212-5 Detrital zircon Lu-Hf results from the Mariposa Formation and Great Valley Group, California, document changing upper-plate conditions during Jurassic-Cretaceous subduction

Session: Reconstructing Earth Surface Processes in Orogenic Systems

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Forearc basin strata preserve a record of plate convergence and related arc magmatism that may help resolve the complex Jurassic-Cretaceous tectonics of California. U-Pb age and Lu-Hf isotopic analysis of detrital zircon from the Late Jurassic Mariposa Formation and the latest Jurassic-Cretaceous Great Valley Group (GVG) can inform three proposed tectonic models for western North America: (1) east-directed subduction and a single continental arc; (2) double-sided subduction, resulting in a continental arc and an offshore island arc; and (3) west-directed subduction of the North American plate beneath an offshore island archipelago. The Mariposa Formation in the Western Sierra Nevada Metamorphic Belt (WSNMB) was deposited 159-148 Ma, with lowermost Mariposa shales interfingering with underlying Late Jurassic volcanic rocks. The Mariposa Formation was folded and metamorphosed during Late Jurassic Nevadan deformation ca. 157-145 Ma.  Nevadan orogenesis was followed by 148-135 Ma unconformable deposition of basal GVG strata on ophiolitic basement rocks west of the WSNMB. Both the Mariposa Formation and GVG contain abundant pre-Mesozoic detrital zircon that link these strata to North America throughout deposition. In addition, both contain abundant Mesozoic arc-derived detrital zircon. Middle to Late Jurassic (ca. 175-155 Ma) detrital zircon from the Mariposa Formation have Ɛ_Hf values that span more than 30 epsilon units. This spread in Ɛ_Hf is consistent with similar ranges documented in metavolcanic rocks of the WSNMB, and indicates that the 175-155 Ma arc incorporated evolved, continental material.  In contrast, detrital zircon from the basal GVG is characterized by abundant ca. 155-135 Ma detrital zircon with juvenile Ɛ_Hf values between +16 and +8.  These results suggest that 155-135 Ma arc magmatism occurred in a (trans)tensional setting during and following Nevadan orogenesis and a westward shift of the arc axis, resulting in the pull up of Ɛ_Hf values. Eastward migration of the arc axis and sediment transport systems during middle to Late Cretaceous time resulted in a largely evolved Ɛ_Hf signature for 110-90 Ma detrital zircon in younger GVG strata. These results are consistent with east-directed subduction and a single continental arc throughout Jurassic-Cretaceous time, and suggest that upper plate conditions within long-lived subduction systems may be highly variable in space and time.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-8233

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