarpment facing the rift and a gentlebackslope with decreasing elevationaway from the rift. Growth of asymmet-ric rift flanks outside zones of large-mag-nitude lithospheric extension is a directconsequence of the crustal thinning andsubsidence that occurs within rift systems[e.g., Weissel & Karner, 1989].

Ongoing studies

The articles presented here represent ini-tial results from a selection of the manydomestic and international studies eluci-dating active processes of the Red Searift. A range of new geological and geo-physical results from some of these stud-ies will be presented in the “Geodynam-ics of Lithospheric Extension” session at

the 2006, Fall AGU Meeting (see p. 22).

ReferencesBosworth, B., P. Huchon, and K. McClay,

2005. The Red Sea and Gulf of Aden ba-sins, Journal of African Earth science, v.43, 334-378.

Chu, D., and G. Gordon, 1998, Current platemotions across the Red Sea, Geophys. J.Int., 135, 313-328.

Omar, G.I., and M.S. Steckler, 1995, Fission-track evidence on the initial opening ofthe Red Sea: two pulses, no propagation,Science, v. 270, 1341-1344.

Reilinger, R., S. McClusky, A. ArRajehi, S.Mahmoud, A. Rayan, W. Ghereab, G.Ogubazghi and A. Al-Aydrus, 2006, Geo-detic constraints on rupturing of continen-tal lithosphere along the Red Sea, MAR-GINS Newsletter 17, 17-21.

Stockli, D.F., E. Szymanski, P. Johnson, F.H.Kattan, K. Kadi, A. Al Shamari, G. Omarand S. Brichau, 2006. Thermochrono-metric constraints on tectonic and geo-morphic evolution of the northern andcentral Saudi Arabian Red Sea Rift Mar-gin, MARGINS Newsletter 17, 10-13.

Weissel, J.K. and G.D. Karner, 1989. Flex-ural uplift of rift flanks due to mechani-cal unloading of the lithosphere duringextension, J. Geophys. Res., 94, 13,919-13,950.

The MARGINS Program offers a $1000 prize for the Outstanding Student Oral Presentation or Poster on MARGINS-related science at the 2006 AGU Fall Meeting. The prize is open to any student from any country who is presenting aMARGINS-related talk or poster for which they are the first author and presenter. MARGINS is an NSF-funded programthat seeks to facilitate outstanding interdisciplinary research, with foci on rupture of continental lithosphere, the (subduc-tion) seismogenic zone, source-to-sink sedimentation, and material and chemical fluxes in the subduction factory. Thisprize highlights the important role of student research in accomplishing MARGINS science goals, and encourages cross-disciplinary input to the MARGINS Program. To be considered, students must apply no later than November 24, 2006 at:www.nsf-margins.org/AGU2006/. The entry must include a brief statement of how the research relates to some aspect ofMARGINS science. This statement is important, as it will be used for pre-screening of entries of relevance, and may beconsidered in the final choice of winner. The winner and any honorable mentions will be notified after the conference andwill be recognized in the MARGINS website and newsletter. Please direct email enquiries to the MARGINS Office:[email protected].

Student prize entrants are encouraged to attend the MARGINS Student and Community Reception, Tuesday, 12December, 6-9 pm , Salon 9, San Francisco Marriott. This informal event will give you an additional opportunity todiscuss your research with MARGINS Prize judges, members of the MARGINS Steering Committee and otherswho study continental margins.

Annual MARGINS Prize forAnnual MARGINS Prize forAnnual MARGINS Prize forAnnual MARGINS Prize forAnnual MARGINS Prize for

Outstanding Student PresentationOutstanding Student PresentationOutstanding Student PresentationOutstanding Student PresentationOutstanding Student Presentation

Page 10 MARGINS Newsletter No. 17, Fall 2006 Red Sea Special Section

The Tertiary Red Sea-Gulf of Suez riftsystem is one of the best-exposed ex-amples of a continental rift (Figure 1) anda prime example of active continentalbreak-up [e.g., Cochran, 1983; Bosworthet al., 2005]. Though much progress hasbeen made in understanding the plate tec-tonic framework and modern strain fieldof the Red Sea [see Reilinger et al., 2006,this newsletter], limited knowledge ofhow extensional strain is spatially andtemporally distributed along the conti-nental margins has made it difficult toadequately evaluate and test models forthe dynamic evolution of this rift system.Traditionally, the timing of growth andexhumation of rift flanks is determinedby identifying erosional products withinthe basin fill. In the Red Sea, however,most of the pre-, syn-, and post- rift sedi-mentary rocks are either deeply buriedwithin the rift, have been uplifted anderoded away, or are poorly dated due tothe scarcity of datable synrift Tertiaryvolcanic rocks, hampering attempts toadequately reconstruct the rifting historyalong the entire margin of the Red Sea.Although the Egyptian and Yemeni mar-gin of the Red Sea has been studied insome detail [e.g., Omar et al., 1987,1989; Menzies et al., 1997], little isknown about the timing of Tertiary rift-ing of the Saudi Arabian margin. In thisunprecedented collaborative project withthe Saudi Geological Survey, we havebeen undertaking a comprehensive low-temperature thermochronometric inves-tigation integrated with structural andgeomorphic studies to determine the tim-ing, origin, and geometry of extensionalfaulting and rift flank exhumation alongthe central and northern Red Sea marginin Saudi Arabia (Figure 1). The primaryaim of this thermochronometric study is

Thermochronometric constraints on tectonic and geomorphic

evolution of the northern and central Saudi Arabian

Red Sea Rift MarginDaniel F. Stockli1, Eugene Szymanski1, Peter Johnson2, Fayek H. Kattan2, Khalid Kadi2, Abdullah Al Shamari2, Gomaa

Omar3, and Stephanie Brichau1

1Dept. of Geology, University of Kansas, Lawrence, Kansas 66045, United States, 2Saudi Geological Survey, Jeddah, 21514, Kingdom of Saudi Arabia, 3Dept. of Earth andEnvironmental Science, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States

to systematically resolve the timing andspatial resolution of extensional faultingand post-rift erosion along the Saudi RedSea margin and to distinguish betweendifferent models proposed for thegeodynamic evolution of the Red Sea riftsystem.

Tectonic and GeologicalEvolution of the Central andNorthern Red Sea

The Saudi Arabian Red Sea coastal mar-gin has experienced a complex geologi-cal, tectonic, and geomorphic history. Themargin is lithologically underlain by anassemblage of Neoproterozoic graniticgneisses, metasedimentary and volcanicrocks, ophiolites, and voluminous syn-to post-kinematic granitic intrusions andfelsic to mafic Pan-African dikes. Base-ment fabrics are dominated by NW-trend-ing foliations in the basement and WNW-to NW-trending basement shear zones ofthe late Pan-African Najd fault system.The Neoproterozoic assemblage is re-gionally unconformably overlain by aflat-lying Paleozoic sedimentary se-quence. The overall picture of the RedSea prior to rifting is that of a low relief,low elevation continental to marinerealm. Subsequent uplift and exhumationrelated to the opening of the Red Sea hascaused erosion of much of the pre-riftstratigraphy.

Volcanism began throughout the RedSea at ~32-30 Ma and appears to largelypredate earliest rift sedimentation. Rapidextension did not start until early Mi-ocene times and is constrained by the old-est definite synrift strata in the northernRed Sea and Gulf of Suez at ~22 Ma [seesummary in Bosworth et al., 2005]. Thistiming for the onset of Red Sea rifting issupported by apatite fission track and (U-

Th)/He data from the Gulf of Suez re-gion recording rapid rift flank exhuma-tion at 21-23 Ma [Omar et al., 1989;Stockli and Bosworth, unpubl.]. Alongthe southern Saudi Arabian margin, how-ever, Bohannon and others [1989] re-ported an onset of rapid extension that isdistinctly older (23-28 Ma), possibly sug-gesting a diachronous onset of riftingalong strike. Menzies and others [1997],on the other hand, obtained apatite fis-sion track ages from the Yemen portionof the Red Sea margin that indicate rapidcooling occurred <25 Ma, post-dating themain period of basaltic volcanism (32-29 Ma). Basaltic volcanism on the SaudiArabian margin has continued from theLate Oligocene throughout the entire de-velopment of the Red Sea and is charac-terized by large flood basalt provincesknown as Harrats. The eruptive centersof the older flows follow a N30°W trendparallel to the Red Sea, while youngerfields are oriented more northerly(N20°W - N10°E) [Coleman et al., 1983;see Nyblade et al., this newsletter]. Ter-tiary dikes, flows, and intrusions on theSaudi coastal plain bracket the age of rift-ing. Along the central Red Sea coast nearJeddah, the earliest synrift volcanic rocksrange from >27 Ma to ~20 Ma [Pallister,1987]. The emplacement of NW-trend-ing dikes at 24-18 Ma has been inter-preted to represent a change in style ofmagmatism indicative of a dramatic in-crease in the extension rate [e.g.,Pallister, 1987].

Geomorphic Evolution ofthe Saudi Arabian Margin

The Saudi Arabian Red Sea margin canbe subdivided into two distinctly differ-ent geomorphic domains: (1) From about21°N to the southern tip of Yemen, the


nificant basalts sequences.The central Saudi Arabian Red Sea

margin comprises erosional surfaces inaddition to the prominent pre-Tertiaryerosional surface dominating southernSaudi Arabia and Yemen. A very distinc-tive surface is well developed betweenthe modern coastal plain and the main riftflank escarpment. It stretches for >500km and dips gently seaward ranging from300-100 m in elevation. A small escarp-ment (50-75 m high) separates this sur-face from the modern coastal plain, whichis often rimmed by uplifted middle Mi-ocene carbonate reefs. This surface likelyrepresents a major wave-cut platformassociated with the middle Miocene tec-tonic reorganization in the Red Sea co-eval with the development of the Gulf ofAqaba transform. New apatite (U-Th)/Hedata from the area between Yanbu andUmm Lujj support a middle to late Mi-ocene age for this erosional surface.North of Al Wajh, additional coastal up-lift related to tectonism along the Aqaba

transform system is expressed by spec-tacular coastal cliffs that appear to in-crease in height from south to north. Newapatite (U-Th)/He data from the area be-tween Dhuba (Duba) and the Gulf ofAqaba exhibit significant post-Miocenecooling and exhumation increasing inmagnitude from south to north.

