3-D geometry of the Moine thrust and its implications for 3-D strain distribution and thrust sheet kinematics

University Of Rochester, Rochester NY

Investigators

Abstract

Most thrust fault traces are arcuate in nature and are comprised of a series of salients and recesses suggesting that thrust faults are, in general, non-planar. Such three- dimensional complexities in the thrust surface geometry may be an important factor controlling the thrust fault kinematics. Consider the analogy of glacial ice flowing over an irregular substrate. The major resistance to movement is not due to the friction between the two materials (which is greatly reduced by pressure-melt water that coats the surface) but due to the distortion of the ice as it flows past surface irregularities. Thrust sheets may behave in a similar manner. Fault rocks are considerably weaker than the host rock and, therefore, the continued deformation of such rocks may not be the factor in large-scale thrust sheet emplacement. The PI suggests that three-dimension irregularities in thrust surface geometry play a major role in thrust sheet kinematics. If this is the case, one should expect that the strain patterns observed along strike of a given thrust fault will reflect the thrust fault geometry. Preliminary studies have shown that both c-axes patterns and relict grain shapes vary along the strike of the fault most likely due to the fault geometry effects. Initial data suggest that deformation is nearly plane strain within a prominent salient; but along the margins of the salient there is a stronger component of flattening strains. However, further examination of the strains in this region are needed to truly resolve this pattern. Three-dimensional finite strain geometries have been determined in this region by measuring relict quartz grain shapes and examining quartz c-axes fabrics. Moreover, it may also be useful to measure final stage incremental strains by determining three-dimensional recrystallized quartz grain shapes and three-dimensional quartz overgrowth geometries. It is only through examining incremental strain histories that the PI can fully understand the kinematics of thrust faults. Nevertheless, current mechanical models do not take into account the existence of non-plane strains and therefore only predict realistic fold-thrust belt behavior to a first- order. In order to make more realistic models for the evolution of fold-and-thrust belts we must start to incorporate the non-plane strain elements of geometrically complex thrust faults.