Physical Properties Architecture of Large Displacement Faults in the Upper Crust: A Synthesis of Sample and Log Based Observations

Abstract:

Large-displacement plate boundary scale faults such as the Alpine Fault develop an internal architecture distinct from that of the host rock due to the effects of accumulated mechanical damage, mineralogical alteration, and potentially contrasting internal stress and pore fluid pressure regimes. The seismic velocity, density, electrical resistivity, and other properties of the faults therefore provide proxy information on parameters relvant to fault strength, locking, and slip. At in situ seismic imaging frequencies (~1 to >100 Hz), fault-hosted low velocity zones (LVZ) are observed in many faults including the San Andreas and Alpine Faults, as well as a number of subduction interface faults, but these signals lack the resolution to resolve the trade-off between length scale and magnitude of velocity anomalies in detail. They also cannot be used to detect the effects of velocity anisotropy in many cases. Borehole and sample-based measurements shed light on the fine-scale architecture of faults, which can in turn delimit permissible interpretations of seismic data. Detailed work on outcrop-derived Alpine Fault samples, DFDP-1 logs and cores, SAFOD, NanTroSEIZE, and JFAST project logs and cores, and other field and borehole examples collectively suggest: (a) brittle fault zones differ greatly in the degree to which their intrinsic elastic properties contrast with those of the wall rock depending on protolith composition, and this strongly affects the degree to which stress or fluid pressure anomalies can be detectable via seismic reflection, refraction, or tomography; (b) fault damage and core (gouge) zones are in some cases distinct from each other in Vp and Vs but not in others; and (c) seismic velocity anisotropy occurs on several nested length scales and is typically weaker in the fault core than in the surrounding damage zone or protolith.