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Home , Caballo Mountains, Camp Rice Formation, Capitan Mountains, Fission track dating, Rio Grande Rift, Sierra de las Uvas

Cooling histories of mountain ranges in the southern based on apatite fission-track analysis—a reconnaissance survey

by Shari A. Kelley, Dept. of Earth and Environmental Sciences, New Institute of Mining & Technology, Socorro, NM 87801-4796; and Charles E. Chapin, Bureau of Mines & Resources, Socorro, NM 87801-4796

Abstract

Fifty-two apatite fission-track (AFT) and two fission-track ages were deter- mined during a reconnaissance study of the cooling and tectonic history of uplifts asso- dated with the southern Rio Grande rift in south-central New Mexico. Mack et al. (1994a, b) proposed that the southern rift has been affected by four episodes of extension beginning at about 35 Ma. The main phases of faulting started in the late , the late , the middle , and the latest Miocene to early , with each phase disrupting earlier rift basins and in some cases reversing the dip of the early rift half- found in the vicinity of the southern . The timing of denudation derived from AFT data in the Caballo, Mud Springs, San Diego, and Dona Ana mountains are consistent with the episodes of uplift and preserved in the Oligocene to Miocene Hayner Ranch and Rincon Formations in the southern Caballo Mountains. Each mountain block studied in the southern rift has a unique history. AFT ages in the rocks on the east side of the record cooling of this mountain block at 21 to 22 Ma in re- sponse to the phase of extension that began in the late Oligocene. Younger AFT ages of 7 to 8 Ma related to the middle Miocene

FIGURE 1—Regional location map for the southern Rio Grande rift; the rift basins are white and the mountain ranges and basins outside of the rift are shaded. The dominant dip of the half-grabens is shown with the strike-and-dip symbols. SAC, Sacramento Mountains; SB, Sierra Blanca; CAPT, ; SA, San Andres Mountains; OR, ; FM, Franklin Mountains; DA, Doña Ana Mountains; SD, San Diego Mountain; CAB, Caballo Mountains; MS, Mud Springs Mountains; UVAS, ; CHUP, Chupadera Mountains; BR, . Locations of Eocene dikes east of Alamogordo and Eocene intrusions discussed in this paper are in italics: TH, Tres Hermanos; 0, Orogrande; C, Cuchillo. Dashed line shows northern extent of Eocene intrusives with ages older than 40 Ma. Cities and towns shown for reference are AL, Alamogordo; EP, El Paso; LC, Las Cruces; T or C, Truth or Consequences. General AFT age ranges are *, 0-10 Ma; *, 10-40 Ma; 0, >40 Ma.

episode of extension are exposed on ranges of the southern Rio Grande systematically with depth (Naeser, 1979; the upthrown side of high-angle faults rift near Las Cruces, New Mexico. Fitzgerald et al., 1995). At cutting the Proterozoic rocks. AFT ages First, Laramide compression temperatures less than 60 to 70°C, from the eastern Organ Mountains are formed west-northwest-trending tracks that are pro duced by the 10-17 Ma and the mean track lengths are uplifts bounded by 238 long; the ages on the west side are 20-29 of U are Ma and the mean track lengths are shorter. southwest-dipping, high-angle reverse retained and annealing is relatively A westward-tilted partia l -annealing zone faults (Seager and Mack, 1986; Seager minor. The AFT ages of detrital grains in for apatite that formed during et al., 1986). This compress i o n a l sedimentary rocks at shallow depths (tem- protracted cooling of the Organ batholith in phase occurred mainly in the peratures <70°C) are equivalent to the late Oligocene to early Mi oc en e is to early Eocene, although or greater than the stratigraphic age preserved in this mountain block. there is evidence for deformation as of the rock unit and the mean track Rapid denudation (200- 400 m/m.y.) and early as Late and as late as tilting of the range occurred in the middle lengths are on the order of 13 to 15 μm. late Eocene. Following Laramide The AFT ages in basement rocks at shallow Miocene. The base of a poorly defined deformation, regional calc-alkaline apatite partial-annealing zone that depth (temperatures <70°C) would reflect volcanism during the latest Eocene to formed du ring burial of southern New the cooling history of the basement prior Mexico in the is preserved in the Oligocene blanketed large portions of southern New Mexico with layers of ash- to the period of stability, and, as with Sacramento Mountains. The earliest phase sedimentary rocks, the mean track of extension is recorded in the AFT data from flow , suggesting that the topo- lengths would be relatively long (>13 Proterozoic rocks exposed at the base of the graphic relief by the middle Sacramento escarpment. Evidence for Oligocene was not as large as it is μm). In the temperature range known significant denudation related to today. was followed by, and as the partial-annealing zone (PAZ), Laramide deformation or late Oligocene was in part coeval with, extension, which is between 60 -70°C and 120- and middle Miocene extension is not leading to the development of the Rio 140°C, depending on the compo sition of observed in the AFT data from t h e Grande rift, a north-trend ing the apatite, the AFT ages and track Sacramento Mountains. AFT data lengths are reduced compared to the original derived from samples in Oligocene to continental rift that extends from Miocene intrusions (Capitan, Sierra northern to northern AFT ages and lengths. Mean track lengths in Blanca, and Black Range) record rapid Mexico. On the basis of the analysis of the PAZ are typically 8 to 13 cooling of these shallowly emplaced plutons. sedimentary deposits in the southern The AFT data from the northern and Caballo Mountains, Dona Ana Mountains, southern Rio Grande rift are similar in many San Diego Mountain, and Sierra de las respects. The trend of young AFT ages Uvas (Fig. 1), Mack et al. (1994a, b) and greater denudation on footwall blocks proposed that the southern rift has adjacent to the master faults been affected by four episodes of controlling the geometry of the half - extension beginning at about 35 Ma. grabens that was observed in northern New Mexico is also found in southern The main phases of faulting started in New Mexico. blocks in the rift the late Eocene, the late Oligocene, the that are bordered by normal faults on middle Miocene, and the latest Miocene two or three sides invariably have young to early Pliocene, with each phase (<12 Ma) AFT ages and high (>10°C disrupting earlier rift basins and in some /Ma) cooling rates. In the places where cases reversing the dip of the early rift Mesozoic AFT ages are preserved, the half-grabens [Note: The Eocene- estimated cooling rates are low (<2°C /Ma) Oligocene boundary used in this paper in both the northern and southern rift. is approximately 33-34 M a a s The primary difference in the cooling histories documented by Swisher and of mountain ranges in the northern and southern rift is the paucity of AFT ages Prothero (1990) and McIntosh et that are older than 30 Ma in the southern a l . (1992b)]. rift. Part of this trend may be a function The nature of the early episodes of rift of sampling bias. extension outside of the area studied Zircon FT and K -Ar data are used to by Mack et al. (1994a, b) is not well examine the distribution of late Mesozoic to known because much of the early Eocene volcanism in southern New Mexico. Miocene to Oligocene extensional A previously unrecognized Cretaceous intrusion was identified southeast of history of the southern rift is buried Hillsboro using zircon FT dating. The beneath younger deposits (Clemons and porphyry from this intrusion was the Osburn, 1986). Consequently, other source of volcanic clasts in the Late techniques must be used to locate Cretaceous to early Tertiary McRae areas affected by early extension Formation in the northeastern Caballo and to understand the timing and the Mountains. In addition, a zircon FT age of amount of denudation associated with 49.6 ±3.8 Ma for the capping these stages of deformation. An at the north end of the San approach that gives spatial and Andres Mountains and K-Ar ages for the temporal data on the development of Orogrande, Cuchillo, and Tres Hermanos stocks are used to constrain the northern vertical relief associated with rift formation limit of 40 Ma volcanism in south- involves determining the cooling history of central New Mexico. rocks in mountain ranges that bound the rift using apatite fission-track (AFT) Introduction analysis (Green et al., 1989). Fission- track annealing in apatite is controlled Recently, detailed geologic primarily by temperature and the mapping and stratigraphic studies by chemical composition of the apatite Seager et al. (1984), Seager and Mack (tracks in chlorapatite completely (1986), Mack and Seager (1990), and anneal at higher temperatures Mack et al. (1994 a, b) have (140°C) than those in fluorapatite constrained the timing and orienta - (120°C); Green et al., 1986; Green, oral tion of compressive and comm. 1996). In a relatively stable extensional events that affected the geological environment where mountain temperatures as a function of depth are now at a maximum, apatite fission- track (AFT) age and track lengths decrease

