FIELD-BASED ANALYSIS OF INTRINSIC AND EXTRINSIC CONTROLS ON FAULT-ZONE DEFORMATION PROCESSES IN POORLY LITHIFIED MATERIALS, WITH IMPLICATIONS FOR HYDROLOGIC STUDIES

Conceptual models of fault-zone architecture and permeability structure are typically based on studies of rocks faulted at depths greater than 3 km and subsequently exhumed. Structures within these faults have been examined to understand the impact of fluids on the mechanics of earthquakes and faulting. At these seismogenic crustal levels, rocks are fully saturated and both T and P are elevated with respect to surface conditions. Rocks deformed at depths of several kilometers or more include both crystalline and fully lithified sedimentary and volcanic rocks; therefore, both intrinsic properties, such as porosity, and extrinsic parameters, such as confining pressure, are different from those of granular materials deformed at near-surface conditions.

Our work indicates that faults in poorly lithified rocks do not exhibit the same fault-zone architecture and permeability structure as faults formed at greater depth in fully lithified rocks. The former contain structures formed in the near-surface environment at low confining pressures, where deformation takes place both above and below the water table. Fault-zone structures in poorly lithified clastic sediments and ash-flow tuffs result from deformation processes which are fundamentally controlled by such variables as grain contact area and strength and consolidation history (including tectonic consolidation). Near-surface fault-zone deformation processes therefore depend generally on material strength, which increases with consolidation, cementation or mineralization, and welding. The dominant deformation processes change with increasing strength from particulate flow, to cataclasis within deformation bands, to fracturing. The first two processes generally reduce saturated permeability, whereas the last one increases it; therefore, fault-zone permeability structure depends critically on fault-zone deformation processes, which can change over time as fault-zone material properties change. Our observations demonstrate the importance of choosing a conceptual model that accurately represents the intrinsic properties (protolith characteristics) of the faulted rock and extrinsic conditions (e.g., depth of faulting) of deformation, both of which influence mode of failure.