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1. Age and provenance relationships between the basal Great Valley Group and its underlying basement: implications for initiation of the Great Valley forearc basin, California, U.S.A.

   https://doi.org/10.2110/jsr.2024.004.2024.004  

   Romero, Mariah C; Orme, Devon A; Surpless, Kathleen D; Ronemus, Chance B; Morrow, Zachary (October 2024, Journal of Sedimentary Research)

   Kaczmarek, Stephen; Sweet, Dustin (Ed.)

   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. 

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2. Thermal history of the northern Great Valley forearc basin, California

   Orme, D.A.; Romero, M.C.; Oleson, E.; Weis, N (August 2023, Geological Society of America Penrose)

   The Great Valley Forearc basin of California preserves >15 km of strata deposited during latest Jurassic-earliest Cretaceous to Eocene sedimentation. Along the western margin of the central-northern Great Valley forearc, the oldest basin strata are preserved as an eastward dipping homoclinal belt. Previous work on the thermal history of the western outcrop belt has constrained sub-normal geothermal gradients (<20C/km) during middle Cretaceous to Eocene time related to subduction refrigeration. However, the timing of maximum burial and subsequent exhumation is restricted to a few local studies. This study applies apatite and zircon (U-Th)/He and apatite fission track thermochronology to quantify maximum burial temperatures and the timing and rate of cooling of latest Jurassic-middle Cretaceous strata of the western homocline and neighboring subsurface along 350 km of the basin margin. Zircon (U-Th)/He dates range from ~167 to 85 Ma, which are either older or bracket corresponding depositional ages. Apatite fission track dates range from ~162 to 90 Ma, with the majority of grains between ~110-90 Ma. All apatite (U-Th)/He dates are less than 50 Ma, with most grains yielding dates between ~40-20 Ma. Preliminary integration of these data into thermal history models indicate that maximum burial temperatures did not exceed 120-180 C. The timing of basin cooling ranges based on locality, with the western outcrop yielding rapid exhumation starting between ~100-65 Ma and subsurface cooling at ~50 Ma. Final cooling to modern temperatures, as constrained by apatite (U-Th)/He dates, generally coincides with the transition to a transform margin after ~30 Ma. 

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3. Late Jurassic paleogeography of the U.S. Cordillera from detrital zircon age and hafnium analysis of the Galice Formation, Klamath Mountains, Oregon and California, USA

   https://doi.org/10.1130/B36810.1  

   Surpless, Kathleen D.; Alford, Ryan W.; Barnes, Calvin; Yoshinobu, Aaron; Weis, Natalee E. (August 2023, Geological Society of America Bulletin)

   The Upper Jurassic Galice Formation, a metasedimentary unit in the Western Klamath Mountains, formed within an intra-arc basin prior to and during the Nevadan orogeny. New detrital zircon U-Pb age analyses (N = 11; n = 2792) yield maximum depositional ages (MDA) ranging from ca. 160 Ma to 151 Ma, which span Oxfordian to Kimmeridgian time and overlap Nevadan contractional deformation that began by ca. 157 Ma. Zircon ages indicate a significant North American continental provenance component that is consistent with tectonic models placing the Western Klamath terrane on the continental margin in Late Jurassic time. Hf isotopic analysis of Mesozoic detrital zircon (n = 603) from Galice samples reveals wide-ranging εHf values for Jurassic and Triassic grains, many of which cannot be explained by a proximal source in the Klamath Mountains, thus indicating a complex provenance. New U-Pb ages and Hf data from Jurassic plutons within the Klamath Mountains match some of the Galice Formation detrital zircon, but these data cannot account for the most non-radiogenic Jurassic detrital grains. In fact, the in situ Cordilleran arc record does not provide a clear match for the wide-ranging isotopic signature of Triassic and Jurassic grains. When compiled, Galice samples indicate sources in the Sierra Nevada pre-batholithic framework and retroarc region, older Klamath terranes, and possibly overlap strata from the Blue Mountains and the Insular superterrane. Detrital zircon age spectra from strata of the Upper Jurassic Great Valley Group and Mariposa Formation contain similar age modes, which suggests shared sediment sources. Inferred Galice provenance within the Klamath Mountains and more distal sources suggest that the Galice basin received siliciclastic turbidites fed by rivers that traversed the Klamath-Sierran arc from headwaters in the retroarc region. Thus, the Galice Formation contains a record of active Jurassic magmatism in the continental arc, with significant detrital input from continental sediment sources within and east of the active arc. These westward-flowing river systems remained active throughout the shift in Cordilleran arc tectonics from a transtensional system to the Nevadan contractional system, which is characterized by sediment sourced in uplifts within and east of the arc and the thrusting of older Galice sediments beneath older Klamath terranes to the east. 

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4. Middle Jurassic to Early Cretaceous orogenesis in the Klamath Mountains Province (Northern California–southern Oregon, USA) occurred by tectonic switching: Insights from detrital zircon U-Pb geochronology of the Condrey Mountain schist

   https://doi.org/10.1130/GES02709.1  

   Chapman, Alan D; Grischuk, Jennifer; Klapper, Meghan; Schmidt, William; LaMaskin, Todd (April 2024, Geosphere)

   NA (Ed.)

