Refining the footwall cooling history of a rift flank uplift, Rio Grande rift, New Mexico

Refining the footwall cooling history of a rift flank

uplift, Rio Grande rift, New Mexico

M. A. House1

Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California, USA

S. A. Kelley

Department of Earth and Environmental Science, New Mexico Institute of Mining and Technology, Socorro, New Mexico,USA

M. Roy

Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico, USA

Received 20 May 2002; revised 11 June 2003; accepted 27 June 2003; published 25 October 2003.

[1] Apatite fission track (AFT) and (U-Th)/He datafrom the Sandia Mountains and Hagan embaymentprovide new insights into the thermal and tectonicevolution of the eastern flank of the Rio Grande rift innorthern New Mexico. AFT and (U-Th)/He data revealrapid cooling in the Sandia Mountains between 22 and17 Ma, followed by a decrease in cooling rate at 16 to14 Ma that temporally corresponds to a hiatus indeposition in the Albuquerque basin. A secondincrease in cooling rate at approximately 14 Ma wasfollowed by continued slow cooling until present.Cooling ages from Jurassic to Permian sandstones inthe Hagan embayment northeast of the SandiaMountains are used to constrain the thermalconditions in Oligocene time that are necessary tomap cooling histories into exhumation histories,thereby providing a limit on the amount of sectionremoved during rift flank development. Thermalmodeling, geologic constraints, and low-temperaturethermochronology are used to demonstrate that theheat flow in the Sandia Mountain region was at least25 mW/m2 higher during Oligocene time compared totoday. Furthermore, at least 3.1 km of material hasbeen exhumed from the Sandia Mountains and 2.4 kmof rock uplift occurred during flexural tilting of theblock since middle Miocene time. INDEX TERMS: 1035

Geochemistry: Geochronology; 8109 Tectonophysics: Continental

tectonics—extensional (0905); 9350 Information Related to

Geographic Region: North America; 9604 Information Related

to Geologic Time: Cenozoic; KEYWORDS: apatite, helium, fission

track, rift flank, Sandia Mountains. Citation: House, M. A., S. A.

Kelley, and M. Roy, Refining the footwall cooling history of a rift

flank uplift, Rio Grande rift, New Mexico, Tectonics, 22(5), 1060,

doi:10.1029/2002TC001418, 2003.

1. Introduction

[2] Rift flank uplifts are ubiquitous features in mostcontinental rift settings [Beaumont, 1978; Brown andPhillips, 1999; Matmon et al., 2000; Morgan et al.,1986; Vening-Meinesz, 1950; Watts et al., 1982]. Theseuplifted footwall blocks are generally separated from axialrift grabens by normal faults and are often associated withconsiderable local relief across the rift-bounding escarp-ment. The strong asymmetry and footwall warping docu-mented in many of these features has been explainedprimarily as the isostatic response to extension, but thetiming of relief production associated with these featuresremains uncertain [Alvarez et al., 1984; Gilchrist andSummerfield, 1990; Vening-Meinesz, 1950; Weissel andKarner, 1989].[3] The chronology of rift-flank exhumation and the

development of associated topographic relief can be recon-structed by using both the basin sedimentary record and thethermal history of footwall rocks [e.g., Fitzgerald andStump, 1997]. However, apatite fission track (AFT) coolingages, which are commonly used to track rift-related exhu-mation of footwall rocks, may significantly predate inde-pendently estimated ages of extension in some rift settings[Kelley and Duncan, 1986]. Pre-rift AFT cooling ages arepreserved in ranges where rift-related exhumation is lessthan 3 to 4.5 km, crustal depths associated with total fissiontrack annealing at 110�C [Kelley et al., 1992]. For example,the abundance of AFT ages recording Laramide deforma-tion (45 to 70 Ma) from some structural highs adjacent tothe Rio Grande rift in New Mexico (e.g., Santa Fe Range,Los Pinos Mountains, Sierra Nacimiento; Figure 1) havebeen interpreted to indicate that a portion of the modernrelief of the ranges is due to earlier tectonism [Kelley andChapin, 1995; Kelley et al., 1992].[4] Incorporating (U-Th)/He ages in apatite with existing

AFT data can refine the chronology of rift-flank deforma-tion and exhumation. The lower closure temperature of this

TECTONICS, VOL. 22, NO. 5, 1060, doi:10.1029/2002TC001418, 2003

1Now at Natural Sciences Division, Pasadena City College, Pasadena,California, USA.

Copyright 2003 by the American Geophysical Union.0278-7407/03/2002TC001418

14 - 1

thermochronometer (�70�C; [Farley, 2000]) means thatthis method will detect exhumation from shallower crustallevels (1.6–2.5 km) and so potentially record more recentstages in the process of rift flank bedrock uplift. Combining(U-Th)/He and AFT ages with sedimentologic data from riftbasins offers the most complete means to reconstruct thedevelopment of continental rifts in general and exhumedrift-flank blocks in particular.[5] We employ this approach in studying the Sandia

Mountains, which define the eastern edge of the Rio Granderift near Albuquerque, New Mexico (Figure 1). Eastwardtilting and warping of this range have been attributed toflexural footwall uplift in response to Neogene faulting [Royet al., 1999]. AFT ages record a cooling episode between�30–15 Ma [Kelley et al., 1992; Kelley and Duncan,1986], consistent with the timing of sedimentation inresponse to the early stages of rifting [Brister and Gries,1994; Chapin and Cather, 1994; Ingersoll et al., 1990;Large and Ingersoll, 1997; Lundahl and Geissman, 1999;

Smith, 2002]. However, the timing of this cooling episodecoincides with a potential reduction in geothermal gradientfollowing Oligocene volcanism in the area, meaning that theamount of exhumation attributed to the AFT data is notwell-constrained [Kelley and Duncan, 1986].[6] We have re-evaluated AFT results for samples orig-

inally studied by Kelley and Duncan [1986] and Kelley etal. [1992], and have selected a subset of these samples for(U-Th)/He analysis as well, with the aim of better docu-menting rift-related exhumation and possibly constrainingits relationship to the topographic growth of rift-flankblocks. These data are supplemented by new results fromthe Hagen embayment to the north (Figure 2), a region thatmay hold the key to pre-rift thermal conditions. The newthermochronologic data for the Sandia Mountains andeastern Rio Grande rift revise previous estimates for theonset of Tertiary exhumation of this block and extend thefootwall cooling history to lower temperatures, permitting amore detailed correlation of the unroofing history of therange to the record of sedimentation in the adjacent basins.

2. Sandia Mountains and Albuquerque Basin

[7] The sedimentary and thermochronometric record ofnorthern New Mexico shows that late Mesozoic tectonicquiescence gave way to Laramide compression, followed byvolcanism, and finally crustal rifting [Baldridge et al., 1995;Chapin and Cather, 1994; Erslev, 2001]. Deposition of theLate Cretaceous Mesaverde Group in the Hagen embaymentsuggests that large parts of this region resided at or near sealevel at end of Mesozoic time (Figure 2 [Beaumont et al.,1956; Mannard, 1975]). Beginning at �75 Ma, Laramidedeformation produced a number of significant basins andbasement-cored uplifts in the region [Dickinson et al., 1988;Seager, 1983; Seager and Mack, 1986]. In the central andwestern Albuquerque basin, Eocene to Early-Middle Oli-gocene pre-rift deposits of the Galisteo-Baca formations andthe unit of Isleta #2 attain a maximum total thickness of2700 m [Lozinsky, 1994; May and Russell, 1994]. Duringthe waning stages of Laramide deformation (31–32 Ma), ashallow intrusive center, the San Pedro-Ortiz porphyry belt,was emplaced east and northeast of the Sandia Mountains[Abbott et al., 2003; Maynard et al., 1990, 1991].[8] Extension across the northern New Mexico segment

of the Rio Grande rift initiated during Late Oligocene time[Chapin and Cather, 1994; Ingersoll et al., 1990; Migginset al., 2001; Morgan et al., 1986] and is manifested by aseries of deep, asymmetrical basins extending through NewMexico and Colorado that are separated by transverseaccommodation zones and are flanked by uplifted structuralblocks [Lewis and Baldridge, 1994]. The east tilted, topo-graphically high-standing Sandia crustal block is oneexample of such a rift flank. The Sandia Mountains occupythe footwall of a west dipping normal fault system boundingthe eastern Albuquerque basin [Russell and Snelson, 1994],and are bounded on the south by the Tijeras fault zone andon the north by the Placitas fault zone [Kelley and Northrop,1975]. Proterozoic (1.4 Ga) Sandia granite and the adjacentmetamorphic Cibola gneiss exposed in the footwall of the

Figure 1. Regional map showing the position of theSandia Mountains within the Rio Grande rift. The shadedarea outlines the basins of the northern Rio Grande rift inNew Mexico. Double lines show the approximate bound-aries between the basins and the strike-and-dip symbolrepresents the general dip of each half-graben [after Chapin,1988]. The fission track data of Kelley et al. [1992] andKelley and Chapin [1995] are shown for reference.

