EARTHQUAKE PETROLOGY: INSIGHTS INTO FAULT SLIP LOCALIZATION AND FAULT HEATING VIA MICRO X-RAY FLUORESCENCE MAPPING AND X-RAY ABSORPTION NEAR EDGE SPECTROSCOPY

Theoretical and laboratory analyses propose a range of dynamic weakening mechanisms to explain fault slip behavior, especially on extremely thin slip surfaces. We use a synchrotron light source to use microscale X-ray fluorescence (μXRF) analytical mapping to investigate μm-scale elemental distribution in faults, and X-Ray Absorption Near Edge Spectroscopy (XANES) to test for elevated temperatures at asperities on fault surfaces. We examine hematite-coated fault surfaces in the footwall of the Wasatch Fault, Utah, and map the distribution of Fe³⁺ and Fe²⁺. XANES analyses indicate Fe³⁺ is reduced to Fe²⁺ at mm long asperities on the faults, and the reduced iron may form several μm deep into the fault zone. We test the thermometry method on a MoS₂-coated fault from New Mexico, where Mo and Y exhibit no valence state transitions, consistent with the low friction coefficient for MoS₂. Modeling of temperature rises on elliptical asperities suggest that the Fe reduction occurs at T> 700°C and creates flash heating that reduces the iron to produce iridescent spots. The XRF thin slip surfaces are connected to anastomosing zones of Fe that weave through a comminuted matrix of hematite, some of which exhibits annealed grains documented by Ault et al. The μXRF mapping in SAFOD Phase III samples reveals intricate elemental distributions that illuminate fault-related textures not visible by conventional forms of microscopy due to ultra-fine grain size and intense gouge blackening. Carbon-rich samples exhibit closely spaced anastomosing slip surfaces, fragmented shear zones, pressure solution seams, calcite-cemented breccia, calcite vein fragments of varying trace element chemistry, and development of Riedel shears. These data show that slip and pressure solution events occur in episodic pulses, suggesting periods of strength recovery between slip events in actively deforming carbon-rich fault zones of the San Andreas Fault.