Geology Presentations Copyright (c) 2013 Utah State University All rights reserved. http://digitalcommons.usu.edu/geology_pres Recent documents in Geology Presentations en-us Wed, 17 Apr 2013 13:35:17 PDT 3600 Rock Properties and Internal Structure of the San Andreas Fault Near ~ 3 km Depth in the SAFOD Borehole Based on Meso- to Micro-scale Analyses of Phase III Whole Rock Core http://digitalcommons.usu.edu/geology_pres/5 http://digitalcommons.usu.edu/geology_pres/5 Wed, 13 Feb 2013 14:35:22 PST We examine the relationships between rock properties and structure within ~ 41 m of PHASE III whole-rock core collected from ~ 3 km depth along the SAF in the San Andreas Fault Observatory at Depth (SAFOD) borehole, near Parkfield, CA.

]]> Kelly Keighley Bradbury et al. Composition and Structure of the San Andreas Fault Observatory at Depth (SAFOD) Phase III Whole-Rock Core: Implications for Fault Zone Deformation and Fluid-Rock Interactions http://digitalcommons.usu.edu/geology_pres/4 http://digitalcommons.usu.edu/geology_pres/4 Wed, 13 Feb 2013 14:35:19 PST We examine the composition and texture of whole-rock core from ~ 3 km depth in the San Andreas Fault Observatory at Depth (SAFOD) borehole, which provides a unique opportunity to characterize in situ rock properties of the near-fault environment, and how these properties vary in an area where deformation is accommodated by aseismic creep and high-rates of microseismicity. Detailed petrography and microstructural analyses coupled with X-Ray Diffraction and X-ray Fluorescence techniques are used to describe composition, alteration, and textures.

All samples record multiple generations of cataclastic deformation in a complexly deformed and altered sequence of fine-grained sheared rocks. Localized shears bound multi-layered zones of medium to ultra-fine grained cataclasite. Phacoidal clasts or porphyroclasts comprised of serpentinite, quartz, and older cataclasite are embedded within the comminuted phyllosilicate-rich gouge. The intensity of damage-related features and the development of a pervasive anastomosing fabric increases towards and within the two active slip zones near ~ 3192 and 3302 m MD. Foliated fabrics alternating with discrete fractures suggest a mixed-mode style of deformation including both ductile flow and brittle deformation processes during fault zone evolution. Deformation at high-strain rates is suggested by the presence of crack-seal veins in clasts, the presence of porphyroclasts, and the development of S-C fabrics in the phyllosilicate- rich gouge.

Evidence for fluid-rock interaction across the fault zone is indicated by depletion of Si and enrichment of MgO, FeO, and CaO; with significant clay alteration and/or growth of neo-mineralized vein fillings and fracture surface coatings. Shear localization may decrease porosity and inhibit fluid flow whereas fracturing may locally facilitate fluid migration and/or chemical alteration within the fault zone. These results constrain hypotheses related to fault zone behavior and broaden our understanding of the processes controlling earthquake nucleation and/or energy adsorption within the SAF. Based on the similarity of our observations to previous results from surface exposures of the SAF, we emphasize the importance of exhumed fault zone studies as proxies for understanding deformation and seismicity in the shallow crust.

]]> Kelly Keighley Bradbury et al. Preservation of the Seismic Cycle in a Continental Low-Angle Normal Fault: West Salton Detachment Fault, USA http://digitalcommons.usu.edu/geology_pres/3 http://digitalcommons.usu.edu/geology_pres/3 Wed, 13 Feb 2013 07:44:07 PST Mitchell R. Prante et al. Multiple Deformation Mechanisms Operating at Seismogenic Depths: Tectonic Pseudotachylyte and Associated Deformation From the Central Sierra Nevada, California http://digitalcommons.usu.edu/geology_pres/2 http://digitalcommons.usu.edu/geology_pres/2 Wed, 13 Feb 2013 07:44:06 PST Mitchell R. Prante et al. Discrete brittle to distributed shearing; Results from analysis of the deep portions of the Cajon Pass Drill Hole http://digitalcommons.usu.edu/geology_pres/1 http://digitalcommons.usu.edu/geology_pres/1 Tue, 12 Feb 2013 07:48:34 PST We performed systematic structural and geochemical analyses on a suite of cored rocks from the vertical Cajon Pass, California drill hole to characterize the deformation and alteration of fault-related rocks. The drill hole lies 4 km northeast of the San Andreas Fault (SAF), and observations of deformed crystalline rock in core and outcrop provide a sample of a 5-km vertical column adjacent to the steeply dipping Cleghorn fault and span the brittle to semi-brittle deformational regime at hydrothermal conditions. The rocks in the upper 500 m of the borehole are composed of sandstones and granitoid augen gneiss, with narrow fault and fracture zones coated with thin seams of laumontite. Below 500 m depth in the core, tonalite gneiss and migmatite contain well-developed discrete brittle faults and fracture zones. Thirty-seven faults are recognized in the core and borehole data; eleven are newly identified here, eight were previously identified in the core, and the remainder are interpreted from borehole image log data. The size of the fault zones intersected by the core controls the extent and nature of deformation. Distribution of faults in the core increase with depth, and fracture densities are greater around fault zones. In the upper 2600 m of the hole, the faults and fractures are typically narrow with thin coatings of alteration products. Prominent fault zones at 2100 - 2300 and 2500 - 2600 m measured depth dip moderately to steeply, and within this fault distributed shearing and alteration textures are common. Microstructures in these fault zones primarily include shear fractures containing a matrix of laumontite with angular to sub-angular clasts within the matrix and record multiple cycles of deformation and alteration. Laumontite mineralization indicates moderate- to high-temperature fluids interacting with the rocks throughout most of the column. The most significant fault observed in the core is an indurated, steep-dipping zone at 3,402 m depth that exhibits evidence of a mixture of brittle and semi-brittle deformation and abundant mineralization and alteration of potassium feldspar and epidote. This fault correlates well with the left-lateral steeply dipping Cleghorn fault, and reflects the interaction between hydrothermal metasomatic alteration and brittle fracture, cataclastic flow, to incipient plastic deformation processes at depth. The interpretation that the fault zone at the bottom of the hole is the Cleghorn fault agrees with stress orientation measurements made there by M D. Zoback and coworkers and indicates that the faults in the drill hole reflect active deformation and alteration associated with northeast-oriented maximum horizontal stress that may drive the left-lateral oblique motion on the Cleghorn fault. The data also show that damage zones associated with faults are present here, and may consist of mixed mode deformation, indicating a long-lived presence of the deformed and altered zones of reduced elastic moduli associated with faults. Simple modeling of the thermodynamics of syntectonic reactions in the fault zones indicate that earthquakes can be the source of heat to drive the reactions, and thus earthquake energy may be consumed in the fault core and damage zone by focused alteration.

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