An Electron Backscatter Diffraction and Cathode-Luminescence Study of
Microstructures in the Damage Zone and the Actively Creeping Core of the
San Andreas Fault Zone
Abstract
Using Electron Backscattered Diffraction (EBSD), we conducted a
preliminary study on 6 core samples from the San Andreas Fault
Observatory at Depth (SAFOD) between 3193-3311m Measured Depth. The
selected samples represent areas of the fault zone in contact with the
actively creeping Central Deforming Zone (CDZ) and Southwestern
Deforming zone (SDZ), with one sample from within the CDZ. The samples
were selected on the basis of containing significant pyrite, and
evidence of multiphase calcite-vein-formation from cathodoluminescence
(CL) imaging. EBSD results identify primary pyrite in the form of
framboids, pyrite-cemented framboid aggregates and spherulitic pyrite
overgrowths. Secondary (hydrothermal) pyrite forms a 15x7mm lenticular
mass in one sample and records abundant evidence of core-and-mantle
subgrain-rotation (SGR) deformation mechanisms in pyrite (Fig. 1),
indicating elevated temperatures >400° C. We find a second
generation of both plastically deformed and undeformed secondary pyrite
that overgrows calcite-filled fractures in boudinaged
Dauphiné-twinned-quartz clasts. Undeformed second-phase pyrite is also
associated in the main secondary pyrite mass with calcite filled
extension-fractures kinematically compatible with extension fractures in
boudinaged quartz. Low-T pyrite cataclasis is also evident. Multiple
generations of e-twinned calcite veins are revealed by CL (Fig. 1). The
twin-density paleo-piezometry suggests differential stresses of 25-233
MPa, consistent with previous estimates and with the present-day
temperature at the SAFOD. However, EBSD also reveals abundant evidence
for dislocation-creep in calcite, commonly including the youngest veins
in the samples. While dislocation creep deformation is possible in
calcite at SAFOD depth, it would involve very high CRSS. Pyrite clearly
records multiple episodes of formation and deformation during decreasing
T from >400° C to present SAFOD conditions. Analysis of the
deformation microstructures in this study also suggests a similar P-T
history for calcite and quartz. Preliminary interpretation of the
results in this study are: 1. SAFOD damage zone rocks close to SDZ and
CDZ deformed, at least episodically, while being exhumed and 2: The
deformation involved neo-mineralization and fluid overpressure events.