DID THE GREAT VALLEY GROUP ACCUMULATE IN A TRANSLATIONAL RATHER THAN STATIONARY FORE ARC BASIN?

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. The Klamath Mountains and Coast Ranges are currently separated by a complex zone of Cretaceous normal faults. 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. Thus the tectonic evolution of the Coast Ranges is distinct from the nearby Klamath and Sierran foothill provinces. 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. Detrital zircon studies of the GVG (DeGraaff-Surpless and others, 2002), although interpreted in a fixed basin context, can be interpreted to indicate that some of this translation occurred during deposition of the GVG and that the GVG deposited unconformably on the Nevadan orogenic belt was not deposited in the same basin as the GVG deposited on the CRO untill arrival of the CRO-GVG fore arc sliver at about 90 Ma.