ABSTRACT This guide begins with an overview of the internal structure and petrology of the Catalina Schist terrane as exposed on Santa Catalina Island, California, followed by a discussion of the tectonic setting and exhumational history of the terrane, and the Cenozoic tectonic and geological evolution of the Inner Borderland, within which it lies. The guide then presents an itinerary for a three-day field trip from 9–11 May 2020. Next, we present a tectonic model for the formation of the Catalina Schist, followed by a discussion of its relationship to the Pelona, Orocopia, Rand, and related schists in southern California. This field trip generally follows the GSA guide published in GSA Field Guide 59 (available at https://pubs.geoscienceworld.org/gsa ): Platt, J.P., Grove, M., Kimbrough, D.L., and Jacobson, C.E., 2020, Structure, metamorphism, and geodynamic significance of the Catalina Schist terrane, in Heermance, R.V., and Schwartz, J.J., eds., From the Islands to the Mountains: A 2020 View of Geologic Excursions in Southern California: Geological Society of America Field Guide 59, p. 165–195, https://doi.org/10.1130/2020.0059(05) .

ABSTRACT This guide begins with an overview of the internal structure and petrology of the Catalina Schist terrane as exposed on Santa Catalina Island, California, followed by a discussion of the tectonic setting and exhumational history of the terrane, and the Cenozoic tectonic and geological evolution of the Inner Borderland, within which it lies. The guide then presents an itinerary for a three-day field trip from 9–11 May 2020. Next, we present a tectonic model for the formation of the Catalina Schist, followed by a discussion of its relationship to the Pelona, Orocopia, Rand, and related schists in southern California.

ABSTRACT The upper Middle Eocene to Lower Miocene Sespe Formation is the youngest part of an ~7-km-thick Cretaceous–Paleogene forearc stratigraphic sequence in coastal southern California. Whereas Upper Cretaceous through Middle Eocene strata of southern California record a transition from local (i.e., continental-margin batholith) to extraregional (i.e., cratonal) provenance resulting from Laramide deformation (75–35 Ma), the Sespe Formation records the reversal of this process and the re-establishment of local sediment sources by Middle Miocene time. In contrast to underlying dominantly marine strata, the Sespe Formation primarily consists of alluvial/fluvial and deltaic sandstone and conglomerate, which represent terminal filling of the forearc basin. Prior to Middle Miocene dissection and clockwise rotation, the Sespe basin trended north-south adjacent to the west side of the Peninsular Ranges. The integration of paleocurrent, accessory-mineral, conglomerate, sandstone, and detrital zircon data tightly constrains provenance. Sespe sandstone deposited in the Late Eocene was supplied by two major rivers (one eroding the Sonoran Desert, to the east, and one eroding the Mojave Desert, Colorado River trough area, and Transition Zone, to the north), as well as smaller local drainages. As the Farallon slab rolled back toward the coast during the Oligocene, the drainage divide also migrated southwestward. During deposition of the upper Sespe Formation, extraregional sources diminished, while the Peninsular Ranges provided increasing detritus from the east and the Franciscan Complex provided increasing detritus from the west (prerotation). As the overall flux of detritus to the Sespe basin decreased and deposition slowed, nonmarine environments were replaced by marine environments, in which the Miocene Vaqueros Formation was deposited. The provenance and paleogeographic information presented herein provides new insights regarding the unique paleotectonic setting of the Sespe forearc from the Late Eocene through earliest Miocene. Nonmarine sedimentation of the Sespe Formation initiated soon after cessation of coastal flat-slab subduction of the Laramide orogeny and terminated as the drainage divide migrated coastward. Competing models for movement along the Nacimiento fault system during the Laramide orogeny (sinistral slip versus reverse slip to emplace the Salinian terrane against the Nacimiento terrane) share the fact that the Peninsular Ranges forearc basin was not disrupted, as it lay south and southwest of the Nacimiento fault system. The northern edge of the Peninsular Ranges batholith formed a natural conduit for the fluvial system that deposited the Sespe Formation.

ABSTRACT Conglomerate-clast analysis, sandstone petrology, and detrital zircon age data determine the provenance and correlation of the Middle Miocene Mint Canyon and Caliente Formations. Detrital zircon age assemblages and sandstone compositions confirm that the Mint Canyon Formation is the upstream equivalent of the southern Caliente Formation and that both are dissimilar to the Punchbowl Formation. The Mint Canyon and Caliente Formations are remnants of an axial drainage system that was likely confined by the ancestral Sierra Pelona/Blue Ridge to the north and ancestral San Gabriel Mountains to the south; sediments derived from these local highlands dominate in the Mint Canyon Formation. This study highlights the importance of integrated-methods analysis. Sandstone detrital modes capture the variability in sandstone composition and the degree of overlap between formations; conglomerate-clast compositional data show differences within the drainage system; distinct detrital zircon age assemblages implicate particular source terranes. Analysis of all these data sets provides a robust and complete characterization of the provenance of this confined drainage system.

