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.

The Pelona-Orocopia-Rand Schists (PORS) of southern California and southwestern Arizona (USA) (Fig. 1) are interpreted as Late Cretaceous to early Paleocene metamorphosed trench sediments and other minor rock types (e.g., Haxel and Dillon, 1978; Jacobson et al., 1988, 2000) subducted during the Laramide orogeny and accreted beneath the lower continental crust during slab flattening of a segment of the Farallon plate (Grove et al., 2003; Saleeby, 2003; Jacobson et al., 2011). Exposures of PORS are dominated by quartzofeldspathic schist with minor mafic schist (metabasalt), ferromanganiferous quartzite (metachert) and marble, and rare pods of actinolite ± talc schist or serpentine schist (e.g., Haxel and Dillon, 1978; Chapman, 2016). This subduction complex is intriguing because it was exhumed as much as several hundred kilometers inland from the former subduction trench (Jacobson et al., 2017), whereas the broadly correlative Franciscan accretionary complex occupies the Coast Ranges of California, in close proximity to the paleo–oceanic trench (Chapman et al., 2016). The Californian PORS were initially recognized as a subduction complex in the late 1960s (Crowell, 1968; Yeats, 1968), and Orocopia Schist was traced eastward into southwestern Arizona by the mid-1970s (Haxel and Dillon, 1978; Haxel et al., 2002). However, a recent discovery of Orocopia Schist at Cemetery Ridge in southwestern Arizona did not occur until 2012 (Haxel et al., 2014; Jacobson et al., 2017), where Orocopia Schist is >300 km inboard of the paleo–oceanic trench, highlighting the extreme scale of subduction underplating during the Laramide orogeny.

Detailed thermochronologic studies of Orocopia Schist in the Gavilan Hills and Orocopia Mountains in California (Fig. 1) revealed two distinct periods of rapid cooling during the early Eocene and the latest Oligocene to early Miocene (Jacobson et al., 2002, 2007), leading to the inference that the Orocopia Schist has undergone two phases of exhumation. The mechanism for the first phase of exhumation remains debated (e.g., Chapman, 2016), whereas the second phase of exhumation of the schist from ∼10–12 km depths to the surface or near surface owes to middle Cenozoic tectonic denudation on low-angle normal faults and erosion (e.g., Haxel et al., 2002; Jacobson et al., 2002, 2007). Some of these areas of unroofed schist have metamorphic core complex–like attributes (Holk et al., 2017), but none are a true metamorphic core complex in that they apparently lack pervasive mylonitic fabrics that formed during the early stages of large-magnitude extension (e.g., Lister and Davis, 1989). In this paper we present geologic mapping and petrographic and geochemical analyses of a newly recognized exposure of Orocopia Schist in the footwall of the northern Plomosa Mountains metamorphic core complex in west-central Arizona. Orocopia Schist in the Plomosa Mountains is particularly interesting and unique because it represents an intersection of two seemingly disparate tectonic elements—a continental margin subduction complex and the interior belt of metamorphic core complexes (Fig. 1).

The northern Plomosa Mountains of west-central Arizona are located within the lower Colorado River extensional corridor (LCREC), a highly extended region in the southern Basin and Range province (Howard and John, 1987). Late Oligocene to Miocene extensional deformation within the LCREC was accomplished primarily by low-angle normal faulting associated with metamorphic core complex development (Spencer and Reynolds, 1989, 1991; Spencer et al., 2018). The belt of metamorphic core complexes in the LCREC trends SE and overlaps a predominantly S-vergent zone of Late Cretaceous crustal shortening known as the Maria fold-and-thrust belt (MFTB) (Reynolds et al., 1986) (Fig. 1). Where the MFTB is cross-cut by the extensional belts of the LCREC, exposures of metamorphic core complexes change from N-S trending to WNW-ESE trending. This coincidence between the change in orientation of extensional belts at the intersection of the MFTB suggests that the LCREC was influenced by the orientation of a crustal welt that formed from Late Cretaceous shortening along the MFTB (Spencer and Reynolds, 1990). The timing of Cretaceous shortening associated with the MFTB is constrained by thrust faults that are clearly cut by granitic intrusions ranging in age from ca. 80 to 70 Ma (Martin et al., 1982; Knapp, 1989; Reynolds et al., 1989; Isachsen et al., 1999; Salem, 2009).

