Variable crustal structure along the Juan de Fuca Ridge ...carbotte/documents/2008CarbotteGC.pdf · Influence of on-axis hot spots and absolute plate motions ... and M. Marjanovic´

Variable crustal structure along the Juan de Fuca Ridge:Influence of on-axis hot spots and absolute plate motions

Suzanne M. CarbotteLamont-Doherty Earth Observatory, Earth Institute at Columbia University, Palisades, New York 10964, USA([email protected])

Mladen R. NedimovicLamont-Doherty Earth Observatory, Earth Institute at Columbia University, Palisades, New York 10964, USA

Department of Earth Sciences, Dalhousie University, Halifax, Nova Scotia B3H 4J1, Canada

Juan Pablo CanalesWoods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA

Graham M. Kent and Alistair J. HardingScripps Institution of Oceanograpy, University of California, San Diego, La Jolla, California 92093, USA

Milena MarjanovicLamont-Doherty Earth Observatory, Earth Institute at Columbia University, Palisades, New York 10964, USA

[1] Multichannel seismic and bathymetric data from the Juan de Fuca Ridge (JDFR) provideconstraints on axial and ridge flank structure for the past 4–8 Ma within three spreading corridorscrossing Cleft, Northern Symmetric, and Endeavour segments. Along-axis data reveal south-to-northgradients in seafloor relief and presence and depth of the crustal magma lens, which indicate a warmeraxial regime to the south, both on a regional scale and within individual segments. For young crust,cross-axis lines reveal differences between segments in Moho two-way traveltimes of 200–300 mswhich indicate 0.5–1 km thicker crust at Endeavour and Cleft compared to Northern Symmetric.Moho traveltime anomalies extend beyond the 5–15 km wide axial high and coincide with distinctplateaus, 32 and 40 km wide and 200–400 m high, found at both segments. On older crust, Mohotraveltimes are similar for all three segments (�2100 ± 100 ms), indicating little difference in averagecrustal production prior to �0.6 and 0.7 Ma. The presence of broad axis-centered bathymetric plateauwith thickened crust at Cleft and Endeavour segments is attributed to recent initiation of ridge axis-centered melt anomalies associated with the Cobb hot spot and the Heckle melt anomaly. Increasedmelt supply at Cleft segment upon initiation of Axial Volcano and southward propagation ofEndeavour segment during the Brunhes point to rapid southward directed along-axis channeling ofmelt anomalies linked to these hot spots. Preferential southward flow of the Cobb and Heckle meltanomalies and the regional-scale south-to-north gradients in ridge structure along the JDFR may reflectinfluence of the northwesterly absolute motion of the ridge axis on subaxial melt distribution.

G3G3GeochemistryGeophysics

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AN ELECTRONIC JOURNAL OF THE EARTH SCIENCES

GeochemistryGeophysics

Geosystems

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Volume 9, Number 8

2 August 2008

Q08001, doi:10.1029/2007GC001922

ISSN: 1525-2027

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http://dx.doi.org/10.1029/2007GC001922

Components: 12,634 words, 16 figures, 2 tables.

Keywords: mid-ocean ridges; Juan de Fuca Ridge; hot spot; multichannel seismic.

Index Terms: 7245 Seismology: Mid-ocean ridges; 7219 Seismology: Seismic monitoring and test-ban treaty verification;

8137 Tectonophysics: Hotspots, large igneous provinces, and flood basalt volcanism.

Received 3 December 2007; Revised 18 April 2008; Accepted 19 May 2008; Published 2 August 2008.

Carbotte, S. M., M. R. Nedimovic, J. P. Canales, G. M. Kent, A. J. Harding, and M. Marjanovic (2008), Variable crustal

structure along the Juan de Fuca Ridge: Influence of on-axis hot spots and absolute plate motions, Geochem. Geophys.

Geosyst., 9, Q08001, doi:10.1029/2007GC001922.

1. Introduction

[2] The largest-scale variations in the structure ofthe world’s mid-ocean ridges (MORs) arise fromthe influence of plate motions and mantle hot spotson mantle upwelling and melt delivery beneath thespreading axis. First-order changes in ridge struc-ture are observed as a function of spreading rate,which reflect the influence of rate of plate separa-tion on mantle upwelling, melt extraction, andcrustal formation [e.g., Macdonald, 1982; Small,1998]. The intersection of MORs with mantle hotspots gives rise to large variations in ridge structureat constant spreading rate that can be comparable tochanges observed across the spreading rate spec-trum [e.g., Iceland, Galapagos, see Ito et al., 2003and references therein]. However, significant var-iations in ridge properties are also observed distantfrom prominent hot spots and along MORs at allspreading rates which are linked to the tectonicsegmentation of ridges defined by discontinuities ata range of scales [e.g., Langmuir et al., 1986;Macdonald et al., 1988; Lin et al., 1990]. Theorigins of this segmentation are not well under-stood and effects of both plate kinematics andsmall-scale compositional or thermal heterogene-ities in the mantle have been considered [e.g.,Schouten et al., 1985; Langmuir et al., 1986;Lonsdale, 1989; Lin and Phipps Morgan, 1992].Studies of spatial and temporal variations in ridgestructure within constant spreading sections of theMOR are needed to understand the contributions ofplate kinematics and mantle processes to ridgesegmentation. The Juan de Fuca Ridge (JDFR) inthe Northeast Pacific is a near constant rate inter-mediate spreading ridge with variable ridge struc-ture and a history of near ridge and on-axis hot spotvolcanism. With northwesterly absolute motion ofthe JDFR axis, the ridge has recently intersectedthe Cobb mantle anomaly leading to the formationof an on-axis hotpot at Axial Volcano [Karsten and

Delaney, 1989; Desonie and Duncan, 1990]. Theridge axis is segmented into a series of discretespreading segments, each with distinct structureand recent tectonic history. From south to north,the seven segments are Cleft, Vance, Axial, Coax-ial, Northern Symmetric or Cobb, Endeavour andMiddle/West Valley segments (Figure 1).

[3] In 2002, we conducted an extensive multichan-nel seismic reflection survey of the JDFR axis andflanks during R/V Maurice Ewing expeditionEW0207 (Figure 1). Data were collected withinthe near axis region [Canales et al., 2005, 2006;Carbotte et al., 2006; Van Ark et al., 2007] as wellas along three, ridge flank transects crossing theCleft, Northern Symmetric and Endeavour seg-ments and extending to crustal ages of 4–8 Ma[Nedimovic et al., 2005b]. The primary motivationfor the flank study was to investigate upper crustalstructure and the role of sediment burial, basementage, and basement relief on crustal alteration due toridge flank hydrothermal circulation (M. Nedimovicet al., Upper crustal evolution along the Juan deFuca Ridge flanks, submitted to Geochemistry,Geophysics, Geosystems, 2008). The Mohorovicicdiscontinuity (Moho) was also well imaged alongthese transects providing constraints on structureof the whole crust within the region. Initial resultsreported by Nedimovic et al. [2005b] focus on thediscovery of sub-Moho reflection events foundboth near the ridge axis (<20 km) as well as nearoff-axis propagator wakes, which may correspondto magma sills emplaced in the Moho transitionzone (MTZ).

[4] In this study, bathymetric and seismic datafrom Cleft, Northern Symmetric and Endeavoursegments are used to characterize crustal structureat these distinct spreading segments. Along-axisseismic data provide constraints on the currentdistribution of magma in the crust, while thecross-axis flank transects provide constraints on

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the history of crustal production within each seg-ment for the past 4–8 Ma. The combined data setindicates a recent history of enhanced crustalproduction at Cleft and Endeavour segments whichwe associate with initiation of nearby on-axis hotspots. Larger-scale regional gradients are observedalong the length of the JDFR in seafloor andsubseafloor structure that cannot be simply attrib-uted to hot spot/melt anomaly proximity and may

reflect influence of plate motions on melt distribu-tion beneath the axis.

