Sierra Nevada-Basin and Range transition near Reno, Nevada: Two-stage development at 12 and 3 Ma

Relative and absolute elevations of the Sierra Nevada and adjacent Basin an d Range province, timing of their differentiation, and location, amount, an d timing of strike-slip movement between them are controversial. The provin cial boundary near Reno developed in two stages. (1) At ca. 12 Ma, the grea ter than or equal to 700 km(2) Verdi-Boca sedimentary basin formed across w hat was to become the boundary, probably as a result of a small-magnitude b ut regional extensional episode that affected much of the western Basin and Range. (2) At 3 Ma, the basin was complexly faulted and folded during a la rger magnitude extensional episode that established the modern Sierran stru ctural and topographic boundary in this area. The boundary is really a tran sition zone with a western edge along the Donner Pass, California, fault zo ne, which is farther west than previously placed. Both episodes appear to h ave resulted from east-west extension only, which suggests that northwest m otion of the Sierra Nevada relative to the Basin and Range shown by geodeti c data began after 3 Ma or was taken up farther east.