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

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 data flank uplift, Rio Grande rift, New Mexico, Tectonics, 22(5), 1060, from the Sandia Mountains and Hagan embayment doi:10.1029/2002TC001418, 2003. provide new insights into the thermal and tectonic evolution of the eastern flank of the Rio Grande rift in northern New Mexico. AFT and (U-Th)/He data reveal 1. Introduction rapid cooling in the Sandia Mountains between 22 and [2] Rift flank uplifts are ubiquitous features in most 17 Ma, followed by a decrease in cooling rate at 16 to continental rift settings [Beaumont, 1978; Brown and 14 Ma that temporally corresponds to a hiatus in Phillips, 1999; Matmon et al., 2000; Morgan et al., deposition in the Albuquerque basin. A second 1986; Vening-Meinesz, 1950; Watts et al., 1982]. These increase in cooling rate at approximately 14 Ma was uplifted footwall blocks are generally separated from axial followed by continued slow cooling until present. rift grabens by normal faults and are often associated with Cooling ages from Jurassic to Permian sandstones in considerable local relief across the rift-bounding escarp- the Hagan embayment northeast of the Sandia ment. The strong asymmetry and footwall warping docu- Mountains are used to constrain the thermal mented in many of these features has been explained conditions in Oligocene time that are necessary to primarily as the isostatic response to extension, but the timing of relief production associated with these features map cooling histories into exhumation histories, remains uncertain [Alvarez et al., 1984; Gilchrist and thereby providing a limit on the amount of section Summerfield, 1990; Vening-Meinesz, 1950; Weissel and removed during rift flank development. Thermal Karner, 1989]. modeling, geologic constraints, and low-temperature [3] The chronology of rift-flank exhumation and the thermochronology are used to demonstrate that the development of associated topographic relief can be recon- heat flow in the Sandia Mountain region was at least structed by using both the basin sedimentary record and the 25 mW/m2 higher during Oligocene time compared to thermal history of footwall rocks [e.g., Fitzgerald and today. Furthermore, at least 3.1 km of material has Stump, 1997]. However, apatite fission track (AFT) cooling been exhumed from the Sandia Mountains and 2.4 km ages, which are commonly used to track rift-related exhu- of rock uplift occurred during flexural tilting of the mation of footwall rocks, may significantly predate inde- pendently estimated ages of extension in some rift settings block since middle Miocene time. INDEX TERMS: 1035 [Kelley and Duncan, 1986]. Pre-rift AFT cooling ages are Geochemistry: Geochronology; 8109 Tectonophysics: Continental preserved in ranges where rift-related exhumation is less tectonics—extensional (0905); 9350 Information Related to than 3 to 4.5 km, crustal depths associated with total fission Geographic Region: North America; 9604 Information Related track annealing at 110C[Kelley et al., 1992]. For example, to Geologic Time: Cenozoic; KEYWORDS: apatite, helium, fission the abundance of AFT ages recording Laramide deforma- track, rift flank, Sandia Mountains. Citation: House, M. A., S. A. tion (45 to 70 Ma) from some structural highs adjacent to Kelley, and M. Roy, Refining the footwall cooling history of a rift the Rio Grande rift in New Mexico (e.g., Santa Fe Range, Los Pinos Mountains, Sierra Nacimiento; Figure 1) have been interpreted to indicate that a portion of the modern relief of the ranges is due to earlier tectonism [Kelley and 1Now at Natural Sciences Division, Pasadena City College, Pasadena, Chapin, 1995; Kelley et al., 1992]. California, USA. [4] Incorporating (U-Th)/He ages in apatite with existing Copyright 2003 by the American Geophysical Union. AFT data can refine the chronology of rift-flank deforma- 0278-7407/03/2002TC001418$12.00 tion and exhumation. The lower closure temperature of this

14 - 1 14 - 2 HOUSE ET AL.: THERMOCHRONOMETRY OF RIO GRANDE RIFT

Smith, 2002]. However, the timing of this cooling episode coincides with a potential reduction in geothermal gradient following Oligocene volcanism in the area, meaning that the amount of exhumation attributed to the AFT data is not well-constrained [Kelley and Duncan, 1986]. [6] We have re-evaluated AFT results for samples orig- inally studied by Kelley and Duncan [1986] and Kelley et al. [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 constraining its relationship to the topographic growth of rift-flank blocks. These data are supplemented by new results from the Hagen embayment to the north (Figure 2), a region that may hold the key to pre-rift thermal conditions. The new thermochronologic data for the Sandia Mountains and eastern Rio Grande rift revise previous estimates for the onset of Tertiary exhumation of this block and extend the footwall cooling history to lower temperatures, permitting a more detailed correlation of the unroofing history of the range to the record of sedimentation in the adjacent basins.

2. Sandia Mountains and Albuquerque Basin

[7] The sedimentary and thermochronometric record of northern New Mexico shows that late Mesozoic tectonic quiescence gave way to Laramide compression, followed by volcanism, and finally crustal rifting [Baldridge et al., 1995; Chapin and Cather, 1994; Erslev, 2001]. Deposition of the Late Cretaceous Mesaverde Group in the Hagen embayment suggests that large parts of this region resided at or near sea Figure 1. Regional map showing the position of the level at end of Mesozoic time (Figure 2 [Beaumont et al., Sandia Mountains within the Rio Grande rift. The shaded 1956; Mannard, 1975]). Beginning at 75 Ma, Laramide area outlines the basins of the northern Rio Grande rift in deformation produced a number of significant basins and New Mexico. Double lines show the approximate bound- basement-cored uplifts in the region [Dickinson et al., 1988; aries between the basins and the strike-and-dip symbol Seager, 1983; Seager and Mack, 1986]. In the central and represents the general dip of each half-graben [after Chapin, western Albuquerque basin, Eocene to Early-Middle Oli- 1988]. The fission track data of Kelley et al. [1992] and gocene pre-rift deposits of the Galisteo-Baca formations and Kelley and Chapin [1995] are shown for reference. the unit of Isleta #2 attain a maximum total thickness of 2700 m [Lozinsky, 1994; May and Russell, 1994]. During the waning stages of Laramide deformation (31–32 Ma), a thermochronometer (70C; [Farley, 2000]) means that shallow intrusive center, the San Pedro-Ortiz porphyry belt, this method will detect exhumation from shallower crustal was emplaced east and northeast of the Sandia Mountains levels (1.6–2.5 km) and so potentially record more recent [Abbott et al., 2003; Maynard et al., 1990, 1991]. stages in the process of rift flank bedrock uplift. Combining [8] Extension across the northern New Mexico segment (U-Th)/He and AFT ages with sedimentologic data from rift of the Rio Grande rift initiated during Late Oligocene time basins offers the most complete means to reconstruct the [Chapin and Cather, 1994; Ingersoll et al., 1990; Miggins development of continental rifts in general and exhumed et al., 2001; Morgan et al., 1986] and is manifested by a rift-flank blocks in particular. series of deep, asymmetrical basins extending through New [5] We employ this approach in studying the Sandia Mexico and Colorado that are separated by transverse Mountains, which define the eastern edge of the Rio Grande accommodation zones and are flanked by uplifted structural rift near Albuquerque, New Mexico (Figure 1). Eastward blocks [Lewis and Baldridge, 1994]. The east tilted, topo- tilting and warping of this range have been attributed to graphically high-standing Sandia crustal block is one flexural footwall uplift in response to Neogene faulting [Roy example of such a rift flank. The Sandia Mountains occupy et al., 1999]. AFT ages record a cooling episode between the footwall of a west dipping normal fault system bounding 30–15 Ma [Kelley et al., 1992; Kelley and Duncan, the eastern Albuquerque basin [Russell and Snelson, 1994], 1986], consistent with the timing of sedimentation in and are bounded on the south by the Tijeras fault zone and response to the early stages of rifting [Brister and Gries, on the north by the Placitas fault zone [Kelley and Northrop, 1994; Chapin and Cather, 1994; Ingersoll et al., 1990; 1975]. Proterozoic (1.4 Ga) Sandia granite and the adjacent Large and Ingersoll, 1997; Lundahl and Geissman, 1999; metamorphic Cibola gneiss exposed in the footwall of the HOUSE ET AL.: THERMOCHRONOMETRY OF RIO GRANDE RIFT 14 - 3