14 - 2 HOUSE ET AL.: THERMOCHRONOMETRY OF RIO GRANDE RIFT

steep western escarpment are unconformably overlain bysandstones and limestones of the Pennsylvanian Sandia andMadera groups [Kelley and Northrop, 1975; Kirby et al.,1994]. These laterally continuous strata are the erosionalremnants of a �2400 m thick section of overlying Paleo-zoic-Mesozoic strata [Kelley and Northrop, 1975], whichare partially preserved in the Hagan embayment and pres-ently define a dip slope along the more gently slopedeastern flank of the Sandia Mountains (Figure 2; approxi-mately 15�–20� east dip).[9] In the southern Albuquerque basin, the oldest rift fill

deposits are interbedded with a 22 Ma basalt [Bachman andMehnert, 1978]. Stratigraphic relationships throughout theAlbuquerque basin reveal episodic sedimentation [Kelley,1977]. The most detailed record of this episodicity is foundin the western Albuquerque basin where magnetostratigra-phy and K-Ar biotite ages for an airfall tuff reveal moderatesedimentation rates (�60 m/m.y.) between 17.3 Ma and16 Ma, followed by a 1.6 Ma hiatus and a subsequentincrease in rate to about 90 m/m.y. between 14 to 12 Ma[Tedford and Barghoorn, 1999]. The sedimentary unitbelow the hiatus is the Zia Formation, an eolianite derivedfrom sources to the west, while the unit above the hiatus isthe fluvial Arroyo Ojito Formation derived from the Naci-miento and southern Jemez Mountains. Although thesesediments cannot be directly tied to the development ofthe Sandia Mountains, they do record the development ofaccommodation space in the Albuquerque Basin. The sed-imentologic record elsewhere in the basin reflects thesevariations in sedimentation rate in less detail. For example,the Miocene Santa Fe Group, which reaches its maximum

thickness of �4200 m adjacent to the Sandia block in theeastern Albuquerque basin, is three times thicker above a16.1 Ma basalt flow in Shell Isleta #2 than it is below theflow [Chapin and Cather, 1994; May and Russell, 1994].Accelerated post-16 Ma sedimentation rates in the Santa FeGroup are also documented in the Socorro area to the south[Cather et al., 1994].[10] In addition to the north trending basin-bounding

faults, the Proterozoic basement along the western scarp ofthe Sandia Mountains is cut by a number of NNW to NNEtrending faults of uncertain offset and age, including theKnife Edge fault [Read et al., 1999] and the La Cueva fault,which parallels the La Luz trail (Figure 2 [Kelley andNorthrop, 1975]). Many of these faults, including the LaCueva fault, are inferred to be pre-Late Paleozoic in agebecause they do not offset the Pennsylvanian sedimentaryrocks [Kelley and Northrop, 1975]. Reverse faults on the eastflanks of the Sandias are inferred to be Laramide in age[Karlstrom et al., 1999]. Quaternary to Holocene faultingcontinues in the Albuquerque Basin today, producing a seriesof structural benches along the eastern margin of the basin[Chapin and Cather, 1994; Connell and Cather, 1999].[11] Warping of the Pennsylvanian Madera Group along

the eastern flank of the Sandia Mountains has been inter-preted to reflect total flexure of the region that has accu-mulated since Paleozoic time, with the majority of thedeflection forming in response to Neogene rifting [Roy etal., 1999]. Flexural models using this datum demonstratethat most of the modern local relief in the Sandia block(2400 m) is the result of extensional unloading by rift-related faults to the west [Roy et al., 1999]. Other workers

Figure 2. Geologic map of the Sandia Mountains and Hagan Embayment after Kelley and Northrop[1975]. Circles indicate locations of samples from the Hagan Embayment (HB) and La Bajada fault area(MAD); Sandia transect (La Luz) is indicated by a heavy line. The open circles are AFT samplesdiscussed by Abbott et al. [2003].

HOUSE ET AL.: THERMOCHRONOMETRY OF RIO GRANDE RIFT 14 - 3

suggest that warping of this unit was accommodated byfaulting to the east and is not purely isostatic flexure [Brownand Phillips, 1999] or may, in part, be produced byLaramide-aged deformation [Karlstrom et al., 1999]. How-ever, paleocurrents in the Paleocene Diamond Tail andEocene Galisteo formations in the Galisteo basin are fromthe northwest and no clasts from the Sandia Mountains arefound in the Galisteo basin to the north [Abbott et al., 1995;Stearns, 1953], suggesting that this range had little or norelief during Early Cenozoic time.

3. Previous Thermochronology From the

Rio Grande Rift and Sandia Mountains

[12] Existing constraints on the exhumational history ofthe Sandia Mountains, as well as other rift-flank blocks that

bound the Rio Grande rift, are largely based on AFT data[Kelley and Chapin, 1995, 1997; Kelley et al., 1992; Kelleyand Duncan, 1986]. Fission tracks are damage zones in acrystal or glass that are formed by spontaneous fission of238U [e.g., Wagner, 1968]. A fission track age is determinedby measuring the density of spontaneous fission tracks andthe U concentration of the sample [e.g., Naeser, 1976].Fission tracks anneal with increasing temperature [Naeserand Faul, 1969]. For most apatites (fluorapatite), annealingoccurs over a temperature range of �60�–110�C [Gleadowet al., 1986; Green et al., 1986], known as the partialannealing zone (PAZ, Figure 3a [Gleadow and Fitzgerald,1987]. In certain situations, when the cooling rate is rapid,the AFT ‘‘age’’ of a sample is interpreted as the approxi-mate time at which the sample cooled to temperatures below�110�C, although annealing may continue until completepassage through temperatures of �60�C. In cases of epi-

Figure 3. Schematic AHE and AFT age-elevation profiles and estimates of bedrock uplift.(a) Punctuated exhumation and bedrock uplift of buried samples residing at shallow levels in theEarth’s crust may result in the preservation of a fossil PAZ-PRZ (heavy gray solid and dashed lines), andinitiation of a new PAZ-PRZ (heavy white solid and dashed lines) at depth. Ages obtained in an elevationprofile preserve the uplifted fossil PAZ-PRZ with the ages ranging between the primary cooling age (t +t1) and age of most recent rock uplift (t1). The amount of modern structural distance between the upliftedfossil PAZ and PRZ zones the depths of the modern zones below the average surface elevation serves asan estimate of the amount of bedrock uplift (Dz) since t1. The elevation over which the PRZ and/or PAZspan can be used to infer the geothermal gradient immediately prior to tectonism. After Gleadow andFitzgerald [1987]. (b) In the case of continued exhumation at a slow to moderate pace, a PAZ or PRZmay not be preserved. Rather, linear AFT or helium age-elevation profiles may be preserved (heavydashed line shows example of helium age-elevation profile). In this case, the slope of the profile providesan indication of the rate of exhumation during the time bracketed by the ages of the samples (t1 to t + t1),provided that neither the geothermal gradient nor the rate of exhumation has changed. ASL = above sealevel and BSL = below sea level.


