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

Accretion of Mesozoic Blueschist-Facies Assemblages on Cedros, San Benitos, and Santa Margarita Islands, West-Central Baja Mexico

Paper number: 35-3
Program: Technical Sessions
Time: 8:00 AM – 8:00 AM

Authors

Abstract

Accretionary assemblages of the Western North American Cordillera are rare south of the U.S.-Mexico border, complicating the reconstruction of margin events in the Mesozoic. Off the present-day coast of west-central and southern Baja Mexico, blueschist-facies metasediments and metabasites crop out on Cedros Island, San Benitos Islands, and Santa Margarita Island and are referred to in aggregate as the Western Baja terrane (WBt). On Cedros Island, the WBt underlies the Middle Jurassic Cedros Island ophiolite and Early to Late Cretaceous Valle Group forearc assemblages. The WBt is composed of fault-bounded, semi-coherent subterranes that each achieved different degree of deformation and metamorphic pressure-temperature paths. Detrital zircon maximum depositional ages (MDAs) from metaclastic rocks of two structurally higher, ocean floor-dominated WBt subterranes range from 91-100 Ma and 95-101 Ma. The structurally lowest, and terrigenous sediment-dominated, subterrane exposed has the youngest (83-92 Ma) MDAs, suggesting it underplated the previously subducted subterranes. The change in dominant lithologies of the subterranes is consistent with accretion recording the progression of sediment source from primarily ocean floor volcanics and sediments to a large delivery of continent-, and mostly arc-, derived sediment load to the trench. All subterranes have a prominent peak from 96-105 Ma, coincident with a period of shortening and flare-up magmatism in the La Posta suite of the Peninsular Ranges Batholith. The short time frame from maximum depositional ages to maximum population peaks suggests a high standing and rapidly eroding arc. The onset of accretion ca. 101 Ma in the WBt coincides with both an increase in sediment load being delivered to the trench and a noted increase in contraction across the active arc margin.

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

doi: 10.1130/abs/2026CD-410914

© Copyright 2026 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.