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Fault Related Deformation of Fine Grained Clastics from the San Andreas Fault Observatory from Depth (Safod) Inferred from Ct Imaging

Sills, David 1; Chester, Judith S.1; Chester, Frederick M.1
1 Center for Tectonophysics and Department of Geology and Geophysics, Texas A&M University, College Station, TX.

Spot cores taken from the San Andreas Fault Observatory at Depth (SAFOD) provide an excellent opportunity to study deformed siliciclastic rocks associated with a long history of seismic and aseismic slip in a continental transform fault. The deviated borehole is located at the southern end of the creeping segment near Parkfield, CA, and transects the entire fault zone at a depth of 2.5 to 2.8 km. Drilling, geological, and geophysical data indicate that the fault is a several hundred meters thick zone of deformed rock containing two main, actively creeping, narrow gouge layers at 3194 and 3301 m measured depth (MD). The mesoscale fabric of the fault zone was characterized through X-ray Computed Tomography (CT) imaging of spot cores (100 mm diameter) taken from across the fault zone and spanning the two main gouge layers. Of the 63.7 m of spot core collected, 36.6 m taken from within the fault zone was scanned at 2 mm spacing with 0.25 mm resolution to render 3D volumes for analysis. Marked changes in lithology and structure occur across the main gouge layers and demarcate three distinct structural-lithic units within the fault zone. West of 3194 m MD is a broad zone of fractured arkosic sandstone that grades eastward into foliated cataclasites, analogous to that seen in exhumed traces of the SAF to the south. This damage zone is cut by microscale fractures and mesoscale conjugate shears that record contraction at high angles to the SAF, and displays evidence of pervasive secondary mineralization. In contrast, the unit between the two main gouge layers is composed of thinly bedded sandstone, siltstone and shale that has experienced bedding parallel extension via interlayer flow and boudinage along high-angle shears and extensional veins. The unit northeast of 3301 m gouge consists of highly fractured and locally microbrecciated siltstone and shale. All three units display mesoscale fabrics consistent with strike-slip, non-coaxial deformation. The different style of deformation likely reflects lithologic control as well as the juxtaposition of units with different burial and deformation history.

 

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009