14 Lost Oceans, Shifting Continents: Interdisciplinary Records of Paleo-Pacific Plate Histories in the American Cordilleras

This session integrates tectonics, petrology, geochronology, stratigraphy, and paleontology to illuminate how Paleo-Pacific plates drove subduction dynamics, crustal evolution, and paleogeographic reconstructions across the complex geological architecture of the American Cordilleras.

Wednesday, 22 April 2026
1:30 PM - 4:00 PM
LMH, Meeting Room

Asymmetric spreading formed a Shatsky-Ojin-Hess conjugate (SOHCO) oceanic plateaux that subducted under North America

Paper number: 14-0
Program: Technical Sessions
Time: 1:30 PM – 1:50 PM

Authors

Abstract

The subduction history along the North American Cordillera during Laramide times is debated and hypothesized to involve subduction of conjugate plateaux related to the Shatsky-Hess LIPs in the central Pacific.  Here we review updated constraints on the Shatsky-Hess LIPs.  We present an updated plate tectonic reconstruction of the Shatsky-Hess LIPs and their conjugates on the Farallon plate, which we call SOHCO (Shatsky-Ojin-Hess conjugates).  We use an optimization routine to produce new poles for asymmetric Farallon-Pacific seafloor spreading during Shatsky-Hess LIP formation; asymmetric spreading has not been considered but is likely due to plume-ridge interactions.  Our SOHCO model implies a ~4300 km-long system of amalgamated conjugate plateaux and seamount chains that is 80% longer than previously modeled.  The SOHCO LIPs subducted continuously under western North America over a protracted 30 Myr period between ~90 Ma to 60 Ma, which may help to explain the extended duration of the long-lived Laramide orogeny from Late Cretaceous to Paleogene. 

We quantitatively predict spatiotemporal SOHCO subduction histories under the Sierra Nevada, Southern California, and Peninsular Ranges batholith belts of California and northern Mexico for testing in future studies.   We particularly highlight a predicted ~2200 km-long conjugate Ojin seamount chain that subducted under the southern California Batholith between 80 to 65 Ma during arc magmatic flareup followed by rapid cooling (Schwartz et al., 2023).  Possible flat-slab subduction would carry the conjugate Ojin seamounts inboard under Arizona during peak porphyry copper genesis 73-56 Ma.  A potentially testable feature of the reconstructed conjugate Ojin seamounts is their southward sweeping subduction from southern California to northernmost Mexico between ~83 Ma to 65 Ma.

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

doi: 10.1130/abs/2026CD-410832

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