sodic exhumation, the pattern of AFT ages from eleva-tion profiles indicates the preservation of a fossil PAZ(Figure 3a), while in cases of continuous cooling, elevationprofiles may yield insight into the rate at which exhumationproceeded (Figure 3b). Thus these data can potentiallyprovide an indication of the magnitude and timing of atleast one, and sometimes multiple, episodes of exhumation[e.g., Fitzgerald and Stump, 1997].[13] Confined fission track lengths constrain the rate and

timing of cooling within the PAZ [Gleadow and Fitzgerald,1987; Green et al., 1989]. Experimental annealing data forfission tracks in apatite [Carlson et al., 1999; Crowley et al.,1991; Laslett et al., 1987] provide the basis for algorithmsdeveloped to constrain the thermal history of each samplewithin the PAZ [Corrigan, 1993; Gallagher et al., 1998;Green et al., 1989; Ketcham et al., 1999]. Thus, track-length measurements from vertical elevation profiles areused to reconstruct a set of cooling histories that potentiallycan be tied to estimates of the rate of exhumation (Figure 3)[Fitzgerald et al., 1995; House et al., 1999].[14] AFT analysis, coupled with geologic constraints, has

been used to identify at least three phases of Mesozoic toCenozoic exhumation in the southern Rocky Mountains,Rio Grande rift and southern High Plains [Kelley andChapin, 1995, 1997; Pazzaglia and Kelley, 1998]. LaramideAFT cooling ages reflecting the first episode are preservedextensively in the Front Range of Colorado [Kelley andChapin, 1997] and are found in the Santa Fe Range, LosPinos Mountains, and Sierra Nacimiento in northern NewMexico [Kelley et al., 1992] (Figure 1). The AFT datafurther show that Laramide deformation was followed by aLate Oligocene to Early Miocene cooling episode that isrecorded along the eastern margin of the Sangre de CristoMountains in northern New Mexico and southern Colorado,in the Pedernal Hills in east central New Mexico, and inTriassic sedimentary rocks as far east as Santa Rosa, NewMexico (Figure 1) [Leonard et al., 2002]. The stress regimeduring the Late Oligocene-Early Miocene cooling event wastransitional between Laramide compression and Rio Granderift extension [Erslev, 2001]. Interpretation of the AFT dataare complicated by the fact that the Mogollon-Datil, SanJuan, Latir, and Ortiz volcanic fields were all active duringthis time interval [Lipman et al., 1986; Maynard et al.,1990, 1991], and regional heat flow may have been high[Kelley, 2002].[15] A period of relative tectonic stability followed this

episode, which in turn was followed by bedrock uplift anderosion associated with rift flank development. AFTages firstreported for samples collected along the west flank of theSandia Mountains were interpreted to represent an episode ofexhumation between �30–15 Ma [Kelley and Duncan,1986]. The cooling history recorded in the Sandia samplespost-dates regional cooling due to volcanism (31 to 32 Ma[Abbott et al., 2003]) in the nearby Cerrillos and Ortizvolcanic fields (Figure 2). Thermal histories derived fromthe age and track length data (mean lengths of 13.9 to 15.1mm)using the model of Corrigan [1991] suggest that the SandiaMountains experienced�3–4.5 kmof erosion since�30Ma,with most occurring after �15 Ma [Kelley et al., 1992].

[16] Most of the AFT data from the Rio Grande rift flankstructural blocks yield ages >16 Ma, yet track-lengthmodels suggest that most of the exhumation of the riftflanks occurred after �15 Ma. Furthermore, the majorinflux of sedimentary material into the Albuquerque basinbegins at this time [Cather et al., 1994; May and Russell,1994]. Interpretations that have been offered to explain theonset of cooling observed in the Sandia AFT data includecontinued deformation following the Laramide events,changing climatic conditions, or epeirogenic bedrock upliftassociated with regional volcanism [Karlstrom et al., 1999;Roy et al., 1999], but post-16 Ma cooling is generallyagreed to reflect rift-related tectonism.[17] Provided that the entire local relief across the eastern

margin of the Rio Grande rift and the Sandia Mountainsblock (�2400 m) was created by extensional faulting andflexural bedrock uplift (as indicated by analysis of thepresent geometry of overlying Paleozoic sediments), thenthe accelerated influx of basement derived detritus into theadjacent Albuquerque basin can only be possible oncegranite basement is exposed. The length of time that ittakes a particle to go from �3000 m in the subsurface to thesurface by exhumation, then into the basin by erosion anddeposition is on the order of 6 m.y. for the rift flanks in NewMexico [Kelley et al., 1992]. The sedimentary influx in theeastern Albuquerque basin post-dates AFT cooling agesfrom the tilted footwall by more than 6 m.y. however,suggesting there may be more to learn regarding theNeogene exhumation and bedrock uplift of this block usingthe lower temperature (U-Th)/He thermochronometer.

4. New Thermochronometry From the

Eastern Margins of the Rio Grande Rift

[18] We have obtained new AFT and apatite (U-Th)/Hecooling ages for several sites along the eastern Rio Granderift: new AFT cooling ages in the Sandia Mountains providetighter constraints than previous work, while AFT ages fromthe late Paleozoic to Mesozoic sedimentary rocks in theHagan embayment were determined in an attempt evaluatethe thermal conditions of the stratigraphic section residingon the Proterozoic basement prior to rifting.[19] In order to document the portion of the exhumation

history that follows that constrained by AFT data, we turn to(U-Th)/He thermochronometry. The extremely low closuretemperature of this isotopic system is critical for resolvingsuch low-temperature thermal histories: for a typical150 micron (prism diameter) apatite grain, the temperatureof complete helium loss is �70�C (assuming a cooling rateof 10�C/m.y. [Farley, 2000]). In actual fact, the transitionbetween quantitative loss and retention of helium in apatiteoccurs over a thermal window of �40�–70�C (Figure 3a;the helium partial retention zone or HePRZ, similar to thePAZ [Wolf et al., 1998]). The (U-Th)/He closure tempera-ture, taken to be the temperature at the base of the HePRZ,scales with apatite grain size (larger grain sizes have higherclosure temperatures). Although the grain-size effect isslight (from 75–150 micron prism diameter, the closure


temperature varies by just 6�C [Farley, 2000]), it can beused to advantage in samples with a range of grain sizes[Reiners and Farley, 2001]. By selecting aliquots with arange of characteristic grain sizes, multiple (U-Th)/He agescan be obtained from a single rock, providing a segment ofthe rock’s cooling history rather than just one point. Thus,an apatite (U-Th)/He age provides an additional informationon the time-temperature history of a specific sample, per-mitting documentation of exhumational histories fromextremely shallow levels in the Earth’s crust.[20] We analyzed samples from two elevation transects

along the western flank of the Sandia Mountains: one alongthe La Luz trail and one that crosses a high angle fault to thenorth known as the Knife Edge fault (Figure 4a). In additionto these transects, three short stratigraphic traverses werecollected from east-dipping sedimentary rocks in the Haganembayment, in the footwalls of the San Francisco and LaBajada faults (Figure 2). One transect is from the block eastof the San Francisco fault and two transects were collectedeast of the La Bajada fault.[21] Results are reported in Table 1 (AFT) and Table 2

((U-Th)/He). For the (U-Th)/He analyses reported here,multi- and single-grain aliquots of apatites from concentratedseparates were selected on the basis of crystal form (typicallyeuhedral to subhedral grains) and clarity (clear, with nooptical evidence for inclusions). Samples were measuredand analyzed at the Caltech Noble Gas Laboratory usingthe furnace extraction method described by Farley et al.[2001] in the case of the La Luz transect, while the Knife

Edge, San Francisco, and La Bajada samples were analyzedusing laser gas extraction [House et al., 2000]. The averagereproducibility of replicated samples is 6.5% (1 s.d.), so wefollow the approach of Farley et al. [2001], and assign anuncertainty figure of 13%/

pN (2s) for samples analyzed N

times.

4.1. La Luz Transect

[22] Kelley and Duncan [1986] reported AFT ages for13 samples from an elevation transect through the SandiaGranite along the La Luz trail (Figure 4a, SAN). Fissiontrack ages were originally obtained using the populationmethod [Naeser, 1976] and track lengths were measured ona few of the samples using the method of Green [1988] withthe bias correction of Laslett et al. [1982]. More recently,Kelley et al. [1992] measured track lengths for the entiresuite.[23] Electron microprobe analyses indicate that the apa-

tite in Sandia Granite is fluorapatite with Cl in the 0.000 to0.017 wt% range and F values of 3.32 to 4.14 wt%. Theanalyses were done at New Mexico Tech on a CAMECASX-100 electron microprobe using a 10 mm beam and acurrent of 15 nA.[24] We found upon re-examination of the apatites that

they are strongly zoned with respect to major and rare earthelements, and uranium. For example, in samples 81SAN01and 81SAN04, several of the apatite grains show composi-tional zoning on SEM backscatter images that is due to

Figure 4. Simplified geologic map with locations of La Luz (SAN) and Knife Edge fault (ASC)samples [after Read et al., 1999]. Tmi = unfoliated dikes of Tertiary age. Heavy gray lines are faults,dashed where inferred.


