Mesozoic–Paleogene Structural Evolution of the Southern U.S. Cordillera As Revealed in the Little and Big Hatchet Mountains, GEOSPHERE; V

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GEOSPHERE Mesozoic–Paleogene structural evolution of the southern U.S. Cordillera as revealed in the Little and Big Hatchet Mountains, GEOSPHERE; v. 14, no. 1 southwest New Mexico, USA doi:10.1130/GES01539.1 Christopher A. Clinkscales1 and Timothy F. Lawton2 1 12 figures; 1 table; 1 supplemental file Department of Geosciences, University of Arizona, 1040 4th Street, Tucson, Arizona 85721, USA 2Centro de Geociencias, Universidad Nacional Autónoma de México, Boulevard Juriquilla 3001, Campus Juriquilla, Juriquilla, QRO 76230, México

CORRESPONDENCE: clinkscales@email.arizona.edu ABSTRACT Upper Eocene–Oligocene ignimbrites and volcaniclastic rocks of the Boot Heel volcanic field of southwestern New Mexico unconformably overlie Lara- CITATION: Clinkscales, C.A., and Lawton, T.F., 2018, Mesozoic–Paleogene structural evolution A Mesozoic to Paleogene polyphase tectonic model presented here for mide syntectonic strata and bury eroded Laramide structures. The distribution of the southern U.S. Cordillera as revealed in the the southern United States (U.S.) Cordillera provides new insight into style of the Paleogene volcanic rocks in the Little Hatchet and Big Hatchet Moun- Little and Big Hatchet Mountains, southwest New and timing of Mesozoic–Paleogene deformation and basin formation in the tains is in part controlled by synmagmatic east-west and northwest-south- Mexico, USA: Geosphere, v. 14, no. 1, p. 162–186, doi:10.1130/GES01539.1. region south of the Colorado Plateau and Mogollon-Datil volcanic field. The east normal faults active from ca. 34 to 27 Ma, the age range of rhyolite dikes model proposes reverse reactivation of Jurassic normal faults during Late intruded along the faults. Two generations of intrusive rocks occupy these

Science Editor: Shanaka de Silva Cretaceous Laramide shortening. It also recognizes late Paleogene east-west– normal faults in the Little Hatchet Mountains: (1) older (ca. 34 Ma) phaneritic Associate Editor: Michael L. Williams and northwest-southeast–trending normal faults formed during a north-south stocks and dikes in the central and southern parts of the range, and (2) younger extensional event that postdated Laramide shortening and preceded Neogene (31–27 Ma) aphanitic latite and rhyolite dikes. East-west–trending faults and Received 3 April 2017 Basin and Range extension. dikes are cut by north-south faults formed during Basin and Range extension. Revision received 23 August 2017 Late Jurassic to Early Cretaceous extension generated northwest-south- The late Eocene–early Oligocene north-south extension provides an important Accepted 20 October 2017 Published online 20 December 2017 east normal faults that formed part of the Border rift system that extended minimum age limit for Laramide shortening, which ended prior to ca. 34 Ma. from southern California to the northwestern Gulf of Mexico. The normal faults cut Mesoproterozoic basement rocks, and localized subsequent uplift of basement rocks during Late Cretaceous fault reactivation that formed north- INTRODUCTION west-southeast–trending Laramide uplifts of southwest New Mexico and southeastern Arizona. The Hidalgo uplift, reconstructed here from structural A comprehensive synthesis of tectonic mechanisms for the Mesozoic– relations in the Little Hatchet and Big Hatchet Mountains of southwestern Cenozoic evolution of the western United States (U.S.) Cordillera requires New Mexico, is bound by bivergent reverse faults that resulted from tec- consideration of the southern U.S. Cordillera. The southern U.S. Cordillera is tonic inversion of a Jurassic–Early Cretaceous graben. The Hidalgo uplift is defined here to encompass the region north of Mexico and south of the Colo- flanked to the north by the Campanian to earliest Maastrichtian Ringbone ba- rado Plateau and Mogollon-Datil volcanic field, a geographic realm now occu- sin, which accumulated synorogenic continental strata and basaltic andesite pied by the Basin and Range province of southeastern Arizona and southern OLD G flows from ca. 75 to 70 Ma. The Ringbone basin was converted from a subsid- New Mexico (Figs. 1 and 2). Prevailing models for the Late Cretaceous–Paleo ing basin in the Little Hatchet Mountains to a volcanic center by ca. 69 Ma, the gene tectonic framework of North America commonly focus on the central emplacement age of an assemblage of shallow, subvolcanic intrusions termed Rocky Mountains of Colorado, Utah, Wyoming, and Montana, the Great Basin the Sylvanite plutonic complex. The basement-involved structural style and of Nevada and Utah (e.g., Dickinson, 2006), and the metamorphic core com- OPEN ACCESS yoked intermontane basin resemble other Laramide uplifts and basins in the plexes of Arizona (e.g., Davis, 1980; Dickinson, 1991). Studies have extensively Rocky Mountain Cordillera and refute alternative Laramide models of strike- addressed various geologic aspects of the southern U.S. Cordillera and Rio slip faulting or regionally extensive horizontal thrust faults in southwestern Grande rift area, from Mesozoic deformation, magmatism, and sedimentation New Mexico, the latter of which fail to account for basement-cored uplifts. A (e.g., Seager et al., 1986; Mack et al., 1986; Lawton and McMillan, 1999; Lucas significant difference with the Rocky Mountain Laramide province is the size and Lawton, 2000; Seager, 2004; Amato et al., 2017) to Paleogene magma- of the uplifts and basins and the close association of southern U.S. Cordilleran tism and deformation (e.g., McIntosh and Bryan, 2000; Copeland et al., 2011); This paper is published under the terms of the structures to nearby Late Cretaceous magmatic centers, which contributed to however, the Mesozoic to Paleogene kinematic history of southwestern New CC‑BY-NC license. interstratified volcanic and volcaniclastic rocks in the basin fill. Mexico lacks a comprehensive geodynamic synthesis.

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flat slab corridor Montana 45° N 45° N

Idaho

Figure 1. Late Jurassic (J) to Cretaceous Wyoming (K) tectonic element map. Select tectonic elements of the western USA Cor- Nevada dillera consist of the following Meso zoic features: (1) approximate location 40° N Laramide 40° N of the Laurentian-Farallon trench plate Forearc Uplifts boundary from Jurassic to Cretaceous; Nevadaplano (2) regional extent of the Mesozoic Cretaceous Subduction Zone Cordilleran magmatic arc and associ- Sevier Orogen ated forearc region (Dickinson, 2006); Thrust Front (3) strike of the Sevier orogenic front Utah Colorado (Dickinson, 2006); (4) location and extent of the Late Jurassic to earliest Arizona New Mexico Late Cretaceous Mogollon Highlands and associated Border rift, with names Colorado slab corridor of main rift basins (Lawton, 2004); and Mogollon Highland flat 35° N Plateau 35° N (5) orientation and location of major basement-cored Late Cretaceous Lara- Fig. 3 mide uplifts (Cross, 1986; Saleeby, 2003; s Seager, 2004). The boundaries of the McCoy Basin flat slab corridor are modified from the flat slab margin interpreted extent of Laramide flat slab Mesozoic to very low-angle subduction of Weil NE-SW Rift Shoulder and Yonkee (2012). The Late Cretaceous Cordilleran Texas Tarahumara arc is noted with italicized Bisbee Basin text. Extent of the Colorado Plateau in Magmatic Arc Chihuahua green. The location of the subregional 30° N Ta L 30° N study area is marked by the red rec rahumaraate K Late J - K tangle (Fig. 3) and occupies the area F Trough Legend arallon Plat referred to in text as the southern U.S. Arc Border Rift Sevier Orogen Cordillera. Thrust Front Sonora e Laramide Orogen Chihuahua Uplift-bounding faults Sabina s Basi Laramide Orogen n Monocline Sinaloa Coahuila 25° N Trajectory Durango Migration Direction of Mesozoic Magmatic Arc of Farallon Plate 500 km Zacatecas

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Metamorphic Core Complex t

50 Ma Montana 45° N olcanic Fron 45° N

Migrating V Idaho 45 Ma

Wyoming Nevada 40 Ma 40° N Laramide 40° N Figure 2. Paleogene tectonic element map. Uplifts Orange dashed lines denote the strike of 35 Ma the Paleogene magmatic arc and associated ages; arrows point in the direction 30 Ma of arc migration (Dickinson, 2002, 2006). 25 Ma San Juan Volcanic Field The Juan de Fuca and Guadalupe plates 20 Ma were formerly contiguous and constituted Utah Colorado the Farallon plate. By the Paleogene, flat Juan de Fuca Neogene slab and normal subduction transitioned Arizona New Mexico Plate amagmatic corridor to a period of slab rollback, which initi- Colorado ated the west-southwest migration of arc magmatism. The subregional map 35° N Mendocino 35° N Mendocino Fracture Zo Plateau (Fig. 3) occupies the area between the late ne Triple Junction Rio Grande Rift (ca. 25 Ma) migrating north Mogollon-Datil Eocene–Oligocene Mogollon-Datil and Fig. 3 Axis Boot Heel volcanic fields (McIntosh et al., Volcanic Field 1992). Black circular to elliptical domains indicate the locations of Cenozoic (ex- Pacific Plate Rivera 20 Ma tensional) metamorphic core complexes. Triple Junction migrating south The relative location of the Mendocino Texas and Rivera triple junctions, and associ- Ocean Ridge/ ated ocean ridge and transform zones, are Transform Zone Boot Heel shown for ca. 25 Ma (Dickinson, 2002). ( ca. 25 Ma) 30° N 30 Ma 30° N 25 Ma Volcanic Field

Legend ~35 Ma Guadalupe Plat Sevier Orogen Sonora Thrust Front Laramide Orogen Chihuahua Coahuila Uplift-bounding faults e Laramide Orogen Sinaloa Monocline 25° N Migration Direction of Durango Paleogene–Neogene Volcanic Front

