cooling histories of mountain ranges in the southern … · cooling histories of mountain ranges in...

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, Capitan Mountains; SA, San Andres Mountains; OR, Organ Mountains; FM, Franklin Mountains; DA, Doña Ana Mountains; SD, San Diego Mountain; CAB, Caballo Mountains; MS, Mud Springs Mountains; UVAS, Sierra de las Uvas; CHUP, Chupadera Mountains; BR, Black Range. 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.

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

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

Abstract

Fifty-two apatite fission-track (AFT) and two zircon 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 Eocene, the late Oligocene, the middle Miocene, and the latest Miocene to early Pliocene, with each phase disrupting earlier rift basins and in some cases reversing the dip of the early rift half-grabens found in the vicinity of the southern Caballo Mountains. 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 erosion preserved in the Oligocene to Miocene Hayner Ranch and Rincon Valley Formations in the southern Caballo Mountains.

Each mountain block studied in the southern rift has a unique history. AFT ages in the Proterozoic rocks on the east side of the San Andres Mountains 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

episode of extension are exposed onthe upthrown side of high-angle faultscutting the Proterozoic rocks. AFT agesfrom the eastern Organ Mountains are10-17 Ma and the mean track lengths arelong; the ages on the west side are 20-29Ma and the mean track lengths are shorter.A westward-tilted part ia l -annea l ing zonefor apat ite that formed duringprotracted cooling of the Organ batholith inthe late Oligocene to early Miocene ispreserved in th is mounta in block.Rapid denudation (200- 400 m/m.y.) andtilting of the range occurred in the middleMiocene. The base of a poorly definedapatite partial-annealing zone thatformed du ring burial of southern NewMexico in the Mesozoic is preserved in theSacramento Mountains. The earliest phaseof extension is recorded in the AFT data fromProterozoic rocks exposed at the base of theSacramento escarpment. Evidence forsignificant denudation related toLaramide deformation or late Oligoceneand middle Miocene extension is notobserved in the AFT data from theSa cram ento Mounta ins . AFT da taderived from samples in Oligocene toMiocene intrusions (Capitan, SierraBlanca, and Black Range) record rapidcooling of these shallowly emplaced plutons.

The AFT data from the northern andsouthern Rio Grande rift are similar in manyrespects. The trend of young AFT agesand greater denudation on footwall blocksadjacent to the master faultscontrol ling the geometry of the ha lf -grabens tha t was observed in northernNew Mexico is also found in southernNew Mexico . Hors t blocks in the riftthat are bordered by normal faults ontwo or three sides invariably have young(<12 Ma) AFT ages and high (>10°C/Ma) cooling rates. In the places whereMesozoic AFT ages are preserved, theestimated cooling rates are low (<2°C /Ma)in both the northern and southern rift.The primary difference in the cooling historiesof mountain ranges in the northern andsouthern rift is the paucity of AFT agesthat are older than 30 Ma in the southernrift. Part of this trend may be a functionof sampling bias.

Zircon FT and K -Ar data are used toexamine the distribution of late Mesozoic toEocene volcanism in southern New Mexico.A previously unrecognized Cretaceousintrusion was identif ied southeast ofHillsboro using zircon FT dating. Theporphyry from this intrusion was thesource of volcanic clasts in the LateCretaceous to early Tertiary McRaeFormation in the northeastern CaballoMountains. In addition, a zircon FT age of49.6 ±3.8 Ma for the rhyolite sill cappingSalinas Peak at the north end of the SanAndres Mountains and K-Ar ages for theOrogrande, Cuchillo, and Tres Hermanosstocks are used to constrain the northernlimit of 40 Ma volcanism in south-central New Mexico.

Introduction

Recently, detailed geologicmapping and stratigraphic studies bySeager et al. (1984), Seager and Mack(1986), Mack and Seager (1990), andMack et al. (1994 a, b) haveconstrained the timing and orienta -tion of Cenozoic compressive andextensional events that affected themountain

ranges of the southern Rio Granderift near Las Cruces, New Mexico .First , Paleogene Laramide compressionformed west-northwest-trendingbasement uplifts bounded bysouthwest-dipping, high-angle reversefaults (Seager and Mack, 1986; Seageret al., 1986). This compress iona lphase occurred main ly in thePaleocene to early Eocene, althoughthere is evidence for deformation asearly as Late Cretaceous and as late aslate Eocene. Following Laramidedeformation, regional calc-alkalinevolcanism during the latest Eocene toOligocene blanketed large portions ofsouthern New Mexico with layers of ash-flow tuff, suggesting that the topo-graphic relief by the middleOligocene was not as large as it istoday. Volcanism was followed by, andwas in part coeval with, extension,leading to the development of the RioGrande rift, a north-trend ingcontinental r i f t that extends fromnorthern Colorado to northernMexico. On the basis of the analysis ofsedimentary deposits in the southernCaballo Mountains, Dona Ana Mountains,San Diego Mountain, and Sierra de lasUvas (Fig. 1), Mack et al. (1994a, b)proposed that the southern rift hasbeen affected by four episodes ofextension beginning at about 35 Ma.The main phases of faulting started inthe late Eocene, the late Oligocene, themiddle Miocene, and the latest Mioceneto early Pliocene, with each phasedisrupting earlier rift basins and in somecases reversing the dip of the early rifthalf-grabens [Note: The Eocene-Oligocene boundary used in this paperis approximately 33-34 M a asdo cu me n te d b y S w is he r a ndPro the ro (1990 ) and McIntosh eta l . (1992b)].