2. Regional Setting and RecentTectonic History

[5] The JDFR is a 480-km-long intermediate-ratespreading center (56 mm/a [Wilson, 1993]) locatedwithin the northeast Pacific between the Blancoand Sovanco fracture zones (Figures 1, 2a, 2b, and2c). The ridge axis migrates to the northwest in theabsolute motion reference frame, oblique to thespreading direction (110�), at �31 mm/a [Smalland Danyushevsky, 2003]. The seven spreadingsegments of the current ridge axis are each �50–100 km long, separated by nontransform offsets upto 30 km in length. All are second-order segmentsin the hierarchy of Macdonald et al. [1988].Segment overlap ranges from 6 km at Cleft/Vance,to >30 km at the Cobb offset between Endeavourand Northern Symmetric segments.

[6] A prominent feature of the JDF region isabundant seamount volcanism and interaction ofthis volcanism with seafloor spreading processes[Davis and Karsten, 1986; Hammond, 1997]. Sea-mounts are found primarily on the Pacific plate,both as isolated edifices and in chains, several ofwhich lie close to and intersect the JDFR axis. TheCobb-Eickelberg seamount chain, formed by mo-tion of the Pacific plate over the Cobb hot spot, isone of the most prominent chains in the region.Axial Volcano lies at the intersection of the Cobb-Eickelberg seamounts with the JDFR and is be-lieved to mark the current location of the Cobb hotspot (Figure 2b). Here, a shallow (1450 m) domeshaped central volcano with a 2 km wide breechedcaldera is bounded by two major rift zones com-posed of elongate volcanic ridges and small volca-nic cones [Johnson and Embley, 1990; Embley etal., 1990]. The South and North Rift Zones areoriented within 15� of spreading direction andextend for �50 km, overlapping with the neighbor-ing Vance and Coaxial ridge segments (Figure 2b).The age difference between seamounts of theCobb-Eickelberg chain and the underlying crust(estimated from radiometric age dating of sea-mount basalts and magnetic anomalies for under-lying crustal age) diminishes eastward toward theJDFR, consistent with the northwestward advanceof the ridge toward a stationary Cobb hot spotsource [Desonie and Duncan, 1990]. Analysis ofbathymetry and magnetic anomaly data indicatethat Axial segment formed within the past 0.5 Mawith an �20 km westward jump of the axis to

Figure 1. Track lines of the 2002 multichannelseismic survey (black lines) superimposed on regionalbathymetry of the Juan de Fuca Ridge (JDFR). Ridgeflank seismic transects are labeled with line numbersand common midpoint (CMP) locations (red dots markevery 1000 CMP). JDF ridge segments and prominentseamount chains are labeled. Blue lines show locationsof magnetic isochrons from Wilson [1993]. Thick blacklines along seismic transects show extent of propagatorpseudofaults highlighted with gray shading in Figures4a–4c. Black arrows show the absolute motion of theridge axis calculated from the HS3-Nuvel1a platemotion model [Gripp and Gordon, 2002]. Ridgemigration velocity varies from �32�, 32 mm/a alongthe southern JDFR to �21�, 28 mm/a along the northernJDFR.


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override the Cobb hotpot [Delaney et al., 1981;Tivey and Johnson, 1990]. Magnetic anomaliesalso indicate that both Vance and Coaxial did notexist as discrete segments from the neighboringCleft and Northern Symmetric segments prior tothe Brunhes anomaly and the current second-ordersegmentation of the JDFR formed since this time(Figure 1) [Wilson, 1993].

[7] The shallow central portion of Endeavour seg-ment coincides with a broad region extending ontothe ridge flanks that marks the projection of theHeckle seamount chain to the ridge axis (Figures 1and 2a). On the basis of morphologic and geo-chemical observations, Karsten et al. [1986] andKarsten and Delaney [1989] suggest that currentmagma supply to this segment is influenced by themantle anomaly associated with the Heckle sea-mounts. The youngest conical seamount edifice inthe Heckle chain is �1.5 Ma. On younger crust,elongate seamount edifices and prominent elongateabyssal hills are observed, which lengthen withdiminished age, indicating increasing interactionbetween the ridge axis and seamount melt source.Other small seamount chains in the region includethe Heck chain adjacent to West Valley segment,Springfield seamounts west of Northern Symmetric

Figure 2a. Close-up bathymetric map of Endeavourand Northern Symmetric segments. Black medium-weight lines mark location of ridge axis; blue linescorrespond to magnetic isochrons. Isochron labels inblue are B/M, Brunhes/Matuyama; J, Jaramilo; 2,anomaly 2; and 2a, anomaly 2a. Bold black cross-axislines labeled a through d show locations of bathymetryprofiles in Figure 2c selected to illustrate characteristicridge morphology near the center and end of these twosegments. Black bold lines with red dots marking every1000 CMP correspond to ridge flank seismic profileshighlighted in Figure 1. White arrows point to theoutward facing slopes that mark the boundaries of axis-centered plateau discussed in text. Black star indicatesprominent west flank hill, which may be former ridgeaxis prior to small ridge jump to current location. Grayshading highlights approximate region of disturbedseafloor topography associated with recent history ofdueling propagation of the Endeavour and NorthernSymmetric segments (see text).

Figure 2b. Close-up bathymetric map from southernNorthern Symmetric to Cleft segments. Annotation as inFigure 2a.



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segment, and the Vance seamounts near the south-ern end of Vance segment (Figure 1).

[8] The tectonic history of the JDFR over the past18 Ma is dominated by a series of ridge propagationevents that have resulted in clockwise rotation ofthe ridge axis by 20� in response to a changingdirection of relative plate motion (Figure 1) [Wilsonet al., 1984; Wilson, 1993]. Southward propagationof the ridge axis established the Blanco TransformFault (TF) at approximately its current location by�5.8 Ma [Wilson, 1993; Embley and Wilson, 1992].Two more recent propagating events terminated atthe Blanco TF and resulted in lengthening of thetransform offset with the most recent phase endingat 1.4 Ma. Over the past several Ma, the location ofthe Blanco TF has jumped north with the mostrecent jump at �0.4 Ma, likely in response to thereadjustment of the JDFR axis associated with riftpropagation [Embley and Wilson, 1992]. Northwardpropagation of the Cobb offset, which separatesEndeavour and Northern Symmetric segments, ini-tiated at �4.5 Ma [Wilson, 1993]. The Cobb offset

reached its maximum northward extent at�0.8 Ma,then retreated to approximately 47�350N as Endeav-our segment propagated southward (Figure 2a)[Shoberg et al., 1991]. Propagation has reversedin the past 100,000 years, with the northern end ofNorthern Symmetric segment currently located at47�460N.

3. Data and Methods

3.1. Seismic Data

[9] Multichannel seismic data were collected with-in three transects oriented perpendicular to theridge axis crossing Endeavour, Northern Symmet-ric and Cleft segments and extending �150 km oneach ridge flank (Figure 1). Along-axis seismicdata also collected during the survey provide con-straints on zero-age crustal structure. Seismic datawere acquired using a 10 air gun tuned array with asource volume of 3005 cu. in., towed at a nominaldepth of 7.5 m, with shots fired every 37.5 m alongtrack. Data were recorded using a 6-km-long 480

Figure 2c. Seafloor depth profiles comparing ridge axis morphology near (left) segment midpoints and (right)segment ends within Endeavour, Northern Symmetric, and Cleft segments (see Figures 2a and 2b for profilelocations). Profiles a, c, and f correspond to ridge flank seismic lines and show depth to top of oceanic crust as well asdepth to seafloor. Sediment cover on the east flank of the ridge partially buries the crustal topography. Blue verticalarrows indicate edges of distinct plateau found at Cleft segment and within central Endeavour segment. Small blackarrow indicates possible former ridge axis location prior to jump to current axis. Red horizontal bars show lateralextent of axial high where present. Green horizontal bars show lateral extent of shallow axial rift graben (see text).



