Deformation near a fault termination, part II: A normal fault in shales

In a previous paper, the strain field in the vicinity of the termination of an experimentally induced fault in clay was described. The present study deals with the strain field associated with the termination of a natural normal fault, and a comparison of the two is made.Samples were taken near the termination of a steeply dipping fault in the Green River Formation, Utah. The preferred orientation of basal planes of clay minerals was measured at 80 points in a three-dimensional array, using an X-ray pole-figure goniometer. From the preferred orientation data, strain was calculated according to the theory of March (1932). Strain remote from the fault, due to compaction alone, was determined bed by bed and subtracted from the strain observed near the fault. In this way the finite strain, presumably associated with faulting only, was determined.The displacement field was found by unstraining and superposing the unstrained and strained configurations. The displacement field, patterns of magnitudes of the second invariant of the strain tensor (being a measure of the shear strain), and those of the principal strains were found. They resemble analogous fields and patterns near strike-slip faults observed in experiments, deduced theoretically, and inferred from field observations on active natural faults, after coordinates are rotated so as to bring the fault planes and slip directions into coincidence.