The primary structural feature of the northern Plomosa Mountains is the Plomosa detachment fault, a gently dipping normal fault responsible for the exhumation of mid-crustal mylonitic rocks that constitute most of the footwall of the northern Plomosa Mountains metamorphic core complex (Fig. 2). The Plomosa detachment fault is the middle of three imbricate low-angle detachment fault systems active during the early to middle Miocene in west-central Arizona. Based on tectonic reconstructions, the Plomosa detachment fault likely accommodated ∼12–17 km of NE-directed extension (Spencer and Reynolds, 1991; Spencer et al., 2018), whereas the Buckskin-Rawhide detachment fault (the structurally highest of the three imbricate detachment faults) accommodated up to 60 km of extension (Spencer and Reynolds 1991; Spencer et al., 2016, 2018). Apatite and zircon fission-track dates suggest that the footwall of the Plomosa detachment fault was exhumed ca. 22–15 Ma (Foster and Spencer, 1992), approximately coeval with initiation and end of detachment faulting in nearby core complexes (Foster and John, 1999; Singleton et al., 2014; Prior et al., 2016).

The Cretaceous Quinn Pass shear zone (new name by Spencer et al., 2018), a segment of the MFTB, lies within the detachment footwall at the southern end of the northern Plomosa Mountains (Fig. 2). This complex zone consists of N- to NE-verging thrust faults and mylonite zones, and E- to SE-trending folds within Paleozoic to Mesozoic sedimentary units (Spencer et al., 2015). The largest structure in this zone is the NE-vergent Deadman thrust, which bounds a mylonite zone as much as ∼500 m thick (Scarborough and Meader, 1983; Steinke, 1997). Less than 2 km south of the Quinn Pass shear zone, a nonconformity beneath moderately to steeply SW-dipping Miocene strata records tilting of the footwall of the Plomosa detachment fault as it was exhumed (Fig. 2).

The first comprehensive geologic study of the northern Plomosa Mountains was by Jemmett (1966) who described the footwall of the Plomosa detachment fault as predominantly gneiss in the southern portion and schist toward the north. Mapping by Scarborough and Meader (1983) described the metamorphic footwall as predominantly compositionally layered gneisses and interpreted several NE-trending folds based on sparse measurements of metamorphic fabrics. They also inferred the Plomosa detachment fault and its footwall to be arched along a large NW-trending antiform, the axis of which they projected just north of the Quinn Pass area, though this structure is not supported by more recent mapping. Spencer et al. (2015) presented the most comprehensive geologic map of the northern Plomosa Mountains, which was the first study to describe the northern Plomosa Mountains as a metamorphic core complex. They include detailed mapping of the highly dissected brittle hanging wall of the Plomosa detachment fault, composed of Paleoproterozoic to Cenozoic granitic and gneissic rocks; Paleozoic, Mesozoic, and Miocene strata; and Miocene volcanic rocks. Strickland et al. (2017) is the first detailed map of the mylonitic footwall of the Plomosa detachment fault, and is the basis for this paper.

Orocopia Schist

The northern portion of the footwall of the Plomosa detachment fault is dominated by mylonitic quartzofeldspathic schist (Fig. 2, unit KPGos) that was recently interpreted as Orocopia Schist by Strickland et al. (2016, 2017). For clarity, we proceed with the conclusion that this mylonitic quartzofeldspathic schist is indeed the Orocopia Schist, while presenting field and petrographic observations and geochemical analyses that support this interpretation (Figs. 3 and 4).

Field and Petrographic Description

The Orocopia Schist comprises ∼10 km2 of the exposed footwall of the Plomosa detachment fault and most commonly crops out as gray flaggy layers with well-developed centimeter-scale layering, or is homogenous with a purple-red color, and commonly has a well-developed S-C′ mylonitic fabric. It is composed dominantly of quartz (26%–50% modal abundance, used throughout) and feldspar (24%–50%), with biotite (8%–34%, variably chloritized) and locally muscovite (2%–20%, generally ∼10%), minor opaque minerals (≤4%), and accessory apatite (≤1%), rutile, zircon, and local garnet (≤0.5% where present) (Supplementary Table 1 in the GSA Data Repository1). Locally the schist contains 0.1–2-m-thick layers of actinolite-bearing schist, which weathers to a golden color and is flaky. In plane-polarized light, biotite in the quartzofeldspathic schist is typically reddish brown, and rutile needles are common in retrograde chlorite. Milky quartz lenses and pods 2–50 cm thick and quartz-feldspar veins with up to ∼10% feldspar are common throughout the Orocopia Schist. Locally the unit contains gray, mica-poor (<10% mica), feldspar-rich (>60% feldspar) layers 10–30 cm thick (Fig. 3A and 4D).

