3D Microstructural Investigation of Experimentally Deformed Smectite-rich San Andreas Fault Gouge from the Southern Deforming Zone (SAFOD)

Abstract:

Smectite clays have been recognized to play an important role in fault zone strength and slip behavior. Due to their interaction with fluids, hydrological properties change and lead to a weakening of the fault. Some of the weakest clay gouge has been recovered from SAFOD core along the active Central Deforming Zone of the San Andreas Fault, with a coefficient of friction as low as µ=0.095. We conducted laboratory shearing experiments to investigate the microstructural fabric development and frictional behavior of powders and natural intact fault gouge to characterize differences in mechanical and hydrological properties caused by varying smectite clay fabrics. In this study we used almost pure clay powder from SAF gouge (<63 µm) and mixed it with varying amounts of quartz or SAF clasts (120-500 µm). We sheared gouge layers in a double-direct shear configuration housed in a pressure vessel, at an effective normal stress of σ'_n = 5 MPa and under saturated conditions with a pore pressure of 2 MPa. We used a relatively low normal stress to allow recovery of material after the experiments for microstructural investigation. We sheared the layers at a constant velocity of 10 µm/s, followed by a series of slide-hold-slide tests to characterize frictional properties. After the experiments all samples were analyzed using a focused ion beam - scanning electron microscope (FIB-SEM) in order to reconstruct the 3D microstructures associated with experimentally induced polished slip surfaces at the micro- to nanometer scale. Initial results document clear stages of clay fabric development related to the clay to clast ratio, which exhibit similarities with naturally developed fabrics. In smectite-dominated mixtures, principle slip surfaces develop parallel to each other, but these features are modified as the clast content increases. With higher clay content strain hardening becomes more prominent. Clay minerals and fabric developed during shear have a strong effect on gouge mechanical and hydrological properties, and therefore likely play a key role in controlling fault gouge slip behavior at seismogenic depth.