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 samples discussed by Abbott et al. [2003]. steep western escarpment are unconformably overlain by thickness of 4200 m adjacent to the Sandia block in the sandstones and limestones of the Pennsylvanian Sandia and eastern Albuquerque basin, is three times thicker above a Madera groups [Kelley and Northrop, 1975; Kirby et al., 16.1 Ma basalt flow in Shell Isleta #2 than it is below the 1994]. These laterally continuous strata are the erosional flow [Chapin and Cather, 1994; May and Russell, 1994]. remnants of a 2400 m thick section of overlying Paleo- Accelerated post-16 Ma sedimentation rates in the Santa Fe zoic-Mesozoic strata [Kelley and Northrop, 1975], which Group are also documented in the Socorro area to the south are partially preserved in the Hagan embayment and pres- [Cather et al., 1994]. ently define a dip slope along the more gently sloped [10] In addition to the north trending basin-bounding eastern flank of the Sandia Mountains (Figure 2; approxi- faults, the Proterozoic basement along the western scarp of mately 15–20 east dip). the Sandia Mountains is cut by a number of NNW to NNE [9] In the southern Albuquerque basin, the oldest rift fill trending faults of uncertain offset and age, including the deposits are interbedded with a 22 Ma basalt [Bachman and Knife Edge fault [Read et al., 1999] and the La Cueva fault, Mehnert, 1978]. Stratigraphic relationships throughout the which parallels the La Luz trail (Figure 2 [Kelley and Albuquerque basin reveal episodic sedimentation [Kelley, Northrop, 1975]). Many of these faults, including the La 1977]. The most detailed record of this episodicity is found Cueva fault, are inferred to be pre-Late Paleozoic in age in the western Albuquerque basin where magnetostratigra- because they do not offset the Pennsylvanian sedimentary phy and K-Ar biotite ages for an airfall tuff reveal moderate rocks [Kelley and Northrop, 1975]. Reverse faults on the east sedimentation rates (60 m/m.y.) between 17.3 Ma and flanks of the Sandias are inferred to be Laramide in age 16 Ma, followed by a 1.6 Ma hiatus and a subsequent [Karlstrom et al., 1999]. Quaternary to Holocene faulting increase in rate to about 90 m/m.y. between 14 to 12 Ma continues in the Albuquerque Basin today, producing a series [Tedford and Barghoorn, 1999]. The sedimentary unit of structural benches along the eastern margin of the basin below the hiatus is the Zia Formation, an eolianite derived [Chapin and Cather, 1994; Connell and Cather, 1999]. from sources to the west, while the unit above the hiatus is [11] Warping of the Pennsylvanian Madera Group along the fluvial Arroyo Ojito Formation derived from the Naci- the eastern flank of the Sandia Mountains has been inter- miento and southern Jemez Mountains. Although these preted to reflect total flexure of the region that has accu- sediments cannot be directly tied to the development of mulated since Paleozoic time, with the majority of the the Sandia Mountains, they do record the development of deflection forming in response to Neogene rifting [Roy et accommodation space in the Albuquerque Basin. The sed- al., 1999]. Flexural models using this datum demonstrate imentologic record elsewhere in the basin reflects these that most of the modern local relief in the Sandia block variations in sedimentation rate in less detail. For example, (2400 m) is the result of extensional unloading by rift- the Miocene Santa Fe Group, which reaches its maximum related faults to the west [Roy et al., 1999]. Other workers 14 - 4 HOUSE ET AL.: THERMOCHRONOMETRY OF RIO GRANDE RIFT