The nature of the early episodes of riftextension outside of the area studiedby Mack et al. (1994a, b) is not wellknown because much of the earlyMiocene to Ol igocene extensionalhisto ry of the southern rift is buriedbeneath younger deposits (Clemons andOsburn, 1986). Consequently, othertechniques must be used to locateareas affected by early extensionand to understand the timing and theamount of denudation associated withthese stages of de formation. Anapproach that gives spatial andtemporal data on the development ofvertical relief associated with rift formationinvolves determining the cooling history ofrocks in mountain ranges that boundthe rift using apatite fission-track (AFT)analysis (Green et al., 1989). Fission-track annealing in apatite is controlledprimarily by temperature and thechemical composition of the apatite(tracks in chlorapatite completelyanneal at higher temperatures(140°C) than those in fluorapatite(120°C); Green et al., 1986; Green, oralcomm. 1996). In a relatively stablegeological environment wheretemperatures as a function of depth arenow at a maximum, apatite fission-track (AFT) age and track lengths decrease

systematically with depth (Naeser, 1979;Fitzgerald et al., 1995). Attemperatures less than 60 to 70°C,tracks that are produced by thespontaneous fission of 238U areretained and annealing is relativelyminor. The AFT ages of detrital grains insedimentary rocks at shallow depths (tem-peratures <70°C) are equivalent toor greater than the stratigraphic ageof the rock unit and the mean tracklengths are on the order of 13 to 15 μm.The AFT ages in basement rocks at shallowdepth (temperatures <70°C) would reflectthe cooling history of the basement priorto the period of stabil ity, and, as withsedimentary rocks, the mean tracklengths would be relatively long (>13μm). In the temperature range knownas the partial-annealing zone (PAZ),which is between 60 -70°C and 120-140°C, depending on the composition ofthe apatite, the AFT ages and tracklengths are reduced compared to the originalAFT ages and lengths. Mean track lengths inthe PAZ are typically 8 to 13

2 February 1997 New Mexico Geology

μm. Finally, at temperatures above 120 to140°C, tracks that are formed are quicklydestroyed and the fission-track age is zero.If, after a period of relative stability, thecrust is rapidly cooled during denudationrelated to a tectonic event, the fossil PAZmay be exhumed and the time of coolingand the paleodepth of the top or bottom ofthe PAZ can be estimated (Gleadow andFitzgerald, 1987). If the tectonically dis-turbed area cools rapidly (>5°C /m.y.), themean track lengths in rocks that are at thebase of the fossil PAZ (below the break-in-slope on the age-depth profile) are gener-ally >14 μm, and the AFT ages from belowthe base of the exhumed PAZ can be usedto estimate the timing of the onset of de-nudation. If the rock column cools moreslowly during uplift and/or erosion, themean track length at the base of the fossilPAZ is typically <14 μm, and the apparentAFT age more loosely constrains timing ofthe initiation of cooling (Foster andGleadow, 1992).

Kelley et al. (1992) demonstrated that inareas undisturbed by late rift volcanism orhydrothermal activity, the timing of denu-dation of a rift flank (e.g., the LadronMountains north of Socorro) derived fromAFT data close ly f i ts the timing ofdenudation determined from thesediment record in the adjacent basinwhere early rift sediments have beenuplifted and exposed. Similarly, in thepresent study, we will examine thecooling histories of the mountain rangesin the vicinity of the early rift basinsdescribed by Mack et al. (1994a, b) andcompare our results with their provenancedata. Because the denudation data fromAFT analysis are consistent with theepisodes of uplift and erosion preserved inareas where the sediment record is wellstudied, the technique can be used todetermine the denudation history ofuplifts adjacent to undissected basinswhere the sediment record is poorlyunderstood.

Fission-track data collected in the north-ern Rio Grande rift of New Mexico andColorado indicate that rocks now exposedin the eastern and western margins of therift cooled during uplift, erosion, andwaning volcanism in a complex, three-dimensional pattern (Kelley et al., 1992).Three interesting trends have emergedfrom the AFT data for the northern RioGrande rift. First, the youngest AFT agesare found in the rift flanks adjacent tolarge normal faults along the deepmargins of the en echelon half-grabensthat make up the present Rio Granderift. Second, promontories that projectinto rift basins at the junction of arcuatenormal faults bounding the deep sideof half grabens (Blanca Peak in theSangre de Cristo Mountains and theLadron Mountains) have some of theyoungest AFT ages observed. Tectonicdenudation on three sides has allowedunusually rapid isostatic uplift in theseareas. Similarly, the

Sangre de Cristo Mountains north ofBlanca Peak are bounded on two sides byactive late Cenozoic faults that haveallowed the rapid isostatic rise of thesemountains. Finally, cooling rates deter-mined from the AFT age and length datahave increased progressively from theLaramide orogenic event (1-4°C/Ma) tothe development of the Rio Grande rift (7-20°C/Ma).

To see if these patterns are characteristic ofthe Rio Grande rift in general, samples forAFT analysis were collected frommountain ranges bordering and withinthe southern Rio Grande rift. The newlycollected AFT age and length data will beused to constrain the cooling histories ofportions of the eastern and western flanksand central horst blocks of the southernrift. The areas of study include theCaballo, Mud Springs, San Diego, Dona Anamountains (central horsts); the Black Range(western flank); the San Andres, Organ,Franklin Mountains (central horsts);and Sierra Blanca, Capitan Mountains,Sacramento Mountains (eastern flank).These new data add to our understandingof vertical motions as a function of time andgeographic location during compressionand extension in the southern Rio Granderift. In addition, the data can be used tocompare and contrast the tectonic andtopographic development of the northernand southern parts of the Rio Grande rift.Only two phases of extensionaldeformation are recognized in thenorthern rift north of Socorro whereasfour phases have been documented in thesouth. The southern rift has undergonemore extension compared to the north, asindicated by the wider zone of deforma-tion and the thinner crust (Keller et al.,1990). Early rift magmatism is sparse tothe north and more common to the south.Furthermore, rifting began about 7 to 8 Maearlier in the southern rift (Mack et al.,1994a, b; Chapin and Cather, 1994).

The principal objectives of this paperare (1) to present new apatite fission-trackages for rocks exposed in the uplifts alongthe eastern and western margins and inthe central horsts of the southern RioGrande rift (Table 1, Fig. 1); (2) to use theAFT data to discuss the local tectonic andcooling history of each mountain blockinvestigated; (3) to integrate the AFT datafrom all the mountain ranges and K-Ardata from intrusives in the southern riftinto a discussion of the tectonic develop-ment of this region during Laramide com-pression, mid-Cenozoic volcanism, andRio Grande rift formation; (4) to use the AFTresults to compare and contrast thedevelopment of the northern and south-ern Rio Grande rift.

In addition, as an experiment, weattempted to more tightly constrain theage of the Eocene Love Ranch Formationin the Jornada del Muerto (LR, Fig. 1) bydetermining the AFT ages of granite clasts

in the formation. The age of the LoveRanch Formation is currently thought tobe between late Paleocene and middleEocene, on the basis of radiometric ages ofthe overlying Rubio Peak Formation (40-43 Ma) and of the age of the youngestunderlying unit (Seager and Clemons,1975); few fossils have been found in theLove Ranch Formation (Seager et al.,1986). By assuming that the Proterozoicrocks from the Laramide Rio Grandeuplift, which acted as the source region forthe Love Ranch Formation, cooled duringdenudation and were transported imme-diately to the basin, we expect the clast AFTages to provide some constraints on thetiming of denudation in the RioGrande uplift and to provide a maximumage for deposition of the Love RanchFormation.

