DETRITAL ZIRCON GEOCHRONOLOGY OF THE GREAT VALLEY GROUP, NORTHERN CALIFORNIA: IMPLICATIONS FOR THE KLAMATH MOUNTAIN/COAST RANGE BOUNDARY

The boundary between the Klamath Mountain and Coast Range provinces has been a subject of debate for over three decades. One of the remarkable features of the boundary is the contrast in the Geological histories of the two areas as exemplified by the Great Valley Group (GVG). The GVG in the southern Klamath Mountains was deposited unconformably on the Nevadan orogenic belt and post-dates Nevadan orogenesis. The GVG in the Coast Ranges, in contrast, has a very different basement and orogenic history . There the ca 165 Ma Coast Range ophiolite (CRO) is depositionally overlain by Late Jurassic volcanic-pelagic rocks, which in turn are depositionally overlain by Tithonian and younger strata of the GVG . This conformable stratigraphic succession was accumulating during the same time frame as Nevadan deformation and metamorphism in the Klamath-Sierran provinces, and it is unaffected by any Late Jurassic deformation . There is an abrupt termination of Nevadan deformation and metamorphism between the Klamath Mountain and Coast Range provinces, as well as the western foothills terrane of the Sierra Nevada and the Coast Ranges. This geometric arrangement of regions affected versus not affected by a major orogenic event is very difficult to explain in terms of any actualistic models for plate boundary tectonism (i.e., how can a forearc escape major shortening that affects the adjacent arc?). One possible explanation is that the Klamath Mountains and Coast Range boundary was originally a plate boundary parallel strike-slip system that has juxtaposed two crustal columns with distinct geologic histories. Thus, there may actually be two GVG basins; one that accumulated on Klamath basement (KGVG) and one that accumulated on Coast Range basement (CGVG). In order to test this hypothesis we have carried out U-Pb detrital zircon geochronology from the KGVG and the CGVG. Our data indicate that they have distinct provenance until the Cenomanian, in support of the two basin and plate boundary strike-slip model.