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channel Syntron digital streamer with receivergroups spaced at 12.5 m. Streamer depth (nominal10 m) and feathering were monitored with 13 depthcontrolling and 11 compass-enhanced DigiCoursebirds, and a GPS receiver on the tail buoy. For theflank transects, data were recorded at a samplingfrequency of 2 ms for 10.24-s-long records. Therecorded signal has a bandwidth ranging from �2to over 100 Hz with a dominant frequency of�10–30 Hz. Nominal common midpoint (CMP)gather spacing is 6.25 m, providing a data tracefold of �80.

[10] The prestack processing sequence for thereflection data includes F-K and bandpass (2–7and 100–125 Hz) filtering, amplitude correctionfor geometrical spreading, and trace editing, fol-lowed by CMP sort, velocity analysis, normal anddip move-out corrections to align the signal forstacking and CMP mute. The poststack processingincludes seafloor mute, primary multiple mute toreduce migration noise, bandpass filtering (from2–7 to 100–125 Hz for shallow sediments to 2–7to 20–40 Hz at Moho depth) and time migration tocollapse diffractions and correctly position reflec-tions. Processing for the layer 2A/2B event, arefracted arrival detected at large source-receiveroffsets [e.g.,Harding et al., 1993;Vera andDiebold,1994] involves a modified processing sequence.Optimal normal move-out velocities that best flat-ten the retrograde branch of the 2A refraction arechosen from constant velocity stacks. The stackedlayer 2A event is time migrated and coherencyfiltered. Surgical mute is then used to extract thelayer 2A event, which is merged with the reflectionsection to form the final, composite seismic image.

Full details on the data processing sequence areprovided by Nedimovic et al. [2005b].

[11] From final migrated sections (Figures 3a, 3b,3c, and 3d) seafloor, top of oceanic crust, base oflayer 2A, Moho, and the reflection from the top ofthe axial magma chamber (AMC) detected beneaththe ridge axis are identified. Estimated pickingerrors are 0.03 s for top of oceanic crust andAMC, and 0.04 s for the base of layer 2A andMoho. Crustal ages along each seismic profile areinterpolated from Wilson [1993]. Interpreted ridgeflank structure is shown in Figures 4a, 4b, and 4c.Along Endeavour and Northern Symmetric trans-ects, two-way traveltimes (TWTT) to the layer 2Aevent are systematically lower on the east flankbecause of eastward increase in seismic velocitieswithin layer 2A. Higher velocities are attributed toenhanced upper crustal alteration associated withthick sediment cover, which blankets the Juan deFuca plate in this region (Nedimovic et al., sub-mitted manuscript, 2008). Because of this markedasymmetry in layer 2A structure, both TWTT toMoho from top of the oceanic crust (Mohocrust) andfrom the layer 2A event (Moho2A) are included inFigures 4a–4c.

[12] Zero-age crustal properties for each ridgesegment including ridge axis depth, on-axis 2Athickness, AMC depth, and percent of segmentwith a detectable AMC, are shown in Figures 5and 6 and summarized in Table 1.

3.2. Bathymetric Data

[13] Bathymetry data used in this study are fromthe Global MultiResolution Topography synthesis(available at http://www.marine-geo.org/services/

Figure 3a. Ridge flank composite seismic profile 1-3-17 crossing Endeavour segment with base of layer 2A event,axial magma lens, and Moho reflections labeled.



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google/create_merc_map.php). In the JDFR re-gion, the original data sources for the compilationinclude Hydrosweep multibeam bathymetric datacollected simultaneous with our seismic survey andduring the 1999 seismic study at Axial Volcano[West et al., 2001], Simrad EM300 data for theaxial region of Cleft segment collected by MBARI[Stakes et al., 2006], and an extensive NOAASeabeam data set collected under the NOAAPMEL Vents program (provided by A. Bobbit).Data are weighted on the basis of data quality (e.g.,EM300 data is used preferentially where available)and gridded at a node interval of �200 m.

4. Results

4.1. Seafloor Structure

[14] Each JDFR segment is characterized by adistinct axial morphology with a shallow (100–200 m) rift or graben located at the center of a

broader 5–15 km wide, 100–400 m axial high(Figure 2c) similar to that found on the fastspreading East Pacific Rise (EPR). The shallowaxial graben appears to be a magmatic rather thantectonically generated structure, linked to dikeintrusion and magma withdrawal from the midcrustmagma lens found at depths of 2–3 km along thisridge [Carbotte et al., 2006]. As a magmaticallygenerated depression linked to the presence of acrustal magma body, the axial graben is moresimilar to the axial summit trough (AST) at theEPR, than to the axial valley found along slowspreading ridges which forms largely by amag-matic extension. However, the axial graben alongthe JDFR differs from the AST at the EPR in that itis a fault bounded structure rather than a volcanocollapse drain-back feature [Fornari et al., 1998]with the structural differences likely due to thethicker brittle layer and longer time intervals be-tween intrusions at the slower spreading JDFR.

Figure 3b. Ridge flank composite seismic profile 32-34 crossing Northern Symmetric segment with base oflayer 2A event, axial magma lens, and Moho reflections labeled.

Figure 3c. Ridge flank composite seismic profile 87-89-73-89a crossing Cleft segment with base of layer 2A event,axial magma lens, and Moho reflections labeled.



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[15] These segment-to-segment changes in themorphology of the axial graben are superimposedon longer-wavelength south-to-north gradients inridge structure with a deeper ridge axis, narroweraxial high and more variable ridge structure to thenorth. Average ridge axis depth for each segmentincreases to the north (Figure 6) and within allsegments except Endeavour, the ridge axis isshallowest at its southern end (Figure 5). AxialVolcano and the central portion of Endeavoursegment are both localized shallow regions alongthe ridge axis, proximal to nearby seamount chains,which are superimposed on the long-wavelengthnorthward deepening trend.

[16] The cross-axis width of the axial high andelevation above surrounding seafloor diminishesfrom south to north (Figures 2a–2c). Whereas theaxial high rises over 400 m above surroundingseafloor at Cleft segment, a more subdued high ispresent at Northern Symmetric and Endeavoursegments (100–300 m). The axial high is widestat Cleft and Vance segments (15 km) comparedwith �5 km at Northern Symmetric and Endeavoursegments (Figure 2c). Variability in ridge structurealso increases to the north. Over the length of

Endeavour segment, axial depth varies by 800 m,compared with less than 200 m at Cleft segment(Figures 5 and 6). Whereas a broad axial high ispresent along the length of Cleft segment withminor difference in cross-axis width and height(Figures 2b and 2c), the axial high diminishes andbecomes difficult to identify toward the ends ofNorthern Symmetric and Endeavour segments(Figures 2a and 2c).

[17] At both Cleft and Endeavour segments, theaxial high is centered within a broader elevatedplateau defined by prominent ridge-parallel out-ward facing slopes present on both sides of the axisthat are up to several hundred meters higher thantypical abyssal hill relief (50–100 m, Figures 2a–2c and 4a). At Cleft segment, conjugate outwardfacing slopes define a plateau that is 200–300 mhigh and 32 km wide centered about the axis,which extends for the full length of the segment.At Endeavour segment the eastern plateau edgecoincides with a partially sediment buried scarp350 m high located 18 km from the axis. This scarpmarks the transition from sediment-free to sedi-ment-covered basement and is identified as animportant hydrogeologic boundary in ODP-IODP

Figure 3d. Close-up of Moho event along each transect within the inner 50 km of the ridge axis. Thin red lineshows our picked Moho locations. Red shaded region corresponds to axis-centered plateau at Cleft and Endeavoursegments highlighted in Figures 4a–4c.



