Computerized X-ray tomography analysis of three-dimensional fault geometries in basement-induced wrench faulting

Abstract

Computerized X-ray tomography applied to sandbox experiments makes it possible to analyze the kinematic evolution, as well as the three-dimensional geometry, of faults in basement-controlled wrench faulting. With increasing basement displacement, the evolution of shear bands developed in the model sand bed is observed as follows. (1) The right-stepping shears that have a ‘cirque’ or ‘shell’ or ‘ship body’ shape develop on both sides of the basement fault. (2) The shears on the one side of the basement fault join those on the other side, and therefore each shear surface forms a helicoidal shape. Shears reach the surface of the sand near or above the basement fault, thereby, en echelon Riedel shears are observed at the surface of the sand. (3) Lower-angle shears and P-shears, lying within the zone defined by the first Riedels, have steeper dips and a less pronounced helicoidal aspect than the Riedels. Such three-dimensional shape and kinematic evolution of shears in the fault experiments agree wellwith the similar faults which propagate up through the unconsolidated sediments from a wrench fault in bedrock as observed in the excavated trenches at some active faults. In the fault experiments, the strike of the Riedel shear to the basement fault in dense sand is larger than that in loose sand, in reasonable agreement with the directions predicted by the Coulomb criterion in terms of the angle of internal friction. Likewise, the geometries of the Riedel shears are considered to depend on the overburden materials in natural cases of wrench faulting.