SPATIAL VARIATION IN FAULT SLIP ZONE THICKNESS

The thickness of a fault slip zone is a primary control on the effectiveness of dynamic processes such as thermal pressurization, elastohydrodynamic lubrication and other thermally activated processes such as melting. The thickness of a single fault slip zone is directly controlled by the geometry of the fault surfaces that bound it on each side. Here, we utilize exceptional exposures of an exhumed seismogenic fault in southern California to characterize slip zone geometry. Maps of cross sectional exposures constructed with the Structure from Motion photogrammetric method illustrate crosscutting relations between multiple layers of ultracataclasite within the fault core and define the evolution of the structures. Contacts between layers exhibit evidence of wear, indicating the layers were slip zones, and the contacts between layers acted as slip surfaces. One continuous layer crosscuts all other structures showing it was the last slip zone to form in the fault. This layer is the thinnest and smoothest, recording significant smoothing on a single structure. Thickness values for individual layers of ultracataclasite as well as the thickness of the inner fault core as a whole were measured at a 3mm interval from digital field maps. Geostatistical analysis of these data was performed to assess the spatial continuity of individual layer thickness. The most recent slip layer ranges 1 to 80 mm thick, and variograms of thickness data from this layer contain a well-defined range. Periodic fluctuations around the sill reveal a characteristic wavelength of 1-3 m. Because this wavelength characterizes the dimension of extremely narrow portions of the slip zone, it may represent the contact asperity size on the fault, similar to D_c from seismological estimates during slip. In addition, order of magnitude fluctuations in slip zone thickness over strike distances of meters will impact the efficiency of thermal pressurization locally on the fault during slip.