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Figure 4a. Line drawing interpretations of seismic horizons detected along both composite seismic profiles withinthe Endeavour segment transect. For each seismic profile, bottom plot shows two-way traveltime to seafloor (blue),top of oceanic crust (green), base of layer 2A (red), axial magma chamber reflection at axis (AMC, orange), andMoho (black). The third plots show traveltime to Moho measured from top of oceanic crust (Mohocrust, black) andfrom base of layer 2A event (Moho2a, red). Thin black horizontal line indicates the average Mohocrust TWTT of2090 ms obtained for all three transects (see text). The second plots show variations in crustal thickness calculatedfrom Moho traveltimes assuming a uniform crustal velocity of 6.7 km/s. The top plots show inferred crustal velocitycalculated assuming uniform crustal thickness of 6.5 km for profile. Vertical black arrows show location of axis. Redshading indicates extent of interpreted 40 km wide plateau-bounding axis at Endeavour segment. Light gray shadingindicates location of propagator psuedofault crossing also shown in map view in Figure 1. Crustal ages in Ma fromWilson [1993] are indicated along each profile in the bottom plots with labeled black dots. Black horizontal lines inthe same plot highlight locations of seamounts and the region of presumed seamount source–influenced crust west ofthe current Endeavour axis.



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FlankFlux studies [Davis et al., 1992]. The westernplateau boundary is �300 m high and is locatedfurther from the axis (21 km). The Endeavourplateau is limited to the central southern portionof the segment and varies in along-axis length,consistent with lengthening of Endeavour segment

during the Brunhes by southward propagation(Figure 2a).

4.2. Shallow to Midcrust Structure

[18] Regional south to north gradients in subsea-floor structure are evident in along-axis seismic

Figure 4b. Line drawing interpretations of seismic horizons detected along both composite seismic profiles withinthe Northern Symmetric segment transect. Annotation as in Figure 4a.



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data, consistent with the regional gradients ob-served in seafloor properties. The depth of theAMC reflection varies from segment to segmentwith the shallowest (excluding Axial) and mostuniform depths found beneath Cleft segment andthe deepest and most variable AMC beneath En-

deavour segment (Figure 6) [Carbotte et al., 2006].Within most segments, the AMC is shallowest atthe southern end of the segment, deepening to thenorth, also consistent with trends in seafloor depth(Figure 5). The proportion of each segment with adetected AMC varies along the JDFR; whereas an

Figure 4c. Line drawing interpretations of seismic horizons detected along both composite seismic profiles withinthe Cleft segment transect. Red shading indicates extent of interpreted 32 km wide plateau bounding axis of Cleftsegment. Rest of annotation as in Figure 4a.



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AMC is imaged beneath 68% of Cleft segment, itis present beneath only the central southern 21% ofEndeavour segment (Table 1).

[19] The thickness of layer 2A varies from segmentto segment. Along the ridge axis, the thinnest andmost uniform 2A is found at Cleft segment with thethickest and most variable 2A beneath Endeavoursegment. Within all segments, seismic layer 2A isthinnest where the AMC is detected and thickenstoward segment ends where the magma lens dis-appears, similar to observations at other ridges[e.g., Kent et al., 1994; Carbotte et al., 2000].Along the ridge flank transects, layer 2A is thickestat Endeavour segment with similar average values atCleft and Northern Symmetric segments (Table 2).Detailed ridge axis surveys show that layer 2Athickness increases within 3–5 km of the axis atCleft segment whereas minor off-axis thickening is

observed at Endeavour segment [Canales et al.,2005; Van Ark et al., 2007].

4.3. Moho Structure

[20] Moho is well imaged on all profiles and can beidentified as close as 1–3 km from the ridge axis ateach segment (Table 2 and Figures 3a–3d and 4a–4c), indicating formation at earliest crustal agessimilar to inferences from the EPR [e.g., Singh etal., 2006]. However, Moho presence and depthwithin the near axis region differ at the 3 segments.At Endeavour and Cleft segments Moho is presentwithin 40–60%of the innermost 20 kmat an averageTWTTof 2190 ± 90 and 2300 ± 40 ms respectively.In contrast, at Northern Symmetric segmentMoho isonly detected beneath 20% of the innermost 20 kmand Moho traveltimes are �200–300 ms lower(Table 2). Scattering associated with the rough

Figure 5. Interpretation of zero-age axial structure along the JDFR from along-axis seismic profiles. (a) AMC depthbelow seafloor. Depths are calculated from reflection traveltimes using velocities of 2.26 km/s for layer 2A and5.5 km/s for layer 2B. (b) Along-axis profile showing depth to seafloor (black), base of layer 2A (blue), and axialmagma chamber reflection (AMC) (red). Figure is modified from Carbotte et al. [2006] and includes data from AxialVolcano [Kent et al., 2003].



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unsedimented axial seafloor presumably contrib-utes to intermittent imaging of Moho at all seg-ments. However, as seafloor relief is comparable atthe three segments, the observed differences inMoho presence are unlikely to be due solely to

seafloor scattering effects and possible differencesin the nature of the crust-mantle transition zone areimplied.

[21] Further on the ridge flanks, average MohoTWTT is similar for all three segments (�2100 ms).Accounting for differences in layer 2A thickness,average traveltimes to Moho (Moho2A) differ by lessthan 35 ms (within the resolution limits of our data).In general, Moho is detected beneath more of theeastern flank of the ridge than the western flank,likely because of improved imaging conditions as-sociated with the thick sediment cover to the east(Figures 3a–3c and 4a–4c).

[22] The transition from greater near-axis Mohotraveltimes at Endeavour and Cleft (2190 and2300 ms respectively, Table 2) to lesser ridge flankvalues (2100 ms) coincides with the edges of thebroad, axis-centered plateaus observed at both ofthese segments. Steps in Moho TWTT coincidentwith the plateau’s bounding slopes are evident online 86 at Cleft segment (Figure 4c) and on theeastern plateau crossing of line 1-3-17 at Endeavour(Figure 4a). LongerMoho TWTT persist beyond thewestern plateau boundary at Endeavour and areassociated with the elongate Heckle seamounts (line18-20-8). Seismic data from additional survey lineswithin Endeavour segment (Figure 1) [VanArk et al.,2007] show that the high near-axis Moho TWTTarerestricted to the plateau region confined to the centralsouth portion of the segment. North and south of theplateau, a shorter Moho TWTT of �2100 ms ismeasured within the near axis region comparableto average ridge flank values (E. Van Ark, personalcommunication, 2007). Seismic lines from the de-tailed Cleft segment survey are presented byCanales et al. [2005] and show that high MohoTWTT of 2200–2400 ms underlie the length of the30 km wide plateau at this segment. High Mohotraveltimes persist to the southern end of Cleftsegment with a bright Moho at 2400 ms belowseafloor evident along line 41 located only 15 kmfrom the southern termination of the segment [seeCanales et al., 2005, Figure 8].