Table

1.ApatiteFissionTrack

DataForSandia

Mountainsa

Sam

ple

Rock

Type

Latitude

Longitude

Elevation,

m

Number

of

Grains

Dated

r s�

105

t/cm

2r i�

106

t/cm

2r d

�105

t/cm

2

Central

Age,

Ma

(±1SE)

P(c

)2,

%

Uranium

Content,

ppm

MeanTrack

Length,mm

(±1SE)

Standard

Deviation

Track

Length

81SAN01

Sandia

granite

3512.27

10626.82

3095

20

1.25(124)

3.68(1826)

1.1806(4605)

19.1

±2.0

>99

38

14.8

±0.3

(101)

1.7

81SAN02

Sandia

granite

3512.30

10626.950

3006

20

1.59(117)

4.12(1517)

1.1812(4605)

21.7

±2.3

96

42

14.3

±0.5

(75)

2.3

81SAN03

Sandia

granite

3512.270

10627.090

2933

20

1.3

(135)

3.81(1980)

1.188(4605)

19.3

±1.9

80

38

14.8

±0.3

(100)

1.4

81SAN04

Sandia

granite

3512.370

10627.240

2805

20

1.97(208)

5.21(2749)

1.1929(4605)

21.5

±1.8

96

53

15.0

±0.3

(100)

1.8

81SAN05

Sandia

granite

3512.470

10627.340

2726

20

1.33(141)

4.14(2189)

1.2054(4605)

18.5

±1.8

96

42

14.7

±0.2

(100)

1.1

81SAN06

Sandia

granite

3512.560

10627.640

2616

20

1.28(149)

3.81(2222)

1.2018(4605)

19.2

±1.8

85

38

14.5

±0.2

(100)

1.3

81SAN07

Sandia

granite

3512.650

10627.86

2549

20

0.92(118)

2.83(1812)

1.2182(4605)

18.9

±2.0

97

28

13.9

±0.4

(100)

281SAN08

Sandia

granite

3512.680

10628.070

2439

20

1.09(112)

3.78(1936)

1.2259(4605)

16.9

±1.8

80

37

14.4

±0.3

(100)

1.6

81SAN09

Sandia

granite

3512.830

10628.400

2354

20

0.99(101)

3.54(1812)

1.2272(4605)

16.3

±1.8

92

35

14.1

±0.4

(100)

281SAN10

Sandia

granite

3512.790

10628.480

2262

20

1.22(119)

4.14(2020)

1.2396(4605)

17.4

±1.8

95

40

14.1

±0.6

(70)

2.7

81SAN13

Sandia

granite

3512.800

10629.360

2012

20

1.76(209)

6.43(3806)

1.2533(4605)

16.4

±1.4

60

62

14.0

±0.4

(100)

1.8

81SAN14

Sandia

granite

3507.750

10628.420

2134

20

1.13(74)

3.6

(1182)

1.2805(4605)

19.1

±2.4

98

34

14.5

±0.4

(50)

1.3

81SAN15

Sandia

granite

3506.630

10629.080

1890

20

0.94(45)

2.93(704)

1.2936(4605)

19.7

±3.1

96

27

14.2

±0.7

(51)

2.6

KnifeEdgeFault

99ASC-2

Sandia

granite

35�14.560

106�28.130

2515

20

0.78(87)

3.17(1778)

1.4151(4613)

16.5

±2.0

97

27

15.4

±1.4

(6)

1.8

99ASC-4

Sandia

granite

35�14.540

106�28.000

2633

20

0.84(94)

3.31(1856)

1.4252(4613)

17.2

±2.0

>99

28

13.8

±1.3

(11)

2.3

99ASC-5

Sandia

granite

35�14.450

106�27.870

2743

20

0.84(71)

3.46(1457)

1.4382(4613)

16.7

±2.2

98

29

14.3

±0.5

(16)

1.1

99ASC-7

Sandia

granite

35�14.640

106�28.450

2499

20

0.81(91)

3.19(1788)

1.4513(4613)

17.6

±2.0

99

26

15.0

±0.8

(13)

1.4

LaBajadaFault

99MAD02

JWestWater

Canyonss.

35�27.320

106�12.880

1706

20

1.31(143)

3.07(1673)

1.238(4613)

25.2

±2.4

95

30

13.9

±0.6

(25)

1.6

99MAD03

TriassicChinle

ss.

35�27.890

106�13.570

1685

20

2.55(286)

4.65(2606)

1.2404(4613)

32.4

±2.4

55

45

14.9±0.9

(22)

2.1

99MAD05

WestWater

Canyonss.

35�29.840

106�13.250

1688

20

1.29(136)

2.26(1194)

1.2396(4613)

33.6

±3.3

98

22

14.0

±1.1

(18)

2.5

99MAD06

TriassicChinle

ss.

35�29.970

106�13.650

1682

20

1.65(79)

3.25(781)

1.2417(4613)

29.9

±3.7

2.5

32

14.3

±1.6

(5)

1.8

99MAD07

Jurassic

Entradass.

35�29.980

106�13.730

1682

20

1.44(62)

2.6

(558)

1.2404(4613)

32.8

±4.6

80

25

12.9

±2.4

(5)

2.7

HaganEmbaym

ent

99HB02

PermianAboss.

35�21.150

106�23.190

1821

20

0.84(99)

3.1(1832)

1.2424(4613)

16.0

±1.8

96

30

15.0

±0.7

(13)

1.4

99HB04

TriassicChinle

ss.

35�21.560

106�21.990

1713

20

1.45(136)

3.74(1759)

1.2436(4613)

22.9

±2.3

70

36

14.3

±1.4

(9)

2.2

aDefinitionsareas

follows:r s,spontaneoustrackdensity;r i,inducedtrackdensity

(reported

inducedtrackdensity

istwicethemeasureddensity).Number

inparenthesisisthenumber

oftrackscountedfor

ages

andfluence

calibrationorthenumber

oftrackmeasuredforlengths;r d,trackdensity

inmuscovitedetectorcoveringCN-6

(1.05ppm);reported

valuedetermined

from

interpolationofvalues

fordetectors

coveringstandardsat

thetopandbottom

ofthereactorpackages

(fluence

gradientcorrection).SE,standarderror.P(c

)2,chi-squared

probability;l f,1.551�

10�10yr�

1;g,0.5;zeta,4772±340(CN6)for

apatite.

Meantracklengthsnotcorrectedforlength

bias[Laslettet

al.,1982].


Table 2. Apatite Helium Resultsa

Sample[U],ppm

[Th],ppm U/Th

[4He],nmol/g

Mass,mg Ft

Radius,mm

Age,Ma(±2s)

AverageAge(±2s)

Sandia Mountains (La Luz and Knife Edge)81SAN01Analysis a 22 83 0.27 2.941 17.5 0.74 46 17.4b 17.3 (1.3)Analysis b 18 51 0.35 2.179 26.6 0.78 59 17.2b

81SAN03Analysis a 17 54 0.31 2.152 25.9 0.77 54 17.2b 17.1 (1.2)Analysis b 23 81 0.28 3.125 42.3 0.80 55 17.0b


Analysis c 19 69 0.28 2.535 20.9 0.75 48 17.5b

Analysis d 24 84 0.29 2.993 21.0 0.75 48 16.8b

Analysis e 21 69 0.30 2.612 25.0 0.76 51 17.2b


Analysis c 27 72 0.37 2.691 2.4 0.74 49 15.3Analysis d 15 51 0.30 2.157 5.3 0.80 63 18.2Analysis e 34 102 0.33 3.383 4.7 0.79 63 13.5




Analysis c 11 41 0.27 1.333 49.0 0.78 56 15.0b

Analysis d 13 30 0.44 1.133 6.9 0.81 69 12.581SAN09 9 30 0.30 0.892 61.6 0.81 65 12.9b 12.9 (1.3)81SAN10 10 37 0.27 1.098 23.2 0.75 48 14.5b 14.5 (1.5)99ASC-4Analysis a 11 43 0.25 1.115 3.9 0.75 57 13.2 16.2 (1.1)Analysis b 8 29 0.26 1.015 2.7 0.72 51 18.1Analysis c 13 42 0.32 1.504 3.6 0.74 57 16.0Analysis d 29 108 0.26 3.211 1.5 0.63 34 17.5

99ASC-5Analysis a 18 72 0.25 2.158 1.6 0.67 43 17.0 16.4 (1.2)Analysis b 28 110 0.25 2.856 2.5 0.68 40 14.4Analysis c 11 45 0.25 1.377 1.8 0.66 40 17.7

99ASC-7Analysis a 21 49 0.43 2.110 2.7 0.72 51 16.5 15.2 (1.1)Analysis b 21 76 0.27 2.076 2.4 0.70 46 14.1Analysis c 6 19 0.31 0.652 6.9 0.78 63 15.1

