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ABSTRACT The California Coast Ranges provide a classic example of a forearc triad comprising (1) a lower plate of the Franciscan subduction complex, and an upper plate of (2) the Coast Range ophiolite, with (3) overlying Great Valley forearc basin deposits (Great Valley Group and overlying strata). This field-trip guide provides a three-day tour of outcrops of the iconic trench-forearc assemblage that provide insight into convergent plate margin processes. Participants will see the contrast between the upper plate Great Valley Group and Coast Range ophiolite with little or no burial metamorphism and penetrative deformation, compared to the Franciscan that exhibits classic subduction interface metamorphism and deformation. Field-trip stops will also examine similarities between the trench-forearc components, including mafic-ultramafic rocks in all three assemblages, as well as mélanges with exotic blocks in the Great Valley Group and Franciscan. The field trip will also view the Coast Range fault that exhumed the Franciscan relative to the Great Valley Group and Coast Range ophiolite primarily in synsubduction time. Relatively little exhumation (<3 km in most areas) has taken place since the non-collisional termination of the subduction episode that formed the Franciscan, Coast Range ophiolite, and Great Valley Group. As a result, the features of these components and their relationships to one another are better preserved than in most of the world’s orogenic belts. In many orogenic belts, it is difficult to distinguish the three forearc components, so the California Coast Ranges provides a rare opportunity to examine such rocks to gain insight into convergent plate margin processes. 

Abstract The Great Valley Forearc (GVF) basin of California, USA, preserves an extensive rock record of the Jurassic–Paleogene tectonic development of the California segment of the North American Cordillera. We present new U-Pb geochronology, zircon and whole-rock geochemistry, and petrographic analyses from the Great Valley Group (GVG), Franciscan subduction complex, and Coast Range Ophiolite (CRO) in the northern San Joaquin Valley to better understand the timing and location of initial forearc sedimentation, and how sediment routing systems may have evolved during Cretaceous time. Basal GVG strata of the Knoxville Formation were deposited ca. 145–140 Ma and are separated by an ~40 m.y. unconformity with overlying strata of the Upper Cretaceous Panoche Formation. Pre-Mesozoic zircon grains are present in both the Knoxville and Panoche formations, but are sparse (0%–7%) compared to other GVG sandstones. Zircon geochemistry records felsic igneous sources (Th/U 0.9–0.2) during both periods of deposition, and epsilon Hf signatures reveal a shift from juvenile to more evolved sources between Knoxville and Panoche deposition. Whole-rock geochemistry shows increasing compositional maturity from latest Jurassic crystallization of the CRO to Early and Late Cretaceous deposition of the GVG. Integrating these data, we present a tectonic model for the northern San Joaquin portion of the GVF basin from ca. 145 Ma to 80 Ma that documents the onset of basin deposition and details sediment pathways during the Early to earliest Late Cretaceous. In addition, we discuss potential drivers for the ~40 m.y. unconformity within the San Joaquin Valley and implications of this work for global forearc basin processes. 

ABSTRACT The Great Valley forearc (GVf) basin, California, records deposition along the western margin of North America during active oceanic subduction from Jurassic through Paleogene time. Along the western GVf, its underlying basement, the Coast Range Ophiolite (CRO), is exposed as a narrow outcrop belt. CRO segments are overlain by the Great Valley Group (GVG), and locally, an ophiolitic breccia separates the CRO from basal GVG strata. New stratigraphic, petrographic, and geochronologic data (3865 detrital and 68 igneous zircon U-Pb ages) from the upper CRO, ophiolitic breccia, and basal GVG strata clarify temporal relationships among the three units, constrain maximum depositional ages (MDAs), and identify provenance signatures of the ophiolitic breccia and basal GVG strata. Gabbroic rocks from the upper CRO yield zircon U-Pb ages of 168.0 ± 1.3 Ma and 165.1 ± 1.2 Ma. Prominent detrital-zircon age populations of the ophiolitic breccia and GVG strata comprise Jurassic and Jurassic–Early Cretaceous ages, respectively, with pre-Mesozoic ages in both that are consistent with sources of North America affinity. Combined with petrographic modal analyses that show abundant volcanic grains (> 50%), we interpret the breccia to be mainly derived from the underlying CRO, with limited input from the hinterland of North America, and the basal GVG to be derived from Mesozoic igneous and volcanic rocks of the Sierra Nevada–Klamath magmatic arc and hinterland. Analysis of detrital-zircon grains from the lower and upper ophiolitic breccia yields MDAs of ∼ 166 Ma and ∼ 151 Ma, respectively. Along-strike variation in Jurassic and Cretaceous MDAs from basal GVG strata range from ∼ 148 to 141 Ma, which are interpreted to reflect diachronous deposition in segmented depocenters during early development of the forearc. The ophiolitic breccia was deposited in a forearc position proximal to North America < 4 Myr before the onset of GVG deposition. A new tectonic model for early development of the GVf highlights the role of forearc extension coeval with magmatic arc compression during the earliest stages of basin development. 

ABSTRACT The Great Valley forearc (GVf) basin, California, records deposition along the western margin of North America during active oceanic subduction from Jurassic through Paleogene time. Along the western GVf, its underlying basement, the Coast Range Ophiolite (CRO), is exposed as a narrow outcrop belt. CRO segments are overlain by the Great Valley Group (GVG), and locally, an ophiolitic breccia separates the CRO from basal GVG strata. New stratigraphic, petrographic, and geochronologic data (3865 detrital and 68 igneous zircon U-Pb ages) from the upper CRO, ophiolitic breccia, and basal GVG strata clarify temporal relationships among the three units, constrain maximum depositional ages (MDAs), and identify provenance signatures of the ophiolitic breccia and basal GVG strata. Gabbroic rocks from the upper CRO yield zircon U-Pb ages of 168.0 ± 1.3 Ma and 165.1 ± 1.2 Ma. Prominent detrital-zircon age populations of the ophiolitic breccia and GVG strata comprise Jurassic and Jurassic–Early Cretaceous ages, respectively, with pre-Mesozoic ages in both that are consistent with sources of North America affinity. Combined with petrographic modal analyses that show abundant volcanic grains (> 50%), we interpret the breccia to be mainly derived from the underlying CRO, with limited input from the hinterland of North America, and the basal GVG to be derived from Mesozoic igneous and volcanic rocks of the Sierra Nevada–Klamath magmatic arc and hinterland. Analysis of detrital-zircon grains from the lower and upper ophiolitic breccia yields MDAs of ∼ 166 Ma and ∼ 151 Ma, respectively. Along-strike variation in Jurassic and Cretaceous MDAs from basal GVG strata range from ∼ 148 to 141 Ma, which are interpreted to reflect diachronous deposition in segmented depocenters during early development of the forearc. The ophiolitic breccia was deposited in a forearc position proximal to North America < 4 Myr before the onset of GVG deposition. A new tectonic model for early development of the GVf highlights the role of forearc extension coeval with magmatic arc compression during the earliest stages of basin development.