[23] Along all flank transects, local Moho travel-time anomalies (100 to 500 ms greater than averageMoho traveltimes) are observed on the young crustside of propagator wakes (Figures 4a–4c). Withinpseudofault zones, Moho is weak to absent. Thelargest Moho traveltime anomalies are observedalong the Cleft transect associated with the youn-gest southward propagator into the Blanco TF. Avariety of factors may contribute to these elevatedMoho traveltimes including thicker crust, slower

Figure 6. Comparison of (a–d) on-axis and (e and f)off-axis crustal structure within individual ridge seg-ments of the JDFR. For all plots, filled solid starsindicate averages measured from ridge flank transects atEndeavour (End), Northern Symmetric (NS), and Cleftsegments and plotted at latitude of axis crossing foreach profile. (a) Ridge axis depth, (b) AMC depth, and(c) on-axis layer 2A thickness. Solid circles in Figures 6a–6c correspond to segment average measured from along-axis seismic profiles shown in Figure 5 and plotted atlatitude of segment midpoint with 1 standard deviation(bars) and total range (open circles) of values for eachsegment (values also given in Table 1). Measurements forridge flank transects (stars) correspond with averagetraveltimes within ±1 km of axis crossing and convertedto depth using Vp = 2.26 km/s. (d) Average near-axiscrustal thickness (TWTT) measured from flank transectswithin 20 km of the axis. (e) Average off-axis layer 2Athickness, measured beyond 1 km from axis crossing andconverted to depth using Vp = 3.2 km/s. Data from the eastflank are excluded because of variable 2A structureassociated with sediment burial (see text). (f) Average off-axis crustal thickness (TWTT) measured beyond 20 kmfrom axis (west flank only).



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crustal velocities due to the presence of ironenriched crustal rocks [e.g., Iturrino et al., 1991],or more fractured and altered crust and possiblyupper mantle. Elsewhere on the ridge flanks localMoho traveltime anomalies (up to 200 ms) areassociated with seamounts including the flanks ofthe Heckle seamounts along the Endeavour transectand a near-axis seamount west of the axis along theNorthern Symmetric transect (Figures 4a and 4b).In addition to these regional variations, fluctuationsin Moho TWTT on the order of 50–100 ms occuron the 5–10 km length scale of the abyssal hillrelief, with greater traveltimes common beneathabyssal hills.

5. Constraints on Crustal Velocities

[24] Reflection data reveal similar average Mohotraveltimes on the flanks of the three ridge seg-

ments (2100 ± 100 ms) beyond 20 km from theaxis. In contrast, crustal traveltimes differ by up to300 ms in the near-axis region, and require signif-icant differences in crustal thickness and/or velocityin young crust (Figures 4a–4c and 6). Refractionstudies conducted in the region since the 1980sprovide some constraints on crustal thickness andseismic velocities relevant for interpreting thereflection results (Figure 7). On the ridge flanks,refraction data have been acquired east of Endeav-our segment [McClymont and Clowes, 2005] andat Cleft segment north of the Blanco FZ centered�130�W and 44�260N [Christeson et al., 2007]. Inboth regions, similar crustal thickness (6.7–7.0 km)is observed for normal crust away from the propa-gator wakes, consistent with the reflection data.Refraction data from the flanks of Northern Sym-metric segment are available from the early exper-iment of McClain and Lewis [1982]. Much thinner

Table 1. Ridge Segment Characteristics

Segment

AverageRidge AxisDeptha (m)

AverageLayer 2AThickness

on Axisb (m)

AverageAMC Depth

BelowSeafloorb (m)

Length ofSegmentc

(km)

Length ofSegmentWith

DetectedAMCd (km)

Percentage ofSegment With

DetectedAMCd (%)

Endeavour 2579 (208) 364 (102) 2580 (330) 90 19 21Northern Symmetric 2546 (123) 291 (64) 2455 (300) 138 76 55Coaxial 2329 (157) 382 (110) 2206 (160) 74 36 49Vance 2431 (37) 313 (54) 2498 (150) 76 40 53Cleft 2266 (24) 247 (40) 2055 (70) 60 41 68

aDepth averages are calculated from evenly spaced along-axis bathymetric profiles digitized using GeoMapApp and the �200 m node

bathymetric grid of the Global MultiResolution Topography synthesis (www.geomapapp.org). The value in parentheses is the standard deviation.bSegment averages are measured from along-axis seismic data shown in Figure 5 and are plotted in Figure 6. The value in parentheses is the

standard deviation.cRidge segment lengths are measured along digitized ridge axis and include zone of segment overlap where present.

dLength of ridge segment with detected AMC represents a minimum value that may underpredict occurrence because of deviations of ship track

above a narrow AMC.

Table 2. Average Layer 2A and Crustal Properties for Three Segment Transects

Segmenta

Near Axis Off-Axis West Flank Off-Axis East Flank

2ATWTTb

(ms)

MohocrustTWTTc

(ms)

Percent Near-Axis RegionWith DetectedMohoc (%)

DistanceFrom Axis toFirst Detected

MohoReflection

(km)

2ATWTTb

(ms)

MohocrustTWTTc

(ms)

2ATWTTb

(ms)

MohocrustTWTTc

(ms)

Endeavour 300 ± 100 2190 ± 90(2190 ± 80)

57 1.5–11 330 ± 80 2100 ± 110 240 ± 90 2010 ± 70

Northern Symmetric 210 ± 30 1980 ± 50(1990 ± 60)

21 3.3–17 280 ± 60 2090 ± 60 210 ± 50 2020 ± 80

Cleft 210 ± 20 2300 ± 40(2250 ± 70)

42 1.2–6.9 290 ± 60 2100 ± 110 340 ± 70 2090 ± 150

aAverages are calculated including both composite ridge flank seismic profiles crossing each segment.

bNear-axis layer 2A TWTT is average calculated for ±1 km about axis; off-axis averages exclude inner ±1 km about axis.

cAverage Mohocrust TWTT calculated for innermost ±10 and ±20 km about axis with values for ±20 km given in parentheses: off-axis averages

exclude inner ±20 km about axis.



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crust (5 km) is reported, but with the sparse sam-pling of the mantle caustic obtained in this study,this value is poorly constrained. The data are alsoinconsistent with our coincident MCS data, whichreveal a prominent Moho reflection at �2 s TWTT,much longer than predicted from their velocitymodel (1.5 s).

[25] Some constraints on seismic velocities in theaxial region near all three transects are also avail-able (e.g., Cudrack and Clowes [1993] and Barclayand Wilcock [2004] for Endeavour, Christeson etal. [1993] for Northern Symmetric, and McDonaldet al. [1994] for Cleft). These studies have limitedresolution within the shallowest crust, but indicatesimilar structure for the upper 1 km at the threesegments, and a lower-velocity, thinner layer 2A onaxis compared with off axis, consistent with thereflection data (Figure 7). Information on seismic

velocities for the lower crust in the on- or near-axisregion is not available in any of these three areas.

6. Discussion

[26] Additional constraints on lower crustal struc-ture will be needed to establish whether greaterMoho TWTT beneath the 32–40 km wide axis-centered plateau at Cleft and Endeavour reflectthicker crust or slower seismic velocities and bothmay plausibly contribute. The magnitude of travel-time anomalies associated with these plateaurequire 0.5–1.0 km thicker crust or seismic veloc-ities that are 0.6 and 0.9 km/s slower within Layer2B/3 (Figures 4a–4c). Assuming that crustal ve-locities depend on temperature as dV/dT = �0.7610�3 km s�1 K�1 [Christensen, 1979], slowervelocities by 0.6–0.9 km/s would require excesscrustal temperatures of �800–1,200�C beneaththese plateau regions relative to further on theridge flanks. Although such temperature anomaliesare expected at the ridge axis, seismic tomographystudies at the faster spreading EPR suggest that thelower crust is quickly cooled by hydrothermalcirculation and that significant thermal anomaliesdo not extend beyond 4–6 km off axis [Dunn etal., 2000]. For the same reason, partial melt is notexpected to be present outside of the axial high-temperature region. From these observations at theEPR, the 32–40 km wide region of greater Mohotraveltimes at Endeavour and Cleft appears toowide to be attributed to axial temperature anoma-lies alone. There is evidence for off-axis volcanismat Cleft segment including young constructionalmounds and low-temperature hydrothermal activityfound 3–4 km from the axis [Stakes et al., 2006].However, even if crustal magma sources for thisoff-axis volcanism exist over these distances, theyare unlikely to account for the much wider regionof Moho traveltime anomalies at this segment.