Low-TemperatureThermochronometry

Over the past three years, our collabora-tive efforts have focused on a compre-hensive thermochronometric study ofexhumed crystalline basement along thecentral and northern Saudi Arabia RedSea rift margin, stretching from the cos-tal escarpment south of Makkah/Jeddahto the northern Gulf of Aqaba (Figure 1).Apatite and zircon (U-Th)/He and apa-tite fission track analysis directly date thetiming of onset of extension and alsoquantify the pre- and post-rift erosionaldenudation of the rift flanks. A criticalaspect of constraining the evolution ofrifting and rift flank exhumation is ac-quiring chronological constraints onfaulting, rift segmentation, and rift local-ization. The answers to these questionshave far-reaching implications for theunderstanding of rifting dynamics and theinterplay between extensional faultingand subsidence during syn-rift sedimen-tation. In addition, integration of detailedthermochronometric data with detailedstructural and kinematic analysis is nec-essary to interpret the temporal con-straints in their proper tectonic frame-work [e.g., Stockli et al., 2005].

We have collected more than 400thermochronometric samples to resolvethe timing of extensional faulting and todifferentiate between different episodesof cooling and exhumation affecting theSaudi Arabian Red Sea margin. Our sam-pling strategy can be summarized by twomajor approaches:

(1) Following the strategy describedby Stockli et al. [2005], a series of de-tailed vertical transects were collectedacross the exhumed crustal blocks andtopographic escarpments along the SaudiRed Sea margin to constrain the timingand spatial distribution of extensional

Figure 1. Shaded digital relief map of the central and northern Red Sea showing locations oflow-temperature thermochronometric samples and vertical sampling transects collected fromthe Saudi Arabian Red Sea margin for this collaborative study. Inset map outlinesNeoproterozoic basement exposures of the Arabian and Nubian Shields and publishedthermochronometric sample locations.

Saudi Red Sea margin is dominated byan impressive continuous ~2-3 km higherosional escarpment. The escarpment isthe result of erosional rift flank retreatand appears to be independent of anystructures in the underlying Precambrian.In central western Saudi Arabia, this up-per surface is a mature erosion surfacethat extends east from the lip of the es-carpment and bevels late Cretaceous-middle Eocene sedimentary rocks. Eastof Makkah (Mecca) this surface is par-tially overlain by Oligo-Miocene Harratflood basalts and is likely late Cretaceousin age. (2) North of Makkah the Red Seaescarpment is discontinuous and less el-evated and is interrupted by paleovalleysthat descend from elevations of 1000-1500 m to the coastal plain. Thesepaleovalleys post-date rifting and areinfilled by ~5-10 Ma flood basalt thatspilled from their eruptive centers eastof the escarpment. We are currently car-rying out detailed 40Ar/39Ar and magne-tite (U-Th)/He dating of geologically sig-

0 500000 1000000

30

00

00

0

Medina

Jeddah

Makkah

Sinai

SAUDI ARABIA

SUDAN

RED SEA

Al Wajh

Gulf of Aqaba

Gulf of Suez

30° 46 °

32°

12°

NubianShield

ArabianShield

22°

Pan-African basement

Published fission track data

Oceanic crust

satellite image extent

EGYPT

Duba

Yanbu

Tabuk 25

00

00

0

Thermochronology

Samples (this study)

2004 Sample Locations (165)



Limit of PhanerozoicCover Sequence

Vertical Transect Locations

25

00

00

0

0 40 80 120 160

Kilometers

N

Hurghada

Quseir

Safaga


faulting and evolution of rift segmenta-tion and localization, as well as rift flankexhumation (Figure 1). The detailed ver-tical transects, spanning up to 3000 m inelevation between Jeddah and the Gulfof Aqaba, allow for detailed reconstruc-tion of the Mesozoic to late Cenozoicthermal history. In addition, verticaltransects were collected in the footwallof the inland Hamd-Jizl halfgraben northof Medina to elucidate the temporal re-lationship between distributed inlandextension and normal faulting within themain Red Sea basin.

(2) Horizontal traverses were col-lected across the entire rift margin fromthe modern coastal plain to the edge ofthe exposed Neoproterozoic ArabianShield (Figure 1). These thermochrono-metric traverses range from ~100 km inlength in northern Saudi Arabia (Gulf ofAqaba and Dhuba) and ~200 km (AlWadj and Yanbu) to ~600 km (Jeddah/Makkah). These long-baseline traverseswere designed to investigate the widthof exhumation related to rifting andcrustal attenuation and to constrain howfar extensional faulting encroaches intothe rift margins beyond the border faultsystems, such as in the Hamd-Jizl basins

north of Medina (Figure 1). Knowledgeof the temporal and spatial distributionof crustal attenuation and the timing offlexural amplification and exhumation ofthe entire width of the rift flank is criti-cal for evaluating the role of processessuch as active versus passive astheno-spheric upwelling, secondary convection,and flexural unloading of the crust, aswell as the distribution of sub-crustallithospheric extension relative to crustalthinning.

ReferencesBohannon, R.G., C.W. Naeser, D.L. Schmidt and

R. A. Zimmermann, 1989, The timing of uplift,volcanism, and rifting peripheral to the RedSea: A case for passive rifting?, J. Geophys.Res., 94, 1683-1701.

Bosworth, B., P. Huchon, and K. McClay, 2005,The Red Sea and Gulf of Aden basins, Journalof African Earth Sciences, v. 43, 334–378.

Cochran, J.R., 1983. A model for the developmentof the Red Sea, Amer. Assoc. Petrol. Geol.Bull., 67, 41-69.

Coleman, R.G., R.T. Gregory, and G.F. Brown,1983. Cenozoic volcanic rocks of SaudiArabia, U.S. Geol. Surv. Open-file Rep. 83-788, 82p.

Menzies, M., K. Gallagher, A. Yelland, and A.J.Hurford, 1997, Volcanic and nonvolcanicrifted margins of the Red Sea and Gulf of


Aden: Crustal cooling and margin evolutionin Yemen, Geochimica et Cosmochimica Acta,vol. 61, 2511-2527.

Nyblade, A., Y. Park, A. Rodgers and A. Al-Amri,2006, Seismic structure of the Arabian Shieldlithosphere and Red Sea margin, MARGINSNewsletter, 17, 13-16.

Omar, G.I., B.P. Kohn, T.M. Lutz and H. Faul,1987, The cooling history of Silurian toCretaceous alkaline ring complexes, southEastern Desert, Egypt, as revealed by fission-track analysis, Earth Planet. Sci. Lett., 83, 94-108.

Omar, G.I., M.S. Steckler, W.R. Buck, and B. P.Kohn, 1989, Fission-track analysis ofbasement apatites at the western margin of theGulf of Suez rift, Egypt: evidence forsynchroneity of uplift and subsidence, EarthPlanet. Sci. Lett., 94, 316-328.

Pallister, J.S., 1987, Magmatic history of Red Searifting: Perspective from the central SaudiArabian coastal plain, Geol. Soc. Amer. Bull.,98, 400-417.

Reilinger, R., S. McClusky, A. ArRajehi, S.Mahmoud, A. Rayan, W. Ghereab, G.Ogubazghi and A. Al-Aydrus, 2006, Geodeticconstraints on rupturing of continentallithosphere along the Red Sea, MARGINSNewsletter, 17, 17-21.

Stockli, D.F., 2005, Application of low-temperature thermochronometry toextensional tectonic settings. In: Reiners, P.,and Ehlers, T. (eds.), Thermochronometry.Reviews in Mineralogy and Geochemistry, vol.58, 420-461.

The Seismogenic Zone of Subduction Thrust FaultsMARGINS Theoretical and Experimental Earth Science Series, Volume 2

Columbia University Press, 2007.Editors: Timothy Dixon (Univeersity of Miami) and J. Casey Moore (University of California-Santa Cruz)

Chapters contributed by 50 MARGINS researchers

• Inside the Subduction Factory www.agu.org/cgi-bin/agubookstore?book=SEGM1389973&search=subduction%20factory

• Rheology and Deformation of the Lithosphere at Continental Margins

www.columbia.edu/cu/cup/catalog/data/023112/0231127383.HTM

Additional MARGINS Volumes Currently AvailableOrder through the web addresses given, or additional information may be found at www.nsf-margins.org/Books.html

COMING SOON


Seismic Structure of the Arabian Shield Lithosphere and

Red Sea MarginAndrew Nyblade1, Yongcheol Park1, Arthur Rodgers2, and Abdullah Al-Amri3

1Department of Geosciences, Penn State University, University Park, PA 16802, 2Seismology Group, Lawrence Livermore National Laboratory, Livermore, CA 94551, 3KingSaud University, Geology Department and Seismic Studies Center, P.O. Box 2455, Riyadh, Saudi Arabia

Introduction

In this MARGINS project we are usingbroadband seismic data from the SaudiArabia National Digital Seismic Network(SANDSN) to investigate crust and up-per mantle structure beneath the easternmargin of the Red Sea and the ArabianShield. The SANDSN has been operatedsince 1998 by the King Abdulaziz Cityfor Science and Technology (KACST),and consists of 38 stations mostly dis-tributed across the Arabian Shield (Fig-ure 1) [Al-Amri and Al-Amri, 1999].Twenty-seven of the stations areequipped with broadband (StreckeisenSTS-2) sensors. Five years of data (1999-2003) from the network have been madeavailable for this project, and the data arebeing used to 1) map first-order structuresurrounding the ruptured Red Sea litho-sphere, 2) evaluate the heterogeneity ofthe continental lithosphere prior to rift-ing, and 3) constrain ambient stress fieldsand lithospheric rheology using localseismicity.

Here we report preliminary findingsfrom a surface wave tomography studyto map differences in upper mantle struc-ture between the eastern margin of theRed Sea, the Arabian Shield, and the Ara-bian Platform (Figure 1). The basementof the study region consists of an amal-gamation of Proterozoic terrains, andacross the Arabian Shield these terrainshave been subjected to Cenozoic upliftand volcanism. The locations of the vol-canic regions are shown in Figure 1a. Theoldest volcanic rocks on the Shield arecontemporaneous with flood basalt vol-canism in Yemen and Ethiopia and theinitiation of rifting in the Red Sea c. 30Ma [Mohr, 1988; Camp et al., 1991;Coleman and McGuire, 1988]. Youngervolcanic rocks (c. 12 Ma to present;Camp and Roobol, 1992) are found in thecentral and northern part of the Shield(Figure 1a). The average elevation across

the Shield is 1 km, but in some areas nearthe Red Sea elevations are as high as 3km. The uplift of the Shield probablyoccurred between 20 and 13 Ma, post-dating the onset of rifting in the Red Seaby at least 10 Ma [McGuire andBohannon, 1989; Bohannon et al., 1989].