Fission-track analysis

Methods

The methods of fission-track analysisused in this study are fully described inKelley et al. (1992). Thirty-one samples ofProterozoic igneous and metamorphicrocks, Paleozoic sedimentary rocks, andEocene to Oligocene plutonic rocks werecollected along reconnaissance traversesthrough ranges within and bordering thesouthern Rio Grande rift (Table 1, Fig. 1). Inaddition, 17 samples were collected in amore detailed study of the Eocene Organbatholith. The criteria used in locating thetraverses included the occurrence of sig-nificant vertical relief and the presence ofrock types containing adequate amounts ofapatite for FT dating. Elevations of thesamples were determined from U.S.Geological Survey 1:24,000-scale topo-graphic maps to an estimated accuracy of ±6m.

Apatite and zircon were separated fromthe samples using standard heavy liquidand magnetic separation techniques. Apa-tite grains were mounted in epoxy, pol-ished to expose the grains, and etched for25 seconds in a 5 M solution of nitric acid toreveal the fission tracks. The zircons weremounted in Teflon, polished, and etched inNaOH/KOH at 230°C for 4.5 hours(90SA01) and 8 hours (90BR05). The grainmounts were then covered with muscovitedetectors and sent to the Texas A&MNuclear Science Center for irradiation. Theneutron flux was calibrated using thezeta technique with Durango apatite andFish Canyon zircon age standards andCorning standard glasses CN-5 and CN-6.The zeta calibration is based on the acceptedages of 27.9 ±0.7 Ma for the Fish CanyonTuff and 31.4 ±0.5 Ma for Durango apatite(Hurford and Green, 1983; Green, 1985).

The confined-track-length distributions inthe apatite grain mounts were determinedusing a microscope fitted with a 100-Xoil-immersion lens, a drawing tube, and adigitizing tablet. The system allows the tracklengths to be measured to approximately±0.2 μm. Horizontal, well-

New Mexico Geology February 1997 3

etched, confined tracks (tracks completelyenclosed within the crystal) in grains withprismatic faces were measured. Theorientation of the tracks with respect tothe c-axis was also measured. Time-temperature estimates were derivedfrom the age and track-length datausing the inverse-modeling algorithmof Corrigan (1991). The values of A, Q,and n used in this investigation arethe composite parameters of Carlson(1990), and an initial track length of15.7 μm is assumed for the models.

Results

The interpretation of the AFT data foreach traverse (Fig. 1) is considered indetail in the following sections. First, we willexamine the AFT data from the Caballo,Mud Springs, San Diego, and Dona AnaMountains and compare the resultswith the interpretations of Mack et al.(1994a, b). We will then discuss the AFTdata from areas where the Cenozoic upliftand sedimentation history are not aswell documented. The cooling ratesestimated from the AFT results are inTable 2. Unfortunate ly, because theuranium content of many of theapatites from the southern rift is low(<5 ppm), detailed track lengthanalysis and thermal modeling waspossible for only nine of the samples.

Caballo, Mud Springs, San Diego,Doña Ana mountains—The Cabal loMountains are an east-dipping faultblock with Proterozoic granitic andmetasedimentary rocks exposed onthe western side (Fig. 2). Paleozoic toMesozoic sedimentary rocks overlie theProterozoic rocks and form the easterndip slope into the Jornada del Muerto. ThePaleozoic sedimentary rocks that formthe crest of the range are dominatedby carbonates lacking apatite, so anage-elevation traverse through therange was not possible. A northwest- tonorth–northwest-trending Laramidebasement-cored uplift is preserved inthe Caballo Mountains; this uplift,which is related to the Rio Grande uplift(Seager and Mack, 1986), shed sedimentsnorthward into a broad Laramidebas in p rese rved as the Love RanchFormation in the modern Jornadade l Muerto. The southern end of the CaballoMountains once floored one of the early,broad rift basins (Seager et al., 1984). Thecooling history recorded by AFT datesfrom the Proterozoic rocks at lowelevation on the west side of the range isdominated by denudation during thelater phases of rift development. TheAFT ages are 5.4 ±0.8 Ma on thenorthern end and 10.8 ±1.8 to 16.1 ±7.3Ma in the south-central part of the range.

Mack et al. (1994a, b) noted that earlyunroofing of the Caballo Mountains isrecorded in the Hayner Ranch Formation(approximately 25 to 16 Ma). The lowerpart of this formation contains clastsfrom the youngest part of the stratigraphic


section (Oligocene Uvas BasalticAndesite and Bell Top tuff); the upperpart of the formation consists of clastsderived from the Eocene Palm Park andLove Ranch Formations. The basal partof the overlying Rincon Valley Formation(about 16 to 6 Ma) is similar to the HaynerRanch in that it includes Uvas, Bell Top,Palm Park, and Love Ranch clasts. Incontrast, in the middle to upper beds ofthe Rincon Valley Formation, clasts ofPermian Abo Formation andPennsylvanian Magdalena Grouplimestones are abundant. No Proterozoicgranite clasts are found in the Rincon Val-ley Formation, except for those thought tobe reworked from the Eocene Love RanchFormation. These observations are consis-tent with the AFT data. The average mod-ern heat flow in the Palomas Basin on thewest side of the mountains is about 95mW/m2 , and the average gradient is onthe order of 30 to 35°C /km (Reiter et al.,1986; Reiter and Barroll, 1990). Assumingthat the average heat f low has notchanged since extension began and as-suming a surface temperature of 15°C, thebase of the apatite PAZ (120°C) was at adepth of 3 to 3.5 km in the late Eocene. Onthe basis of the geologic cross-sectionA-A' of Seager et al. (1982), at least 3 km ofPaleozoic to Middle Cenozoic sedimenta-ry and volcanic rocks may have overlainthe Proterozoic rocks prior to denudationof the Caballo Mountains. The AFT agesfrom low-elevation Proterozoic rocks inthe south-central Caballo Mountainsrecord cooling through the PAZ for apatiteduring deposition of the Rincon ValleyFormation at the time the Proterozoicrocks were still in the subsurface. TheseProterozoic rocks were exposed at the sur-face later; Proterozoic detritus is commonin the early Pliocene to middle PleistocenePalomas Formation (Lozinsky and Haw-ley, 1986; Mack and Seager, 1990) in thePalomas Basin to the west. The decrease inAFT age to the north and depositionaltrends of Proterozoic detritus imply thatthe fault that was active during lateOligocene to Miocene time along thesoutheastern margin of the Caballo Moun-tains propagated northward during latestMiocene to Pliocene time (G. Mack, writ-ten comm. 1996).