[27] On the basis of these considerations, thickercrust is a more likely explanation for the enhancedMoho traveltimes beneath the axis-centeredplateaus found at Endeavour and Cleft segments.Inferred crustal thickness anomalies of 0.5–1 km(Figures 4a and 4c) are close to those expected forisostatic compensation of the roughly 100–200 mof excess relief associated with these featuresrelative to the surrounding abyssal hill topography.For isostatically compensated excess topography ofdd = 100–200 m, crustal density r = 2700 kg/m3

and mantle density r = 3100 kg/m3, the expectedexcess crustal thickness is dd/0.24 = 380–770 m[Turcotte and Schubert, 2002].

Figure 7. Compilation of velocitymodels obtained fromprevious seismic refraction studies within Cleft, NorthernSymmetric, and Endeavour segments. (a) Velocitymodels for ridge flank crust at Endeavour [McClymontand Clowes, 2005] and Northern Symmetric [McClainand Lewis, 1982]. Horizontal arrows indicate crust-mantle boundary. Depth to Moho from the recentreflection/refraction study of Christeson et al. [2007] atCleft segment is indicated with black arrow. (b) Velocitymodels for axial crust from refraction studies of Cudrackand Clowes [1993] at Endeavour, Christeson et al.[1993] at Northern Symmetric, and McDonald et al.[1994] at Cleft.



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[28] The presence of thicker crust beneath theseaxis-centered plateaus at Cleft and Endeavourcompared with typical ridge flank values impliesa recent increase in crustal production at thesesegments. As discussed below, we attribute theserecent changes to initiation of ridge axis-centeredmelt anomalies associated with the Cobb hot spotand the Heckle seamount source giving rise toenhanced melt production at Cleft and Endeavoursegments respectively.

6.1. Enhanced Crustal Production Alongthe Southern JDFR and PossibleAssociation With Cobb Hot Spot

[29] Assuming a constant spreading rate of 56 mm/a, a maximum age of 0.57 Ma for onset ofenhanced crustal production at Cleft segment isestimated from the 32 km width of the axis-centered plateau. The uniform width of this plateauto its southernmost extent and the greater Mohotraveltimes found on seismic lines to within 15 kmof the Blanco FZ [Canales et al., 2005], indicate arapid increase in crustal production along thelength of the segment at this time. Conjugateoutward facing slopes define a plateau of similarwidth (age) at the adjacent Vance segment to thenorth (Figure 2b) and a contemporaneous increasein magma supply to Vance segment is implied.Seismic data are available for the Vance segmentplateau and indicate excess crustal thickness rela-tive to our average ridge flank values, although theinferred anomaly (�100 m) is smaller than that atCleft segment (up to 300 m) [Canales et al., 2005].From their study of magnetic and bathymetric data,Delaney et al. [1981] estimate that Axial segmentinitiated within the Brunhes anomaly at �0.5 Mawith a 20 km westward jump of the axis to overridethe Cobb hot spot. This timing is close to theestimated age of onset of enhanced crustal produc-tion at Cleft and Vance segments and we infer thatenhanced melt production beneath these segmentsinitiated rapidly linked to development of this on-axis hot spot.

[30] The morphology of Coaxial segment, locatedimmediately north of Axial, suggests recent onsetof magma ‘‘oversupply’’ to this ridge segment aswell, linked to proximity to Axial Volcano. Thissegment is associated with a prominent plateaudistinct from surrounding seafloor, that rises400–600 m and which tapers in width from16 km near Axial to 10 km at its northern end,�75 km north of the summit of Axial (Figure 2b)[Embley et al., 2000]. Elevated plateau shoulders

form half pear shapes in plan view, and near thenorth rift zone of Axial segment have the appear-ance of a bisected seamount presumably linked tothe abundant nearby seamounts and the Axial meltanomaly.

[31] From two-dimensional modeling of an along-axis bathymetry and gravity profile constructedfrom archived ship track data, Hooft and Detrick[1995] infer a mantle thermal anomaly of 30–40�Cand a zone of 1–2 km thicker crust extending�100 km along axis beneath Axial and adjacentCoaxial segment, which they attribute to effects ofthe Cobb hot spot. Preliminary analysis of gravitydata collected simultaneous with our seismic sur-vey support thickened crust within the near axisregion at Cleft segment but not Northern Symmetric(Figure 8). Residual gravity anomalies calculatedfor the flank transects assuming a constant thick-ness crust indicate lower densities are needed atthe axis of Cleft segment relative to the other twosegments consistent with thicker crust (or warmercrust and/or mantle) at Cleft (Figure 8a). Incomparison, models calculated using crustal thick-nesses constrained by the seismic observationsexplain most of the difference in residual anoma-lies at the axis of the three segments (Figure 8b).More comprehensive analysis of gravity datacollected during our study will be presentedelsewhere.

[32] Considered together these bathymetric, gravi-ty, and seismic data reveal the presence of elevatedplateau with thickened crust centered on the ridgeaxis from Cleft to Coaxial that initiated close intime to onset of the on-axis hot spot at AxialVolcano. Bathymetric swells and gravity anomaliesindicative of thicker crust are commonly observedwhere hot spots approach, intersect, and recedefrom mid-ocean ridges and are attributed to flow ofplume mantle toward and along ridges [e.g., Itoand Lin, 1995]. From their analysis of bathymetricand gravity anomalies in the vicinity of severalridge-crossing hot spots Ito and Lin [1995] findthat the magnitude and along-axis extent of theseanomalies are greatest when the hot spot is cen-tered beneath the axis. Modeling studies of ridge-plume interactions indicate that, under conditionsof low viscosities in the shallow mantle and thinplume layers, the topography of the lithosphere atridges may effectively channel flow of plumemantle leading to ‘‘pipe-like’’ along-axis flow[e.g., Sleep, 1996; Ribe et al., 1995; Georgen andLin, 2003]. Rapid initiation of enhanced crustalproduction along the southern JDFR with onset of



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Axial Volcano implies efficient pipe-like flowlinked to the on-axis hot spot, low shallow mantleviscosities, and a thin layer of anomalous mantle.

[33] In contrast to the geophysical observations,geochemical data suggest that Cobb hot spot influ-ence is limited toAxial segment. Unlikemost hot spotseamount chains, basalts from the Cobb-Eickelbergchain have essentially normal MORB-like isotopiccompositions, with no evidence of isotopic enrich-ment relative to basalts from the adjacent JDFR. Onthe basis of the lack of isotopic enrichment, earlygeochemical studies concluded that the Cobb hot spotwas primarily a thermal anomaly rather than a com-positionally distinct enriched mantle plume [Desonieand Duncan, 1990; Rhodes et al., 1990]. However,more recent detailed geochemical studies demon-strate that the Cobb hot spot does produce basaltswith distinct geochemical characteristics includingenrichments in alkalis and highly incompatible ele-ments [Chadwick et al., 2005; Rhodes et al., 1990].Using these characteristics as chemical tracers of theCobb source, Chadwick et al. [2005] conclude thatCobb hot spot influence is confined to Axial segmentwith gradients along the rift zones of Axial Volcanoindicative of enhanced fractional crystallization withincreasing distance from the volcano summit. Geo-chemical data from the 1993 eruption at Coaxialsegment indicate the magmatic plumbing systembeneath Coaxial at present is separate from that ofAxial Volcano [Embley et al., 2000]. Similarly, thereis no geochemical evidence of hot spot influence onsegments to the south of Axial segment [e.g., Rhodeset al., 1990].

