Pulverization Texturein Fault Damage Zones: A result of Implosion Damage or Dynamic Compressive Stresses?

Abstract:

Micro-brecciation, or pulverization, is recognized as a fundamental component of the architecture and damage products of many large faults, although the precise mechanisms to produce this damage are debated, with both compressive and tensile mechanisms proposed. We characterized several sites along the San Jacinto fault, southern California, where the total depth of exhumation for the life history of the fault can be determined, to study the confining stresses required for pulverization. In basement rock near Anza, where exhumation is less than 100 m, granitic dikes injected into schist of the Burnt Valley Complex are pulverized out to several meters from the fault core, whereas the schist is brecciated at the macro-scale and contains narrow centimeter-thick seams of black cataclasite. Similar relationships are observed in Horse Canyon, which is exhumed about 400 m below a regional Tertiary erosion surface, where granitic dikes emplaced into schist are pulverized out to distances of several tens of meters from the fault core. These observations imply that very low confining stress is required for micro-brecciation in granitic rock.

Unconsolidated sandstones (alluvial fan deposits) along the SJF in Rock House Canyon are undeformed where the deposits are exhumed by about 70 m, but show incipient pulverization (high-density, sub-grain cracking) at 120 m depth of exhumation. Cracks oriented perpendicular to the fault formed in individual quartz and feldspar grains out to a few meters from the fault core. These observations suggest that the confining stress required for onset of pulverization in unconsolidated deposits is on the order of 2-2.5 MPa. As the tensile strength of quartz is an order of magnitude higher than these confining stresses, the most likely mechanism that is producing this damage is dynamic compressive stresses during passage of the rupture front.