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

Southern Origin of the Franciscan Complex and Implications for Baja BC

Paper number: 18-3
Program: Technical Sessions
Time: 8:45 AM – 9:05 AM

Authors

Abstract

The “Baja British Columbia” hypothesis suggests that a large micro-continent, called “Insular”, was located at Californian latitudes at 90 Ma, before being translated thousands of kilometres northwards during the late Cretaceous. However, reconciling this model with the geological record of the Franciscan Complex has been a longstanding challenge. The record of subduction accretion in the Franciscan during the Cretaceous, combined with an absence of evidence for a collisional event, poses a problem to reconstructions where Insular is located outboard of the Franciscan. We identify four possible logical solutions to this situation: (a) Insular is located outboard of the Franciscan Complex and disrupts subduction accretion, (b) Insular is still offshore and accretes later, (c) Insular is located further north than southern California, and (d) the Franciscan Complex was located further south than southern California. We explore the fourth possibility where the Franciscan Complex was also located hundreds of km further south and subsequently translated northwards – similar to Baja BC. We revisit paleomagnetic constraints from the Calera and Laytonville Limestones, showing that a southern location for the Franciscan terrane predicts Farallon plate velocities that are slower and more consistent with plate reconstructions than previous studies. This model is analogous to the modern tectonic setting of Sumatra, where oblique subduction results in northward translation of the accretionary complex along inboard strike-slip faults. In this region, Himalayan sediments are transported along the trench for thousands of kilometres, indicating that a southern origin of the Franciscan Complex can still be consistent with detrital zircon constraints. This scenario reconciles the paleomagnetic data from British Columbia without predicting a Cretaceous collision in the Franciscan Complex, and should be investigated in further detail.

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

doi: 10.1130/abs/2026CD-410847

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