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

In Situ Development of Jurassic Arcs along the California Margin during Localized Extension and Convergence

Paper number: 18-9
Program: Technical Sessions
Time: 10:40 AM – 11:00 AM

Authors

Abstract

Some plate tectonic models for Jurassic arc volcanism and adjacent basins in the western Sierra Nevada describe a tectonic history dominated exclusively by regional convergence, with large-scale crustal shortening forming during exotic arc collision with the North American margin. Other models have proposed a history of transtension and transpression along the margin during the Late Triassic through Jurassic. Based on our recent research and interpretations from the literature we suggest that the Late Triassic –Jurassic rocks of the northwestern Sierra Nevada foothills underwent extension within and adjacent to volcanic arcs, both coeval with and prior to localized shortening. Both modes of deformation likely were diachronous and heterogeneous along and across strike. 

Key new observations and zircon data refute large-scale arc – continent collision in this region. We have documented a depositional contact locally exists between Jurassic arc volcanic and volcaniclastic rocks and deformed chert – argillite - serpentinite basement. Poorly sorted and immature clastic rocks containing basement clasts as well as volcanic lithic fragments and crystals are locally preserved along the contact suggesting a topographically irregular basement existed during arc volcanism. This boundary was previously interpreted as a major thrust coinciding with collision of an exotic island arc with the North American margin. The eastern margin of the Jurassic foothills arc contains coarse to fine clastic rock with abundant coarse olistoliths of basement and volcanic arc rocks. The intensity of the low-grade fabric in the olistolith-rich unit varies greatly along and across strike. We suggest these observations are more compatible with a transtensional or extensional tectonic setting overprinted locally by shortening in the upper plate of an east-dipping subduction rather than a proposed west-dipping subduction zone that evolved into a collisional plate boundary in the Late Jurassic. The tectonic setting must have been near the current margin of North America based on the abundance and ages of Precambrian and Paleozoic zircon in Late Triassic coarse pebble conglomerate and Late Jurassic volcaniclastic rocks. Our interpretation of the Jurassic margin development combining intermittent Jurassic extension with convergence has similarities to tectonic models proposed by several researchers for the Klamath Mountains and appears compatible with some interpretations of the Blue Mountains province. This presentation will explore the northern continuation of this type of plate boundary into Canada.

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

doi: 10.1130/abs/2026CD-410839

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