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

Home , Granite Wash Mountains, Ibex Peak (Arizona), La Posa Plain, Plomosa Mountains, Riverside Mountains

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 characterized 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 metachert, 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

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 as the Maria et al., 2014; Prior et al., 2016). fold-and-thrust belt (MFTB) (Reynolds et al., 1986) (Fig. 1). Where the The Cretaceous Quinn Pass shear zone (new name by Spencer et al., MFTB is cross-cut by the extensional belts of the LCREC, exposures of 2018), a segment of the MFTB, lies within the detachment footwall at the metamorphic core complexes change from N-S trending to WNW-ESE southern end of the northern Plomosa Mountains (Fig. 2). This complex trending. This coincidence between the change in orientation of exten- zone consists of N- to NE-verging thrust faults and mylonite zones, and sional belts at the intersection of the MFTB suggests that the LCREC E- to SE-trending folds within Paleozoic to Mesozoic sedimentary units was influenced by the orientation of a crustal welt that formed from Late (Spencer et al., 2015). The largest structure in this zone is the NE-vergent Cretaceous shortening along the MFTB (Spencer and Reynolds, 1990). Deadman thrust, which bounds a mylonite zone as much as ~500 m thick

Geological Society of America | LITHOSPHERE | Volume 10 | Number 6 | www.gsapubs.org 724

Geologic map and cross section of the footwall Plomosa detachment fault Elevation (m) of the Plomosa detachment fault Nic 1500 1000 500 0 -500 Miocene intrusive Miocene A’ complex volcanic units A’ tion

Nic Nv 21 c MzPzq (NE) 21 Ni 22 Orocopia Miocene KoPG s 20 Schist sedimentary units

hment KoPG s Ns Nic c 17 27 Orocopia Paleozoic and Mesozoic 24 metabasalt sedimentary units 18 *MzPzq too thin to illustrate in cross sec 28 K aPGm MzPzs 25 Plomosa deta Figure 2. Geologic map and Paleoproterozoic to 17 Interlayered mylonitic Mesozoic granitic and 18 cross section of the footwall 17 quartzite and marble gneissic rocks 17 of the Plomosa detachment KoPG s

m C

G 5 P s fault. Units in the left col- MzPzq MzXg K a G B P umn are the result of this 19 Ko 11 study. Units in the right col-

W 16

umn are from Spencer et al. Gneiss 15 13 Nic Undi erentiated (2015). Only the Orocopia hanging wall units KXgn 10 Schist and footwall units in

114°08’ and Quaternary Nic 34 sediment contact with the schist are 33°53’ N 20 described in the text. Refer Mylonite 6 to Strickland et al. (2017)

am 7 for descriptions of all units. KoPG s PG K 14 Small foliation and bed- KXgn ding symbols south of the KXgn mylonitic front are derived from Spencer et al. (2015). KXgn 33 11 S/D—Strike and Dip; T/P— A 27 Trend and plunge. Dotted 28 14 31 14 lines are concealed con- 29 tacts. The Quinn Pass shear 25 12 zone comprises units MzXg Plomosa and MzPzs below the non- detachment fault 10 21 24 conformity. Labels A–C in 18 the cross-section: (A) Depth ) of the top of the Orocopia contac t Possible (SW Mylonitic front 42 extents of Schist below the early KXgn Nic A Miocene surface: 3–4 km. Undi erentiated Quaternary 40 (B) Structural thickness of B’ sediment 2 km the exposed portion of the Faults Thrust faults B’ Orocopia Schist: ~1.8 km. (NNE) 17 18 KXgn c (C) Minimum structural

11 Ni thickness of the Orocopia 19 KXgn

Nonconformity Dikes 31 Schist as projected in the

(between MzXg and Ns) 33 Nic 38 Front cross-section: ~2.7 km. Mylonitic

38 Mudersbach A 40 n 20 KXgn granite

Mylonitic foliation (S/D) with KXg 18 Mylonitic lineation (T/P) 55 Nic MzPzs (represents average of surrounding 80 MzPzs measurements where applicable)

17 MzPzs 64 MzXg

Horizontal Horizontal mylonitic mylonitic 70 58 onconformity

foliation lineation 53 Quinn N 52 Pass Discrete shear zone dip direction Ns with top-to-SW displacement ) 38 78 MzXg (SSW

Gneissic or tectonic foliation (S/D) 65 79 Nv 64 B 64 50

61 35 0

27 Ns 500 32 -500 1500 1000 Bedding (S/D) (m) Elevation 55 B Nv

Geological Society of America | LITHOSPHERE | Volume 10 | Number 6 | www.gsapubs.org 725

(Scarborough and Meader, 1983; Steinke, 1997). Less than 2 km south veins with up to ~10% feldspar are common throughout the Orocopia of the Quinn Pass shear zone, a nonconformity beneath moderately to Schist. Locally the unit contains gray, mica-poor (<10% mica), feldspar- steeply SW-dipping Miocene strata records tilting of the footwall of the rich (>60% feldspar) layers 10–30 cm thick (Fig. 3A and 4D). Plomosa detachment fault as it was exhumed (Fig. 2). A stratigraphic-test drill hole (La Posa Federal 1A) located ~8 km west-northwest of the exposed footwall (Fig. 1) and centered in the north- PREVIOUS MAPPING ern La Posa Plain encountered metamorphic basement beneath 720 m of sediment. The lithologic log of this drill hole describes the metamorphic The first comprehensive geologic study of the northern Plomosa Moun- basement as schist with abundant quartz and two micas, which resembles tains was by Jemmett (1966) who described the footwall of the Plomosa the Orocopia Schist. The description lacks any mention of hornblende and detachment fault as predominantly gneiss in the southern portion and is thus distinctly unlike gneiss, which composes much of the crystalline schist toward the north. Mapping by Scarborough and Meader (1983) bedrock in the Plomosa Mountains to the southeast and in the Mesquite described the metamorphic footwall as predominantly compositionally Mountains to the northwest, suggesting that the Orocopia Schist at the layered gneisses and interpreted several NE-trending folds based on sparse northern Plomosa Mountains may extend at least 8 km further west- measurements of metamorphic fabrics. They also inferred the Plomosa northwest in the subsurface. detachment fault and its footwall to be arched along a large NW-trending Five hallmarks of the Orocopia Schist as described by Haxel and Dil- antiform, the axis of which they projected just north of the Quinn Pass lon (1978) are present in the schist of the northern Plomosa Mountains: area, though this structure is not supported by more recent mapping. 1. The schist has a dominantly quartzofeldspathic composition and Spencer et al. (2015) presented the most comprehensive geologic map gray flaggy appearance. of the northern Plomosa Mountains, which was the first study to describe 2. Graphitic plagioclase poikiloblasts are widespread throughout the the northern Plomosa Mountains as a metamorphic core complex. They schist (Figs. 3C and 4A), are generally 1–5 mm in size, and have a dark include detailed mapping of the highly dissected brittle hanging wall of gray color, locally with a bluish tint. In thin section, the foliation defined the Plomosa detachment fault, composed of Paleoproterozoic to Cenozoic by graphite is preserved within poikiloblasts in some samples (Fig. 4A). granitic and gneissic rocks; Paleozoic, Mesozoic, and Miocene strata; The presence of graphitic plagioclase is a key identifier of Orocopia Schist. and Miocene volcanic rocks. Strickland et al. (2017) is the first detailed 3. A 3.5-km-long unit of amphibolite schist interlayered with Orocopia map of the mylonitic footwall of the Plomosa detachment fault, and is Schist separates the main body of Orocopia Schist from gneiss (Fig. 2, the basis for this paper. unit KPGam; Fig. 3D) and is distinguished from the immediately adjacent gneiss by common interlayers of Orocopia Schist and quartzite and GEOLOGY OF THE FOOTWALL OF THE PLOMOSA lack of gneissic layering. The amphibolite is composed of ~60%–80% DETACHMENT FAULT hornblende, 15%–30% plagioclase, ~5% quartz, and 2%–3% titanite, with accessory biotite, garnet, opaque minerals, and apatite. This mineral Orocopia Schist assemblage records peak metamorphism in the amphibolite facies. Garnets have been resorbed (Fig. 4E) and altered to plagioclase + biotite + epi- The northern portion of the footwall of the Plomosa detachment fault dote + hornblende(?). This unit is similar to amphibolite layers observed is dominated by mylonitic quartzofeldspathic schist (Fig. 2, unit KPGos) in Orocopia Schist of other localities, which are generally interpreted as that was recently interpreted as Orocopia Schist by Strickland et al. (2016, metamorphosed basalt, consistent with geochemical analysis of three 2017). For clarity, we proceed with the conclusion that this mylonitic samples from the northern Plomosa Mountains (see section Compara- quartzofeldspathic schist is indeed the Orocopia Schist, while presenting tive Geochemistry of Orocopia Metabasalt, Actinolitite, and Metachert). field and petrographic observations and geochemical analyses that support 4. Quartzite layers 3–30 cm thick with small (~0.5 mm) reddish-orange this interpretation (Figs. 3 and 4). manganiferous garnet are common within the Orocopia amphibolite. The quartzite is generally weathered to a rusty red color, though some out- Field and Petrographic Description crops have clear alternating white and dark gray layers (Fig. 3E). Locally The Orocopia Schist comprises ~10 km2 of the exposed footwall of quartzite is tightly folded within the amphibolite (Fig. 3D). Garnet also the Plomosa detachment fault and most commonly crops out as gray locally appears as masses within quartzite (Fig. 4F). Quartzite of a very flaggy layers with well-developed centimeter-scale layering, or is homog- similar appearance in Orocopia Schist exposures in southeast California enous with a purple-red color, and commonly has a well-developed S-C′ are interpreted as metachert (e.g., Haxel and Dillon, 1978), which is mylonitic fabric. It is composed dominantly of quartz (26%–50% modal supported by the geochemical analysis of six samples from the northern abundance, used throughout) and feldspar (24%–50%), with biotite (8%– Plomosa Mountains (see section Comparative Geochemistry of Orocopia 34%, variably chloritized) and locally muscovite (2%–20%, generally Metabasalt, Actinolitite, and Metachert). ~10%), minor opaque minerals (≤4%), and accessory apatite (≤1%), rutile, 5. Actinolitite pods, 5 cm to 1.5 m wide, are widely scattered through- zircon, and local garnet (≤0.5% where present) (Supplementary Table 1 out the schist (Fig. 3F). These pods are composed of ~98% coarse-grained in the GSA Data Repository1). Locally the schist contains 0.1–2-m-thick green actinolite, with accessory talc, quartz, opaque minerals, and/or layers of actinolite-bearing schist, which weathers to a golden color and feldspar. We found 24 such actinolitite pods, which appear to be dis- is flaky. In plane-polarized light, biotite in the quartzofeldspathic schist tributed uniformly throughout the schist body (Strickland et al., 2017). is typically reddish brown, and rutile needles are common in retrograde Locally, several smaller pods are aligned parallel to the trend of mylonitic chlorite. Milky quartz lenses and pods 2–50 cm thick and quartz-feldspar lineations. Layers of actinolite-bearing schist ~0.1–2 m thick commonly

1 GSA Data Repository Item 2018358, which includes the DR Spreadsheet: Raw zircon U-Pb ages of igneous samples (“raw data” tab) and details of methods (“meta- data” tab); Supplementary Table 1: Thin-section analysis of samples from the footwall of the Plomosa detachment fault; and Supplementary Figure 1: Locations and coordinates (North American Datum of 1983, Universal Transverse Mercator zone 11N) of geochemical and geochronological samples from the northern Plomosa Mountains, is available at http://www.geosociety.org/datarepository/2018 or on request from [email protected].