A stratigraphic-test drill hole (La Posa Federal 1A) located ∼8 km west-northwest of the exposed footwall (Fig. 1) and centered in the northern La Posa Plain encountered metamorphic basement beneath 720 m of sediment. The lithologic log of this drill hole describes the metamorphic basement as schist with abundant quartz and two micas, which resembles the Orocopia Schist. The description lacks any mention of hornblende and is thus distinctly unlike gneiss, which composes much of the crystalline bedrock in the Plomosa Mountains to the southeast and in the Mesquite Mountains to the northwest, suggesting that the Orocopia Schist at the northern Plomosa Mountains may extend at least 8 km further west-northwest in the subsurface.

Five hallmarks of the Orocopia Schist as described by Haxel and Dillon (1978) are present in the schist of the northern Plomosa Mountains:

1. The schist has a dominantly quartzofeldspathic composition and gray flaggy appearance.

2. Graphitic plagioclase poikiloblasts are widespread throughout the schist (Figs. 3C and 4A), are generally 1–5 mm in size, and have a dark gray color, locally with a bluish tint. In thin section, the foliation defined by graphite is preserved within poikiloblasts in some samples (Fig. 4A). The presence of graphitic plagioclase is a key identifier of Orocopia Schist.

3. A 3.5-km-long unit of amphibolite schist interlayered with Orocopia Schist separates the main body of Orocopia Schist from gneiss (Fig. 2, unit KPGam; Fig. 3D) and is distinguished from the immediately adjacent gneiss by common interlayers of Orocopia Schist and quartzite and lack of gneissic layering. The amphibolite is composed of ∼60%–80% hornblende, 15%–30% plagioclase, ∼5% quartz, and 2%–3% titanite, with accessory biotite, garnet, opaque minerals, and apatite. This mineral assemblage records peak metamorphism in the amphibolite facies. Garnets have been resorbed (Fig. 4E) and altered to plagioclase + biotite + epidote + hornblende(?). This unit is similar to amphibolite layers observed in Orocopia Schist of other localities, which are generally interpreted as metamorphosed basalt, consistent with geochemical analysis of three samples from the northern Plomosa Mountains (see section Comparative Geochemistry of Orocopia Metabasalt, Actinolitite, and Metachert).

4. Quartzite layers 3–30 cm thick with small (∼0.5 mm) reddish-orange manganiferous garnet are common within the Orocopia amphibolite. The quartzite is generally weathered to a rusty red color, though some outcrops have clear alternating white and dark gray layers (Fig. 3E). Locally quartzite is tightly folded within the amphibolite (Fig. 3D). Garnet also locally appears as masses within quartzite (Fig. 4F). Quartzite of a very similar appearance in Orocopia Schist exposures in southeast California are interpreted as metachert (e.g., Haxel and Dillon, 1978), which is supported by the geochemical analysis of six samples from the northern Plomosa Mountains (see section Comparative Geochemistry of Orocopia Metabasalt, Actinolitite, and Metachert).

5. Actinolitite pods, 5 cm to 1.5 m wide, are widely scattered throughout the schist (Fig. 3F). These pods are composed of ∼98% coarse-grained green actinolite, with accessory talc, quartz, opaque minerals, and/or feldspar. We found 24 such actinolitite pods, which appear to be distributed uniformly throughout the schist body (Strickland et al., 2017). Locally, several smaller pods are aligned parallel to the trend of mylonitic lineations. Layers of actinolite-bearing schist ∼0.1–2 m thick commonly include actinolitite pods within them, as well as minor talc and rare layers of talc schist.

Metamorphic Fabrics

Mylonitic L-S and L>S fabrics are pervasive throughout the Orocopia Schist of the northern Plomosa Mountains. Centimeter- to decimeter-scale tight to isoclinal folds with attenuated limbs and thickened hinges are common and have axes parallel to mylonitic lineations and axial surfaces that range from upright to recumbent. On average, mylonitic foliations of the Orocopia Schist dip shallowly to the SW (Fig. 5B). C′ shear bands are very common in the schist, and all 56 kinematic indicators documented in outcrop or thin section indicate top-to-the-NE shear. Mylonitic lineations are very systematic with an average orientation of trend 224°, plunge 19° (Fig. 6B).

Comparative Geochemistry of Orocopia Metabasalt, Actinolitite, and Metachert

The geochemistry and origin of the minor metabasalt, actinolitite, and metachert of the PORS have been discussed by Haxel et al. (1987, 2002), Jacobson et al. (1988), Dawson and Jacobson (1989), Moran (1993), Chapman (2016), and Haxel and Jacobson (2017). Here we use new geochemical data to establish that these rock types in the Orocopia Schist of the Plomosa Mountains are much like those of the PORS as a whole.