[34] Together, these geochemical studies suggest amuch narrower extent of Axial hot spot influencealong the JDFR than that inferred from the bathy-metric and gravity anomalies, and seismicallyestimated crustal thickness. This decoupling be-tween geochemical and geophysical anomaliesalong mid-ocean ridges with nearby hot spots isobserved elsewhere including the Mid AtlanticRidge in the vicinity of the Iceland and Azoreshot spots [e.g., Ito et al., 2003] and along theGalapagos Spreading Center [Cushman et al.,2004]. For example, whereas geochemical dataindicate the mantle source signature of the Azoreshot spot extends to the Hayes FZ [Schilling et al.,1983; Debaille et al., 2006], the thermal effect ofthe hot spot estimated from bathymetric and grav-ity anomalies extends over 800 km further south to�26�300N [Thibaud et al., 1998; Ito et al., 1997].The origin of these discrepancies between geo-chemical and geophysical indicators associatedwith ridge–hot spot interaction are poorly under-stood. The role of volatiles, in particular water, onmelting and chemical extraction are likely to be

Figure 8. Residual gravity anomalies calculated forEndeavour, Northern Symmetric, and Cleft ridge flankprofiles in Figures 3a–3d (a) assuming a uniform crustalthickness of 6.5 km and (b) using variable crustalthickness constrained by Moho observations fromFigures 4a–4c. Residual anomalies correspond toobserved gravity minus theoretical gravity obtained foreach case with depths to all other crustal layersconstrained by the seismic data (seafloor, top of oceaniccrust, base of layer 2A). Horizontal axis is distancealong profile in kilometers away from ridge axis. For allmodels, densities within layer 2A are determined from2A velocities of Nedimovic et al. [2005a] using theempirical density-velocity relation of Carlson andRaskin [1984]. Constant densities are assumed for allother crustal layers (1.03 g/cm3 for water, 1.9 g/cm3 forsediments, 2.75 g/cm3 for layer 2B/3, and 3.3 g/cm3 formantle). No corrections are included for cooling ofcrust-mantle zone away from the ridge axis. Negativeresidual anomalies are obtained for the axial regionalong all profiles, with the largest (most negative)anomalies at Cleft segment. (a) residual anomaliesobtained assuming constant 6.5 km thick crust for allsegments are �10 mGal lower at the axis of Cleftsegment compared to Northern Symmetric segment andindicate that significantly lower densities (or thickercrust) are required at Cleft. (b) Using the variable crustalthickness constrained by the seismic observations,differences in residual anomalies at the axis of the threesegments are minimal (3 mGal lower at Cleft segment).



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important [Asimow and Langmuir, 2003; Cushmanet al., 2004]. Entrained ambient mantle associatedwith upwelling mantle plumes may also contribute.

6.2. Enhanced Crustal Production atEndeavour Segment Associated WithHeckle Seamount Source

[35] From Moho traveltime anomalies we estimatethat crust beneath the �40 km wide plateau atEndeavour segment is �0.5–1.0 km thicker thanthe average thickness beneath the ridge flanks(Figure 4a). The age of crust at the plateau edges,estimated from magnetic anomalies, is 0.71 Maand we interpret development of this plateau asresulting from the northwesterly migrating JDFRoverriding the melt anomaly associated with theHeckle seamount chain just after onset of theBrunhes anomaly (Figure 2a). Prior to the Brunhes,Northern Symmetric segment had propagatedsteadily north for �3 Ma. However, propagationdirection reversed during the Brunhes and Endeav-our segment rapidly propagated south �35 km(Figure 2a). This southward propagation may bedirectly linked to enhanced ridge axis melt supplyand along-axis channeling of the Heckle anomalyfollowing ridge capture. At young crustal ages(<200,000 years) there is evidence for a smalleastward jump of the Endeavour axis (Figures 2aand 2c) which could reflect readjustment of theplate boundary to remain approximately centeredon the Heckle melt anomaly with ongoing north-westerly migration of the JDFR. Magnetic datashow that the Brunhes anomaly is not symmetricabout the current axis, and is 3 km wider on thewest flank (Figure 2a). The prominent ridge cen-tered 5.5 km west of the ridge axis has similarmorphology and dimensions to the current axis,including a <1.0 km wide central rift graben, and isthe inferred former ridge location (Figures 2a and2c). A conjugate partial ridge is present east of thecurrent axis within the southern part of the segmentindicating that the axis jump was confined withinthe axial rift of this region. Renewed northwardpropagation of Northern Symmetric segment hasoccurred in the past 100,000 years [Shoberg et al.,1991], which may reflect a current waning ininfluence of the Heckle anomaly on melt produc-tion at Endeavour segment as this anomaly istransferred to the east flank of the ridge.

[36] Lavas from Endeavour segment are unusualalong the JDFR in that they are on average themost enriched in incompatible elements and mostdiverse in compositional types [Karsten, 1988;

Woodcock et al., 2007]. Recent detailed samplingwithin the axial graben floor and shoulders ofEndeavour segment indicate that the wide varietyof MORB chemical types require variations inextents of melting as well as multiple mantlesources [Woodcock et al., 2007]. Karsten [1988]attributed the presence of highly enriched lavas atEndeavour to contribution of the Heckle meltinganomaly to spreading ridge melts. However, sub-sequent geochemical sampling of Heckle and thenearby Heck seamounts to the north indicate theselavas come from a highly depleted mantle sourceand there is no evidence that the enriched compo-nent sampled at the Endeavour axis is linked toHeckle melts [Leybourne and Van Wagoner, 1991,Cousens et al., 1995]. On the basis of thesegeochemical data the origin of enhanced meltingassociated with formation of the Heckle seamountchain appears to require a shallow thermal anomalywithin a depleted mantle source rather than anenriched compositional heterogeneity [Leybourneand Van Wagoner, 1991]. The Vance seamountslocated on the western flank of Vance segment aresimilar in that they are more depleted than theadjoining ridge axis lavas although enrichedMORB compositions are also found [Wendt etal., 2007]. Although the nature of mantle processesthat give rise to small off-axis seamount chainsincluding the Heckle (and Vance) are not wellunderstood, the prevailing view is that they arelinked to small-scale thermal anomalies and heter-ogeneities within the shallow mantle [Scheirer andMacdonald, 1995;Wilson, 1992]. Morphologic andseismic observations, along with the recent tectonichistory of the region provide strong support forincreasing interaction at younger crustal ages be-tween the Heckle melt source and the ridge axis,consistent with migration of the JDFR toward thismantle heterogeneity. Interaction between the meltsource for the Vance seamounts and the adjacentridge axis is evident in the field of small volcaniccones on the west flank of Vance segment and thecurrent offset axial volcanic ridges centered withinthe current axial rift (Figure 2b) [Canales et al.,2005]. In addition to Axial hot spot, this meltanomaly may also contribute to increased crustalproduction over the past 0.57 Ma along the south-ern JDFR.

[37] Comparison of the structural anomalies atEndeavour segment with those at Cleft to Coaxialsegments indicates that the geophysical manifesta-tions of ridge–melt anomaly interaction along theJDFR are similar and analogous to those observedat much larger on-axis hot spots. Although the



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scale of impact differs, in each region, an elongateridge-parallel plateau with thickened crust is found,indicating enhanced ridge-axis melt production andrapid along-axis propagation of the melt anomalyfollowing ridge capture.