The Mud Springs Mountains (Fig. 2) are asmall, east-dipping intrarift horst blockwith a small outcrop of Proterozoic meta-sedimentary rocks exposed along thesouthwest side; the metamorphic rocksare overlain by Paleozoic sedimentaryrocks. As in the case of the northern Ca-ballo Mountains, the AFT age of 8.4 ±3.8Ma for the Proterozoic rocks records rapidcooling during the later phases of rifting.

San Diego Mountain (Fig. 2) is anothersmall intrarift horst block. The northernmarginal thrust fault of the Laramide RioGrande uplift, which brought Proterozoicrocks near the surface, is exposed alongthe north side of the mountain(Seager,1986). The area subsequently

formed part of the early rift basinpreserved in the southern CaballoMountains. The early rift basin was thenbroken up during the later phases of riftdevelopment. The AFT age of 6.5 ±2.2 Maon Proterozoic granite appears to be theresult of cooling during the latest episodeof normal faulting in the late Miocene.Late Quaternary uplift of this block isrecorded by a 30-m offset in the CampRice Formation (Seager, 1986). Seager(1986) proposed that the highly silicifiedoutcrop of Camp Rice Formation thatrests on the Proterozoic granite representsa fossil hot-spring deposit. Anomalouslyhigh modern heat flow in a shallow-flowsystem northwest of San Diego Mountainwas observed by Reiter et al. (1978). Thewater in the fossil hydrothermal systemapparently was not hot enough, orthe system was so short lived, that it didnot completely reset the AFT ages toages younger than the Quaternary CampRice Formation.

One sample from a dacite dike on theeast side of the Dona Ana Mountains (Fig.3), which may be part of a small cauldronwithin the Organ cauldron (Seager andMcCurry, 1988), was collected to investi-gate the cooling history of this range. AnAFT age of 18.4 ±4.4 Ma was determined.Mack et al. (1994a, b) see evidence foruplift of the Dona Ana Mountains begin-ning with the deposition of the RinconValley Formation. Pennsylvanian PantherSeep Formation and Tertiary rhyoliteclasts that are unique to the Doña AnaMountains are found in the Rincon ValleyFormation but not in the underlyingHayner Ranch Formation. Because theAFT and provenance results from thesouthern Caballo area are compatible, wewill now apply this technique to areas inthe southern rift where the Oligocene toMiocene uplift and sedimentation historyare concealed by younger basin fill.

Black Range-The remnants of theEmory cauldron, which is part of the largeMogollon-Datil volcanic field, are expos-ed in the Black Range on the west side ofthe Palomas Basin (Fig. 2). Volcanismbegan in the area about 38 Ma and thecauldron formed with the eruption of thevoluminous 35.2 Ma Kneeling Nun Tuff(Elston, 1989; McIntosh et al., 1992a). Thecaldera floor was subsequently domedupward by resurgence, and the moatbetween the caldera wall and dome waslargely filled with younger lava and pyro-clastic flows from local and distantsources (Elston, 1989). Tuffs preserved inthe Black Range are 35.2 to 28.1 Ma(McIntosh et al., 1992a). Most of the faultsrelated to cauldron collapse have beenreactivated during rift-relateddeformation.

A Proterozoic granitic outcrop north ofKingston was sampled as p art of thisstudy, but this particular granite containsfluorite, not apatite so no AFT data are

available. The two highest-elevation sam-ples are from Tertiary andesitic intrusivessoutheast of Kingston. Maggiore (1981)determined a zircon FT age of 34.0 ±2.8Ma for the andesite porphyry sill at thelocation of 90BR04 (Table 1). The AFT ageof 36.6 ±8.1 Ma is concordant with the zir-con FT age and is indicative of rapid cool-ing of this shallowly emplaced sill. In con-trast, the dacite intrusive exposed atslightly higher elevation (90BR03) is sig-nificantly younger (19.4 ±2.2 Ma) than90BR04. The eastern margin of the Emorycauldron, which is located adjacent to thissample, has been reactivated a number oftimes following the collapse of the caul-dron (Abitz, 1986), complicating the cool-ing history of this area.

The two lower-elevation samples arefrom a small horst block east of Hillsboro.One sample (90BR01, Table 1), with anAFT age of 21.7 ±3.6 Ma, is from the 73.4±2.5 Ma (K-Ar age; Hedlund, 1974)Copper Flat stock. The long mean track-length measurements for this sample indi-cate rapid cooling. The other sample(90BR05), which is from a small outcroplabeled Tii on the map of Seager et al.(1982), surprisingly yielded Cretaceousapatite and zircon FT ages (Table 1).Because the zircon FT age of 68.0 ±5.4 Mais nearly concordant with the AFT age of63.7 ±4.5 Ma and the mean track lengthsare long (Table 1), the intrusive cooledrapidly (>40°C /Ma) from the blockingtemperature of 240 ±50°C (Hurford, 1986)for fission tracks in zircon to approximate-ly 120°C during the Late Cretaceous. Sinceabout 65 Ma, the cooling rate slowed to2°C /Ma. This outcrop has not been signif-icantly heated since the Cretaceous.

Despite the complex structures and agepatterns in this area, the AFT data providesome constraints on the cooling history ofthe Black Range. Much of the thermal his-tory of this area is related to cooling fol-lowing mid-Cenozoic igneous activity inthis region. The discovery of a previouslyunknown Cretaceous intrusion yielding a64 Ma AFT age suggests that post -Cretaceous burial of the horst block south-east of Hillsboro was not great enough toreset the AFT age, which is congruouswith the thin (<1 km) Cenozoic volcanicand sedimentary section preserved in thisarea (G. Mack, written comm. 1996). How-ever, the Copper Flat stock to the northremained in the subsurface and began tocool in the early Miocene. These observa-tions are corroborated by the distributionof volcanic clasts in the Late Cretaceous toearly Tertiary McRae Formation exposedin the northeastern Caballo Mountains.Clasts that are chemically identical to theporphyry in the recently discoveredCretaceous intrusion have been found inthis unit, but clasts from the Copper Flatstock have not been detected in the McRaeFormation (N. McMillan, written comm.1995).