Three samples of metabasalt from the single large body in the Plomosa Mountains (Fig. 2) and six samples of accompanying Fe-Mn metachert have been analyzed by the U.S. Geological Survey (USGS) Central Minerals Environmental Resources Science Center, Analytical Chemistry Project: major elements by wavelength-dispersive X-ray fluorescence (XRF), and trace elements by inductively coupled plasma (ICP) mass spectrometry or ICP atomic-emission spectrometry (Table 1). Samples for ICP analysis were prepared by two methods, acid dissolution and sinter-dissolution, yielding duplicate analyses for most elements. Selection of data for these elements then followed the scheme outlined by Haxel et al. (2018). Three whole-rock samples of Plomosa actinolitite were analyzed by XRF and ICP at ALS Laboratories in Reno, Nevada (USA). For comparison with these new analyses from the Plomosa Mountains, we use data for some 100 samples of PORS metabasalt, actinolitite, and metachert from several areas, chiefly Cemetery Ridge and Trigo Mountains, southwest Arizona; and Picacho district, Gavilan Hills, Chocolate Mountains, Orocopia Mountains, Blue Ridge, eastern San Gabriel Mountains, Sierra Pelona, and Rand Mountains, southern California.

Metabasalt.Dawson and Jacobson (1989) found that PORS metabasalts comprise two groups. Group 1, dominant, is characterized by chondrite-normalized rare earth element (REE) spectra that are flattish or slightly light REE (LREE)-depleted [(Ce/Yb)cn = 0.4–1.8] and thus geochemically resembles normal and transitional mid-ocean-ridge basalt (MORB) (Klein, 2005). Group 2 patterns are slightly LREE enriched [i.e., have gentle negative slopes, (Ce/Yb)cn = 2.8–3.9]; this subordinate group is like enriched MORB or intraplate basalt. The three Plomosa Mountains samples have virtually flat REE spectra [(Ce/Yb)cn = 0.9–1.0] and belong to group 1 (Fig. 7).

The composition of PORS metabasalts, including those in the Plomosa Mountains, is tholeiitic rather than calcalkaline or alkaline (Fig. 8). Commonly used tectonic discrimination diagrams (including several not shown), based upon presumably immobile trace elements (Pearce, 2014), indicate that PORS metabasalts have affinities to normal MORB and, to a lesser extent, enriched MORB and/or tholeiitic oceanic intraplate basalt (Fig. 9). The metabasalts lack affinity to alkaline-intraplate or magmatic-arc basalt. These geochemical comparisons point to two conclusions. First, Plomosa Mountains metabasalt is indistinguishable from PORS metabasalt in general. Second, PORS metabasalts are derived from common oceanic basalt, of at least two types. Similar varieties of basalt, along with 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 (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 (Haxel et al., 2015, 2018). At Cemetery Ridge, meter-size veins and pods of actinolitite are part of an assemblage of metasomatic rocks produced by mechanical and fluid-mediated interaction of peridotite and enclosing quartzofeldspathic schist (Epstein et al., 2016, 2018). This actinolite 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 reaction zones, from which they acquired their actinolitite.

Figure 10 compares concentrations of eleven compatible to moderately incompatible (Sr, Zn, Ga, Al) major and trace elements in Plomosa Mountains actinolitite with those in actinolite veins at Cemetery Ridge and typical Orocopia actinolitite pods in the southern Chocolate Mountains (Dillon, 1976). Except for unusually low Ni in one Plomosa Mountains sample, patterns for actinolitite from the three areas are similar, consistent with a common origin. Of course, congruence of Ca simply reflects the composition of actinolite. On the other hand, proportions of Mg and Fe could vary widely (through the series tremolite-actinolite-ferroactinolite), but do not; all eight samples are similarly magnesian, with molar MgO / (MgO + FeO*) = 0.85 ± 0.03. Nor is the observed general agreement of the other eight elements, nonessential constituents of actinolite, required by stoichiometry. In Cemetery Ridge actinolitite, these elements reside partly in actinolite and partly in accessory minerals such as albite, pleonaste, olivine, and serpentine; elsewhere in the PORS talc (at least) is an accessory phase.

Metachert. Previous and ongoing studies reveal that PORS metachert (and associated siliceous marble, here not considered separately) comprises three principal components: dominant biogenic and subordinate detrital and hydrothermal. For present purposes, the most informative view of metachert geochemistry is provided by shale-normalized REE spectra. Rare earth elements are contributed mainly by the biogenic and hydrothermal components, which derive their REE from material dissolved or suspended in seawater. The most common type of REE pattern is that shown in Figure 11. (These patterns are corrected by removal of the minor detrital component, as explained in the caption; see also Haxel et al., 1987.) Most PORS metacherts, including those in the Plomosa Mountains, display pronounced negative Ce anomalies. Median Ce/Ce* = 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 deep negative Ce anomalies are diagnostic of derivation from seawater, which is characterized by similar anomalies (Fig. 11). Even though the Orocopia metachert of the Plomosa Mountains has been carried several hundred kilometers inland by low-angle subduction and undergone at least two episodes (Late Cretaceous and Miocene) of amphibolite-facies metamorphism, it retains a chemical palimpsest of its oceanic origin.

