COMPARING SIMPLE- AND PURE-SHEAR MODELS FOR ANCESTRAL ROCKY MOUNTAINS DEFORMATION

Numerous mechanisms have been introduced to explain Ancestral Rocky Mountains (ARM) deformation. Here, we present a hypothesis emphasizing internal body rotation in a simple-shear-like mechanism to explain formation of the ARM orogen. Recent studies interpret the ARM as a network of structures formed by a pure-shear mechanism, and this does explain larger basins with well-defined proximal uplifts at the plate-margin. Additionally, present understanding relies upon late Precambrian features including the Wichita mega-shear to govern the strain pattern, but this stops short of providing an explanation for the structural character of ARM deformation within the continent interior.

The simple shear model presented here addresses some of the outstanding issues with pure shear/preexisting weakness conceptions of ARM deformation. Modern systems, like the Quinghai-Lanzou-North China basin transition in China, demonstrate that a single governing tectonic style can produce varied types of structural deformation. Importantly, strain location may be influenced by the preexisting structures, but does not need to directly overlap with these in space.

Numerical experiments were conducted to test the independence of the strain patterns produced by pure-shear and simple-shear from the distribution of material properties within deformed blocks. Preliminary results suggest there may be commonalities in the strain patterns between blocks deformed by the same shear mechanism, regardless of the distribution of lithosphere properties. Importantly, in left-lateral deformation, simple shear consistently forms distinct parallelogram geometries of strain with maximum internal angles well in excess of 100 degrees. The trends of the Uncompahgre uplift in Utah-Colorado and the Wichita uplift of Oklahoma-Texas form a similar angle. Simple shear also predicts major strike slip faults traversing the sides of the block to which forces are applied, and this may explain the Nemaha Ridge and a less well-known fault system crossing N-S through Nevada. On the other hand, pure-shear generates strain geometries with internal angles close to 90 degrees, and there is no evidence for extensive strike-slip systems that may through-cut parallel to the stressed walls in these experiments.