La Bajada Fault99MAD02Analysis a 9 28 0.32 0.955 3.7 0.71 51 16.2 17.7(1.6)Analysis b 12 15 0.81 1.319 6.3 0.79 69 19.2

99MAD05 14 54 0.26 2.042 6.5 0.78 63 18.2 18.2 (2.4)

Hagen Embayment99HB02Analysis a 25 11 2.37 1.823 12.0 0.83 83 14.6 (1.9)Analysis b 2 3 0.74 0.125 5.2 0.78 66 9.7 (1.3)

99HB04Analysis a 37 6 5.74 3.342 13.5 0.84 86 19.2 (2.5)Analysis b 7 24 0.27 0.498 7.8 0.79 69 9.4 (1.2)

aFt and radius are mass-weighted values. Age is corrected for alpha-ejection [Farley et al., 1996]. Values in parentheses are 2suncertainties computed using method of Farley et al. [2001]. No average ages are reported for San Francisco fault samples asdescribed in text.

bSamples outgassed using a resistance furnace; all others were outgassed using a Nd-YAG laser [House et al., 2000].


differences in SiO2 content (0.40 to 1.04 wt% in some zonescompared to 0.04 to 0.33 wt%). More important, most of theapatites from the top of the elevation profile (81SAN01 to08) have uranium concentrated on the rim of the apatitegrains (Figure 5), although some grains with high uraniumcores were also observed. Consequently, we re-evaluatedthese samples using the external detector method [Naeser,1979].[25] The new AFT ages are shown with track lengths of

Kelley et al. [1992] in Table 1. The ages range from �22 Maat high elevation to 16 Ma at low elevation; this age range issignificantly narrower than that reported by Kelley andDuncan [1986]. The largest age discrepancies between thetwo data sets come from the highest elevation samples on theprofile, where the uranium zonation in the apatite is mostpronounced. Where the uranium concentrations in the apa-tites are more uniform, the ages are the same within error.The steeper slope of the age-elevation profile (Figure 6a) is

more consistent with the rapid cooling rate suggested by thelong mean track lengths of 13.9 to 15.0 mm measured byKelley et al. [1992].[26] A subset of nine of the La Luz samples was selected

for apatite (U-Th)/He thermochronometry. Replicate analy-ses were obtained for all but two samples, 81SAN09 and81SAN10, for which insufficient material was available(Table 2 and Figure 6a). In most cases, replicates were ingood agreement but multiple replicates of samples81SAN04, 81SAN05 and 81SAN08 exhibited considerablescatter outside the expected uncertainty. This scatter mayreflect compositional complexities in the apatite grainsthemselves, as described above. The (U-Th)/He dates are12.9 to 18.5 Ma and are consistently younger than the AFTage for each sample, displaying a positive correlationbetween age and elevation with a small break in slope ata modern elevation of �2550 m (Figure 6a).

4.2. Knife Edge Fault Transect

[27] Four samples of Sandia Granite were collected alonga short transect to the north of the La Luz trail (Figure 4a;ASC). This suite of samples crosses the Knife Edge fault, asteeply dipping breccia and pseudotachylite zone that sep-arates the high-elevation cliff face (locally known as TheShield) of the western escarpment of the Sandia Mountainsfrom the lower-elevation, less rugged portion of the escarp-ment [Read et al., 1999]. This fault corresponds to atopographic break and may be rift-related. The AFT datafrom the Knife Edge fault transect are consistent with agesfrom the La Luz trail to the south. AFT ages for thesesamples range from 16.5 to 17.6 Ma, and generally increasewith sample elevation. No obvious break in age is observedas the transect crosses the Knife Edge fault, suggesting thatthere has not been significant offset across this structuresince �16 Ma.[28] Three samples from the Knife Edge profile (ASC4, 5

and 7) contained sufficient quantities of quality material forreplicated single-crystal analyses. (U-Th)/He ages are 15.2–16.4 Ma and are also positively correlated with elevation.While there is no obvious break in age observed across theKnife Edge fault, the samples in the elevation rangecorresponding to the La Luz profile seem to display similarcurvature, suggesting a possible reduction in exhumationrate.

4.3. La Bajada Fault

[29] The second area that we targeted was the Permian toMesozoic sediments exposed near the La Bajada fault inthe Hagan embayment, north of the Sandia Mountains(Figure 4a). The Hagan embayment is unique in that muchof the Paleozoic, Mesozoic, and early Cenozoic sedimentarysection that likely covered central New Mexico prior torifting is preserved here [Kelley and Northrop, 1975].Therefore, cooling ages from this region provide the meansto obtain limits on the pre-rifting thermal structure of theregion, as well as additional insight into regional progres-sions in rift-related faulting.[30] Ten samples of Triassic to Cretaceous sandstone

along two traverses through the footwall of the La Bajada

Figure 5. Photos of induced fission tracks in muscovitedetectors, showing the zonation of uranium in the apatitefrom the Sandia Granite.


fault were collected for analysis, but only five containedenough apatite for dating purposes. Unlike the clear, euhe-dral crystals from the Sandia granite, apatites from this suitewere variably frosted and abraded, as expected from detritalapatite crystals. Along the southern traverse, the AFT agefor the Salt Wash Canyon member of the Jurassic MorrisonFormation is 25.2 ± 2.4 Ma and the (U-Th)/He age is 17.7 ±1.6 Ma. The AFT age of the underlying Triassic Chinlesandstone is 32.4 ± 2.4 Ma. The AFT ages are not clearlycorrelated with stratigraphic position or elevation (Figure 6c)because the Chinle Formation contains chlorine-rich grainsthat tend to retain fission tracks to temperatures of about140�C, leading to a mix of AFT age populations andgenerally older central ages. Note that the Chi-squaredstatistic in Table 1 is low for the Chinle samples, indicativeof multiple age populations [Galbraith, 1981]. The samplesalong the northern profile were collected in the La Bajadafault zone (MAD07), �0.1 km east of the fault (MAD06)

and 0.7 km east of the fault (MAD05). AFT ages along thenorthern traverse (29.9–32.8 Ma) are the same withinuncertainty and are similar to those to the south, as is thesingle (U-Th)/He age from this profile (18.2 ± 2.4 Ma).[31] Sufficient numbers of confined tracks for meaningful

analysis were found in only three samples. The mean tracklengths are long (13.9–14.9 mm) with narrow standarddeviations, suggestive of rapid cooling. Cooling rates of3�–5�C/m.y. are calculated using the AFT and (U-Th)/Heages.

4.4. San Francisco Fault

[32] The AFT age of the Permian Abo sandstone collectednear the San Francisco fault zone is 16.0 ± 1.8 Ma and the(U-Th)/He analysis yields two age populations, one at 14.6 ±1.9 Ma and the other at 9.7 ± 1.3 Ma. Similarly, the TriassicChinle further up-section has an AFT age of 22.9 ± 2.3 Ma

Figure 6. Apatite helium and fission track ages from the Sandia Mountains, New Mexico. Apatitehelium ages are shown by solid circles and apatite fission track ages are shown by open circles. Errors areshown (2s) for both data sets. The triangles correspond to two samples south of the La Luz traverse.(a) Age versus elevation relationship for Sandia samples in present position. Boxes represent the KnifeEdge profile, while circles represent the La Luz transect. (b) Relative elevations of Sandia samples aftercorrection for 15� of eastward tilting. (c) Apatite helium and fission track ages from the footwall of the LaBajada-San Francisco fault. Helium ages are shown by solid symbols and fission track ages are shown byopen symbols. Boxes represent samples from the La Bajada footwall, while circles represent samplesfrom the San Francisco footwall. Triangles are small grain size helium ages.


and two (U-Th)/He age populations, 19.2 ± 2.5 Ma and 9.4 ±1.2 Ma. The AFT and older (U-Th)/He ages both correlatewell with stratigraphic position and indicate rapid cooling,consistent with the few mean track length measurements forthese samples. In both samples, there is a correlation betweengrain size and (U-Th)/He age such that older ages correspondto larger grains (�83–86 micron radii; Table 2), whileyounger ages are found in the smaller (66–69 micron radii)grains. Part of the range in ages observed here can beexplained by the control of grain size on closure temperature.For example, a cooling rate of 1�C/m.y. suggests a range ofclosure temperature of 61�–64�C for grains in this sizerange, resulting in age differences of �3 m.y.. Higher cool-ing rates would reduce this range of temperatures and theresulting age differences. However, the range in ages that weobserve is larger than this, suggesting that either these rocksresided for a longer period of time within the HePRZ, or thatthere may be some other factor controlling the (U-Th)/He agein these samples. The fact that U and Th contents also appearto be correlated with grain size suggests that these grainsreflect two distinct detrital populations.

