18 Crossing Borders: Large-scale tectonic models of the North American Cordillera

This session focusses on the Phanerozoic evolution of the North American Cordillera, soliciting studies that address large-scale tectonic models associated with obliquely convergent and divergent movement along the margin.

Thursday, 23 April 2026
8:00 AM - 12:00 PM
LMH, Isla Carmen

Cretaceous Kinematic Evolution of Southern California and the Baja-BC Problem

Paper number: 18-5
Program: Technical Sessions
Time: 9:25 AM – 9:45 AM

Authors

Abstract

Paleomagnetic data from the Insular and related terranes in the western Canadian and northern U.S. Cordillera indicate large-magnitude (~4000 km) northward translations along or adjacent (offshore) to western North America in the Late Cretaceous. In some widely debated ‘Baja-BC’ models, the Insular terranes are hypothesized to have collided with southern California and northern Baja California before being translated northward along dextral faults from 85–55 Ma. However, a major unresolved problem with Baja-BC controversy is the absence of recognized dextral faults in southern California capable of accommodating such large-magnitude northward translations.

 

Here we synthesize new data from all known Mesozoic shear zones and faults within the southern California region – specifically, the Sawmill, Tumamait, Alamo Mountain–Piru Creek, Black Belt, and Eastern Peninsular Ranges shear zones – along with previously published data from the Cuyamaca–Laguna Mountain shear zone. These shear zones represent a system of intra-arc ductile shear zones and thrust faults active during the middle to Late Cretaceous. Field observations, microstructures, electron backscatter diffraction–derived crystallographic vorticity axis analysis, and titanite petrochronology reveal two distinct deformation phases: an early phase (120–90 Ma) of mostly arc-normal shortening with a minor dextral component, and a later phase (90–70 Ma) of widespread sinistral transpressional deformation accompanied by high-flux arc magmatism. We find no evidence for large-magnitude, dextral shear zones that could have translated Insular terranes northward. Instead, our findings support “offshore” models in which the Insular terranes were translated northward along structures farther west with little to no interaction with the southern California margin.

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

doi: 10.1130/abs/2026CD-410998

© 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.