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 the Earth’s crust may result in the preservation of a fossil PAZ-PRZ (heavy gray solid and dashed lines), and initiation of a new PAZ-PRZ (heavy white solid and dashed lines) at depth. Ages obtained in an elevation profile 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 uplifted fossil PAZ and PRZ zones the depths of the modern zones below the average surface elevation serves as an estimate of the amount of bedrock uplift (Dz) since t1. The elevation over which the PRZ and/or PAZ span can be used to infer the geothermal gradient immediately prior to tectonism. After Gleadow and Fitzgerald [1987]. (b) In the case of continued exhumation at a slow to moderate pace, a PAZ or PRZ may not be preserved. Rather, linear AFT or helium age-elevation profiles may be preserved (heavy dashed line shows example of helium age-elevation profile). In this case, the slope of the profile provides an 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 sea level and BSL = below sea level. suggest that warping of this unit was accommodated by bound the Rio Grande rift, are largely based on AFT data faulting to the east and is not purely isostatic flexure [Brown [Kelley and Chapin, 1995, 1997; Kelley et al., 1992; Kelley and Phillips, 1999] or may, in part, be produced by and Duncan, 1986]. Fission tracks are damage zones in a Laramide-aged deformation [Karlstrom et al., 1999]. How- crystal or glass that are formed by spontaneous fission of ever, paleocurrents in the Paleocene Diamond Tail and 238U [e.g., Wagner, 1968]. A fission track age is determined Eocene Galisteo formations in the Galisteo basin are from by measuring the density of spontaneous fission tracks and the northwest and no clasts from the Sandia Mountains are the U concentration of the sample [e.g., Naeser, 1976]. found in the Galisteo basin to the north [Abbott et al., 1995; Fission tracks anneal with increasing temperature [Naeser Stearns, 1953], suggesting that this range had little or no and Faul, 1969]. For most apatites (fluorapatite), annealing relief during Early Cenozoic time. occurs over a temperature range of 60–110C[Gleadow et al., 1986; Green et al., 1986], known as the partial annealing zone (PAZ, Figure 3a [Gleadow and Fitzgerald, 3. Previous Thermochronology From the 1987]. In certain situations, when the cooling rate is rapid, Rio Grande Rift and Sandia Mountains the AFT ‘‘age’’ of a sample is interpreted as the approxi- mate time at which the sample cooled to temperatures below [12] Existing constraints on the exhumational history of 110C, although annealing may continue until complete the Sandia Mountains, as well as other rift-flank blocks that passage through temperatures of 60C. In cases of epi- HOUSE ET AL.: THERMOCHRONOMETRY OF RIO GRANDE RIFT 14 - 5 sodic exhumation, the pattern of AFT ages from eleva- [16] Most of the AFT data from the Rio Grande rift flank tion profiles indicates the preservation of a fossil PAZ structural blocks yield ages >16 Ma, yet track-length (Figure 3a), while in cases of continuous cooling, elevation models suggest that most of the exhumation of the rift profiles may yield insight into the rate at which exhumation flanks occurred after 15 Ma. Furthermore, the major proceeded (Figure 3b). Thus these data can potentially influx of sedimentary material into the Albuquerque basin provide an indication of the magnitude and timing of at begins at this time [Cather et al., 1994; May and Russell, least one, and sometimes multiple, episodes of exhumation 1994]. Interpretations that have been offered to explain the [e.g., Fitzgerald and Stump, 1997]. onset of cooling observed in the Sandia AFT data include [13] Confined fission track lengths constrain the rate and continued deformation following the Laramide events, timing of cooling within the PAZ [Gleadow and Fitzgerald, changing climatic conditions, or epeirogenic bedrock uplift 1987; Green et al., 1989]. Experimental annealing data for associated with regional volcanism [Karlstrom et al., 1999; fission tracks in apatite [Carlson et al., 1999; Crowley et al., Roy et al., 1999], but post-16 Ma cooling is generally 1991; Laslett et al., 1987] provide the basis for algorithms agreed to reflect rift-related tectonism. developed to constrain the thermal history of each sample [17] Provided that the entire local relief across the eastern within the PAZ [Corrigan, 1993; Gallagher et al., 1998; margin of the Rio Grande rift and the Sandia Mountains Green et al., 1989; Ketcham et al., 1999]. Thus, track- block (2400 m) was created by extensional faulting and length measurements from vertical elevation profiles are flexural bedrock uplift (as indicated by analysis of the used to reconstruct a set of cooling histories that potentially present geometry of overlying Paleozoic sediments), then can be tied to estimates of the rate of exhumation (Figure 3) the accelerated influx of basement derived detritus into the [Fitzgerald et al., 1995; House et al., 1999]. adjacent Albuquerque basin can only be possible once [14] AFT analysis, coupled with geologic constraints, has granite basement is exposed. The length of time that it been used to identify at least three phases of Mesozoic to takes a particle to go from 3000 m in the subsurface to the Cenozoic exhumation in the southern Rocky Mountains, surface by exhumation, then into the basin by erosion and Rio Grande rift and southern High Plains [Kelley and deposition is on the order of 6 m.y. for the rift flanks in New Chapin, 1995, 1997; Pazzaglia and Kelley, 1998]. Laramide Mexico [Kelley et al., 1992]. The sedimentary influx in the AFT cooling ages reflecting the first episode are preserved eastern Albuquerque basin post-dates AFT cooling ages extensively in the Front Range of Colorado [Kelley and from the tilted footwall by more than 6 m.y. however, Chapin, 1997] and are found in the Santa Fe Range, Los suggesting there may be more to learn regarding the Pinos Mountains, and Sierra Nacimiento in northern New Neogene exhumation and bedrock uplift of this block using Mexico [Kelley et al., 1992] (Figure 1). The AFT data the lower temperature (U-Th)/He thermochronometer. further show that Laramide deformation was followed by a Late Oligocene to Early Miocene cooling episode that is recorded along the eastern margin of the Sangre de Cristo 4. New Thermochronometry From the Mountains in northern New Mexico and southern Colorado, Eastern Margins of the Rio Grande Rift in the Pedernal Hills in east central New Mexico, and in Triassic sedimentary rocks as far east as Santa Rosa, New [18] We have obtained new AFT and apatite (U-Th)/He Mexico (Figure 1) [Leonard et al., 2002]. The stress regime cooling ages for several sites along the eastern Rio Grande during the Late Oligocene-Early Miocene cooling event was rift: new AFT cooling ages in the Sandia Mountains provide transitional between Laramide compression and Rio Grande tighter constraints than previous work, while AFT ages from rift extension [Erslev, 2001]. Interpretation of the AFT data the late Paleozoic to Mesozoic sedimentary rocks in the are complicated by the fact that the Mogollon-Datil, San Hagan embayment were determined in an attempt evaluate Juan, Latir, and Ortiz volcanic fields were all active during the thermal conditions of the stratigraphic section residing this time interval [Lipman et al., 1986; Maynard et al., on the Proterozoic basement prior to rifting. 1990, 1991], and regional heat flow may have been high [19] In order to document the portion of the exhumation [Kelley, 2002]. history that follows that constrained by AFT data, we turn to [15] A period of relative tectonic stability followed this (U-Th)/He thermochronometry. The extremely low closure episode, which in turn was followed by bedrock uplift and temperature of this isotopic system is critical for resolving erosion associated with rift flank development. AFTages first such low-temperature thermal histories: for a typical reported for samples collected along the west flank of the 150 micron (prism diameter) apatite grain, the temperature Sandia Mountains were interpreted to represent an episode of of complete helium loss is 70C (assuming a cooling rate exhumation between 30–15 Ma [Kelley and Duncan, of 10C/m.y. [Farley, 2000]). In actual fact, the transition 1986]. The cooling history recorded in the Sandia samples between quantitative loss and retention of helium in apatite post-dates regional cooling due to volcanism (31 to 32 Ma occurs over a thermal window of 40–70C (Figure 3a; [Abbott et al., 2003]) in the nearby Cerrillos and Ortiz the helium partial retention zone or HePRZ, similar to the volcanic fields (Figure 2). Thermal histories derived from PAZ [Wolf et al., 1998]). The (U-Th)/He closure tempera- the age and track length data (mean lengths of 13.9 to 15.1 mm) ture, taken to be the temperature at the base of the HePRZ, using the model of Corrigan [1991] suggest that the Sandia scales with apatite grain size (larger grain sizes have higher Mountains experienced 3–4.5 km of erosion since 30 Ma, closure temperatures). Although the grain-size effect is with most occurring after 15 Ma [Kelley et al., 1992]. slight (from 75–150 micron prism diameter, the closure 14 - 6 HOUSE ET AL.: THERMOCHRONOMETRY OF RIO GRANDE RIFT

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. temperature varies by just 6C[Farley, 2000]), it can be Edge, San Francisco, and La Bajada samples were analyzed used to advantage in samples with a range of grain sizes using laser gas extraction [House et al., 2000]. The average [Reiners and Farley, 2001]. By selecting aliquots with a reproducibility of replicated samples is 6.5% (1 s.d.), so we range of characteristic grain sizes, multiple (U-Th)/He ages follow the approach of Farleyp et al. [2001], and assign an can be obtained from a single rock, providing a segment of uncertainty figure of 13%/ N(2s) for samples analyzed N the rock’s cooling history rather than just one point. Thus, times. an apatite (U-Th)/He age provides an additional information on the time-temperature history of a specific sample, per- mitting documentation of exhumational histories from 4.1. La Luz Transect extremely shallow levels in the Earth’s crust. [22] Kelley and Duncan [1986] reported AFT ages for [20] We analyzed samples from two elevation transects 13 samples from an elevation transect through the Sandia along the western flank of the Sandia Mountains: one along Granite along the La Luz trail (Figure 4a, SAN). Fission the La Luz trail and one that crosses a high angle fault to the track ages were originally obtained using the population north known as the Knife Edge fault (Figure 4a). In addition method [Naeser, 1976] and track lengths were measured on to these transects, three short stratigraphic traverses were a few of the samples using the method of Green [1988] with collected from east-dipping sedimentary rocks in the Hagan the bias correction of Laslett et al. [1982]. More recently, embayment, in the footwalls of the San Francisco and La Kelley et al. [1992] measured track lengths for the entire Bajada faults (Figure 2). One transect is from the block east suite. of the San Francisco fault and two transects were collected [23] Electron microprobe analyses indicate that the apa- east of the La Bajada fault. tite in Sandia Granite is fluorapatite with Cl in the 0.000 to [21] Results are reported in Table 1 (AFT) and Table 2 0.017 wt% range and F values of 3.32 to 4.14 wt%. The ((U-Th)/He). For the (U-Th)/He analyses reported here, analyses were done at New Mexico Tech on a CAMECA multi- and single-grain aliquots of apatites from concentrated SX-100 electron microprobe using a 10 mm beam and a separates were selected on the basis of crystal form (typically current of 15 nA. euhedral to subhedral grains) and clarity (clear, with no [24] We found upon re-examination of the apatites that optical evidence for inclusions). Samples were measured they are strongly zoned with respect to major and rare earth and analyzed at the Caltech Noble Gas Laboratory using elements, and uranium. For example, in samples 81SAN01 the furnace extraction method described by Farley et al. and 81SAN04, several of the apatite grains show composi- [2001] in the case of the La Luz transect, while the Knife tional zoning on SEM backscatter images that is due to Table 1. Apatite Fission Track Data For Sandia Mountainsa