The most distinctive feature of PORS metachert is its ferromanganiferous character, commonly apparent in hand specimen through the presence of metamorphic spessartine (or piedmontite) and magnetite. This enhancement in Mn and Fe (and Ni, Cu, and Zn) owes to the hydrothermal component. Enrichment of Mn, relative to detrital background, is on average roughly ten times that of Fe (Fig. 12). Plomosa Mountains metachert 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.

Further observations. Although geochemically quite similar to their counterparts in Orocopia Schist elsewhere in southwestern Arizona, Plomosa Mountains metabasalt and metachert differ in their field setting. In most Arizonan Orocopia Schist, metabasalt and metachert form thin layers, typically no more than one or two meters thick, and are loosely associated, in that separate layers of metabasalt and metachert typically crop out within 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 strained by Late Cretaceous and Miocene deformation so that its original size and shape are uncertain, this metabasalt mass is clearly much larger than any other known in southwestern Arizona. The next largest, at Cemetery Ridge, is ∼60 m in maximum exposed dimension. In this respect, the Orocopia Schist of the Plomosa Mountains appears more like the Californian Pelona or Orocopia Schist in such places as the San Gabriel Mountains, Sierra Pelona, and southern Chocolate Mountains, where metabasalt is more voluminous and the association of metabasalt and metachert more intimate (Ehlig, 1958, 1981; Muehlberger and Hill, 1958; Dillon, 1976; Jacobson, 1983).

Northern Plomosa Intrusive Complex

A newly recognized Miocene intrusive complex, here referred to as the Northern Plomosa intrusive complex (NPIC), parallels in plan view the Plomosa detachment fault in the northern portion of the footwall, and has a total exposure of ∼5 km2 (Fig. 2, unit Nic) (Strickland et al., 2017).

Field Description

The NPIC has a composition of (1) felsic, leucocratic biotite tonalite, granodiorite, and rare granite (altogether totaling ∼60% of the unit); and (2) intermediate hornblende-biotite diorite, with lesser quartz diorite, quartz monzodiorite, and rare quartz monzonite (Figs. 13 and 14). The bulk of this unit appears as layered tabular bodies approximately parallel to mylonitic foliation (Fig. 14A), though nonmylonitic diorite or quartz diorite dikes locally cut across well-foliated layers. Mylonitic to protomylonitic leucocratic dikes and sills 3 cm to 2 m thick are common within the Orocopia Schist (Figs. 14C and 14E). Intrusions of intermediate composition are rare in the Orocopia Schist and record evidence of magma mingling, with centimeter- to decimeter-scale irregular flattened mafic blobs within rocks of a more intermediate composition (Fig. 14D). Highly strained felsic NPIC is in some locations difficult to distinguish from mica-poor mylonitic Orocopia Schist, as both may be relatively rich in quartz and have a homogenous appearance.

Metamorphic Fabrics

The majority of the NPIC is mylonitic, generally with a shallowly SW-dipping mylonitic foliation parallel to that of the Orocopia Schist (Figs. 2 and 5C), and mylonitic fabrics in NPIC leucocratic dikes intruded into the Orocopia Schist parallel fabrics in the adjacent schist. Mylonitic leucocratic intrusions of the NPIC are also commonly isoclinally folded within the schist, with attenuated limbs and thickened hinges, indicating that these dikes are pre- or synmylonitic. All 17 kinematic indicators documented in outcrop or thin section indicate top-to-the-NE shear. Mylonitic lineations of the NPIC have an average orientation of trend 211°, plunge 11°, similar to the average orientation of mylonitic lineations of the Orocopia Schist (Fig. 6C).

Age of Intrusion and Mylonitization

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 (USA) (Fig. 15; Table 2; refer to the Data Repository for raw ages and detailed methods). Euhedral zircons from the NPIC were mounted on sticky tape and ablated through their outer rim into the core, producing a depth versus age profile, though overgrowth rims were not identified in these samples. Reduced age data were analyzed with Isoplot software (Ludwig, 2003). Ages reported are the weighted mean from individual 206Pb/238U ages that overlap with concordia at 2σ error and have <10% uncertainty.

