HR: 0800h
AN: T21A-0454    [Abstracts]
TI: Mesoscale Structure and Lithology of the SAFOD Phase I and II Core Samples
AU: * Almeida, R
EM: ralmeida@geo.tamu.edu
AF: Center for Tectonophysics, Department Geology & Geophysics, Texas A&M University, College Station, TX 77843-3115 United States
AU: Chester, J S
EM: chesterj@geo.tamu.edu
AF: Center for Tectonophysics, Department Geology & Geophysics, Texas A&M University, College Station, TX 77843-3115 United States
AU: Chester, F M
EM: chesterf@geo.tamu.edu
AF: Center for Tectonophysics, Department Geology & Geophysics, Texas A&M University, College Station, TX 77843-3115 United States
AU: Kirschner, D L
EM: dkirschn@eas.slu.edu
AF: Department Department of Earth and Atmospheric Science, Saint Louis University, St. Louis, MO 63103 United States
AU: Waller, T D
EM: tjwaller@neo.tamu.edu
AF: Center for Tectonophysics, Department Geology & Geophysics, Texas A&M University, College Station, TX 77843-3115 United States
AU: Moore, D E
EM: dmoore@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd, Menlo Park, CA 94025 United States
AB: Core was taken from the SAFOD drill hole during Phase I and Phase II drilling. Almost complete recovery was obtained from the Phase 1 cored intervals of 4798-4824' (1476-1484 m) and 10025-10063' (3085-3096 m) measured depth, MD, and 50% recovery was realized during Phase II from 13090-13116' (4028-4036 m) MD. Although fractured and locally fragmented during the coring process, the core samples are fairly continuous across each interval. After removal of the protective sleeves, we prepared wrap-around 1:1 map tracings of the core and characterized the lithologies and mesoscopic deformation features visible on the core surfaces. The 4798-4824' cored interval consists of a medium-grained hornblende-biotite granodiorite with leucocratic phenocrysts and lenses that are weakly foliated in some places. Small shears, fractures, and veins that record both high- and low-temperature deformation are present in this cored interval. The dominant brittle deformation features are a series of subvertical fractures and moderately dipping shears, both of which contain some secondary mineral fill and display centimeter(s)-thick halos of low-grade alteration and staining of the granodiorite host. The upper section of the 10025-10063' cored interval is composed of pebble conglomerate to coarse-grained arkosic sandstone with lithic fragments of granite, sandstone, siltstone, and volcanic clasts. The beds are massive, well-cemented, and contain rare cobble-sized clasts. The lower section is a fine-grained, well-cemented arkosic sandstone that grades down-hole into a fine- to very fine-grained siltstone. Bedding is indistinct throughout this section. This interval is crosscut by numerous fracture sets that record multiple stages of deformation and fluid infiltration. The dominant brittle features include irregular, somewhat diffuse cataclastic bands, up to two cm thick, that are oriented at high angles to the core axis, and thinner, dark colored shear fractures that are up to several mm thick. Shear fractures are common in the pebble conglomerate and coarse sandstone, and display consistent kinematic indicators and preferred orientations. Two minor faults are present in this depth interval. On the basis of juxtaposition of different rock types, these faults likely have displacements of at least several meters. The 13090-13116' interval consists of shale with a few thin beds of siltsone and very fine-grained sandstone. Graded bedding, fossils fragments, and bioturbated sections are present. Deformation features include numerous small veins, scaly fabric, and some polished slip surfaces. Overall, the deformation intensity of the cored intervals is consistent with the inferred location of the sampled sections along the margins of the San Andreas fault zone. These cored sections are being compared to FMI logs to determine the true orientation of fabric elements.
DE: 8010 Fractures and faults
DE: 8012 High strain deformation zones
DE: 8025 Mesoscopic fabrics
DE: 8106 Continental margins: transform
DE: 8164 Stresses: crust and lithosphere
SC: Tectonophysics [T]
MN: Fall Meeting 2005