Abstract: Initiation of the Great Valley Forearc Basin: Constraints from Basin Analysis and Sandstone Petrofacies

INGERSOLL, RAYMOND V.

Three competing models predict very different Late Jurassic paleogeography and paleotectonics in northern California. Model 1: arc-arc collision during the Sierran phase of the Nevadan orogeny, followed quickly by and overlapping with intraarc spreading to form the Josephine ophiolite, and followed by the Klamath phase of the Nevadan orogeny as the Franciscan Trench initiated in the former backarc basin underlain by the Great Valley ophiolite. Model 2: continuous subduction of oceanic crust formed by mid-ocean spreading, with the Great Valley forearc basin initiating by westward jumping of the subduction zone. Model 3: initiation of the Great Valley forearc following transtensional rifting of a preexisting forearc.

Two types of analysis of the Jurassic part of the Great Valley Group can be used to test these models: basin history and architecture, and sandstone petrofacies. Model 1 predicts that basal GVG overlies backarc ophiolite, remnant-arc volcanics and accreted intraoceanic-arc basement. Model 2 predicts that basal GVG overlies a pre-Nevadan subduction complex and mid-ocean ophiolite. Model 3 predicts that basal GVG overlies rifted pre-Nevadan forearc basement, including newly formed forearc crust.

Model 1 predicts that sandstone should have a mixture of recycled orogenic and undissected-magmatic-arc provenance. Models 2 and 3 predict that sandstone should have only dissected- and undissected magmatic-arc provenance.

Model 1 is most useful in explaining all known characteristics of the Great Valley forearc basin.

Search and Discovery Article #90945©1997 AAPG Pacific Section Meeting, Bakersfield, California