Paleogene–Neogene Zacatecas Metamorpic Core Complex 500 km

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We define the Late Cretaceous to Paleogene southern U.S. Cordillera as genic plateau and hinterland, named the Nevadaplano (DeCelles, 2004); and a region characterized by narrow basement-cored uplifts and intermontane (4) the Sierra Nevada volcanic arc and forearc region (Dickinson, 2006). To basins south of the Colorado Plateau. Laramide basement uplifts and adja- the south of the central Rocky Mountains, in present-day Arizona and south- cent basins in the southern U.S. Cordillera resemble those of the central Rocky eastern California, the Sevier fold-thrust belt terminated along strike near the Mountains; however, some striking differences exist. The uplift-to-basin wave- northwestern extent of the Mogollon Highlands. These highlands represent length of structures in southern New Mexico is ~50% smaller than contempo- the northern rift shoulder of the Late Jurassic to Early Cretaceous Border rift raneous structures in central Wyoming. Furthermore, magmatism accompa- system, which included the McCoy, Bisbee, Chihuahua, and Sabinas basins nied shortening. Basement-involved shortening occurred inboard of the active and ultimately connected to the nascent Gulf of Mexico basin (Bilodeau, Tarahumara volcanic arc in Sonora, Mexico (Fig. 1; González-León et al., 2011), 1982; Dickinson et al., 1986; Lawton and McMillan, 1999; Stern and Dickinson, and active plutonic centers occurred adjacent to the flanks of uplifts and vol 2010; Fig. 1). canic complexes developed within Laramide intermontane basins. Late Jurassic through mid-Late Cretaceous deformation varied along Two contiguous mountain ranges in southwesternmost New Mexico, the strike in the southwestern U.S., with shortening in the Basin and Range re- Little Hatchet and Big Hatchet Mountains, referred to herein as the Hatchet gion (Sevier orogen) passing southward to extensional deformation in the ranges, contain well-exposed structural features that define a tectonic style southern U.S. Cordillera. A Late Jurassic to Early Cretaceous belt of exten- representative of the southern U.S. Cordillera (Fig. 3). These ranges host a sional basins, collectively termed the Border rift, trended northwest-south- relatively complete stratigraphic section of Paleozoic to Mesozoic rocks, the east from present-day southern California, southern Arizona, and New Mex- thickest Laramide syntectonic stratigraphic section in southern New Mex- ico (Fig. 4), and extended into northeastern Mexico (Dickinson et al., 1986; ico, a suite of dated Precambrian and Mesozoic rocks and Paleogene dikes Lawton and McMillan, 1999; Mickus et al., 2009; Spencer et al., 2011). Uplifted and plutons, and extensive exposures of pre-Cenozoic structural features, rift blocks along the northern edge of the rift system formed the Mogollon strata, and magmatic systems (Zeller, 1970; Hodgson, 2000; Clinkscales and Highlands, a significant drainage divide (e.g., Bilodeau, 1986; Lawton et al., Lawton, 2014). We present geologic evidence demonstrating that structural 2014) between the southern end of the Cordilleran foreland basin and Border relations in southern New Mexico are inconsistent with transcurrent or rift. Upper Jurassic to lower Cretaceous continental, marginal-marine, and wrench-style structures (e.g., Hodgson, 2000) and ramp-flat thrust geom shallow-marine deposits and associated magmatism record active crustal ex- etries (e.g., Corbitt and Woodward, 1973; Drewes, 1988). Field and geo- tension to post-rift thermal subsidence (Garrison and McMillan, 1999; Mauel chronologic data presented here also provide evidence for widespread, et al., 2011; Spencer et al., 2011). Proposed mechanisms for rifting include previously unrecognized east-west normal faults in the area that record extension or transform motion associated with the opening of the Gulf of a Paleogene, pre–Basin and Range synmagmatic extensional event that Mexico (Bilodeau, 1982) and extension due to backarc (Lawton and McMillan, correlates with other Cordilleran extensional systems (e.g., Davis, 1980; 1999; Dickinson and Lawton, 2001) or interarc processes (Stern and Dickin- Coney and Harms, 1984; Gans, 1997). Our results place new temporal and son, 2010) related to rollback of the Farallon and Mezcalera plates (Dickinson kinematic constraints on the Mesozoic to Paleogene evolution of the region and Lawton, 2001). south of the Colorado Plateau, a region critical to understanding timing and A transition from post-rift thermal subsidence to foreland basin sub- style of Laramide deformation and subsequent crustal extension, and pro- sidence occurred by late Albian–Cenomanian time. This change in basin vides context for future studies addressing the evolution of the Paleogene dynamics is recorded by increased tectonic subsidence rates (Mack, 1987; landscape that can be linked to paleoclimate and elevation studies in the Clinkscales and Lawton, 2014), changes in detrital provenance signaled Great Basin (e.g., Mix et al., 2011). by volcanic-lithic compositions, and systematic differences in paleocurrents and facies from lower and upper Cretaceous rocks (Mack et al., 1986; Machin, 2013). In the Little Hatchet Mountains of southwestern New Mex- GEOLOGIC BACKGROUND ico, the change from rift basin to foreland basin is recorded by the deposition of more than 1400 m of fluvial and marginal marine strata of the The principal structural belts defining the Jurassic to Paleogene paleo Mojado Formation and Mancos Shale (e.g., Lucas and Lawton, 2005). Silici- geography of the west-central U.S. Cordillera and Rocky Mountains (present- clastic strata of the Mojado Formation and correlative Beartooth Quartzite day California, Colorado, Montana, Nevada, Utah, and Wyoming) broadly in the Burro Mountains and Cookes Range (Fig. 3) record a volcanic and consist of the following tectonic elements, from east to west (Fig. 1): (1) base- recycled orogen source terrane with east-directed paleocurrents sourced ment-involved Laramide block uplifts and flanking intermontane basins from a retroarc fold-thrust belt to the west (Mack, 1987; Machin, 2013). By (Dickinson and Snyder, 1978; Dickinson et al., 1988; Lawton, 2008); (2) a mid-Cenomanian time, the foreland basin in southern New Mexico was north-south frontal thrust belt and associated foreland basin of the Sevier contiguous northward with the Cordilleran foreland basin of northern New orogen (Lawton, 1994; DeCelles and Coogan, 2006); (3) an extensive oro- Mexico, Colorado, and Utah.

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Figure 3. Geologic map of southwestern New Mexico (scale 1:500,000) 40° 40° and southeastern Arizona (scale 1:1,000,000). Northwest-southeast olcanic Field Laramide structures and their in- 30°30° ferred positions across ranges are Mogollon-Datil V dashed and correlate to exposed N BR structural and stratigraphic rela- 20° 20° Pgv Rio Grande Rift tionships, well penetrations, and Mz 120° 110° 100° 90° 80° subregional extrapolations (Seager, Copper Flat 2004). The names of the principal 33°0′0″N porphyry Pz 33°03 ′0″N Laramide uplifts and basins are italicized. Note the dominant rock expo- CM sures in the region consist of Paleo- AH gene–Neogene volcanic rocks (pink). SCR Pz The Little and Big Hatchet Moun- Silver City tains (study area) offer a unique Love Ranch Basin SA exposure of Paleozoic and Mesozoic BM rocks along a north-south transect Rio Grande Uplift offset by various northwest-south- Arizona New Mexico east Laramide structures, and ex- pC CR pose the roots of the Hidalgo uplift Potrillo Uplif RM and syn-Laramide strata of the Lordsburg Las Cruces Ringbone Basin. Geologic map of Kv Potr OM New Mexico from Green and Jones pC illo Basi (1997); geologic map of Arizona is t Rio Grande Klondike BasiVM n from Richard et al. (2000). The geo- PY FL graphic information system map n format was made available through Luna Uplif Ci Ludington et al. (2005). Labeled Fig. 5 32°0′0″N t R 32°03 ′0″N ranges include Animas Hills (AH), if Ringbo CD t Animas Mountains (AN), Big Hatchet n Texas Mountains (BH), Black Range (BR), CH e Basin TH PM El Paso Burro Mountains (BM), Chiricahua PL HidalgoLH UpliftSR Mountains (CH), Caballo Mountains Mz (CM), Cookes Range (CR), Cedar Pgv—Paleogene–Neogene volcanic rocks Mountain Range (CD), East Potrillo AN Mountains (PM), Florida Mountains BH Little Ha (FL), Little Hatchet Mountains (LM), Pgv Kv—Upper Cretaceous–lower Paleogene igneous rocks Top Organ Mountains (OM), Peloncillo Ba t Mountains (PL), Pyramid Mountains sin Mz—Mesozoic Rocks: Pre- and Syn-Laramide (PY), Robledo Mountains (RM), San Andres Mountains (SA), Silver City Pz—Paleozoic sedimentary rocks Range (SCR), Sierra Rica Mountains MEXICO Laramide (SR), Tres Hermanas Mountains Uplift-bounding Ci—Cambrian–Ordovician igneous rocks (TH), and Victorio Mountains (VM). 0410 20 0 Reverse/Thrust faults km dashed where inferred pC—Precambrian igneous and metamorphic Rocks

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40°0°° 40° d Volcanic Fiel 30° 30° Mogollon-Datil N Approximate limit of 20° 20° lower Cretaceous– 120° 110° 100° 90° 80° lowermost Upper 33°0′0″N Cretaceous rocks 33°03 ′0″N

Silver City

Arizona New Mexico

Lordsburg Las Cruces

Fig. 5 32°0′0″N 32°03 ′0″N Texas El Paso

Lower–lowermost Upper Cretaceous Subcrop Map ? Upper Jurassic (Bisbee Group) rocks Upper Paleozoic rocks lower Paleozoic rocks MEXICO Precambrian igneous and metamorphic rocks Border Rift 0 10 20 40 kkm Normal faults

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Figure 4. Lower–lowermost upper Cretaceous subcrop map. Colored areas refer to the subcrop relationships for lower–lowermost upper Cretaceous rocks in the southern New Mexico and southeastern Arizona. Dashed faults refer to present-day location of major normal faults of the Border rift, defined by subcrop relationships and major Laramide reverse faults. The approximate limit of lower Cretaceous Bisbee Group rocks is indicated by dashed line to the north. Modified from Lawton (2000). Refer to Figure 3 caption for sources on background geologic maps.

Late Cretaceous to Eocene Laramide orogenesis involved the breakup the Little Hatchet Mountains and southwest-verging structures in the Big of the Cordilleran foreland by uplift of basement-involved blocks along re- Hatchet Mountains (Seager, 1983, 2004). Recent studies integrating struc- verse and thrust faults (Dickinson and Snyder, 1978; Cross, 1986; Seager and tural-stratigraphic relations with U-Pb dating of syn-Laramide clastics and Mack, 1986; Dickinson et al., 1988; Mack and Clemons, 1988). Geotectonic interbedded tuffs demonstrate that no significant hiatus occurs between models invoke flat slab subduction as the driving mechanism for far-field the Ringbone and Skunk Ranch Formations. Instead, the unconformity rep- Laramide deformation (Dickinson and Snyder, 1978; English et al., 2003), resents a progressive unconformity along intrabasinal thrust faults with no which occurred 700–1500 km from the trench, and a general null in arc mag- evidence of Paleogene deposition and major shift in Laramide kinematics matism along the flat slab corridor (Saleeby, 2003). Factors controlling the (Clinkscales and Lawton, 2014). shallowing of the Farallon plate include the subduction of younger, buoyant Late Cretaceous deformation in southern New Mexico occurred nearly oceanic crust and/or subduction of an aseismic ridge or thick oceanic pla- coeval with northeastward migration of arc magmatism from coastal Sonora, teau (Henderson et al., 1984; Liu et al., 2010; Heller and Liu, 2016; Copeland Mexico, to southern New Mexico from ca. 115 to 69 Ma. This geographic et al., 2017). shift in arc volcanism is recorded by (1) ca. 115–92 Ma volcanic rocks and Laramide uplifts consist of thick-skinned, basement-involved features that plutons of the Peninsular Ranges in Baja California and coastal Sonora (Wet- generally flank intermontane basins dominated by alluvial, fluvial, and lacus- more et al., 2003); (2) post–90 Ma volcaniclastic and plutonic rocks of the trine deposits. The orientation and distribution of many Laramide uplifts in the Tarahumara arc in northern Sonora, Mexico (González-León et al., 2011); central Rocky Mountains are inferred to be controlled by antecedent structural and (3) upper Cretaceous volcanic rocks and plutons in southwestern New lineaments (e.g., Marshak et al., 2000; Stone, 2002). Similarly, Jurassic normal Mexico (e.g., Hidalgo Formation and Sylvanite plutonic complex, discussed faults of the Border rift system, locally the Bisbee basin in New Mexico and Ari herein). Late Cretaceous to Paleogene magmatism in southern New Mex- zona, were reactivated as reverse faults during Laramide shortening (Lawton, ico is argued to be related to three magmatic arc episodes (e.g., McMillan, 2000; Bayona and Lawton, 2003). 2004) interpreted to have occurred from 76 to 70 Ma, 61 to 57 Ma, and 46 to Laramide uplifts and basins in southern New Mexico are oriented 40 Ma (Amato et al., 2017). Igneous rocks bracketed for the oldest episode, northwest-southeast and bounded by reverse faults that generally verge between ca. 76 and 70 Ma, crop out within and near the Little and Big Hatchet northeast (Fig. 3; Seager, 2004) and contain upper Cretaceous to Paleogene Mountains, including exposures in the central and northern Little Hatchet continental and volcanic deposits (Basabilvazo, 2000; Seager et al., 1997; Mountains, Lordsburg, the Burro Mountains, Silver City area, and Animas Lawton, 2008; Jennings et al., 2013; Amato et al., 2017). The distribution of Hills (Fig. 3). Northeastward migration of Cretaceous magmatism and vol- these uplifts and basins is rendered uncertain by Cenozoic basin fill that ex- canism is inferred as a result of progressive shallowing of the Farallon plate tensively buries the Laramide syntectonic rocks and structures; moreover, (e.g., Coney and Reynolds, 1977). structural overprinting by multiple deformation episodes also confounds The Paleogene was a period of major tectonic reconfiguration from interpretation of the kinematic history. Nevertheless, exposures are suffi- shortening to extension, accommodated by changing convergent plate cient to establish the structural and depositional boundaries of Laramide kinematics along western Laurentia (Coney and Reynolds, 1977). Prevailing uplifts and basins (Seager et al., 1997; Seager, 2004; Clinkscales and Law- models for post-Sevier and post-Laramide deformation and magmatism in- ton, 2014). Early studies in the region characterized shortening structures voke Cordilleran-scale extension (Atwater, 1970; Coney and Harms, 1984) as ramp-flat thrust faults akin to a thin-skinned deformation style similar and onset of silicic magmatism (e.g., McIntosh et al., 1992; McIntosh and to contemporaneous structures in the Sevier orogenic belt of the U.S. Cor- Bryan, 2000). Regionally diachronous crustal extension occurred roughly dillera (Corbitt and Woodward, 1973; Lawton, 1985; DeCelles, 2004). Later synchronously with a migration of arc magmatism south-southwestward studies in the northern Little Hatchet Mountains interpreted Laramide defor from Idaho to Nevada across the Great Basin and westward from New Mex- mation as a thick-skinned deformation event that occurred in two discrete ico across Arizona and Sonora, Mexico (Gans, 1997; Dickinson, 2006). These pulses, a Late Cretaceous shortening event accompanied by basement up- arc belts nearly converged ca. 20 Ma at an amagmatic corridor west of the lift and later Eocene transcurrent deformation that reactivated the former Colorado Plateau and east of the incipient Mendocino triple junction and basement-involved faults (Hodgson, 2000). The two pulses were defined early San Andreas fault system (Dickinson, 2002). The Boot Heel volcanic in the Little Hatchet Mountains on the basis of inferred discrete Campa- field in southern New Mexico (Fig. 2) is associated with the westward mi- nian–Maastrichtian and Eocene ages for the Laramide syntectonic Ring- gration of arc volcanism through the southern U.S. Cordillera and is re- bone and Skunk Ranch Formations, respectively (e.g., Lawton et al., 1993; corded by thick rhyolite to rhyodacite ignimbrites and lavas erupted during Basabilvazo, 2000). Late Cretaceous deformation was attributed to south- two discrete phases, from 35.2 to 32.7 Ma and 27.6 to 26.8 Ma (McIntosh and west-northeast shortening, whereas Eocene deformation was attributed Bryan, 2000). An ignimbrite in the Little Hatchet Mountains with a 40Ar/39Ar to strike-slip displacement on basement-involved faults that formed a full (sanidine) age of 32.6 ± 0.16 Ma was emplaced during the first magmatic positive flower structure, consisting of northeast-verging structures in pulse (McIntosh and Bryan, 2000).

