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Abstract: Ophiolitic Basement to the Great Valley Forearc basin, California, from Seismic and Gravity Data: Implications for Crustal Growth at the North American Continental Margin


A velocity model (from a 200-km-long east-west reflection/refraction profile collected in northern California in 1993), further constrained by density and magnetic models, reveals an ophiolite (Great Valley Ophiolite, (GVO)) underlying the Great Valley, which in turn is underlain by a westward extension of lower-density continental crust - Sierran affinity material (SAM). Our final model, achieved by using an integrated modeling approach; first modeling the seismic-refraction data to obtain a velocity model, and then modeling the long-wavelength features of the gravity data to obtain a density model that is constrained in the upper-crust by our velocity model, reveals the crustal section of GVO (7-8 km thick), the GVO relict oceanic Moho (11-16 km depth) and 5-7 km-thickness of GVO mantle, which dips west into the present-day mantle. The GVO does not extend west beneath the Coast Ranges, instead it extends only as far as the western margin of the Great Valley. There is 16-18 km thickness of lower-density material beneath the GVO mantle which we interpret as SAM, with a second, deeper, 'present-day' continental Moho at about 34 km depth. At midcrustal depths, the boundary between the eastern extent of the GVO and the western extent of SAM is a near-vertical velocity and density discontinuity about 80 km east of the western margin of the Great Valley. Our model has important implications for crustal growth at the North American continental margin, since we suggest that a thick ophiolite sequence was obducted onto continental material, probably during the Jurassic Nevadan orogeny, so that the Great Valley basement is oceanic crust above oceanic mantle vertically stacked above continental crust and continental mantle.

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