14: Origin of the northern Basin and Range province: Implications from the geology of its eastern boundary

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Abstract
Dynamic crustal processes in the Basin and Range province during the late Cenozoic are characterized by regional uplift, extensional block faulting, and fundamentally basaltic volcanism. The products of these processes are well expressed along the boundary with the western Colorado Plateaus where they provide special insight into the evolution of these two provinces. In Utah, the uplift and associated displacement on major faults along the western Colorado Plateaus has been proceeding at a vigorous rate (as much as 400 m/m.y.) over the past few million years. In Arizona, on the other hand, tectonism seems to have stagnated since a period of major uplift in the Pliocene, or earlier. The inception of fundamentally basaltic volcanism has migrated from central Arizona northward along the boundary at a rate of about 3 cm /yr since middle Miocene time, and eastward into the Colorado Plateaus at a slower rate, about 1 cm/yr. Regional features of the Basin and Range province important to any interpretive model include (1) an east-west geologic and geophysical discontinuity at approximately lat 37°N and (2) bilateral symmetry about a north-south axis in topography, gravity, basaltic volcanism, and regional upwarp of flanking Sierra Nevada and Colorado Plateaus. We believe that the region from the Sierra Nevada to the Colorado Plateaus is a broad crustal upwarp whose central part—the Basin and Range province—has been collapsing by block faulting. Upwarping, lateral east-west extension, faulting, and basaltic volcanism have operated concurrently as a by-product of upwelling asthenospheric mantle. There is evidence that after inception of block faulting in the middle Miocene throughout the Basin and Range province, a resurgent northward-migrating wave of dynamic processes developed. This produced a zone of broad upwarp and extensional faulting, followed by a zone of faulting and basaltic volcanism, and, in the southern wake, a zone of reduced tectonism where erosion and sedimentation are the dominant geologic processes. The cause of widespread upwelling of the asthenosphere to initiate block faulting is unresolved, whereas the resurgent northward-migrating wave of Basin and Range dynamics may be related to thermal instabilities in the asthenosphere that are associated with a northward-moving hole in the Farallon plate which is descending beneath the southwestern United States.

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