PLATE BOUNDARY PARALLEL TRANSPORT OF FORE ARC BASIN STRATA: AN EXAMPLE FROM THE CRETACEOUS OF NORTH AMERICA

In Northern California the Great Valley Group (GVG), which was deposited on the Coast Range Ophiolite (CRO) and its Late Jurassic cover, spans the entire Cretaceous period, some 80 Ma of earth history and is classically considered to have accumulated in a stationary fore arc basin . This non translational view requires that plate convergence remained essentially orthogonal for the entire Cretaceous. However, plate reconstructions suggest significant oblique convergence during this time period which would have led to margin parallel translation of the fore arc region. Using modern fore arc translation rates of 1 to 2 cm per year (Jarrad, 1986), permits up to as much as 1600 km of translation. Geologic evidence that supports a translational model for the Great Valley fore arc basin is as follows. In the Klamath Mountains of northern California the ca 160 ma Josephine ophiolite and its Late Jurassic cover were deformed and metamorphosed during the Late Jurassic Nevadan Orogeny. In this region the GVG unconformably overlies the Nevadan orogenic belt. In contrast the ca 160 Ma CRO and its Late Jurassic cover did not experience the Nevadan Orogeny and there is no angular unconformity between the CRO and the GVG. In addition, the ca 160 Ma Smartville Complex and Late Jurassic Mariposa Formation, now located due east of the CRO in the Sierran foothills, were deformed during the Nevadan Orogeny. In order to account for this regional tectonic distinction, the CRO must have been translated into its current position by a margin parallel strike-slip system which originally separated the Coast Range province from the Klamath Mountain and Sierran provinces. Our new detrital zircon data suggest that significant strike slip movement occurred in the Late Jurassic/Early Cretaceous. Hauterivian through Albian detrital zircon ages of the GVG from the northern Coast Ranges are distinct from the GVG deposited on the Klamath Mountains, whereas Cenomanian strata have significant similarities. This distinction is present in both Mesozoic and pre-Mesozoic detrital zircon populations. We suggest that the Coast Range GVG and Klamath GVG were two separate basins brought together by plate boundary parallel strike slip faulting. Cenomanian strata of the GVG appears to represent an overlap sequence tying the two basins together in the Late Cretaceous.