LATE MIOCENE DEVELOPMENT OF THE NORTHWESTERN MARGIN OF THE BASIN AND RANGE PROVINCE IN NORTHWESTERN NEVADA AND NORTHEASTERN CALIFORNIA

New geologic, geophysical, and geochronologic data demonstrate that the northwestern boundary of the modern Basin and Range Province formed during the late Miocene in a region of previously unextended crust. Across the ~220-km-wide region between the Santa Rosa and Warner Ranges, conformable sequences of 35–15 Ma volcanic rocks are cut by only a single generation of high-angle normal faults that accommodated ~23 km of total east-west extension. Fission-track, (U-Th)/He, geologic, and structural data from the Pine Forest Range show that faulting there began at 11–12 Ma, progressed at a relatively constant rate until at least 3 Ma, and has continued until near the present time. The Santa Rosa Range to the east experienced a similar history, although the amount of post-6 Ma faulting there is not as well-defined. Less complete data from adjacent ranges permit a similar timing for faulting, and we infer that extensional faulting in northwestern Nevada began everywhere at 12 Ma and has continued until recently. Faulting in the Warner Range in northeastern California can only be constrained to have begun between 14 and 3 Ma, but ongoing geologic and thermochronologic studies of the range-bounding fault suggest significantly younger and more rapid extension than in northwestern Nevada. Compared to the central Nevada Basin and Range, extension in northwestern Nevada began more recently, is of lesser total magnitude, and represents significantly less supracrustal strain. Fission-track data from Cretaceous plutons emplaced at ~5 km depth indicate that erosional exhumation of the pre-Tertiary basement surface took place mainly during the Late Cretaceous and was followed by a long period of little deformation until 12 Ma. The low magnitude of extension and early Tertiary stability suggest that the modern ~30 km thick crust in northwestern Nevada was only slightly thicker (~34 km) prior to extension and was no thicker than ~37 km in the Late Cretaceous. This stands in contrast to the both the central part of the northern Basin and Range and southern Sierra Nevada, where thick Cretaceous crust is thought to have supported high surface elevations well into the Cenozoic.