RESEARCH Orocopia Schist in the Northern Plomosa Mountains, West-Central Arizona

RESEARCH Orocopia Schist in the northern Plomosa Mountains, west-central Arizona: A Laramide subduction complex exhumed in a Miocene metamorphic core complex E.D. Strickland1, J.S. Singleton1, and G.B. Haxel2,3 1DEPARTMENT OF GEOSCIENCES, COLORADO STATE UNIVERSITY, FORT COLLINS, COLORADO 80523, USA 2U.S. GEOLOGICAL SURVEY, FLAGSTAFF, ARIZONA 86001, USA 3GEOLOGY PROGRAM, SCHOOL OF EARTH SCIENCES AND ENVIRONMENTAL SUSTAINABILITY, NORTHERN ARIZONA UNIVERSITY, FLAGSTAFF, ARIZONA 86011, USA ABSTRACT We document field relationships, petrography, and geochemistry of a newly identified exposure of Orocopia Schist, a Laramide subduction complex, in the northern Plomosa Mountains metamorphic core complex of west-central Arizona (USA). This core complex is character- ized by pervasive mylonitic fabrics associated with early Miocene intrusions. The quartzofeldspathic Orocopia Schist records top-to-the-NE mylonitization throughout its entire ~2–3 km structural thickness and 10 km2 of exposure in the footwall of the top-to-the-NE Plomosa detachment fault. The schist of the northern Plomosa Mountains locally contains graphitic plagioclase poikiloblasts and scattered coarse- grained actinolitite pods, both of which are characteristic of the Orocopia and related schists. Actinolitite pods are high in Mg, Ni, and Cr, and are interpreted as metasomatized peridotite—an association observed in Orocopia Schist at nearby Cemetery Ridge. A 3.5-km-long unit of amphibolite with minor interlayered ferromanganiferous quartzite is localized along a SE-dipping contact between the Orocopia Schist and gneiss. Based on their lithologic and geochemical characteristics, we interpret the amphibolite and quartzite as metabasalt and meta chert, respectively. The top of the Orocopia Schist is only ~3–4 km below a ca. 21 Ma tuff in the footwall of the Plomosa detachment fault, suggesting that a major Paleogene exhumation event brought the schist to upper-crustal depths after it was subducted in the latest Cretaceous but before most Miocene core complex exhumation. The Orocopia Schist in the northern Plomosa Mountains is located near the center of the Maria fold-and-thrust belt, which likely represented a crustal welt in the Late Cretaceous. The keel of this crustal welt may have been sheared off by the shallowly dipping Farallon slab prior to underplating of rheologically weak Orocopia Schist. Paleogene exhumation of the Orocopia Schist in the northern Plomosa Mountains is consistent with extensional exhumation recorded in Orocopia Schist in the Gavilan Hills of southeasternmost California, which shortly postdated schist underplating, suggesting that subduction of schist may have triggered Paleogene extension in the region. LITHOSPHERE; v. 10; no. 6; p. 723–742; GSA Data Repository Item 2018358 | Published online 18 October 2018 https://doi.org/10.1130/L742.1 INTRODUCTION and Orocopia Schist was traced eastward into southwestern Arizona by the mid-1970s (Haxel and Dillon, 1978; Haxel et al., 2002). However, a The Pelona-Orocopia-Rand Schists (PORS) of southern California and recent discovery of Orocopia Schist at Cemetery Ridge in southwestern southwestern Arizona (USA) (Fig. 1) are interpreted as Late Cretaceous Arizona did not occur until 2012 (Haxel et al., 2014; Jacobson et al., to early Paleocene metamorphosed trench sediments and other minor 2017), where Orocopia Schist is >300 km inboard of the paleo–oceanic rock types (e.g., Haxel and Dillon, 1978; Jacobson et al., 1988, 2000) trench, highlighting the extreme scale of subduction underplating during subducted during the Laramide orogeny and accreted beneath the lower the Laramide orogeny. continental crust during slab flattening of a segment of the Farallon plate Detailed thermochronologic studies of Orocopia Schist in the Gavilan (Grove et al., 2003; Saleeby, 2003; Jacobson et al., 2011). Exposures of Hills and Orocopia Mountains in California (Fig. 1) revealed two distinct PORS are dominated by quartzofeldspathic schist with minor mafic schist periods of rapid cooling during the early Eocene and the latest Oligocene (metabasalt), ferromanganiferous quartzite (metachert) and marble, and to early Miocene (Jacobson et al., 2002, 2007), leading to the inference rare pods of actinolite ± talc schist or serpentine schist (e.g., Haxel and that the Orocopia Schist has undergone two phases of exhumation. The Dillon, 1978; Chapman, 2016). This subduction complex is intriguing mechanism for the first phase of exhumation remains debated (e.g., Chap- because it was exhumed as much as several hundred kilometers inland man, 2016), whereas the second phase of exhumation of the schist from from the former subduction trench (Jacobson et al., 2017), whereas the ~10–12 km depths to the