CRUSTAL STRUCTURE, GEOPHYSICAL MODELS AND CONTEMPORARY TECTONISM OF THE COLORADO PLATEAU

Surface wave dispersion and seismic refraction data show that the crust of the interior of Colorado Plateau is approximately 45 km thick. This thickness is significantly greater than that found in the Basin and Range Province (~30 km) which bounds the plateau on the west and south. Results from recent seismic studies indicate that the Rio Grande rift, which bounds the plateau on the east, also has a thinner crust (30-35 km) than the plateau. The northern boundary of the plateau is apparently not associated with a major change in crustal thickness. In general, belts of active seismicity and late Cenozoic faulting and volcanism are associated with those boundaries of the Colorado Plateau which involve substantial crustal thinning. At both the northwestern and southwestern boundaries of the plateau, seismic data indicate that thinner Basin and Range crust extends as much as 100 km into the plateau. Thus, it appears that zones of extension (rifting?) bounding the plateau appear to be growing at the plateau's expense. Surface wave and seismic refraction data indicate that the crustal structure of the interior of the Colorado Plateau is typical of stable continental areas. However, Pn (upper mantle) velocities appear to be lower (7.8 km s-1) than would be expected in a stable region. Silica geothermometry indicates that the average heat flow for the plateau is 55 mWm-2 (1.3 HFU). Thermal and gravity models of the plateau indicate the thickness of the lithosphere to be approximately 60 km, a thickness which is intermediate between those of the Basin and Range and Great Plains. This thickness for the lithosphere is consistent with both seismic and electrical conductivity data and may explain the elevation difference between the Colorado Plateau and the Great Plains. © 1979.