Geological Society of America | LITHOSPHERE | Volume 10 | Number 6 | www.gsapubs.org 726

A B

C D

4 cm E F

Figure 3. Photographs of the Orocopia Schist and features therein, at the northern Plomosa Mountains. (A) Homogenous, reddish-purple Orocopia Schist with two feldspar-rich layers (above the hammer handle, extending from the left- to right-hand side of the photo), which are parallel to the mylonitic foliation. Ribbons of quartz + feldspar cross behind the hammer handle. Hammer is 35 cm long. (B) Gray flaggy layers of Orocopia Schist, with a brown to tan desert varnish. Hammer for scale on outcrop. (C) Graphitic plagioclase; pencil is pointing to an example. (D) Slab of Orocopia metabasalt from the northern Plomosa Mountains, with folded foliation. The orange-red portion at the bottom is metachert, which has been squeezed into the fold hinge of the metabasalt and incorporated into one of the limbs. (E) Exposure of metachert located within Orocopia metabasalt, with light and dark banding parallel to the mylonitic foliation. (F) Coarse-grained green actinolitite pod within the Orocopia Schist.

Geological Society of America | LITHOSPHERE | Volume 10 | Number 6 | www.gsapubs.org 727

A B

C D

E F

Figure 4. Photomicrographs of samples of Orocopia Schist collected from the northern Plomosa Mountains. (A) Graphitic plagioclase poikiloblast in a sample of Orocopia Schist (graphite is the disseminated opaque mineral) (plane-polarized light, PPL) (sample 0316- P39a). White arrows in A–C indicate the direction top of photomicrograph was sheared relative to the bottom. (B) Quartzofeldspathic schist with biotite and muscovite (cross-polarized light, XPL) (sample 155-P9). S-C fabric is visible, which records top-to-the-NE sense of shear. (C) Quartz- and feldspar-rich example of Orocopia Schist demonstrating mylonitic fabric. Sigma-clast bordered by oblique quartz grain-shape fabric records top-to-the-NE sense of shear (XPL, 1λ plate inserted) (sample 1116-P9). (D) Feldspar-rich Orocopia Schist showing a composition dominantly of feldspar, with minor biotite and quartz (XPL) (sample 0316-P81b). (E) Orocopia metabasalt from the northern Plomosa Mountains, showing mineralogy dominantly of hornblende and feldspar, with minor quartz and titanite (PPL) (sample 0217-P41). The remnants of two resorbed garnets are seen in the upper-right corner. (F) Metachert showing masses of very fine-grained garnet (PPL) (sample 1016-P178).

Geological Society of America | LITHOSPHERE | Volume 10 | Number 6 | www.gsapubs.org 728

include actinolitite pods within them, as well as minor talc and rare lay- or thin section indicate top-to-the-NE shear. Mylonitic lineations are very ers of talc schist. systematic with an average orientation of trend 224°, plunge 19° (Fig. 6B).

Metamorphic Fabrics Comparative Geochemistry of Orocopia Metabasalt, Actinolitite, Mylonitic L-S and L>S fabrics are pervasive throughout the Orocopia and Metachert Schist of the northern Plomosa Mountains. Centimeter- to decimeter-scale The geochemistry and origin of the minor metabasalt, actinolitite, and tight to isoclinal folds with attenuated limbs and thickened hinges are com- metachert of the PORS have been discussed by Haxel et al. (1987, 2002), mon and have axes parallel to mylonitic lineations and axial surfaces that Jacobson et al. (1988), Dawson and Jacobson (1989), Moran (1993), range from upright to recumbent. On average, mylonitic foliations of the Chapman (2016), and Haxel and Jacobson (2017). Here we use new Orocopia Schist dip shallowly to the SW (Fig. 5B). C′ shear bands are very geochemical data to establish that these rock types in the Orocopia Schist common in the schist, and all 56 kinematic indicators documented in outcrop of the Plomosa Mountains are much like those of the PORS as a whole.

A All mylonitic foliations (poles) B Mylonitic foliations in Orocopia Schist (poles) N N

2 n = 620 n = 243 Solid line = cylindrical best t. e3 = T/P: 228°, 19° Dashed line = avg. foliation plane (S/D: 116°, 10° SW) Dashed line = avg. foliation plane (S/D: 130°, 19° SW) C Mylonitic foliations in the NPIC (poles) D Mylonitic foliations in gneiss (poles) N N

3 n = 122 n = 214

Solid line = cylindrical best t. e3 = T/P: 139°, 08° Solid line = cynlindrical best t. e3 = T/P: 028°, 05° Dashed line = avg. foliation plane (S/D: 099°, 13° S) Dashed line = avg. foliation plane (S/D: 334°, 05° NE)

Figure 5. Stereoplots of poles to mylonitic foliations from the footwall of the Plomosa detachment fault. Red polygons are 1% area contours. Average foliation planes are from e1. All stereoplots were created with Stereonet 9.9.5 for Windows (Allmendinger, 2017). avg.—average; e1—maximum eigenvector; e3—minimum eigenvector; NPIC—Northern Plomosa intrusive complex; S/D—strike and dip; T/P—trend and plunge.

Geological Society of America | LITHOSPHERE | Volume 10 | Number 6 | www.gsapubs.org 729

A All mylonitic lineations B Mylonitic lineations in Orocopia Schist N N

n = 566 n = 217

e1 = T/P: 220°, 09° e1 = T/P: 224°, 19°

C Mylonitic lineations in the NPIC D Mylonitic lineations in gneiss N N

n = 122 n = 210

e1 = T/P: 211°, 11° e1 = T/P: 038°, 05°

Figure 6. Stereoplots of mylonitic lineations from the footwall of the Plomosa detachment fault. Red polygons are 1% area contours. e1—maximum eigenvector; T/P—trend and plunge; NPIC—Northern Plomosa intrusive complex.

Three samples of metabasalt from the single large body in the Plomosa comparison with these new analyses from the Plomosa Mountains, we use Mountains (Fig. 2) and six samples of accompanying Fe-Mn metachert data for some 100 samples of PORS metabasalt, actinolitite, and metachert have been analyzed by the U.S. Geological Survey (USGS) Central Min- from several areas, chiefly Cemetery Ridge and Trigo Mountains, south- erals Environmental Resources Science Center, Analytical Chemistry west Arizona; and Picacho district, Gavilan Hills, Chocolate Mountains, Project: major elements by wavelength-dispersive X-ray fluorescence Orocopia Mountains, Blue Ridge, eastern San Gabriel Mountains, Sierra (XRF), and trace elements by inductively coupled plasma (ICP) mass Pelona, and Rand Mountains, southern California. spectrometry or ICP atomic-emission spectrometry (Table 1). Samples for Metabasalt. Dawson and Jacobson (1989) found that PORS metabasalts ICP analysis were prepared by two methods, acid dissolution and sinter- comprise two groups. Group 1, dominant, is characterized by chondrite- dissolution, yielding duplicate analyses for most elements. Selection of normalized rare earth element (REE) spectra that are flattish or slightly light

data for these elements then followed the scheme outlined by Haxel et REE (LREE)-depleted [(Ce/Yb)cn = 0.4–1.8] and thus geochemically resem- al. (2018). Three whole-rock samples of Plomosa actinolitite were ana- bles normal and transitional mid-ocean-ridge basalt (MORB) (Klein, 2005). lyzed by XRF and ICP at ALS Laboratories in Reno, Nevada (USA). For Group 2 patterns are slightly LREE enriched [i.e., have gentle negative

Geological Society of America | LITHOSPHERE | Volume 10 | Number 6 | www.gsapubs.org 730

TABLE 1. REPRESENTATIVE WHOLE-ROCK ANALYSES OF MINOR ROCK TYPES IN THE OROCOPIA SCHIST OF THE NORTHERN PLOMOSA MOUNTAINS, ARIZONA (USA) Metabasalt Actinolitite Metachert Sample P188 PM4 PM5/P41 P57 P421 P274 PM6 PM2 PM1 number

Major-element oxides

SiO2 49.80 48.30 49.20 56.5 55.2 80.6 82.8 83.3 85.6

TiO2 1.34 1.63 1.86 0.06 0.03 0.19 0.13 0.14 0.14

Al2O3 13.20 13.80 13.80 2.94 3.61 3.06 2.52 2.51 3.16 † Fe2O3 3.86 4.43 4.49 6.39 5.06 7.23 5.36 4.88 2.23 FeO 7.14 8.43 9.28 2.76 0.63 2.01 1.30 MnO 0.17 0.18 0.23 0.25 00.231.802.401.340.43 MgO 6.40 7.08 6.38 19.2 20.3 1.03 0.28 1.19 1.01 CaO 10.3 9.87 10.1 11.0 11.2 1.84 1.96 1.69 3.61

