SCEC2021 Plenary Talk, Earthquake Geology

What can hematite (U-Th)/He thermochronometry tell us about fault mechanics in the shallow crust?

Alexis K. Ault

Oral Presentation

2021 SCEC Annual Meeting, SCEC Contribution #11142
Above the seismogenic zone, earthquake ruptures may propagate to the surface along discrete fault planes or earthquake energy is attenuated in fault damage zones. Forecasting earthquake hazards requires knowledge of the slip distribution and mechanical behavior of the shallow crust. Hematite, a common secondary mineral along faults, is amenable to (U-Th)/He (hematite He) thermochronometry that informs the timing, temperatures, depths, and rates of fault slip. Hematite rotary shear experiments at ambient conditions and seismic slip rates show low coefficients of quasi-static and dynamic friction. Natural samples from two exhumed, seismogenic faults imply a broad range of hematite rheology and slip rates at shallow crustal depths. The Painted Canyon fault zone in the Mecca Hills, CA (part of a positive flower structure adjacent to the southern San Andreas fault), is characterized by dense networks of ~1-10 cm-diameter, striated hematite-coated slip surfaces developed in Proterozoic basement. Some surfaces exhibit hematite injection veins into underlying host rock, indicating initial formation by pore fluid overpressure. Slip surface hematite comprises nm-thick platelets in textures akin to foliated phyllosilicates. Hematite He dates record ~0.8-0.4 Ma mineralization at <1.5 km depth. These observations indicate distributed deformation by slow slip events and/or creep that potentially accommodated and dampened radiated seismic energy from large earthquakes. In contrast, the Hurricane normal fault near La Verkin, UT, is expressed as a semi-continuous, m^2 mirrored fault surface extending >1 km along strike that cuts silicified Triassic conglomerate. Here, hematite exhibits nanoscale deformation textures reflecting cataclasis and amorphization of pre-existing hematite and surrounding rock, followed by new hematite growth and solidification of an amorphous silica layer during seismic slip. Hematite He dates document mineralization and fault slip ~0.6-0.4 Ma at ~300 m depth. Mechanical and hydrothermal processes weaken fault materials in a comparatively strong host rock, resulting in repeated up-dip propagation of earthquake ruptures along a discrete fault surface. This comparison highlights how hematite textures and thermochronometry reveal an array of structures and evolving mechanical properties that may promote or inhibit rupture propagation in the shallow crust.