Number Standard of Central Uranium Mean Track Deviation 5 6 5 2 Elevation, Grains rs  10 ri  10 rd  10 Age, Ma P(c) , Content, Length, mm Track Sample Rock Type Latitude Longitude m Dated t/cm2 t/cm2 t/cm2 (±1 SE) % ppm (±1 SE) Length

81SAN01 Sandia granite 35 12.27 106 26.82 3095 20 1.25 (124) 3.68 (1826) 1.1806 (4605) 19.1 ± 2.0 >99 38 14.8 ± 0.3 (101) 1.7 RIFT GRANDE RIO OF THERMOCHRONOMETRY AL.: ET HOUSE 81SAN02 Sandia granite 35 12.30 106 26.950 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 35 12.270 106 27.090 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 35 12.370 106 27.240 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 35 12.470 106 27.340 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 35 12.560 106 27.640 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 35 12.650 106 27.86 2549 20 0.92(118) 2.83(1812) 1.2182 (4605) 18.9 ± 2.0 97 28 13.9 ± 0.4 (100) 2 81SAN08 Sandia granite 35 12.680 106 28.070 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 35 12.830 106 28.400 2354 20 0.99 (101) 3.54(1812) 1.2272 (4605) 16.3 ± 1.8 92 35 14.1 ± 0.4 (100) 2 81SAN10 Sandia granite 35 12.790 106 28.480 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 35 12.800 106 29.360 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 35 07.750 106 28.420 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 35 06.630 106 29.080 1890 20 0.94(45) 2.93 (704) 1.2936 (4605) 19.7 ± 3.1 96 27 14.2 ± 0.7 (51) 2.6

Knife Edge Fault 99ASC-2 Sandia granite 3514.560 10628.130 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 3514.450 106 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

La Bajada Fault 99MAD02 J West Water Canyon ss. 3527.320 10612.880 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 Triassic Chinle ss. 3527.890 10613.570 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 West Water Canyon ss. 3529.840 10613.250 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 Triassic Chinle ss. 3529.970 10613.650 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 Entrada ss. 3529.980 10613.730 1682 20 1.44 (62) 2.6 (558) 1.2404(4613) 32.8 ± 4.6 80 25 12.9 ± 2.4 (5) 2.7

Hagan Embayment 99HB02 Permian Abo ss. 3521.150 10623.190 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 Triassic Chinle ss. 3521.560 10621.990 1713 20 1.45 (136) 3.74 (1759) 1.2436(4613) 22.9 ± 2.3 70 36 14.3 ± 1.4 (9) 2.2

a Definitions are as follows: rs, spontaneous track density; ri, induced track density (reported induced track density is twice the measured density). Number in parenthesis is the number of tracks counted for ages and fluence calibration or the number of track measured for lengths; rd, track density in muscovite detector covering CN-6 (1.05 ppm); reported value determined from interpolation of values for detectors 2 10 1 covering standards at the top and bottom of the reactor packages (fluence gradient correction). SE, standard error. P(c) , chi-squared probability; lf,1.551Â 10 yr ; g, 0.5; zeta, 4772 ± 340 (CN6) for apatite. Mean track lengths not corrected for length bias [Laslett et al., 1982]. 14 - 7 14 - 8 HOUSE ET AL.: THERMOCHRONOMETRY OF RIO GRANDE RIFT

Table 2. Apatite Helium Resultsa

[U], [Th], [4He], Mass, Radius, Age, Average Sample ppm ppm U/Th nmol/g mg Ft mm Ma(±2s) Age(±2s)

Sandia Mountains (La Luz and Knife Edge) 81SAN01 Analysis 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 81SAN03 Analysis 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 81SAN04 Analysis a 21 75 0.28 2.481 29.3 0.77 53 15.3b 17.1 (0.8) Analysis b 18 66 0.27 2.617 21.5 0.76 54 18.9b 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 81SAN05 Analysis a 16 51 0.31 2.028 32.0 0.78 56 17.0b 15.9 (0.5) Analysis b 19 60 0.32 2.095 21.4 0.75 49 15.6b Analysis c 27 72 0.37 2.691 2.4 0.74 49 15.3 Analysis d 15 51 0.30 2.157 5.3 0.80 63 18.2 Analysis e 34 102 0.33 3.383 4.7 0.79 63 13.5 81SAN06 Analysis a 22 74 0.30 3.046 36.3 0.78 56 18.4b 18.5 (1.3) Analysis b 13 43 0.30 1.777 23.4 0.78 56 18.5b 81SAN07 Analysis a 11 37 0.30 1.151 26.3 0.79 59 13.4b 12.9 (0.9) Analysis b 21 66 0.32 1.829 25.5 0.76 50 12.3b 81SAN08 Analysis a 13 49 0.27 1.369 44.7 0.79 60 13.2b 13.9 (0.5) Analysis b 16 59 0.27 1.941 49.0 0.79 60 15.0b 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 12.5 81SAN09 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-4 Analysis 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 18.1 Analysis c 13 42 0.32 1.504 3.6 0.74 57 16.0 Analysis d 29 108 0.26 3.211 1.5 0.63 34 17.5 99ASC-5 Analysis 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 14.4 Analysis c 11 45 0.25 1.377 1.8 0.66 40 17.7 99ASC-7 Analysis 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 14.1 Analysis c 6 19 0.31 0.652 6.9 0.78 63 15.1

La Bajada Fault 99MAD02 Analysis 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 Embayment 99HB02 Analysis 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) 99HB04 Analysis 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)

a Ft and radius are mass-weighted values. Age is corrected for alpha-ejection [Farley et al., 1996]. Values in parentheses are 2s uncertainties computed using method of Farley et al. [2001]. No average ages are reported for San Francisco fault samples as described in text. bSamples outgassed using a resistance furnace; all others were outgassed using a Nd-YAG laser [House et al., 2000]. HOUSE ET AL.: THERMOCHRONOMETRY OF RIO GRANDE RIFT 14 - 9

more consistent with the rapid cooling rate suggested by the long mean track lengths of 13.9 to 15.0 mm measured by Kelley 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 and 81SAN10, for which insufficient material was available (Table 2 and Figure 6a). In most cases, replicates were in good agreement but multiple replicates of samples 81SAN04, 81SAN05 and 81SAN08 exhibited considerable scatter outside the expected uncertainty. This scatter may reflect compositional complexities in the apatite grains themselves, as described above. The (U-Th)/He dates are 12.9 to 18.5 Ma and are consistently younger than the AFT age for each sample, displaying a positive correlation between age and elevation with a small break in slope at a modern elevation of 2550 m (Figure 6a).

