212 Reconstructing Earth Surface Processes in Orogenic Systems

Advances and applications of detrital and stratigraphic records for reconstructing lithospheric, climatic, and earth surface interactions over geologic time scales.

Tuesday, 21 October 2025
1:30 PM - 5:30 PM
HBGCC, 303AB

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

Paper number: 212-6
Program: Technical Sessions
Time: 2:50 PM – 3:10 PM

Authors

Abstract

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 ƐHf values that span more than 30 epsilon units. This spread in ƐHf 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 ƐHf 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 ƐHf values. Eastward migration of the arc axis and sediment transport systems during middle to Late Cretaceous time resulted in a largely evolved ƐHf 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 Programs. Vol. 57, No. 6

doi: 10.1130/abs/2025AM-413096

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