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Show David M. Miller U. S Geological Survey 345 Middlefield Road, MS- 975 Menlo Park, CA 94025 dmiller@ usgs. gov CRETACEOUS TO QUATERNARY GEOLOGIC EVOLUTION OF THE GREAT BASIN Introduction The Great Basin largely coincides with the northern Basin and Range geologic province, which is characterized by alternating mountains and basins, generally high altitude, and active extensional tectonics. This province defies simple description, however, because Basin- and- Range extensional tectonism varies across the region and because rocks within it were inherited from several cross- cutting previous geologic provinces. As a result, although the geologic evolution of the Great Basin is better known for recent times because the rock record is fairly complete, its evolution must be cast in a longer time frame using less complete records from the past to appreciate it fully. This paper considers the Great Basin from the middle of the Cretaceous period ( roughly 100 million years ago; Ma) to the Quaternary period ( the last 2 million years) and casts the geologic evolution in terms of paleogeography and development of lakes. General Concepts Geology has both direct and indirect influence on the development and maintenance of lakes. Direct influences include controls on soils and landforms ( raw materials, rates of erosion and deposition, addition of volcanic materials) and tectonic controls on local topography and climate ( rainfall and snow/ ice accumulation, rain shadows, basin development, river capture). Indirect effects are largely tectonic and include distant ultra- high mountain and sub- sea topography that affects global atmospheric and oceanic circulation patterns, in turn influencing global climate patterns, and shifting climates caused by plate tectonic movements- wholesale movement of continental masses between the tropics and the poles. The Great Basin region evolved geologically from mountain- building to mountain collapse over the last 100 million years. From before 100 Ma to about 50 Ma, the region probably looked much like the modern Andes of central South America, where the oceanic plate is subducting beneath the continent and causes considerable volcanism and contractional tectonics within the continental plate. Starting at about 50 Ma, these high mountains began to collapse as subduction slowed and then stopped. Collapse was accompanied by volcanism, voluminous at times, and manifested itself in the geologic record in several different ways. From about 45 to 20 Ma, volcanism swept southward across the future Great Basin and the crust extended in a roughly east- west direction in and near magmatic complexes. After about 18 Ma, the entire region extended east- west, essentially by the Sierra Nevada migrating west with respect to the Wasatch Mountains and Colorado Plateau. From 17 Ma to about 8 Ma, this extension resulted in broad basins, many occupied by lakes, that were much wider than the present basins. Beginning about 10 to 7 Ma, modern basins and ranges started to take form, with much of the detail of the present topography developing since ~ 3 Ma. Cretaceous and Paleocene- Mountain- building Before about 80 Ma, the future Great Basin was a three- part orogen, with a magmatic arc ( future batholith) on the west, followed to the east by an internal orogen where deep burial of crust led to high- grade metamorphism, and farther east to a frontal fold and thrust belt ( Sevier orogenic belt). This orogen was driven by subduction beneath the west edge of North America. After about 80 Ma, in the region north of Las Vegas and south of central Idaho, the plate began to subduct more rapidly and along a much shallower trajectory, extinguishing that part of the magmatic arc and causing mountain- building much farther east in the present Rocky Mountains ( Laramide orogeny). This mountain- building overlapped spatially and temporally with the frontal fold and thrust belt along a zone on the east margin of the present Great Basin. Within the Sevier and Laramide mountain systems, orogenic basins formed as the mountains reacted dynamically to the |