ABSTRACT Blocks of variably serpentinized oceanic mantle peridotite (harzburgite, olivine orthopyroxenite, and dunite) are entrained within a latest Cretaceous (“Laramide”) low-angle subduction channel, the Orocopia Schist, exposed at Cemetery Ridge, southwest Arizona. Oceanic peridotite, serpentinized by seawater, is strikingly out of place in this region of Paleoproterozoic to Jurassic continental crust. Correspondingly, the Cemetery Ridge peridotite contains four serpentinization-related sulfide or intermetallic minerals quite unusual for an inland, continental tectonic setting: pentlandite, cobalt pentlandite, heazlewoodite, and awaruite. The peridotite also contains two Ni-arsenide minerals, orcélite and maucherite; and, less commonly, the sulfides pyrrhotite, bismuthinite, bornite, and parkerite(?). These minerals form tiny to small (~3–100 μm) grains sparsely scattered amongst the profusion of serpentine and magnetite produced by serpentinization of olivine; many grains are enclosed in magnetite. The three principal sulfide minerals at Cemetery Ridge are pentlandite [(Ni,Fe) 8.93 S 8 ] in all samples studied, and cobalt pentlandite [(Co,Ni,Fe) 9.01 S 8 ] and heazlewoodite (Ni 2.98 S 2 ) in many or most. Pentlandite and cobalt pentlandite form a discontinuous series to 74 molar percent of the Co end member. High-Co pentlandite has systematically elevated Ni/Fe. Pyrrhotite (Fe 7.88 S 8 ) occurs only in one high-S sample. Although orcélite (Ni 4.71 As 2 ) and maucherite (Ni 11.06 As 8 ) are volumetrically rare, one or both are found in most samples. Awaruite (Ni 5 Fe) is extremely rare, a few minute blebs in two samples. Sulfide assemblages at Cemetery Ridge indicate the highly reduced pore fluid typical of serpentinization. Sulfur was introduced into Cemetery Ridge peridotite during the early stages of serpentinization, as one aspect of a general enrichment in fluid-mobile elements in a suprasubduction (mantle-wedge) environment. Further serpentinization was accompanied by progressive desulfurization, with concomitant transformation from minerals of higher to lower sulfidation, and partial transfer of first Fe then Ni from sulfide to oxide phases. Five Ni or Ni-Co sulfide or arsenide minerals at Cemetery Ridge are new and unexpected for Arizona, where serpentinite is quite rare and Ni or Co deposits unknown. Sulfide minerals and assemblages at Cemetery Ridge are comparable to those of serpentinites in such places as the California and Oregon Coast Ranges and Mid-Atlantic Ridge, emphasizing the uniqueness of the tectonic setting of the subducted oceanic peridotite at Cemetery Ridge, and enhancing the status of the Orocopia and related schists as the world’s type (first-recognized, best-known) low-angle paleosubduction complex.

The source of volcanic material in the Upper Triassic Chinle Formation on the Colorado Plateau has long been speculated upon, largely owing to the absence of similar-age volcanic or plutonic material cropping out closer than several hundred kilometers distant. These strata, however, together with Upper Triassic formations within El Antimonio and Barranca Group sedimentary rocks in northern Sonora, Mexico, yield important clues about the inception of Cordilleran magmatism in Triassic time. Volcanic clasts in the Sonsela Member of the Chinle Formation range in age from ca. 235 to ca. 218 Ma. Geochemistry of the volcanic clasts documents a hydrothermally altered source region for these clasts. Detrital zircons in the Sonsela Member sandstone are of similar age to the clasts, as are detrital zircons from the El Antimonio and Barranca Groups in Sonora. Most noteworthy about the Colorado Plateau Triassic zircons, however, are their Th/U ratios, which range from ~1 to 3.5 in both clast and detrital zircons. Thorium/uranium ratios in the Sonoran zircons, in contrast, range from ~0.4 to ~1. These data, together with rare-earth-element geochemistry of the zircons, shed light on likely provenance. Geochemical comparisons support correlation of clasts in the Sonsela Member with Triassic plutons in the Mojave Desert in California that are of the same age. Zircons from these Triassic plutons have relatively low Th/U ratios, which correspond well with values from El Antimonio and Barranca Group sedimentary rocks, and support derivation of the strata, at least in part, from northern sources. The Sonsela Member zircons, in contrast, match Th/U values obtained from Proterozoic through Miocene volcanic, volcaniclastic, and plutonic rocks in the eastern and central Mojave Desert. Similarly, rare-earth-element compositions of zircons from Jurassic ignimbrites in the Mojave Desert, though overlapping those of zircons from Mojave Desert plutons, also closely resemble those from Sonsela Member zircons. We use these data to speculate that erosion of Triassic volcanic fields in the central to eastern Mojave Desert shed detritus that became incorporated into the Chinle Formation on the Colorado Plateau.