San Andres Mountains—The SanAndres Mountains (Figs. 1, 3) are a west-tilted fault block, with Paleozoic sedimen-tary rocks (primarily carbonates) restingon

Proterozoic igneous and metamorphicrocks. Virtually nothing has beenpublished on the Cenozoic uplift history ofthe San Andres Mountains for tworeasons.

First, the range is bounded on both sidesby undissected basins, the Jornada delMuerto on the west and the Tularosa Basinon the east. Second, the San AndresMoun-


Mountains are within White SandsMissi le Range, so access is restricted.AFT samples were co l le cted fromthree trave rses through theProterozoic rocks exposed on the eastside of the San Andres Mountains. Inaddition, samples were collected fromthe undated rhyol i te s i l l e xposedon Salinas Peak near the north endof the range. The zircon FT age of therhyolite sill is 49.6 ±3.8 Ma (90SA01,Table 1); no apatite was recovered fromthe rhyolite.

The AFT ages of approximately 21 to22 Ma (Table 1; Fig. 3) were determinedfrom Proterozoic rocks at the northend of the range. A traverse throughDead Man Canyon (Fig. 3) in the centralpart of the range yielded AFT ages of 7 to22 Ma. The approximately 22 Ma agesare generally found in thestratigraphically highest part of theProterozoic section a short distancebelow the Cambrian Bliss Sandstoneand the 15 to 7 Ma ages are fromstratigraphically lower levels. Anexception to this generalization is90SA15 (8.9 ±2 Ma), which is locatedon the upthrown side of a west-dippingnormal fault that offsets Paleozoicand Proterozoic rocks in this area(Seager et al., 1982). The southern tra-verse is in the structurallycomplicated Little San NicolasCanyon area (Fig. 3; Roths, 1991) .Again, the ages in this canyon arebetween 8 and 21 Ma, but in thiscase, the youngest ages strictly coin-c ide with north -trending, h igh-angle faults cutting the Proterozoicgranites and gneiss. Roths (1991)mapped these faults as thrust faults,but the AFT data imply that thesefaults have been reactivated duringextension, with the youngest agesexposed on the upthrown block. Onepossible interpretation of thislimited AFT data set is that the SanAndres Mountains started coolingduring an early phase of extensionand that a later phase of rapidcooling is recorded in samples nearhigh-angle faults in the Dead ManCanyon and San Nicolas Canyon areas(Table 2).

Organ Mountains —The OrganMountains (Fig. 3) are, in large part,the erosional remnant of the lateEocene Organ cauldron. The Organbatholith intrudes Proterozoic rocks,Paleozoic sedimentary rocks, and theEocene Orejon Andesite, as well as thepyroclastic flows and lavas associatedw i th the fo rm at i on o f thecau ld ron (Seager, 1981). The Organbatholith cons i s t s o f f o u r p l u t o n i cp h a s e s , two hypabyssal phases, anda variety of silicic dikes (Seager,1981). The silicic magma thatformed the Organ bathol i th wasemplaced at shal low depths, andthe upper part of this magma bodyerupted to produce three large ash-flowtuff units. Approximately 3 km of tuffswere deposited and preserved in thesubsiding cauldron. These tuffs wereerupted between 35.7 to 36.2 Ma, onthe basis of 40Ar /39Ar ages and

paleomagnetic data of McIntosh et al.(1992a). The cauldron likely developedin an extensional setting because normalfaults that o ffset the tuffs areintruded by batholithic rocks (Seager andMcCurry, 1988). The final phase ofvolcanism in the area is represented by300 m of rhyolitic to trachytic lavas.Subsequently, the area has been blockfaulted and rotated about 15-20°W.Most of the mineralization in the OrganMountains is related to emplacement ofthe batholith (Seager, 1981), butmineralized veins and dikes that cutthe Sugarloaf Peak quartz monzonitein the northern part of the batholithmay have formed after these rockscrystallized. The dikes and quartzmonzonite have been intenselysericitized in the Organ miningdistrict in the northeastern part of thebatholith (Fig. 4).

The AFT ages from the Organ batholithrange from about 10 to 29 Ma. The agesdo not show a signi f icant increase

with increased elevation, but theages on the west side are generallyolder than those on the east side (Figs. 4,5), a trend that is cons is tent wi th thewestward d ip o f the batholith. Theage-elevation data and the track-lengthdata for the east-side samples (81OR05,06, 08, 21, 22) indicate a relativelysimple, rapid (7-11°C / Ma; Table 2, Fig.5) cool ing history for this part ofthe batholith. Sample 81OR15 at BaylorPass is significantly younger than theadjacent samples. The track-length datafor 81OR15 indicate that it cooledrapidly (12°C /Ma; Table 2). Track-length data for nearby samples81OR13 (13.6 ±1.3, n=24) and81OR20 (11.8 ±1.6, n=24; Fig. 5)suggest that the west-side samplesexperienced protracted coolingduring the late Oligocene prior torapid tilting and denudation of thebatholith 14 to 17 Ma. The sinuous


l ine on Fig. 4 marks theapproximate boundary between theuplifted, rotated zone of protractedcooling (PAZ) on the west s ide fromrocks that were more rapidly cooledon the east side.

Franklin Mountains -Pam Roths(oral comm. 1991) provided a sampleof Proterozoic Thunderbird rhyolitefrom high elevation on the FranklinMountains, a west-t i l ted fault b locksouth of the Organ Mountains (Fig.1). The AFT age for this low-uraniumapatite is 51.8 ±15.8 Ma. This relativelyold AFT age from high elevation suggeststhe possibility that an uplifted PAZmay be present in the Frank l inMountains. An age-elevation traverseis needed to investigate this possibility.

