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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
  • Alan Boyle,
  • Jafar Hadizadeh
Alan Boyle
University of Liverpool

Corresponding Author:[email protected]

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Jafar Hadizadeh
University of Louisville
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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.