Although a great deal of work hasbeen done to understand the origin ofCenozoic uplift and volcanism in theArabian Shield, the development of thesefeatures in relation to rifting in the RedSea remains enigmatic and must be as-certained before the tectonic evolution ofthe Red Sea rift can be fully understood.The surface uplift and volcanism are gen-erally assumed to be due to hot, buoyantmaterial in the upper mantle that mayhave eroded the base of the lithosphere[Camp and Roobol, 1992]. However, thelateral and vertical extent of the thermalanomaly in the upper mantle under theShield is uncertain, as is its relationshipto rifting in the Red Sea.

Previous seismic work in the regionhas revealed low seismic velocities in theupper mantle beneath the Shield [e.g.,Sandvol et al., 1998; Mellors et al., 1999;Rodgers et al., 1999; Debayle et al.,2001; Julia et al., 2003; Benoit et al.,2003], consistent with the presence of amantle thermal anomaly. Global tomo-graphic models suggest that the regionof low seismic velocities could extendfrom shallow upper mantle depths acrossthe transition zone into the lower mantle[e.g., Ritsema et al., 1999; Debayle et al.,2001; Zhao, 2001; Grand, 2002].Daradich et al. [2003] have used thesemodels to suggested that the uplift of theShield is caused by thermally buoyantmantle rising from the core-mantleboundary all the way to the surface. Otherstudies, however, have found little evi-dence for thinning of the transition zoneunder the Shield, [e.g., Kumar et al.,2002; Benoit et al, 2003], suggesting that

the low velocities, and hence thermalanomaly, do not extend as deep as thetransition zone.

Hansen et al. [2006] recently pub-lished results from shear wave splittinganalyses using data from the SANDSNstations. Their results show a N-S fastpolarization direction across the Shield(Figure 1c), similar to the results fromWolfe et al. [1999] for the Saudi ArabianPASSCAL experiment stations [Vernonet al., 1996]. The N-S pattern of fast po-larization directions is not easy to ex-plain by flow in the mantle in thedirection of plate motion or to fossilanisotropy in the Proterozoic lithosphere,and consequently Hansen et al. [2006]have attributed it to a combination ofplate and density driven flow in the as-thenosphere. The density driven flow isassociated with warm material from theAfar hotspot moving to the northwestchanneled by the thinner lithosphere un-der the Red Sea, implying the existenceof a thermal anomaly within the upperpart of the mantle beneath the Shield.

Surface Wave Tomography

To investigate further the depth and lat-eral extent of the low velocity region inthe upper mantle beneath the easternmargin of the Red Sea and the ArabianShield, we have conducted surface wavetomography using measurements ofRayleigh wave phase velocities. In addi-tion to data from the SANDSN network,we have included data from two seismicexperiments in Ethiopia (the EthiopianBroadband Seismic Experiment, Nybladeand Langston, 2002; EAGLE, Maguireet al., 2003), the Saudi Arabia PASSCALExperiment [Vernon et al., 1996], andseveral permanent seismic stations in theregion (Figure 1a).

Interstation phase velocities were ob-tained using the method described byLawrence et al. [2006], which is based


Figure 1. (a) Map of Arabian Peninsula and surrounding regions showing topography (grey scale), seismic station locations, Cenozoicvolcanic fields, and the outline of the Arabian Shield. (b) Ray path coverage for 120 sec. period Rayleigh waves. (c) Phase velocity variationsfor 120 sec. period Rayleigh waves with shear-wave splitting results from Hansen et al. [2006] superimposed. (d) Checkerboard resolutiontest for Rayleigh wave phase velocity variations at 120 sec. period. The dotted lines show the +/- 9% contour interval for the input checkerboardanomalies. The blue and grey shaded regions show the recovered structure.

on an array analysis technique modifiedfrom Menke and Levin [2002], to solvefor the variation in ray path from a greatcircle. When applied to the available data,we obtained 900 or more interstationphase velocity curves, each constructedfrom interstation phase velocities mea-sured at up to 30 period bands from 16 to

180 s. The best ray coverage is obtainedbetween periods of 60 and 120 s. Usingthe phase velocities measurements, wehave preformed inversions using a leastsquares algorithm to construct phase ve-locity maps, and we have tested the reso-lution of the maps using standardcheckerboard methods.

A preliminary result, using about 80%of the available data, is shown in Figures1b, c and d. The ray coverage is bestacross the Shield and the northern andcentral parts of the Platform. The num-ber of rays, as well as the density of cross-ing ray paths, degrades in the Red Seaand in the southern part of the Platform.


Because most of the teleseismic earth-quakes come from the east or the west,there are relatively few ray paths orientednorth-south. Inclusion of interstationmeasurements between the KACST andEthiopian stations in the inversion wasnecessary to improve the density of cross-ing ray paths.

The phase velocity maps betweenperiods of 65 and 120 s show similar ve-locity variations across the study regionand have similar resolution. Because weare primarily interested in lithosphericmantle structure under the Shield, weshow in Figure 1c the map for 120 s,which is broadly indicative of structuredeeper than about 100-120 km. The mapshows a simple pattern of lower-than-average phase velocities beneath theShield, as well as to the north of theShield near the Gulf of Aqaba. Faster-than-average velocities are found beneaththe Platform surrounding the Shield.

Preliminary resolution tests indicatethat anomalies on the order of a few hun-dred kilometers in wavelength can beresolved. Figure 1d shows the resultsfrom a 400x400 km checkerboard test;clearly anomalies of this size, which aresomewhat smaller than the dimensionsof the Shield, can be resolved.

Discussion

Our preliminary results suggest that themantle lithosphere everywhere beneaththe Shield to the east of the Red Sea rifthas been modified thermally, and thatthere is a fairly abrupt change in lithos-pheric structure across the Shield-Plat-form boundary. The reduction inRayleigh wave phase velocities at 120 speriod under the Shield compared to thePlatform is between 5 and 8%. Similarresults have been reported recently by us[Park et al., 2005] from an S body wavetomography of the region using theSANDSN data, and preliminary resultsfrom S receiver function analysis of theSANDSN data also indicate the presenceof thermally modified upper mantle be-neath the eastern margin of the Red Seaand the Shield [Hansen, personal com-munication]. Previous tomographic im-ages of the Arabian Peninsula using re-

gional data sets indicate the presence ofthermally perturbed mantle lithosphereunder the Shield [Benoit et al., 2003;Debayle et al, 2001], but they do notshow an abrupt change in lithosphericstructure across the Shield-Platformboundary, as suggested by our prelimi-nary results (Figure 1).

How do these results advance our gen-eral understanding of rift processes incontinental settings? And more specifi-cally, how do they help to achieve the

zone thickness beneath the ArabianShield, Geophysical Research Letters, 30,doi:10.1029/2002GL016436.

Bohannon, R. G., C. W. Naeser, D. L. Schmidt,and R. A. Zimmermann, 1989, The timing ofuplift, volcanism, and rifting peripheral to theRed-Sea - A case for passive rifting, Journalof Geophysical Research-Solid Earth andPlanets, 94, 1683-1701.

Camp, V. E., M. J. Roobol, T. H. Dixon, E. R.Ivins, and B. J. Franklin, 1991, Tomographicand volcanic asymmetry around the Red Sea;constraints on rift models; discussion and re-ply [modified], Tectonics, 10, 649-656.

Camp, V. E., and M. J. Roobol, 1992, Upwellingasthenosphere beneath Western Arabia and itsregional implications, Journal of Geophysi-cal Research-Solid Earth, 97, 15255-15271.

Coleman, R. G. and A. V. McGuire, 1988, Magmasystems related to the Red-Sea opening,Tectonophysics, 150, 77-100.

Daradich, A., J. X. Mitrovica, R. N. Pysklywec,S. D. Willett, and A. M. Forte, 2003, Mantleflow, dynamic topography, and rift-flank up-lift of Arabia, Geology, 31, 901-904.

Debayle, E., J. J. Leveque, and M. Cara, 2001,Seismic evidence for a deeply rooted low-ve-locity anomaly in the upper mantle beneaththe northeastern Afro/Arabian continent, Earthand Planetary Science Letters, 193, 423-436.

Grand, S. P., 2002, Mantle shear-wave tomogra-phy and the fate of subducted slabs, Philo-sophical transactions. Mathematical, physi-cal, and engineering sciences, 360, 2475-2491.

Hansen, S., S. Schwartz, A. Al-Amri, and A.Rodgers, 2006, Combined plate motion anddensity driven flow in the asthenosphere be-neath Saudi Arabia: Evidence from shear-wave splitting and seismic anisotropy, Geol-ogy, 34, doi:10.1130/G22713.1.

Julia, J., C. J. Ammon, and R. B. Herrmann, 2003,Lithospheric structure of the Arabian Shieldfrom the joint inversion of receiver functionsand surface-wave group velocities,Tectonophysics, 371, 1-21.

Kumar, M. R., D. S. Ramesh, J. Saul, D. Sarkar,and R. Kind, 2002, Crustal structure and up-per mantle stratigraphy of the Arabian shield,Geophysical Research Letters, 29,doi:10.1029/2001GL014530.

Lawrence, J. F., D. A. Wiens, A. A. Nyblade, S.Anandakrishnan, P. J. Shore, and D. Voigt,2006, Rayleigh wave phase velocity analysisof the Ross Sea, Transantarctic Mountains, andEast Antarctica from a temporary seismographarray, Journal of Geophysical Research-SolidEarth, 111, B06302, doi:10.1029/2005JB003812.

Maguire, P. K. H., C. J. Ebinger, G. W. Stuart, G.D. Mackenzie, K. A. Whaler, J. M. Kendall,M. A. Khan, C. M. R. Fowler, S. L.Kelemperer, G. R. Keller, S. Harder, T.

science goals of the MARGINS Ruptur-ing Continental Lithosphere initiative?Certainly, the rheology of the continen-tal lithosphere on the eastern margin ofthe Red Sea has been thermally weak-ened, and thus the post-rift thermal evo-lution of the lithosphere has likely beenaffected. But beyond that, answers tothese questions depend in large part onwhether the thermal anomaly imaged inthe upper mantle to the east of the RedSea extends under the Red Sea. If thethermal anomaly under the shield devel-oped about 10 Ma after the initiation ofrifting, then it is not clear how this couldhave influenced strain partitioning at thetime of rifting. However, if the anoma-lous mantle structure under the Shieldextends beneath the Red Sea, then thetectonic development of the Red Sea andShield would appear to be linked.