Abstract This field guide describes a two-and-one-half day transect, from east to west across southern California, from the Colorado River to the San Andreas fault. Recent geochronologic results for rocks along the transect indicate the spatial and temporal relationships between subarc and retroarc shortening and Cordilleran arc magmatism. The transect begins in the Jurassic(?) and Cretaceous Maria retroarc fold-and-thrust belt, and continues westward and structurally downward into the Triassic to Cretaceous magmatic arc. At the deepest structural levels exposed in the southwestern part of the transect, the lower crust of the Mesozoic arc has been replaced during underthrusting by the Maastrichtian and/or Paleocene Orocopia schist.

The Orocopia Schist of the Orocopia Mountains is part of the regionally extensive Pelona-Orocopia-Rand Schist terrane, which is generally interpreted as a relict subduction complex underplated beneath southern California and southwestern Arizona during the latest Cretaceous–early Cenozoic Laramide orogeny. The schist in the Orocopia Mountains forms the lower plate of the Orocopia Mountains detachment fault and has an exposed structural thickness of ∼1.5 km. Prograde metamorphism occurred in the albite-epidote amphibolite facies, although the upper half of the section exhibits a strong greenschist-facies retrograde overprint. A mylonite zone just a few meters thick is present at the top of the schist. The upper plate of the Orocopia Mountains detachment fault is divided into one mappable unit consisting of Proterozoic gneiss widely intruded by 76 Ma leucogranite and a second unit dominated by anorthosite-syenite with minor amounts of leucogranite. Both the leucogranite-gneiss and anorthosite-syenite units are locally cut by faults that may be genetically related to the Orocopia Mountains detachment fault. None of the rocks in the upper plate exhibit evidence of ductile deformation related to movement on the detachment fault. 40 Ar/ 39 Ar analysis of the schist yielded total gas ages of 54–50 Ma for hornblende, 52–34 Ma for muscovite, 33–14 Ma for biotite, and 25–24 Ma for K-feldspar. A single apatite fission track sample yielded an age of 16 Ma. The above results, combined with multidiffusion domain (MDD) analysis of K-feldspar, indicate two major episodes of cooling: one beginning at ca. 52–50 Ma, the other starting at ca. 24–22 Ma. The early Cenozoic phase of cooling is attributed to subduction refrigeration combined with erosional and tectonic denudation. The greenschist-facies retrogression of the schist probably occurred at this time. The middle Cenozoic cooling event is thought to be the result of normal-sense slip on the Orocopia Mountains detachment fault. The thin mylonite at the top of the schist probably formed in association with this structure. The early and middle Cenozoic events each appear to have contributed substantially to the 30–35 km of total exhumation required to bring the schist from its maximum depth of underthrusting to the surface. Most 40 Ar/ 39 Ar ages from the upper plate fall into the following ranges: 76–69 Ma for hornblende, 75–56 Ma for biotite, and 78–42 Ma for K-feldspar. One apatite fission track age of 27 Ma was obtained from the anorthosite-syenite unit. MDD thermal histories for K-feldspar vary significantly with structural position, implying the presence of at least one major structural break within the upper plate. The distinctly old ages for the upper plate compared to the schist indicate that the former was exhumed to relatively shallow crustal levels by latest Cretaceous to early Cenozoic time. The upper plate was juxtaposed against the schist in the earliest Miocene by slip on the Orocopia Mountains detachment fault. The two-stage cooling and exhumation history for the Orocopia Schist in the Orocopia Mountains is virtually identical to that inferred recently for the Gavilan Hills to the southeast based upon a similar thermochronologic analysis. Combined with preliminary investigations of several additional bodies of schist exposed along the Chocolate Mountains anticlinorium of southeastern California and southwestern Arizona, these data provide strong evidence for a major middle Cenozoic extensional event throughout the region. The inferred middle Cenozoic extensional faults are folded by the Chocolate Mountains anticlinorium. This contradicts a recent model for erosional unroofing of the Orocopia Schist, which predicts that the Chocolate Mountains anticlinorium developed primarily during the early Cenozoic.

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Full article available in PDF version.

Full article available in PDF version.