Print this pageAAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Densification and Alteration of Siliciclastics Adjacent to the San Andreas Fault at SAFOD: Implications for Seismic Velocity Structure and Fluid Flow
(1) Geology and Geophysics, Texas A&M University, College Station, TX.
Spot core from the San Andreas Fault Observatory at Depth (SAFOD) borehole provides the opportunity to quantify and characterize damage and mineral alteration of siliciclastics adjacent to an active, large displacement fault. Arkosic, coarse-grained, pebbly sandstone, and fine-grained sandstone and siltstone were retrieved from 2.55 km depth. The samples represent the damaged wall rock of the fault zone approximately 130 m west of the active creeping trace of the San Andreas Fault. In situ conditions at the cored depth include a temperature of approximately 90 °C, high magnitude differential stress directed at high angles to the fault, and a pore fluid of Ca-Na-K brine. Samples show evidence for densification through grain-scale fracture and crushing, as well as fluid-assisted processes of crack-sealing, dissolution-precipitation, and alteration-neocrystallization. The core is cut by numerous faults and displays evidence of distinct, repeating episodes of dilation and cementation. Subsidiary faults are grouped into three size classes: 1) small incipient faults, 1 to 2 mm wide, that record early fault development, 2) intermediate subsidiary faults, 2 to 3 mm wide, that show cataclastic grain size reduction and flow, extensive cementation, and alteration of host particles, and 3) large subsidiary faults that have cataclastic zones up to 10 mm wide. Although the small faults show preferred orientations consistent with the in situ stress, the microscale deformation away from the small faults appears to record more isotropic consolidation. Petrographic study, including particle size analysis, was facilitated by back-scattered electron imaging to distinguish fractures and grain characteristics, and elemental mapping to distinguish phases. The mineral assemblage is dominated by quartz, oligoclase, potassium feldspar, and new albite and laumontite. In general, grain shapes and size distributions indicate K-spar and quartz are primarily deformed through fracture, whereas oligoclase has altered producing albite and extensive cementation by laumontite. Current work is aimed at 1) quantifying the evolution of particle size reduction by mineral alteration, mineral growth, and comminution, 2) determining whether cements are locally derived or reflect advective transport along the damaged zone of the fault, and 3) quantifying the degree to which changes in seismic velocity across the fault result from mineral alteration, densification, and dilatant fracturing.