More detailed imaging of the uppermantle under the Red Sea and the Ara-bian Shield is clearly needed. Comple-tion of the surface and body wavetomography studies underway at PennState, King Saud University, andLawrence Livermore National Lab usingthe SANDSN data should provide imageswith improved resolution of upper mantlestructure across the region, letting us de-termine if the low velocity structure inthe upper mantle under the Shield extendsbeneath the Red Sea.

ReferencesAl-Amri, M., and A. Al-Amri, 1999, Configura-

tion of the seimolographic networks in SaudiArabia, Seismological Research Letters, 70,322-331.

Benoit, M. H., A. A. Nyblade, J. C. VanDecar,and H. Gurrola, 2003, Upper mantle Pwave velocity structure and transition See “Seismic” cont. on pg. 25


Geodetic constraints on Rupturing of Continental Lithosphere along

the Red SeaRobert Reilinger1, Simon McClusky1, Abdullah ArRajehi2, Salah Mahmoud3, Ali Rayan3, Woldai Ghebreab4, Ghebrebrhan

Ogubazghi4, and A. Al-Aydrus5

1Department of Earth, Atmospheric, and Planetary Sciences, MIT, Cambridge, MA 02139 USA, 2King Abdulaziz City for Science and Technology, Riyadh, Kingdom of SaudiArabia, 3National Research Institute of Astronomy and Geophysics, Helwan, Cairo, Egypt, 4Asmara University, Asmara, Eritrea, 5Faculty of Science, Sana’a University,Yemen

Our project uses the Global PositioningSystem (GPS) to monitor and quantifypatterns and rates of tectonic and mag-matic deformation associated with activerifting of the continental lithosphere in

the northern Red Sea and the transitionto sea floor spreading in the central RedSea. Figure 1 shows the current networkand preliminary velocities for GPS siteswith sufficient observations, together

with GPS-velocities from our broadercollaborative project (Reilinger et al.,2006). As briefly described here, thesenew data allow us to: 1) identify, andquantify present-day motion of the Sinaimicro-plate that is distinct from bothNubia and Arabia; 2) determine moreprecisely the motion of the Arabian platerelative to Nubia and Eurasia (Euler vec-tors); 3) establish baseline observationsto quantify the spatial distribution of de-formation associated with rupturing ofthe continental lithosphere as a functionof the stage of rifting (i.e., along strikeof the rift); and 4) quantify the relation-ships between deformational processesaround the periphery of the Arabian plateto provide new constraints on the dynam-ics of Arabia plate motion and associatedrifting along the Red Sea. In addition,independent but coordinated observa-tions in Eritrea are beginning to quantifybifurcation of rifting in the southern RedSea near the Afar Rift/Rift/Rift TripleJunction, promising to provide powerfulnew constraints on the dynamics of RedSea rifting and the influence of rheologyon the kinematic response of the lithos-phere.

Sinai Micro-plate[Mahmoud et al., 2005]

GPS survey sites in the Sinai Peninsulashow northerly motion relative to Africa(Nubia) at 1.4 ± 0.8 mm/yr north and 0.4± 0.8 mm/yr west (Figure 1). Continu-ous IGS GPS sites in Israel, west of theDead Sea fault [Wdowinski et al., 2004]show a similar northerly sense of motionrelative to Nubia (2.4 ± 0.6 mm/yr northand 0.04 ± 0.7 mm/yr east), suggestingthat the entire Sinai Block south of Leba-non is characterized by northward trans-lation relative to the Nubian plate. Wedevelop an elastic block model con-strained by the GPS results that is con-


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Figure 1. Red Sea and surroundings showing idealized plate boundaries (Arabia-Nubia-Somalia, dark blue; Arabia and Nubia-Eurasia, light blue) and GPS-derived velocities relativeto Eurasia with 1-sigma error ellipses. The boxes show the locations of Figures 2 (north) and3. The inset, modified from McQuarrie et al. (2003), shows the motion of Arabia and Nubiarelative to Eurasia since 56 Ma along with the GPS-derived motion for Arabia extrapolatedto this time interval.


Figure 2. Elastic block model for the Sinai micro-plate showing residual GPS velocities and95% confidence ellipses, and fault slip rates (fault-normal slip rates in brackets, positive forleft lateral and compression). The inset shows the trade-off between locking depth and sliprate for the model, indicating the best fit for a slip rate of 4.3 mm/yr and a 13 km lockingdepth for the central DSF (see Mahmoud et al., 2005 for details).

sistent with regional tectonics and allowsus to estimate slip rates for Sinai bound-ing faults (Figure 2). The substantial, andperhaps unanticipated left-lateral strike-slip motion within the Gulf of Suez issupported by geologic analyses that in-dicate a late Pleistocene change in theregional stress field from rift-normal toN-S extension [Bosworth and Strecker,1997]. These observations indicate thatthe Sinai Peninsula and Levant regioncomprise a separate sub-plate sand-wiched between the Arabian and Nubianplates. The relatively recent change inGulf of Suez extension provides an im-portant new constraint on the dynamicsof rifting in the N Red Sea.

Arabia Plate Motion

The separation of Arabia from Nubia andassociated extension in the Red Sea ini-tiated in the Miocene roughly simulta-neously with the onset of continental col-lision between Arabia and Eurasia alongthe Bitlis-Zagros suture zone (inset, Fig-ure 1). Collision continues today as evi-denced by the intense seismic activityalong the borders of the Arabian plate.We initiated GPS observations in theKingdom of Saudi Arabia by establish-ing 5 continuously recording stations(Figure 1). These stations, IGS stationsin Bahrain and Damascus, and surveysites in SE Turkey, Oman, and Yemen,constrain Arabia motion and provideboundary conditions on the distributionof extension along the Red Sea plateboundary. The GPS velocities are con-sistent with coherent motion of the Ara-bian plate with internal deformation be-low the current resolution of our mea-surements (~ 1-2 mm/yr). The GPS-de-termined Euler vectors for Arabia-Nubia,and Arabia-Somalia relative motions areindistinguishable from geologic Eulervectors determined from marine mag-netic anomalies in the Red Sea and Gulfof Aden [Chu and Gordon, 1998, 1999],implying that, to the resolution of ourobservations, spreading in the central RedSea is primarily confined to the centralrift (±10-20%). This contrasts with theN Red Sea and Gulf of Suez where ex-tension appears more spatially distrib-

uted. Furthermore, Arabia-Eurasia rela-tive motion from GPS is equal withinuncertainties to relative motion deter-mined from plate reconstructions sug-gesting that Arabia plate motion has re-mained constant (±10%) during at leastthe past ~10 Ma (Figure 1, inset). Thenew constraints on Arabia motion pro-vide corresponding strong constraints onslip rates for the faults bounding the plate,including the Red Sea rift, Dead Sea fault,Makran subduction, faults along theZagros Mountains, and the EastAnatolian Fault (EAF) that separates theArabian and Anatolian plates. Surpris-ingly, we find that while the EAF is char-acterized by predominantly left-lateralstrike slip, it also shows a small compo-

nent of extension. Likewise we find ex-tension in the direction of relative platemotion north of the Arabian plate in theLesser Caucasus. These observations ap-pear to be incompatible with classic “in-dentor/extrusion” models for present-daydeformation within this continental col-lision zone.

Baseline Measurements ofthe Spatial Character ofExtension Along the Red Sea

Figure 1 shows survey sites establishedalong profiles perpendicular to the RedSea spreading axis in Egypt (collectedand analyzed by the National ResearchInstitute of Astronomy and Geophysics -

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NRIAG), Saudi Arabia, and Eritrea. Ex-cept in the vicinity of the Sinai Penin-sula/northernmost Red Sea and in thesouthern Red Sea adjacent to Eritrea, thesurvey sites have an insufficient obser-vation history to determine reliable ve-locities. In addition, we anticipate smallstrains along the coast for the central RedSea based on the approximate coinci-dence of GPS-plate motion and seafloorspreading estimates of spreading rates (<~2mm/yr). More definitive conclusionsabout the distribution of extension willrequire repeat surveys over sufficienttime for the anticipated small signals toemerge from the GPS uncertainties.

Plate Dynamics andRupturing of the Continen-tal Lithosphere

An overarching objective of our geodeticresearch is to establish an observationalbasis to constrain better the dynamics ofplate motions and continental deforma-tion. This is a difficult problem involv-ing trade-offs between driving forces andrheology that has occupied Earth scien-

tists since the advent of Plate Tectonics.However, we believe that the fundamen-tally new kinematic observations pro-vided by space-based geodesy, new quan-titative geological techniques that allowprecise dating of tectonic events, andadvances in seismology that are quanti-fying lithospheric and mantle structureand anisotropy, provide powerful newconstraints on dynamic and rheologicalEarth models.

We alluded above to the implicationsof Arabia-Anatolia relative motion for theextrusion model [Reilinger et al., 2006].For the Red Sea, a fundamental questionconcerns the relative importance of ridge“push” by the Red Sea and Gulf of Adenridges and the Afar hot spot, and slab“pull” along the Makran subduction zone(and Zagros?) for driving the separationof NU and AR and the collision of ARwith Eurasia. In the case of ridge push,we would expect intra-plate, ridge-nor-mal compression within Arabia, whileslab pull would induce trench-normalextension. While at present we prefermodels where subduction dominates the

dynamics, we continue to develop thegeodetic observations necessary to quan-tify present-day strain of Arabia to evalu-ate better the importance of different platedriving forces.

Bifurcation of Rifting in theSouthern Red Sea

Although our MARGINS project is con-fined to the simpler part of the rift in thenorthern and central Red Sea, ourEritrean partners have taken the initia-tive to extend our geodetic network tothe southern Red Sea (Figure 1). Thispart of the Nubia-Arabia plate boundaryincludes the Danakil Block and adjacentDepression that appear to result from bi-furcation of the rift north of the Afarplume (Figure 3). This extension of ournetwork is highly valuable in that it pro-vides a basis to investigate further theinfluence of lithospheric rheology on rift-ing processes.