Four intrusions dated via U-Pb zircon geochronology yielded Miocene ages (Fig. 15; Table 2). Sample 0316-P71x is from a protomylonitic granodiorite sill that intrudes the Orocopia Schist (Figs. 14E and 14F). Eight concordant zircon ages out of 31 are Miocene, yielding a weighted mean age of 22.3 ± 0.5 Ma (Mean square of weighted deviates [MSWD] = 2.1). The older zircons are inherited, with ages ranging from Proterozoic to Paleogene. Samples 0316-P80b and 0316-P81a, from mylonitic granodiorite dikes within the Orocopia Schist, have similar weighted mean ages of 22.6 ± 0.3 Ma (MSWD = 2.4) and 22.8 ± 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 nonmylonitic diorite collected ∼1.5 km north of the Quinn Pass shear zone and structurally above a mylonitic front in the footwall. This diorite has entirely Miocene zircon ages with a weighted mean age of 20.5 ± 0.2 Ma (MSWD = 1.8).

At the outcrop scale, intrusions within the Orocopia Schist commonly appear to have undergone high strain, exhibiting attenuation, boudinage, and isoclinal folding, yet at the thin-section scale these intrusions are protomylonitic or record only minor quartz dynamic recrystallization (e.g., sample 0316-P71x; Figs. 14E and 14F), suggesting that they intruded during mylonitization and deformed primarily as magma or crystal mush. The mapping of pervasive mylonitic fabrics and the determination of early Miocene ages for the synmylonitic intrusions represent the first documentation of the northern Plomosa Mountains as a metamorphic core complex with Miocene mylonitic fabrics. Moreover, the presence of mylonitic Miocene dikes and sills throughout the Orocopia Schist, and the parallelism of their mylonitic fabrics with the schist, suggest that the observed mylonitic fabric of the Orocopia Schist in the northern Plomosa Mountains is also dominantly of early Miocene age. Mylonitization across the footwall must have ceased prior to cooling below zircon and apatite fission-track closure temperatures ca. 18–14 Ma (Foster and Spencer, 1992).


Field Observations

Crystalline gneiss dominates the central and southern portion of the footwall of the Plomosa detachment fault (Figs. 2 and 16). Based on the similarity of this unit to other such gneisses in west-central Arizona (e.g., Bryant and Wooden, 2008), the protolith is likely a mix of Proterozoic, Jurassic, and Cretaceous rocks. The gneiss dominantly comprises alternating layers ∼3–30 cm thick of hornblende amphibolite, biotite-poor tonalite or granodiorite, and well-foliated and lineated hornblende biotite tonalite or granodiorite. Locally, packets of well-foliated gneiss are 3–10 m thick, defined by 1-mm- to 1.5-cm-thick layers of alternating felsic and mafic compositions. Amphibolite and hornblende-rich tonalite or granodiorite are locally several meters thick, and amphibolite commonly exhibits pinch-and-swell or boudinage structures. The unit locally includes leucogranite layers 1–10 cm thick with recrystallized ribbons of quartz and plagioclase.

Metamorphic Fabrics

Mylonitic fabrics are pervasive throughout the majority of the gneiss (composing ∼14 km2 of the gneiss in the footwall). However, the mylonitic foliation of the gneiss is discordant to that of the Orocopia Schist and NPIC, dipping on average shallowly to moderately NW and E, defining an antiform with an average fold axis of trend 028°, plunge 05° (Fig. 5D). Mylonitic lineations are typically defined by stretched quartz, aligned hornblende, and streaks of biotite. A gradational mylonitic front is present in the southern portion of the gneiss (Fig. 2), defining a transition from mylonite to protomylonite to nonmylonitic gneiss with sparse discrete shear zones. Thirty of 32 kinematic indicators documented in outcrop or thin section indicate top-to-the-NE shear, with the two top-to-the-SW indicators observed as discrete shear zones near the mylonitic front. Top-to-the-SW (antithetic) discrete shear zones have been documented near the mylonitic front in other Arizona core complexes (e.g., Reynolds and Lister, 1990). Mylonitic lineations in the gneiss have a nearly identical NE-SW trend as in the Orocopia Schist and NPIC, with an average orientation of trend 038°, plunge 05° (Fig. 6D).

Contact with the Orocopia Schist

The gneiss is juxtaposed against the Orocopia Schist along a tectonic contact where Orocopia metabasalt is concentrated (Fig. 2). The tectonic contact between the Orocopia Schist and the gneiss dips moderately to the SE—as interpreted from the orientation of gneissic foliation of Orocopia metabasalt, which generally varies from subvertical to gently SE dipping (Fig. 17)—and is undulatory with decimeter- to meter-scale folds with axes parallel to the strike of the folded gneissic foliation. The gneissic foliation of the metabasalt and adjacent gneiss at the contact is discordant to the overall orientation of mylonitic foliation in the Orocopia Schist and NPIC, which generally dips shallowly SW, and to the mylonitic foliation of the gneiss, which generally dips shallowly to moderately NW or E (Fig. 2). However, mylonitic foliations of all units are locally rotated into parallelism with the contact, and the tectonic contact was the locus of NPIC Miocene intrusions. The concordance of adjacent mylonitic foliations to the contact, the undulating nature of the contact, and the concentration of mylonitic Miocene intrusions along the contact all suggest it has been greatly overprinted by Miocene deformation. Thus, its original age and tectonic significance are now obscure.

