245 Rock Deformation and the Dynamics of Mountain Building: A Session Honoring the Scientific Contributions of John P. Platt, Part I

We welcome contributions inspired by the diverse research of John Platt in structural geology and tectonics showing how the rock record can be used to illuminate the fundamental processes of crustal deformation and mountain building.

Wednesday, 22 October 2025
8:00 AM - 12:00 PM
HBGCC, 217D

Re-evaluating plate tectonic reconstructions of the Franciscan complex, USA, and along-strike regions using ocean plate stratigraphy

Paper number: 245-7
Program: Technical Sessions
Time: 9:40 AM – 9:55 AM

Authors

Abstract

The Franciscan complex is part of the classic paleo-forearc ‘triad’ of the California Coast Ranges.  Published plate tectonic reconstructions of the Franciscan complex present conflicting views on the identity, age, and geometry of subducting plates.  Here we predict subducting oceanic plate ages along the Franciscan complex from end-member plate models using a recent algorithm (Williams et al., 2021).  We compare the subducting plate and accretionary ages of the models to ocean plate stratigraphy (OPS) ages from the Franciscan complex (Wakabayashi, 2025), which preserve a tectono-sedimentary record of the subducting oceanic plate and its interaction with the continental margin from ~160 to ~30 Ma.  We discuss comparisons to OPS records from along-strike regions including Baja California and Washington-British Columbia.   

‘Andean’-style plate models traditionally place the Franciscan complex within the continental forearc during long-lived, continuous, eastward subduction of a single oceanic plate (Farallon) under western North America since ~170 Ma (e.g., Engebretson et al., 1985; Müller et al., 2022).    In contrast, a second plate model class we call ‘double subduction’ reconstructs subduction of a now-vanished ocean basin, followed by the Farallon plate, under western North America during the Franciscan (e.g., Tikoff et al., 2023; Fuston et al., accepted).  Finally, ‘archipelago’ models consider a more complex history whereby the older (pre-123 Ma) Franciscan complex was located along an intra-oceanic subduction zone located ~1000 km south of its present latitude, transferred to the Farallon plate at 115 Ma and moved northeastwards, was progressively accreted to the Guerrero block along western North America until ~100 Ma, and then was driven northwards to its present location by the Eocene (Clennett et al., 2020).  Our analysis shows that Andean-style plate models require progressive younging of subducting plate ages along the Franciscan from the mid-Jurassic until Farallon-Pacific spreading ridge subduction at ~30 Ma; these predictions do not match well with Franciscan OPS age trends.  We discuss the likelihood that a second, ‘pre-Farallon’ subducting plate is needed during Franciscan history to explain the abrupt change in OPS age trends at ca. 95/100 Ma recorded in the Marin Headlands. California.

Geological Society of America Abstracts with Programs. Vol. 57, No. 6

doi: 10.1130/abs/2025AM-413179

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