Na2O 2.82 2.99 2.18 0.32 0.26 0.09 0.08 0.30 0.31

K2O 0.80 0.91 1.00 0.07 0.04 0.16 0.05 0.45 0.08

P2O5 0.14 0.18 0.18 <0.01 <0.010.650.390.390.25

H2O 1.7 1.3 1.5 2.07 2.04 0.60.4 0.50.6

CO2 1.6 0.1 0.2 0.20.7 0.21.1 Total§ 99.2 99.5 100.7 99.9 98.8 100.599.499.199.9

Trace elements Sc 44.7 47.4 47.0 6. 2. 7.74.8 6.05.1 V 340. 362. 441. 50. 25.164.97.588.487.7 Cr 140. 120. 120. 1700. 1580.42. 43.31. 30. Co 40.2 49.5 51.1 44. 52.11. 510.611. 38.7 Ni 61. 62. 61. 1290. 1080.60. 50.66. 27. Pd (ng/g) <1 <1 <1 7. 11.8.11. Cu 11. 31. 35. 3. 4. 130. 84.85. 186. Zn 76.4 107. 117. 80. 72.119.45.597.226.5 Ga 18.8 19.1 19.9 5.3 5.98.606.446.356.52 Zr 89.6 100. 108.9.<274.048.751.852.2 Hf 2. 3. 3. 0.2 <0.2 1. <1 1. 1. Nb 5.9 4.8 5.7 0.5 <0.2 4.42.0 3.72.4 Ta 0.80 0.91 0.86 0.1 <0.1 0.25 0.16 0.16 0.22 Mo 1.26 0.64 0.73 1. <1 9.67 7.94 4.79 3.55 W 0.8 0.5 0.7 <1 2. 0.30.3 0.10.4 Sn 1.9 1.2 1.4 1. 2. <0.2 0.40.7 0.5 Th 0.9 0.3 0.4 0.09 0.05 4.12.2 2.93.1 Ba 256. 112. 114. 59.5 19.7 1570.704.2540. 92. La 5.9 5.2 5.5 1.0 0.876.644.353.736.0 Ce 13.5 13.4 14.5 2.6 1.243.625.028.624.5 Pr 2.31 2.37 2.52 0.48 0.20 20.9 12.7 14.4 10.2 Nd 11.3 12.5 13.0 2.2 0.986.752.558.740.7 Sm 3.6 4.0 4.3 0.82 0.28 17.2 11.3 12.2 8.40 Eu 1.29 1.51 1.51 0.32 0.32 4.17 2.75 3.21 2.10 Gd 4.93 6.12 5.99 0.72 0.36 17.7 12.9 12.9 9.28 Tb 0.87 1.05 0.99 0.14 0.07 2.18 1.83 1.85 1.32 Dy 5.77 7.06 7.15 0.96 0.43 12.7 11.4 10.7 7.59 Ho 1.23 1.51 1.55 0.19 0.10 2.59 2.26 2.12 1.57 Er 3.55 4.25 4.17 0.59 0.35 6.70 6.06 5.52 3.96 Tm 0.56 0.65 0.66 0.09 0.06 0.96 0.86 0.74 0.60 Yb 3.6 4.4 4.4 0.64 0.43 5.95.0 4.94.0 Lu 0.55 0.68 0.64 0.10 0.06 0.90 0.76 0.71 0.60 Y 35.1 39.6 38.1 5.9 3.270.659.759.539.7 Note: Major-element oxides are in mass percent; trace elements, µg/g, except Pd, ng/g. Blank—not determined. † For actinolitite samples, Fe2O3 represents total Fe and H2O loss on ignition. § Major-oxide total, including Cr2O3, NiO, and BaO.

Geological Society of America | LITHOSPHERE | Volume 10 | Number 6 | www.gsapubs.org 731

slopes, (Ce/Yb)cn = 2.8–3.9]; this subordinate group is like enriched MORB 1976). Except for unusually low Ni in one Plomosa Mountains sample, or intraplate basalt. The three Plomosa Mountains samples have virtually patterns for actinolitite from the three areas are similar, consistent with a

flat REE spectra [(Ce/Yb)cn = 0.9–1.0] and belong to group 1 (Fig. 7). common origin. Of course, congruence of Ca simply reflects the composi- The composition of PORS metabasalts, including those in the Plo- tion of actinolite. On the other hand, proportions of Mg and Fe could vary mosa Mountains, is tholeiitic rather than calcalkaline or alkaline (Fig. 8). widely (through the series tremolite-actinolite-ferroactinolite), but do not; Commonly used tectonic discrimination diagrams (including several not all eight samples are similarly magnesian, with molar MgO / (MgO + FeO*) shown), based upon presumably immobile trace elements (Pearce, 2014), = 0.85 ± 0.03. Nor is the observed general agreement of the other eight indicate that PORS metabasalts have affinities to normal MORB and, to a elements, nonessential constituents of actinolite, required by stoichiometry. lesser extent, enriched MORB and/or tholeiitic oceanic intraplate basalt In Cemetery Ridge actinolitite, these elements reside partly in actinolite (Fig. 9). The metabasalts lack affinity to alkaline-intraplate or magmatic- and partly in accessory minerals such as albite, pleonaste, olivine, and arc basalt. These geochemical comparisons point to two conclusions. First, serpentine; elsewhere in the PORS talc (at least) is an accessory phase. Plomosa Mountains metabasalt is indistinguishable from PORS metaba- Metachert. Previous and ongoing studies reveal that PORS meta salt in general. Second, PORS metabasalts are derived from common chert (and associated siliceous marble, here not considered separately) oceanic basalt, of at least two types. Similar varieties of basalt, along with comprises three principal components: dominant biogenic and subordinate some other igneous rock types apparently not represented in the PORS, are found as blocks within Franciscan mélanges (MacPherson et al., 1990). Actinolitite. One of the hallmarks of PORS is pods of coarsely bladed pale-green actinolitite like that reported here from the Plomosa Mountains A 4 (Fig. 3F). Although high concentrations of Cr and Ni, ~1000–2000 μg/g, had indicated some connection with ultramafic rocks, origin of this actinolitite was obscure until discovery of bodies of well-preserved peridotite within the Orocopia Schist at Cemetery Ridge, southwestern Arizona 3 (Haxel et al., 2015, 2018). At Cemetery Ridge, meter-size veins and pods of actinolitite are part of an assemblage of metasomatic rocks produced Tholeiitic by mechanical and fluid-mediated interaction of peridotite and enclos- trend

ing quartzofeldspathic schist (Epstein et al., 2016, 2018). This actinolite T 2 formed much as described by Harlow and Sorensen (2005). Although serpentinized peridotite is uncommon in the PORS, and well-preserved peridotite rare, somewhere along their subduction path all of the schists must have come in contact with peridotite enveloped by metasomatic 1 reaction zones, from which they acquired their actinolitite. Figure 10 compares concentrations of eleven compatible to moderately Calcalkaline incompatible (Sr, Zn, Ga, Al) major and trace elements in Plomosa Moun- trend tains actinolitite with those in actinolite veins at Cemetery Ridge and typical 0 Orocopia actinolitite pods in the southern Chocolate Mountains (Dillon, 0 1 2 3 4 FeO* / MgO

1.5 100 B PORS metabasalt PORS metabasalt

s Plomosa Mtns (Gp1) e Plomosa Mtns

it r Other, Group 1 Other ranges

n Oceanic

h alkaline

/ 1.0

/ Oceanic

e Nb tholeiitic

0.5 10

0.0 3 0.00 0.05 0.10 0.15 0.20 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Zr / Figure 7. Chondrite-normalized rare earth element spectra of Figure 8. Examples of diagrams showing geochemical classifica- group 1 Pelona-Orocopia-Rand Schists (PORS) metabasalt. tion of Pelona-Orocopia-Rand Schists (PORS) metabasalt (after Normalizing values from Pourmand et al. (2012): chondritic Floyd and Winchester, 1975; Miyashiro and Shido, 1975; Kep- abundance × 1.33. pie et al., 2012). Major elements are renormalized volatile free.

Geological Society of America | LITHOSPHERE | Volume 10 | Number 6 | www.gsapubs.org 732

10 10 Orocopia actinolitite A y Continental Plomosa Mtns (n = 3)

magmatic e Cemetery Ridge (3)

l arc t S. Chocolate Mtns (2)

MORB - OIB arra

1 e n Alkaline

Oceanic ansitional / Tr magmatic e1 t

i Th

arc i

holeiitic o

T n

0.1 t t

E-MORB Ac

PORS metabasalt N-MORB Plomosa Mtns Other ranges 0.01 0.01 0.1 1 10 0.1 Ta / Yb Mg Ca Sr Cr Mn Fe Co Ni Zn Ga Al Figure 10. Concentration of eleven major and trace elements 600 (in periodic table order) in Orocopia Schist actinolitite from three areas, normalized to concentration in depleted mantle B (Salters and Stracke, 2004). 500 Magmatic arc basalt hundred kilometers inland by low-angle subduction and undergone at 400 least two episodes (Late Cretaceous and Miocene) of amphibolite-facies metamorphism, it retains a chemical palimpsest of its oceanic origin.

V (µg/g) 300 The most distinctive feature of PORS metachert is its ferromanganif- MORB erous character, commonly apparent in hand specimen through the presence of metamorphic spessartine (or piedmontite) and magnetite. This 200 enhancement in Mn and Fe (and Ni, Cu, and Zn) owes to the hydrothermal Ocean-island component. Enrichment of Mn, relative to detrital background, is on aver- basalt age roughly ten times that of Fe (Fig. 12). Plomosa Mountains metachert 100 has much the same range of Fe and Mn as other PORS metachert. Three Plomosa samples have among the highest concentrations of Mn and Fe; three samples are medial. 0 0 5 10 15 20 Further observations. Although geochemically quite similar to their Ti (µg/g) / 1000 counterparts in Orocopia Schist elsewhere in southwestern Arizona, Plo- Figure 9. Examples of discrimination diagrams showing petro mosa Mountains metabasalt and metachert differ in their field setting. In tectonic affinities of Pelona-Orocopia-Rand Schists (PORS) most Arizonan Orocopia Schist, metabasalt and metachert form thin layers, metabasalt (after Pearce, 1982, 1983; Shervais, 1982; Chiari et typically no more than one or two meters thick, and are loosely associated, al., 2011). MORB—mid-ocean-ridge basalt; N-MORB—normal in that separate layers of metabasalt and metachert typically crop out within MORB; E-MORB—enriched MORB; OIB—ocean-island basalt. several tens of meters of one another. In contrast, in the Plomosa Mountains metachert is generally interlayered with metabasalt. Furthermore, the single metabasalt body in the Plomosa Mountains is 3.5 km long. Though highly detrital and hydrothermal. For present purposes, the most informative strained by Late Cretaceous and Miocene deformation so that its original size view of metachert geochemistry is provided by shale-normalized REE and shape are uncertain, this metabasalt mass is clearly much larger than any spectra. Rare earth elements are contributed mainly by the biogenic and other known in southwestern Arizona. The next largest, at Cemetery Ridge, hydrothermal components, which derive their REE from material dis- is ~60 m in maximum exposed dimension. In this respect, the Orocopia solved or suspended in seawater. The most common type of REE pattern Schist of the Plomosa Mountains appears more like the Californian Pelona or is that shown in Figure 11. (These patterns are corrected by removal of Orocopia Schist in such places as the San Gabriel Mountains, Sierra Pelona, the minor detrital component, as explained in the caption; see also Haxel and southern Chocolate Mountains, where metabasalt is more voluminous et al., 1987.) Most PORS metacherts, including those in the Plomosa and the association of metabasalt and metachert more intimate (Ehlig, 1958, Mountains, display pronounced negative Ce anomalies. Median Ce/Ce* = 1981; Muehlberger and Hill, 1958; Dillon, 1976; Jacobson, 1983). 0.15 (where Ce* represents Ce interpolated between La and Pr) for all 22 PORS samples shown, and 0.17 for Plomosa Mountains metachert. These Northern Plomosa Intrusive Complex deep negative Ce anomalies are diagnostic of derivation from seawater, which is characterized by similar anomalies (Fig. 11). Even though the A newly recognized Miocene intrusive complex, here referred to as Orocopia metachert of the Plomosa Mountains has been carried several the Northern Plomosa intrusive complex (NPIC), parallels in plan view