Structural Depth and Thickness of the Orocopia Schist

A nonconformity between early Miocene strata and pre-Cenozoic crystalline rocks in the footwall of the Plomosa detachment fault dips ∼55° SW (Fig. 2) due to tilting of the footwall during Miocene exhumation. To determine the age of this nonconformity, we dated an ash-fall tuff bed directly overlying granite in the footwall (sample 16-3-1). The weighted mean age of concordant zircons from the tuff is 21.1 ± 0.2 Ma (MSWD = 1.6), nearly identical to the 21.1 ± 0.3 Ma TuffZirc age determined using the algorithm of Ludwig (2003). We interpret this nonconformity to represent the surface near the inception of detachment faulting at ca. 22–20 Ma in the west-central Arizona core complexes (e.g., Foster and John, 1999; Singleton et al., 2014; Prior et al., 2016), and prior to ca. 19 Ma rapid extension in the adjacent Whipple Mountains core complex (Gans and Gentry, 2016). Projecting perpendicular to the nonconformity (assumed to be horizontal ca. 21 Ma) suggests that the Orocopia Schist was only 3–4 km below the surface near the time of initiation of slip on the Plomosa detachment fault (Fig. 2, projection A). If the Plomosa detachment fault initiated as early as 22 Ma, possibly several kilometers of early Miocene schist exhumation occurred prior to deposition of the tuff (assuming a slip rate of ∼3 km/m.y.; Foster and John, 1999), in which case the paleo-depth to the top of the schist may have been ∼5–6 km at the inception of detachment faulting. In either case, the geothermal gradient must have been very high (≥80 °C/km) to result in mylonitization of the NPIC and Orocopia Schist at these shallow depths. The exposed portion of the Orocopia 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 that the schist extended at least 10–12 km below the ca. 21 Ma surface near the inception of detachment faulting.

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 Plomosa Mountains is the Orocopia Schist. This conclusion is also strongly supported by detrital zircon geochronology (Seymour et al., 2018). The northern Plomosa Mountains is a metamorphic core complex with Miocene 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 presents 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 near the margin of the schist. The Plomosa detachment fault then initiated along the schist and NPIC, exhuming these rocks from depths as great as 10–12 km to the surface.

Orocopia Schist: A Subhorizontal Layer above the Farallon Slab

Geologists have questioned whether the PORS form a continuous layer sandwiched between the subducting Farallon plate and overlying crust, or whether the curvilinear exposure along the Chocolate Mountains anticlinorium (Fig. 1) implies that the Orocopia Schist forms only a narrow belt in the subsurface and is not laterally continuous in the direction of subduction (Haxel et al., 2002). Documentation of Orocopia Schist at the northern Plomosa Mountains provides additional constraint on the geometry and extent of the subducted PORS. The Orocopia Schist in the northern Plomosa Mountains and Cemetery Ridge are both >300 km from the paleo–oceanic trench in the subduction direction. These exposures confirm that the Orocopia Schist was transported far inland and indicate that a continuous layer once extended from these areas to the Chocolate Mountains anticlinorium (Fig. 1) (Haxel et al., 2015; Jacobson et al., 2017), though has since been dissected by large-magnitude Miocene extension. While several geologists still question the interpretation of Laramide low-angle subduction (e.g., Maxson and Tikoff, 1996; Hildebrand, 2015; Tikoff et al., 2016), the presence of subducted schist so far inboard of the plate boundary is difficult to explain without a shallowly dipping Farallon slab.

