27-12 The thermochronologic record of the transition from Late Cretaceous shallow subduction to an Oligocene transform margin in the San Gabriel Mountains of southern California

Session: Evolution of Cordilleran-type orogenic systems

Presenting Author:

Karissa Vermillion

Authors:

Vermillion, Karissa Brianne¹, Grove, Marty², Jacobson, Carl E.³, Jepson, Gilby ⁴, Murphy, Michael A.⁵, Peplinski, Ryan ⁶, Copeland, Peter C.⁷

(1) Department of Earth and Atmospheric Sciences, University of Houston, Houston, TX, USA, (2) Department of Earth and Planetary Sciences, Stanford University, Palo Alto, CA, , (3) Department of Earth and Space Sciences, West Chester University, West Chester, PA, USA; Department of the Earth, Atmosphere, and Climate, Iowa State University, Ames, Iowa, USA, (4) Department of Earth, Marine, and Environmental Sciences, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina, USA, (5) Department of Earth and Atmospheric Sciences, University of Houston, La Canada Flintridge, CA, , (6) Department of Geological Sciences, California Polytechnic University Pomona, Pomona, California, USA, (7) Department of Earth and Atmospheric Sciences, University of Houston, Houston, TX, USA,

Abstract:

The Late Cretaceous to present-day lithospheric evolution of the southwestern North American cordillera involved a transition from NE-directed, shallow subduction of Farallon oceanic crust to the present-day NW-oriented, dextral San Andreas fault (SAF) transform margin. In southern California, this record is exceptionally well preserved by rocks surrounding the Sierra Pelona anticlinorium in the western San Gabriel Mountains (SGM). There, the trench-derived, Late Cretaceous Pelona Schist was accreted to the base of an upper-plate assemblage of Proterozoic craton, Triassic–Cretaceous arc rocks, and Late Cretaceous–Paleogene forearc strata. The sequence is unconformably overlain by Oligocene–Miocene rift deposits formed during initial development of the SAF transform. Here we present thermal histories of SGM basement constrained by ⁴⁰Ar/³⁹Ar results from muscovite, biotite, and K-feldspar together with apatite fission-track analysis. Data was acquired from both basement exposures and derivative Oligocene–Miocene alluvial cobbles. Regional cooling (70–60 Ma) of SGM basement through the muscovite and biotite Ar partial retention zones coincided with the inferred time of underthrusting and accretion of the Pelona Schist (< 70 Ma) based upon equivalent ⁴⁰Ar/³⁹Ar closure and detrital zircon U-Pb age data from the schist. Thermal histories of SGM crust were constrained from K-feldspar using the multiple diffusion domain (MDD) method and apatite fission-track confined track length distributions. Generally, MDD models support Late Cretaceous–Early Cenozoic cooling followed by upper crustal residency (~225–125°C) prior to a second phase of rapid cooling that we ascribe to extension-related exhumation recorded by Vaquez Fm. cobbles. The denudation history of SGM basement differed along opposing flanks of the Sierra Pelona lower-plate exposure. North of Sierra Pelona, SGM basement cooled earlier and experienced prolonged residence in the mid-to-upper crust at ~225°C from 70–35 Ma and are overlain by Maastrichtian–Paleocene marine strata. SGM basement south of Sierra Pelona represent deeper crustal levels. Initial denudation of these rocks from Late Cretaceous depths occurred between 60–40 Ma depending upon proximity to faults bounding the Sierra Pelona antiformal lower-plate exposure. Similar relationships are exhibited by correlative rocks in the Orocopia Mountains east of the SAF. Both K-feldspar and AFT thermal models from cobbles and basement indicate crustal residence at 200–150°C until the onset of Late Oligocene rifting and exhumation of SGM basement as recorded by clasts within the Vasquez Fm.

Geological Society of America Abstracts with Programs. Vol. 58, No. 3, 2026

© Copyright 2026 The Geological Society of America (GSA), all rights reserved.