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Several Rocky Mountain uplifts have collapsed, usually back down the root zone of the thrust that raised the buckled slabs. Basement is juxtaposed against sedimentary fills and subcrust is juxtaposed against granitic crust. Thus, uplifts have "anti-roots" and strong positive gravity anomalies with slabs held up by strength rather than buoyancy, making them susceptible to collapse. The Rio Grande rift trends along the crests of older uplifts. Collapse is accentuated by regional uplift that removes basin fills. This substitution of "air for rock" increased gravity stressing.
Large gravity faults, including the San Luis basin boundary fault with relief that may exceed 20,000 ft (6,100 m), are caused by this mechanism. Their large size may be caused by a significant increment of displacement above the isostatic equilibrium position added to the normal buoyancy mechanism that drives these faults.
The north side of the Brown's Park graben in the Uinta Mountains
bears this relationship to the Uinta thrust, increasing in distance from the thrust trace as stratigraphic throw and amount of overhang of the thrust increases. A plunging section of the South Granite Mountains fault system in the area of Ferris Mountain provides a downplunge cross section in which the collapse fault can be seen to join and "back down" the root zone of the thrust. Thus the outer thrust lip "hangs up" and is left standing higher than the core of the uplift. Distance between the two types of faults provides an estimate of the amount of overhang.
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