39 The Mesozoic Convergent Margin of Alta and Baja California

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
1:30 PM - 5:30 PM
LMH, Deli

Metamorphism, Sedimentary Recycling, and Exhumation of High-Pressure Rocks in the Cedros Subduction Complex, Cedros Island, Baja California, Mexico

Paper number: 39-9
Program: Technical Sessions
Time: 3:30 PM – 3:45 PM

Authors

Abstract

High-pressure rocks from subduction complexes are key records of the physical and chemical processes that operate on the subduction interface, but interpretation of these records requires accurate structural understanding of where they formed in the subduction zone and the mechanisms by which they were exhumed. We present new geologic mapping, outcrop-scale observations, and geochronology from subduction-zone assemblages at Punta Prieta Ridge on Cedros Island, Baja California (Mexico), to investigate the history of subduction, exhumation, and structural assembly of these rocks. The rocks of Punta Prieta Ridge are exposed in the footwalls of high- and low-angle normal faults that carry Cretaceous forearc basin strata and attenuated mantle sections of the Cedros Island Ophiolite in their hangingwalls. The footwall rocks are subduction-zone assemblages organized into distinct nappes that decrease in metamorphic grade and degree of strain structurally downward. Garnet-amphibolite and blueschist blocks within the subduction complex yield 40Ar/39Ar cooling ages between 172 and 144 Ma and are hosted in siliciclastic rocks which yield detrital zircon maximum depositional ages between 92 and 72 Ma. Zircon (U-Th)/He cooling ages of blocks and metasedimentary matrix collected through different structural levels of the complex indicate multiple events of exhumation to shallow crustal levels between 93 and 54 Ma, which we attribute to changes in wedge dynamics during subduction. Single grain Apatite (U-Th)/He cooling ages range from 37 to 4 Ma and define first quartile ages between 17 and 5 Ma and mean ages between 20 and 17 Ma, which we interpret to date exhumation along Neogene high-angle normal faults that define the topography on the island and were activated after subduction had ceased.

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

doi: 10.1130/abs/2026CD-410955

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