5. Discussion of Results

5.1. La Luz and Knife Edge Transects

[33] The correlation between cooling age and elevation inAFT and (U-Th)/He ages along the Sandia La Luz andKnife Edge transects is consistent with cooling of theSandia Mountain block through temperatures of �110�–70�C between �22–14 Ma. Apparently rapid coolingbetween 22 and �17 Ma is indicated by high elevationsamples (>2700 m). An offset in both the AFT and (U-Th)/He age elevation trends at �2500–2600 m elevation onboth the La Luz and Knife Edge profiles may reflect a briefreduction in cooling rate at �16 Ma (Figure 6a). In bothprofiles, cooling ages below �2500–2600 m are largelyage-invariant with elevation, suggesting a return to rapidcooling after �16 Ma.[34] The AFT age and track length data summarized in

Table 1 and the AFTSolve algorithm [Ketcham et al., 1999]were used to determine thermal histories for the SandiaMountain samples. Grey swaths shown on Figure 7 containpossible thermal histories that fit the AFT age and tracklength data for the highest and lowest elevation samples forwhich we have both AFT and (U-Th)/He data on the La Luztraverse. The lower elevation sample requires a two-phasecooling history, such that an early episode of cooling at�17 Ma is followed by second acceleration at �14 Ma(Figure 7b). In contrast, the thermal history for the highestsample does not record the second episode of acceleratedcooling, perhaps because this particular sample had alreadycooled below the closure temperature for both AFT and(U-Th)/He systems and was at or very near the surface by17 Ma (Figure 7a).[35] The first rapid cooling episode indicated by the

modeling results for sample 81SAN10 and the AFT and(U-Th)/He age-elevation profiles in general is broadlyconsistent with the timing of influx of basement-derivedmaterial in the Santa Fe Group of the adjacent Albuquerque

basin [e.g., Cather et al., 1994; May and Russell, 1994].The timing of the reduction in cooling rate indicated bymodeling track length data from sample 81SAN10 and fromthe (U-Th)/He data from the vertical profile roughly corre-sponds to the time of a sedimentary hiatus (16–14.4 Ma) inthe Santa Fe Group in the western Albuquerque basin[Tedford and Barghoorn, 1999]. Note that the AFT ageand track length data for sample 81SAN10 are equally wellfit by thermal histories that do not include a rate decrease[e.g., Kelley et al., 1992], but the step in the cooling historyis suggested by the break in slope along the (U-Th)/He age-elevation profile (Figure 6a) and is consistent with the81SAN10 (U-Th)/He age.[36] Leeder and Gawthorpe [1987] have explored the

timing of sedimentation in a basin with respect to the timingof footwall uplift. The Leeder and Gawthorpe [1987] modelof sedimentation in rift basins suggests that during times ofintense tectonism along range front faults and increasedsubsidence of the basin adjacent to the footwall block, afine-grained playa facies commonly accumulates along themountain front and erosion and tilting occurs on the

Figure 7. Range of possible thermal histories that fit tracklength and age data. Histories derived using the model ofKetchum et al. [1999].


hanging wall dip slope. During times of relative quiescence,sediments eroded from the mountain front onlap the hang-ing wall dip slope and sedimentation on the dip slope isrenewed, usually above an angular unconformity. Thesection measured by Tedford and Barghoorn [1999] is onthe hanging wall dip slope, but no angular discordance ismentioned in their description. The model of Leeder andGawthorpe [1987] may not be directly applicable to theAlbuquerque Basin because the sediments on either side ofthe hiatus are not derived from the Sandia Mountains.

5.2. Hagan Embayment Results

[37] Cooling ages across the Hagan embayment suggestthat spatially variable deposition and possibly, a progressionin faulting controlled the rate and time at which this regioncooled (Figure 6c). AFT ages from east of the La Bajadafault are nearly synchronous with the age of volcanism (31to 32 Ma) in the nearby Cerrillos and Ortiz fields, so theymay reflect cooling following locally elevated heat flowaround these volcanic centers. Long track lengths in thesesamples indicate that this cooling was rapid, but therelatively large difference between AFT and (U-Th)/Heages (�18 Ma) in these rocks suggest that cooling slowedprior to passage through �70�C. This later stage of coolingrecorded by the (U-Th)/He data coincided with depositionof more than 953 m of conglomerate, sandstone andmudstone in the hanging wall of the San Francisco fault,in the region west of the La Bajada fault. Deposition of thismaterial, derived from the Ortiz Mountains in the footwallof the La Bajada fault, began at �25 Ma and continued untilafter 11 Ma [Connell and Cather, 1999; Stearns, 1953].Thus, the (U-Th)/He ages from the footwall of the LaBajada fault may reflect exhumation in response to motionacross the La Bajada fault.[38] Farther west, cooling was in progress in the footwall

of the San Francisco fault at �23–15 Ma and may havecontinued as late as �10 Ma. The AFT age and track-lengthdata, combined with the small difference between the AFTand (U-Th)/He ages of the larger grains for these sandstonesindicate that cooling in this part of the basin was quiterapid. This small range in AFT and (U-Th)/He ages for agiven sample implies a rapid rate of cooling similar to thatpreserved in samples from the Sandia Mountain block,suggesting that these younger ages may reflect a westwardprogression of faulting across the Hagen embayment.Alternatively, the relatively younger helium ages in thefootwall of the San Francisco fault may reflect delayedcooling related to the large influx of clastic debris derivedfrom the east at this time (see above). In either case, the9.4–9.7 Ma (U-Th)/He ages of the smaller grains in thesandstones suggest that the cooling rate decreased dramat-ically after �15 Ma in the San Francisco fault footwall.

6. Paleotemperature Estimates From

Hagan Embayment Data

[39] Cooling documented by thermochronometric dataand track length models in the Sandia Mountains may be

attributed to exhumation of the footwall block driven bybedrock uplift across the range-bounding normal faultsystem, as well as erosion and isostatic adjustments drivenby increased local topographic gradients across the rift flank[Kelley et al., 1992]. However, in order to map thesecooling histories into exhumation histories (and perhapsgain some insight into the development of topographicrelief), we must have some understanding of the geothermalgradient prior to exhumation. If the AFT ages from the LaBajada region predate rift-related exhumation, then theymay provide a means to estimate of the geothermal gradientprior to exhumation as well as an upper limit on themagnitude of exhumation.[40] Figure 8 shows a reconstruction of a range of

possible temperatures in the northern Sandia Mountains/Hagan embayment area at the end of Cretaceous time(Figure 8a) and at the end of volcanic activity in Ortizand San Pedro volcanic fields during Oligocene time(Figure 8b). The thickness of the Mesozoic to Paleozoicsection used in these reconstructions is taken from Stearns[1953]. The thickness of the preserved Paleocene to Mio-cene section varies considerably across the area (400–2000 m [Stearns, 1953]) so an average thickness of�1000 m is used in this calculation. Thermal conductivityvalues for the predominant lithologies in each unit are basedon published estimates for similar lithologies [Carter et al.,1998]. The temperatures are calculated using the thermalresistance method [Bodell and Chapman, 1982; Bullard,1939] for a range of heat flow values. A heat flow of63 mW/m2 is assumed for the tectonically stable situation atthe end of the Mesozoic, prior to Laramide deformation, anda heat flow of 105 mW/m2 is assumed during peak riftingand volcanism in the area, translating to average gradientsof 22� and 35�C/km, respectively. The modern heat flow inthe northern Sandia Mountains is �80 mW/m2. This valuecorresponds to a modern gradient of 29�C/km. Note that thelow-thermal conductivity Mancos Shale has an importantimpact on the thermal structure of the area and that thegradient through the shale interval is �50�C/km.[41] These reconstructions show that temperatures in the

northern Sandia Mountains likely were not high enough toanneal the fission tracks near the top of the Proterozoicbasement at the end ofMesozoic time if a heat flow consistentwith a tectonically stable crust is assumed (Figure 8a, lightline). The only way to heat the basement just below the GreatUnconformity (separating Pennsylvanian strata from theunderlying Sandia granite) to temperatures affecting theAFT systematics prior to Laramide deformation is to havean elevated heat flow of 105 mW/m2 (Figure 8a, heavy line),an unlikely scenario given geologic history described earlier.Consequently, the base of the PAZ at this time (110�C) wasmost probably at a depth of �4000 m in the subsurface andthe base of the PRZ (70�C) was at about 2000 m depth, nearthe top of the Madera Group, at the end of Mesozoic time.The calculations demonstrate that an increase in regional heatflow during middle Cenozoic time is required to explain theAFT data.[42] During Oligocene time, regional volcanism generally

elevated the heat flow in New Mexico and southern Colo-


rado [Kelley, 2002] and at least 1000 m, and perhapsas much as 2000 m, of material was deposited on top ofthe Cretaceous Mesa Verde Formation. Figure 8b showsthe range of temperatures for a conservative 1000 mthickness of Laramide and post-Laramide section and heatflows ranging from the modern 80 mW/m2 to a high of105 mW/m2. The minimum heat flow required to heat theSandia granite just below the unconformity to temperaturesabove 110�C, assuming 1000 m of burial, is 70 mW/m2.