2 February 1997 New Mexico Geology μm. Finally, at temperatures above 120 to Sangre de Cristo Mountains north of in the formation. The age of the Love 140°C, tracks that are formed are quickly Blanca Peak are bounded on two sides by Ranch Formation is currently thought to destroyed and the fission-track age is zero. active late Cenozoic faults that have be between late Paleocene and middle If, after a period of relative stability, the allowed the rapid isostatic rise of these Eocene, on the basis of radiometric ages of crust is rapidly cooled during denudation mountains. Finally, cooling rates deter- the overlying Rubio Peak Formation (40- related to a tectonic event, the PAZ mined from the AFT age and length data 43 Ma) and of the age of the youngest may be exhumed and the time of cooling have increased progressively from the underlying unit (Seager and Clemons, and the paleodepth of the top or bottom of Laramide orogenic event (1-4°C/Ma) to 1975); few have been found in the the PAZ can be estimated (Gleadow and the development of the Rio Grande rift (7- (Seager et al., Fitzgerald, 1987). If the tectonically dis- 20°C/Ma). 1986). By assuming that the Proterozoic turbed area cools rapidly (>5°C /m.y.), the rocks from the Laramide Rio Grande mean track lengths in rocks that are at the To see if these patterns are characteristic of uplift, which acted as the source region for base of the fossil PAZ (below the break-in- the Rio Grande rift in general, samples for the Love Ranch Formation, cooled during slope on the age-depth profile) are gener- AFT analysis were collected from denudation and were transported imme- ally >14 μm, and the AFT ages from below mountain ranges bordering and within diately to the basin, we expect the clast AFT the base of the exhumed PAZ can be used the southern Rio Grande rift. The newly ages to provide some constraints on the to estimate the timing of the onset of de- collected AFT age and length data will be timing of denudation in the Rio nudation. If the rock column cools more used to constrain the cooling histories of Grande uplift and to provide a maximum slowly during uplift and/or erosion, the portions of the eastern and western flanks age for deposition of the Love Ranch mean track length at the base of the fossil and central horst blocks of the southern Formation. PAZ is typically <14 μm, and the apparent rift. The areas of study include the AFT age more loosely constrains timing of Caballo, Mud Springs, San Diego, Dona Ana the initiation of cooling (Foster and mountains (central horsts); the Black Range Fission-track analysis Gleadow, 1992). (western flank); the San Andres, Organ, Franklin Mountains (central horsts); Methods and Sierra Blanca, Capitan Mountains, Kelley et al. (1992) demonstrated that in Sacramento Mountains (eastern flank). The methods of fission-track analysis areas undisturbed by late rift volcanism or These new data add to our understanding used in this study are fully described in hydrothermal activity, the timing of denu- of vertical motions as a function of time and Kelley et al. (1992). Thirty-one samples of dation of a rift flank (e.g., the Ladron geographic location during compression Proterozoic igneous and metamorphic Mountains north of Socorro) derived from and extension in the southern Rio Grande rocks, sedimentary rocks, and AFT data closely fits the timing of rift. In addition, the data can be used to Eocene to Oligocene plutonic rocks were denudation determined from the compare and contrast the tectonic and collected along reconnaissance traverses record in the adjacent basin topographic development of the northern through ranges within and bordering the where early rift have been and southern parts of the Rio Grande rift. southern Rio Grande rift (Table 1, Fig. 1). In uplifted and exposed. Similarly, in the Only two phases of extensional addition, 17 samples were collected in a present study, we will examine the deformation are recognized in the more detailed study of the Eocene Organ cooling histories of the mountain ranges northern rift north of Socorro whereas batholith. The criteria used in locating the in the vicinity of the early rift basins four phases have been documented in the traverses included the occurrence of sig- described by Mack et al. (1994a, b) and south. The southern rift has undergone nificant vertical relief and the presence of compare our results with their more extension compared to the north, as rock types containing adequate amounts of data. Because the denudation data from indicated by the wider zone of deforma- apatite for FT dating. Elevations of the AFT analysis are consistent with the tion and the thinner crust (Keller et al., samples were determined from U.S. episodes of uplift and erosion preserved in 1990). Early rift magmatism is sparse to Geological Survey 1:24,000-scale topo- areas where the sediment record is well the north and more common to the south. graphic maps to an estimated accuracy of ±6 studied, the technique can be used to Furthermore, rifting began about 7 to 8 Ma m. determine the denudation history of earlier in the southern rift (Mack et al., Apatite and zircon were separated from uplifts adjacent to undissected basins 1994a, b; Chapin and Cather, 1994). the samples using standard heavy liquid where the sediment record is poorly and magnetic separation techniques. Apa- understood. tite grains were mounted in epoxy, pol- The principal objectives of this paper ished to expose the grains, and etched for Fission-track data collected in the north- 25 seconds in a 5 M solution of nitric acid to ern Rio Grande rift of New Mexico and are (1) to present new apatite fission-track ages for rocks exposed in the uplifts along reveal the fission tracks. The were Colorado indicate that rocks now exposed the eastern and western margins and in mounted in Teflon, polished, and etched in in the eastern and western margins of the NaOH/KOH at 230°C for 4.5 hours rift cooled during uplift, erosion, and the central horsts of the southern Rio Grande rift (Table 1, Fig. 1); (2) to use the (90SA01) and 8 hours (90BR05). The grain waning volcanism in a complex, three- mounts were then covered with muscovite dimensional pattern (Kelley et al., 1992). AFT data to discuss the local tectonic and cooling history of each mountain block detectors and sent to the A&M Three interesting trends have emerged investigated; (3) to integrate the AFT data Nuclear Science Center for irradiation. The from the AFT data for the northern Rio neutron flux was calibrated using the Grande rift. First, the youngest AFT ages from all the mountain ranges and K-Ar data from intrusives in the southern rift zeta technique with Durango apatite and are found in the rift flanks adjacent to Fish Canyon zircon age standards and large normal faults along the deep into a discussion of the tectonic develop- ment of this region during Laramide com- Corning standard glasses CN-5 and CN-6. margins of the en echelon half-grabens pression, mid-Cenozoic volcanism, and The zeta calibration is based on the accepted that make up the present Rio Grande ages of 27.9 ±0.7 Ma for the Fish Canyon rift. Second, promontories that project Rio Grande rift formation; (4) to use the AFT results to compare and contrast the Tuff and 31.4 ±0.5 Ma for Durango apatite into rift basins at the junction of arcuate (Hurford and Green, 1983; Green, 1985). normal faults bounding the deep side development of the northern and south- ern Rio Grande rift. The confined-track-length distributions in of half grabens (Blanca Peak in the the apatite grain mounts were determined Sangre de Cristo Mountains and the using a microscope fitted with a 100-X Ladron Mountains) have some of the In addition, as an experiment, we oil-immersion lens, a drawing tube, and a youngest AFT ages observed. Tectonic attempted to more tightly constrain the digitizing tablet. The system allows the track denudation on three sides has allowed age of the Eocene Love Ranch Formation lengths to be measured to approximately unusually rapid isostatic uplift in these in the (LR, Fig. 1) by ±0.2 μm. Horizontal, well- areas. Similarly, the determining the AFT ages of clasts

