Geophysical and geological characterization of Cenozoic tectonism and magmatism across the northwestern margin of the Basin and Range province.

摘要

The tectonically-active eastern and western boundaries of the northern Basin and Range have been studied extensively, yet comparatively little is known about this province's more diffuse northwestern margin that marks the transition from recent (12 Ma to present) low-magnitude (≤20%) extension in northwestern Nevada to relatively unextended volcanic plateaus in northwestern-most Nevada, northeastern California, and southern Oregon. This work advances our understanding of this region's Cenozoic tectonic evolution in two ways: (1) by constraining its extensional and magmatic history through geologic mapping and geochronology, and (2) by delimiting the crustal structure and composition through active-source seismology and potential-field data. Geologic mapping and geochronology from the Black Rock Range in northwestern Nevada document volcanism spanning 35-16 Ma. This Cenozoic section is broadly conformable and dips gently (∼5°-10°) to the northwest indicating the range experienced no significant tilting, and presumably no significant extension, between 35 and 16 Ma. Moderate tilting and concomitant uplift occurred after 16 Ma, consistent with the tectonic histories of nearby ranges. Approximately 90 km west of the Black Rock Range, our reflection profiling images the 35° present-day dip of the Surprise Valley Fault that marks the westernmost boundary of the Basin and Range and, together with geological data, constrains its total slip (≥7 km) and time-averaged slip rate (1-2 mm/yr). Regarding regional crustal structure, we use seismic-velocity and potential-field modeling to document a ∼20% variation in crustal thickness, from ∼37 km under northeastern California to ∼31 km under northwestern Nevada. This degree of thinning is consistent with the amount of extension recorded in the upper crust in northwestern Nevada, suggesting the crustal response to extension was relatively homogeneous over the entire crustal column. Our modeling also images an 80 km-wide zone of unusually low upper-crustal velocities (∼5.9-6.1 km/s) that coincides with the surface location of Cretaceous granites, apparently locating the elusive northern extension of the Sierra Nevada batholith through northwestern Nevada for the first time in the subsurface. Combining these geological and geophysical data, we reconstruct the late Cretaceous to present crustal evolution of this region, documenting an interplay between magmatic addition to the crust, erosional exhumation, sedimentation, and extension that has reversed the direction of crustal thinning from a west-facing continental margin to an east-facing interior basin margin over this time interval.