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New Mexico Geological Society Downloaded from: http://nmgs.nmt.edu/publications/guidebooks/69 Constraining timing of extension in the southern Rio Grande Rift and basin and range using apatite and zircon (U-Th)/He thermochronology J. Biddle and Ricketts, J. W., Amato, J.M., 2018, pp. 127-135 Supplemental data available: http://nmgs.nmt.edu/repository/index.cfm?rid=2018002 in: Las Cruces Country III, Mack, Greg H.; Hampton, Brian A.; Ramos, Frank C.; Witcher, James C.; Ulmer-Scholle, Dana S., New Mexico Geological Society 69th Annual Fall Field Conference Guidebook, 218 p. This is one of many related papers that were included in the 2018 NMGS Fall Field Conference Guidebook. Annual NMGS Fall Field Conference Guidebooks Every fall since 1950, the New Mexico Geological Society (NMGS) has held an annual Fall Field Conference that explores some region of New Mexico (or surrounding states). Always well attended, these conferences provide a guidebook to participants. Besides detailed road logs, the guidebooks contain many well written, edited, and peer-reviewed geoscience papers. These books have set the national standard for geologic guidebooks and are an essential geologic reference for anyone working in or around New Mexico. Free Downloads NMGS has decided to make peer-reviewed papers from our Fall Field Conference guidebooks available for free download. Non-members will have access to guidebook papers two years after publication. Members have access to all papers. This is in keeping with our mission of promoting interest, research, and cooperation regarding geology in New Mexico. However, guidebook sales represent a significant proportion of our operating budget. Therefore, only research papers are available for download. Road logs, mini-papers, maps, stratigraphic charts, and other selected content are available only in the printed guidebooks. Copyright Information Publications of the New Mexico Geological Society, printed and electronic, are protected by the copyright laws of the United States. No material from the NMGS website, or printed and electronic publications, may be reprinted or redistributed without NMGS permission. Contact us for permission to reprint portions of any of our publications. One printed copy of any materials from the NMGS website or our print and electronic publications may be made for individual use without our permission. Teachers and students may make unlimited copies for educational use. Any other use of these materials requires explicit permission. This page is intentionally left blank to maintain order of facing pages. New Mexico Geological Society Guidebook, 69th Field Conference, Las Cruces Country III, 2018, p. 127-135. 127 CONSTRAINING TIMING OF EXTENSION IN THE SOUTHERN RIO GRANDE RIFT AND BASIN AND RANGE USING APATITE AND ZIRCON (U-TH)/HE THERMOCHRONOLOGY JULIAN BIDDLE1, JASON W. RICKETTS1, AND JEFFREY M. AMATO2 1Department of Geological Sciences, The University of Texas at El Paso, 500 West University Ave. El Paso, TX 79968 2Department of Geological Sciences, New Mexico State University, Las Cruces, NM 88003 ABSTRACT—We sampled rocks for zircon (ZHe) and apatite (AHe) (U-Th)/He thermochronology from seven mountain ranges across the Rio Grande rift-Basin and Range transition zone in southeastern Arizona, southern New Mexico, and western Texas. Individual AHe ages (n=23) range from 8–26 Ma, and ZHe ages (n=42) range from 19–649 Ma. Samples from the Basin and Range province, west of the Cookes Range and the Florida Mountains (southwestern New Mexico), have a small spread in ZHe ages, whereas samples from the southern Rio Grande rift yield a wide range of ZHe ages that is related to a corresponding spread in effective uranium (eU). Forward and inverse modeling suggests that cooling from temperatures >200°C in the southeastern Basin and Range may have been due to a combination of tectonic ex- humation and mid-Cenozoic igneous activity, whereas extension in the southern Rio Grande rift exhumed rocks from depths corresponding to temperatures <200°C. However, basins in the southern Rio Grande rift are up to 3 km deep, whereas the southeastern Basin and Range is characterized by basins with less than 700 m of basin fill. While further work is needed to fully understand the effects of normal faulting vs. igneous activity on thermochronologic data in southwestern New Mexico, these observations may be a reflection of different styles of ex- tension in each region, separated by a N–S trending boundary in southern New Mexico. Core complex style extension involving low-angle normal faults and mid-crustal detachments are common within the Basin and Range. In contrast, extension in the southern Rio Grande rift may have been accomplished through high-angle faults, which would favor the formation of deep basins, but which did not exhume rocks from depths corresponding to temperatures >200°C. INTRODUCTION ing a N–S trending zone of extension along the weakened crust that developed into the Rio Grande rift (Ricketts et al., 2016). The Rio Grande rift