Spreading in the southern Red Sea isthought to deviate towards the west, southof about 17°N, forming the Danakil De-pression that is separated from the RedSea proper by the intervening NW-SEstriking Danakil Block [e.g., Chu andGordon, 1998] (Figure 3). The Red Searift and Danakil Depression do not ap-pear to be connected by a transform faultas is typical for offset ridges in moremature ocean basins. We propose a kine-matic model for the S Red Sea that in-cludes a “Danakil micro-plate”, ashypothesized by Chu and Gordon [1998],and shown in Figure 3. The model in-volves linearly increasing spreading ratesfrom N to S within the Depression andlinearly decreasing rates along the RedSea rift such that their sum is equal tothe full Arabia-Nubia spreading rate.While the model is very preliminary andpoorly constrained by the available GPSdata, it has a number of interesting fea-tures, including: 1) the proposed Danakilmicro-plate boundaries correspond wellwith seismic activity; 2) the “junction”where the Red Sea rift “bifurcates” at ~17°N involves negligible deformation ofthe SW Red Sea, consistent with the ab-sence of any well defined tectonic fea-tures on the sea floor; 3) the model results

Figure 3. Elastic block model for the S Red Sea and Afar region. The model is highly idealizedand still poorly constrained by GPS. The model involves coherent rotation of the Danakilmicro-plate with extension increasing from N-S on the western plate boundary and decreasingfrom N-S along the Red Sea.

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in coherent rotation of the Danakil block,consistent with its geological structureand aseismic character, and 4) the modelinvolves coherent rotation of the Arabianplate, consistent with the absence of tec-tonic deformation or significant seismicactivity in Yemen. Interestingly, the widthof the Danakil depression at the latitudeof the Afar triple junction (~12°N) isroughly 300 km. Arabia-Nubia relativeplate motion at this latitude is about 17mm/yr. If AR-NU relative motion hasbeen roughly constant since the initiationof Red Sea rifting, as indicated by geo-logical studies (Figure 1 inset), the de-pression would develop in ~18 Myr,roughly the time that Arabia is thoughtto have separated from Nubia(McQuarrie et al., 2003). This impliesthat the Danakil micro-plate has been anintegral part of rifting/triple junction pro-cesses throughout the history of separa-

tion of the Arabian and Nubian plates.

References

Bosworth, W., and M.R. Strecker, 1997,Stress field changes in the Afro-Ara-bian rift system during the Mioceneto Recent period, Tectonophysics,278, 47-62.

Chu, D., and G. Gordon, 1998, Currentplate motions across the Red Sea,Geophys. J. Int., 135, 313-328.

Chu, D., and G. Gordon, 1999, Evidencefor motion between Nubia and Soma-lia along the Southwest Indian ridge,Nature, 398, 64-66.

Mahmoud, S., R. Reilinger, S. McClusky,P. Vernant, and A. Tealeb, 2005, GPSevidence for northward motion of theSinai block: Implications for E. Medi-terranean tectonics, Earth and Plan-etary Sci. Lett., 238, 217-227.

McQuarrie, N., J.M. Stock, C. Verdel,

and B.P. Wernicke, 2003, Cenozoicevolution of the Neotethys and impli-cations for the causes of plate mo-tions, Geophys. Res. Lett., 30, doi10.1029/2003GL017992.

Reilinger, R., et al., 2006, GPS con-straints on continental deformation inthe Africa-Arabia-Eurasia continentalcollision zone and implications for thedynamics of plate interactions, J.Geophys. Res., 111, doi:10.1029/2005/JB004051.

Wdowinski, S., Y. Bock, G. Baer, L.Prawirodirdjo, N. Bechor, S. Naaman,R. Knafo, F. Forrai, & Y. Melzer,2004, GPS measurements of currentcrustal movements along the DeadSea Fault, J. Geophys. Res., 109,B05403, doi:10.1029/2003JB002640.

Seismogenesis and Subduction Fluxes in the Middle America

Subduction Zone: The role of IODP and ORIONAGU Fall Meeting, 2006, Mon., 11 Dec., 8 pm onward, Salon A3, San Francisco Marriott

This public mini-workshop will focus on interconnections between the dynamics of bothseismic and aseismic slip processes and along-strike variations in subduction fluxes (fluids,sediments, and chemical species) through the Middle America arc and fore-arc. How do thephysical and hydrologic structure and composition of the incoming plate and fore-arc con-trol both fluxes and seismogenic processes along the Middle America Trench? Can thesecontrols be quantified sufficiently to make predictions from Central America to other non-accretionary subduction zones? Topics addressing these questions will include: physical,geophysical and hydrological properties of the system; material/chemical fluxes; integra-tion of onland and offshore studies; relation of laboratory and numerical studies to fieldobservations; and development of new technologies and methodologies.

This mini-workshop immediately follows the MARGINS Izu-Bonin-Marianas mini-work-shop in the same location. Food and drink will be provided at both events.

A MARGINS Interdisciplinary Mini-Workshop

Conveners: K. Brown (University of California, San Diego), N. Bangs (U. Texas atAustin), S. Schwartz (U. California, Santa Cruz)

Page 20 MARGINS Newsletter No. 17, Fall 2006 AGU 2006

In this section we highlight several MARGINS events taking place at the 2006 AGU Fall meeting, 11-15 December,Moscone Center West, San Francisco. This will be the first AGU meeting with the new Boston University MARGINSOffice at the helm, and they are looking forward to an interesting week for MARGINS at AGU.

As the Washington University MARGINS Office closes its doors at the end of its three year rotation, on pages 22-25we include our last attempt to pick out AGU sessions of special interest to MARGINS Newsletter readers. We hopeyou find it useful, and Meredith Berwick and Paul Wyer are sorry to be missing this year’s meeting.

As usual, the MARGINS Student Prize will run throughout the week, with a special emphasis on prize participants atthe MARGINS Student and Community Reception (Tues., 12 Dec., 6-9 pm, Salon 9, San Francisco Marriott). Thisreception is a chance for members of the community to mingle in a relaxed environment, and will feature Programupdates from the new MARGINS Chair, Geoff Abers, and NSF Marine Geology and Geophysics Program Director,Bilal Haq.

Interdisciplinary mini-workshops for Izu-Bonin-Marianas (see p. 5) and Central America (see p. 19) during AGUweek represent the first responses to the MARGINS Office solicitation. The call is repeated on p. 26. We hope theAGU mini-workshops will attract a wide audience and guarantee the success of future events of this type.

The MARGINS Database continues at Lamont Doherty Earth Observatory. You can visit them and demo some oftheir data and tools at booth #329 in the AGU Exhibition Hall.

Finally, we are pleased to promote the IODP Town Hall Meeting, Thurs., 14 Dec., 7:30 pm, Salons 4-6, San FranciscoMarriott. Topics in the IODP program will include: Charting IODP Investigations; Planning Workshops; and anUpdate on IODP Platforms. IODP is actively interested in input and proposals from beyond the traditional oceandrilling community; we hope this will encourage a wide range of scientists reading this newsletter to attend.

Calendar:• MARGINS Interdisciplinary Mini-Workshop on the Izu-Bonin-Marianas Subduction Factory Focus Site

(Mon., 11 Dec., 6-8 pm, Salon A3, San Francisco Marriott)• Seismogenesis and Subduction Fluxes in the Middle America Subduction Zone: The role of IODP and

ORION (Mon., 11 Dec., 8 pm onward, Salon A3, San Francisco Marriott)• MARGINS Student and Community Reception (Tues., 12 Dec., 6-9 pm, Salon 9, San Francisco Marriott)

MARGINS at the 2006 AGU Fall Meeting

MARGINS Database, Fall MARGINS Database, Fall AGU 2006AGU 2006Come visit the MARGINS database group:

AGU Exhibition Hall, Booth #329

Explore the MARGINS database. Download data. See demos of

GeoMapApp - a free, versatile tool for plotting and visualizing data.

http://www.marine-geo.org/margins/

The Washington University MARGINS Office, October 2006


MARGINS Student and Community ReceptionMARGINS Student and Community ReceptionMARGINS Student and Community ReceptionMARGINS Student and Community ReceptionMARGINS Student and Community ReceptionMARGINS Student and Community ReceptionMARGINS Student and Community Reception

AGU Fall Meeting 2006AGU Fall Meeting 2006AGU Fall Meeting 2006AGU Fall Meeting 2006AGU Fall Meeting 2006AGU Fall Meeting 2006AGU Fall Meeting 2006

With comments on the MARGIN Program’s status from NSF ProgramDirector, Bilal Haq and MARGINS Chair, Geoff Abers.

Tuesday, 12 December, 6-9 pm, Salon 9, San Francisco Marriott

The MARGINS Reception at Fall AGU is an open event, welcoming par-ticipants in MARGINS-funded studies and all others with an interest in theprogram. This year, the reception will highlight the participants in the MAR-GINS Student Prize by offering space to display and discuss posters pre-senting their research. Students are encouraged to use this event as an op-portunity to further share their results and interact with a wide spectrum ofMARGINS scientists.

There will be ample time to mingle, with food and drink courtesy of Bos-ton University and the new Boston MARGINS Office. Among those presentwill be Geoff Abers (MARGINS Chair), members of the MARGINS Steer-ing Committee, and Program Directors for MARGINS from the NationalScience Foundation (NSF).

Geoff Abers and NSF Program Director, Bilal Haq, will use this event as anopportunity to welcome those present, introduce the new MARGINS Of-fice, and update the community on MARGINS funding and other issues,including a discussion of the recent change to the Red Sea status and otherancillary sites.


