CENOZOIC TECTONISM IN THE CENTRAL BASIN AND RANGE - MOTION OF THE SIERRAN GREAT VALLEY BLOCK

According to geologic reconstructions, the motion of the Sierran-Great Valley block with respect to the Colorado Plateau was mainly westerly at more than 20 mm/yr from 16 to 10 Ma, changing to northwest or NNW since 8 to 10 Ma, at an average rate of 15 mm/yr. These kinematics are consistent with two other independent methods of determining the posi tion of the block since 20 Ma-reconstructions based on paleomagnetic d ata from range blocks that bound the Basin and Range on the west, and a revised history of Pacific-North America plate motion based on a glo bal plate circuit (Atwater and Stock, 1998, this issue). The plate-tec tonic reconstruction shows a change to more northerly motion between t he Pacific and North American plates at similar to 8 Ma, in concert wi th the motion of the Sierran-Great Valley block. Moreover, the northea st limit of extant oceanic crust (as indicated by the reconstruction o f the continental geology) tracks closely with the southwest limit of extant continental crust (as indicated by the positions of oceanic pla tes) since 20 Ma. The coordination between plate motions and the intra plate geology suggests that plate-boundary forces strongly influenced deformation within the continent.