Calcite-sealed microbreccias and fracture networks in SAFOD gouge outside the active creep zones: A CL study

Abstract:

A suite of 11 samples from the San Andreas Fault Observatory at Depth (SAFOD) is studied using SEM Cathodoluminescence (CL) and optical imaging. The studied samples represent ‘currently inactive’ rocks of the San Andreas Fault (SAF) outside the actively creeping intervals known as the SDZ and CDZ. The samples were selected from SW side of the SDZ, NE side of the CDZ, and between the two zones, in 3188-3311 m Measured Depth interval. Our main objective is a preliminary interpretation of calcite-sealed breccias and fracture network microstructures with respect to their P-T conditions. The results are analyzed via trace element ratios, spectral luminescence measurements, and crosscutting relationship of the vein calcite. We noted that in the particular case of using crosscutting relationship in conjunction with contrasting calcite luminescence, the latter effect does not necessarily indicate temporally distinct vein-sealing events since in some samples the luminescence contrast boundaries do not coincide with crystal growth fronts, changes in twin spacing or any clear optical discontinuity. Furthermore, in some samples, veins with different luminescence are found mutually crosscutting, which indicates contemporaneous availability of calcite-saturated fluids with different trace element ratios (notably the Fe/Mn). The calcite crystallization is mostly by blocky and elongate blocky growth in open fractures of implosion microbreccias as well as foliation-normal antitaxial veins that define pressure solution cleavage. Microstructures indicating granular gouge fluidization and clay-rich injections in block-and-matrix gouge, Jigsaw puzzle textures, and secondary pyrite growths are found associated with the calcite veining. In a sample NE of the CDZ, we find apparent folding deformation of calcite veins with >3 micron twin lamellae. The results suggest that gouge microstructures outside the currently active creep zones, at least in part, represent microseismic activity and deformation mechanisms that are operative below the SAFOD depths within the currently established creep regime in the SAF.