Recognition of crustal extension in the Basin and Range Province: A history.

The Basin and Range Province of the western United States has inspired or augmented many important geological ideas but is most famous for the concept of crustal extension. Acceptance of the concept was a slow process. Prior to the recognition of plate tectonics, extension was unexpected and hard to understand; when mapping of the region began in the 1860s, mountain ranges were assumed to be the products of crustal compression due to contraction of the Earth. The first mapping suggested that ranges are anticlinal folds, but the idea of uplift along vertical range-bounding faults arose shortly thereafter. There was recognition starting in the 1880s that at least some faults were normal-sense rather than vertical, which implied extension, but there were decades of dissent by geologists who still promoted the anticlinal-folding idea, thought that inclined faults were reverse faults, or believed that the basins and ranges were created by water or wind or glacial erosion rather than tectonics. Normal faulting gradually became widely accepted, but comprehension of its meaning was hindered by apparent thrust faults in parts of the province that suggested contractional deformation. These faults would later be interpreted as low-angle normal faults, and crustal extension became widely, though not universally, accepted in the 1960s as magnetic anomalies confirmed seafloor spreading. Roughly a century elapsed between the first geologic mapping and general acceptance of crustal extension. A parallel evolution of thought occurred in Africa as a consensus arose that rift valleys were products of extension [ABSTRACT FROM AUTHOR]