Sessions directly addressingMARGINS themesOS: Studies of Sediment Transfer FromLand Through the Ocean and into theStratigraphic RecordBecause of the global diversity in tectonicsetting, climate, and fluid-energy dissipation,the fate of fluvial discharge in continentalmargins is complex and our ability to quan-tify and predict the fluxes of dissolved andparticulate substances over space and timerequires a concerted interdisciplinary effort.This session is intended to provide a venuefor the broad community investigating theproduction, transport, storage and accumu-lation of terrestrial and marine materialsalong the source-to-sink pathways in theworld’s continental margins. OS12A, OS13D,OS14A (MCS 206), OS23A, OS23B (MCWLevel 2)OS: New Algorithms and Models for Flu-vial and Coastal Sediment-Transport andSurface DynamicsContinuing advances in our understanding offluid dynamics, particle behavior, bed char-acteristics and the evolution of morphologyare resulting in new algorithms and modelsfor sediment transport. This session will fo-cus on emerging concepts in sediment dy-namics that are appearing in process-orientedmodels. Presentations describing observa-tions of novel processes, presenting newanalyses of sediment-transport processes, ordescribing algorithms for incorporating in-creasingly realistic dynamics into practicalmodels are welcomed. OS24A (MCW 3009)T: The Geodynamics of Lithospheric Ex-tensionThis session focuses on the geodynamics oflithospheric extension at all scales. Contri-butions are invited that elucidate the evolu-tion of extensional tectonics, from a tectonic,sedimentary, magmatic and metamorphicpoint of view, and over the whole range ofgeodynamic conditions, from post-orogenicextension of overthickened crustal belts, tocontinental break-up and oceanic spreading.T23F, T24A (MCW 3004), T31B, T31D(MCW Level 1)

T: New Observations From the MantleWedge: Consequences for Water, Petrol-ogy, Melt, and FlowThis session seeks to highlight new con-straints on structure, flow, dehydration andmelting processes within the subduction zonemantle wedge. Papers from seismology,geodynamical modeling, mineral physics,geochemistry, and petrology are encouraged.T21G, T22C, T33E (MCS 302)T: New Perspectives on the SeismogenicZone: From the Surface to Depth andModern to AncientMost of the world’s largest earthquakes andtsunamis initiate in subduction zones, yet ourunderstanding of fault zone processes is lim-ited. Recent studies of modern and ancientsubduction zones as part of the SeismogenicZone Experiment (SEIZE) and related stud-ies provide new perspectives on these pro-cesses. This session welcomes results ofonland field work on ancient prisms, onlandand offshore geophysical investigations,modeling and laboratory studies, and seaf-loor/subseafloor observations that provideinsight on seismogenic zone processes. T12C,T13G (MCS 302), T21A (MCW Level 1)T: Extensional Processes Leading to theFormation of Basins and Rifted Margins,From Volcanic to Magma-LimitedNew observations and models allow us toinvestigate the processes responsible for con-tinental extension and lithospheric rupture inunprecedented detail. Key questions that needto be addressed on all rifted margins concernthe style of the early phases of extension,delineating the factors that are most impor-tant in controlling strain localization and par-titioning throughout rifting (e.g., pre-exist-ing weaknesses, detachment and/or rollinghinge faults, syn-rift magmatism, etc.), andunderstanding how variations in rheologywith depth influence the style of rifting andfinal breakup. T51C, T52C (MCW 3004),T53A (MCW Level 2)T: Development of the Gulf of Californiaand Other Young Divergent Plate Bound-aries Along Tectonically Active ContinentMarginsLithosphere ruptures in continent interiors

and along active continental margins. TheGulf of California is an oblique-divergentplate boundary formed along a volcanic arcand between older magmatic arcs. Recentonshore and offshore, and surface to mantlestudies, from MARGINS and other projectshave made major advances in understandingthe history of the plate boundary and pro-cesses of rifting. Contributions are invitedfrom the Gulf of California and other youngrifts along active continental margins. T41D(MCW Level 2)V: Lessons From the Izu-Bonin-Marianaand Central American Subduction Facto-riesPresentations are invited that discuss all as-pects of the Izu-Bonin-Mariana and CentralAmerican subduction zones, including sub-duction input, forearc processes and the ori-gin and evolution of magma and crust. Top-ics might include: character and origin of thecrust; relationship between subduction inputand output in the forearc and volcanic arc;how differences in the mantle wedge or sub-duction parameters affect magma composi-tion; reasons for spatial and temporal varia-tions in magma composition, includingvolatiles; and flux estimates. V41B (MCW,Level 2), V51F, V52B (MCW 3010)

Other sessions relevant toMARGINS scienceB: Advances in Process Understanding andImplications of Exchanges Across the Sedi-ment-Water InterfaceThe depth of surface-water penetration, re-tention time within shallow subsurface envi-ronments, and the solute flux within thesesubsurface environments relative to water andsolute flux at the surface are all importantwhen addressing the solute retention/trans-formations in these environments. This ses-sion seeks to bring together a combinationof coupled field and modeling approachesthat addresses both the physical processesaltering solute transport across sediment-water interfaces and the resulting bio-geochemical implications. B22C (MCW3004), B23A (MCW Level 2)

Sessions Related to MARGINS Science

at the Fall 2006 AGU Meeting6The extensive list of sessions in AGU’s Fall Meeting program can be daunting, so each year the MARGINS Office assembles a list ofsessions that we think may be of special interest to the MARGINS community. The concise summaries included with our subjective choicesare edited excerpts from the original AGU session abstracts (www.agu.org/meetings/fm06).AGU Code Key: Section: Day of Week (1-5): Session Time (1X: 8:00; 2X: 10:20; 3X: 13:40; 4X: 16:00). E.g., OS12A = Ocean Sciences,Monday, Session 2A (10:20). MCS = Moscone Center South; MCW = Moscone Center West. Please refer to the AGU meeting program toverify session times and locations.

AGU 2006


ED: Successful Partnerships in GeoscienceEducation: Past and PresentStarting in the 1970s the Earth Science com-munity embarked on innovative programs toincrease diversity in the geosciences. Newprograms that grew from these early pro-grams took on best practices and innovativeeducational components that are now incor-porated in programs for all children andteachers. This session will look back at someof these programs in a historical frameworkand will look forward by examining the suc-cess of current programs that aim to increasethe participation of diverse students in theearth and environmental sciences. ED33C(MCS 310), ED51A (MCW Level 2)ED: Hands-on, Inquiry-Based Classroomand Laboratory Assignments: BringingResearch in Earth-Surface Processes andHydrology to K-12 and UndergraduateStudentsPresenters in this session will introduce a labexercise, demonstration, or hands-on activ-ity that they have designed or adapted for usein their K-12 or undergraduate geoscienceclassroom or lab. In addition to their oral orposter presentation, the presenters will dis-play their activity, lab exercise, or demon-stration in the “Hands-on Gallery” during theposter session, so that participants in the par-allel teacher workshop can test the materi-als. ED51D (MCS 274), ED53A (MCW Level2)ED: Facilitating Undergraduate Researchin the Geosciences: Classroom Innovationsthat Encourage and Support Student In-vestigationsA wide range of projects supported by theNational Science Foundation’s Course, Cur-riculum, and Laboratory Improvement(CCLI), Geoscience Education, and COSEEprograms, NASA programs, and other fund-ing sources have sought to bring the tools andmethods of geoscience research into under-graduate classrooms. This session seeks tohighlight those classroom approaches thathave been effective in aiding undergraduatestudents in the transition from learner to in-vestigator. ED33B (MCW Level 2)H: Soft Computing Tools for Hydrologi-cal ModelingSoft computing is an emerging computationalapproach which integrates several artificialintelligence methodologies to deal with un-certainty and imprecision associated withmodeling and understanding hydrologic sys-tems. This session will invite presentationsthat will focus on applications of soft com-puting techniques to deal with a variety ofproblems within the field of surface and sub-

surface hydrology at different spatial and tem-poral scales. H23D (MCW Level 2)H: Coastal Geomorphology andMorphodynamicsCoastal environments, including sandyshores, rocky coasts, marshes, estuaries anddeltas, evolve in response to winds, waves,currents, tides, and changes in relative sea-level. This session will focus on large-scalecoastal evolution (scales >> m and >> days),exploring feedbacks between changes inmorphology and forcing agents, including theinfluence of the geological framework oncoastal dynamics. H31I, H32C (MCW 3002),H33B (MCW Level 2)H: Hillslope, Glacial, and Drainage BasinPostersThis is a general poster session on processesthat affect the form and function of the sur-face of the Earth. These processes, in whichphysics, chemistry, and biology have roles,occur over a wide range of temporal and spa-tial scales, and include fluvial, Aeolian, andcoastal sediment transport and the resultingerosion and sedimentation; hillslope massmovements; glacial and periglacial activity;weathering and pedogenesis; surface mani-festations of volcanism and tectonism; andhuman activities that modify the surface ofthe Earth. H53B (MCW Level 1)IN: Standardizing Fine-Grained Access toGeoscience DataWeb services provide the opportunity for on-demand open access to observational andother geoscientific data in support of scien-tific investigations. However, the style of datarequired varies significantly across the geo-sciences. This topical session is intended toprovide a forum for investigators working ondata models and structures, schemas and ser-vice interfaces in support of fine-grained dataaccess within the geosciences. IN51B (MCWLevel 2), IN53C (MCW 3020)IN: Standards-Based InteroperabilityAmong Tools and Data Services in theEarth SciencesGroups are actively developing interoperabledata access, analysis and display systemsbased on evolving international standards,such as the Open Geospatial Consortium(OGC) protocol set. Presentations and dem-onstrations for this session are encouragedfor GALEON, GSN and similarinteroperability efforts. There will be a spe-cial electronic poster area set up for live,online demonstrations of theseinteroperability technologies. IN42A (MCS309), IN43C (MCW Level 2)NG: Autogenic Dynamics in LandscapeEvolution and the Geologic Record

Nonlinearity and noisiness in sediment trans-port systems produce fractal surfaces anddynamic unpredictability. This internally-generated (“autogenic”) variability acts as afilter to obscure the signal of environmental(“allogenic”) change. Stratigraphy is the ac-cumulated record of these partially-preservedsurfaces and hence is a signal that has beenfurther filtered. The goal of this session is tobring together research that explores the char-acteristic length and time scales of autogenicprocesses and their influence on the evolu-tion of landscapes and the stratigraphicrecord. NG43D (MCW Level 2), NG53A(MCW 3022)NS: Applications of Near-Surface Geo-physics in Coastal EnvironmentsNear-surface geophysical methods are usedincreasingly in support of coastal geologicaland engineering studies, resource manage-ment, and hazard mitigation research. Seis-mic, electrical, resistivity, and electromag-netic (EM and GPR) methods provide high-resolution information for investigation ofcoastal geomorphology and stratigraphy,geoarchaeological context, hazard assess-ment, geotechnical characterization, andgroundwater exchange in coastal aquifers.This session welcomes contributions in theseareas. NS24A (MCS 220), NS31B (MCWLevel 2)OS: Nearshore ProcessesFor over 30 years, our understanding ofnearshore processes has developed and grownin large part from the pioneering work of Dr.Edward B. Thornton and others. This sessioninvites abstracts that focus on the dynamicsof waves, currents, turbulence, and sedimenttransport from the beach face to the shelfbreak along both sandy and muddy coastlines.Abstracts covering all aspects of nearshoreprocesses research are welcome. OS21D,OS22B, OS23D (MCW 3009)OS: Cabled Ocean Observatories: NovelScience Experiments, Technologies andData Management SystemsCabled ocean observatories can transform theocean sciences with the introduction ofpower, high bandwidth communication,elaborate sensor networks, and abundant real-time data return spanning decades. Existingand emerging observatories include LEO 15,VENUS, MARS, NEPTUNE, ARENA andESONET. The session will consider the novelcommunity science experiments, new andmodified technologies, and complex datamanagement systems handling the abundant(Gb/sec) real-time data stream, includingvideo and HDTV. The profound impact ofthe scientific, technological, educational and