   The Klamath Mountains Province of Northern California and southern Oregon, USA, consists of generally east-dipping terranes assembled via Paleozoic to Mesozoic subduction along the western margin of North America. The Klamath Mountains Province more than doubled in mass from Middle Jurassic to Early Cretaceous time, due to alternating episodes of extension (e.g., rifting and formation of the Josephine ophiolite) and shortening (e.g., Siskiyou and Nevadan events). However, the tectonic mechanisms driving this profound Mesozoic growth of the Klamath Mountains Province are poorly understood. In this paper, we show that formation of the Condrey Mountain schist (CMS) of the central Klamath Mountains Province spanned this critical time period and use the archive contained within the CMS as a key to deciphering the Mesozoic tectonics of the Klamath Mountains Province. Igneous samples from the outer CMS subunit yield U-Pb zircon ages of ca. 175–170 Ma, which reflect volcanic protolith eruptive timing. One detrital sample from the same subunit contains abundant (~54% of zircon grains analyzed) Middle Jurassic ages with Paleozoic and Proterozoic grains comprising the remainder and yields a maximum depositional age (MDA) of ca. 170 Ma. These ages, in the context of lithologic and thermochronologic relations, suggest that outer CMS protoliths accumulated in an outboard rift basin and subsequently underthrust the Klamath Mountains Province during the Late Jurassic Nevadan orogeny. Five samples of the chiefly metasedimentary inner CMS yield MDAs ranging from 160 Ma to 130 Ma, with younger ages corresponding to deeper structural levels. Such inverted age zonation is common in subduction complexes and, considering existing K-Ar ages, suggests that the inner CMS was assembled by progressive underplating over a >10 m.y. timespan. Despite this age zonation, age spectra derived from structurally shallow and deep portions of the inner CMS closely overlap those derived from the oldest section of the Franciscan subduction complex (South Fork Mountain schist). These relations suggest that the inner CMS is a composite of South Fork Mountain schist slices that were sequentially underplated beneath the Klamath Mountains Province. The age, inboard position, and structural position (i.e., the CMS resides directly beneath Jurassic arc assemblages with no intervening mantle) of the CMS suggest that these rocks were emplaced during one or more previously unrecognized episodes of shallow-angle subduction restricted to the Klamath Mountains Province. Furthermore, emplacement of the deepest portions of the CMS corresponds with the ca. 136 Ma termination of magmatism in the Klamath Mountains Province, which we relate to the disruption of asthenospheric flow during slab shallowing. The timing of shallow-angle subduction shortly precedes that of the westward translation of the Klamath Mountains Province relative to correlative rocks in the northern Sierra Nevada Range, which suggests that subduction dynamics were responsible for relocating the Klamath Mountains Province from the arc to the forearc. In aggregate, the above relations require at least three distinct phases of extension and/or rifting, each followed by an episode of shallow-angle underthrusting. The dynamic upper-plate deformation envisioned here is best interpreted in the context of tectonic switching, whereby slab steepening and trench retreat alternate with slab shallowing due to recurrent subduction of buoyant oceanic features. 

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5. Record of the Late Jurassic Cordilleran margin from detrital zircon analysis of the Galice and Mariposa Formations, California and Oregon

   https://doi.org/10.1130/2025.2565(10)  

   Surpless, Kathleen D; Barnes, Calvin; Yoshinobu, Aaron (September 2025, Geological Society of America)

   Gordon, Stacia M; Miller, Robert B; Rusmore, Margi E; Tikoff, Basil (Ed.)

   ABSTRACT Details of the Late Jurassic tectonic evolution of the North American continental margin remain controversial, but a clear understanding of Late Jurassic tectonics is essential for understanding subsequent terrane accretion and displacement. Upper Jurassic strata of the Galice Formation in the western Klamath Mountains province and the Mariposa Formation in the Western Sierra Nevada metamorphic province were deposited along the margin of North America during this critical time. The Galice and Mariposa Formations have long been correlated, and these strata are the youngest rocks deformed during Late Jurassic Nevadan deformation in Oregon and California. Published and new detrital zircon age and εHf data sets from the Galice Formation (N = 30; n-age = 7287; n-Hf = 876) and Mariposa Formation (N = 13; n-age = 3656; n-Hf = 484) confirm previous correlations between Galice and Mariposa strata and require that abundant continentally derived zircon reached both basins with the onset of turbidite deposition ca. 159 Ma. However, subtle differences between the pre-Mesozoic age distributions and Mesozoic zircon εHf values compiled for each basin reveal nuances in provenance that can be directly related to the location of each basin relative to sediment sources within the magmatic arc and retroarc region. Mixture modeling indicates that the relative latitudinal position of the Galice and Mariposa basins with respect to their source regions can account for the differences in source contributions to each basin, and our results indicate <200 km of post-Jurassic dextral displacement within the Sierra Nevada magmatic arc. These geochemical and age-based provenance results, combined with depositional age constraints for each basin, are most consistent with models for the Late Jurassic Nevadan orogeny that call on changing plate kinematics during eastward subduction that resulted in periods of transtension and transpression along the margin, rather than westward subduction of the North American plate beneath an island archipelago or double subduction of the Mezcalero plate. 

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