New Mexico Geology February 1997 3 4 February 1997 New Mexico Geology New Mexico Geology February 1997 5 etched, confined tracks (tracks completely enclosed within the ) in grains with prismatic faces were measured. The orientation of the tracks with respect to the c-axis was also measured. Time- temperature estimates were derived from the age and track-length data using the inverse-modeling algorithm of Corrigan (1991). The values of A,Q, and n used in this investigation are the composite parameters of Carlson (1990), and an initial track length of 15.7 μm is assumed for the models.

Results

The interpretation of the AFT data for each traverse (Fig. 1) is considered in detail in the following sections. First, we will examine the AFT data from the Caballo, Mud Springs, San Diego, and Dona Ana Mountains and compare the results with the interpretations of Mack et al. (1994a, b). We will then discuss the AFT data from areas where the Cenozoic uplift and sedimentation history are not as well documented. The cooling rates estimated from the AFT results are in Table 2. Unfortunately, because the content of many of the apatites from the southern rift is low (<5 ppm), detailed track length analysis and thermal modeling was possible for only nine of the samples. Caballo, Mud Springs, San Diego, Doña Ana mountains—The Caballo Mountains are an east-dipping block with Proterozoic granitic and metasedimentary rocks exposed on the western side (Fig. 2). Paleozoic to Mesozoic sedimentary rocks overlie the Proterozoic rocks and form the eastern dip slope into the Jornada del Muerto. The Paleozoic sedimentary rocks that form the crest of the range are dominated by carbonates lacking apatite, so an age-elevation traverse through the range was not possible. A northwest- to north–northwest-trending Laramide basement-cored uplift is preserved in the Caballo Mountains; this uplift, which is related to the Rio Grande uplift (Seager and Mack, 1986), shed sediments northward into a broad Laramide basin preserved as the Love Ranch Formation in the modern Jornada de l Muerto. The southern end of the Caballo Mountains once floored one of the early, broad rift basins (Seager et al., 1984). The cooling history recorded by AFT dates from the Proterozoic rocks at low elevation on the west side of the range is dominated by denudation during the late r phases of rift development. The AFT ages are 5.4 ±0.8 Ma on the northern end and 10.8 ±1.8 to 16.1 ±7.3 Ma in the south-central part of the range. Mack et al. (1994a, b) noted that early unroofing of the Caballo Mountains is recorded in the Hayner Ranch Formation (approximately 25 to 16 Ma). The lower part of this formation contains clasts from the youngest part of the stratigraphic

