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

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Most thrust fault traces are arcuate in nature and are comprised of a series ofsalients and recesses suggesting that thrust faults are, in general, non-planar. Such three-dimensional complexities in the thrust surface geometry may be an important factorcontrolling the thrust fault kinematics. Consider the analogy of glacial ice flowing overan irregular substrate. The major resistance to movement is not due to the frictionbetween the two materials (which is greatly reduced by pressure-melt water that coats thesurface) but due to the distortion of the ice as it flows past surface irregularities. Thrustsheets may behave in a similar manner. Fault rocks are considerably weaker than thehost rock and, therefore, the continued deformation of such rocks may not be the factor inlarge-scale thrust sheet emplacement. The PI suggests that three-dimension irregularities inthrust 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 willreflect the thrust fault geometry. Preliminary studies have shown that both c-axespatterns and relict grain shapes vary along the strike of the fault most likely due to thefault geometry effects. Initial data suggest that deformation is nearly plane strainwithin a prominent salient; but along the margins of the salient there is a strongercomponent of flattening strains. However, further examination of the strains in thisregion are needed to truly resolve this pattern. Three-dimensional finite strain geometrieshave been determined in this region by measuring relict quartz grain shapes andexamining quartz c-axes fabrics. Moreover, it may also be useful to measure final stageincremental strains by determining three-dimensional recrystallized quartz grain shapesand three-dimensional quartz overgrowth geometries. It is only through examiningincremental strain histories that the PI can fully understand the kinematics of thrust faults.Nevertheless, current mechanical models do not take into account the existence ofnon-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 beltswe must start to incorporate the non-plane strain elements of geometrically complexthrust faults.