Geological Society of America | LITHOSPHERE | Volume 10 | Number 6 | www.gsapubs.org 733

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu 10 10 PORS: Ce/Ce* ≈ 0.09–0.19, PORS metachert (La/Yb) ≈ 0.57–0.72 Plomosa Mtns sn e = 0.50 0.20 (median) Other ranges Mn/F 1.0 0.06

-normalized 1 0.02 (mass %)

Mn 0.1

PORS metasandstone 0.1 (median) PORS metachert and 0.01 Dilution, dominantly siliceous marble by biogenic component, × 0.11 Plomosa Mtns Detrital component Other ranges Nondetrital concentration, shale of metachert (median) 0.01 10¯³ 10¯ 0.1 1 Fe (mass %) 10 30 Ce/Ce* ≈ 0.08 Figure 12. Concentration of Fe and Mn in 41 samples of Pelona- 10¯ Orocopia-Rand Schists (PORS) metachert (and associated shal e North Pacic seawater / siliceous marble). Median composition of PORS metasandstone represents 52 analyses. Approximate composition of the detri- 10¯ tal component is calculated assuming that it has the same Fe/Al awater and Mn/Al as metasandstone and that Al is entirely detrital. Se

10¯ La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Quartz Figure 11. Most common type of shale-normalized rare earth element (REE) spectra for Pelona-Orocopia-Rand Schists (PORS) metachert (and associated siliceous marble). REE concentrations are corrected by subtracting the estimated minor Quartz-rich (≤5%) detrital component (which contributes disproportion- Granitoid ately to Ce), yielding a closer approximation to the spectra of the main carriers of REEs, the detrital and hydrothermal components. Approximate composition of the detrital com-

e ponent is calculated assuming that it has the same REE/Al, for t i To each REE, as PORS metasandstone (52 samples), and that Al is Gran na ar Grano- lit Granite e entirely detrital. Normalization: post-Archean Australian shale diorite eldsp (PAAS) (Pourmand et al., 2012). Small positive Eu anomalies F ali k are an artifact of shale normalization, as negative chondrite- Al normalized Eu anomalies of PORS metachert are smaller than

that of PAAS. Spectrum of dissolved REEs in North Pacific Alkali Feldspar Quartz Diorite Quartz Syenite seawater (Alibo and Nozaki, 1999), depth 1200 m, is shown Quartz Quartz Quartz Syenite Monzonite Monzodiorite for comparison; note difference in vertical scale. Alkali Feldspar Diorite/Gabbro/ Syenite SyeniteMonzonite Monzodiorite Anorthosite Alkali Feldspar Plagioclase

the Plomosa detachment fault in the northern portion of the footwall, and Figure 13. Quartz–alkali feldspar–plagioclase diagram for the Northern 2 has a total exposure of ~5 km (Fig. 2, unit Nic) (Strickland et al., 2017). Plomosa intrusive complex (NPIC). Red polygon represents the general range of compositions for the NPIC based on inspection of 26 thin sections. Field Description The NPIC has a composition of (1) felsic, leucocratic biotite tonalite, granodiorite, and rare granite (altogether totaling ~60% of the unit); and mafic blobs within rocks of a more intermediate composition (Fig. 14D). (2) intermediate hornblende-biotite diorite, with lesser quartz diorite, Highly strained felsic NPIC is in some locations difficult to distinguish quartz monzodiorite, and rare quartz monzonite (Figs. 13 and 14). The from mica-poor mylonitic Orocopia Schist, as both may be relatively rich bulk of this unit appears as layered tabular bodies approximately par- in quartz and have a homogenous appearance. allel to mylonitic foliation (Fig. 14A), though nonmylonitic diorite or quartz diorite dikes locally cut across well-foliated layers. Mylonitic to Metamorphic Fabrics protomylonitic leucocratic dikes and sills 3 cm to 2 m thick are common The majority of the NPIC is mylonitic, generally with a shallowly within the Orocopia Schist (Figs. 14C and 14E). Intrusions of intermedi- SW-dipping mylonitic foliation parallel to that of the Orocopia Schist ate composition are rare in the Orocopia Schist and record evidence of (Figs. 2 and 5C), and mylonitic fabrics in NPIC leucocratic dikes intruded magma mingling, with centimeter- to decimeter-scale irregular flattened into the Orocopia Schist parallel fabrics in the adjacent schist. Mylonitic

Geological Society of America | LITHOSPHERE | Volume 10 | Number 6 | www.gsapubs.org 734

A B

C D

E F

Figure 14. Photographs and a photomicrograph of samples from the Northern Plomosa intrusive complex (NPIC). (A) Example of the layered tabular bodies of alternating felsic and intermediate compositions, characteristic of the NPIC. These layers are parallel to the mylonitic foliation, and may have been transposed during mylonitization. Hammer for scale. (B) Slab of a leucocratic mylonitic or protomylonitic sample representing the “core” of the NPIC. (C) Isoclinally folded leucocratic intrusion within Orocopia Schist. Pencil for scale. (D) Intermediate hornblende-bearing intrusion with evidence of magma mingling. (E) Apparently highly strained intrusion within Orocopia Schist. Pencil for scale. (F) Photomicrograph (cross-polarized light) of an intrusive sample (sample 0316-P71x) from rocks similar to and nearby the outcrop shown in E, showing very low strain and a weak foliation defined by aligned biotite, in contrast to the highly strained appearance of the rock in outcrop.

Geological Society of America | LITHOSPHERE | Volume 10 | Number 6 | www.gsapubs.org 735

leucocratic intrusions of the NPIC are also commonly isoclinally folded alternating layers ~3–30 cm thick of hornblende amphibolite, biotite- within the schist, with attenuated limbs and thickened hinges, indicating poor tonalite or granodiorite, and well-foliated and lineated hornblende that these dikes are pre- or synmylonitic. All 17 kinematic indicators docu- biotite tonalite or granodiorite. Locally, packets of well-foliated gneiss mented in outcrop or thin section indicate top-to-the-NE shear. Mylonitic are 3–10 m thick, defined by 1-mm- to 1.5-cm-thick layers of alternating lineations of the NPIC have an average orientation of trend 211°, plunge felsic and mafic compositions. Amphibolite and hornblende-rich tonalite 11°, similar to the average orientation of mylonitic lineations of the Oro- or granodiorite are locally several meters thick, and amphibolite com- copia Schist (Fig. 6C). monly exhibits pinch-and-swell or boudinage structures. The unit locally includes leucogranite layers 1–10 cm thick with recrystallized ribbons of Age of Intrusion and Mylonitization quartz and plagioclase. Here we report U-Pb zircon isotopic ages for the NPIC determined at the Laser Ablation ICPMS Laboratory at the University of Texas at Austin Metamorphic Fabrics (USA) (Fig. 15; Table 2; refer to the Data Repository for raw ages and Mylonitic fabrics are pervasive throughout the majority of the gneiss detailed methods). Euhedral zircons from the NPIC were mounted on (composing ~14 km2 of the gneiss in the footwall). However, the mylonitic sticky tape and ablated through their outer rim into the core, producing foliation of the gneiss is discordant to that of the Orocopia Schist and a depth versus age profile, though overgrowth rims were not identified NPIC, dipping on average shallowly to moderately NW and E, defining in these samples. Reduced age data were analyzed with Isoplot software an antiform with an average fold axis of trend 028°, plunge 05° (Fig. 5D). (Ludwig, 2003). Ages reported are the weighted mean from individual Mylonitic lineations are typically defined by stretched quartz, aligned 206Pb/238U ages that overlap with concordia at 2σ error and have <10% hornblende, and streaks of biotite. A gradational mylonitic front is present uncertainty. in the southern portion of the gneiss (Fig. 2), defining a transition from Four intrusions dated via U-Pb zircon geochronology yielded Mio- mylonite to protomylonite to nonmylonitic gneiss with sparse discrete cene ages (Fig. 15; Table 2). Sample 0316-P71x is from a protomy- shear zones. Thirty of 32 kinematic indicators documented in outcrop lonitic granodiorite sill that intrudes the Orocopia Schist (Figs. 14E and or thin section indicate top-to-the-NE shear, with the two top-to-the-SW 14F). Eight concordant zircon ages out of 31 are Miocene, yielding a indicators observed as discrete shear zones near the mylonitic front. Top- weighted mean age of 22.3 ± 0.5 Ma (Mean square of weighted devi- to-the-SW (antithetic) discrete shear zones have been documented near ates [MSWD] = 2.1). The older zircons are inherited, with ages ranging the mylonitic front in other Arizona core complexes (e.g., Reynolds and from Proterozoic to Paleogene. Samples 0316-P80b and 0316-P81a, Lister, 1990). Mylonitic lineations in the gneiss have a nearly identical from mylonitic granodiorite dikes within the Orocopia Schist, have NE-SW trend as in the Orocopia Schist and NPIC, with an average ori- similar weighted mean ages of 22.6 ± 0.3 Ma (MSWD = 2.4) and 22.8 entation of trend 038°, plunge 05° (Fig. 6D). ± 0.5 Ma (MSWD = 2.2), respectively, and have only a few xenocrystic ages (1 of 19 and 4 of 14, respectively). Sample 0217-P16 is from a Contact with the Orocopia Schist nonmylonitic diorite collected ~1.5 km north of the Quinn Pass shear The gneiss is juxtaposed against the Orocopia Schist along a tectonic zone and structurally above a mylonitic front in the footwall. This contact where Orocopia metabasalt is concentrated (Fig. 2). The tectonic diorite has entirely Miocene zircon ages with a weighted mean age of contact between the Orocopia Schist and the gneiss dips moderately to the 20.5 ± 0.2 Ma (MSWD = 1.8). SE—as interpreted from the orientation of gneissic foliation of Orocopia At the outcrop scale, intrusions within the Orocopia Schist commonly metabasalt, which generally varies from subvertical to gently SE dipping appear to have undergone high strain, exhibiting attenuation, boudinage, (Fig. 17)—and is undulatory with decimeter- to meter-scale folds with and isoclinal folding, yet at the thin-section scale these intrusions are pro- axes parallel to the strike of the folded gneissic foliation. The gneissic tomylonitic or record only minor quartz dynamic recrystallization (e.g., foliation of the metabasalt and adjacent gneiss at the contact is discordant sample 0316-P71x; Figs. 14E and 14F), suggesting that they intruded to the overall orientation of mylonitic foliation in the Orocopia Schist and during mylonitization and deformed primarily as magma or crystal mush. NPIC, which generally dips shallowly SW, and to the mylonitic foliation of The mapping of pervasive mylonitic fabrics and the determination of the gneiss, which generally dips shallowly to moderately NW or E (Fig. 2). early Miocene ages for the synmylonitic intrusions represent the first However, mylonitic foliations of all units are locally rotated into paral- documentation of the northern Plomosa Mountains as a metamorphic lelism with the contact, and the tectonic contact was the locus of NPIC core complex with Miocene mylonitic fabrics. Moreover, the presence Miocene intrusions. The concordance of adjacent mylonitic foliations to of mylonitic Miocene dikes and sills throughout the Orocopia Schist, the contact, the undulating nature of the contact, and the concentration and the parallelism of their mylonitic fabrics with the schist, suggest of mylonitic Miocene intrusions along the contact all suggest it has been that the observed mylonitic fabric of the Orocopia Schist in the northern greatly overprinted by Miocene deformation. Thus, its original age and Plomosa Mountains is also dominantly of early Miocene age. Mylonitiza- tectonic significance are now obscure. tion across the footwall must have ceased prior to cooling below zircon and apatite fission-track closure temperatures ca. 18–14 Ma (Foster and Structural Depth and Thickness of the Orocopia Schist Spencer, 1992). A nonconformity between early Miocene strata and pre-Cenozoic crystalline rocks in the footwall of the Plomosa detachment fault dips ~55° SW Gneiss (Fig. 2) due to tilting of the footwall during Miocene exhumation. To deter- mine the age of this nonconformity, we dated an ash-fall tuff bed directly Field Observations overlying granite in the footwall (sample 16-3-1). The weighted mean Crystalline gneiss dominates the central and southern portion of the age of concordant zircons from the tuff is 21.1 ± 0.2 Ma (MSWD = 1.6), footwall of the Plomosa detachment fault (Figs. 2 and 16). Based on nearly identical to the 21.1 ± 0.3 Ma TuffZirc age determined using the the similarity of this unit to other such gneisses in west-central Arizona algorithm of Ludwig (2003). We interpret this nonconformity to represent (e.g., Bryant and Wooden, 2008), the protolith is likely a mix of Protero- the surface near the inception of detachment faulting at ca. 22–20 Ma in zoic, Jurassic, and Cretaceous rocks. The gneiss dominantly comprises the west-central Arizona core complexes (e.g., Foster and John, 1999;