is one of the world’s major intracra- In addition, the relationship between the Rio Grande rift tonic rifts, extending more than 1,000 km from Colorado to and the adjacent Basin and Range province remains poorly un- Mexico (e.g., Keller et al., 1990). Although the rift has been derstood. Some argue that the Rio Grande rift is a not a distinct extensively studied, there remain fundamental questions re- structural feature and is simply part of the larger Basin and garding its origin. Understanding the timing of extension in Range province (e.g., Baldridge et al., 1984). Physiographi- different parts of the rift can provide important constraints on cally, the southern Rio Grande rift resembles the immediately possible causes of rifting. Although the timing of earliest ex- proximal Basin and Range because extension is more diffuse tension is somewhat unconstrained, changing patterns in sed- and affects a wider region (Fig. 1). However, the southern imentation, magmatism, and style of deformation all suggest Rio Grande rift in southern New Mexico can still be distin- that the region was subjected to early extension by 35–30 Ma guished from the Basin and Range because it has higher heat (Smith, 2004), and rifting is ongoing (Berglund et al., 2012). flow, deeper basins with Neogene sedimentary fill, and an Several competing geodynamic models for the initiation of the abundance of Quaternary faults (Seager and Morgan, 1979; rift have been proposed to account for this period of extension. Keller et al., 1990; Nakai et al., 2017). Constraining the timing Some models suggest that Rio Grande rift extension is driven of extension across the southern Rio Grande rift – Basin and by mantle forces, either through large-scale mantle upwelling Range transition is therefore important for addressing a variety based on mantle tomography (Moucha et al., 2008) or small- of structural questions related to the underlying driving forces scale mantle convection at the edge of the stable Great Plains that have caused extension, and for investigating the nature of province (van Wijk et al., 2008). Other models rely on crustal the boundary separating the two structural domains. forces to drive extension, and include clockwise rotation of the Colorado Plateau (Hamilton, 1981; Chapin and Cather, 1994; TECTONIC SETTING Landman and Flowers, 2013), collapse of over-thickened Rio Grande Rift continental crust (Cordell, 1978; Eaton, 1986), and initiation of a transform setting along the western margin of the North The Rio Grande rift is a region of lithospheric extension American plate (Dickinson and Snyder, 1979; Baldridge et al., situated between the Basin and Range Province and Colora- 2006). One model suggests that detachment of a piece of the do Plateau to the west and the Great Plains to the east. The Farallon plate beneath the Rio Grande region in New Mexico rift trends approximately north-south from northern Colorado enhanced asthenospheric upwelling in the slab window, creat- to northern Mexico, and has a distinctive structural style of Figure 1 128 BIDDLE, RICKETTS, AND AMATO Sample Pz-Mz Ng-Q Basin Fill Locations Cookes !"# Range 16CR01 Sierra de Cz Igneous pC Quaternary las Uvas $"%&'( Fault 16CR02 Malpais Mimbres Basin Organ Mtns Basin Little Hatchet Mesilla )*&+(,- 16PE01 Franklin Mtns 16FL04 16CH02 16FL05 Mtns 16CH01 E. Potrillo Basin 32°N Playas Basin Cedar Florida Deming Basin Mtns Mtns 17LH03 Mtns 17FR04 . "%-/ Chiricahua 16EP01 17FR03 Hueco Bolson Mtns 17LH02 17FR02 17LH01 Basin Animas N Peloncillo Mtns 015 30 60 90 120 Mtns Big km Animas Hatchet Mtns 109°W 108°W 107°W 106°W FIGURE 1. Simplified geologic map of southeastern Arizona, southern New Mexico, and western Texas highlighting sample locations. The thick dashed line is the proposed boundary between the Basin and Range and Rio Grande rift from Woodward et al. (1978). Uplifted ranges are in black text, and basins are named in gray text. asymmetrical half-graben basins bounded by normal faults Basin and Range Province (e.g., Chapin and Cather, 1994; Lewis and Baldridge, 1994). Half-grabens are typically bordered by master normal faults The Basin and Range is a significant extensional province that accumulate a majority of fault displacement and result along western North America. The region’s characteristic in uplift of the footwall block, typically exposing Precambri- physiographic appearance is controlled by systems of alternat- an crystalline rocks. From northern Colorado to central New ing horsts and grabens (Stewart, 1971). Large-magnitude Eo- Mexico, the rift is a discrete, narrow feature that consists of cene-late Miocene extension in the Basin and Range resulted in interconnected basins that have widths less than ~80 km. South a belt of metamorphic core complexes that extends from British of the Socorro “constriction” (Kelley, 1952), the rift abrupt- Columbia, Canada, to northern Mexico (Coney, 1980; Axen et ly widens, and its physical expression becomes more diffuse. al., 1993; Constenius, 1996; Dickinson, 2002, 2009).

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