METHODS Copper Dick fault (Zeller, 1970). Monzonite and quartz monzonite rocks are phaneritic with medium-grained crystals of elongate plagioclase, orthoclase, Compiled geologic maps (Figs. 5 and 6) for the ranges include our geological amphibole, and local quartz. Diorite intrusions contain minerals similar to mapping in the Little Hatchet Mountains, conducted at scales of 1:10,000 and those of the monzonite rocks but tend to have a darker aphanitic groundmass 1:24,000, and evaluation of previously published geological maps in both ranges (Zeller, 1970). Diorite indicated on the geologic map of Zeller (1970) is Jurassic (Zeller, 1970; Drewes, 1991; Hodgson, 1991; 2000). Cross-section construction basalt intruded by the Sylvanite plutonic complex (Lawton and Harrigan, 1998; was based on projected map contacts, dip measurements, and local measured Clinkscales and Lawton, 2014). sections and aided by use of 2D Move (https://www .mve.com /software /move) An alkali feldspar granite collected from a <10 m2 fault block along the west- software (Fig. 7). Stratigraphic nomenclature, age, and principal rock types are ern trace of the Copper Dick fault yielded a U-Pb zircon age of 34 ± 1 Ma (Fig. summarized in Figure 8. Newly published geochronology from the Little Hatchet 9D; MSWD = 0.18, n = 11). Compositional and texturally similar granites crop Mountains includes U-Pb zircon ages for five samples analyzed by laser abla- out along both segments of the Copper Dick fault. These granite exposures tion–multicollector–inductively coupled–mass spectrometry (LA-MC-ICP-MS) at do not exceed 10 m2 and consist of equigranular monocrystalline quartz and the Arizona LaserChron Center (Tucson, Arizona). Specifications on testing sam- orthoclase phenocrysts, microcrystalline plagioclase, and subordinate biotite. ple reproducibility, accuracy, analytical procedures, and uncertainties with LA- The granite along the fault is sheared and brecciated, indicating emplacement MC-ICP-MS at the Arizona LaserChron center were described in more detail else- during protracted fault movement. Previous maps interpreted these granite where (Gehrels et al., 2008; Gehrels, 2012). We also cite unpublished 40Ar/39Ar intrusions along the Copper Dick fault as Precambrian (Hodgson, 2000) or Cre- ages from rhyolite dikes (Cleary, 2004) in the central domain of the Little Hatchet taceous–Paleogene (Zeller, 1970) fault slivers; our U-Pb zircon age data confirm Mountains (Table 1). Previously published and unpublished 40Ar/39Ar ages were a latest Eocene to early Oligocene age. recalculated using the ArArCALC program (Koppers, 2002). A porphyritic granite with finely crystalline groundmass of quartz, plagio clase, and orthoclase yielded a weighted mean U-Pb age of 29 ± 1 Ma (Fig. 9E; MSWD = 0.74, n = 8). The granite comprises nearly equal proportions and GEOCHRONOLOGY phenocrysts of quartz with embayed rims (7%–10%), plagioclase (10%), and orthoclase (20%), and forms a conspicuous east-west–trending dike as much Clinkscales and Lawton (2017), Mesozoic – Paleogene Structural Evolution of the southern US Cordillera as revealed in the Little and Big Hatchet Mountains, southwest New Mexico, USA: U-Pb geochronologic analyses by Laser-Ablation Multicollector ICP Mass Spectrometry Isotope ratios Apparent ages (Ma) Analysis U206Pb U/Th 206Pb* ±207Pb*±206Pb* ±error 206Pb* ±207Pb*±206Pb* ±Best age± (ppm)204Pb 207Pb* (%)235U* (%)238U(%) corr.238U* (Ma) 235U (Ma) 207Pb* (Ma) (Ma) (Ma) Unpublished U-Pb geochronologic data that provide new insight on ages as 40 m wide in the southern part of the Little Hatchet Mountains (Zeller, 1970). 1.30.10.1.15 60 64724 2.013.40144.0 1.8751 4.80.18232.6 0.55 1079.3 26.2 1072.3 31.7 1058.3 80.5 1058.3 80.5 1.30.10.1.02 1872099102.3 13.30750.7 1.8442 1.80.17801.6 0.91 1056.0 16.0 1061.4 11.9 1072.4 14.7 1072.4 14.7 1.30.10.1.13 1632287652.4 13.29040.9 1.8825 1.80.18151.6 0.88 1074.9 16.0 1074.9 12.1 1075.0 17.1 1075.0 17.1 1.30.10.1.06 10396177 2.213.27731.5 1.8663 2.40.17971.8 0.77 1065.418.01069.215.71077.030.31077.030.3 of faults, plutonism, and structural relations in the Little Hatchet Mountains are The dike resembles the Granite Pass Granite in texture and mineral content. 1.30.10.1.04 10353712.2 13.27622.8 1.7130 4.60.16493.6 0.79 984.232.91013.429.41077.157.01077.157.0 1.30.10.1.01 1531636532.1 13.22710.9 1.9486 3.40.18693.2 0.96 1104.7 32.8 1098.0 22.5 1084.6 18.5 1084.6 18.5 1.30.10.1.14 2602307201.7 13.20000.8 1.8753 2.40.17952.3 0.94 1064.422.41072.416.01088.716.01088.716.0 1.30.10.1.05 1641652502.4 13.16911.0 1.8798 1.60.1795 1.20.751064.511.51074.010.31093.420.41093.420.4 40 39 1.30.10.1.08 1651142682.2 13.12111.3 1.8650 2.10.17751.7 0.81 1053.2 16.8 1068.8 14.1 1100.7 25.0 1100.7 25.0 described here. U-Pb geochronologic data for samples analyzed as part of this The age is statistically younger than a K-feldspar Ar/ Ar age of 32.74 ± 1.30.10.1.09 11897702.2 13.09153.5 1.9067 4.30.18102.5 0.58 1072.624.21083.428.31105.269.51105.269.5 1.30.10.1.03 10139046 2.713.01661.8 1.7343 3.50.16373.0 0.86 977.527.31021.322.61116.736.01116.736.0 1.30.10.1.10 20950895 2.112.92503.5 1.8196 3.90.17061.8 0.45 1015.3 16.7 1052.6 25.7 1130.7 69.7 1130.7 69.7 1 1.30.10.1.12 16333152.3 12.82476.2 1.8021 8.00.16765.0 0.63 999.046.21046.252.11146.2123.6 1146.2 123.6 study are provided in the Supplemental Item . A metamorphosed alkali feld- 0.18 Ma for the Granite Pass pluton recalculated from a legacy age of 32.32 ± spar granite (14GP03) collected adjacent to the Granite Pass–Windmill thrust 0.16 Ma reported by Channell et al. (2000; Table 1). 1Supplemental Item. Includes LA-ICP-MS U-Th-Pb zircon data from the Arizona LaserChron Center at system in the southern Little Hatchet Mountains (Fig. 6) yielded a weighted the University of Arizona. Please visit http://doi.org mean age of 1090 ± 15 Ma (Fig. 9A; mean square of weighted deviates, MSWD = /10.1130/GES01539 .S1 or the full-text article on www 0.34, n = 5). The rock consists of a granular mosaic of orthoclase (60%), quartz STRUCTURAL RELATIONS OF THE HATCHET RANGES .gsapubs.org to view the Supplemental Item. (30%), and plagioclase (7%) with clots of intergrown biotite and hematite that represent an altered ferromagnesian mineral, perhaps hornblende, and yield a The Little Hatchet Mountains constitute a north-south horst bounded by color index of ~8. This is the first reported occurrence of Proterozoic basement north-trending normal faults. The topography of the range and mapped fault north of Granite Pass in the Little Hatchet Mountains. relationships indicate that the bounding faults are discontinuous along strike An aplite granite dike (1.1.30.10) collected near Granite Pass in the southern with fault terminations giving way to relay or transfer zones similar to those Little Hatchet Mountains yielded a weighted mean age of 1085 ± 13 Ma (Fig. described by Faulds and Varga (1998). The Big Hatchet Mountains, oriented 9B; MSWD = 0.29, n = 13). Aplite dikes intrude a rapakivi granite host and are roughly north-south to northwest-southeast, are separated from the Little of similar age, as indicated by overlapping U-Pb zircon ages (1077 ± 18 Ma; Hatchet Mountains by surficial deposits at Hatchet Gap (Fig. 5); however, Amato and Mack, 2012), and can be differentiated from the host granite by a northwest-southeast faults exposed in the Big Hatchet Mountains and on its finely crystalline groundmass with abundant quartz phenocrysts, and lack of northeastern flank impart a control on the topography of the range and may K-feldspar megacrysts. Precambrian aplite dikes are almost exclusively com- be related to extension that predated east-west Basin and Range extension. posed of quartz, orthoclase, and finely crystalline biotite. North-south faults demonstrably offset both northwest-southeast reverse A quartz monzonite sample from the Sylvanite plutonic complex (Fig. 5) and thrust faults and east-west and northwest-southeast normal faults. East- yielded a U-Pb zircon weighted mean age of 69 ± 1 Ma (Fig. 9C; MSWD = 0.80, west– and northwest-southeast–trending normal faults displace Laramide n = 44). The Sylvanite plutonic complex is composed of diorite, monzonite, shortening structures. The two sets of normal faults serve to expose a variety and quartz monzonite that intrude Jurassic to Cretaceous strata south of the of Laramide structural levels discussed in the text.

GEOSPHERE | Volume 14 | Number 1 Clinkscales and Lawton | Mesozoic–Paleogene structural evolution of the southern U.S. Cordillera Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/14/1/162/4035211/162.pdf 169 by guest on 24 September 2021 on 24 September 2021 by guest Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/14/1/162/4035211/162.pdf Research Paper the major structural elements. Cross sections are shown in Figure 7. modified from Zeller (1970), and Clinkscales Lawton Hodgson (2000), (2014). The geology of the Big Hatchet Mountains is simplified from Drewes (1991) and illustrates Figure 5. Bedrock geologic map of Little and Big Hatchet Mountains. Major faults discussed in text are labeled. Geologic map of the Little Hatchet Mountains is adapted and Little and Big Hatchet Mountains Bedrock Geologic Map Bisbee Group Laramide Syntectonic Unit s A Big Hatchet Mountains Map Unit Little Hatchet Mountains Map Units 48 48 48 Map Symbols A ′ Pennyslvanian–lower Permian IPh—Horquilla Limestone Pce—undif Prcse—undi B El Paso Formation M Cambrian–Mississippian: Pzl—undif U Kbg—undif KPgu—Upper Cretaceous (?)–Paleogene strata Pgv—Paleogene volcaniclastic rocks Syncline Normal fault—dashed where inferre Overturned Strike and Dip of Bedding Thrust/Reverse fault—dashed where inferre Foliation Strike and Di pCg—Mesoproterozoic (ca. 1.1 Ga) igneous rocks Strike and Dip of Bedding Formational contact—dashed where inferre Overturned Fold Anticline Cross-section lines pCg—Mesoproterozoic (ca. 1.1 Ga) igneous rocks P Pzu—undif Jb—Jurassic Broken Jug Formation Kh—Cretaceous Hell-to-Finish Formation Ku—Cretaceous U-Bar Formation Km—Cretaceous Mojado Formation Kma—Cretaceous Mancos Shal Kr—Cretaceous Ringbone Formation Ks—Cretaceous Skunk Ranch Formation Khv—Cretaceous Hidalgo Formation Kqm—Cretaceous Sylvanite Complex— Pgv—Paleogene volcaniclastic rocks Pgg–Paleogene granite and quartz monzonite Pge—Paleogene Eureka Pluton—diorite Pgl/r—Paleogene–Neogene (?) latite/rhyolite dike Ngs—Neogene (?) silicified strata Percha Shal Escabrosa Limestone Paradise Formation Earp Formation Colina Limestone Epitaph Dolomit Scherrer Formation Concha Limestone Rainvalley Formation Glance Conglomerat Hell-to-Finish Formation Mojado Formation liss Sandstone Cambrian Bliss Sandstone Permian Earp Formation ontoya Group -Bar Formation ennsylvanian Horquilla Limestone quartz monzonite and diorite Axis Axis ferentiated lower Paleozoic strata ferentiated lowermost Permian strata ferentiated Cretaceous Bisbee Group ferentiated Paleozoic strata ff erentiated lower Permian strata e Axis e p e

e d s d d

Mojado Thrust Thrust Canyon Canyon fault fault Dick faul t Dick faul t Bull fault fault Copper 0