outreach opportunities will be examined.OS31C (MCW Level 2), OS34F (MCS 220)OS: Communicating Broadly: Perspec-tives and Tools for Ocean, Earth and At-mospheric ScientistsAcross the geosciences, opportunities aboundfor researchers to communicate with peoplewho have distinct and sometimes divergentinterests – journalists, resource managers,environmentalists, policy makers, philanthro-pists, educators, and industry leaders, to namejust a few. In this session, scientists and oth-ers within the academic community are in-vited to reflect on their experiences with suchcommunication. Presentations that describeresources for building scientists’ communi-cation skills – for example, organizations,programs, workshops, courses and publica-tions – are also highly encouraged. U41F,U42C, U43D, U44C (MCW 3006)S: Geophysical Structure and Dynamics ofthe Western United StatesBroad ranges of geological and geophysicaldatasets are currently being collected andgeodynamical models are in development toprovide new constraints on the structure anddynamics of the western United States. Therich and complex tectonic processes and his-tory of this region are still under significantdebate, thus necessitating a host of newmultidisciplinary examinations. New obser-vations and models from seismology,geodynamics, mineral physics, and petrologyas they relate to this region are encouraged,particularly those which marry complemen-tary datasets and methodologies. S43A (MCWLevel 2), S51D, S52A (MCW 3009), S54A(MCS 305)S: The July 17, 2006 Java Earthquake andTsunami: What Are We Learning?The precise rupture processes and control-ling mechanism of such slow source tsunamievents as the July 17, 2006 Java Earthquakeand Tsunami are poorly understood, in partbecause they likely occur in a weak subduc-tion interface that is often considered to beunable to nucleate earthquake rupture. Thissession invites contributions that seek to im-prove identification and our understanding ofthis and other such events, and the processesthat control their tsunami generation. S14A(MCW 3009), S21A (MCW Level 1)T: Episodic Tremor and Slip: Correlatoinsto Geologic and Geophysical Segmentationin CascadiaMany different types of observations fromthe Cascadia Subduction Zone suggest along-arc segmentation. The purpose of this ses-sion is to bring together investigators from

different disciplines to review and updateobservations of arc (sensu lato) segmentationin order to explore links among disparate datasets and provide insights into the underlyingprocesses that control the observed segmen-tation. T53G (MCS 300)T: GeoFrame: A Geologic Framework forEarthScope’s USArrayThe GeoFrame initiative is a new geologicventure that focuses on the construction, sta-bilization, and modification of the NorthAmerican continent through time. The pur-pose of this session is to present recent re-search in the areas identified during a recentEarthScope workshop (a mega swath includ-ing Cascadia, the Northern Rockies, the BlackHills/Great Plains, the Superior Province inthe US, the Mid-Continent region, and thecentral Appalachians and a long swath alongthe Walker Lane trend) and to provide a fo-rum for the community to present researchresults, in addition to making suggestions andmodifications to the target areas. T42A (MCS301), T43C (MCW Level 2)T: Seismogenesis and Tsunami Hazards of“Aseismic” Island ArcsThe tectonic environment of the Nicobar andAndaman Islands section of the Sumatra-Andaman Earthquake rupture zone is nottypical of subduction zones that experiencegiant earthquakes, and yet it experienced slipequivalent to an earthquake of magnitude 9or greater. Our inability to explain why thissegment experienced such large-scale ruptureprompts us to reconsider whether the Chil-ean-Mariana paradigm for seismic vs.aseismic subduction is adequate for under-pinning assessments of tsunami hazard in is-land arcs and ocean basins bordered by them.This session aims at assessing the currentstate of knowledge of seismogenesis and tsu-nami hazards in island arcs, especially thosein tectonic environments that are thought tonot favor the occurrence of giant earthquakes.T21F (MCS 301), T23A (MCW Level 1)T: Phenomenology, Mechanisms, and Haz-ard Implications of Episodic Aseismic Slip,Tremor, and EarthquakesRepeating episodes of aseismic fault slip,seismic tremor, and perhaps associated earth-quake rate changes are best documented insubduction zones around the world, but havealso been observed in a growing number ofother geologic settings. This session seekspresentations that critically examine the ob-servational constraints on these phenomena,compare their differences from region to re-gion, test explanatory physical models, anddiscuss their implications for assessing vol-

canic and earthquake hazards. T54A (MCS300)T: New Observations of Dike InjectionEpisodes in Extensional TerrainsIn September 2005, a seismic swarm aroundthe Dabbahu (Boina) rift segment in Afar,Ethiopia, was associated with the intrusionof a 60 km long dike, up to 8 m wide, alongthe entire rift segment. This is the largest rift-ing episode to have occurred subaerially sincethe Krafla (Iceland 1975-1984) and Asal-Ghoubbet (Djibouti, 1978) episodes, and of-fers a unique opportunity to learn aboutcrustal growth at divergent plate boundaries.This session invites contributions that de-scribe results from the broad spectrum ofgeophysical and geological techniques thathave been applied to subaerial and subma-rine rifting episodes. T33E (MCW 3002),T41B (MCW Level 2)T: Postcollisional ExtensionWithin the last quarter century, our under-standings of post-collisional continental ex-tension have improved substantially. How-ever, there are still many important questionssuch as the nature and geometry of initialnormal faults formed during post-collisionalextension, and the nature and depth of theductile-brittle transition. It is also still debatedwhether the post-collisional continental ex-tension is initiated by simple shear or pureshear. The main purpose of this session is tobring together researchers working on theproblems of post-collisional continental ex-tension and associated structures such asmetamorphic core complexes in differentparts of the world. T33B (MCW Level 1),T41E (MCS 301)U: Communicating Broadly: Perspectivesand Tools for Ocean, Earth and Atmo-spheric ScientistsAcross the geosciences, opportunities aboundfor researchers to communicate with peoplewho have distinct and sometimes divergentinterests – journalists, resource managers,environmentalists, policy makers, philanthro-pists, educators, and industry leaders, to namejust a few. In this session, scientists and oth-ers within the academic community are in-vited to reflect on their experiences with suchcommunication. Presentations that describeresources for building scientists’ communi-cation skills – for example, organizations,programs, workshops, courses and publica-tions – are also highly encouraged. U41F,U42C, U43D, U44C (MCW 3006)U: Consequences of Subduction and theEvolution of the MantleThe chemical and physical properties of sub-

AGU 2006


ducted oceanic lithosphere are not well-known, but control many aspects of Earth’sconvective/tectonic state as well as it’s ther-mal history. This session welcomes studiesfrom geochemistry, geodynamics, seismol-ogy and mineral physics, and in particularstudies that combine approaches from differ-ent disciplines, in an effort to integrate thenature of subduction with deep mantle pro-cesses. U11A, U12A, U13B, U14B (MCS308), U12A (MCW Level 2)U: The Indian Ocean Tsunami 2004: TwoYears On.Based on the enormous amount of new workon the Andaman-Sumatra earthquakes andtsunamis, the objective of the session is tobring together scientists from all disciplinesto present their research in the region and tofoster and encourage multidisciplinary col-laboration. Applications of the research in theIndian Ocean to other areas are also encour-aged. U44A, U51B, U52A (MCS 308), U53A(MCW Level 1), U53C (MCS 308)U: Large Distributed Arrays of Geophysi-cal and Environmental SensorsUsing modern technological systems, it isnow possible to monitor the Earth and itsspace environment with increasing accuracyand frequency, and to receive the data with

near-real-time promptness, using very largearrays of data acquisition and data transpor-tation links. The purpose of this special ses-sion is to bring together working representa-tives involved with very large distributed ar-rays of sensors in order to foster communi-cation about the practicalities of operatingsuch systems, to discuss theoretical issues thatmight pertain to their management and fu-ture development, and to promote coopera-tion and coordination. U41B (MCW Level 2)V: To What Depth Can Continental Crustbe Subducted: Observations From Ultra-high-Pressure Metamorphic Rocks, Ex-periments, and Numerical ModelingUltra-high pressure metamorphic rocks(UHPM) are the best natural laboratory tostudy to what depth continental crust may besubducted. The main task of this session is toformulate clear statements: (1) what do wereally know from the UHPM rocks aboutdeep subduction of the crustal material; (2)how is modern experimental and numericalmodeling consistent with the “facts” collectedfrom natural rocks? V31A (MCW Level 1),V43F, V44B (MCW 3011)V: Observations and Interpretations ofLow-Frequency Earthquakes in Volcanicand Nonvolcanic Environments

Recent observations and research have high-lighted that ordinary stick-slip failure mayproduce LF earthquakes in certain volcanicsettings due to exceptionally high strain rateswithin the magma, low rupture velocities,and/or complexities in the path betweensource and seismometer. The goals of thissession are to investigate the range of mecha-nisms that may produce LF events and therange of settings in which various types ofLF events occur. V41A (MCW Level 1), V52A(MCW 3012)

Other sessions of interestT: Interpreting the Tectonics of the PacificRim Using Plate Kinematics and Slab Win-dow Volcanism. T51C (MCW Level 2), T53E(MCS 301)V: Crystal-scale records of magmatic pro-cesses. V51B (MCW Level 2), V53E (MCW303)V: Origin, Storage, and Transport of Wa-ter in Earth’s Mantle. V41D (MCW Level2), V53F, V54A (MCS 304)V: The Dynamic Reaction: Interactions ofMetamorphic Reactions and Deformationin Nature, Experiments, and Models. V31C(MCW Level 1), V33F (MCW 3012)

AGU Fall Meeting mini-workshopson pp. 5 and 19).

13. Other matters:• The new MARGINS Office at

Boston University would officiallytake over operations on October 1,2006. The MSC and NSF ProgramOfficers thanked Paul Wyer andMeredith Berwick for their role inthe success of the MARGINSOffice at Washington Universityin Saint Louis and their continuingsupport as the new office takesover.

• Interim Chair, Doug Wiens wasthanked for providing directionand continuity at WashingtonUniversity since Julie Morrisbegan her rotation as NSF-OCEDivision Director in April, 2006.

• John Milliman was thanked for hisdedicated commitment as herotated off the Steering Commit-tee.

P- and S-wave velocity heterogeneity beneaththe Arabian Peninsula, EOS, Trans. AGU, FallMeeting supplement, F1410.

Ritsema, J., H. J. van Heijst, and J. H. Woodhouse,1999, Complex shear wave velocity structureimaged beneath Africa and Iceland, Science,286, 1925-1928.

