<P> The tectonic mechanisms responsible for lithospheric extension in the Basin and Range province are controversial, and several competing hypotheses attempt to explain it . Key events preceding Basin and Range extension in the western United States include a long period of compression due to the subduction of the Farallon Plate under the west coast of the North American continental plate which stimulated the thickening of the crust . Most of the pertinent tectonic plate movement associated with the province occurred in Neogene time and continues to the present . By Early Miocene time, much of the Farallon Plate had been consumed, and the seafloor spreading ridge that separated the Farallon Plate from the Pacific Plate (East Pacific Rise) approached North America . In the Middle Miocene, the East Pacific Rise was subducted beneath North America ending subduction along this part of the Pacific margin; however, the Farallon Plate continued to subduct into the mantle . The movement at this boundary divided the East Pacific Rise and spawned the San Andreas transform fault, generating an oblique strike - slip component . Today, the Pacific Plate moves north - westward relative to North America, a configuration which has given rise to increased shearing along the continental margin . </P> <P> The tectonic activity responsible for the extension in the Basin and Range is a complex and controversial issue among the geoscience community . The most accepted hypothesis suggests that crustal shearing associated with the San Andreas Fault caused spontaneous extensional faulting similar to that seen in the Great Basin . However, plate movement alone does not account for the high elevation of the Basin and Range region . The western United States is a region of high heat flow which lowers the density of the lithosphere and stimulates isostatic uplift as a consequence . Lithospheric regions characterized by elevated heat flow are weak and extensional deformation can occur over a broad region . Basin and Range extension is therefore thought to be unrelated to the kind of extension produced by mantle upwelling which may cause narrow rift zones, such as the Afar Triple Junction . Geologic processes that elevate heat flow are varied, however some researchers suggest that heat generated at a subduction zone is transferred to the overriding plate as subduction proceeds . Fluids along fault zones then transfer heat vertically through the crust . This model has led to increasing interest in geothermal systems in the Basin and Range, and requires consideration of the continued influence of the fully subducted Farallon plate in the extension responsible for the Basin and Range Province . </P> <P> In some localities in the Basin and Range, metamorphic basement is visible at the surface . Some of these are metamorphic core complex (MCC), an idea that was first developed based on studies in this province . A metamorphic core complex occurs when lower crust is brought to the surface as a result of extension . MCCs in the Basin and Range were not interpreted as being related to crustal extension until after the 1960s . Since then, similar deformational patterns have been identified in MCCs in the Basin and Range and has led geologists to examine them as a group of related geologic features formed by Cenozoic crustal extension . The study of metamorphic core complexes has provided valuable insight into the extensional processes driving Basin and Range formation . </P> <P> Prior to the Eocene Epoch (55.8 ± 0.2 to 33.9 ± 0.1 Ma) the convergence rate of the Farallon and North American Plates was fast, the angle of subduction was shallow, and the slab width was huge . During the Eocene the Farallon Plate subduction - associated compressive forces of the Laramide, Sevier and Nevada orogenies ended, plate interactions changed from orthogonal compression to oblique strike - slip, and volcanism in the Basin and Range Province flared up (Mid-Tertiary ignimbrite flare - up). It is suggested that this plate continued to be underthrust until about 19 Ma, at which time it was completely consumed and volcanic activity ceased, in part . Olivine basalt from the oceanic ridge erupted around 17 Ma and extension began . </P>

Which of the following was a geologic factor in the formation of the basin and range