CONSEQUENCES OF THE MOJAVE WAISTLAND

Imagine an Andean-style orogen, with a forearc, arc, and backarc fold-and-thrust belt. Allow one segment of that orogen to have a narrow arc-to-craton distance limiting the cratonward growth of the fold-and-thrust belt. What might be the consequences? If the arc-craton distance decreases uniformly along the orogen, then the narrow segment will see far more rapid crustal thickening. The consequences of that would start with a highland likely to be a major source of sediment. Disruption of the fold-and-thrust belt would follow as gravitational stresses balance compressional ones. As the crust warms, the lower crust might be driven to flow into neighboring areas, expanding uplift even into areas with little or no surface deformation. Further warming might lead to extensional faulting and melting of the lower crust. Depending on the overall magnitude of shortening, profound tectonism of the crust is likely.

This scenario is not hypothetical: the Mojave Desert occupies the narrowest part of the Cretaceous Cordilleran orogen (hence, the “waistland”). Unusual uplift seems likely: parts of this region were the source of sediments that travelled far into central Utah. Also, it is adjacent to the southwestern Colorado Plateau, which was uplifted and deeply incised in latest Cretaceous time with minimal faulting. The Sevier belt in this area shut down by 80 Ma, but the oddly oriented Maria fold-and-thrust belt emerged instead. Large extensional faults have been documented in several places. The pattern of Cretaceous plutons younging to the east, seen to both the north and south, is unrecognizeable in the Mojave. Peraluminous granites reflecting crustal melts are near metaluminous plutons that seem quite arc-like. And, of course, this region saw emplacement of the Rand-Pelona-Orocopia schists, making the overall structure of the crust as complex as anywhere in the continent.

Viewed from this perspective, the peculiar geology of the Mojave region need not be a product of unusual subduction processes (e.g., plateau subduction) but instead might reflect the presence of an unusually recalcitrant lithosphere in the western Colorado Plateau interacting with the very long-lived contractional Cordilleran orogen.