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Show tug- of- war between crustal thickening driven by overthrusting, nearby depressions from this loading, and crustal thinning driven by gravitational instability. Examples of basinal sequences include the Newark Canyon, Green River, Claron, and Baseline Formations and Flagstaff Limestone. The basins probably formed at high altitudes, and contained broad shallow lakes and marshes bordered by steep alluvial piedmonts. Eocene and Oligocene- Collapse of the mountains and initial continental extension Beginning about 55 to 50 Ma ( middle Eocene), subduction slowed and the enormously thickened continental crust began protracted extension, a process still continuing. A southward- building volcanic arc, perhaps akin to the modern Cascades, formed along the future Sierra Nevada. East of this arc, belts of volcanism formed in the extending back- arc, where they swept southward in pace with the volcanic arc. Extreme local extension took place in many intrusive/ extrusive centers as crust locally collapsed. Basins associated with extending areas were probably low relief but may have been at high altitude. The overall progression of paleogeography during this time period was from isolated basins to integrated drainages over broad regions of subdued relief. The continental divide may have lain through the center of the Great Basin. Eocene ( 45 to 35 Ma) volcanism and extension is restricted to the northern Great Basin. Volcanism was andesitic to rhyolitic, with a few widespread ignimbrite deposits. Basins generally formed as successors to earlier orogenic basins, receiving deposits less than 1 km thick. Basins were broad, and many contained lakes and marshes. Examples are the White Sage, Sheep Pass, and Elko basins. In general, basinal deposits were not tilted even though extension locally occurred in the areas of these basins. Along the eastern fringe of the Great Basin, successor basins include the Bridger, Uinta, and Claron. Oligocene ( 35 to 20 Ma) volcanism and extension migrated south to the central Great Basin as a belt about 100 km wide that stretched across the entire Great Basin. As with the Eocene, volcanism was andesitic to rhyolitic, but many intense ignimbrite eruptions occurred, particularly between 28 and 21 Ma. Although lake and continental sediments intervene between ash flows in places, few large sedimentary basins formed. The Titus Canyon Formation and its equivalents through the Death Valley area represent one of the few examples. Lack of Oligocene deposits in the northern and southern Great Basin, including tuff expected from voluminous eruptions in the central Great Basin, suggests that these areas were undergoing erosion and were externally drained. The widespread ignimbrites mark low- relief surfaces across much of the Great Basin. Miocene- Continuing extension and widespread volcanism From roughly 20 to 10 Ma, block faulting and bimodal basalt- rhyolite volcanism developed regionwide, indicating a major change in Great Basin tectonics. Magmatism for the first time included the entire Great Basin, and wholesale extensional faulting across the entire Great Basin is indicated by many examples of normal faults and nearby thick basinal deposits. During this time period, several migrating foci of intense magmatism developed, three of which form or lie near the boundaries of the Great Basin: eastern Snake River Plain ( ENE direction; - 17 to 0 Ma), central Oregon ( WNW direction; - 10 to 0 Ma), and eastern California ( NNW direction; 18 to 0 Ma). Normal faulting accommodated roughly east- west extension that generally was not large, but locally was more extreme; metamorphic core complexes formed in the Miocene in several places, including those that initiated during earlier, more localized extension. The faulting led to internal drainage in most of the Great Basin, as indicated by extensive sedimentary sections of this age ( Humboldt, Salt Lake, Horse Spring, Ricardo, and Esmerelda Formations) that formed, at least across the northern Great Basin, in broad basins perhaps three times as wide as current basins. Basins accumulated strata as thick as 2 to 4 km across the northern Great Basin, but were less deep farther south. Volcanogenic sediments overwhelmed many of the basins, but lacustrine facies are common throughout the Great Basin. Pliocene and Quaternary- Basin and Range topography and tectonics Roughly 9 to 5 Ma ( commonly about 7 Ma), a change in tectonics led to the formation of precursors to modern basins and ranges. A small shift in the direction of plate convergence apparently led to a change to a regional WNW- oriented extension direction. The Great Basin differentiated from adjacent provinces by the relative uplift of marginal mountains such as the Sierra Nevada and Wasatch Mountains, and the adjacent Colorado Plateau. However, the Great |