Playas V

alley Ngs Spring faul Spring faul Livermore Reverse fault Reverse fault Windmill Granite Pass Reverse fault Reverse fault 24 108°30′0″ W Kr Ks Pgv Kh Ku 108°30 ′ B B′ Figure N Beacon Hill Ks Jb 0 ″ W t t Kr fault fault Kr 6 Khv Kr Hatchet Gap Hatchet Gap Mine Canyon Kqm Reverse fault Reverse fault Kh Reverse fault Reverse fault Pzu Ku Km Peak (2550 m) Peak (2550 m) Big Ta Big Hatchet Kma fault fault Pge Kr 8 Pzu Kh Ku Pgg Ku km Ku Km pCg nk Km Kh Pzl Jb 108°25′0″ W Km Kh 108°25′0″ W Pzu Hatchet Gap A′ IPh Peak (2024 m) Peak (2024 m) A pCg IPh Howells Pgv Pzl Howells Ridge Thrust faul Howells Ridge Thrust faul Copper Dick faul t Copper Dick faul t Hachita Hidalgo Reverse faul t Hidalgo Reverse faul t Ringbone Reverse Pr Coyote Hill s cs e We faul t faul t Kbg ll Synclin pCg Hachita V alley e 108°20′0″ W Prcse 108°20′0″ W Pce IPh Map Location New Mexico t t Kb g Sierra Rica Mts. KPg u Pg v Pz l N 108°15′0″ W 108°15′0″ W 31°35′0″ N 31°40′0″ N 31°45′0″ N 31°50′0″ N 31°55′0″ N 31°30′0′N

GEOSPHERE | Volume 14 | Number 1 Clinkscales and Lawton | Mesozoic–Paleogene structural evolution of the southern U.S. Cordillera 170 on 24 September 2021 by guest Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/14/1/162/4035211/162.pdf Research Paper Mercator; NAD 27—North American Datum 1927. tions shown by red dots. A ca. 29 Ma granite dike crosscuts the Windmill fault. Map location is shown in Figure 5. USGS—U.S. Geological Survey; UTM—Universal Transverse Pass and Windmill faults, are present in the map area. Both faults are defined by ca. 1.1 Ga igneous rocks in hanging wall, substantiated by U-Pb zircon ages with sample loca - Figure 6. Geologic map of southern Little Hatchet Mountains near Granite Pass. The two principal reverse faults that define the northern flank of the Hidalgo uplift, the Granite Geologic Map of Southern Little Hatchet Mountains 3517000 3518000 3519000 Hidalgo County 86 Granite Pass Reverse Fault 83 Pgg Pe s Pel

Qal 81 70 IPh 78 82 65 50 78 85 Pgqm 85 1090 ± 15 Ma , New Mexico 80 70 14GP03 75 57 89 Pes 76 55 Figure 7C Cross section 29 ± 1 Ma 74000 Qal 50 59 41 53 pCg 51 07 71 Pel 14GP01 82 80 46 65 74 51 Windmill Reverse Faul 80 , 64 80 Cb 77 32 01 Pgqm UTM Datum: NAD 27 1:24,000 USGS Hachita Peak Quadrangl 45 Pel IPh Pg d Pgg 46 85 50 Km pC g t 70 48 48 1085 ± 13 Ma 1.30.10. 1 Limestone IPh—Pennsylvanian Horquilla Pe—Permian Earp Formation Km—Cretaceous Mojado Formation m onzonit Pgg—Paleogene granite and quartz / diorite dik Pgqm/d—Paleogene quartz monzonite pCg—Mesoproterozoic (ca. 1.1 Ga) Cb—Cambrian Bliss Sandston 50 Geochronology Sampl Overturned Strike and Dip Strike and Dip of Bedding where inferred Formational contact—dashed where inferre Normal fault—dashed where inferre Thrust/Reverse fault—dashed 4100 Pel–limestone Pes–siltstone igneous rocks 0 e e d d

N e km

GEOSPHERE | Volume 14 | Number 1 Clinkscales and Lawton | Mesozoic–Paleogene structural evolution of the southern U.S. Cordillera 171 Research Paper

A Big Hatchet Mountains Hatchet Gap Little Hatchet Mountains Coyote Hills Howells Ridge fault Big Tank Mine Canyon Hatchet Gap Granite Pass Copper Dick Howells Well Hidalgo Ringbone fault system fault Syncline SW fault gap in section gap in section fault gap in section fault fault fault NE 3000 m Kr 2000 m Kh Km Pzu Khv Ku* IPh pC Pgv 1000 m Kh* IPh Ku Pce Ku Km Km Km Pzl Jb Prcse IPh Pzl Kh 0 m Pzu ? Ku Jb Pce pC Ku Kqm Hidalgo Kh Pgg Pzu IPh Kh Jb fault

2000 m 4000 m 1:1 v.e. NW-SE Granite Pass Normal fault Windmill Legend Kh—Hell-to-Finish Formation B C fault fault *Kh shown in Figure 5 as Kbg Copper Dick Bull Canyon Howells Ridge Hidalgo gap in section Pgg—Paleogene granite and quartz monzonite S fault fault fault fault N Pzl Jb—Broken Jug Formation Prcse 2000 m Bull Canyon Graben Pgv—Paleogene volcaniclastic rocks Mojado fault Pgv Prcse—undifferentiated lower Permian strata Pgv Km Khv Pce Ks Kqm—Cretaceous Sylvanite Complex— Pce—undifferentiated lowermost Permian quartz monzonite and diorite Kh Ks Kr strata 1000 m Khv Pce Km Kr pC Ks—Skunk Ranch Formation IPh minor IPh—Horquilla Limestone Km Kr fault splay Kr—Ringbone Formation Jb Kma Kr Ku Km Pzl Pzl—undifferentiated lower Paleozoic strata 0 m Km Ku pC Kma—Mancos Shale Kh Ku Ku Jb pCg—Mesoproterozoic (ca. 1.1 Ga) igneous rocks Pzu Kh Km—Mojado Formation Kh Ku ? Pzu 1000 m Ku—U-Bar Formation 1:1 v.e. *Ku shown in Figure 5 as Kbg Reactivated Faults 1000 m 2000 m 1:1 v.e.

Figure 7. (A) Big and Little Hatchet Mountains A-A’ cross section. Gaps in cross section correspond to areas with no bedrock exposure and offsets in cross section. These gaps are measured to scale and interpretations in the intervening gaps are interpolated to illustrate possible geologic continuity. Dips and formation contacts are projected into the cross section; however, subsurface geology is very poorly defined and based on surface relations. (B) Cross-section B-B’ in the Little Hatchet Mountains. Dips and formation contacts are projected into the cross section. (C) Cross section of Granite Pass and Windmill fault zone in the southern Little Hatchet Mountains with Pgg (Paleogene granite and quartz monzonite) intrusion omitted to clarify structural relations. Cross section is noted by dashed box A. Intrusion contact marked by thin dashed lines. Precambrian rocks in the hanging wall of the Windmill fault are shown to illustrate reverse separation relations recorded along strike to the east with a subsidiary thrust splay.

Little Hatchet Mountains Southern Domain

We define three structural domains in the Little Hatchet Mountains (Fig. The southern domain consists of faulted Proterozoic igneous rocks 10): (1) a southern domain from Hatchet Gap north to the Windmill fault; (2) a and Paleozoic strata. Rapakivi granite and aplitic dikes ca. 1.1 Ga (Amato central domain between the Windmill fault on the south and the Copper Dick and Mack, 2012; this study) are exposed in the southernmost Little Hatchet fault on the north; and (3) a northern domain that extends from the Copper Mountains and at Hatchet Gap. The northwest-southeast Hatchet Gap nor- Dick fault to the southernmost Coyote Hills. The exposed strata (Fig. 8) consist mal fault juxtaposes footwall Precambrian rocks and lower Paleozoic strata of (1) fault-bound horses of upper Paleozoic rocks; (2) upper Jurassic and lower against Pennsylvanian Horquilla Limestone with an estimated fault throw to upper Cretaceous rocks of the Bisbee Group, including the Broken Jug (for- of 2200 m, calculated on the basis of omitted lower Paleozoic strata (Fig. 7). merly Broken Jug Limestone of Lasky, 1947), Hell-to-Finish, U-Bar, and Mojado Near Granite Pass, Precambrian igneous rocks are present in the hanging Formations (Lucas et al., 2001), and Mancos Shale; (3) upper Cretaceous syn- wall of both the Granite Pass and Windmill faults. The Granite Pass fault em- orogenic Laramide strata of the Ringbone and Skunk Ranch Formations and places Precambrian rocks against overturned Horquilla Limestone. The mini volcanic Hidalgo Formation; and (4) upper Eocene to Oligocene ignimbrites mum throw on the Granite Pass fault, estimated from the cumulative thick- and interbedded epiclastic strata. ness of lower Paleozoic strata (e.g., Zeller, 1965; Drewes, 1991), is ~2000 m.

Big Hatchet MountainsLittle Hatchet Mountains

Paleogene volcaniclastic rocks unconformity Paleogene volcaniclastic rocks Upper Cretaceous (?)–Paleogene clastics (~120 m) unconformity unconformity Skunk Ranch Formation (850 m) Late Cretaceous: Maastrichtian ? Basaltic Andesite flow represents tongue of Hidalgo Formation Mojado Formation Early Cretaceous: Albian–Cenomanian Strata

Ringbone Formation (1600 m) U-Bar Formation Figure 8. Generalized stratigraphic Synorogenic Laramide Late Cretaceous: Campanian–Maastrichtian Early Cretaceous: Aptian–Albian ? columns of Big (left) and Little (right) Hatchet Mountains. The Big Hatchet Mountains expose the most complete Paleozoic section in the region (Zeller, 1965; Hell-to-Finish Formation unconformity Drewes, 1991), whereas the Little Early Cretaceous: Neocomian–Aptian Mancos Shale (100 m) Hatchet Mountains expose an Late Cretaceous: Cenomanian extensive upper Jurassic to Cretaceous Bisbee Group (Kbg) lower–upper Cretaceous section Glance Conglomerate from Drewes, 1991 Mojado Formation (1200 m) with the thickest known Lara- *no complete stratigraphic section in the Big Hatchet Mountains unconformity Early Cretaceous–Late Cretaceous: mide synorogenic deposits in lower Permian strata (875 m) Albian–Cenomanian the southern U.S. Cordillera (e.g., Lucas and Lawton, 2000). The Rainvalley Formation Laramide synorogenic Hidalgo Concha Limestone Formation is not illustrated in the Scherrer Formation Little Hatchet Mountains section. p Epitaph Dolomite U-Bar Formation (970 m) An incomplete section of lower to Early Cretaceous: Aptian–Albian upper Cretaceous (Bisbee Group) lowermost Permian strata (460 m) rocks is exposed in the southern- Colina Limestone most Big Hatchet Mountains and Earp Formation only partial exposures are present in the range, so no individual Bisbee Grou Hell-to-Finish Formation (525 m) thicknesses are denoted. Refer to Early Cretaceous: Neocomian–Aptian Horquilla Limestone (1060 m) Little Hatchet Mountains column Pennyslvanian–Lowermost Permian for Bisbee Group formation thicknesses. No upper Jurassic rocks

Paleozoic Strata have been identified or previously lower Paleozoic strata (1085 m) Broken Jug Formation (1200 m) recognized (e.g., Drewes, 1991; Cambrian–Mississippian: Late Jurassic: Oxfordian–Tithonian Lawton and Harrigan, 1998) in the Paradise Formation Big Hatchet Mountains. Escabrosa Limestone Percha Shale 1000 m Montoya Group unconformity El Paso Formation Horquilla and Earp Formations Bliss Sandstone (partially exposed) nonconformity reverse/thrust fault contact Mesoproterozoic (ca. 1.1 Ga) igneous rocks Mesoproterozoic (ca. 1.1 Ga) igneous rocks

Principal Lithologies Sandstone and siliciclastic rocks Limestone Limestone, chert bearing Basalt and Basaltic Andesite flows

Fine grained shale and siltstone Conglomerate Rapakivi Granite and dikes Ignimbrite and volcaniclastics rocks

