35 The Mesozoic Convergent Margin of Alta and Baja California (Posters)

This session will cover aspects including the structure, petrology, geochronology, and stratigraphy of the Mesozoic convergent margin of North America from Alta California to Baja California Sur.

Friday, 24 April 2026
8:00 AM - 5:30 PM
LMH, 5th Floor Chapel

Late Cretaceous–Eocene Cooling and Exhumation of the Peninsular Ranges Batholith (S. CA) Constrained by Apatite U–Pb and (U–Th)/He Analyses

Paper number: 35-1
Program: Technical Sessions
Time: 8:00 AM – 8:00 AM

Authors

Abstract

The Peninsular Ranges Batholith (PRB) is a segment of the continental magmatic arc that formed along the western margin of North America during the Cretaceous and now extends from southern California to Baja California. Previous low-temperature thermochronology documents Late Cretaceous–Paleogene cooling near Mount San Jacinto and rapid Miocene-Pliocene exhumation along the southern segment of the late Cenozoic San Jacinto fault. A lack of low-temperature thermochronology data along the late Cenozoic Elsinore fault and central San Jacinto fault has limited our ability to determine transform margin versus older subduction margin processes in driving exhumation of the PRB. To address this uncertainty, we targeted samples along the traces of these faults to evaluate the spatial and temporal trends in pluton crystallization and exhumation.

We present new LA-ICP-MS zircon and apatite U–Pb geochronology and apatite (U–Th)/He (He) thermochronology to constrain the timing of PRB pluton emplacement, cooling, and exhumation. Zircon and apatite U–Pb ages indicate mid- to Late Cretaceous pluton emplacement and cooling within the upper crust. Apatite He dates from eight samples range from ca. 76–7 Ma, with most falling between ca. 76–40 Ma, recording Late Cretaceous to mid-Eocene cooling. Samples from Coyote Canyon yield younger cooling ages ranging from ca. 43–7 Ma, consistent with previously reported thermochronologic results along the southern San Jacinto fault. Although modern regional topography is characterized by strike-slip fault-bounded ranges, the thermochronologic data do not support significant exhumation driven by Miocene to recent fault activity. Instead, we interpret these ages to reflect cooling and exhumation related to the transition from normal-angle subduction to Laramide shallow slab subduction.

The new U-Pb and apatite He data, coupled with published thermochronology data, help reconcile the evolution of the PRB, which further refines our understanding of the tectonic processes that characterize the region. Future work will involve additional apatite and zircon U–Pb analyses and thermal history modeling to better constrain the rates and magnitude of exhumation.

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

doi: 10.1130/abs/2026CD-410857

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