Geological Society of America | LITHOSPHERE | Volume 10 | Number 6 | www.gsapubs.org 736

0.11 28 0316-P71x 0316-P71x 27 22.3 ± 0.5 Ma (MSWD=2.1) 0.09 26

Pb 25 20 6

/ 24 0.07 Pb 23 ge (Ma) 20 7 A 22 0.05 21

0.03 19 220 240260 280 300 320 340

0316-P80b 0316-P80b 0.09 25 22.6 ± 0.3 Ma (MSWD=2.4)

24 Pb

20 6 0.07 / 23 Pb ge (Ma) A 20 7 22 0.05

0.03 220 240 260 280 300 320 340 20 0316-P81a Figure 15. Uranium-lead (U-Pb) zircon 0316-P81a 26 data plots from igneous samples. Left: 0.09 22.8 ± 0.5 Ma (MSWD=2.2) Inverse concordia (Tera-Wasserburg) 25 plots with 2σ error ellipses. Dashed

Pb gray ellipses do not overlap concor- 0.07

20 6 24 dia at 2σ and were excluded from / the weighted mean age calculations. Pb e (Ma) Four ages from sample 0316-P71x

20 7 23 0.05 Ag with >10% uncertainty in 206Pb/238U

22 age are excluded from the plot. Right: Weighted mean 206Pb/238U ages from 0.03 all concordant ages with <10% error. 220 240260 280 300 320 340 21 White bars are ages rejected as out- 0217-P16 liers by the Isoplot weighted mean 0217-P16 23 2σ criteria. MSWD—Mean square of 0.09 20.5 ± 0.2 Ma (MSWD=1.8) weighted deviates.

22 Pb 0.07 20 6 / 21 Pb e (Ma) 20 7 0.05 Ag 20

19 0.03 220 240260 280 300 320 340

16-3-1 16-3-1 0.11 23 21.1 ± 0.2 Ma (MSWD=1.6)

0.09 22 Pb 20 6 / Pb 0.07 e (Ma) 21 20 7 Ag

0.05 20

0.03 220 240260 280 300320 340 19 238U/206Pb

Geological Society of America | LITHOSPHERE | Volume 10 | Number 6 | www.gsapubs.org 737

TABLE 2. ZIRCON U-PB AGES OF IGNEOUS SAMPLES, NORTHERN PLOMOSA INTRUSIVE COMPLEX, ARIZONA (USA) Sample name Rock type UTM coordinates Concordant 206Pb/238U ages (zone 11 N) Weighted mean age (Ma)Tuff zircon age (Ma) 0316-P71x Granodiorite, Northing 3758338 22.25 ± 0.53 Ma 22.20 +1.10/-0.46 Ma protomylonitic sill Easting 769383 1 of 10 rejected 1 of 10 rejected MSWD = 2.1 0316-P80b Granodiorite, Northing 3757321 22.61 ± 0.34 Ma 22.58 +0.95/-0.18 Ma mylonitic dike Easting 769528 0 of 16 rejected 0 of 16 rejected MSWD = 2.4 0316-P81a Granodiorite, Northing 3757544 22.82 ± 0.45 Ma 23.10 +0.28/-0.73 Ma mylonitic dike Easting 769283 0 of 11 rejected 1 of 11 rejected MSWD = 2.2 0217-P16 Diorite, Northing 3746181 20.49 ± 0.16 Ma 20.45 +0.20/-0.18 Ma nonmylonitic Easting 767928 2 of 29 rejected 3 of 29 rejected MSWD = 1.8 16-3-1 Tuff Northing 3742963 21.07 ± 0.15 Ma 21.10 +0.25/-0.27 Ma Easting 768831 2 of 27 rejected 3 of 27 rejected MSWD = 1.6 Note: UTM—Universal Transverse Mercator; MSWD—Mean square of weighted deviates. Datum is North American Datum 1983 (NAD83).

Singleton et al., 2014; Prior et al., 2016), and prior to ca. 19 Ma rapid schist exhumation occurred prior to deposition of the tuff (assuming a extension in the adjacent Whipple Mountains core complex (Gans and slip rate of ~3 km/m.y.; Foster and John, 1999), in which case the paleo- Gentry, 2016). Projecting perpendicular to the nonconformity (assumed depth to the top of the schist may have been ~5–6 km at the inception of to be horizontal ca. 21 Ma) suggests that the Orocopia Schist was only detachment faulting. In either case, the geothermal gradient must have 3–4 km below the surface near the time of initiation of slip on the Plomosa been very high (≥80 °C/km) to result in mylonitization of the NPIC and detachment fault (Fig. 2, projection A). If the Plomosa detachment fault Orocopia Schist at these shallow depths. The exposed portion of the Oro- initiated as early as 22 Ma, possibly several kilometers of early Miocene copia Schist has a structural thickness of ~1.8 km (Fig. 2, projection B), but minimum structural thickness as projected in cross section is ~2.7 km (Fig. 2, projection C). Projecting perpendicular to the nonconformity to the top of the northeasternmost exposure of the Orocopia Schist suggests A that the schist extended at least 10–12 km below the ca. 21 Ma surface near the inception of detachment faulting.

DISCUSSION

Orocopia Schist in the Plomosa Mountains Metamorphic Core Complex

Based on field observations and geochemical evidence, we conclude that the 10 km2 exposure of quartzofeldspathic schist in the northern Plo- mosa Mountains is the Orocopia Schist. This conclusion is also strongly B supported by detrital zircon geochronology (Seymour et al., 2018). The northern Plomosa Mountains is a metamorphic core complex with Mio- cene mylonitic fabrics that are structurally several kilometers thick and encompass the Orocopia Schist, as demonstrated by synmylonitic early Miocene intrusions common throughout the schist. Orocopia Schist in other localities underwent final exhumation along normal faults in the Miocene but involving little to no penetrative strain (e.g., Gatuna fault, southeasternmost California, USA; Jacobson et al., 2002). This study pres- ents the first documentation of Orocopia Schist exhumed within a Miocene metamorphic core complex, with Miocene mylonitization through the entire structural thickness of the schist. The presence of Orocopia Schist in the northern Plomosa Mountains is linked to development of the Plomosa Mountains metamorphic core complex. Prior to initiation of the Plomosa detachment fault, the rheologically weak Orocopia Schist extended from a paleo-depth of 3–6 km to at least 10–12 km, and early Miocene magma emplacement was focused Figure 16. Photographs of gneiss from the northern Plomosa Mountains. (A) Slab of mylonitic gneiss. (B) Isoclinally folded near the margin of the schist. The Plomosa detachment fault then initiated gneiss. Folds are consistent with top-to-the-NE sense of shear along the schist and NPIC, exhuming these rocks from depths as great as (NE to the right). 10–12 km to the surface.

Geological Society of America | LITHOSPHERE | Volume 10 | Number 6 | www.gsapubs.org 738

A Orocopia amphibolite gneissic foliation B N

n = 10

Dashed line = avg. foliation plane (S/D: 037°, 52° SW)

Figure 17. (A) Stereoplot of gneissic foliation of Orocopia amphibolite (metabasalt). Measurements were collected from along the contact of the Orocopia Schist and gneiss, within unit KPGam of Figure 2. avg.—average; S/D—strike and dip. (B) Photograph of subvertical undulatory foliation of Orocopia metabasalt (unit KPGam of Fig. 2), with a subvertical leucocratic sill left of John Singleton.

