Major Structures of the Rocky Mountains of Colorado and Utah

This paper describes the major structures of Colorado and Utah and presents a theory of origin based on new knowledge of the layering and constitution of the upper mantle and lower crust. It proposes that the Ancestral Rockies and the more modem ones of Cretaceous and early Tertiary age of both the shelf of Colorado and eastern Utah and the miogeosyncline of western Utah are the result of vertical uplifts of the silicic crust. The uplifts are caused by the rise, from the upper mantle, of basalt in scattered places to the base of the silicic crust. This rise domed the silicic crust and the overlying sedimentary veneer. The surficial structures are viewed as gravity-caused mass movements along the flanks of the uplifts and in addition, particularly in the miogeosyncline of western Utah, as synclinoria of Paleozoic strata between uplifts where folding and considerable bedding plane thrusting occurs. The theory also relates widespread magmatism to tectonism in a reasonable fashion. It recognizes that the Rocky Mountains have been a region of Cenozoic regional uplift in which possibly more energy was required than for the building of individual uplifts. This, too, is related to a transformation of the uppermost mantle into a lighter density state. And finally, the Basin and Range faulting of western Utah is viewed as the result of the rise of sufficient basalt from the mantle to form a continuous layer under the silicic crust of the miogeosyncline thus bringing into existence a new framework of forces. The surficial structures are framed about the primary uplifts, but the entire silicic crust becomes attenuated toward the Pacific as it is activated by a component of gravity on a mobilized lower crust