DO HEMATITE PRECIPITATION TEXTURES SEED SUBSEQUENT SLIP RATES AND MECHANISMS?

Fault damage zones at ~1-3 km depth may host microseismicity or inelastic deformation that influences the propagation and arrest of earthquake ruptures. Shallow, basement-hosted fault damage in the Wasatch fault zone (WFZ), UT, and southern San Andreas fault system exposed in Mecca Hills, CA, have a similar intensity of minor, mm-thick, hematite-coated fault surfaces that are texturally similar within each damage zone. However, fundamentally different hematite slip surface textures and grain morphologies between each damage zone raise questions of how and why this common fault zone mineral may behave so differently at broadly comparable depths.

Hematite precipitation textures vary depending on mineralization depth and thus temperature, fluid temperature, rock type, stress field, and strain rate. We compare new and published textural observations of mm-thick hematite slip surfaces in the WFZ and Mecca Hills to evaluate the role of initial precipitation textures on subsequent fault slip processes. Previously reported hematite (U-Th)/He thermochronometry data patterns support seismic slip along WFZ fault mirrors and slow slip along Mecca Hills hematite slip surfaces. Mirrored hematite surfaces in the WFZ crosscut deformed granite and develop within hematite veins and hematite-cemented breccia with tabular, ~1-10 μm x ~10-100 μm plates. Nanoparticle morphologies indicate spatially-variable coseismic temperature rise, recrystallization, and sintering at the slip interface, and underlying vestigial (ultra)cataclasite reveals extreme grain size reduction of vein platelets during slip. In Mecca Hills, networks of hematite slip surfaces dissect chlorite- and clay-rich basement and exhibit basal hematite injection veins and layered veinlets. Foliated, ~10-85 nm x ~200 nm-1 μm hematite plates are not comminuted and appear to deform by interplate sliding – exhibiting textures similar to those observed for phyllosilicate-rich surfaces. Hematite textures inform the frictional stability of these surfaces, which is impacted by both the hematite and adjacent rock properties. We suggest initial hematite grain size, aspect ratio, and fabric influence slip rates and rheology, with coarse-grained and anisotropic, nanoscale plates seeding subsequent fast and slow slip, respectively.