Orocopia Schist: A Subhorizontal Layer above the Farallon Slab transformation kinetics were too sluggish to convert the metabasalt to ecologite before exhumation beneath the crustal welt. If subducted below Geologists have questioned whether the PORS form a continuous a crustal welt that was hypothetically 50 km thick, the Orocopia Schist at layer sandwiched between the subducting Farallon plate and overlying the northern Plomosa Mountains would have needed to be exhumed ~45 crust, or whether the curvilinear exposure along the Chocolate Mountains km through the crust in the Paleogene. Possible interactions of the crustal anticlinorium (Fig. 1) implies that the Orocopia Schist forms only a nar- welt with the Farallon plate and Orocopia Schist that do not involve sub- row belt in the subsurface and is not laterally continuous in the direction duction to >45 km include: (1) the bottom of the thick crustal welt was of subduction (Haxel et al., 2002). Documentation of Orocopia Schist at sheared off by the shallowly dipping Farallon slab prior to underplating of the northern Plomosa Mountains provides additional constraint on the the Orocopia Schist (Fig. 18B; Spencer et al., 2018); or (2) the Orocopia geometry and extent of the subducted PORS. The Orocopia Schist in the Schist extruded into the middle crust before reaching the crustal welt (Fig. northern Plomosa Mountains and Cemetery Ridge are both >300 km from 18C). Alternatively, the lower crust and lithospheric mantle may have the paleo–oceanic trench in the subduction direction. These exposures delaminated prior to schist underplating (e.g., Wells and Hoisch, 2008). confirm that the Orocopia Schist was transported far inland and indicate Even if the Orocopia Schist was not subducted beneath a significant crustal that a continuous layer once extended from these areas to the Choco- welt, and was underplated to only 30 km depth, significant Paleogene late Mountains anticlinorium (Fig. 1) (Haxel et al., 2015; Jacobson et exhumation of the Orocopia Schist would have been necessary for it to al., 2017), though has since been dissected by large-magnitude Miocene reach the upper crust prior to final exhumation by Miocene detachment extension. While several geologists still question the interpretation of faulting. Our interpretation for Paleogene exhumation of Orocopia Schist Laramide low-angle subduction (e.g., Maxson and Tikoff, 1996; Hilde in the northern Plomosa Mountains is supported by Paleogene exhumation brand, 2015; Tikoff et al., 2016), the presence of subducted schist so far documented in other locations of PORS in southern California and south- inboard of the plate boundary is difficult to explain without a shallowly western Arizona (e.g., Haxel et al., 2002; Jacobson et al., 2002; Grove dipping Farallon slab. et al., 2003). Detailed thermochronologic studies of Orocopia Schist at the Gavilan Hills and Orocopia Mountains (Fig. 1) revealed a Paleogene Paleogene Exhumation of the Orocopia Schist period of rapid cooling ca. 52–43 Ma (Jacobson et al., 2002, 2007), consistent with normal-sense slip on the Paleogene Chocolate Mountains fault The presence of Orocopia Schist within 3–6 km of the surface prior to (Oyarzabal et al., 1997), coupled with a significant component of erosional early Miocene detachment faulting remains a puzzle. The northern Plo- exhumation. Latest Cretaceous to early Paleogene NE-SW extension is mosa Mountains is within the Maria fold-and-thrust belt (Fig. 1), an area also inferred in the nearby Dome Rock Mountains, Harcuvar Mountains, presumed to have formed a crustal welt in the Late Cretaceous (Spencer and Buckskin-Rawhide Mountains (Fig. 1) (Boettcher et al., 2002; Wong and Reynolds, 1990) prior to subduction of the Orocopia Schist ca. 73 et al., 2013; Singleton and Wong, 2016). These ranges apparently lack Ma (Seymour et al., 2018; Fig. 18A). Orocopia metabasalt at the northern Orocopia Schist, but evidence for Laramide-age extension in this region Plomosa Mountains lacks evidence for eclogite-facies metamorphism, supports the interpretation that Paleogene exhumation was widespread suggesting that it was never subducted to >45 km (≥1.3 GPa) or that phase throughout southern California and western Arizona.

Geological Society of America | LITHOSPHERE | Volume 10 | Number 6 | www.gsapubs.org 739

80 Ma Rand Schist Plomosa Mountains A

Subduc Continental crust ted oceanic plateau Crustal welt

Lithospheric mantle

200 km Asthenosphere

70 Ma Orocopia Schist B Figure 18. Cross-sectional cartoons illustrat- Continental crust ing the proposed methods of underplating of the Orocopia Schist (blue) without subducting beneath the crustal welt of the Maria Lithospheric mantle fold-and-thrust belt (labeled as crustal welt). (A) Starting configuration at 80 Ma showing Asthenosphere the crustal welt. (B, C) Proposed models. No vertical exaggeration is intended for crustal thickness and angles of subduction; topogra- Crustal welt sheared o or delamination of welt ~80 Ma phy, however, is greatly exaggerated.

70 Ma Orocopia Schist C Continental crust

Lithospheric mantle

Asthenosphere Extrusion of schist

Paleogene extension following subduction of the Orocopia Schist layer extending from the Chocolate Mountains anticlinorium northeast could have been triggered by rheological weakening of the crust from to the northern Plomosa Mountains and Cemetery Ridge, providing com- (1) hydration associated with fluid flux from the Farallon slab and sub- pelling evidence that Orocopia Schist was emplaced above a shallowly ducted sediments (e.g., Wells and Hoisch, 2008); and/or (2) emplacement dipping Farallon slab. In the northern Plomosa Mountains, the 3–4 km of weak schist in the lower crust, leading to gravitational collapse of the paleo-depth of the Orocopia Schist at ca. 21 Ma necessitates a major thickened crust. Therefore, the subduction and exhumation of weak schist Paleogene exhumation event, supporting Paleogene extensional exhuma- not only may have controlled the location and geometry of the northern tion documented by studies of Orocopia Schist at other localities as well Plomosa Mountains metamorphic core complex, but it may also have as studies of nearby core complexes. The association of Orocopia Schist been responsible for regional latest Cretaceous to Paleogene extension. with Paleogene exhumation suggests that the subduction of rheologically weak schist beneath previously thickened crust may have been the trigger CONCLUSIONS for major Paleogene exhumation in west-central Arizona.

We have documented Orocopia Schist and a Miocene intrusive com- ACKNOWLEDGMENTS plex in the footwall of the Plomosa detachment fault, and have dem- This study was funded by USGS EDMAP grant G16AC00142 and National Science Foundation Tectonics Program award 1557265 to J. Singleton. The authors thank Alan Chapman, Carl onstrated that the northern Plomosa Mountains is a metamorphic core Jacobson, Marty Grove, Kirsten Sauer, and an anonymous reviewer for their insightful com- complex with Miocene mylonitic fabrics that record a consistent top-to- ments, which have improved the quality of this manuscript. Nikki Seymour was instrumental the-NE sense of shear. Synmylonitic early Miocene intrusions in the schist to our geochronology efforts by reducing the age data and providing additional knowledge and support. Andrew Griffin provided valuable field support and geological expertise while demonstrate that the footwall of the Plomosa detachment fault, and likely mapping with E. Strickland for nearly a month in the northern Plomosa Mountains. the entire structural thickness of the Orocopia Schist, records penetrative mid-crustal strain coeval with Miocene core complex development. REFERENCES CITED The documentation of Orocopia Schist in the northern Plomosa Moun- Alibo, D.S., and Nozaki, Y., 1999, Rare earth elements in seawater: Particle association, shale- tains provides important insight into the spatial and temporal emplacement normalization, and Ce oxidation: Geochimica et Cosmochimica Acta, v. 63, p. 363–372, https://doi.org/10.1016/S0016-7037(98)00279-8. of subducted schist beneath southwestern Arizona and southeastern Cali- Allmendinger, R.W., 2017, Stereonet 9.9.5 for Windows: http://www.geo.cornell.edu/geology/ fornia. We conclude that the Orocopia Schist once formed a continuous faculty/RWA/programs/stereonet.html (accessed October 2015).

Geological Society of America | LITHOSPHERE | Volume 10 | Number 6 | www.gsapubs.org 740