Paleogene Exhumation of the Orocopia Schist

The presence of Orocopia Schist within 3–6 km of the surface prior to early Miocene detachment faulting remains a puzzle. The northern Plomosa Mountains is within the Maria fold-and-thrust belt (Fig. 1), an area presumed to have formed a crustal welt in the Late Cretaceous (Spencer and Reynolds, 1990) prior to subduction of the Orocopia Schist ca. 73 Ma (Seymour et al., 2018; Fig. 18A). Orocopia metabasalt at the northern Plomosa Mountains lacks evidence for eclogite-facies metamorphism, suggesting that it was never subducted to >45 km (≥1.3 GPa) or that phase transformation kinetics were too sluggish to convert the metabasalt to ecologite before exhumation beneath the crustal welt. If subducted below a crustal welt that was hypothetically 50 km thick, the Orocopia Schist at the northern Plomosa Mountains would have needed to be exhumed ∼45 km through the crust in the Paleogene. Possible interactions of the crustal welt with the Farallon plate and Orocopia Schist that do not involve subduction to >45 km include: (1) the bottom of the thick crustal welt was sheared off by the shallowly dipping Farallon slab prior to underplating of the Orocopia Schist (Fig. 18B; Spencer et al., 2018); or (2) the Orocopia Schist extruded into the middle crust before reaching the crustal welt (Fig. 18C). Alternatively, the lower crust and lithospheric mantle may have delaminated prior to schist underplating (e.g., Wells and Hoisch, 2008). Even if the Orocopia Schist was not subducted beneath a significant crustal welt, and was underplated to only 30 km depth, significant Paleogene exhumation of the Orocopia Schist would have been necessary for it to reach the upper crust prior to final exhumation by Miocene detachment faulting. Our interpretation for Paleogene exhumation of Orocopia Schist in the northern Plomosa Mountains is supported by Paleogene exhumation documented in other locations of PORS in southern California and southwestern Arizona (e.g., Haxel et al., 2002; Jacobson et al., 2002; Grove et al., 2003). Detailed thermochronologic studies of Orocopia Schist at the Gavilan Hills and Orocopia Mountains (Fig. 1) revealed a Paleogene period of rapid cooling ca. 52–43 Ma (Jacobson et al., 2002, 2007), consistent with normal-sense slip on the Paleogene Chocolate Mountains fault (Oyarzabal et al., 1997), coupled with a significant component of erosional exhumation. Latest Cretaceous to early Paleogene NE-SW extension is also inferred in the nearby Dome Rock Mountains, Harcuvar Mountains, and Buckskin-Rawhide Mountains (Fig. 1) (Boettcher et al., 2002; Wong et al., 2013; Singleton and Wong, 2016). These ranges apparently lack Orocopia Schist, but evidence for Laramide-age extension in this region supports the interpretation that Paleogene exhumation was widespread throughout southern California and western Arizona.

Paleogene extension following subduction of the Orocopia Schist could have been triggered by rheological weakening of the crust from (1) hydration associated with fluid flux from the Farallon slab and subducted sediments (e.g., Wells and Hoisch, 2008); and/or (2) emplacement of weak schist in the lower crust, leading to gravitational collapse of the thickened crust. Therefore, the subduction and exhumation of weak schist not only may have controlled the location and geometry of the northern Plomosa Mountains metamorphic core complex, but it may also have been responsible for regional latest Cretaceous to Paleogene extension.

We have documented Orocopia Schist and a Miocene intrusive complex in the footwall of the Plomosa detachment fault, and have demonstrated that the northern Plomosa Mountains is a metamorphic core complex with Miocene mylonitic fabrics that record a consistent top-to-the-NE sense of shear. Synmylonitic early Miocene intrusions in the schist demonstrate that the footwall of the Plomosa detachment fault, and likely the entire structural thickness of the Orocopia Schist, records penetrative mid-crustal strain coeval with Miocene core complex development.

The documentation of Orocopia Schist in the northern Plomosa Mountains provides important insight into the spatial and temporal emplacement of subducted schist beneath southwestern Arizona and southeastern California. We conclude that the Orocopia Schist once formed a continuous layer extending from the Chocolate Mountains anticlinorium northeast to the northern Plomosa Mountains and Cemetery Ridge, providing compelling evidence that Orocopia Schist was emplaced above a shallowly dipping Farallon slab. In the northern Plomosa Mountains, the 3–4 km paleo-depth of the Orocopia Schist at ca. 21 Ma necessitates a major Paleogene exhumation event, supporting Paleogene extensional exhumation documented by studies of Orocopia Schist at other localities as well as studies of nearby core complexes. The association of Orocopia Schist with Paleogene exhumation suggests that the subduction of rheologically weak schist beneath previously thickened crust may have been the trigger for major Paleogene exhumation in west-central Arizona.

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 Jacobson, Marty Grove, Kirsten Sauer, and an anonymous reviewer for their insightful comments, which have improved the quality of this manuscript. Nikki Seymour was instrumental 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 mapping with E. Strickland for nearly a month in the northern Plomosa Mountains.

1GSA Data Repository Item 2018358, which includes the DR Spreadsheet: Raw zircon U-Pb ages of igneous samples (“raw data” tab) and details of methods (“metadata” 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 editing@geosociety.org.
Gold Open Access: This paper is published under the terms of the CC-BY-NC license.