6 February 1997 New Mexico Geology section (Oligocene Uvas Basaltic formed part of the early rift basin available. The two highest-elevation sam- and Bell Top tuff); the upper preserved in the southern Caballo ples are from Tertiary andesitic intrusives part of the formation consists of clasts Mountains. The early rift basin was then southeast of Kingston. Maggiore (1981) derived from the Eocene Palm Park and broken up during the later phases of rift determined a zircon FT age of 34.0 ±2.8 Love Ranch Formations. The basal part development. The AFT age of 6.5 ±2.2 Ma Ma for the andesite porphyry sill at the of the overlying Rincon Valley Formation on Proterozoic granite appears to be the location of 90BR04 (Table 1). The AFT age (about 16 to 6 Ma) is similar to the Hayner result of cooling during the latest episode of 36.6 ±8.1 Ma is concordant with the zir- Ranch in that it includes Uvas, Bell Top, of normal faulting in the late Miocene. con FT age and is indicative of rapid cool- Palm Park, and Love Ranch clasts. In Late uplift of this block is ing of this shallowly emplaced sill. In con- contrast, in the middle to upper beds of recorded by a 30-m offset in the Camp trast, the dacite intrusive exposed at the Rincon Valley Formation, clasts of Rice Formation (Seager, 1986). Seager slightly higher elevation (90BR03) is sig- and (1986) proposed that the highly silicified nificantly younger (19.4 ±2.2 Ma) than Magdalena Group outcrop of that 90BR04. The eastern margin of the Emory are abundant. No Proterozoic rests on the Proterozoic granite represents cauldron, which is located adjacent to this granite clasts are found in the Rincon Val- a fossil hot-spring deposit. Anomalously sample, has been reactivated a number of ley Formation, except for those thought to high modern heat flow in a shallow-flow times following the collapse of the caul- be reworked from the Eocene Love Ranch system northwest of San Diego Mountain dron (Abitz, 1986), complicating the cool- Formation. These observations are consis- was observed by Reiter et al. (1978). The ing history of this area. tent with the AFT data. The average mod- water in the fossil hydrothermal system ern heat flow in the Palomas Basin on the apparently was not hot enough, or The two lower-elevation samples are west side of the mountains is about 95 the system was so short lived, that it did from a small horst block east of Hillsboro. mW/m2 , and the average gradient is on not completely reset the AFT ages to One sample (90BR01, Table 1), with an the order of 30 to 35°C /km (Reiter et al., ages younger than the Quaternary Camp AFT age of 21.7 ±3.6 Ma, is from the 73.4 1986; Reiter and Barroll, 1990). Assuming Rice Formation. ±2.5 Ma (K -Ar age; Hedlund, 1974) that the average heat flow has not Copper Flat stock. The long mean track- changed since extension began and as- One sample from a dacite on the length measurements for this sample indi- suming a surface temperature of 15°C, the east side of the Dona Ana Mountains (Fig. cate rapid cooling. The other sample base of the apatite PAZ (120°C) was at a 3), which may be part of a small cauldron (90BR05), which is from a small outcrop depth of 3 to 3.5 km in the late Eocene. On within the Organ cauldron (Seager and labeled Tii on the map of Seager et al. the basis of the geologic cross-section McCurry, 1988), was collected to investi- (1982), surprisingly yielded Cretaceous A-A' of Seager et al. (1982), at least 3 km of gate the cooling history of this range. An apatite and zircon FT ages (Table 1). Paleozoic to Middle Cenozoic sedimenta- AFT age of 18.4 ±4.4 Ma was determined. Because the zircon FT age of 68.0 ±5.4 Ma ry and volcanic rocks may have overlain Mack et al. (1994a, b) see evidence for is nearly concordant with the AFT age of the Proterozoic rocks prior to denudation uplift of the Dona Ana Mountains begin- 63.7 ±4.5 Ma and the mean track lengths of the Caballo Mountains. The AFT ages ning with the deposition of the Rincon are long (Table 1), the intrusive cooled from low-elevation Proterozoic rocks in Valley Formation. Pennsylvanian Panther rapidly (>40°C /Ma) from the blocking the south-central Caballo Mountains Seep Formation and Tertiary rhyolite temperature of 240 ±50°C (Hurford, 1986) record cooling through the PAZ for apatite clasts that are unique to the Doña Ana for fission tracks in zircon to approximate- during deposition of the Rincon Valley Mountains are found in the Rincon Valley ly 120°C during the Late Cretaceous. Since Formation at the time the Proterozoic Formation but not in the underlying about 65 Ma, the cooling rate slowed to rocks were still in the subsurface. These Hayner Ranch Formation. Because the 2°C /Ma. This outcrop has not been signif- Proterozoic rocks were exposed at the sur- AFT and provenance results from the icantly heated since the Cretaceous. face later; Proterozoic detritus is common southern Caballo area are compatible, we in the early Pliocene to middle will now apply this technique to areas in Despite the complex structures and age (Lozinsky and Haw- the southern rift where the Oligocene to patterns in this area, the AFT data provide ley, 1986; Mack and Seager, 1990) in the Miocene uplift and sedimentation history some constraints on the cooling history of Palomas Basin to the west. The decrease in are concealed by younger basin fill. the Black Range. Much of the thermal his- AFT age to the north and depositional tory of this area is related to cooling fol- trends of Proterozoic detritus imply that Black Range-The remnants of the lowing mid-Cenozoic igneous activity in the fault that was active during late this region. The discovery of a previously Oligocene to Miocene time along the Emory cauldron, which is part of the large Mogollon-Datil volcanic field, are expos- unknown Cretaceous intrusion yielding a southeastern margin of the Caballo Moun- 64 Ma AFT age suggests that post - tains propagated northward during latest ed in the Black Range on the west side of the Palomas Basin (Fig. 2). Volcanism Cretaceous burial of the horst block south- Miocene to Pliocene time (G. Mack, writ- east of Hillsboro was not great enough to ten comm. 1996). began in the area about 38 Ma and the cauldron formed with the eruption of the reset the AFT age, which is congruous voluminous 35.2 Ma Kneeling Nun Tuff with the thin (<1 km) Cenozoic volcanic The Mud Springs Mountains (Fig. 2) are a (Elston, 1989; McIntosh et al., 1992a). The and sedimentary section preserved in this small, east-dipping intrarift horst block floor was subsequently domed area (G. Mack, written comm. 1996). How- with a small outcrop of Proterozoic meta- upward by resurgence, and the moat ever, the Copper Flat stock to the north sedimentary rocks exposed along the between the caldera wall and was remained in the subsurface and began to southwest side; the metamorphic rocks largely filled with younger lava and pyro- cool in the early Miocene. These observa- are overlain by Paleozoic sedimentary clastic flows from local and distant tions are corroborated by the distribution rocks. As in the case of the northern Ca- sources (Elston, 1989). Tuffs preserved in of volcanic clasts in the Late Cretaceous to ballo Mountains, the AFT age of 8.4 ±3.8 the Black Range are 35.2 to 28.1 Ma early Tertiary McRae Formation exposed Ma for the Proterozoic rocks records rapid (McIntosh et al., 1992a). Most of the faults in the northeastern Caballo Mountains. cooling during the later phases of rifting. related to cauldron collapse have been Clasts that are chemically identical to the reactivated during rift-related porphyry in the recently discovered San Diego Mountain (Fig. 2) is another deformation. Cretaceous intrusion have been found in small intrarift horst block. The northern this unit, but clasts from the Copper Flat marginal of the Laramide Rio stock have not been detected in the McRae A Proterozoic granitic outcrop north of Formation (N. McMillan, written comm. Grande uplift, which brought Proterozoic Kingston was sampled as p art of this 1995). rocks near the surface, is exposed along study, but this particular granite contains the north side of the mountain fluorite, not apatite so no AFT data are (Seager,1986). The area subsequently

New Mexico Geology February 1997 7 San Andres Mountains —The San Proterozoic igneous and metamorphic First, the range is bounded on both sides Andres Mountains (Figs. 1, 3) are a west- rocks. Virtually nothing has been by undissected basins, the Jornada del tilted , with Paleozoic sedimen- published on the Cenozoic uplift history of Muerto on the west and the Tularosa Basin tary rocks (primarily carbonates) restingon the San Andres Mountains for two on the east. Second, the San Andres reasons. Moun-

