Cenozoic tectonism in the central Basin and Range; magnitude, rate, and distribution of upper crustal strain

Abstract

A wealth of pre-Cenozoic geologic markers across the central Basin and Range province, defined mainly by thrust faults developed within a miogeoclinal wedge, makes it one of the best areas on Earth to reconstruct large-scale continental extension. We use this ~100,000 km^2 region, bounded by the southern Sierra Nevada on the west and the Colorado Plateau on the east, to illustrate the concepts of correlation and misalignment for regional strain markers and of strain compatibility and kinematic viability in palinspastic reconstructions. The results indicate ~250 to 300 km of west-northwest motion of the Sierra away from the Plateau, accommodated by both crustal thinning and north-south shortening. Tertiary intermontane basin deposits and mineral cooling ages of deeply exhumed rocks constrain the overall kinematics of motion. Most of the westward motion occurred between 16 and 5 Ma, at rates near 2 cm/yr, slowing to 1 to 1.5 cm/yr in the last 5 Ma. We have quantified the partitioning of strain between vertical crustal thinning (via normal faults) and map-view plane strain (via conjugate strike-slip faults) by placing a grid of 10 x 10 km elements on a retrodeformed map of the region and measuring the increase in area of grid elements between the undeformed and present-day grids. This analysis yields a maximum finite elongation of the Basin and Range at 36°N to 37°N of 3.4, oriented N73°W. Map-view area balance shows that 20 percent of this elongation is compensated by ~north-south shortening, and 80 percent by crustal thinning. This yields an average thinning factor for the upper crust of 2.7 between the Sierra and Plateau, inconsistent with the hypothesis that Neogene deformation in the central Basin and Range is predominantly dextral-shear plane strain. These results, in concert with the observation that the southern Sierra Nevada has similar crustal thickness to the central Basin and Range, support the hypothesis of large-scale eastward flow of Sierran deep crust during extension.