OPHIOLITIC BASEMENT TO THE GREAT VALLEY FORE-ARC BASIN, CALIFORNIA, FROM SEISMIC AND GRAVITY-DATA - IMPLICATIONS FOR CRUSTAL GROWTH AT THE NORTH-AMERICAN CONTINENTAL-MARGIN

The nature of the Great Valley basement, whether oceanic or continenta l, has long been a source of controversy, A velocity model (derived fr om a 200-km-long east-west reflection-refraction profile collected sou th of the Mendocino triple junction, northern California, in 1993), fu rther constrained by density and magnetic models, reveals an ophiolite underlying the Great Valley (Great Valley ophiolite), which in turn i s underlain by a westward extension of lower-density continental crust (Sierran affinity material), We used an integrated modeling philosoph y, first modeling the seismic-refraction data to obtain a final veloci ty model, and then modeling the long-wavelength features of the gravit y data to obtain a final density model that is constrained in the uppe r crust by our velocity model, The crustal section of Great Valley oph iolite is 7-8 km thick, and the GI eat Valley ophiolite relict oceanic Moho is at 11-16 km depth, The Great Valley ophiolite does not extend west beneath the Coast Ranges, but only as far as the western mel-gin of the Great Valley, where the 5-7-km-thick Great Valley ophiolite ma ntle section dips west into the present-day mantle. There are 16-18 hn of lower-density Sierran affinity material beneath the Great Valley o phiolite mantle section, such that a second, deeper, ''present-day'' c ontinental Moho is at about 34 km depth, At mid-crustal depths, the bo undary between the eastern extent of the Great Valley ophiolite and th e western extent of Sierran affinity material is a near-vertical veloc ity and density discontinuity about SO km cast of the western margin o f the Great Valley. Our model has important implications for crustal g rowth at the North American continental margin, We suggest I-hat a thi ck ophiolite sequence was obducted onto continental material, probably during the Jurassic Nevadan orogeny, so that the Great Valley basemen t is oceanic crust above oceanic mantle vertically stacked above conti nental crust and continental mantle.