8 February 1997 New Mexico Geology Mountains are within White Sands Missile Range, so access is restricted. AFT samples were collected from three traverses through the Proterozoic rocks exposed on the east side of the San Andres Mountains. In addition, samples were collected from the undated rhyolite sill exposed o n Salinas Peak near the north end of the range. The zircon FT age of the rhyolite sill is 49.6 ±3.8 Ma (90SA01, Table 1); no apatite was recovered from the rhyolite. The AFT ages of approximately 21 to 22 Ma (Table 1; Fig. 3) were determined from Proterozoic rocks at the north end of the range. A traverse through Dead Man Canyon (Fig. 3) in the central part of the range yielded AFT ages of 7 to 22 Ma. The approximately 22 Ma ages are generally found in the stratigraphically highest part of the Proterozoic section a short distance below the Bliss and the 15 to 7 Ma ages are from stratigraphically lower levels. An exception to this generalization is 90SA15 (8.9 ±2 Ma), which is located on the upthrown side of a west-dipping normal fault that offsets Paleozoic and Proterozoic rocks in this area (Seager et al., 1982). The southern tra- verse is in the structurally complicated Little San Nicolas Canyon area (Fig. 3; Roths, 1991). Again, the ages in this canyon are between 8 and 21 Ma, but in this case, the youngest ages strictly coin- cide with north -trending, high - angle faults cutting the Proterozoic and . Roths (1991) mapped these faults as thrust faults, but the AFT data imply that these faults have been reactivated during extension, with the youngest ages exposed on the upthrown block. One possible interpretation of this limited AFT data set is that the San Andres Mountains started cooling during an early phase of extension and that a later phase of rapid cooling is recorded in samples near high-angle faults in the Dead Man Canyon and San Nicolas Canyon areas paleomagnetic data of McIntosh et al. with increased elevation, but the (Table 2). (1992a). The cauldron likely developed ages on the west side are generally Organ Mountains —The Organ in an extensional setting because normal older than those on the east side (Figs. 4, Mountains (Fig. 3) are, in large part, faults that offset the tuffs are 5), a trend that is consistent with the the erosional remnant of the late intruded by batholithic rocks (Seager and westward dip of the batholith. The Eocene Organ cauldron. The Organ McCurry, 1988). The final phase of age-elevation data and the track-length batholith intrudes Proterozoic rocks, volcanism in the area is represented by data for the east-side samples (81OR05, Paleozoic sedimentary rocks, and the 300 m of rhyolitic to trachytic lavas. 06, 08, 21, 22) indicate a relatively Eocene Orejon Andesite, as well as the Subsequently, the area has been block simple, rapid (7-11°C / Ma; Table 2, Fig. pyroclastic flows and lavas associated faulted and rotated about 15-20°W. 5) cooling history for this part of w i t h t h e f o rmation of the Most of the mineralization in the Organ the batholith. Sample 81OR15 at Baylor c a u l d r o n (Seager, 1981). The Organ Mountains is related to emplacement of Pass is significantly younger than the batholith consists of four plutonic the batholith (Seager, 1981), but adjacent samples. The track-length data p h a s e s , t w o hypabyssal phases, and mineralized veins and dikes that cut for 81OR15 indicate that it cooled a variety of silicic dikes (Seager, the Sugarloaf Peak quartz monzonite in the northern part of the batholith rapidly (12°C /Ma; Table 2). Track- 1981). The silicic that length data for nearby samples formed the Organ batholith was may have formed after these rocks crystallized. The dikes and quartz 81OR13 (13.6 ±1.3, n=24) and emplaced at shallow depths, and 81OR20 (11.8 ±1.6, n=24; Fig. 5) the upper part of this magma body monzonite have been intensely sericitized in the Organ mining suggest that the west-side samples erupted to produce three large ash-flow experienced protracted cooling tuff units. Approximately 3 km of tuffs district in the northeastern part of the batholith (Fig. 4). during the late Oligocene prior to were deposited and preserved in the rapid tilting and denudation of the subsiding caul dron. These tuffs were The AFT ages from the Organ batholith batholith 14 to 17 Ma. The sinuous erupted between 35.7 to 36.2 Ma, on range from about 10 to 29 Ma. The ages the basis of 40Ar / 39Ar ages and do not show a significant increase

New Mexico Geology February 1997 9 line on Fig. 4 marks the age on amphibole determined by Moore et 1991; Campbell et al., 1994). An AFT approximate boundary between the al. (1991), indicating rapid cooling of age from low elevation on the eastern uplifted, rotated zone of protracted this intrusion at shallow levels in the margin of the pluton is 27.9 ±4.3 Ma. cooling (PAZ) on the west side from crust. Similarly, an AFT age of 23.8 Because the AFT age is concordant rocks that were more rapidly cooled ±2.6 Ma and high -mean-track length with the 40Ar/ 39Ar age on feldspar, on the east side. of 14.4 ±0.4 μm (Table 1) for the Bonito this shallowly emplaced plu ton cooled Franklin Mountains -Pam Roths Lake stock (26.6 ±1.4 Ma K-Ar on rapidly. (oral comm. 1991) provided a sample biotite; Moore et al., 1991) suggest The Sacramento Mountains (Figs. 1, of Proterozoic Thunderbird rhyolite rapid cooling of this intrusion 3), an east-tilted fault block that from high elevation on the Franklin (90SB02; Fig. 3). marks the transition from the Mountains, a west-tilted fault block In contrast to the high -elevation southeastern Rio Grand e rift to the south of the Organ Mountains (Fig. samples with their rapid, simple Great Plains, are dominated by 1). The AFT age for this low-uranium cooling histories, the lower-elevation Paleozoic carbonates, so the apatite is 51.8 ±15.8 Ma. This relatively samples had a more protracted, and opportunity for collection of samples with old AFT age from high elevation suggests perhaps complicated, cooling history abundant apatite and zircon is the possibility that an uplifted PAZ following emplacement. The AFT dates somewhat limited. Attempts to recover may be present in the Franklin of 17.5 and 22.6 Ma reported by Allen apatite from sandstone lenses in the Mountains. An age-elevation traverse and Foord (1991) for Black Mountain , Pennsylvanian, and is needed to investigate this possibility. stock (BMS, Fig. 3) are significantly Permian carbonate sections exposed Sierra Blanca, Capitan, Sacramento younger than the 37.8 and 34.6 Ma on the west side of the range were mountains- Sierra Blanca (Figs. 1, 40Ar/39Ar ages on hornblende and unsuccessful. Apatite was found in 3), which lies north of the plagioclase, respectively, for this Proterozoic metasedimentary rocks S a c ra m e n to Mountains, forms part intrusion and its associated dikes. No south of Alamogordo at the base of of the eastern margin of the rift and is track-length data were presented for the west-facing Sacramento the highest mountain in the southern these samples. An attempt was made to escarpment, in the Permian Abo rift (3,658 m). Sierra Blanca consists determine the AFT age of a syenite Formation near High Rolls, and in primarily of Eocene (38 Ma) andesitic porphyry dike near Three Rivers Proterozoic intrusive rocks in the Bent volcanic rocks overlying Cretaceous to campground along the western base dome. Although there is insufficient Paleocene sedimentary rocks. Four of Sierra Blanca, but the apatite con - material for a complete traverse Oligocene-Eocene (25-38 Ma) syenite- tains abundant defects that obscured through the range, the datable samples do monzonite-granite stocks intrude the sed- the fission tracks. provide some information on imentary and volcanic rocks The 28.3 ±0.1 Ma Capitan t h e Cenozoic cooling history of this (Thompson, 1972; Moore et al., 1991). p l u to n (Campbell et al., 1994) lies mountain block. The AFT data from the stocks can be northeast of Sierra Blanca (Fig. 1). This The highest-elevation sample, 90SAC04 used to examine the post-Oligocene large granitic batholith is oriented (Table 1, Fig. 3), which is from the cooling history of this part of the east-west and parallels the Capitan Permian Abo Formation, has an AFT eastern flank of the rift. Apatite ages for lineament (Chapin et al., 1978a). age of 112.7 ±25.3 Ma. On the basis the syenite porphyry part of the Three Fluid-inclusion data indicates that the of limited track -length data (11.4 ±2.8 Rivers stock at high elevation are 26.5 Capitan pluton, the top of which now μm; n=12), this rock unit likely resided ±5.1 and 32.8 ±5.2 Ma (905B03 and stands about 1 km above the surrounding in the PAZ for a sub- 905B04; Fig. 3). These AFT ages are nearly plains, was intruded beneath at least 1 concordant with the 29.9 ±1.8 Ma K-Ar km of rock (Allen and McLemore,