TABLE 1. IGNEOUS ROCK AGES, LITTLE HATCHET MOUNTAINS Age Unit description, sample ID (Ma) (1σ uncertainty unless indicated) System Location* Reference Alkali feldspar granite, sample 14GP03 1090 ± 15 (weighted mean; 2σ uncertainty) U-Pb, zircon31. 784533, –108.467050 This study Rapakivi granite, southern Little Hatchet 1077 ± 4 (concordia age; 2σ uncertainty) U-Pb, zircon31. 765922, –108.456364 Amato and Mack (2012) Precambrian Mountains, sample 11LHM-1 basement Aplite dike in rapakivi granite, sample 1.30.10.1 1085 ± 13 (weighted mean; 2σ uncertainty) U-Pb, zircon31. 764040, –108.456490 This study 1080 ± 14 (concordia age; 2σ uncertainty) Tuff in lower Mancos Shale, sample 11BQ18 97.2 ± 1.6 Ma (weighted mean; 2 uncertainty) U-Pb, zircon31. 863651, –108.462991 Machin (2013) σ Cretaceous (georeferenced location foreland basin from map in Machin, 2013) Skunk Ranch Formation (middle member), 70.38 ± 0.48 (weighted mean; 2σ uncertainty) U-Pb, zircon31. 852063, –108.482052 Jennings et al. (2013) ash-fall tuff, sample 1.5.27.10 Skunk Ranch Formation (middle member), 70.63 ± 0.70 (weighted mean; 2σ uncertainty) U-Pb, zircon31. 864879, –108.484800 Jennings et al. (2013) ash-fall tuff, sample 10GJ-2 Skunk Ranch Formation (middle member), 71.44 ± 0.53 (weighted mean; 2σ uncertainty) U-Pb, zircon31. 865032, –108.483401 Jennings et al. (2013) ash-fall tuff, sample 10GJ-1 Ringbone Formation (upper member), ash-fall 73.31 ± 0.71 (weighted mean; 2σ uncertainty) U-Pb, zircon31. 873648, –108.485872 Clinkscales and Lawton tuff, sample 15.8.28.10 (2014) Laramide Ringbone Formation (middle member), ash-fall 73.41 ± 0.97 (weighted mean; 2σ uncertainty) U-Pb, zircon31. 891571, –108.476051 Clinkscales and Lawton syntectonic tuff, sample 10.11.22.10 (2014) clastics and Hidalgo Formation (Hidalgo Volcanics of Zeller, 71.44 ± 0.38 (legacy) 40Ar/39Ar hornblende 31.934617, –108.465517Lawton et al. (1993); volcanism 1970), sample NM-679 72.37 ± 0.38 (Earthtime recalculation) Yo ung et al. (2000) Hidalgo Formation (Hidalgo Volcanics of Zeller, 70.69 ± 0.44 (legacy) 40Ar/39Ar hornblende 31.919900, –108.512567Young et al. (2000) 1970), sample 99PP3 71.61 ± 0.44 (Earthtime recalculation) Hidalgo Formation (Hidalgo Volcanics of Zeller, 70.53 ± 0.48 (legacy) 40Ar/39Ar hornblende 31.936917, –108.496150 Young et al. (2000) 1970), sample HDV-127 71.45 ± 0.48 (Earthtime recalculation) Sylvanite intrusive complex (quartz monzonite), 68.76 ± 0.61 (weighted mean; 2σ uncertainty) U-Pb, zircon31. 857377, –108.477121 This study sample 16.1.17. 10 Diorite intruding Eureka intrusive complex, 35.5 ± 1.7 (legacy) 40Ar/39Ar groundmass north of Hidalgo fault; precise Channell et al. (2000) sample EUREKA 35.96 ± 1.7 (Earthtime recalculation) location unknown Granite in Copper Dick fault zone, sample 33.8 ± 0.7 (weighted mean; 2σ uncertainty); U-Pb, zircon31. 850324, –108.484868 This study 2.1.31.10 33.1 ± 2.8 (lower intercept; 2σ uncertainty) Ignimbrite tuff (Gillespie Tuff), sample 682 32.62 ± 0.16 (legacy) 40Ar/39Ar sanidine 31.901000, –108.505000 McIntosh and Bryan 33.04 ± 0.16 (Earthtime recalculation) (2000) Granite at Granite Pass, sample GP-23 32.33 ± 0.16 (legacy, weighted mean of biotite and 40Ar/39Ar biotite and Granite Pass; precise Channell et al. (2000) potassium feldspar ages) potassium feldspar location unknown§ 32.74 ± 0.16 (Earthtime recalculation) Paleogene East-west rhyolite dike, sample SPLD-5 31.20 ± 0.57 (legacy) 40Ar/39Ar hornblende 31.825083, –108.430733 Cleary (2004) magmatism 31.61 ± 0.57 (Earthtime recalculation) (appendix B**) Northeast-southwest granite dike, sample 28.9 ± 0.4 (weighted mean; 2σ uncertainty) U-Pb zircon31. 786733, –108.467900 This study 14GP01 East-west rhyolite dike, sample SPLD-4 28.47 ± 0.47 (legacy) 40Ar/39Ar groundmass31. 826033, –108.431067 Cleary (2004) 28.84 ± 0.47 (Earthtime recalculation) (appendix B**) East-west rhyolite dike, sample SPLD-2 28.18 ± 0.97 (legacy) 40Ar/39Ar qroundmass31. 841017, –108.432550 Cleary (2004) 28.84 ± 0.97 (Earthtime recalculation) (appendix B**) East-west rhyolite dike, sample SPLD-1 26.80 ± 0.98 (legacy) 40Ar/39Ar groundmass† 31.837767, –108.430767 Cleary (2004) 27.15 ± 0.98 (Earthtime recalculation) (appendix B**) *Latitude, Longitude: Datum WGS 84 (World Geodetic System 1984). †No plateau age calculated; stated age is total gas age. §Approximate location: Sample collected in section 23, Township 29 south, Range 16 west in southern Little Hatchet Mountains. **Ages from Lisa Peters’ appendix 8 report in Cleary (2004).

A Alkali feldspar granite (14GP03) B Aplite granite dike (1.30.10.1) Weighted Mean: Weighted Mean: 1240 1400 1090 ± 15 Ma 1085 ± 13 Ma Box heights are 2σ Box heights are 2σ 1200 MSWD = 0.34, n = 5 1300 MSWD = 0.29, n = 13

1160 1200 1120

1100 AGE (Ma ) 1080

1000 1040

1000 900

960 800

C Quartz monzonite (16.1.17.10) D Alkali feldspar granite (2.1.31.10)

82 41

78 39

74 37

70 35

66 33 AGE (Ma )

62 31

58 29 Weighted Mean: Weighted Mean:

54 68.76 ± 0.61 Ma 27 33.80 ± 0.74 Ma Box heights are 2σ Box heights are 2σ MSWD = 0.80, n=45 MSWD = 0.18, n=11 50 25 E Porphyritic granite dike (14GP01)

Figure 9. Weighted mean age plots. MSWD—mean square of 30 weighted deviates. (A) Alkali feldspar granite at Windmill fault. (B) Aplite granite at Granite Pass fault. (C) Quartz monzonite from Sylvanite plutonic complex. (D) Alkali feldspar granite along Copper Dick fault. (E) Porphyritic granite dike that crosscuts Windmill fault. Sample locations for A, B, and E are shown AGE (Ma ) 28 in Figure 6 and for C and D are shown in Figure 10. Plots were generated with Isoplot (Ludwig, 2003).

26 Weighted Mean: 28.93 ± 0.41 Ma Box heights are 2σ MSWD = 0.74, n = 8 24

Bedrock Geologic Map 108°30′0″W 108°25′0″W 108°20′0″W Km Coyote Hills Pgv Coyote Hills Fault Little Hatchet Mountains Beacon Hill Kr Coyote Hills fault Map Units Kr fault Ku Khv Ngs—Neogene (?) silicified strata Ngs 31°55′0″N Kr Ku Pge Ringbone Reverse Pgl/r—Paleogene–Neogene (?) latite/rhyolite dike Kh fault Pge—Paleogene Eureka Pluton—diorite Kh Northern Pgg—Paleogene granite and quartz monzonite Pgv Kr Km Domain Pgv—Paleogene volcaniclastic rocks Mojado Hidalgo Reverse fault Kqm—Cretaceous Sylvanite Complex— Thrust Ks Km fault Ku Howells Ridge Thrust fault quartz monzonite and diorite Ku Laramide Syntectonic Units Bull Fig. 11 Kr Khv—Cretaceous Hidalgo Formation Canyon Skunk Kma fault Ranch Ks Copper Dick fault Ks—Cretaceous Skunk Ranch Formation Pzu Figure 10. Structural domains of the Little fault Copper Dick fault Kr—Cretaceous Ringbone Formation Jb Kh Hatchet Mountains. The Little Hatchet Copper Kh Mountains are subdivided into three main Dick fault Kqm 31°50′0″N Kma—Cretaceous Mancos Shale Jb structural domains: (1) southern domain 2.1.31.10 16.1.17.10 Bisbee Group between Hatchet Gap and the Windmill 34 ± 1 Ma Livermore 69 ± 1 Ma Km—Cretaceous Mojado Formation Spring fault fault; (2) central domain between the Ku—Cretaceous U-Bar Formation Central Domain Windmill fault and Copper Dick fault; and Kh—Cretaceous Hell-to-Finish Formation Kh (3) northern domain between the Copper Dick fault and northwest-southeast Coyote Jb—Jurassic Broken Jug Formation Ku Hills fault, which defines the southern boundary of the Coyote Hills. Pzu—undifferentiated Paleozoic strata pCg Km Permian Earp Formation Windmill Pzu Reverse fault Windmill fault system Pennsylvanian Horquilla Limestone Pzu Cambrian Bliss Sandstone Pgg

pCg—Mesoproterozoic (ca. 1.1 Ga) igneous rocks Granite Pass Reverse fault 31°45′0″N Map Symbols pCg Formational contact—dashed where inferred Southern Domain

48 Strike and Dip of Bedding

48 Overturned Strike and Dip of Bedding Hatchet Gap Pzu 48 pCg New Mexico Foliation Strike and Dip N fault Hatchet Gap Thrust/Reverse fault—dashed where inferred IPh fault Normal fault—dashed where inferred Hatchet Gap N Syncline Axis Map Location Anticline Axis 0248 km Overturned Fold Axis

The Windmill fault emplaces Precambrian rocks against the lower Cretaceous with a 40Ar/39Ar age of ca. 33 Ma (Table 1; Channell et al., 2000), is petrologically Mojado Formation (Figs. 5 and 6). A subsidiary splay of the Windmill fault similar to the dated granitic dike (14GP01) that crosscuts the Windmill fault and (Fig. 6) juxtaposes the Earp Formation in the hanging wall of the splay against northwest-southeast normal fault. Mojado Formation in the footwall. The minimum throw on the Windmill fault, estimated from the cumulative thickness of Paleozoic strata to the upper part Central Domain of the Mojado Formation, assuming an absence of Jurassic Broken Jug strata due to southward pinchout of Jurassic beds, is ~4200 m. Despite an absence North of Granite Pass, upper Jurassic to Cretaceous strata of the Bisbee of unambiguous evidence for subsequent extensional reactivation along the Group (Lawton and Olmstead, 1995; Lucas and Lawton, 2005) comprise an Windmill fault, its orientation parallel to a nearby northwest-southeast nor- extensive south-southwest–dipping panel that dominates the central domain. mal fault suggests that the observed stratigraphic displacement is a minimum. The central domain contains the only recognized exposures of Jurassic Bro- The Granite Pass and Windmill faults, constituting the Granite Pass–Windmill ken Jug Formation in southwestern New Mexico (Lawton and Harrigan, 1998; fault system, are separated by an unnamed northwest-southeast normal fault, Lucas and Lawton, 2000), but correlative upper Jurassic strata are present in which is truncated by the Granite Pass pluton (Fig. 6). The Granite Pass pluton, the Chiricahua Mountains of southeast Arizona (Figs. 3 and 4; Lawton and Olm-