Boettcher, S.S., Mosher, S., and Tosdal, R.M., 2002, Structural and tectonic evolution of Mesozoic Haxel, G.B., Wittke, J.H., Epstein, G.S., and Jacobson, C.E., 2018, Serpentinization-related basement-involved fold nappes and thrust faults in the Dome Rock Mountains, Arizona, in nickel, iron, and cobalt sulﬁde, arsenide, and intermetallic minerals in an unusual inland Barth, A., ed., Contributions to Crustal Evolution of the Southwestern United States: Geologi- tectonic setting, southern Arizona, USA, in Ingersoll, R.V., Graham, S.A., and Lawton, cal Society of America Special Paper 365, p. 73–97, https://doi .org /10 .1130 /0 -8137 -2365 -5 .73. T.F., eds., Tectonics, Sedimentary Basins, and Provenance: A Celebration of the Career Bryant, B., and Wooden, J.L., 2008, Geology of the northern part of the Harcuvar complex, of William R. Dickinson: Geological Society of America Special Paper 540, https://doi.org west-central Arizona: U.S. Geological Survey Professional Paper 1752, 52 p. /10.1130/2018.2540(04) (in press). Chapman, A.D., 2016, The Pelona-Orocopia-Rand and related schists of southern California: Hildebrand, R.S., 2015, Dismemberment and northward migration of the Cordilleran orogen: A review of the best-known archive of shallow subduction on the planet: International Baja-BC resolved: GSA Today, v. 25, no. 11, p. 4–11, https://doi.org/10.1130/GSATG255A.1. Geology Review, v. 59, p. 664–701, https://doi.org/10.1080/00206814.2016.1230836. Holk, G.J., Grove, M., Jacobson, C.E., and Haxel, G.B., 2017, A two-stage fluid history for the Chapman, A.D., Jacobson, C.E., Ernst, W.G., Grove, W., Dumitru, T., Hourigan, J., and Ducea, Orocopia Schist and associated rocks related to flat subduction and exhumation, south- M.N., 2016, Assembling the world’s type shallow subduction complex: Detrital zircon eastern California: A cold and dry metamorphic core complex: Geological Society of geochronologic constraints on the origin of the Nacimiento block, central California Coast America Abstracts with Programs, v. 49, no. 6, https://doi .org /10 .1130 /abs /2017AM -302680. Ranges: Geosphere, v. 12, p. 533–557, https://doi.org/10.1130/GES01257.1. Howard, K.A., and John, B.E., 1987, Crustal extension along a rooted system of imbricate low- Chiari, M., Djerić, N., Garfagnoli, F., Hrvatović, H., Krstić, M., Levi, N., Malasoma, A., Marroni, angle faults: Colorado River extensional corridor, California and Arizona, in Coward, M.P., M., Menna, F., Nirta, G., Pandolfi, L., Principi, G., Saccani, E., Stojadinović, U., and Trivić, Dewey, J.F., and Hancock, P.L., eds., Continental Extensional Tectonics: Geological Society B., 2011, The geology of the Zlatibor-Maljen area (western Serbia): A geotraverse across of London Special Publication 28, p. 299–311, https://doi .org /10 .1144 /GSL .SP .1987 .028 .01 .19. the ophiolites of the Dinaric-Hellenic collisional belt: Ofioliti, v. 36, p. 139–166. Isachsen, C.E., Gehrels, G.E., Riggs, N.R., Spencer, J.E., Ferguson, C.A., Skotnicki, S.J., and Crowell, J.C., 1968, Movement histories of faults in the Transverse Ranges and speculations Richard, S.M., 1999, U-Pb geochronologic data from zircons from eleven granitic rocks on the tectonic history of California, in Dickinson, W.R., and Grantz, A., eds., Proceedings in central and western Arizona: Arizona Geological Survey Open-File Report 99-5, 30 p. of Conference on Geologic Problems of San Andreas Fault System: Stanford University Jacobson, C.E., 1983, Structural geology of the Pelona Schist and Vincent thrust, San Gabriel Publications in the Geological Sciences 11, p. 323–341. Mountains, California: Geological Society of America Bulletin, v. 94, p. 753–767, https:// Dawson, M.R., and Jacobson, C.E., 1989, Geochemistry and origin of mafic rocks from the doi.org/10.1130/0016-7606(1983)94<753:SGOTPS>2.0.CO;2. Pelona, Orocopia, and Rand Schists, southern California: Earth and Planetary Science Jacobson, C.E., Dawson, M.R., and Postlethwaite, C.E., 1988, Structure, metamorphism, and Letters, v. 92, p. 371–385, https://doi.org/10.1016/0012-821X(89)90061-7. tectonic significance of the Pelona, Orocopia, and Rand Schists, southern California,in Dillon, J.T., 1976, Geology of the Chocolate and Cargo Muchacho Mountains, southeastern- Ernst, W.G., ed., Metamorphism and Crustal Evolution of the Western United States (Rubey most California [Ph.D. thesis]: Santa Barbara, University of California, 405 p. Volume VII): Englewood Cliffs, New Jersey, Prentice-Hall, p. 976–997. Ehlig, P.L., 1958, The geology of the Mount Baldy region of the San Gabriel Mountains, Cali- Jacobson, C.E., Barth, A.P., and Grove, M., 2000, Late Cretaceous protolith age and provenance fornia [Ph.D. thesis]: Los Angeles, University of California, 195 p. of the Pelona and Orocopia Schists, southern California: Implications for evolution of the Ehlig, P.L., 1981, Origin and tectonic history of the basement terrane of the San Gabriel Cordilleran margin: Geology, v. 28, p. 219–222, https://doi .org /10 .1130 /0091 -7613 (2000)28 Mountains, central Transverse Ranges, in Ernst, W.G., ed., The Geotectonic Development <219:LCPAAP>2.0.CO;2. of California (Rubey Volume I): Englewood Cliffs, New Jersey, Prentice-Hall, p. 253–283. Jacobson, C.E., Grove, M., Stamp, M.M., Vucic, A., Oyarzabal, F.R., Haxel, G.B., Tosdal, R.M., Epstein, G.S., Haxel, G.B., Wittke, J.H., and Jacobson, C.E., 2016, Unusual metasomatic rocks and Sherrod, D.R., 2002, Exhumation history of the Orocopia Schist and related rocks associated with subducted mantle peridotite in the Orocopia Schist at Cemetery Ridge, in the Gavilan Hills area of southeasternmost California, in Barth, A., ed., Contributions southwest Arizona: Geological Society of America Abstracts with Programs, v. 48, no. 4, to Crustal Evolution of the Southwestern United States: Geological Society of America https://doi.org/10.1130/abs/2016CD-272077. Special Paper 365, p. 129–154, https://doi.org/10.1130/0-8137-2365-5.129. Epstein, G.S., Haxel, G.B., Wittke, J.H., and Jacobson, C.E., 2018, Formation of intermediate Jacobson, C.E., Grove, M., Vućić, A., Pedrick, J.N., and Ebert, K.A., 2007, Exhumation of the and ultrabasic metasomatic rocks within a reaction zone surrounding subducted perido- Orocopia Schist and associated rocks of southeastern California: Relative roles of ero- tite, Cemetery Ridge, southwest Arizona: Geological Society of America Abstracts with sion, synsubduction tectonic denudation, and middle Cenozoic extension, in Cloos, M., Programs, v. 50, no. 5, https://doi.org/10.1130/abs/2018RM-314046. Carlson, W.D., Gilbert, M.C., Liou, J.G., and Sorensen, S.S., eds., Convergent Margin Ter- Floyd, P.A., and Winchester, J.A., 1975, Magma type and tectonic setting discrimination us- ranes and Associated Regions: A Tribute to W.G. Ernst: Geological Society of America ing immobile elements: Earth and Planetary Science Letters, v. 27, p. 211–218, https://doi Special Paper 419, p. 1–37, https://doi.org/10.1130/2007.2419(01). .org/10.1016/0012-821X(75)90031-X. Jacobson, C.E., Grove, M., Pedrick, J.N., Barth, A.P., Marsaglia, K.M., Gehrels, G.E., and Nourse, Foster, D.A., and John, B.E., 1999, Quantifying tectonic exhumation in an extensional orogen J.A., 2011, Late Cretaceous early Cenozoic tectonic evolution of the southern California with thermochronology: Examples from the southern Basin and Range Province, in Ring, margin inferred from provenance of trench and forearc sediments: Geological Society U., Brandon, M.T., Lister, G.S., and Willett, S.D., eds., Exhumation Processes: Normal of America Bulletin, v. 123, p. 485–506, https://doi.org/10.1130/B30238.1. Faulting, Ductile Flow and Erosion: Geological Society of London Special Publication Jacobson, C.E., Hourigan, J.K., Haxel, G.B., and Grove, M., 2017, Extreme latest Cretaceous 154, p. 343–364, https://doi.org/10.1144/GSL.SP.1999.154.01.16. low-angle subduction: Zircon ages from Orocopia Schist at Cemetery Ridge, southwest Foster, D.A., and Spencer, J.E., 1992, Apatite and zircon fission-track dates from the northern Arizona, USA: Geology, v. 45, p. 951–954, https://doi.org/10.1130/G39278.1, https://doi Plomosa Mountains, La Paz County, west-central Arizona: Arizona Geological Survey .org/10.1130/G39278.1. Open-File Report 92-9, 11 p. Jemmett, J.P., 1966, Geology of the northern Plomosa Mountain range, Yuma County, Arizona Gans, P.B., and Gentry, B.J., 2016, Dike emplacement, footwall rotation, and transition from mag- [unpublished Ph.D. thesis]: Tucson, University of Arizona, 128 p. matic to tectonic extension in the Whipple Mountains metamorphic core complex, south- Keppie, J.D., Dostal, J., Murphy, J.B., Galaz-Escanilla, G., Ramos-Arias, M.A., and Nance, eastern California: Tectonics, v. 35, p. 2564–2608, https://doi .org /10 .1002 /2016TC004215. R.D., 2012, High pressure rocks of the Acatlán Complex, southern Mexico: Large-scale Grove, M., Jacobson, C.E., Barth, A.P., and Vucic, A., 2003, Temporal and spatial trends of Late subducted Ordovician rifted passive margin extruded into the upper plate during the Cretaceous–early Tertiary underplating of Pelona and related schist beneath southern Cali- Devonian–Carboniferous: Tectonophysics, v. 560–561, p. 1–21, https://doi.org/10.1016/j fornia and southwestern Arizona, in Johnson, S.E., Patterson, S.R., Fletcher, J.M., Girty, .tecto.2012.06.015. G.H., Kimbrough, D.L., and Martin-Barajas, A., eds., Tectonic Evolution of Northwestern Klein, E.M., 2003, Geochemistry of the igneous oceanic crust, in Rudnick, R.L., ed., Treatise Mexico and the Southwestern USA: Geological Society of America Special Paper 374, on Geochemistry, Volume 3: The Crust: Oxford, Elsevier, p. 433–463, https://doi .org /10 p. 381–406, https://doi.org/10.1130/0-8137-2374-4.381. .1016/B0-08-043751-6/03030-9. Harlow, G.E., and Sorensen, S.S., 2005, Jade (nephrite and jadeitite) and serpentinite: Meta- Knapp, J.H., 1989, Structural development, thermal evolution, and tectonic significance of somatic connections: International Geology Review, v. 47, p. 113–146, https://doi.org a Cordilleran basement thrust terrane, Maria fold and thrust belt, west-central Arizona /10.2747/0020-6814.47.2.113. [Ph.D. thesis]: Cambridge, Massachusetts Institute of Technology, 267 p. Haxel, G., and Dillon, J., 1978, The Pelona-Orocopia Schist and Vincent–Chocolate Mountain Lister, G.S., and Davis, G.A., 1989, The origin of metamorphic core complexes and detach- thrust system, southern California, in Howell, D.G., and McDougall, K.A., eds., Mesozoic ment faults formed during Tertiary continental extension in the northern Colorado River Paleogeography of the Western United States: Pacific Section, Society of Economic Pa- region, U.S.A.: Journal of Structural Geology, v. 11, p. 65–94, https://doi .org/10.1016/0191 leontologists and Mineralogists Pacific Coast Paleogeography Symposium 2, p. 453–469. -8141(89)90036-9. Haxel, G.B., and Jacobson, C.E., 2017, Geochemistry of metachert, siliceous marble, and (newly Ludwig, K.R., 2003, User’s manual for Isoplot 3.00: A geochronological toolkit for Microsoft recognized) meta-umber in the Pelona-Orocopia-Rand Schist low-angle subduction com- Excel: Berkeley Geochronology Center Special Publication 4, 74 p. plex, southern California and southwest Arizona: Geological Society of America Abstracts MacPherson, G.J., Phipps, S.P., and Grossman, J.N., 1990, Diverse sources for igneous blocks with Programs, v. 49, no. 6, https://doi.org/10.1130/abs/2017AM-296690. in Franciscan melanges, California Coast Ranges: The Journal of Geology, v. 98, p. 845– Haxel, G.B., Budahn, J.R., Fries, T.L., King, B.-S.W., White, L.D., and Aruscavage, P.J., 1987, 862, https://doi.org/10.1086/629457. Geochemistry of the Orocopia Schist, southeastern California: Summary, in Dickinson, Martin, D.L., Krummenacher, D., and Frost, E.G., 1982, K-Ar geochronologic record of Me- W.R., and Klute, M.A., eds., Mesozoic Rocks of Southern Arizona and Adjacent Areas: sozoic and Tertiary tectonics of the Big Maria–Little Maria–Riverside Mountains terrane, Arizona Geological Society Digest 18, p. 49–64. in Frost, E.G., and Martin, D.L., eds., Mesozoic-Cenozoic Tectonic Evolution of the Colo- Haxel, G.B., Jacobson, C.E., Richard, S.M., Tosdal, R.M., and Grubensky, M.J., 2002, The Oro- rado River Region, California, Arizona, and Nevada: San Diego, California, Cordilleran copia Schist in southwest Arizona: Early Tertiary oceanic rocks trapped or transported far Publishers, p. 519–557. inland, in Barth, A., ed., Contributions to Crustal Evolution of the Southwestern United Maxson, J., and Tikoff, B., 1996, Hit-and-run collision model for the Laramide orogeny, west- States: Geological Society of America Special Paper 365, p. 99–128, https://doi.org/10 ern United States: Geology, v. 24, p. 968–972, https://doi.org/10.1130/0091-7613(1996)024 .1130/0-8137-2365-5.99. <0968:HARCMF>2.3.CO;2. Haxel, G.B., Jacobson, C.E., and Wittke, J.H., 2014, Mantle peridotite in newly discovered Miyashiro, A., and Shido, F., 1975, Tholeiitic and calc-alkaline series in relation to the behav- far-inland subduction complex, southwest Arizona: Initial report: International Geology iors of titanium, vanadium, chromium, and nickel: American Journal of Science, v. 275, Review, v. 57, p. 871–892, https://doi.org/10.1080/00206814.2014.928916. p. 265–277, https://doi.org/10.2475/ajs.275.3.265.

