RECONSTRUCTING THE AGES OF OCEAN BASINS ALONG THE ANCIENT CALIFORNIA AND NEVADA MARGIN

In reconstructions of the history of western North America, a major question remains inadequately answered: how old was the ocean crust of the eastern Pacific and its marginal basins during the major tectonic events of the Paleozoic and Mesozoic? Around the Pacific rim, the age of the ocean crust strongly influences the style of modern tectonic processes at plate boundaries. It probably did so in the past, as well. The large USGS radiolarian collection, whose development was shepherded by D.L. Jones, provides enough data for northern California and Nevada to begin to answer the question. For example, the ocean crust along the Middle and early Late Jurassic margin of California was probably very young. Murchey and Blake interpreted “anomalous” eastward-younging directions of Jurassic ocean crustal fragments in the Franciscan Complex as evidence for the Californian arrival of a spreading ridge during the Middle to Late Jurassic. It was temporally related to the formation of Middle Jurassic ophiolites and the transpressive Nevadan orogeny. In this model, the trailing edge of a Panthalassan plate was subducted, the subduction zoned stepped westward, and the leading edge of the Farallon plate began to be subducted. Prior to the Nevadan orogeny, continous sequences of Permian to Late Triassic radiolarites accumulated in basins west of the Eastern Klamath-Sierran island arcs. These strata, as well as Early Jurassic radiolarites and limestone-capped Permian seamounts, were preserved in Western Klamath Mountains terranes. The continuous sequences indicate that ocean basins at least 60 million years old lay along the Pacific side of the island arcs during the Late Triassic and (or) Early Jurassic. During the Permian, on the other hand, there is scant evidence at these latitudes for old ocean crustal material on the oceanward side of the arcs. In contrast, a great deal of evidence exists for a very old marginal basin, the Havallah-Schoonover Basin of Nevada, between the Permian volcanic island arcs and the North American margin. The basin, locally as old as latest Devonian and Early Mississippian and containing continuous depositional sequences spanning 80 to 100 million years, was closing by the Early Triassic, with vergence to the east. This event was the Sonoma orogeny.