stead, 1995; Olmstead and Young, 2000). Conglomerate in the Broken Jug For- along the Livermore Spring fault, as recorded by fault slickenlines (53°, 223°, mation has north-south elongated pebbles, stretched by a factor of 3 (Cleary, rake = 37°). 2004), along with local mylonitic fabrics. The stretched-pebble conglomerates The east-west trending Bull Canyon graben lies between the Copper Dick are most apparent near the Copper Dick fault. The Bisbee Group is intruded fault to the south and the Bull Canyon fault to the north. The graben is offset by the Late Cretaceous Sylvanite Complex, near which the strata are extensively at the Livermore Spring fault, with the antithetic graben-bounding faults of thermally metamorphosed. Bisbee strata are locally displaced by minor east- the footwall more closely spaced than in the western hanging wall. East of the west, west-northwest–east-southeast–trending normal faults with <10 m of off- Livermore Spring fault, the Bull Canyon graben contains a syncline that in- set, rhyolite dikes, and thrust faults. Conjugate west-northwest–east-southeast cludes Ringbone Formation unconformable upon a narrow (~100 m) exposure and east-northeast–west-southwest rhyolite dikes that intrude Bisbee Group of Mancos Formation. West of the Livermore Spring fault, the Skunk Ranch For- strata yielded 40Ar/39Ar ages between 32 and 27 Ma (Table 1; Cleary, 2004). mation directly overlies the U-Bar Formation. No Ringbone Formation is pres- Paleozoic strata that underlie the Broken Jug Formation are locally exposed ent west of the fault, and U-Bar limestone boulders locally dominate the basal adjacent to the eastern segment of the Copper Dick fault. conglomeratic beds of the Skunk Ranch Formation. Upper Eocene–Oligocene volcanic rocks unconformably overlying the Skunk Ranch Formation contain Northern Domain a basal lag of Skunk Ranch boulders. North of the Bull Canyon fault and the Mojado thrust fault, the Skunk Ranch Formation overlies the Ringbone Forma- The northern domain of the Little Hatchet Mountains is bounded on tion (Basabilvazo, 2000). Volcanic and volcaniclastic strata in the Bull Canyon the south by the Copper Dick fault and on the north by a prominent north- graben constitute the thickest preserved Paleogene section in the Little Hatchet west-southeast normal fault that juxtaposes Mojado Formation against Oligo- Mountains. No Paleogene rocks are exposed east of the Livermore Spring fault cene ignimbrites of the Coyote Hills (Fig. 5). The east-west Copper Dick fault or south of the Copper Dick fault. has a north dip of ~60° and a down-to-the-north minimum stratigraphic dis- The segment of the Livermore Spring fault between the Bull Canyon and placement along its western segment of 3000 m, estimated from offset lower Copper Dick faults displays older-on-younger stratigraphic separation despite to upper Cretaceous strata. Total offset along the eastern segment of the fault the fault’s normal displacement. Hanging wall U-Bar Formation is juxtaposed is accommodated across two east-west normal faults (Fig. 7A) and across one against Ringbone Formation, leading previous workers to interpret this seg- dominant fault on its western segment (Fig. 7B). ment of the Livermore Spring fault as a thrust fault (Zeller, 1970), potentially The synorogenic the Ringbone and Skunk Ranch Formations (Fig. 8) oc- related to Eocene transpression along the Copper Dick fault (Hodgson, 2000). cupy the northern domain of the Little Hatchet Mountains (Basabilvazo, 2000; Normal offset on the Livermore Spring fault is unambiguous north and south Hodgson, 2000) and unconformably overlie Bisbee Group strata and a local ex- of this central segment; moreover, the southern continuation of the fault rep- posure of Mancos Shale (Lucas and Lawton, 2005). The Ringbone and super resents the western range-bounding fault, indicating its young age (Fig. 5). jacent Skunk Ranch Formations consist of (1) alluvial and fluvial conglomer- The folded Howells Ridge thrust fault parallels both limbs of the Howells ates containing unmetamorphosed clasts of Bisbee Group and Paleozoic rocks Well syncline and displays striking changes in stratigraphic separation along that record an unroofing sequence of Cretaceous to Paleozoic strata (Clink- strike. On the north flank of the syncline, the fault trends northwest-southeast scales and Lawton, 2014); (2) braided fluvial volcanic-lithic sandstones; and and is offset by the north-south Beacon Hill normal fault. West of the Bea- (3) lacustrine rocks intercalated with ash-fall tuffs (Lawton et al., 1993; Jennings con Hill fault, the Howells Ridge fault juxtaposes the U-Bar Formation over et al., 2013; Clinkscales and Lawton, 2014). Bisbee Group strata are exposed in the Ringbone Formation, displaying unambiguous reverse separation (Fig. the west-plunging Howells Well syncline and as discontinuous fault-bounded 7B), whereas to the east, the fault juxtaposes the upper part of the U-Bar For- slivers along thrust faults. Isolated fault blocks (~10 m2) of mylonitic U-Bar For- mation over the Hell-to-Finish and lower U-Bar Formations and thus displays mation limestone crop out along the Copper Dick fault and commonly consist normal separation. Laramide synorogenic strata are absent east of the Bea- of marble with granular texture and local foliation with an average 73° (NE) con Hill fault, except for a thin, local exposure of lowermost Ringbone For- strike, subparallel to the Copper Dick fault. Fault sheared granite, in domains mation, which unconformably overlies the Hell-to-Finish Formation adjacent which rarely exceed 10 m2, is present along both segments of the Copper Dick to the fault (Basabilvazo, 2000), indicating that Hell-to-Finish likely underlies fault zone. One of these granite exposures was dated at 34 Ma (Fig. 9D). Ringbone Formation west of the Beacon Hill fault (Fig. 7B). The Ringbone and The northern structural domain south of the Hidalgo fault can be further Hidalgo formations crop out west of the Beacon Hill fault in an asymmetric divided into western and eastern sectors across the north-trending Livermore syncline (Fig. 5). Spring fault. The Livermore Spring fault displaces the western segment of the The northern domain can be further divided at the NW-trending Hidalgo Copper Dick fault southward relative to its eastern segment in the footwall of fault, a prominent northeast-verging reverse fault that juxtaposes Bisbee and the Livermore Spring fault. The strike separation between the western and Ringbone Formation strata against the Hidalgo Formation. Along the central to eastern traces of the Copper Dick fault is likely augmented by oblique slip eastern trace of the Hidalgo fault, the Hell-to-Finish Formation was emplaced

over upper Cretaceous Hidalgo Formation, but west of the Beacon Hill fault, CONTRASTING STRUCTURAL LEVELS AND RELATIONS the Ringbone Formation is thrust over the Hidalgo Formation. The Hidalgo Formation crops out extensively north of the Hidalgo fault. Structural relations in the Hatchet ranges provide key evidence for pre-Lara North of the Hidalgo fault, the Bisbee Group and Ringbone Formation are lim- mide and post-Laramide fault evolution and kinematics. The regional cross ited to the northeastern part of the range, where they are folded into an anti- section (Fig. 7A) demonstrates that the Hatchet ranges expose four recogniz- cline-syncline pair adjacent to the Ringbone thrust fault (Fig. 5; Zeller, 1970; able structural levels, which include, from deepest to shallowest: (1) a deep Hodgson, 2000). The Hidalgo Formation unconformably overlies the Hell-to- structural level consisting of Precambrian rocks exposed between the Granite Finish, U-Bar, and Ringbone formations and is intruded by Oligocene diorite Pass–Windmill fault system and the Hatchet Gap fault; (2) an intermediate level dikes of the Eureka intrusive complex (Channell et al., 2000). The Ringbone of southwest-dipping Paleozoic rocks in the Big Hatchet Mountains north of the Formation unconformably overlies the U-Bar Formation in the hanging wall of Big Tank fault; (3) a shallower level that consists of southwest-dipping Bisbee the Ringbone fault, which emplaces Ringbone, Hell-to-Finish, and U-Bar strata Group strata in the central structural domain of the Little Hatchet Mountains over the Mojado Formation. The absence of the Mojado Formation from the and in the footwall of the Big Tank fault in the Big Hatchet Mountains; and (4) a hanging wall near the Mojado thrust trace (Zeller, 1970) indicates thrust dis- shallow level where Laramide syntectonic strata and ignimbrites are exposed placement and resulting erosion of the Mojado Formation prior to Ringbone in the northern domain of the Little Hatchet Mountains (Fig. 7B) and in down- deposition. Displacement along the Ringbone fault was of sufficient magni- dropped fault blocks along east-west and northwest-southeast normal faults. tude to exhume ~1200 m of Mojado Formation. The deepest structural level between the Granite Pass and Hatchet Gap faults (Figs. 5 and 10) exposes Grenville-age (ca. 1.1 Ga) igneous rocks and a nonconformable contact with overlying Cambrian Bliss Sandstone. The ap- Big Hatchet Mountains proximately southwest-dipping Paleozoic section of the Big Hatchet Moun- tains nonconformably overlies Precambrian rocks and generally defines the The Big Hatchet Mountains contain a nearly complete section of lower and backlimb of the northeast-vergent Hidalgo uplift (Fig. 7A). The Hidalgo uplift upper Paleozoic strata that generally strike northwest-southeast and dip south- is bounded to the south by the Mine Canyon and Big Tank faults. These southwest (Zeller, 1965). The oldest strata are in the north-northeast part of the range west-vergent reverse faults are of lesser total displacement than the com- where lower Paleozoic strata overlie Proterozoic rocks on a nonconformity bined Granite Pass and Windmill faults and thus are considered as secondary (Fig. 5). The most striking geologic formation in the Big Hatchet Mountains structures to the main northeast-vergent reverse faults, which emplace Pre- is the Pennsylvanian Horquilla Limestone, which defines many of the highest cambrian rocks over Paleozoic and lower Cretaceous rocks. The opposed ver- promontories and peaks in the range, including Big Hatchet Peak at 2550 m gence of the Mine Canyon–Big Tank and Granite Pass–Windmill fault systems (Drewes, 1991). The structural geology comprises northwest-southeast–strik- created an asymmetric bivergent wedge with the Big Hatchet Mountains in ing reverse and thrust faults that generally verge southwest, and a younger the core of the uplift. This bivergent style is similar to Laramide basement- set of northwest-southeast–trending normal faults. The two most prominent involved uplifts of the central Rocky Mountain region, where seismic reflec- reverse faults include the Mine Canyon and Big Tank faults (Fig. 5). The Mine tion data and subsurface well control augment extensive field exposures Canyon fault is a complex southwest-verging fault zone of imbricate horse for detailed cross-section construction (e.g., southern Beartooth Mountains; blocks that duplicate the Horquilla Formation and overlying Permian strata. Neely and Erslev, 2009). The Big Tank fault, interpreted as a reverse fault by Zeller (1965), emplaces Paleozoic strata are inferred to underlie the Little Hatchet Mountains, as in- lower Permian rocks, in the northern hanging wall, over folded Bisbee Group dicated by thin slivers of Paleozoic rocks near Granite Pass and local exposures strata (Drewes, 1991). The Bisbee Group consists of a stratigraphic succession beneath Jurassic strata near the Copper Dick fault (Figs. 5 and 10; Zeller, 1970; similar to that observed in the Little Hatchet Mountains (Fig. 8) except that Lawton and Harrigan, 1998). Although of uncertain thickness and extent, the Jurassic strata are absent from the range; instead, lower Cretaceous rocks di- Paleozoic strata regionally thin northward away from the axis of the Paleozoic rectly overlie Permian strata (Zeller, 1965; Drewes, 1991). Pedregosa basin, now largely inverted in the Big Hatchet Mountains (Thomp- Northwest-southeast normal faults are present throughout the range. The son et al., 1978). age of these normal faults is inferred to be equivalent to northwest-trending The central domain of the Little Hatchet Mountains, between the Wind- faults and dikes in the Little Hatchet Mountains (Fig. 5). This inference is based mill and Copper Dick faults, represents the southernmost footwall block of the on similar fault orientations and juxtaposition relations. Paleogene volcanic Hidalgo uplift that underlies the Late Cretaceous Laramide unconformity. In rocks in the southeastern Big Hatchet Mountains occupy the hanging walls of turn, the Copper Dick fault juxtaposes a full section of pre-Laramide Meso- the northwest-southeast normal faults and are correlative to ignimbrites ex- zoic strata in the structurally higher southern footwall block (central domain) posed in the central domain of the Little Hatchet Mountains (McIntosh and against Laramide synorogenic strata of the Ringbone basin and overlying Bryan, 2000). Paleogene ignimbrite strata only preserved in the hanging wall (northern do-