4.2. Knife Edge Fault Transect

[27] Four samples of Sandia Granite were collected along a short transect to the north of the La Luz trail (Figure 4a; ASC). This suite of samples crosses the Knife Edge fault, a steeply dipping breccia and pseudotachylite zone that sep- arates the high-elevation cliff face (locally known as The Shield) of the western escarpment of the Sandia Mountains from the lower-elevation, less rugged portion of the escarp- ment [Read et al., 1999]. This fault corresponds to a topographic break and may be rift-related. The AFT data from the Knife Edge fault transect are consistent with ages from the La Luz trail to the south. AFT ages for these samples range from 16.5 to 17.6 Ma, and generally increase with sample elevation. No obvious break in age is observed as the transect crosses the Knife Edge fault, suggesting that there has not been significant offset across this structure since 16 Ma. [28] Three samples from the Knife Edge profile (ASC4, 5 Figure 5. Photos of induced fission tracks in muscovite and 7) contained sufficient quantities of quality material for detectors, showing the zonation of uranium in the apatite replicated single-crystal analyses. (U-Th)/He ages are 15.2– from the Sandia Granite. 16.4 Ma and are also positively correlated with elevation. While there is no obvious break in age observed across the Knife Edge fault, the samples in the elevation range corresponding to the La Luz profile seem to display similar differences in SiO2 content (0.40 to 1.04 wt% in some zones compared to 0.04 to 0.33 wt%). More important, most of the curvature, suggesting a possible reduction in exhumation apatites from the top of the elevation profile (81SAN01 to rate. 08) have uranium concentrated on the rim of the apatite grains (Figure 5), although some grains with high uranium 4.3. La Bajada Fault cores were also observed. Consequently, we re-evaluated [29] The second area that we targeted was the Permian to these samples using the external detector method [Naeser, Mesozoic sediments exposed near the La Bajada fault in 1979]. the Hagan embayment, north of the Sandia Mountains [25] The new AFT ages are shown with track lengths of (Figure 4a). The Hagan embayment is unique in that much Kelley et al. [1992] in Table 1. The ages range from 22 Ma of the Paleozoic, Mesozoic, and early Cenozoic sedimentary at high elevation to 16 Ma at low elevation; this age range is section that likely covered central New Mexico prior to significantly narrower than that reported by Kelley and rifting is preserved here [Kelley and Northrop, 1975]. Duncan [1986]. The largest age discrepancies between the Therefore, cooling ages from this region provide the means two data sets come from the highest elevation samples on the to obtain limits on the pre-rifting thermal structure of the profile, where the uranium zonation in the apatite is most region, as well as additional insight into regional progres- pronounced. Where the uranium concentrations in the apa- sions in rift-related faulting. tites are more uniform, the ages are the same within error. [30] Ten samples of Triassic to Cretaceous sandstone The steeper slope of the age-elevation profile (Figure 6a) is along two traverses through the footwall of the La Bajada 14 - 10 HOUSE ET AL.: THERMOCHRONOMETRY OF RIO GRANDE RIFT

Figure 6. Apatite helium and fission track ages from the Sandia Mountains, New Mexico. Apatite helium ages are shown by solid circles and apatite fission track ages are shown by open circles. Errors are shown (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 Knife Edge profile, while circles represent the La Luz transect. (b) Relative elevations of Sandia samples after correction for 15 of eastward tilting. (c) Apatite helium and fission track ages from the footwall of the La Bajada-San Francisco fault. Helium ages are shown by solid symbols and fission track ages are shown by open symbols. Boxes represent samples from the La Bajada footwall, while circles represent samples from the San Francisco footwall. Triangles are small grain size helium ages. fault were collected for analysis, but only five contained and 0.7 km east of the fault (MAD05). AFT ages along the enough apatite for dating purposes. Unlike the clear, euhe- northern traverse (29.9–32.8 Ma) are the same within dral crystals from the Sandia granite, apatites from this suite uncertainty and are similar to those to the south, as is the were variably frosted and abraded, as expected from detrital single (U-Th)/He age from this profile (18.2 ± 2.4 Ma). apatite crystals. Along the southern traverse, the AFT age [31] Sufficient numbers of confined tracks for meaningful for the Salt Wash Canyon member of the Jurassic Morrison analysis were found in only three samples. The mean track Formation is 25.2 ± 2.4 Ma and the (U-Th)/He age is 17.7 ± lengths are long (13.9–14.9 mm) with narrow standard 1.6 Ma. The AFT age of the underlying Triassic Chinle deviations, suggestive of rapid cooling. Cooling rates of sandstone is 32.4 ± 2.4 Ma. The AFT ages are not clearly 3–5C/m.y. are calculated using the AFT and (U-Th)/He correlated with stratigraphic position or elevation (Figure 6c) ages. because the Chinle Formation contains chlorine-rich grains that tend to retain fission tracks to temperatures of about 4.4. San Francisco Fault 140C, leading to a mix of AFT age populations and generally older central ages. Note that the Chi-squared [32] The AFT age of the Permian Abo sandstone collected statistic in Table 1 is low for the Chinle samples, indicative near the San Francisco fault zone is 16.0 ± 1.8 Ma and the of multiple age populations [Galbraith, 1981]. The samples (U-Th)/He analysis yields two age populations, one at 14.6 ± along the northern profile were collected in the La Bajada 1.9 Ma and the other at 9.7 ± 1.3 Ma. Similarly, the Triassic fault zone (MAD07), 0.1 km east of the fault (MAD06) Chinle further up-section has an AFT age of 22.9 ± 2.3 Ma HOUSE ET AL.: THERMOCHRONOMETRY OF RIO GRANDE RIFT 14 - 11 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 correlate well with stratigraphic position and indicate rapid cooling, consistent with the few mean track length measurements for these samples. In both samples, there is a correlation between grain size and (U-Th)/He age such that older ages correspond to larger grains (83–86micronradii;Table2),while younger ages are found in the smaller (66–69 micron radii) grains. Part of the range in ages observed here can be explained by the control of grain size on closure temperature. For example, a cooling rate of 1C/m.y. suggests a range of closure temperature of 61–64C for grains in this size range, resulting in age differences of 3 m.y.. Higher cool- ing rates would reduce this range of temperatures and the resulting age differences. However, the range in ages that we observe is larger than this, suggesting that either these rocks resided for a longer period of time within the HePRZ, or that there may be some other factor controlling the (U-Th)/He age in these samples. The fact that U and Th contents also appear to be correlated with grain size suggests that these grains reflect two distinct detrital populations.

