MICROSTRUCTURAL DEVELOPMENT DURING DISPLACEMENT AND DYNAMIC WEAKENING OF SMECTITE-RICH FAULT GOUGE

Until recently, fault zones composed primarily of velocity-strengthening clay minerals were believed to be incapable of initiating or propagating seismic rupture. The shallow seismic slip behavior of the 2011 Mw 9.0 Tohoku earthquake, high-speed shear experiments, and dynamic rupture models collectively show that seismic rupture can propagate along clay-rich faults because they weaken dramatically at seismic slip rates. The San Andreas Fault Observatory at Depth (SAFOD) program recovered smectite-rich gouge from the Southwest Deforming Zone (SDZ) and Central Deforming Zone (CDZ), which are believed to accommodate most of the displacement along the central segment of the San Andreas Fault. We present and quantify the microstructural evolution of CDZ gouge during high-velocity shear experiments and constrain the displacement- and velocity-weakening.

We tested room-dry and water-wet gouge at room temperature using a high-speed rotary apparatus. The sample was flaked to ~600 μm and 1 mm thick layers were sheared at a normal stress of 0.3-1.5 MPa, velocity of 0.1-1.3 m/s, and to 1-20 m displacement. Petrographic, scanning electron microscope (SEM), and transmission electron microscope (TEM) images were collected from radial cut petrographic slides, which show structures in slip-perpendicular sections, and tangential slides, which show structures in slip-parallel sections.

We identified and mapped four microstructural units, similar to those defined by Kitajima et al. (2010), using optical microscopy. The development of a composite planar (C-S) fabric (Unit 2) from the initial microstructure (Unit 1) occurs during the first 1m of slip coincident with an increase in strength. Subsequent displacement weakening to steady-state strength occurs during continued shear of Unit 2, and subsequently with progressive development of a localized foliated slip-zone (Unit 4) and associated fluidized material (Unit 3). We interpret that displacement and dynamic weakening are caused by slip along clay-foliation assisted by shear-heating pressurization of pore fluid in wet gouge and by additional grain-size reduction and possible decomposition of dry gouge. The shape preferred orientations of clasts and clay particles were measured in SEM and TEM images and support processes inferred from petrographic analyses.