surface or near surface owes to middle Cenozoic broadly correlative Franciscan accretionary complex occupies the Coast tectonic denudation on low-angle normal faults and erosion (e.g., Haxel et Ranges of California, in close proximity to the paleo–oceanic trench al., 2002; Jacobson et al., 2002, 2007). Some of these areas of unroofed (Chapman et al., 2016). The Californian PORS were initially recognized schist have metamorphic core complex–like attributes (Holk et al., 2017), as a subduction complex in the late 1960s (Crowell, 1968; Yeats, 1968), but none are a true metamorphic core complex in that they apparently Geological© 2018 The SocietyAuthors. of Gold America Open |Access: LITHOSPHERE This paper | Volume is published 10 | underNumber the 6 terms| www.gsapubs.org of the CC-BY-NC license. 723 Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/10/6/723/4548862/723.pdf by guest on 26 September 2021 STRICKLAND ET AL. | Orocopia Schist in the northern Plomosa Mountains, west-central Arizona RESEARCH CA NV AZ POR subduction complexes Rand-Pelona Orocopia Bakersfield metamorphic core complex t 35°N Garlock faul San Andreas fault Transition zone Maria fold-thrust belt BR Basin-and-Range Los Angeles MM HV LP PM DR OR Phoenix CM CR GH 33°N 100 km 118°W San Diego 114°W Figure 1. Distribution of Pelona-Orocopia-Rand Schists (PORS) subduction complexes in southern California and southwestern Arizona (USA), and locations of the northern Plomosa Mountains (PM) and other ranges mentioned in the text (modified from Haxel et al., 2014). Base map is colored by elevation and derived from GeoMapApp (http://www.geomapapp.org). Maria fold-and-thrust belt outline is modified from Spencer and Reynolds (1990). Red polygons indicate Miocene metamorphic core complexes. Bold black lines are major Quaternary strike-slip faults. BR—Buckskin-Rawhide Mountains; CM—Chocolate Mountains; CR—Cemetery Ridge; DR—Dome Rock Mountains; GH—Gavilan Hills; HV—Harcuvar Mountains; MM—Mesquite Mountains; OR—Orocopia Mountains; PM—Plomosa Mountains. Black dot labeled LP is the location of the drill hole La Posa Federal 1A. CA—Califor- nia; NV—Nevada; AZ—Arizona. lack pervasive mylonitic fabrics that formed during the early stages of The timing of Cretaceous shortening associated with the MFTB is con- large-magnitude extension (e.g., Lister and Davis, 1989). In this paper strained by thrust faults that are clearly cut by granitic intrusions ranging we present geologic mapping and petrographic and geochemical analyses in age from ca. 80 to 70 Ma (Martin et al., 1982; Knapp, 1989; Reynolds of a newly recognized exposure of Orocopia Schist in the footwall of the et al., 1989; Isachsen et al., 1999; Salem, 2009). northern Plomosa Mountains metamorphic core complex in west-central The primary structural feature of the northern Plomosa Mountains is Arizona. Orocopia Schist in the Plomosa Mountains is particularly inter- the Plomosa detachment fault, a gently dipping normal fault responsible esting and unique because it represents an intersection of two seemingly for the exhumation of mid-crustal mylonitic rocks that constitute most disparate tectonic elements—a continental margin subduction complex of the footwall of the northern Plomosa Mountains metamorphic core and the interior belt of metamorphic core complexes (Fig. 1). complex (Fig. 2). The Plomosa detachment fault is the middle of three imbricate low-angle detachment fault systems active during the early to GEOLOGIC SETTING middle Miocene in west-central Arizona. Based on tectonic reconstruc- tions, the Plomosa detachment fault likely accommodated ~12–17 km The northern Plomosa Mountains of west-central Arizona are located of NE-directed extension (Spencer and Reynolds, 1991; Spencer et al., within the lower Colorado River extensional corridor (LCREC), a highly 2018), whereas the Buckskin-Rawhide detachment fault (the structurally extended region in the southern Basin and Range province (Howard and highest of the three imbricate detachment faults) accommodated up to John, 1987). Late Oligocene to Miocene extensional deformation within 60 km of extension (Spencer and Reynolds 1991; Spencer et al., 2016, the LCREC was accomplished primarily by low-angle normal faulting 2018). Apatite and zircon fission-track dates suggest that the footwall of associated with metamorphic core complex development (Spencer and the Plomosa detachment fault was exhumed ca. 22–15 Ma (Foster and Reynolds, 1989, 1991; Spencer et al., 2018). The belt of metamorphic Spencer, 1992), approximately coeval with initiation and end of detach- core complexes in the LCREC trends SE and overlaps a predominantly ment faulting in nearby core complexes (Foster and John, 1999; Singleton S-vergent zone of Late Cretaceous crustal shortening known

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