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
GODFREY, NICOLA J., BRUCE B. BEAUDOIN, SIMON L. KLEMPERER
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