10 February 1997 New Mexico Geology Orogrande area (Table 3) and to the New Mexico. Two acknowledged Lara- age of dikes and sills in the mide-aged intrusive centers in the Sacramento Mountain escarp ment southern rift are the Salado Hills west of Cloudcroft (45.3 ±2.2 Ma, stock in the Cuchillo Mountains Asquith, 1974; Fig. 1). (Chapin et al., 1978) and the Copper Flat stock (Hedlund, 1974) in the Black Range. The identification of a Discussion previously unknown Cretaceous intrusion in the Black Range is of interest for stantial period of time prior to The reconnaissance nature of this two reasons. The first is economic, denudation. The Proterozoic rocks in study and the lack of datable rocks at given the intrusion's proximity to the Bent area to the north have AFT high elevations in several of the the Copper Flat porphyry copper ages that are in the 52 to 67 Ma mountain ranges in the southern deposit. Second, the source of the range, and the mean track lengths volcanic clasts in the Late rift preclude detailed analysis of the Cretaceous to early Tertiary McRae are also short (10.1 ±2.6 μm; n=13). Like timing of episodes of extension and the Abo samples, these age and Formation along the eastern flank of rates of denudation associated with the Caballo Mountains has been the length data are characteristic of each tectonic event that affected samples found in a PAZ that likely subject of some debate (N. southern New Mexico. Despite the short- McMillan, written comm. 1993). This formed at a time of relative tectonic comings of the available data, the stability during Mesozoic burial of the intrusion (or one like it) appears to results do permit some qualitative be the source for the clasts. Further area (Fig. 6). Bowsher (1991) proposed, analysis. on the basis of facies distribution geochronologic studies on the ea st patterns near Sierra Blanca, that side of the Black Range may help Laramide deformation identify additional Cretaceous uplift in the Sacramento Mountains plutons. may have started in the early In contrast to data from the Mesozoic; however, the AFT age and Early to middle Eocene volcanism is not northern Rio Grande rift and the common in northern and central length data indicate that little Southern Rocky Mountains provinces denudation was associated with this New Mexico but is widespread in (Kelley et al., 1992; Kelley and Chapin, southern New Mexico and in the Trans- minor . The 35 to 41 Ma 1995), cooling related to Laramide Pecos belt of west Texas. The K-Ar ages AFT ages and the mean track length deformation is not well represented in the for four intrusions in southern New of 16.3 ±4.2 μm (n=3) from the base southern rift AFT data. The preservation Mexico (Table 3, Fig. 1) and the of the western Sacramento in the Sacramento Mountains of a dikes and sills east of Alamogordo escarpment can be used to infer that probable uplifted PAZ that formed (Asquith, 1974), the 49.6 ±3.8 Ma the Mesozoic PAZ was brought toward during Mesozoic burial and the lack of zircon FT age on the Salinas Peak rhy- the surface by uplift and erosion 50 to 70 Ma ages with relatively long olite sill in the San Andres Mountains, during the earliest phase of extension mean track lengths like those found in and the 49.6 ±11.7 Ma age on the and was brought to the surface by the the Front Range of Colorado (Kelley probable tuff in the sinkhole near latest phase of rifting. The preservation and Chapin, 1995) suggests that Cloudcroft further establish the of old ages at high elevation (Fig. 6) Laramide deformation was relatively limits of Eocene volcanism. The dashed indicate that this mountain range was minor along the eastern margin of the line of Fig. 1 represents the not strongly uplifted early in the rift. The few minor folds and faults of northernmost extent of pre -40 Ma development of the rift. Further work possible Laramide age that are volcanism in south-central New on the Permian red beds in this area is present in the western escarpment of the Mexico. As noted by Christiansen needed to test this interpretation. Sacramento Mountains apparently did and Yeats (1992) and Humphreys (1995), the age of volcanism decreases An interesting deposit that appears little to disrupt the Mesozoic P AZ. to be an intensely altered volcanic ash progressively from 40 to 50 Ma in west Else where in the southern rift, cooling Texas, northern Mexico, and southern was discovered in a sinkhole in the related to Laramide deformation has Permian Yeso Formation on the New Mexico to about 21 Ma in southern been overprinted by later volcanism and Nevada along a west—northwestward outskirts of Cloud-croft in the extension. Sacramento Mountains. Although trend across the southern Basin and this fine-grained deposit contains no Range province. The new age s help Late Cretaceous to middle Eocene identifiable glass shards or zircon, it constrain the post-Laramide geometry of does contain subhedral grains of volcanism the Farallon slab beneath apatite with an AFT age of 49.3 ±11.7 Ma southern New Mexico (Severinghaus Late Cretaceous to Paleocene and Atwater, 1990; Humphreys, 1995). (CL ash, Table 1). This age is similar to the volcanic centers are rare in central and K-Ar age of the oldest intrusive in the northern