main). We infer that the central and northern domains of the Little Hatchet Mountains are two structural levels within the footwall of the Hidalgo uplift. Recognition of the north-south–trending Livermore Spring normal fault Future Livermore requires a reinterpretation of the Copper Dick fault. The two segments of the A Spring fault Copper Dick fault were previously interpreted as parts of a steep, continuous W E Laramide right-lateral strike-slip fault that passed through an abrupt left-step Formations restraining bend (Zeller, 1970; Hodgson, 2000) at what is interpreted here as Ks Ks—Skunk Ranch the Livermore Spring fault. The former interpretation derived from apparent Ku Ku Kr—Ringbone emplacement of the U-Bar Formation over the Ringbone Formation at the fault bend. Structural relations demonstrate normal displacement on the Copper Kma—Mancos Shale Dick fault, and sheared upper Eocene–Oligocene granite was likely intruded Skunk Ranch Km—Mojado along the fault zone during active fault dilation. Mylonitic U-Bar limestone in fault Ku—U-Bar the fault zone likely formed in the presence of recrystallization temperatures Kr between 300 and 400 °C (e.g., Bernabe and Brace, 1990). We infer that the Mojado fault Kh—Hell-to-Finish Paleogene granite intrusions, mylonitic marble blocks, and north-south elon- Km gated limestone pebbles in the Broken Jug Formation indicate high geother- Kma mal gradients during north-south extension, which took place during latest Ku Eocene–Oligocene time on the basis of the ages of the granite intrusion (Fig. Kh 9D) and rhyolite dikes elsewhere in the range (Table 1). Extension was thus synmagmatic and of sufficient magnitude to generate normal faults, some thrust faults eroded in uplifted footwall with throws exceeding 3000 m (e.g., Copper Dick fault). Moreover, overlap- of Livermore Spring fault B Skunk Ranch ping ages of the Oligocene silicic igneous rocks in the range, including the 34 ± fault Livermore Spring 1 Ma granite intrusion along the Copper Dick fault, a 33 Ma ignimbrite asso- fault ciated with the thick volcaniclastic section in the Bull Canyon graben, and the N 44 32–27 Ma rhyolite dikes, indicate likely synextensional deposition of Oligocene ~50 volcanic rocks. Neogene displacement on the Livermore Spring fault served to expose Ksl variable structural depths of intrabasinal Laramide thrust faults. The U-Bar For- Kr Ku Ku mation in the hanging wall of the Mojado thrust system, whereas preserved west of the Livermore Spring fault, is now eroded east of the fault, where only Kma the subthrust Ringbone Formation is exposed (Fig. 11). Bounding faults of Mojado fault Km the Bull Canyon graben are more closely spaced east of the Livermore Spring fault, as expected for antithetically dipping normal faults exposed at a deeper Kr level. The deeper eastern block of the Bull Canyon graben exposes subthrust Ku strata, whereas the shallower, western block exposes the structurally higher thrust sheets. These intrabasinal thrust sheets were active during or after Ring- bone deposition, resulting in exhumation and erosion of the Ringbone and Mojado Formations from the hanging wall of the Mojado thrust and subse- Kh quent burial of the U-Bar Formation by the Skunk Ranch Formation containing basal conglomerate beds dominated by U-Bar boulders. Figure 11. Conceptual sketch drawing for older-on-younger juxtaposition across the north-south Livermore Spring fault. Cross-sectional line is shown in Figure 10. (A) Structure before offset Extensive exposures of the Hidalgo Formation (Young et al., 2000) occur along the Livermore Spring fault. The Mojado and Skunk Ranch faults define thrust sheets with north of the Hidalgo fault. The Hidalgo Formation is age correlative with the U-Bar Formation in the hanging wall overriding Ringbone Formation. Formation abbreviations upper member of the Ringbone Formation and the Skunk Ranch Formation as in Figure 10. (B) Thrust sheets with U-Bar Formation in thrust hanging-wall blocks are offset (Clinkscales and Lawton, 2014). The thick section of Hidalgo Formation, which by the Livermore Spring fault. This resulted in older-on-younger juxtaposition of structurally higher thrust sheets emplaced against lower plate Ringbone Formation. is mostly restricted to north of the Hidalgo fault, suggests that this fault may have served as an intrabasinal partitioning structure within the Laramide Ring- bone basin.

DISCUSSION rived from the eroded core of the Hidalgo uplift in the Big Hatchet Mountains with possible minor contribution from the southern domain of Little Hatchet Pre-Laramide Structure and Basin Configuration Mountains. However, the absence of thermal alteration of source rocks prior to synorogenic clast production indicates that the Sylvanite intrusive complex, Correlation of Bisbee Group strata and lower Cretaceous subcrop relations only ~1 m.y. younger than uppermost preserved Laramide strata, immediately reveal the rift geometry of the Bisbee basin in southern New Mexico (Fig. 4). postdated Ringbone and Skunk Ranch deposition. This observation, along with Bisbee Group strata crop out in nearby ranges, including the Animas, Burro, the thick basaltic andesite flows assigned to the Hidalgo Formation, indicates Chiricahua, and Peloncillo Mountains, and the Cookes Range (Fig. 3). In the that by the earliest Maastrichtian, the Ringbone basin was converted from Little Hatchet Mountains, a thick Bisbee Group section, the thickest in the re- a subsiding depocenter dominated by alluvial, fluvial, and lacustrine sedi gion, includes the upper Jurassic Broken Jug Formation (1228 m), a thick U-Bar mentation to a volcanic center. No evidence of early Paleogene deformation or section (~970 m versus ~240 m north of Copper Dick fault), and thick Mojado associated syntectonic clastic deposition is recognized in southwesternmost Formation (~1200 m). The upper Jurassic to lower Cretaceous Bisbee Group New Mexico. However, Paleogene clastic strata are documented elsewhere in thins from a thick former basin keel in the Little Hatchet Mountains (Lucas and the region, notably the northern Florida and Victorio Mountains (Lobo Forma- Lawton, 2000; Lawton, 2004; Machin, 2013), north to the Burro Mountains and tion; De los Santos et al., 2016) and along the Rio Grande rift near Las Cruces, Cookes Range to an area of less accommodation on the margin of the basin. New Mexico (Love Ranch Formation; Seager et al., 1997). Upper Cretaceous strata equivalent to the upper part of the Mojado Formation The dimensions, orientations, and extents of the southern Cordilleran Lara- in the Burro Mountains unconformably overlie Precambrian rocks (Mack et al., mide structures and basins are a result of the antecedent structural architecture 1986; Lawton, 2004; Machin, 2013). of the Border rift. The geographic extent of Laramide uplifts and basins in south- The thinning of upper Jurassic to upper Cretaceous strata north of the Little ern New Mexico is projected from exposed reverse and thrust structures, com- Hatchet Mountains can be attributed to the fault block geometry of the Bisbee parison of stratigraphic sections across ranges, and sparse well control (Hodg- basin and proximity to the rift shoulder of the Mogollon Highlands (Fig. 1). In ad- son, 2000; Seager, 2004). Despite the relative position of the Tarahumara arc to dition, a similar north-northeast thinning trend for lower-upper Cretaceous rocks the Hidalgo uplift and Ringbone basin, the foreland of southern New Mexico can be attributed to the geometry of a post-rift early Late Cretaceous foreland did not develop into a thin-skinned retroarc fold-thrust system. We infer that if basin (Mack, 1987; Clinkscales and Lawton, 2014). The transition to a foreland the preexisting Bisbee rift structures had not been present, the region may have basin setting is represented by the thick (>1200 m) Mojado Formation and Ceno- been more akin to other retroarc thin-skinned provinces (e.g., Sevier fold-thrust manian marine, ammonite-bearing facies of the Mancos Formation (Lucas and belt), but the presence of preexisting, basement-involved normal faults facili- Lawton, 2000, 2005), deposited in a basin foredeep situated in the Little Hatchet tated fault reactivation and uplift of granitic Mesoproterozoic basement blocks Mountains, and thinner correlative Beartooth Formation in the Burro Mountains and the development of bivergent block uplifts (Fig. 12). In addition, the Lara- deposited in a forebulge or distal foredeep position (Mack, 1987; Machin, 2013). mide uplifts in southern New Mexico have a shorter uplift-to-basin wavelength than contemporaneous structures in the central Rocky Mountain region and are Laramide Clastics, Volcanism, and Uplift Style of lesser areal extent. As a simple comparison, the Bighorn Mountains of Mon- tana and Wyoming are ~55 km wide in the northeast-southwest direction and Contemporary arc volcanism is recorded in the Laramide syntectonic rocks flanked by the ~100-km-wide Bighorn basin. In contrast, the width of the Hidalgo of the Little Hatchet Mountains. Ash-fall tuffs and basaltic andesite are inter- uplift, projected between its flanking reverse faults in the northeast-southwest bedded with continental syntectonic deposits (Basabilvazo, 2000; Jennings direction, is ~20 km and the maximum width of the Ringbone basin is <~40 km et al., 2013), and the thick (>1700 m) ca. 70 Ma Hidalgo Formation, consist- (approximate distance between Granite Pass fault and Luna uplift; Fig. 3). Fur- ing dominantly of basaltic andesite flows (Young et al., 2000). The syntectonic thermore, the current areal extent of the Hidalgo uplift and Ringbone basin is a Laramide section was deposited between ca. 75 and 70 Ma (Clinkscales and maximum width if post-Laramide extension is considered (Fig. 12). Lawton, 2014), and emplacement of the Sylvanite plutonic complex ca. 69 Ma The occurrence of synorogenic volcanic rocks, the coeval Tarahumara arc in appears to postdate, or coincide with, the waning stages of Laramide deposi- Sonora, Mexico, and nearby magmatism (Fig. 3) are major factors that differ- tion in the Ringbone basin. The intrusion is also likely postdepositional based entiate the Laramide orogen in southern New Mexico from contemporaneous on comparison of intruded strata in the footwall of the Copper Dick fault and uplifts in the central Rocky Mountains. Late Cretaceous (Campanian) Laramide synorogenic conglomerate clasts of the Ringbone and Skunk Ranch Forma- shortening in southern New Mexico encompassed an area from the Hidalgo up- tions. Strata near the Sylvanite complex are pervasively thermally metamor- lift to the Rio Grande uplift with active local magmatism from ca. 76 to 69 Ma phosed and all Bisbee strata in the central domain show some evidence of (Amato et al., 2017; this study). By the end of the Campanian (ca. 69 Ma), the thermal alteration, whereas clasts in conglomerate of the Ringbone and Skunk Ringbone basin in the Little Hatchet Mountain area was no longer a subsiding Ranch Formations are not metamorphosed. These clasts were dominantly de- depocenter accumulating continental clastics, but a transformed volcanic center.

GEOSPHERE | Volume 14 | Number 1 Clinkscales and Lawton | Mesozoic–Paleogene structural evolution of the southern U.S. Cordillera Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/14/1/162/4035211/162.pdf 180 by guest on 24 September 2021 on 24 September 2021 by guest Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/14/1/162/4035211/162.pdf Research Paper the Jb is unknown; however, the section likely thins northward onto higher former rift blocks. The Broken Jug Formation (Jb) does not crop out in the Big Hatchet Mountains and thickest known exposures occur south of the Copper Dick fault. The northern limit of zoic section and the nature of the Paleozoic and Mesozoic contact north of the Granite Pass–Windmill fault system; therefore, the Paleozoic section is not restored in detail. indicate of shortening. ~8 km Limited Paleozoic exposures in the Little Hatchet Mountains do not permit robust inferences regarding the stratigraphic thickness of the Paleo - the northern hanging wall. (C) Schematic pre-Laramide line length restoration. Line lengths for Mojado and U-Bar Formations were preserved across the cross section and ening. This line of section does not intersect Laramide synorogenic strata directly north of the Copper Dick fault; however, Laramide synorogenic rocks are only preserved in the asymmetry of bivergent Hidalgo uplift. The Laramide unconformity represents a conceptual erosional base over the main uplift that existed during Laramide short shown as A-A’ in Figure 5. (B) Paleogene extension and magmatism restored. Fault offset on the Big Tank and Mojado faults is restored to simplify section. Section illustrates verti Figure 12. Structural restoration of the Little and Big Hatchet Mountains with preserved formation thicknesses and line length. Formation labels as in Figures 5 and 10 (v.e.— Legend all cross sections to same scale and U-Bar Formations based on Mojado line length restoration Prcse—undi Jb—Broken Jug Formation Khv—Cretaceous Hidalgo Formatio Permian strata *Kh in Figure 5 part of Kbg Kh—Hell-to-Finish Formation *Ku in Figure 5 part of Kbg Ku—U-Bar Formation Km—Mojado Formatio Pgv—Paleogene volcaniclastic rocks Ranch Formations R ingbone and Skunk Kr and Ks—Upper Cretaceous quartz monzonite and diorite Kqm—Cretaceous Sylvanite Comple quartz monzonite Pgg—Paleogene granite and IPh—Horquilla Limestone Permian strata Pce—undi strata Pzl—undif Pzu—undi (Little Hatchet Mountains only) i gneous rock s pCg—Mesoproterozoic (ca. 1.1 Ga) cal exaggeration). (A) Present-day structure with dip tadpoles and projected contacts based on field relations. Line of section is identical to Figure 7A and location is ~8 km shortening ferentiated lower Paleozoic ff ff erentiated lowermost erentiated Paleozoic strata ff erentiated lower pC Pzl IP h Pce Prcse Kh Ku Km n

n x SW 0 m 1000 m 2000 m 3000 m C A Paleogene extension restore d B 10000 m Prcse Big T Km on Big T Ku fault Kh Fault offset Pc e IP h Present Da y Prcs e Pre-Laramide Restoration, ank Ku Kh Km Laramide Uplif t Pz l restored ank fault 1:1 v. Pc e IP h Mine Canyon fault system Pce e. IP h Pzl Big Hatchet Mountains IPh pC ? ca. 1.1 Ga Hidalgo Uplift pC datum To Hatchet Ga p faul t are unknown. Depth to Mesoproterozoic basement The thickness and continuity of the Paleozoic sectio Little Hatchet Mountains are only partially exposed Paleozoic strata in the location of th Ma 33 Pg g Km Ku ? fault restored Granite Pass Granite Pass Kh Km p Mojado Formation Ku Kh faul t crop out south of Copper Dick faul no Laramide syntectonic clastics Jb Copper Dic Pzu erosion and deposition Syntectoni c NW Normal faul t Windmill faul t -SE faul t Jb 69 Ma Kq m Kq m k

t Little Hatchet Mountain Jb Ringbone Basi n down-plunge position in synclin Syntectonic clastics crop out in Kr + Ks Howells Synclin Km Laramide Unconformit e We e Pz u ll Kh v unconformity Laramide Future Ku e

