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

Assessing the basement age of the Midriff Islands in Bahia de Los Angeles, Baja California

Paper number: 39-7
Program: Technical Sessions
Time: 3:00 PM – 3:15 PM

Authors

Abstract

The basement of several of the small islands located at the mouth of Bahia de Los Angeles, just southwest of the Canal de Ballenas transform, has been mapped and studied only in reconnaissance fashion. They have been proposed to be Paleozoic metasediments (Martin-Barajas et al., 2022) based on similarities with Devonian rocks on the coast at Calamajué (Campbell and Crocker, 1993), ~100 km to the northwest. We carried out preliminary characterization of the basement rocks at Isla Ventana and Isla Smith, and identified tabular sandstones with subarkose compositions, shales and cherts with a green-schist facies metamorphic overprint. The structural style is characterized by intense isoclinal folding with “fish-hook” style refolding and the presence of several generations of quartz veins in various stages of transposition (sometimes representing up to ~50% of the outcrop). We propose an alternative correlation with the Triassic Varales Unit of the Arteaga Complex on mainland Mexico, which has a similar lithological assemblage (Centeno-Garcia et al., 2025). Even though these rocks would have been ~1000 km to the southeast in a pre-Gulf of California framework, they would have occupied a similar structural position at the margin of the early Mesozoic North America.  We collected samples for detrital U-Pb zircon geochronology from both islands and will compare our results to the well-characterized Arteaga Complex detrital spectra to test the correlation and shed light on the Triassic paleogeography of the North American Cordillera.

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

doi: 10.1130/abs/2026CD-411077

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