New Mexico Geology February 1997 11 Rift extension Mountain or the Mud Springs The oldest of the four phases of Mountains, have much younger ages extension is recorded by the low- than the uplifts on the flanks of the rift. elevation AFT data in the Sacramento Mountains. Given the preservation of Comparison of northern and the older AFT ages in the Sacramento southern rift Mountains, little denuda tion is associated with this event. The Many of the trends observed in extensional episodes that began in the late the northern Rio Grande rift are also Oligocene and middle Miocene were not present in the southern rift. As observed in the Sacramento discussed above, the response of the Mountains, but waning stages of the fault blocks adjacent to large faults in late Oligocene episode may be the half-grabens that form the Rio reflected in the cooling history of the Grande rift is similar in both the Black Mountain stock south of Sierra northern and southern . Th e cooling Blanca. In contrast, cooling related to the of the mountain blocks is controlled earliest phase of extension is not observed by denudation related to flexural in the AFT data from the other mountain isostatic uplift of the fault-bounded ranges in this study; instead the data ranges (Kelley et al., 1992). Intrarift are dominated by the affects of the horst blocks in the southern rift that late Oligocene and middle Miocene are bordered by normal faults on two or episodes of extension. The cooling rates three sides, like San Diego Mountain or of 7 to 12°C /m.y. associated with Mud Springs Mountains, invariably middle Miocene extension in the have young (<12 Ma) AFT ages and high Organ Mountains imply denudation rates (>10°C /Ma) cooling rates. In the northern of 200 to 400 m/m.y., assuming a rift, AFT ages of 10 to 20 Ma and rapid geothermal gradient of 30°C/km. The cooling rates are commonly associated latest phase of extension, which began with blocks bounded on two or more in the latest Miocene to Pliocene, is sides by normal faults. Furthermore, in poorly represented in the AFT data the few places where Mesozoic AFT ages are because of the lag between the time preserved, the estimated cooling rates that cooling commenc es at depths of 2 to are low (<2°C /Ma) in both the northern 3 km and the time that the rocks and southern rift. reach the surface. The late Miocene Figure 7 summarizes the AFT data ages in the northern Caballo, Mud from the southern and northern rift. Springs, San Andres, and San Diego As noted by Chapin (1971) and mountains suggest rapid denudation Baldridge et al. (1983), the Rio Grande (250-600 m /m.y.) associated with the rift widens south of the Ladron latest phase of rifting. Mountains. The rift consists of a single The trend of young AFT ages and half- at any given latitude north greater denudation on the margin of of the Ladron Mountains, but it is the fault blocks adjacent to the deep made up of nested or multiple half- side of the half-grabens observed in gra bens to the south. Consequently, northern New Mexico is also found the Ladron Mountains were used as the in southern New Mexico. For example, boundary between the northern and using gravity data and limited drill- southern rifts. The primary difference hole information, Lozinsky and Bauer in the cooling histories of mountain (1991) found that the Tularosa Basin is ranges in the northern and southern underlain by two sub -parallel half- rift is illustrated in the his tograms of grabens. The eastern half-graben Fig. 7. The AFT ages that are older than adjacent to the Sacramento Mountains 30 Ma are more common in the northern is a relatively shallow bench tilted to Rio Grande rift than they are in the the east (maximum of 1200 m of fill). southern rift. Part of this trend may be a The western half-graben adjacent to the function of sampling bias. The southern San Andres-Organ Mountains is rift has not been as extensively sampled deeper (at least 1,800 m of fill) and is as the northern rift, and most of the tilted to the west (Lozinsky and Bauer, southern-rift samples are from the intrarift 1991). The AFT ages of 7 to 22 Ma are horst blocks. Furthermore, the upper present in the San Andres Mountains parts of the Sacramento, San Andres, and adjoining the deep side of the western Caballo Mountains are composed half-graben. In contrast, the AFT ages primarily of Paleozoic carbonates that are 35 to 112 Ma in the Sacra mento lack apatite for AFT dating. If datable Mountains. The AFT data from the rocks were present along the crests of Sierra Blanca and Capitan intrusive these ranges, older AFT ages like those above, cooling related to the earliest cen ters reflect rapid cooling of these found in the northern rift would and latest phases of rifting is not well shallowly emplaced plutons. In the probably be observed. Although part represented in the AFT data, but the Palomas Basin, the youngest ages are in of the trend is related to sampling late Oligocene and middle Miocene the Caballo and Mud Springs bias, the combined effects of regional vol- episodes are prominent in Fig. 7A, Mountains, next to the master fault canism and greater extension in the suggesting that larger amounts of for the Palomas Basin. Although the southern rift have apparently led to the denudation occurred during these AFT age distribution in the Black Range lack of preservation of ages older than phases of extension compared with the is complex, all of the ages are older than the 30 Ma in the southern rift. earliest episode. Similarly, the rift - AFT ages in the Caballo Mountains. The dashed lines in Fig. 7A depict related AFT ages from the northern rift Furthermore, the central horst blocks the timing of the initiation of each cluster in the late Oligocene to middle bounded by normal faults on two o r episode of rifting found in the southern Miocene range. three sides, such as San Diego Caballo Mountains. As discussed

