Slickenside

Abstract: A new technique is derived to invert slickenside data for the stress field that caused the faulting episode. This inversion is simplified by the assumption that the magnitude of the tangential traction on the various fault planes, at the time of rupture, is similar. Study of three normal faulting regimes shows that the inversion derived with this assumption yields results that closely match older inversions that did not include the assumption. Hence the assumption may be valid and is shown to be justified by analyzing a simple fracture criterion. Application of slip data inversions is extended from faulting regimes to the slip on bedding plane faults in folding regimes. Comparison of the inversion results with the geometry of the folds shows this application to be successful, greatly increasing the number of data sets that can be used to find the paleostress field.

Abstract: To allow focal mechanisms to be inverted for the stress field requires a different inversion algorithm than for slickenside data because focal mechanisms do not represent fault slip data unless one can decide which nodal plane is the fault plane. If one can decide which nodal plane is the fault plane, then the focal mechanisms can be inverted with the slickenside inversion algorithm. This decision cannot always be made, so algorithms for inverting focal mechanisms for the stress field are studied. These algorithms either use both of the possible fault planes or attempt to choose the correct fault plane while determining the stress tensor. Simulated focal mechanisms are made from slickenside data and used to provide a control study for the focal mechanism inversion algorithms. The results of this control study show that focal mechanisms can be inverted to find the best stress tensor, but the resolution is decreased unless the fault planes can be picked a priori. The resolution can also be increased by including constraints on the magnitude of the tangential traction on the fault plane. Therefore, using focal mechanisms to study small variations in the stress field requires that other data (e.g., studies of the hypocenters, surface faulting, or structural information concerning the region) be introduced to pick which of the nodal planes is the fault plane. This study also introduces the method of bootstrap resampling to the statistics of this problem. The non-Gaussian nature of the data makes the nonparametric formulation of the bootstrap approach ideal for this problem.

Abstract: This article reviews geologic and other evidence constraining the thickness of the principal slip zone (PSZ) that accommodates the bulk of coseismic shear displacement during an individual rupture event. Surface deformation from rupturing may occupy swaths tens of meters or more in width, but trenches across active faults generally reveal that incremental slip is accommodated by a PSZ that is tens of centimeters or less in thickness. Geomorphic evidence, coupled with the observations from trenching, suggest a PSZ may stay well localized for distances of several kilometers through many rupture episodes. Mine exposures and exhumed fault zones demonstrate that PSZs separating different lithologies within the “fault core,” although contained within “damage zones” of variably fractured rock ranging up to hundreds of meters in thickness, often comprise just a few centimeters of gouge/ultracataclasite that have accommodated large finite displacements (>1 km). Microstructural studies demonstrate incremental slip localized still further down to 1–10 mm, as do other fault-rock assemblages (slickensides and slickenfibers, fault-veins of pseudotachylyte friction-melt, intravein septa in hydrothermal fault infills). The accumulated evidence indicates that localization of coseismic shearing to less than 10 cm on planar faults is widespread throughout the crustal seismogenic zone, with extreme localization to less than 1 cm not uncommon. However, some distributed coseismic shear may also develop, especially at rupture irregularities. Coseismic reduction of shear resistance from friction-melting (Δ T ∼ 1000°C) or from transient thermal pressurization of aqueous fluids (Δ T ∼ 100°C) requires slip during moderate-to-large earthquakes ( u > 1 m) to be restricted to narrow zones, respectively a few centimeters or tens of centimeters in thickness. Given the evidence for slip localization, the apparent scarcity of pseudotachylyte suggests either that seismic friction-melting is a rare phenomenon, or that pseudotachylyte is only rarely preserved in recognizable form within mature hydrated fault zones.

Abstract: Fractures within granodiorite of the central Sierra Nevada, California, were studied to elucidate the mechanics of faulting in crystalline rocks, with emphasis on the nucleation of new fault surfaces and their subsequent propagation and growth. Within the study area the fractures form a single, subparallel array which strikes N50°–70°E and dips steeply to the south. Some of these fractures are identified as joints because displacements across the fracture surfaces exhibit dilation but no slip. The joints are filled with undeformed minerals, including epidote and chlorite. Other fractures are identified as small faults because they display left-lateral strike slip separations of up to 2 m. Slickensides, developed on fault surfaces, plunge 0°–20° to the east. The faults occur parallel to, and in the same outcrop with, the joints. The faults are filled with epidote, chlorite, and quartz, which exhibit textural evidence of shear deformation. These observations indicate that the strike slip faults nucleated on earlier formed, mineral-filled joints. Secondary, dilational fractures propagated from near the ends of some small faults contemporaneously with the left-lateral slip on the faults. These fractures trend 25°±10° from the fault planes, parallel to the direction of inferred local maximum compressive stress. The faults did not propagate into intact rock in their own planes as shear fractures. Rather, adjacent faults were linked together by secondary, dilational fractures. Extensive secondary fracturing between faults produced larger fault zones that accommodate 10–100 m of left-lateral slip. As deformation progressed, faulting evolved from relatively short, closely spaced faults to longer, more widely spaced fault zones.