Sierra Blanca, Capitan, Sacramentomountains- Sierra Blanca (Figs. 1,3), wh ich l ie s no r th o f theSac ramento Mountains, forms partof the eastern margin of the rift and isthe highest mountain in the southernrift (3,658 m). Sierra Blanca consistsprimarily of Eocene (38 Ma) andesiticvolcanic rocks overlying Cretaceous toPaleocene sedimentary rocks. FourOligocene-Eocene (25-38 Ma) syenite-monzonite-granite stocks intrude the sed-imentary and volcanic rocks(Thompson, 1972; Moore et al., 1991).The AFT data from the stocks can beused to examine the post-Oligocenecooling history of this part of theeastern flank of the rift. Apatite ages forthe syenite porphyry part of the ThreeRivers stock at high elevation are 26.5±5.1 and 32.8 ±5.2 Ma (905B03 and905B04; Fig. 3). These AFT ages are nearlyconcordant with the 29.9 ±1.8 Ma K-Ar

age on amphibole determined by Moore etal. (1991), indicating rapid cooling ofthis intrusion at shallow levels in thecrust. Similarly, an AFT age of 23.8±2.6 Ma and high-mean-track lengthof 14.4 ±0.4 μm (Table 1) for the BonitoLake stock (26.6 ±1.4 Ma K-Ar onbiotite; Moore et al., 1991) suggestrapid cooling of this intrusion(90SB02; Fig. 3).

In contrast to the high -elevationsamples with their rapid, simplecooling histories, the lower-elevationsamples had a more protracted, andperhaps complicated, cooling historyfollowing emplacement. The AFT datesof 17.5 and 22.6 Ma reported by Allenand Foord (1991) for Black Mountainstock (BMS, Fig. 3) are significantlyyounger than the 37.8 and 34.6 Ma40Ar/39Ar ages on hornblende andplagioclase, respectively, for thisintrusion and its associated dikes. Notrack-length data were presented forthese samples. An attempt was made todetermine the AFT age of a syeniteporphyry dike near Three Riverscampground along the western baseof Sierra Blanca, but the apatite con-tains abundant defects that obscuredthe fission tracks.

The 28 .3 ±0 .1 Ma Cap i tanp lu ton (Campbell et al., 1994) liesnortheast of Sierra Blanca (Fig. 1). Thislarge granitic batholith is orientedeast-west and parallels the Capitanlineament (Chapin et al., 1978a).Fluid-inclusion data indicates that theCapitan pluton, the top of which nowstands about 1 km above the surroundingplains, was intruded beneath at least 1km of rock (Al len and McLemore ,

1991; Campbell et al., 1994). An AFTage from low elevation on the easternmargin of the pluton is 27.9 ±4.3 Ma.Because the AFT age is concordantwith the 40Ar/ 39Ar age on feldspar,this shallowly emplaced pluton cooledrapidly.

The Sacramento Mountains (Figs. 1,3), an east-tilted fault block thatmarks the transit ion from thesoutheastern Rio Grande rift to theGreat Plains, are dominated byPaleozoic carbonates, so theopportunity for collection of samples withabundant apatite and zircon issomewhat limited. Attempts to recoverapatite from sandstone lenses in theOrdov ic ian, Pennsylvanian, andPermian carbonate sections exposedon the west side of the range wereunsuccessful. Apatite was found inProterozoic metasedimentary rockssouth of Alamogordo at the base ofthe west-facing Sacramentoescarpment, in the Permian AboFormation near High Rolls, and inProterozoic intrusive rocks in the Bentdome. Although there is insufficientmaterial for a complete traversethrough the range, the datable samples dop r o v i d e s o m e i n f o r m a t i o n o nt h e Cenozoic cooling history of thismountain block.

The highest-elevation sample, 90SAC04(Table 1, Fig. 3), which is from thePermian Abo Formation, has an AFTage of 112.7 ±25.3 Ma. On the basisof limited track -length data (11.4 ±2.8μm; n=12), this rock unit likely residedin the PAZ for a sub-


stantial period of time prior todenudation. The Proterozoic rocks inthe Bent area to the north have AFTages that are in the 52 to 67 Marange, and the mean track lengthsare also short (10.1 ±2.6 μm; n=13). Likethe Abo samples, these age andlength data are characteristic ofsamples found in a PAZ that likelyformed at a time of relative tectonicstability during Mesozoic burial of thearea (Fig. 6). Bowsher (1991) proposed,on the basis of facies distributionpatterns near Sierra Blanca, thatuplift in the Sacramento Mountainsmay have started in the earlyMesozoic; however, the AFT age andlength data indicate that littledenudation was associated with thisminor disturbance. The 35 to 41 MaAFT ages and the mean track lengthof 16.3 ±4.2 μm (n=3) from the baseof the western Sacramentoescarpment can be used to infer thatthe Mesozoic PAZ was brought towardthe surface by uplift and erosionduring the earliest phase of extensionand was brought to the surface by thelatest phase of rifting. The preservationof old ages at high elevation (Fig. 6)indicate that this mountain range wasnot strongly uplifted early in thedevelopment of the rift. Further workon the Permian red beds in this area isneeded to test this interpretation.

An interesting deposit that appearsto be an intensely altered volcanic ashwas discovered in a sinkhole in thePermian Yeso Formation on theoutskirts of Cloud-croft in theSacramento Mountains. Althoughthis fine-grained deposit contains noidentifiable glass shards or zircon, itdoes contain subhedral grains ofapatite with an AFT age of 49.3 ±11.7 Ma(CL ash, Table 1). This age is similar to theK-Ar age of the oldest intrusive in the

Orogrande area (Table 3) and to theage of dikes and sills in theSacramento Mountain escarpmentwest of Cloudcroft (45.3 ±2.2 Ma,Asquith, 1974; Fig. 1).

Discussion

The reconnaissance nature of thisstudy and the lack of datable rocks athigh elevations in several of themountain ranges in the southernri f t preclude detailed analysis of thetiming of episodes of extension andrates of denudation associated witheach tectonic event that affectedsouthern New Mexico. Despite the short-comings of the available data, theresults do permit some qualitativeanalysis.

Laramide deformation

In contrast to data from thenorthern Rio Grande rift and theSouthern Rocky Mountains provinces(Kelley et al., 1992; Kelley and Chapin,1995), cooling related to Laramidedeformation is not well represented in thesouthern rift AFT data. The preservationin the Sacramento Mountains of aprobable uplifted PAZ that formedduring Mesozoic burial and the lack of50 to 70 Ma ages with relatively longmean track lengths like those found inthe Front Range of Colorado (Kelleyand Chapin, 1995) suggests thatLaramide deformation was relativelyminor along the eastern margin of therift. The few minor folds and faults ofpossible Laramide age that arepresent in the western escarpment of theSacramento Mountains apparently didlittle to disrupt the Mesozoic P AZ.Elsewhere in the southern rift, coolingrelated to Laramide deformation hasbeen overprinted by later volcanism andextension.

