LITHOSPHERIC GRAVITATIONAL POTENTIAL-ENERGY AND PAST OROGENESIS - IMPLICATIONS FOR CONDITIONS OF INITIAL BASIN AND RANGE AND LARAMIDE DEFORMATION

Gravitational body forces (i.e., buoyancy forces) have come to be seen as critical to the evolution of orogens, Nevertheless, constraining t he role of body forces in specific geologic scenarios is made difficul t by the substantial number of poorly constrained physical parameters needed to fully relate forces to deformation. By separating the calcul ation of buoyancy forces from the calculation of the resulting deforma tion, models based on relatively simple descriptions of the lithospher e can yield geologically useful constraints. Among these are the impor tance of paleoelevation in driving syn- and postcontractional extensio n and in localizing contractional strain. Although such phenomena have been considered in more complex models of continental deformation, th e simpler analysis presented here clearly establishes first-order limi ts on lithospheric structures and paleoelevations consistent,vith buoy ancy-driven deformation, In the early Cenozoic Great Basin of the west ern United States, we show that the low elevations inferred in much of the geologic literature are inconsistent with a body-force origin for observed extensional tectonism, East of the Colorado Plateau, localiz ation of Laramide deformation coincides.with pre-laramide subsidence o f the Western Interior seaway. This subsidence prestressed the Lithosp here, making the Southern Rocky Mountains the weak link in responding to regional compressional stress.