Rodgers, A., W. Walter, R. Mellors, A. M. S. Al-Amri and Y. S. Zhang, 1999, Lithosphericstructure of the Arabian Shield and Platformfrom complete regional waveform modelingand surface wave group velocities, Geophys.J. Int., 138, 871-878.

Sandvol, E., D. Seber, M. Barazangi, F. Vernon,R. Mellors, and A. Al-Amri, 1998, Lithos-pheric seismic velocity discontinuities beneaththe Arabian Shield, Geophysical ResearchLetters, 25, 2873-2876.

Vernon, F., R. Mellors, J. Berger, A. Edelman, A.Al-Amri, J. Zollweg, and C. Wolfe, 1996,Observations from regional and teleseismicearthquakes recorded by a deployment ofbroadband seismometers in the Saudi ArabianShield, EOS Trans. AGU, 77, 478.

Wolfe, C. J., F. L. Vernon, and A. Al-Amri, 1999,Shear-wave splitting across western SaudiArabia: The pattern of upper mantle anisot-ropy at a Proterozoic shield, Geophysical Re-search Letters, 26, 779-782.

Zhao, D., 2001, Seismic structure and origin ofhotspots and mantle plumes, Earth And Plan-etary Science Letters, 192, 251-265.

Furman, K. Mickus, L. Asfaw, A. Ayele, andB. Abebe, 2003, Geophysics project in Ethio-pia studies continental breakeup, EOS Trans.AGU, 84, 342-343.

McGuire, A. V., and R. G. Bohannon, 1989, Tim-ing of mantle upwelling - evidence for a pas-sive origin for the Red-Sea rift, Journal ofGeophysical Research-Solid Earth and Plan-ets, 94, 1677-1682.

Mellors, R. J., V. E. Camp, F. L. Vernon, A. M. S.Al-Amri, and A. Ghalib, 1999, Regional wave-form propagation in the Arabian Peninsula,Journal of Geophysical Research-Solid Earth,104, 20221-20235.

Menke, W. and V. Levin, 2002, Anomalous sea-ward dip of the lithosphere-asthenosphereboundary beneath northeastern USA detectedusing differential-array measurements ofRayleigh waves, Geophysical Journal Inter-national, 149, 413-421.

Mohr, P. A., B. Zanettin, and Anonymous, 1988,The Ethiopian flood basalt province, in Con-tinental Flood Basals, edited by J. D.Macdougal, Kluwer Academic, Boston, Mass.,65-110.

Nyblade, A. A., and C. A. Langston, 2002, Broad-band seismic experiments probe the East Af-rican rift, EOS Trans. AGU, 83, 405-408.

Park, Y., A. A. Nyblade, A. Rodgers, A. Al-Amri,2005, Tomographic imaging of upper mantle

“Highlights” cont. from pg. 7 “Seismic” cont. from pg. 15

-PW, October 2006


Call for interdisciplinary MARGINS mini-workshops

The MARGINS Office and Steering Committee aim to support efforts that expedite synthesis of results from MARGINS sciencein the various focus areas and initiatives. To this end, the MARGINS Office offers to help MARGINS funded investigatorsorganize and fund mini-workshops held at national meetings for the purpose of bringing together a group of multi-disciplinaryinvestigators to synthesize results to date. Such mini-workshops can be associated with GSA, AGU, or other national meetingsat which your research area is well represented. They can be 3-4 hour workshops one evening after sessions, or half-day to day-long sessions before or after the meeting. They can bring together multiple investigators from a single focus site or from bothfocus sites within an initiative, or can address a theme that transcends initiatives, according to what makes the most scientificsense and where there is the greatest need.

There are some ground rules for such mini-workshops, as discussed below. The rules are intended to maximize the benefit ofsuch workshops to a larger scientific community and emphasize opportunities for interdisciplinary integration, as opposed toproviding a venue for a single or few proponent groups to meet. Once the MARGINS Steering Committee approve a mini-workshop proposal, arrangements will be made as follows:

1. The MARGINS Office will provide the cost of a meeting room, presentation equipment and non-alcoholic refreshments, and will work with the meeting conveners and local hotels to make logistical arrangements. Regretfully, the MARGINS Office CANNOT afford to provide travel or lodging costs for participants. Alcoholic refreshments may be served as a goodwill gesture from another appropriate organization (a convener’s home institution, for example), but CANNOT be subsidized using MARGINS Office funds.2. Workshop conveners are responsible for developing the science program, communicating with workshop participants on scientific matters and working with the MARGINS Office to arrange logistics.3. Any MARGINS Office supported mini-workshop will be open to all interested parties and will be advertised via the MARGINS mailing list, MARGINS website, and meeting program.4. Workshop conveners will provide a brief write-up of the major results of the workshop for dissemination via the MARGINS website and newsletter, so that important outcomes may be shared with the larger community.

If you are interested in hosting a mini-workshop, coordinate with your colleagues, and then send the MARGINS Office a 1-2page outline of your meeting plan as soon as possible. Requests should generally come not later than 3 months ahead of themeeting. The MARGINS Steering Committee will review the submitted proposal before the MARGINS Office will agree tosupport a synthesis mini-workshop. Your write-up should include:

• Scientific rationale for the meeting and reasons for its timeliness.• Evidence that a wide group of interdisciplinary researchers would be able to attend.• A draft scientific program for the mini-workshop.• The national meeting with which the mini-workshop would be associated.• The format (evening, half-day or full day, pre- or post-meeting) desired and acceptable dates.• Size of meeting envisioned.• Anticipated cost items (meeting space, refreshments, A/V equipment, etc.). Note that a detailed budget for these

costs is not initially required.

Thank you for considering such an undertaking, and we look forward to hearing from you.

With best regards,

The MARGINS Office and Steering Committee


Geoffrey Abers, ChairDepartment of Earth SciencesBoston University675 Commonwealth AvenueBoston, MA 02215Tel: (617) 353-2616e-mail: [email protected]

Peter van KekenDepartment of Geological Sciences425 East University AvenueUniversity of MichiganAnn Arbor, MI 48109-1063Tel: (734) 764-1497e-mail: [email protected]

Paul J. UmhoeferDepartment of Geology, Box 4099Northern Arizona UniversityFlagstaff, AZ 86011Tel: (928) 523-6464e-mail: [email protected]

Uri S. ten BrinkU.S. Geological Survey384 Woods Hole Rd.Woods Hole, MA 02543 USATel: (508) 457-2396e-mail: [email protected]

Steve KuehlVirginia Institute of Marine ScienceCollege of William and MaryHolben House Room 201Cloucester Point, VA 23062Tel: (804) 684-7118e-mail: [email protected]

Lincoln PratsonDivision of Earth and Ocean SciencesDuke University203 Old Chem Bldg, Box 90230Durham, NC 27708-0277Tel: (919) 681-8077e-mail: [email protected]

Don ReedDepartment of GeologySan Jose State UniversitySan Jose, CA 95192-0102Tel: (408) 924-5036e-mail: [email protected]

James GillEarth Sciences DepartmentUniversity of California-Santa Cruz137 Applied Sciences1156 High StreetSanta Cruz, CA 95064Tel: (831) 459-2425e-mail: [email protected]

Julia K. MorganDepartment of Earth Science, MS-1266100 Main StreetRice UniversityHouston, TX 77005-1892Tel: (713) 348-6330Houston, TX 77005-1892e-mail: [email protected]

W. Roger BuckLamont-Doherty Earth ObservatoryRt. 9WPalisades, NY 10964Tel: (845) 365-8592e-mail: [email protected]

Mike BlumDepartment of Geology and GeophysicsE235 Howe-Russell Geosciences ComplexLouisiana State UniversityBaton Rouge, LA 70803Tel: (225) 578-5735e-mail: [email protected]

W. Steven HolbrookDepartment of Geology and GeophysicsUniversity of WyomingEarth Sciences Building 3016Laramie, WY 82071-3006Tel: (307) 766-2724e-mail: [email protected]

Liz ScreatonDepartment of Geological SciencesUniversity of Florida241 Williamson, Box 112120Gainesville, FL 32611Tel: (352) 392-4612e-mail: [email protected]

Jeff RyanDepartment of GeologyUniversity of South Florida4202 East Fowler Ave., SCA 528Tampa, FL 33620-5201Tel: (813) 974-1598/6287e-mail: [email protected]

MARGINS Steering Committee

Bilal HaqMarine Geology and Geophysics Program

Division of Ocean SciencesTel: (703) 292-8582Fax: (703) 292-9085

e-mail: [email protected]

Rodey BatizaOcean Drilling Program

Division of Ocean SciencesTel: (703) 292-8581Fax: (703) 292-9085


William LeemanPetrology and GeochemistryDivision of Earth Sciences

Tel: (703) 292-7411Fax: (703) 292-9025


NSF Program DirectorsNational Science Foundation, 4201 Wilson Boulevard, Arlington, Virginia 22230

Please refer to the MARGINS website for updates to this information.

MARGINS OfficeBoston University, Department of Earth Sciences

675 Commonwealth Ave., Boston, MA 02215Tel: (617) 358-4624, Fax: (617) 353-3290, E-mail: [email protected],Website: www.nsf-margins.org

Anne TrehuCollege of Oceanic and Atmospheric SciencesOregon State UniversityOceanography and Admin. Bldg. 104Corvallis, OR 97331-5503Tel: (503) 737-2655e-mail: [email protected]

This newsletter is supported by the National Science Foundation underGrant No. OCE 03-25002. Any opinions, findings, and conclusionsexpressed in it are those of the authors and do not necessarily reflect theviews of the National Science Foundation.

MARGINS OfficeWashington University in St. Louis

1 Brookings Drive, CB 1169

St. Louis, MO 63130 USA

Non-Profit

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2006

This newsletter was produced and distributed on behalf of MARGINS by the out-goingMARGINS Office at Washington University in St. Louis:Editor: P. Wyer <[email protected]>Composition, graphics, and sub-editing: M. Berwick <[email protected]>

Farewell from the Washington University

MARGINS Office. We’ve enjoyed our run

and will miss working with you.

Best wishes to the new MARGINS Office at

Boston University. If your three years yield

as many challenges, solutions and rewards

as ours, we know you’ll have a great time!

Documents

Published bi-annually by the Report on MARGINS Workshop ...rjstern/egypt/PDFs... · Report on MARGINS Workshop: Interpreting Upper-Mantle Images James B. Gaherty1, Greg Hirth2, and