Late Cretaceous to middle Eocenevolcanism

Late Cretaceous to Paleocenevolcanic centers are rare in central andnorthern

New Mexico. Two acknowledged Lara-mide-aged intrusive centers in thesouthern rift are the Salado Hillsstock in the Cuchillo Mountains(Chapin et al., 1978) and the CopperFlat s tock (Hedlund, 1974) in theBlack Range. The identification of apreviously unknown Cretaceous intrusionin the Black Range is of interest fortwo reasons. The first is economic,given the intrusion's proximity tothe Copper Flat porphyry copperdeposit. Second, the source of thevolcanic clasts in the LateCretaceous to early Tertiary McRaeFormation along the eastern flank ofthe Caballo Mountains has been thesubject of some debate (N.McMillan, written comm. 1993). Thisintrusion (or one like it) appears tobe the source for the clasts. Furthergeochronologic studies on the ea stside of the Black Range may helpidentify additional Cretaceousplutons.Early to middle Eocene volcanism is notcommon in northern and centralNew Mexico but is widespread insouthern New Mexico and in the Trans-Pecos belt of west Texas. The K-Ar agesfor four intrusions in southern NewMexico (Table 3, Fig. 1 ) and thedikes and si l ls east o f Alamogordo(Asquith, 1974), the 49.6 ±3.8 Mazircon FT age on the Salinas Peak rhy-olite sill in the San Andres Mountains,and the 49.6 ±11.7 Ma age on theprobable tuff in the sinkhole nearCloudcroft further establish thelimits of Eocene volcanism. The dashedline of Fig. 1 represents thenorthernmost extent of pre -40 Mavolcanism in south-central NewMexico. As noted by Christiansenand Yeats (1992) and Humphreys(1995), the age of volcanism decreasesprogressively from 40 to 50 Ma in westTexas, northern Mexico, and southernNew Mexico to about 21 Ma in southernNevada along a west—northwestwardtrend across the southern Basin andRange province. The new ages helpconstrain the post-Laramide geometry ofthe Fara l lon s lab beneathsouthe rn New Mexico (Severinghausand Atwater, 1990; Humphreys, 1995).


Rift extensionThe oldest of the four phases of

extension is recorded by the low-elevation AFT data in the SacramentoMountains. Given the preservation ofthe older AFT ages in the SacramentoMountains, little denuda tion isassociated with this event. Theextensional episodes that began in the lateOligocene and middle Miocene were notobserved in the SacramentoMountains, but waning stages of thelate Oligocene episode may bereflected in the cooling history of theBlack Mountain stock south of SierraBlanca. In contrast, cooling related to theearliest phase of extension is not observedin the AFT data from the other mountainranges in this study; instead the dataare dominated by the affects of thela te Ol igocene and middle Mioceneepisodes of extension. The cooling ratesof 7 to 12°C /m.y. associated withmiddle Miocene extension in theOrgan Mountains imply denudation ratesof 200 to 400 m/m.y., assuming ageothermal gradient of 30°C/km. Thelatest phase of extension, which beganin the latest Miocene to Pliocene, ispoorly represented in the AFT databecause of the lag between the timethat cooling commences at depths of 2 to3 km and the time that the rocksreach the surface. The late Mioceneages in the northern Cabal lo , MudSpr ings , San Andres, and San Diegomountains suggest rapid denudation(250-600 m /m.y.) associated with thelatest phase of rifting.

The trend o f young AFT ages andgreater denudation on the margin ofthe fault blocks adjacent to the deepside of the half-grabens observed innorthern New Mexico is also foundin southern New Mexico. For example,using gravity data and limited drill -hole information, Lozinsky and Bauer(1991) found that the Tularosa Basin isunderlain by two sub -parallel half-grabens. The eastern half-grabenadjacent to the Sacramento Mountainsis a relatively shallow bench tilted tothe east (maximum of 1200 m of fill).The western half-graben adjacent to theSan Andres-Organ Mountains isdeeper (at least 1,800 m of fill) and istilted to the west (Lozinsky and Bauer,1991). The AFT ages of 7 to 22 Ma arepresent in the San Andres Mountainsadjoining the deep side of the westernhalf-graben. In contrast, the AFT agesare 35 to 112 Ma in the SacramentoMountains. The AFT data from theSierra Blanca and Capitan intrusivecenters reflect rapid cooling of theseshallowly emplaced plutons. In thePalomas Basin, the youngest ages are inthe Caballo and Mud SpringsMountains, next to the maste r fau l tfo r the Pa lomas Bas in . Although theAFT age distribution in the Black Rangeis complex, all of the ages are older than theAFT ages in the Caballo Mountains.Furthermore, the central horst blocksbounded by normal faults on two o rt h r e e s i d e s , s u c h a s S a n D i e g o

Mountain or the Mud SpringsMountains, have much younger agesthan the uplifts on the flanks of the rift.

Comparison of northern andsouthern rift

Many of the trends observed inthe northern Rio Grande rift are alsopresent in the southern rift. Asdiscussed above, the response of thefault blocks adjacent to large faults inthe half-grabens that form the RioGrande rift is similar in both thenorthern and southern rifts. The coolingof the mountain blocks is contro l ledby denudation related to flexuralisostatic uplift of the fault-boundedranges (Kelley et al., 1992). Intrarifthorst blocks in the southern rift thatare bordered by normal faults on two orthree sides, like San Diego Mountain orMud Springs Mountains, invariablyhave young (<12 Ma) AFT ages and high(>10°C /Ma) cooling rates. In the northernrift, AFT ages of 10 to 20 Ma and rapidcooling rates are commonly associatedwith blocks bounded on two or moresides by normal faults. Furthermore, inthe few places where Mesozoic AFT ages arepreserved, the estimated cooling ratesare low (<2°C /Ma) in both the northernand southern rift.