However, our limited collection of samples from the Haganembayment indicates that the entire section below andincluding the Jurassic Morrison Formation in the vicinityof the La Bajada fault was at temperatures above 110�Cprior to 35 Ma. An elevated heat flow value of 105 mW/m2

alone does not heat the Morrison to the required temper-atures. Rather, an additional 500 m of burial with a heatflow value of 105 mW/m2 are needed to raise the Morrisonto 110�C. If the modern heat flow is assumed, an addition

Figure 8. Paleotemperatures for the Hagen Embayment. (a) Estimated temperatures at the end ofCretaceous time assuming normal heat flow of 63 mW/m2 (light line) and an elevated heat flow of105 mW/m2 (heavy line). Unit thicknesses from Stearns [1953] and thermal conductivity values fromCarter et al. [1998]. The solid circles represent parts of the section that have AFT data indicatingpaleotemperatures >110�C prior to �35 Ma. The dashed line shows 110�C for reference. (b) Estimatedtemperatures during Oligocene time assuming a heat flow of 105 mW/m2 and an average thickness of1000 m for Laramide and post-Laramide section (shown in gray [Stearns, 1953]).


1500 m of section is needed to elevate paleo-temperaturessufficiently to reset the Hagen embayment AFT ages.

7. Elevated Regional Heat Flow During

Oligocene Time

[43] The calculations above indicate that at least 3200 m,and perhaps as much as 4700 m of section, has beenremoved from the top of the Madera Limestone, the rangecapping unit at the crest of the modern Sandia Mountains,since Oligocene time (Figure 8b). Furthermore, regionalheat flow was at least �25 mW/m2 higher than it is today(Figure 8b). Now we are faced with the challenge ofseparating cooling associated with decreasing heat flowfrom cooling associated with exhumation.[44] As alluded to in the previous section, the elevated

late Oligocene to early Miocene heat flow recorded by theAFT data in the Hagan embayment appears to be related toa large middle Cenozoic heat flow anomaly that influencedpaleotemperatures on the High Plains of northeastern NewMexico and southeastern Colorado [Kelley and Chapin,1995; Kelley, 2002; Leonard et al., 2002]. AFT ages derivedfrom Triassic sandstone exposed at the surface betweenAmarillo, Texas, and Santa Fe, New Mexico, generallydecrease toward the west [Leonard et al., 2002]. Apatitefrom Triassic sandstone east of Santa Rosa, New Mexico,(Figure 1) have short mean track lengths and mixed agepopulations, characteristic of samples originating fromwithin the PAZ. Surprisingly, surface exposures of Triassicsandstone between Santa Rosa, which is located 185 kmeast of Albuquerque, and the eastern margin of the RioGrande rift yield AFT ages on the order of 25 to 30 Ma. Inother words, the base of a middle Cenozoic PAZ observedin the drillholes in Oklahoma [Carter et al., 1998] and innortheastern New Mexico [Leonard et al., 2002] comes tothe surface near Santa Rosa (Figure 9).[45] A decline in regional heat flow following extensive

Oligocene-Miocene volcanism has been proposed byMorgan et al. [1986] and Perry et al. [1993]. Perry etal. [1993] used Nd compositions of rhyolite from thewestern United States to suggest that lower crustaltemperatures decreased by �300�C at about 25 to 20 Ma.Here we examine the cooling history of the upper 2 to 4 kmof the crust associated with decreasing middle to lower

crustal temperatures using simple 2-D analytical [e.g.,Lachenbruch et al., 1976] and numerical models. Themodels are designed to examine two concepts. First, can adecline in regional heat flow account for the rapid coolingrecorded by the low temperature thermochronometers in theSandia Mountains? Second, how did volcanism in the OrtizMountains and Cerillos Hills influence the cooling history ofthe Sandia Mountains?[46] We summarize our findings for models that incorpo-

rate hypothetical thermal source geometries that can poten-tially match the observed rock cooling rates in the SandiaMountains and the differential uplift (relative to the surface)and tilting of the Middle Cenozoic partial annealing zone onthe High Plains of NewMexico. A 10 km thick, 1300�C, heatsource with a half-width of 160 km was turned on for 10 Maand then allowed to cool. The top of the heat source wasplaced at depths of 30 km, 20 km, and 10 km, and wascentered under the Sandia Mountains. The sources need tohave a minimum half width of 160 km to match the SantaRosa AFT data on the tilted PAZ. At depths of 2–4 km, theaverage estimated burial depth for the Triassic to upperProterozoic section in the Sandia Mountains prior to exhu-mation, temperatures rise only 20�–40�C above backgroundtemperatures and the cooling rate is slow (�1�C/m.y.) for asource at 30 km. Temperatures at 4 km in the Sandia areaexceed 200� to 370�C and cooling rates are 5� to 20�C/m.y. ifshallower (10–20 km) sources are assumed. Current seismicimaging of the Albuquerque region does not have sufficientresolution to identify potential middle to lower crustalstructures that may represent an equilibrated mafic magmachamber beneath the Sandia Mountains. Additionally, grav-ity and magnetic data from central New Mexico do notindicate shallow, broad (160 km half-width), mafic sourcesin the crust in this region. The Rio Grande rift is currentlyunderlain by at least two active magma chambers of mafic tointermediate composition, beneath the Jemez Mountains andbeneath the Socorro region [e.g., Rinehart and Sanford,1981; Balch et al., 1997; Fialko and Simons, 2001; Lutteret al., 1995], suggesting the importance of intrusive events inrifting. Although we recognize that cooling of an Oligoceneage, shallow mafic magma chamber may potentially explainthe low-temperature thermochronology above, present geo-physical imaging does not support this interpretation.[47] Alternatively, a cooler (900�C), long-lived (10 m.y.

duration) source representing an intermediate composition

Figure 9. Illustration of the geometry of the middle Cenozoic apatite fission track partial annealing zone(PAZ) with respect to the Rio Grande rift, the Sandia Mountains and the southern High Plains. Modifiedfrom Roy et al. [1999] and Leonard et al. [2002].


magma chamber at 10 km produces temperatures >225�Cand rapid cooling rates (�10�C/m.y.) at 4 km and temper-atures >120�C and rates of 5�C/m.y. at 2 km. Instantaneouscooling of an 800�C source at 10 km yields temperatures of�180�C at 4 km and a cooling rate of 6�C/m.y. Thus, it ispossible to produce rapid relaxation of isotherms if a sourceat 10 km is assumed. The high temperatures (>180�C)predicted by these models at 4 km appear to be recordedby 40Ar/39Ar potassium feldspar data for the Sandia Granite[Harrison and Burke, 1988]. However, there is no compel-ling geophysical evidence for the cooled remnants of a largemagma body a depth of 10 km between Albuquerque andSanta Rosa [e.g., Karlstrom et al., 2002]. The slow coolingrates associated with the cooling of deeper sources (>10 km)do not match the observed rates.[48] The remnants of very shallow intermediate composi-

tion intrusions are preserved in Ortiz Mountains and Cer-rillos Hills. Maynard [1995] notes that the Ortiz magmasintrude rocks as young as Eocene Galisteo Formation andthat the intrusions are in the form of laccoliths. The La Luztraverse is 30 km from the exposed edge of the Ortizintrusions and the intrusions have an exposed half width of12 km. If we assume that the laccoliths were emplaced at adepth of 1 km, that the magma was initially at 1000�C, andthat laccoliths are fed by a magma chamber at 10 km with ahalf width of 12 km, then the temperatures due to localmagmatism increase only 3�C above background in theSandia Mountains, if cooling is instantaneous. If magmatismwas continuous for 1 Ma, the maximum time permitted bythe 40Ar/39Ar data of Abbott et al. [2003], then temperaturesin the Sandia Mountains could have increased as much as8�C above background. Peak temperatures are reached about5 Ma after magmatism ends and the cooling rates at thisdistance are quite slow, well below 1�C/m.y.[49] In summary, both regionally and locally elevated

heat flow have certainly influenced temperatures in theSandia Mountain region, but the rapid cooling ratesrecorded by the AFT and (U-Th)/He data in the SandiaMountains are not merely a reflection of isotherm relaxationfollowing magmatism. We would argue that the rapid cool-ing observed in the low temperature thermochronology datais controlled primarily by exhumation.