Kh Jb Pz u Hidalgo faul Kh v Ringbone faul Pz u Kh Ku Kr Km . t s y n t NE Pin

GEOSPHERE | Volume 14 | Number 1 Clinkscales and Lawton | Mesozoic–Paleogene structural evolution of the southern U.S. Cordillera 181 Research Paper

Late Cretaceous magmatism in southern New Mexico has been attributed 1984). Metamorphic core complexes similar to those in southern Arizona have to a migrating volcanic arc that attended the progressive shallowing of the not been identified in the Little and Big Hatchet Mountains, but the east-west Farallon plate (McMillan, 2004); nevertheless, when assessed in detail, the normal faults may represent an earliest embryonic core complex (e.g., Rick- distribution and age of Late Cretaceous plutons in southern New Mexico sug- etts et al., 2015) or the structurally highest fault segment of a metamorphic gest no systematic younging trend to the northeast or batholith parallel to complex. However, low-angle normal faults, with similar northwest-southeast the trench. For example, the Copper Flat porphyry system in the Animas Hills orientations, are present at Mahoney Ridge in the Florida Mountains (Fig. 3), (Fig. 3) has a ca. 75 Ma age (McLemore et al., 2000), but is located northeast which were mapped as low-angle thrust faults with younger-on-older separa- of the younger 70–69 Ma volcanic centers for the Hidalgo Formation and Syl- tion (Brown and Clemons, 1983). vanite plutonic complex in the Little Hatchet Mountains and Late Cretaceous Models explaining latest Eocene–Oligocene extension in southwest New copper porphyry systems in southeastern Arizona (e.g., Lang and Titley, 1998). Mexico must take into account the large-volume silicic volcanism and magma- In contrast, Cretaceous magmatism in central Sonora, Mexico, appears to fol- tism of the period, recorded by the local Boot Heel (e.g., McIntosh and Bryan, low an expected northeast younging trend with plutonic and volcanic rocks 2000) and Mogollon-Datil volcanic fields (McIntosh et al., 1992). Magmatism parallel to the trench (González-León et al., 2011). We suggest that Late Creta- and extension occurred in pulses (McIntosh and Bryan, 2000), as indicated by ceous magmatism in southern New Mexico and southeastern Arizona resulted ca. 34 Ma intrusions along the Copper Dick fault and Granite Pass pluton, and from localized convection along the southeast margin of a subducting oce- ca. 27 Ma latite dikes (Cleary, 2004). These intrusions are age correlative to ig- anic plateau (Liu et al., 2010), perhaps at a tear fault at the edge of the classic nimbrite rocks ubiquitous throughout the region (Fig. 3). Although an unequiv- Late Cretaceous flat slab corridor (Fig. 1; e.g., Valencia-Moreno et al., 2017). ocal relationship between east-west and northwest-southeast normal faults The resultant northeast-southwest magmatic belt along the flat slab margin and expanded growth ignimbrite sections in hanging-wall blocks cannot be would have been contemporaneous with trench-parallel arc volcanism in cen- firmly established in the Little and Big Hatchet Mountains, the occurrence and tral Sonora, Mexico. However, by the Eocene the volcanic arc appears to have preservation of the thickest ignimbrite exposures in the hanging walls suggest migrated northeast into southern New Mexico (McMillan, 2004), suggesting a synkinematic association. progressive shallowing of the Farallon plate in the Paleogene and attendant On the basis of thick tilted sections of Oligocene ignimbrites near Silver magmatic activity. Future investigations should consider Late Cretaceous mag- City, New Mexico (Fig. 3), some workers have inferred that Laramide shorten- matism in southern New Mexico in relation to the southeast margin of the Late ing continued as late as the Oligocene (Copeland et al., 2011; Tomlinson et al., Cretaceous Laramide flat slab corridor and concomitant Laramide structures 2013). Thickening of the ignimbrite section near Silver City is interpreted to and porphyry complexes in southeastern Arizona. have taken place on the hanging walls of blind thrust faults as part of growth monoclinal structures. In contrast, the thickest preserved sections of Eocene– Paleogene Extension and Magmatism Oligocene ignimbrite rocks in the Hatchet ranges are located within grabens associated with unambiguous normal faults, where the ignimbrites deposi- In the Great Basin region of the western U.S., a southwestward sweep in tionally truncate Laramide thrust faults and unconformably overlie Laramide arc magmatism occurred from ca. 50 Ma in Idaho and Montana, 40 Ma in cen- syntectonic strata. Shortening-related growth in the ignimbrite section near tral Nevada, and ca. 20 Ma in southern Nevada (Fig 2; Dickinson, 2002, 2013). Silver City may reveal local strain complexities contemporaneous with exten- Paleoele vation and paleoclimatic models for the Great Basin region indicate sion in the Hatchet ranges; nevertheless, our analysis contravenes a short- that this southward sweep in Paleogene magmatism was coincident with a pro- ening origin for the ignimbrite growth monoclines. We speculate that these tracted migration in surface uplift and metamorphic core complex formation tilted Oligocene sections are of extensional origin, as tilted footwall blocks or (e.g., Mix et al., 2011); however, the timing for development of high-elevation, monoclines cored by blind normal faults, based on late Eocene–Oligocene rugged topography in the Great Basin region is still unresolved, as indicated subregional to regional deformation trends (Fig. 2) and the similar age and by detailed mapping and geochronology that challenge the ages, stratigraphic orientation for northwest-southeast and east-west surface-breaking normal positions, and implied significance of rock samples used in previous stable faults in the Little Hatchet Mountains. No field evidence in the Hatchet ranges isotope studies (Lund Snee et al., 2016). The Cordillera of the southwestern or other nearby ranges corroborates Oligocene shortening; instead, evidence U.S. and Sonora, Mexico, likewise underwent a diachronous west to south- indicates regional Oligocene extension. west sweep in magmatism, extension, and surface uplift(?) from southern New Late Eocene–Oligocene extension and magmatism in the Hatchet ranges Mexico ca. 34 Ma (Coney and Reynolds, 1977; Dickinson, 2002; this study), likely resulted from a combination of factors ultimately associated with chang- through Arizona and Sonora, Mexico, from 30 to ca. 20 Ma (Dickinson, 2002, ing plate interactions between the Farallon and North American plates (e.g., 2006). In southern Arizona and Sonora, the westward migration of magmatism Coney and Harms, 1984; Engebretson et al., 1984). Extension in the region is coincides with the development of large-magnitude extensional fault systems part of a regional trend of backarc extension that took place as the Farallon (Gans, 1997) and metamorphic core complexes (Davis, 1980; Coney and Harms, plate foundered westward (Humphreys, 1995; Dickinson, 2002), accompanied

by asthenospheric flow into the mantle wedge during plate retreat. Upwell- fined by bivergent thrust and reverse faults, is exposed in southwesternmost ing asthenosphere led to the voluminous silicic magmatism of the Boot Heel New Mexico. This interpretation is in stark contrast to proposed thin-skinned (e.g., McIntosh and Bryan, 2000) and Mogollon-Datil volcanic fields (McIntosh deformation throughout the southern U.S. Cordilleran region (e.g., Corbitt et al., 1992). Extension that accompanied latest Eocene–Oligocene magmatism and Woodward, 1973; Drewes, 1988). However, thin-skinned deformation in may have been induced by crustal weakening caused by renewed magmatism southern Arizona (e.g., thrust structures in the San Pedro trough area; Dick- associated with the rollback of the Farallon plate, and gravitational instability inson, 1991) may have been contemporaneous to the Laramide structures in of the upper crust as a response to crustal thickening accompanying heat ad- the Hatchet Mountains, and future studies can resolve the spatial distribu- vection (e.g., Liu, 2001) and/or magmatic additions to the crust. Furthermore, tion of these disparate structural styles. Regional shortening ended by early it is likely that variations in normal fault orientations from east-west to north- Eocene time (ca. 52 Ma), as indicated by local structural relations and basin west-southeast could have been controlled by local perturbations around vol- development in southern New Mexico. Late Cretaceous faults and folds are canic centers (e.g., Nieto-Samaniego et al., 1999) and/or slight strain rotation displaced by latest Eocene to Oligocene east-west– and northwest-southeast– related to different pulses of magmatism. trending dikes and normal faults, with some normal fault throws exceeding On the basis of the close temporal relationship between Paleogene silicic 3000 m. Late Eocene–Oligocene extension was synmagmatic, as indicated magmatism and north-south extension in southwestern New Mexico, pre– by (1) east-west–trending rhyolite dikes, some intruded along normal faults, Basin and Range extension was evidently not associated with simple gravi- (2) sheared Oligocene granite intrusions along the Copper Dick normal fault, tational instability in a high-elevation hinterland plateau as suggested for the and (3) thick contemporary ignimbrite sections in Eocene–Oligocene grabens. Sevier hinterland, or Nevadaplano (Fig. 1; e.g., Druschke et al., 2011). We con- Eocene–Oligocene arc magmatism is attributed to asthenospheric upwelling clude that extension was not a result of gravitational collapse of a high-stand- and crustal melting as the Farallon plate foundered westward, following the ing Laramide region because the onset of Paleogene magmatism and exten- shallower slab subduction phase of the Laramide orogeny. A west to south- sion ca. 34 Ma took place ~18 m.y. after the youngest record of syn-Laramide west migration of the Paleogene magmatic front led to thermal weakening, deposition in the region ca. 52 Ma, corresponding to the uppermost inferred extension, and possibly surface uplift, that continued from southwest New age of the Lobo Formation (De los Santos et al., 2016). This does not preclude Mexico (ca. 34 Ma) to the metamorphic core complexes of Sonora, Mexico, crustal thickening as an important mechanism leading to extension; however, and Arizona. This west and southwest migration of arc magmatism was con- crustal thickening and eventual instability were likely associated with cumula- temporaneous with a southwest sweep in magmatism in the Great Basin re- tive thickening that resulted from Late Cretaceous shortening and magmatism, gion. Future integrative tectonic studies on the southern U.S. Cordillera could and culminated in magmatic additions to, and thermal weakening of, the crust address crustal thickness and attendant paleoaltimetric changes from the Late during the Paleogene. Cretaceous and Paleogene. Ultimately, these studies, linked to the Great Basin region, would shed light on the climatic and surface evolution of the greater Western Cordillera prior to the development of the San Andreas fault system CONCLUSIONS and opening of the Basin and Range and Rio Grande rift.

Four generations of Jurassic through Oligocene structures in the Hatchet ranges of southwesternmost New Mexico provide a template for the Mesozoic ACKNOWLEDGMENTS to Paleogene kinematic evolution of those ranges and regional crustal evolu- Laboratory and field research was supported by 2010–2011 grants by the New Mexico Geological tion of the southern U.S. Cordillera. Laramide uplifts and basins of southern Society Grant-in-Aid Program, the Geological Society of America Research Grants Program, and the R.E. Clemons Field Research Fund at New Mexico State University to Clinkscales. U-Pb zircon New Mexico were the result of basement-involved deformation in the pres- geochronology was conducted at the University of Arizona LaserChron Center, supported by Na- ence of active plutonic centers. Late Cretaceous magmatism in southern New tional Science Foundation grant EAR-0732436. We gratefully acknowledge laboratory assistance Mexico may have been the result of convection along the southeast margin by Jeff Amato and Evan Kochelek. The 2D Move software provided by Midland Valley aided in cross-section construction. We thank Mike Williams and an anonymous reviewer for constructive of a subducting oceanic plateau contemporaneous with arc volcanism in cen- comments that improved the clarity of this manuscript. tral Sonora, Mexico. Local volcanism and smaller uplift-to-basin wavelengths distinguish the southern U.S. Cordillera from coeval Laramide structures in the central Rocky Mountains. The narrow uplift-to-basin configuration is a REFERENCES CITED consequence of the distribution of Jurassic–Early Cretaceous normal faults Amato, J.M., and Mack, G.H., 2012, Detrital zircon geochronology from the Cambrian–Ordovician of the Border rift. The existence of pre-Laramide normal faults is recorded Bliss Sandstone, New Mexico: Evidence for contrasting Grenville-age and Cambrian sources by local thickness variations of upper Jurassic to lower Cretaceous strata as on opposite sides of the Transcontinental Arch: Geological Society of America Bulletin, v. 124, p. 1826–1840, https://doi .org /10 .1130 /B30657 .1 . well as regional northward thinning of Mesozoic strata and abrupt changes Amato, J.M., Mack, G.H., Jonell, T.N., Seager, W.R., and Upchurch, G.R., 2017, Onset of the Lara- in lower Cretaceous subcrop. An inverted graben within the Bisbee basin, de- mide orogeny and associated magmatism in southern New Mexico based on U-Pb geochro-

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