255 Broad Applications of Thermochronology to Understanding Geologic Rates and Processes Through the Sedimentary Record

This session showcases the many ways mid-to-low temperature thermochronometers can be used in isolation or with additional chronometers to understand geologic processes preserved in the sedimentary record such as sediment provenance, source exhumation, burial and/or erosion history.

Wednesday, 22 October 2025
8:00 AM - 12:00 PM
HBGCC, 304C

Basement and detrital thermochronology, western San Gabriel Mountains, southern California: evidence for the missing crustal section

Paper number: 255-7
Program: Technical Sessions
Time: 9:55 AM – 10:10 AM

Authors

Abstract

The southern California margin records the transition from Late Cretaceous flat slab subduction to Neogene transform faulting along the San Andreas fault system (SAFS). Initial 24-18 Ma extension along the expanding SASF transform produced half-graben sedimentation, volcanism, and basement uplifts. Evidence of this extension is well preserved in the western San Gabriel Mountains, where the Oligocene-Miocene Vasquez Formation contains 5 km of local basement-derived sediment and intercalated volcanics. The magnitude and timing of cooling driven by extension is critical in elucidating the thermal history of the region prior to the initiation of the SAFS. New 40Ar/39Ar (muscovite, biotite, k-spar) thermochronology from arc basement that structurally overlies subducted and accreted Pelona Schist and apatite fission-track thermochronology from Vasquez Fm. clasts are reported from the western San Gabriel Mountains. This consolidated data set permits comparison of basement and basin thermal histories to better understand the evolution of the SAFS in this region.

40Ar/39Ar (K-spar, biotite, muscovite) and apatite-fission-track thermochronology results from the Sierra Pelona region indicate generally slow cooling from the Late Cretaceous to ~24 Ma, consistent with a combination of subduction refrigeration and exhumation caused by flab slab subduction. Western San Gabriel basement K-feldspar 40Ar/39Ar thermal history results indicate regional < 5°C/m.y. cooling from 250-150°C between 60-25 Ma. Apatite fission-track data from Vasquez Fm. clasts record both this earlier slow cooling phase and more rapid subsequent (>10°C/m.y.) cooling from ca. 140-60 °C during the Oligocene.  Preservation of both cooling signals reflects progressive Oligocene erosion of the basement during deposition of the Vasquez Fm.

Combined basement and basin thermal histories require considerable (5-10 km) basement denudation prior to deposition of the Vasquez Fm. In the Miocene, thermal history models require subsequent heating up to 80°C which we suggest is due to the deposition of thick (ca. 3 km) submarine fans during rapid subsidence. These marine strata were largely eroded during 10-6 Ma rapid uplift of San Gabriel basement due to the onset of Late Miocene-Pliocene transpression that inverted the Vasquez Basin. Based on our thermal history models we suggest at least 2 km of basement and 3 km of marine sediments were removed since ~25 Ma.

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

doi: 10.1130/abs/2025AM-414064

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