Geological Society of America | LITHOSPHERE | Volume 10 | Number 6 | www.gsapubs.org 741

Moran, A.E., 1993, The effect of metamorphism on the trace element composition of sub- Singleton, J.S., Stockli, D.F., Gans, P.B., and Prior, M.G., 2014, Timing, rate, and magnitude ducted oceanic crust and sediment [Ph.D. thesis]: Houston, Texas, Rice University, 378 p. of slip on the Buckskin-Rawhide detachment fault, west central Arizona: Tectonics, v. 33, Muehlberger, W.R., and Hill, H.S., 1958, Geology of the central Sierra Pelona, Los Angeles p. 1596–1615, https://doi.org/10.1002/2013TC003517. County, California: American Journal of Science, v. 256, p. 630–643, https://doi.org/10 Spencer, J.E., and Reynolds, S.J., 1989, Middle Tertiary tectonics of Arizona and adjacent .2475/ajs.256.9.630. areas, in Jenny, J.P., and Reynolds, S.J., 1989, Geologic Evolution of Arizona: Arizona Oyarzabal, F.R., Jacobson, C.E., and Haxel, G.B., 1997, Extensional reactivation of the Chocolate Geological Society Digest 17, p. 539–574. Mountains subduction thrust in the Gavilan Hills of southeastern California: Tectonics, Spencer, J.E., and Reynolds, S.J., 1990, Relationship between Mesozoic and Cenozoic tectonic v. 16, p. 650–661, https://doi.org/10.1029/97TC01415. features in west-central Arizona and adjacent southeastern California: Journal of Geo- Pearce, J.A., 1982, Trace element characteristics of lavas from destructive plate boundar- physical Research, v. 95, p. 539–555, https://doi.org/10.1029/JB095iB01p00539. ies, in Thorpe, R.S., ed., Andesites: Orogenic Andesites and Related Rocks: New York, Spencer, J.E., and Reynolds, S.J., 1991, Tectonics of mid-Tertiary extension along a tran- Wiley, p. 525–548. sect through west central Arizona: Tectonics, v. 10, p. 1204–1221, https://doi .org/10.1029 Pearce, J.A., 1983, Role of the sub-continental lithosphere in magma genesis at active con- /91TC01160. tinental margins, in Hawkesworth, J.A., and Norry, M.J., eds., Continental Basalts and Spencer, J.E., Youberg, A., Love, D., Pearthree, P.A., Steinke, T.R., and Reynolds, S.J., 2015, Mantle Xenoliths: Nantwich, UK, Shiva, p. 230–249. Geologic map of the Bouse and Ibex Peak 7 1/2′ quadrangles, La Paz County, Arizona: Pearce, J.A., 2014, Immobile element fingerprinting of ophiolites: Elements, v. 10, p. 101–108, Arizona Geological Survey Digital Geologic Map DGM-107, version 2.0, scale 1:24,000. https://doi.org/10.2113/gselements.10.2.101. Spencer, J.E., Reynolds, S.J., Scott, R.J., and Richard, S.M., 2016, Shortening in the upper Pourmand, A., Dauphas, N., and Ireland, T.J., 2012, A novel extraction chromatography and plate of the Buckskin-Rawhide extensional detachment fault, southwestern U.S., and im- MC-ICP-MS technique for rapid analysis of REE, Sc and Y: Revising CI-chondrite and plications for stress conditions during extension: Tectonics, v. 35, p. 3119–3136, https:// Post-Archean Australian Shale (PAAS) abundances: Chemical Geology, v. 291, p. 38–54, doi.org/10.1002/2016TC004345. https://doi.org/10.1016/j.chemgeo.2011.08.011. Spencer, J.E., Singleton, J.S., Strickland, E., Reynolds, S.J., Love, D., Foster, D.A., and John- Prior, M.G., Stockli, D.F., and Singleton, J.S., 2016, Miocene slip history of the Eagle Eye de- son, R., 2018, Geodynamics of Cenozoic extension along a transect across the Colorado tachment fault, Harquahala Mountains metamorphic core complex, west-central Arizona: River extensional corridor, southwestern USA: Lithosphere, v. 10, https://doi.org/10.1130 Tectonics, v. 35, p. 1913–1934, https://doi.org/10.1002/2016TC004241. /L1002.1. Reynolds, S.J., and Lister, G.S., 1990, Folding of mylonitic zones in Cordilleran metamorphic Steinke, T.R., 1997, Geologic map of the eastern Plomosa Pass area: Arizona Geological Sur- core complexes: Evidence from near the mylonitic front: Geology, v. 18, p. 216–219, https:// vey Contributed Map CM-97-A, scale 1:3570. doi.org/10.1130/0091-7613(1990)018<0216:FOMZIC>2.3.CO;2. Strickland, E.D., Singleton, J.S., Seymour, N.M., and Wong, M.S., 2016, Evidence for Orocopia Reynolds, S.J., Spencer, J.E., Richard, S.M., and Laubach, S.E., 1986, Mesozoic structures in Schist in the footwall of the Plomosa Mountains metamorphic core complex, west-central west-central Arizona, in Beatty, B., and Wilkinson, P.A.K., eds., Frontiers in Geology and Arizona: Geological Society of America Abstracts with Programs, v. 48, no. 7, https://doi Ore Deposits of Arizona and the Southwest: Arizona Geological Society Digest 16, p. 35–51. .org/10.1130/abs/2016AM-283161. Reynolds, S.J., Spencer, J.E., Laubach, S.E., Cunningham, D., and Richard, S.M., 1989, Geo- Strickland, E.D., Singleton, J.S., Griffin, A.T.B., and Seymour, N.M., 2017, Geologic map of the logic map, geologic evolution, and mineral deposits of the Granite Wash Mountains, northern Plomosa Mountains metamorphic core complex, Arizona: Arizona Geological west-central Arizona: Arizona Geological Survey Open-File Report 89-04 (updated Sep- Survey Contributed Map CM-17-A, 1 map sheet, scale 1:10,000. tember 1993), 52 p. Tikoff, B., Moores, E.M., and Hole, J.A., 2016, The recurring myth of Farallon flat-slab sub- Saleeby, J., 2003, Segmentation of the Laramide Slab: Evidence from the southern Sierra duction: Geological Society of America Abstracts with Programs. v. 48, no. 7, https://doi Nevada region: Geological Society of America Bulletin, v. 115, p. 655–668, https://doi.org .org/10.1130/abs/2016AM-284634. /10.1130/0016-7606(2003)115<0655:SOTLSF>2.0.CO;2. Wells, M.L., and Hoisch, T.D., 2008, The role of mantle delamination in widespread Late Cre- Salem, A.C., 2009, Mesozoic tectonics of the Maria fold and thrust belt and McCoy Basin, taceous extension and magmatism in the Cordilleran orogen, western United States: southeastern California: An examination of polyphase deformation and synorogenic re- Geological Society of America Bulletin, v. 120, p. 515–530, https://doi .org /10 .1130 /B26006 .1. sponse [Ph.D. thesis]: Albuquerque, University of New Mexico, 284 p. Wong, M.S., Singleton, J.S., Baughman, J., and Bunting, K.C., 2013, Evidence for Miocene re- Salters, V.J.M., and Stracke, A., 2004, Composition of the depleted mantle: Geochemistry activation of a Late Cretaceous to Early Tertiary shear zone in the Harcuvar and Buckskin- Geophysics Geosystems, v. 5, Q05B07, https://doi.org/10.1029/2003GC000597. Rawhide metamorphic core complexes, Arizona: Geological Society of America Abstracts Scarborough, R., and Meader, N., 1983, Reconnaissance geology of the northern Plomosa Moun- with Programs, v. 45, no. 7, p. 523. tains: Arizona Bureau of Geology and Mineral Technology Open-File Report 83‑24, 35 p. Yeats, R.S., 1968, Southern California structure, sea-floor spreading, and history of the Pacific Seymour, N.M., Strickland, E.D., Singleton, J.S., Stockli, D.F., and Wong, M.S., 2018, Laramide basin: Geological Society of America Bulletin, v. 79, p. 1693–1702, https://doi .org/10.1130 subduction and metamorphism of the Orocopia Schist, northern Plomosa Mountains, /0016-7606(1968)79[1693:SCSSSA]2.0.CO;2. west-central Arizona: Insights from zircon U-Pb geochronology: Geology, v. 46, p. 847–850, https://doi.org/10.1130/G45059.1. Shervais, J.W., 1982, Ti-V plots and the petrogenesis of modern and ophiolitic lavas: Earth and Planetary Science Letters, v. 59, p. 101–118, https://doi .org /10 .1016 /0012 -821X (82)90120 -0. Singleton, J.S., and Wong, M.S., 2016, Polyphase mylonitization in the Harcuvar and Buck- MANUSCRIPT RECEIVED 11 MARCH 2018 skin-Rawhide metamorphic core complexes, west-central Arizona: Geological Society of REVISED MANUSCRIPT RECEIVED 26 JULY 2018 America Abstracts with Programs, v. 48, no. 4, https://doi .org /10 .1130 /abs /2016CD -274076. MANUSCRIPT ACCEPTED 7 SEPTEMBER 2018

Geological Society of America | LITHOSPHERE | Volume 10 | Number 6 | www.gsapubs.org 742

Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/10/6/723/4548862/723.pdf by guest on 26 September 2021