8. Constraints of the Timing of Footwall

Tilting and Initiation of Exhumation

[50] The orientation of Pennsylvanian limestones that capthe eastern flanks of the Sandia Mountains, as well as theaverage slope of the east flank of the range provide anupper limit on the magnitude of post-22 Ma eastward tiltingof the Sandia Mountains (Figure 4b [Kelley and Northrop,1975]). The �2400 m of bedrock uplift that results fromthis tilting, while an upper limit, accounts for much of themodern topographic relief across the Sandia Mountainstoday (Figure 4b). Stratigraphic relationships in the Haganembayment suggest that tilting of the Sandia block initiatedin the Middle Oligocene and continued through the Pleis-tocene and was coincident with much of the footwall

cooling recorded by our thermochronometric data. Forexample, an angular unconformity between the 36–27 MaEspinaso Formation and the basal Tanos Formation of theSanta Fe Group containing a 25.4 ± 0.3 Ma basalt flowconstrains the timing of some of the tilting in this area to bemiddle Oligocene [Connell and Cather, 1999]. The uppermember of the Santa Fe Group in the Hagan embayment,the Blackshare Formation, conformably overlies the TanosFormation and contains an 11.6 ± 0.4 Ma ash bed. Thedip of the Blackshare Formation continuously decreasesup-section from 27� to 36� NE in the lower part of thesection to 4� to 16� NE above the ash bed. The flat-lyingTuerto Formation overlying this sequence interfingers witha 2.8 Ma basalt from the Cerros del Rio volcanic fields,indicating that all tilting pre-dates 2.8 Ma. Provided thattilting of these basin strata is somehow related with tiltingof the Sandia block (supported by the similarity in coolingages at both sites), then constraints from the Haganembayment suggest that tilting roughly coincides withand partially post-dates the cooling of the Sandia footwall.[51] Together, the thermochronologic and stratigraphic

observations can be used to reconstruct the chronology ofexhumation and possibly topographic growth, along theeastern flank of the Rio Grande rift. At end of the Laramide,the Madera Formation was buried below �2000 m ofsediment and rocks of the La Luz and Knife Edge transectswere at temperatures of 80� to 90�C. The addition of at least1000 m of material during Early Cenozoic time and elevatedheat flow and burial during Oligocene volcanism furtherheated the sample transects. Ages of 22–16 Ma record anepisode of cooling that brings the upper parts of La Luz andKnife Edge transects above HePRZ by 16 Ma. After areduction in cooling rate at 14 to 16 Ma, cooling is renewed14 to 13 Ma. After this time, the thermochronometric dataoffer no insight into the rate of cooling, but the sedimentaryrecord shows that tilting was complete by �3 Ma.[52] Ehlers et al. [2001] point out that a number of

competing processes occur during footwall exhumation.The 70� and 110�C isotherms are warped toward shallowerdepths by the advection of warmer rocks during exhumationof the footwall, while sedimentation decreases heat flow onthe hanging wall. Juxtaposition of the hot footwall againstthe cold hanging wall results in lateral cooling of thefootwall block. Furthermore, the presence of low thermalconductivity sediments in the basin can cause a thermalconductivity refraction effect [Kelley and Duncan, 1986].Fortunately, the La Luz and Knife Edge traverses are setback 2 to 3 km from the range bounding fault, thusminimizing some of the hanging wall effects. Ehlers et al.[2001] calculate that the exhumation rate determined fromage-elevation data generally overpredict model based exhu-mation rates. They also note that at exhumation rates of 3 to5 mm/year, the AFT and (U-Th)/He ages form near verticallines on elevation versus age plots, such as those observedin the Sandia Mountains, indicating that the data are lesssensitive to topography and surface processes at highexhumation rates.[53] As mentioned previously, one possible interpretation

of the change in cooling rate seen in the (U-Th)/He data is


that it represents true decrease in the exhumation rate ofthe Sandias that fortuitously coincides with a hiatus in thesedimentary record in the western Albuquerque Basin and asubsequent, loosely constrained, post-14 to 16 Ma increasein sedimentation rate in the Albuquerque Basin. The modelof Ehlers et al. [2001] raises another possibility. Say, forexample, that there is a previously unrecognized Oligoceneintrusive body under the Sandia Mountains/AlbuquerqueBasin at a depth of 10 km. Perhaps the early 22 to 17 Marapid cooling recorded in the Sandia Mountains is due torelaxation of isotherms following magmatism. The apparentdecrease in cooling rate displayed by the (U-Th)/He datamay in fact signal the beginning of exhumation as isothermsare swept upward during the initial stages of footwalladvection, as described by Ehlers et al. [2001]. Therenewed cooling of the footwall would begin once thepositions of the isotherms are stabilized and cooling dueto erosion prevails.[54] Additional data are needed to distinguish between

the possibilities. Cores (only cuttings are available now) ofthe Santa Group in the deep part of the Albuquerque Basinnear the range front are needed to more tightly constrain theexhumation history of the Sandia Mountains. Detailedgeophysical data are required to rule out the possibility ofa previously unrecognized upper crustal magma chamber.Additional low-temperature thermochonologic data areneeded from other profiles through the range to obtain amore three-dimensional view of the cooling history of theSandia Mountains.

9. Conclusions

[55] New (U-Th)/He and AFT data from the SandiaMountains and Hagan embayment, when coupled withregional geologic and thermochronologic data, offer newinsights into possible thermal conditions prior to rifting, aswell as better constraints on the timing and rate of cooling

of the eastern rift flank. Geologic constraints demonstratethat �2.4 km of section covered the Sandia granite at theend of Cretaceous time when the area was tectonicallystable and near sea level. Temperatures in the Sandia granitemay have been high enough to reset (U-Th)/He systematics,but they were not hot enough to completely reset AFT ages.Laramide sedimentation, as well as deposition of Oligocenevolcaniclastic rocks added 1.0 to 2.5 km of section. Addi-tional burial caused temperatures in the granite to rise above110�C; however, burial alone does not easily explain young(25 to 30 Ma) ages in the Triassic to Jurassic section of theHagan embayment or the 25 Ma ages in Triassic sandstonenear Santa Rosa. Elevated regional heat flow on the order of105 mW/m2 during Oligocene time is required to resolvethe pattern of AFT ages.[56] Low-temperature thermochronometric data from the

Sandia Granite on the west face of the Sandia Mountainsrecord rapid cooling (>10�C/m.y.) between 22 and 17 Ma, adecrease in cooling rate 16 to 14 Ma, and renewed cooling14 to 13 Ma. Models involving heat sources representingOligocene intrusions suggest that isotherm relaxation fol-lowing regional magmatism might be responsible for therapid initial cooling rates, provided that the intrusive bodiesare shallow (<20 km) and very wide (160 km half-width).Sources located at depths >20 km in the crust cool tooslowly to match the observations. Current geophysicalimaging, however, precludes the instrusion geometriesneeded to fit the Sandia and Santa Rosa AFT data. Ourfavored interpretation is that the low-temperature coolingdata record exhumation of at least 3.1 km of material fromabove the Madera Limestone and that 2.4 km of rock upliftoccurred during flexural tilting of the rift flank in the SandiaMountains.

[57] Acknowledgments. The paper benefited from the thorough andthoughtful reviews of Craig Jones and Steve Cather. Conversations withFrank Pazzaglia, Karl Karlstrom, Chuck Chapin, and Sean Connell pro-vided helpful insights.

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��M. A. House, Natural Sciences Division, Pasadena

City College, 1570 East Colorado Boulevard, BuildingE, Room 210, Pasadena, CA 91106, USA. ([email protected])

S. A. Kelley, Department of Earth and Environ-mental Science, New Mexico Institute of Mining andTechnology, Socorro, NM 87801, USA. ([email protected])

M. Roy, Department of Earth and PlanetarySciences, University of New Mexico, Albuquerque,NM 87131-1116, USA. ([email protected])


Documents

Refining the footwall cooling history of a rift flank uplift, Rio Grande rift, New Mexico