5. Discussion of Results 5.1. La Luz and Knife Edge Transects

[33] The correlation between cooling age and elevation in AFT and (U-Th)/He ages along the Sandia La Luz and Knife Edge transects is consistent with cooling of the Sandia Mountain block through temperatures of 110– 70C between 22–14 Ma. Apparently rapid cooling between 22 and 17 Ma is indicated by high elevation samples (>2700 m). An offset in both the AFT and (U-Th)/ He age elevation trends at 2500–2600 m elevation on both the La Luz and Knife Edge profiles may reflect a brief Figure 7. Range of possible thermal histories that fit track reduction in cooling rate at 16 Ma (Figure 6a). In both length and age data. Histories derived using the model of profiles, cooling ages below 2500–2600 m are largely Ketchum et al. [1999]. age-invariant with elevation, suggesting a return to rapid cooling after 16 Ma. [34] The AFT age and track length data summarized in basin [e.g., Cather et al., 1994; May and Russell, 1994]. Table 1 and the AFTSolve algorithm [Ketcham et al., 1999] The timing of the reduction in cooling rate indicated by were used to determine thermal histories for the Sandia modeling track length data from sample 81SAN10 and from Mountain samples. Grey swaths shown on Figure 7 contain the (U-Th)/He data from the vertical profile roughly corre- possible thermal histories that fit the AFT age and track sponds to the time of a sedimentary hiatus (16–14.4 Ma) in length data for the highest and lowest elevation samples for the Santa Fe Group in the western Albuquerque basin which we have both AFT and (U-Th)/He data on the La Luz [Tedford and Barghoorn, 1999]. Note that the AFT age traverse. The lower elevation sample requires a two-phase and track length data for sample 81SAN10 are equally well cooling history, such that an early episode of cooling at fit by thermal histories that do not include a rate decrease 17 Ma is followed by second acceleration at 14 Ma [e.g., Kelley et al., 1992], but the step in the cooling history (Figure 7b). In contrast, the thermal history for the highest is suggested by the break in slope along the (U-Th)/He age- sample does not record the second episode of accelerated elevation profile (Figure 6a) and is consistent with the cooling, perhaps because this particular sample had already 81SAN10 (U-Th)/He age. cooled below the closure temperature for both AFT and [36] Leeder and Gawthorpe [1987] have explored the (U-Th)/He systems and was at or very near the surface by timing of sedimentation in a basin with respect to the timing 17 Ma (Figure 7a). of footwall uplift. The Leeder and Gawthorpe [1987] model [35] The first rapid cooling episode indicated by the of sedimentation in rift basins suggests that during times of modeling results for sample 81SAN10 and the AFT and intense tectonism along range front faults and increased (U-Th)/He age-elevation profiles in general is broadly subsidence of the basin adjacent to the footwall block, a consistent with the timing of influx of basement-derived fine-grained playa facies commonly accumulates along the material in the Santa Fe Group of the adjacent Albuquerque mountain front and erosion and tilting occurs on the 14 - 12 HOUSE ET AL.: THERMOCHRONOMETRY OF RIO GRANDE RIFT hanging wall dip slope. During times of relative quiescence, attributed to exhumation of the footwall block driven by sediments eroded from the mountain front onlap the hang- bedrock uplift across the range-bounding normal fault ing wall dip slope and sedimentation on the dip slope is system, as well as erosion and isostatic adjustments driven renewed, usually above an angular unconformity. The by increased local topographic gradients across the rift flank section measured by Tedford and Barghoorn [1999] is on [Kelley et al., 1992]. However, in order to map these the hanging wall dip slope, but no angular discordance is cooling histories into exhumation histories (and perhaps mentioned in their description. The model of Leeder and gain some insight into the development of topographic Gawthorpe [1987] may not be directly applicable to the relief), we must have some understanding of the geothermal Albuquerque Basin because the sediments on either side of gradient prior to exhumation. If the AFT ages from the La the hiatus are not derived from the Sandia Mountains. Bajada region predate rift-related exhumation, then they may provide a means to estimate of the geothermal gradient 5.2. Hagan Embayment Results prior to exhumation as well as an upper limit on the magnitude of exhumation. [37] Cooling ages across the Hagan embayment suggest that spatially variable deposition and possibly, a progression [40] Figure 8 shows a reconstruction of a range of in faulting controlled the rate and time at which this region possible temperatures in the northern Sandia Mountains/ cooled (Figure 6c). AFT ages from east of the La Bajada Hagan embayment area at the end of Cretaceous time fault are nearly synchronous with the age of volcanism (31 (Figure 8a) and at the end of volcanic activity in Ortiz to 32 Ma) in the nearby Cerrillos and Ortiz fields, so they and San Pedro volcanic fields during Oligocene time may reflect cooling following locally elevated heat flow (Figure 8b). The thickness of the Mesozoic to Paleozoic around these volcanic centers. Long track lengths in these section used in these reconstructions is taken from Stearns samples indicate that this cooling was rapid, but the [1953]. The thickness of the preserved Paleocene to Mio- relatively large difference between AFT and (U-Th)/He cene section varies considerably across the area (400– ages (18 Ma) in these rocks suggest that cooling slowed 2000 m [Stearns, 1953]) so an average thickness of prior to passage through 70C. This later stage of cooling 1000 m is used in this calculation. Thermal conductivity recorded by the (U-Th)/He data coincided with deposition values for the predominant lithologies in each unit are based of more than 953 m of conglomerate, sandstone and on published estimates for similar lithologies [Carter et al., mudstone in the hanging wall of the San Francisco fault, 1998]. The temperatures are calculated using the thermal in the region west of the La Bajada fault. Deposition of this resistance method [Bodell and Chapman, 1982; Bullard, 1939] for a range of heat flow values. A heat flow of material, derived from the Ortiz Mountains in the footwall 2 of the La Bajada fault, began at 25 Ma and continued until 63 mW/m is assumed for the tectonically stable situation at the end of the Mesozoic, prior to Laramide deformation, and after 11 Ma [Connell and Cather, 1999; Stearns, 1953]. 2 Thus, the (U-Th)/He ages from the footwall of the La a heat flow of 105 mW/m is assumed during peak rifting Bajada fault may reflect exhumation in response to motion and volcanism in the area, translating to average gradients across the La Bajada fault. of 22 and 35C/km, respectively. The modern heat flow in the northern Sandia Mountains is 80 mW/m2. This value [38] Farther west, cooling was in progress in the footwall of the San Francisco fault at 23–15 Ma and may have corresponds to a modern gradient of 29C/km. Note that the continued as late as 10 Ma. The AFT age and track-length low-thermal conductivity Mancos Shale has an important data, combined with the small difference between the AFT impact on the thermal structure of the area and that the and (U-Th)/He ages of the larger grains for these sandstones gradient through the shale interval is 50C/km. indicate that cooling in this part of the basin was quite [41] These reconstructions show that temperatures in the rapid. This small range in AFT and (U-Th)/He ages for a northern Sandia Mountains likely were not high enough to given sample implies a rapid rate of cooling similar to that anneal the fission tracks near the top of the Proterozoic preserved in samples from the Sandia Mountain block, basement at the end of Mesozoic time if a heat flow consistent suggesting that these younger ages may reflect a westward with a tectonically stable crust is assumed (Figure 8a, light progression of faulting across the Hagen embayment. line). The only way to heat the basement just below the Great Alternatively, the relatively younger helium ages in the Unconformity (separating Pennsylvanian strata from the footwall of the San Francisco fault may reflect delayed underlying Sandia granite) to temperatures affecting the AFT systematics prior to Laramide deformation is to have cooling related to the large influx of clastic debris derived 2 from the east at this time (see above). In either case, the an elevated heat flow of 105 mW/m (Figure 8a, heavy line), 9.4–9.7 Ma (U-Th)/He ages of the smaller grains in the an unlikely scenario given geologic history described earlier. sandstones suggest that the cooling rate decreased dramat- Consequently, the base of the PAZ at this time (110C) was ically after 15 Ma in the San Francisco fault footwall. most probably at a depth of 4000 m in the subsurface and the base of the PRZ (70C) was at about 2000 m depth, near the top of the Madera Group, at the end of Mesozoic time. 6. Paleotemperature Estimates From The calculations demonstrate that an increase in regional heat Hagan Embayment Data flow during middle Cenozoic time is required to explain the AFT data. [39] Cooling documented by thermochronometric data [42] During Oligocene time, regional volcanism generally and track length models in the Sandia Mountains may be elevated the heat flow in New Mexico and southern Colo- HOUSE ET AL.: THERMOCHRONOMETRY OF RIO GRANDE RIFT 14 - 13

Figure 8. Paleotemperatures for the Hagen Embayment. (a) Estimated temperatures at the end of Cretaceous time assuming normal heat flow of 63 mW/m2 (light line) and an elevated heat flow of 105 mW/m2 (heavy line). Unit thicknesses from Stearns [1953] and thermal conductivity values from Carter et al. [1998]. The solid circles represent parts of the section that have AFT data indicating paleotemperatures >110C prior to 35 Ma. The dashed line shows 110C for reference. (b) Estimated temperatures during Oligocene time assuming a heat flow of 105 mW/m2 and an average thickness of 1000 m for Laramide and post-Laramide section (shown in gray [Stearns, 1953]). rado [Kelley, 2002] and at least 1000 m, and perhaps However, our limited collection of samples from the Hagan as much as 2000 m, of material was deposited on top of embayment indicates that the entire section below and the Cretaceous Mesa Verde Formation. Figure 8b shows including the Jurassic Morrison Formation in the vicinity the range of temperatures for a conservative 1000 m of the La Bajada fault was at temperatures above 110C thickness of Laramide and post-Laramide section and heat prior to 35 Ma. An elevated heat flow value of 105 mW/m2 flows ranging from the modern 80 mW/m2 to a high of alone does not heat the Morrison to the required temper- 105 mW/m2. The minimum heat flow required to heat the atures. Rather, an additional 500 m of burial with a heat Sandia granite just below the unconformity to temperatures flow value of 105 mW/m2 are needed to raise the Morrison above 110C, assuming 1000 m of burial, is 70 mW/m2. to 110C. If the modern heat flow is assumed, an addition 14 - 14 HOUSE ET AL.: THERMOCHRONOMETRY OF RIO GRANDE RIFT

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. Modified from Roy et al. [1999] and Leonard et al. [2002].