12 February 1997 New Mexico Geology Love Ranch clasts 5. The AFT data derived from samples cauldron complex in the Emory cauldron; in in Oligocene to Miocene intrusions Clemons, R. E., King, W. E., Mack, G. H., and The original intent of this part of the (Capitan, Sierra Blanca, and Black Zidek, J. (eds.), Truth or Consequences Region: New Mexico Geological Society, Guidebook 37, investigation was to try to put Range) record rapid cooling of these pp. 161-166. constraints on the age of the Love Ranch shallowly emplaced plutons. Formation. Unfortunately, the AFT 6. The AFT data from the northern and Allen, M. S., and Foord, E. E., 1991, Geological, results do nothing to clarify the age of southern Rio Grande rift are similar geochemical and isotopic characteristics of the in many respects. The trend of young Lincoln County porphyry belt, New Mexico: the Love Ranch; however, the data can implications for regional and mineral be used to address the thermal AFT ages and greater denudation on the margin of fault blocks adjacent to deposits; in Barker, J. M., Kues, B. S., Austin, G. history of the southern Jornada del S., and Lucas, S. G. (eds.), Geology of the Sierra Muerto. The AFT ages of three granitic the master faults controlling the geometry Blanca, Sacramento, and Capitan Ranges, New clasts from the Love Ranch of the half-grabens that was observed Mexico: New Mexico Geological Society, Guide- in northern New Mexico is also book 42, pp. 97-102.. Formation range from 13.2 ±3.1 Ma to found in southern New Mexico. Horst 22.7 ±0.9 Ma, significantly younger than blocks in the rift that are bordered by Allen, M. S., and McLemore, V. T., 1991, The geolo- the stratigraphic age of the unit. The gy and petrogenesis of the Capitan pluton, New normal faults on two or three side s Mexico; in Barker, J. M., Kues, B. S., Austin, G. S., spread in ages may be related to invariably have young (<12 Ma) AFT differences in apatite composition and Lucas, S. G. (eds.), Geology of the Sierra ages and high (>10°C /Ma) cooling Blanca, Sacramento, and Capitan Ranges, New among the clasts. The high-mean- rates. In the places where Mesozoic Mexico: New Mexico Geological Society, Guide- track lengths for these samples AFT ages are preserved, the book 42, pp. 115 -128. indicate that the clasts were heated estimated cooling rates are low (<2°C Asquith, G. B., 1974, Petrography and petrogenesis to temperatures above 120°C following /Ma) in both the northern and of Tertiary camptonites and diorites, Sacramento deposition in the basin, completely southern rift. The primary difference Mountains, New Mexico: New Mexico Bureau of annealing pre-existing fission tracks. in the cooling histories of mountain Mines and Mineral Resources, Circular 141, 6 pp. Heating of the samples occurred during ranges in the northern and Baldridge, W. S., Bartov, Y., and Kron, A., 1983, burial by about 2 km of volcanic and southern rift is the paucity of AFT of the Rio Grande rift and south- volcaniclastic rocks in a part of the ages that are older than 30 Ma in the eastern , New Mexico and Jornada del Muerto that has elevated southern rift. Part of this trend may be a ; supplement to Riecker, R. E. (ed.), Rio Grande Rift: Tectonics and Magmatism: Wash- heat flow (115-127 mW / m2; Reiter and function of sampling bias. 7. A previously unrecognized ington, D.C., American Geophysical Union, scale Barroll, 1990). The samples 1:500,000. subsequently cooled relatively rapidly Cretaceous intrusion was identified Bowsher, A. L., 1991, Some effects of during exhumation in the early to middle southeast of Hillsboro using zircon FT dating. The porphyry from this basement on the development of the Sacramento Miocene. Mountains; in Barker, J. M., Kues, B. S., Austin, G. Conclusions intrusion was the source of volcanic S., and Lucas, S. G. (eds.), Geology of the Sierra clasts in the Late Cretaceous to early Blanca, Sacramento, and Capitan Ranges, New 1. The AFT data from the Caballo, Mud Tertiary McRae Formation located in the Springs, San Diego, and Doña Ana Mexico: New Mexico Geological Society, Guide- northeastern Caballo Mountains. book 42, pp. 81-89. mountains and the sedimentologic 8. A zircon FT age of 49.6 ±3.8 Ma on the data from the Oligocene to Miocene Campbell, A. R., Heizler, M. T., and Dunbar, N. W., rhyolite sill exposed on Salinas Peak at the 1994, 40Ar /39Ar dating of fluid inclusions in Hayner Ranch and Rincon Valley north end of the San Andres quartz from the Capitan pluton, New Mexico Formations (Mack et al., 1994a, b) are Mountains and K-Ar ages of the (abs.): PACROFI, v. 5, p. 11. consistent. Orogrande, Cuchillo, and Tres Carlson, W. D., 1990, Mechanisms and kinetics of 2. The 21 to 22 Ma AFT ages in the Hermanos stocks constrain the apatite fission-track annealing: American Proterozoic rocks on the east side of northern limit of pre-40 Ma volcanism in Mineralogist, v. 75, pp. 1120-1139. the San Andres Mountains record cooling south-central New Mexico. Cather, S. M., Chamberlin, R. M., Chapin, C. E., in response to the phase of extension ACKNOWLEDGMENTS—This project was and McIntosh, W. C., 1994, Stratigraphic conse- quences of episodic extension in the Lemitar that began in the late Oligocene. supported by NSF grant EAR8804203 Mountains, central Rio Grande rift; in Keller, G. Younger AFT ages of 7 to 8 Ma related and by the New Mexico Bureau of Mines R., and Cather, S. M. (eds.), Basins of the Rio to the middle Miocene episode of and Mineral Resources. Thanks to Grande Rift: Structure, Stratigraphy, and Tectonic extension are exposed on the upthrown William Seager for donating some of Setting: Geological Society of America, Special side of high-angle faults cutting the his samples from the Organ batholith Paper 291, pp. 157-170. Proterozoic rocks. and for his help in collecting samples in Chapin, C. E., 1971, The Rio Grande rift, pt. 1: mod- 3. The AFT ages from the eastern Organ the San Andres Mountains . Sam Seek ifications and additions; in James, H. L. (ed.), San Mountains are 10 to 17 Ma and the helped us gain access to the San Luis Basin (Colorado): New Mexico Geological mean track lengths are long; the ages Andres Mountains on White Sands Society, Guidebook 22, pp. 191-201. on the west side are 20 to 29 Ma and Missile Range, and range riders Rod Chapin, C. E., and Cather, S. M., 1994, Tectonic set- the mean track lengths are shorter. A Pino and Chris Ortega provided ting of the axial basins of the northern and cen- westward-tilted partial-annealing zone for transportation during our visit. tral Rio Grande rift; in Keller, G. R., and Cather, Paul Bauer collected the Proterozoic S. M. (eds.), Basins of the Rio Grande Rift: apatite that formed during protracted Structure, Stratigraphy, and Tectonic Setting: cooling of the Organ batholith in the samples from the Sacramento Mountains. Pam Roths provided Geological Society of America, Special Paper 291, early Miocene to late Oligocene is 5-25 samples from the Little San Nicolas pp. . preserved in this mountain block. Rapid Chapin, C. E., Chamberlin, R. M., Osburn, G. R., denudation (200-400 m/m.y.) and and Dead Man Canyon areas in the San Sanford, A. R., and White, D. W., 1978a, Explora- tilting of the range occurred in the middle Andres Mountains and from the Franklin tion framework of the Socorro geothermal area, Miocene. Mountains. Thanks to Nancy New Mexico; in Chapin, C. E., Elston, W. E., and McMillan for her discussions James, H. L. (eds.), Field guide to selected caul- 4. The base of a poorly defined apatite concerning the McRae Formation. The drons and mining districts of the Datil-Mogollon partial-annealing zone that formed during samples were irradiated under the volcanic field, New Mexico: New Mexico Geo- Mesozoic burial of southern New DOE reactor share program at the logical Society, Special Publication 7, pp. 115-130. Mexico is preserved in the Texas A&M Nuclear Science Sacramento Mountains. The earliest Chapin, C. E., Jahns, R. H., Chamberlin, R. M., and Center. We appreciate the con- Osburn, G. R., 1978b, First-day road log from phase of extension is recorded in the structive reviews of Greg Mack and an Socorro to Truth or Consequences via Magdalena AFT data from Proterozoic rocks exposed anonymous reviewer. and Winston; in Chapin, C. E., Elston, W. E., and at the base of the Sacra mento James, H. L. (eds.), Field guide to selected caul- escarpment. Evidence for significant drons and mining districts of the Datil-Mogollon References denudation related to Laramide volcanic field, New Mexico: New Mexico Geo- logical Society, Special Publication 7, pp. 1-31. deformation, or late Oligocene and middle Abitz, R. J., 1986, Geology of mid-Tertiary volcanic Miocene extension is not observed in rocks of the east-central Black Range, Sierra the AFT data. County, New Mexico: Implications for a double

New Mexico Geology February 1997 13 Christiansen, R. L., and Yeats, R. L., 1992, Post-La- W., Karlstrom, K. E., and Harrison, B. (eds.), in the Jornada del Muerto: a region of crustal ramide geology of the U.S. Cordilleran region; in Geology of the Santa Fe Region: New Mexico thinning in the Rio Grande rift without upper Burchfiel, B. C., et al. (eds.), The Cordilleran Geological Society, Guidebook 46, pp. 87-96. crustal extension: Tectonophysics, v. 174, pp. Orogen: Conterminous U.S.: Geological Society Kelley, S. A., Chapin, C. E., and Corrigan, J., 1992, 183-195. of America, Geology of North America, v. G-3, Late Mesozoic to Cenozoic cooling histories of Reiter, M., Shearer, C., Edwards, C. L., 1978, Geo- pp. 261-406. the flanks of the northern and central Rio Grande thermal anomalies along the Rio Grande rift in Clemons, R. E., and Osburn, G. R., 1986, Geology rift, Colorado and New Mexico: New Mexico New Mexico: Geology, v. 6, pp. 85-88. of the Truth or Consequences area; in Clemons, Bureau of Mines and Mineral Resources, Bulletin Reiter, M., Eggleston, R. E., Broadwell, B. R., and R. E., King, W. E., Mack, G. H., and Zidek, J. 145,39 pp. Minier, J., 1986, Estimates of terrestrial heat flow (eds.), Truth or Consequences Region: New Mex- Laslett, G. M., Kendall, W. S., Gleadow, A. J. W., from deep petroleum tests along the Rio Grande ico Geological Society, Guidebook 37, pp. 69-81. and Duddy, I. R., 1982, Bias in measurement of rift in central and southern New Mexico: Journal Corrigan, J. D., 1991, Deriving thermal history fission-track length distribution: Nuclear Tracks, of Geophysical Research, v. 91, pp. 6225-6245. information from apatite fission-track data: v. 6, p. 79-85. Seager, W. R., 1981, Geology of the Organ Moun- Journal of Geophysical Research, v. 96, pp. Lozinsky, R. P., and Bauer, P. 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