Figure 7 summarizes the AFT datafrom the southern and northern rift.As noted by Chapin (1971) andBaldridge et al. (1983), the Rio Granderift widens south of the LadronMountains. The rift consists of a singlehalf-graben at any given latitude northof the Ladron Mountains, but it ismade up of nested or multiple half-grabens to the south. Consequently,the Ladron Mountains were used as theboundary between the northern andsouthern rifts. The primary differencein the cooling histories of mountainranges in the northern and southernrift is illustrated in the his tograms ofFig. 7. The AFT ages that are older than30 Ma are more common in the northernRio Grande rift than they are in thesouthern rift. Part of this trend may be afunction of sampling bias. The southernrift has not been as extensively sampledas the northern rift, and most of thesouthern-rift samples are from the intrarifthorst blocks. Furthermore, the upperparts of the Sacramento, San Andres, andCaballo Mountains are composedprimarily of Paleozoic carbonates thatlack apatite for AFT dating. If datablerocks were present along the crests ofthese ranges, older AFT ages like thosefound in the northern rift wouldprobably be observed. Although partof the trend is related to samplingbias, the combined effects of regional vol-canism and greater extension in thesouthern rift have apparently led to thelack of preservation of ages older than30 Ma in the southern rift.

The dashed lines in Fig. 7A depictthe timing of the initiation of eachepisode of rifting found in the southernCaballo Mountains. As discussed

above, cooling related to the earliestand latest phases of rifting is not wellrepresented in the AFT data, but thelate Oligocene and middle Mioceneepisodes are prominent in Fig. 7A,suggesting that larger amounts ofdenudation occurred during thesephases of extension compared with theearliest episode. Similarly, the rift -related AFT ages from the northern riftcluster in the late Oligocene to middleMiocene range.


Love Ranch clasts

The original intent of this part of theinvestigation was to try to putconstraints on the age of the Love RanchFormation. Unfortunately, the AFTresults do nothing to clarify the age ofthe Love Ranch; however, the data canbe used to address the thermalhistory of the southern Jornada delMuerto. The AFT ages of three graniticclasts from the Love RanchFormation range from 13.2 ±3.1 Ma to22.7 ±0.9 Ma, significantly younger thanthe stratigraphic age of the unit. Thespread in ages may be related todifferences in apatite compositionamong the clasts. The high-mean-track lengths for these samplesindicate that the clasts were heatedto temperatures above 120°C followingdeposition in the basin, completelyannealing pre-existing fission tracks.Heating of the samples occurred duringburial by about 2 km of volcanic andvolcaniclastic rocks in a part of theJornada del Muerto that has elevatedheat flow (115-127 mW / m2; Reiter andBarroll, 1990). The samplessubsequently cooled relatively rapidlyduring exhumation in the early to middleMiocene.

Conclusions1. The AFT data from the Caballo, Mud

Springs, San Diego, and Doña Anamountains and the sedimentologicdata from the Oligocene to MioceneHayner Ranch and Rincon ValleyFormations (Mack et al., 1994a, b) areconsistent.

2. The 21 to 22 Ma AFT ages in theProterozoic rocks on the east side ofthe San Andres Mountains record coolingin response to the phase of extensionthat began in the late Oligocene.Younger AFT ages of 7 to 8 Ma relatedto the middle Miocene episode ofextension are exposed on the upthrownside of high-angle faults cutting theProterozoic rocks.

3. The AFT ages from the eastern OrganMountains are 10 to 17 Ma and themean track lengths are long; the ageson the west side are 20 to 29 Ma andthe mean track lengths are shorter. Awestward-tilted partial-annealing zone forapatite that formed during protractedcooling of the Organ batholith in theearly Miocene to late Oligocene ispreserved in this mountain block. Rapiddenudation (200-400 m/m.y.) andtilting of the range occurred in the middleMiocene.

4. The base of a poorly defined apatitepartial-annealing zone that formed duringMesozoic burial of southern NewMexico is preserved in theSacramento Mountains. The earliestphase of extension is recorded in theAFT data from Proterozoic rocks exposedat the base of the Sacra mentoescarpment. Evidence for significantdenudation re lated to Laramidedeformation, or late Oligocene and middleMiocene extension is not observed inthe AFT data.

5. The AFT data derived from samplesin Oligocene to Miocene intrusions(Capitan, Sierra Blanca, and BlackRange) record rapid cooling of theseshallowly emplaced plutons.

6. The AFT data from the northern andsouthern Rio Grande rift are similarin many respects. The trend of youngAFT ages and greater denudation onthe margin of fault blocks adjacent tothe master faults controlling the geometryof the half-grabens that was observedin northern New Mexico is alsofound in southern New Mexico. Horstblocks in the rift that are bordered bynormal faults on two or three side sinvariably have young (<12 Ma) AFTages and high (>10°C /Ma) coolingrates. In the places where MesozoicAFT ages are preserved, theestimated cooling rates are low (<2°C/Ma) in both the northern andsouthern rift. The primary differencein the cooling histories of mountainranges in the northern andsouthern rift is the paucity of AFTages that are older than 30 Ma in thesouthern rift. Part of this trend may be afunction of sampling bias.

7. A previously unrecognizedCretaceous intrusion was identifiedsoutheast of Hillsboro using zircon FTdating. The porphyry from thisintrusion was the source of volcanicclasts in the Late Cretaceous to earlyTertiary McRae Formation located in thenortheastern Caballo Mountains.

8. A zircon FT age of 49.6 ±3.8 Ma on therhyolite sill exposed on Salinas Peak at thenorth end of the San AndresMountains and K-Ar ages of theOrogrande, Cuchillo, and TresHermanos stocks constrain thenorthern limit of pre-40 Ma volcanism insouth-central New Mexico.

ACKNOWLEDGMENTS—This project wassupported by NSF grant EAR8804203and by the New Mexico Bureau of Minesand Mineral Resources. Thanks toWilliam Seager for donating some ofhis samples from the Organ batholithand for his help in collecting samples inthe San Andres Mountains . Sam Seekhelped us gain access to the SanAndres Mountains on White SandsMissile Range, and range riders RodPino and Chris Ortega providedtransporta t ion during our vis i t .Paul Bauer collected the Proterozoicsamples from the SacramentoMountains. Pam Roths providedsamples from the Little San Nicolasand Dead Man Canyon areas in the SanAndres Mountains and from the FranklinMountains. Thanks to NancyMcMillan for her discussionsconcerning the McRae Formation. Thesamples were irradiated under theDOE reactor share program at theTexas A&M Nuclear ScienceCenter. We appreciate the con-structive reviews of Greg Mack and ananonymous reviewer.

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