6.3. Absolute Plate Motions and SubaxisMelt Distribution

[38] Bathymetric studies of fast and intermediatespreading ridges reveal a distinct asymmetry inridge axis depths near discontinuities linked to thedirection of absolute motion of the ridge axis[Carbotte et al., 2004; Supak et al., 2007]. Leadingsegments (those offset at a discontinuity in thedirection of ridge absolute motion) are consistentlyshallower, with higher inferred magma supplyrelative to the adjoining trailing segments. Theserecent studies focus on the local depth asymmetriesobserved across discontinuities. However, withinthe framework of the large-scale segmentation ofthe JDFR, the regional-scale south-to-north gra-dients observed along the ridge in axis depth andcross-axis dimensions of the axial high are alsoconsistent with this pattern; the shallow, broadCleft segment is a leading segment whereas thedeeper, narrow Endeavour is a trailing segment.Seismic data reveal gradients along the JDFR insubseafloor structure consistent with the bathymet-ric trends; the crustal magma lens is shallower andmore uniform in depth beneath the southern JDFRsegments whereas it is deeper on average andpresent at a wider range of depths at NorthernSymmetric and Endeavour (Figure 6). With theexception of Axial Volcano, the leading Cleftsegment is associated with the shallowest, andmost persistent magma lens along the JDFR. Incontrast, a midcrust magma lens is only detectedbeneath 20% of the trailing Endeavour segmentand is possibly only present because of ridgecapture of the Heckle melt anomaly (Table 1 andFigure 6). Our data indicate enhanced crustalproduction during the Brunhes at both of thesesegments linked to local melt anomalies. However,proximity to these anomalies alone cannot explainthe regional-scale along-axis trends. Seafloor mor-phological parameters and magma lens propertiesindicate a warmer axial regime at Cleft than eitherVance or Coaxial segments, both of which arelocated closer to the on-axis hot spot at AxialVolcano. In addition, inferred crustal thicknesseswithin the inner 30 km about the axis are higher atCleft than at Vance segment indicating greatercrustal production at this southernmost segmentin the recent past.

[39] Carbotte et al. [2004] propose that the influ-ence of absolute motion of the spreading plateboundary on mantle melting may give rise todifferences in melt availability to adjacent leadingand trailing segments through melt tapping acrossthe bounding discontinuity. This mechanism isexpected to be limited to the near-discontinuityregion unless significant large-scale along-axisredistribution of mantle melt occurs. Additionalfactors linked to the absolute motion of the ridgeaxis may contribute to the larger-scale along-axisvariations observed at the JDFR. Davis and Currie[1993] note that the deep rift morphology of thenorthernmost JDFR could reflect diminished mag-ma supply linked to the northwestward motion ofthe ridge over the asthenosphere in the wake of theExplorer plate. In this scenario, mantle melt avail-ability at the trailing Endeavour and West Valleysegments may be limited by prior upwelling be-neath the adjacent leading segment (South ExplorerRidge) as the segmented ridge axis sweepsobliquely over the deeper mantle. An alternatehypothesis proposed by Schouten et al. [1987] isthat absolute motion of the plates may give rise tosubcrustal along-axis asthenospheric flow at a rateand direction equal to the axis-parallel componentof motion of the hot spot frame relative to the ridgeaxis (Figure 9). Along-axis flow is predicted fromthis model only if the absolute motion of the ridgeaxis is oblique to the spreading direction. Theabsolute motion of the JDFR axis is oriented38�–49� oblique to spreading direction (110�)and southward directed along-axis flow is pre-dicted (Figure 9a).

[40] From morphologic, gravity, and seismic obser-vations along the southern JDFR we infer influenceof the Cobb hot spot on melt supply to the JDFRwhich reaches over twice as far south of the currenthot spot location at Axial Volcano as to the north.The southward propagation of Endeavour segmentsince ridge capture of the Heckle anomaly, which isevident in magnetic and morphological data, indi-cates southward directed along-axis channeling ofthis melt anomaly as well. Preferential along-axischanneling of these melt anomalies to the southcould be linked to the northwestward migration ofthe JDFR, following the model of Schouten et al.[1987] (Figure 9). Perhaps damming of southwarddirected channeled subaxial flow by the large-offset Blanco TF [e.g., Georgen and Lin, 2003]plays a role in the excess crustal production at Cleftsegment compared with Vance. Beneath mostJDFR segments there is a notable asymmetry inAMC depths with shallower magma lens to the



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south indicating a warmer axial regime (Figure 5).This warmer-to-the-south trend could also reflectsouthward directed subaxial asthenospheric flowinduced by absolute motion of the ridge axis.

7. Conclusions

[41] Seismic reflection and bathymetric data fromthe JDFR reveal a recent history of ridge–hot spotinteraction which we associate with the Cobb hotspot and the Heckle melt anomaly. In both casesthe physical manifestations of ridge–melt anomalyinteraction include elongated axis-parallel plateauelevated a few hundreds of meters above surround-ing seafloor and associated with thickened crustconsistent with isostatic compensation of the ex-cess topography. Given the available age con-straints, onset of enhanced crustal productionclosely follows onset of the on-axis hot spotindicating rapid along-axis channeling of anoma-lous mantle. The scale of impact inferred from thestructural anomalies differs in the two regions,presumably because of differences in melt anomalyflux. In both areas, the impact of these meltanomalies on the ridge axis regime may be primar-ily thermal rather than compositional. The along-axis extent of Cobb hot spot influence estimatedfrom the geophysical anomalies is much wider thanthat inferred from the geochemical data and anom-alous melting of ambient mantle is inferred.

[42] An important conclusion of this study is thatthe structure and tectonic history of Endeavoursegment, including the current distribution of meltbeneath the axis and southward propagation duringthe early Brunhes, are directly linked to presence ofthe Heckle melt anomaly. There is also evidencefor a small recent ridge jump to the east, whichmay reflect transfer of the Heckle anomaly to theeastern flank of the ridge with ongoing northwest-erly migration of the JDFR. A return to northwardpropagation of Northern Symmetric segment in thepast 100,000 years may reflect waning influence ofthe Heckle anomaly on magma supply at Endeav-our segment as the ridge now migrates away fromthis anomaly.

[43] Seismic and bathymetric data reveal south-to-north gradients in ridge morphological parametersand melt distribution within the crust, which indi-cate a general trend of warmer to the south withinindividual ridge segments and along the JDFR as awhole. These trends cannot simply be attributed tohot spot proximity and may be linked to influenceof the absolute motion of the JDFR on mantlemelting and melt distribution beneath the ridgeaxis. Along-axis asthenospheric flow induced bythe obliquely migrating JDFR or variations inmantle fertility due to prior partial melting andmelt extraction as the ridge continuously migratesto the northwest over the deeper mantle couldcontribute to the along-axis gradients observed.Regional variations in ridge properties along the

Figure 9. Model for influence of absolute plate motions on ridge–melt anomaly interaction. (a) Predicted directionof subaxial asthenospheric (SAA) flow beneath the JDFR axis following Schouten et al. [1987] arising from absolutemotion of the spreading plate boundary relative to the underlying deeper mantle. Relative motion of the Pacific andJuan de Fuca plates is shown in blue, absolute motion of Pacific plate is shown in purple, and Juan de Fuca plate isshown in light blue. Predicted motion of the mantle (M) referenced to the ridge axis at R is shown in red with axis-parallel and -perpendicular components indicated. Velocity vectors are calculated from absolute plate motion modelHS3-Nuvel1a for the JDFR axis at 48�N [Gripp and Gordon, 2002]. (b) Schematic illustration showing a segmentedMOR approaching and overriding localized melting anomalies in the shallow mantle associated with off-axisseamount chains. Absolute motion of the plate boundary to the northwest gives rise to preferential southward directedalong-axis flow (red arrows) of the captured melt anomalies. Figure modified from Carbotte et al. [2004].



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JDFR relative to the large-scale segmentation andabsolute motion of the axis, are consistent withobservations elsewhere along the global MOR andsupport the notion that plate kinematics plays animportant role in the magmatic segmentation ofridges.

Acknowledgments

[44] Thanks to Mike Perfit, Gail Christeson, and Associate

Editor Peter Van Keken for their careful reviews which greatly

improved the manuscript. This work was supported by U.S.

National Science Foundation grants OCE00-02488 to S.M.C.,

OCE06-48303 to S.M.C. and M.R.N., OCE-0648923 to J.P.C.,

and OCE00-02600 to G.M.K. and A.J.H. This is LDEO

contribution 7152.

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