1500 m of section is needed to elevate paleo-temperatures crustal temperatures using simple 2-D analytical [e.g., sufficiently to reset the Hagen embayment AFT ages. Lachenbruch et al., 1976] and numerical models. The models are designed to examine two concepts. First, can a decline in regional heat flow account for the rapid cooling 7. Elevated Regional Heat Flow During recorded by the low temperature thermochronometers in the Oligocene Time Sandia Mountains? Second, how did volcanism in the Ortiz Mountains and Cerillos Hills influence the cooling history of [43] The calculations above indicate that at least 3200 m, the Sandia Mountains? and perhaps as much as 4700 m of section, has been [46] We summarize our findings for models that incorpo- removed from the top of the Madera Limestone, the range rate hypothetical thermal source geometries that can poten- capping unit at the crest of the modern Sandia Mountains, tially match the observed rock cooling rates in the Sandia since Oligocene time (Figure 8b). Furthermore, regional Mountains and the differential uplift (relative to the surface) heat flow was at least 25 mW/m2 higher than it is today and tilting of the Middle Cenozoic partial annealing zone on (Figure 8b). Now we are faced with the challenge of the High Plains of New Mexico. A 10 km thick, 1300C, heat separating cooling associated with decreasing heat flow source with a half-width of 160 km was turned on for 10 Ma from cooling associated with exhumation. and then allowed to cool. The top of the heat source was [44] As alluded to in the previous section, the elevated placed at depths of 30 km, 20 km, and 10 km, and was late Oligocene to early Miocene heat flow recorded by the centered under the Sandia Mountains. The sources need to AFT data in the Hagan embayment appears to be related to have a minimum half width of 160 km to match the Santa a large middle Cenozoic heat flow anomaly that influenced Rosa AFT data on the tilted PAZ. At depths of 2–4 km, the paleotemperatures on the High Plains of northeastern New average estimated burial depth for the Triassic to upper Mexico and southeastern Colorado [Kelley and Chapin, Proterozoic section in the Sandia Mountains prior to exhu- 1995; Kelley, 2002; Leonard et al., 2002]. AFT ages derived mation, temperatures rise only 20–40C above background from Triassic sandstone exposed at the surface between temperatures and the cooling rate is slow (1C/m.y.) for a Amarillo, Texas, and Santa Fe, New Mexico, generally source at 30 km. Temperatures at 4 km in the Sandia area decrease toward the west [Leonard et al., 2002]. Apatite exceed 200 to 370C and cooling rates are 5 to 20C/m.y. if from Triassic sandstone east of Santa Rosa, New Mexico, shallower (10–20 km) sources are assumed. Current seismic (Figure 1) have short mean track lengths and mixed age imaging of the Albuquerque region does not have sufficient populations, characteristic of samples originating from resolution to identify potential middle to lower crustal within the PAZ. Surprisingly, surface exposures of Triassic structures that may represent an equilibrated mafic magma sandstone between Santa Rosa, which is located 185 km chamber beneath the Sandia Mountains. Additionally, grav- east of Albuquerque, and the eastern margin of the Rio ity and magnetic data from central New Mexico do not Grande rift yield AFT ages on the order of 25 to 30 Ma. In indicate shallow, broad (160 km half-width), mafic sources other words, the base of a middle Cenozoic PAZ observed in the crust in this region. The Rio Grande rift is currently in the drillholes in Oklahoma [Carter et al., 1998] and in underlain by at least two active magma chambers of mafic to northeastern New Mexico [Leonard et al., 2002] comes to intermediate composition, beneath the Jemez Mountains and the surface near Santa Rosa (Figure 9). beneath the Socorro region [e.g., Rinehart and Sanford, [45] A decline in regional heat flow following extensive 1981; Balch et al., 1997; Fialko and Simons, 2001; Lutter Oligocene-Miocene volcanism has been proposed by et al., 1995], suggesting the importance of intrusive events in Morgan et al. [1986] and Perry et al. [1993]. Perry et rifting. Although we recognize that cooling of an Oligocene al. [1993] used Nd compositions of rhyolite from the age, shallow mafic magma chamber may potentially explain western United States to suggest that lower crustal the low-temperature thermochronology above, present geo- temperatures decreased by 300C at about 25 to 20 Ma. physical imaging does not support this interpretation. Here we examine the cooling history of the upper 2 to 4 km [47] Alternatively, a cooler (900C), long-lived (10 m.y. of the crust associated with decreasing middle to lower duration) source representing an intermediate composition HOUSE ET AL.: THERMOCHRONOMETRY OF RIO GRANDE RIFT 14 - 15 magma chamber at 10 km produces temperatures >225C cooling recorded by our thermochronometric data. For and rapid cooling rates (10C/m.y.) at 4 km and temper- example, an angular unconformity between the 36–27 Ma atures >120C and rates of 5C/m.y. at 2 km. Instantaneous Espinaso Formation and the basal Tanos Formation of the cooling of an 800C source at 10 km yields temperatures of Santa Fe Group containing a 25.4 ± 0.3 Ma basalt flow 180C at 4 km and a cooling rate of 6C/m.y. Thus, it is constrains the timing of some of the tilting in this area to be possible to produce rapid relaxation of isotherms if a source middle Oligocene [Connell and Cather, 1999]. The upper at 10 km is assumed. The high temperatures (>180C) member of the Santa Fe Group in the Hagan embayment, predicted by these models at 4 km appear to be recorded the Blackshare Formation, conformably overlies the Tanos by 40Ar/39Ar potassium feldspar data for the Sandia Granite Formation and contains an 11.6 ± 0.4 Ma ash bed. The [Harrison and Burke, 1988]. However, there is no compel- dip of the Blackshare Formation continuously decreases ling geophysical evidence for the cooled remnants of a large up-section from 27 to 36 NE in the lower part of the magma body a depth of 10 km between Albuquerque and section to 4 to 16 NE above the ash bed. The flat-lying Santa Rosa [e.g., Karlstrom et al., 2002]. The slow cooling Tuerto Formation overlying this sequence interfingers with rates associated with the cooling of deeper sources (>10 km) a 2.8 Ma basalt from the Cerros del Rio volcanic fields, do not match the observed rates. indicating that all tilting pre-dates 2.8 Ma. Provided that [48] The remnants of very shallow intermediate composi- tilting of these basin strata is somehow related with tilting tion intrusions are preserved in Ortiz Mountains and Cer- of the Sandia block (supported by the similarity in cooling rillos Hills. Maynard [1995] notes that the Ortiz magmas ages at both sites), then constraints from the Hagan intrude rocks as young as Eocene Galisteo Formation and embayment suggest that tilting roughly coincides with that the intrusions are in the form of laccoliths. The La Luz and partially post-dates the cooling of the Sandia footwall. traverse is 30 km from the exposed edge of the Ortiz [51] Together, the thermochronologic and stratigraphic intrusions and the intrusions have an exposed half width of observations can be used to reconstruct the chronology of 12 km. If we assume that the laccoliths were emplaced at a exhumation and possibly topographic growth, along the depth of 1 km, that the magma was initially at 1000C, and eastern flank of the Rio Grande rift. At end of the Laramide, that laccoliths are fed by a magma chamber at 10 km with a the Madera Formation was buried