DEVELOPMENT OF STRUCTURAL RELIEF IN BROAD EXTENDED DOMAINS: THE POTENTIAL ROLE OF DEFORMATION IN THE MIDDLE-CRUST

The California Continental Borderland and Basin and Range Provinces are each broad extended terrains with a large amount of structural relief. Detailed examination and comparison of the inner-northeastern part of the Borderland with the Virgin River Valley area of the Basin-and-Range show that upper crustal strain is highly varied in each province and that simple stress-strain models do not adequately explain the complex patterns of crustal thinning and relief generation. The borderland has thin crust (18-25 km), a gently east-sloping Moho, a well-developed basin-ridge physiography, prominent northwest-oriented linear faults, rapidly subsiding basins that lack normal-faulted margins, and rapidly rising ridges where stress and strain patterns indicate local crustal shortening. Although roughly 25% of inner borderland is locally emergent, most of the province is subsiding and remains below sea level or is slightly higher where sedimentation exceeds subsidence. The Virgin River area has relatively thin crust (about 30 km), a level Moho, and a well-developed basin-and-ridge physiography that includes the deepest basin in the province. The described magnitude of extensional strain is roughly the same in the ranges as it is deep beneath the basin. The Phanerozoic section, although tectonically thin, is equally preserved in the ranges and deep in the basin. In both provinces the amount of structural relief is equal to 25% or more of the total crustal thickness. Diverse strain patterns such as strike-slip and shortening strain are evident in both provinces as well. Since this diversity is contemporaneous, models advocating temporally distinct stages of deformation with changing stress fields are probably unrealistic in either case. It is also hard to explain the high vertical relief in either province by simple extensional models. The structural relief and vertical deformation are best explained by thinning and flow that occurred in the middle part of the crust. Differences in basal traction across the transition between ductile and elastic rocks may explain much of the strain heterogeneity in the upper crust in each case.