below 2000 m of half width of 12 km, then the temperatures due to local sediment and rocks of the La Luz and Knife Edge transects magmatism increase only 3C above background in the were at temperatures of 80 to 90C. The addition of at least Sandia Mountains, if cooling is instantaneous. If magmatism 1000 m of material during Early Cenozoic time and elevated was continuous for 1 Ma, the maximum time permitted by heat flow and burial during Oligocene volcanism further the 40Ar/39Ar data of Abbott et al. [2003], then temperatures heated the sample transects. Ages of 22–16 Ma record an in the Sandia Mountains could have increased as much as episode of cooling that brings the upper parts of La Luz and 8C above background. Peak temperatures are reached about Knife Edge transects above HePRZ by 16 Ma. After a 5 Ma after magmatism ends and the cooling rates at this reduction in cooling rate at 14 to 16 Ma, cooling is renewed distance are quite slow, well below 1C/m.y. 14 to 13 Ma. After this time, the thermochronometric data [49] In summary, both regionally and locally elevated offer no insight into the rate of cooling, but the sedimentary heat flow have certainly influenced temperatures in the record shows that tilting was complete by 3 Ma. Sandia Mountain region, but the rapid cooling rates [52] Ehlers et al. [2001] point out that a number of recorded by the AFT and (U-Th)/He data in the Sandia competing processes occur during footwall exhumation. Mountains are not merely a reflection of isotherm relaxation The 70 and 110C isotherms are warped toward shallower following magmatism. We would argue that the rapid cool- depths by the advection of warmer rocks during exhumation ing observed in the low temperature thermochronology data of the footwall, while sedimentation decreases heat flow on is controlled primarily by exhumation. the hanging wall. Juxtaposition of the hot footwall against the cold hanging wall results in lateral cooling of the footwall block. Furthermore, the presence of low thermal 8. Constraints of the Timing of Footwall conductivity sediments in the basin can cause a thermal Tilting and Initiation of Exhumation conductivity refraction effect [Kelley and Duncan, 1986]. Fortunately, the La Luz and Knife Edge traverses are set [50] The orientation of Pennsylvanian limestones that cap back 2 to 3 km from the range bounding fault, thus the eastern flanks of the Sandia Mountains, as well as the minimizing some of the hanging wall effects. Ehlers et al. average slope of the east flank of the range provide an [2001] calculate that the exhumation rate determined from upper limit on the magnitude of post-22 Ma eastward tilting age-elevation data generally overpredict model based exhu- of the Sandia Mountains (Figure 4b [Kelley and Northrop, mation rates. They also note that at exhumation rates of 3 to 1975]). The 2400 m of bedrock uplift that results from 5 mm/year, the AFT and (U-Th)/He ages form near vertical this tilting, while an upper limit, accounts for much of the lines on elevation versus age plots, such as those observed modern topographic relief across the Sandia Mountains in the Sandia Mountains, indicating that the data are less today (Figure 4b). Stratigraphic relationships in the Hagan sensitive to topography and surface processes at high embayment suggest that tilting of the Sandia block initiated exhumation rates. in the Middle Oligocene and continued through the Pleis- [53] As mentioned previously, one possible interpretation tocene and was coincident with much of the footwall of the change in cooling rate seen in the (U-Th)/He data is 14 - 16 HOUSE ET AL.: THERMOCHRONOMETRY OF RIO GRANDE RIFT that it represents true decrease in the exhumation rate of of the eastern rift flank. Geologic constraints demonstrate the Sandias that fortuitously coincides with a hiatus in the that 2.4 km of section covered the Sandia granite at the sedimentary record in the western Albuquerque Basin and a end of Cretaceous time when the area was tectonically subsequent, loosely constrained, post-14 to 16 Ma increase stable and near sea level. Temperatures in the Sandia granite in sedimentation rate in the Albuquerque Basin. The model may have been high enough to reset (U-Th)/He systematics, of Ehlers et al. [2001] raises another possibility. Say, for but they were not hot enough to completely reset AFT ages. example, that there is a previously unrecognized Oligocene Laramide sedimentation, as well as deposition of Oligocene intrusive body under the Sandia Mountains/Albuquerque volcaniclastic rocks added 1.0 to 2.5 km of section. Addi- Basin at a depth of 10 km. Perhaps the early 22 to 17 Ma tional burial caused temperatures in the granite to rise above rapid cooling recorded in the Sandia Mountains is due to 110C; however, burial alone does not easily explain young relaxation of isotherms following magmatism. The apparent (25 to 30 Ma) ages in the Triassic to Jurassic section of the decrease in cooling rate displayed by the (U-Th)/He data Hagan embayment or the 25 Ma ages in Triassic sandstone may in fact signal the beginning of exhumation as isotherms near Santa Rosa. Elevated regional heat flow on the order of are swept upward during the initial stages of footwall 105 mW/m2 during Oligocene time is required to resolve advection, as described by Ehlers et al. [2001]. The the pattern of AFT ages. renewed cooling of the footwall would begin once the [56] Low-temperature thermochronometric data from the positions of the isotherms are stabilized and cooling due Sandia Granite on the west face of the Sandia Mountains to erosion prevails. record rapid cooling (>10C/m.y.) between 22 and 17 Ma, a [54] Additional data are needed to distinguish between decrease in cooling rate 16 to 14 Ma, and renewed cooling the possibilities. Cores (only cuttings are available now) of 14 to 13 Ma. Models involving heat sources representing the Santa Group in the deep part of the Albuquerque Basin Oligocene intrusions suggest that isotherm relaxation fol- near the range front are needed to more tightly constrain the lowing regional magmatism might be responsible for the exhumation history of the Sandia Mountains. Detailed rapid initial cooling rates, provided that the intrusive bodies geophysical data are required to rule out the possibility of are shallow (<20 km) and very wide (160 km half-width). a previously unrecognized upper crustal magma chamber. Sources located at depths >20 km in the crust cool too Additional low-temperature thermochonologic data are slowly to match the observations. Current geophysical needed from other profiles through the range to obtain a imaging, however, precludes the instrusion geometries more three-dimensional view of the cooling history of the needed to fit the Sandia and Santa Rosa AFT data. Our Sandia Mountains. favored interpretation is that the low-temperature cooling data record exhumation of at least 3.1 km of material from above the Madera Limestone and that 2.4 km of rock uplift 9. Conclusions occurred during flexural tilting of the rift flank in the Sandia [55] New (U-Th)/He and AFT data from the Sandia Mountains. Mountains and Hagan embayment, when coupled with regional geologic and thermochronologic data, offer new [57] Acknowledgments. The paper benefited from the thorough and thoughtful reviews of Craig Jones and Steve Cather. Conversations with insights into possible thermal conditions prior to rifting, as Frank Pazzaglia, Karl Karlstrom, Chuck Chapin, and Sean Connell pro- well as better constraints on the timing and rate of cooling vided helpful insights.

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