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Rock Properties and Internal Structure of the San Andreas Fault Near Utah State University DigitalCommons@USU Geosciences Presentations Geosciences 12-2010 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 Kelly Keighley Bradbury Utah State University James P. Evans Utah State University Follow this and additional works at: https://digitalcommons.usu.edu/geology_pres Part of the Geology Commons Recommended Citation Bradbury, Kelly Keighley and Evans, James P., "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" (2010). Geosciences Presentations. Paper 5. https://digitalcommons.usu.edu/geology_pres/5 This Poster is brought to you for free and open access by the Geosciences at DigitalCommons@USU. It has been accepted for inclusion in Geosciences Presentations by an authorized administrator of DigitalCommons@USU. For more information, please contact [email protected]. TA41A-2099 Rock properties and Internal Structure of the San Andreas Fault at ~ 3km depth in the SAFOD Borehole: Mesoscopic to Microscopic Analyses of Phase III Whole Rock Core By Kelly Keighley Bradbury ([email protected]) and James P. Evans ([email protected]), Department of Geology, Utah State University, 4505 Old Main Hill, Logan, UT I. Introduction III. Mesoscopic to Microscopic Core-Based Studies V. Whole-Rock Geochemistry We examine the relationships between rock properties and structure within ~ 41 m Lithology & Mineralogical Composition Deformation-Related Features of PHASE III whole-rock core collected from ~ 3 km depth along the SAF in the San Hole E Core Images b c Lithologies encountered include (reported in measured core depths): d 3150 m 3192.5 m 3195.8 m Major Elements a Andreas Fault Observatory at Depth (SAFOD) borehole, near Parkfield, CA. Arkosic sandstones [3141 - 3144.5 m and 3145.8 - 3152.6 m] XRF results from SAFOD Sheared black silty shale [3144.5 - 3145.7 m] Direct mesoscale observations of the core are integrated with detailed petrography 1906 M 7.8 Black ultrafine-grained rocks [3192.9 – 3196.4 m] cuttings data for the main San Francisco Phyllosilicate-rich (± serpentinite-bearing) block-in-matrix or mélange and microstructural analyses coupled with X-Ray Diffraction and X-ray Fluorescence 1989 M 6.9 fault rocks [3186.7 - 3196.4 m; 3198.4 - 3311.8 m] borehole (D. Kirschner, NA Plate 3151 m techniques to document variations in composition, alteration, and structures that Alternating beds and/or blocks of siltstone, sandstone, and shale 3142 m 3144.6 m 3151 m SAFOD 2004;1966 M ~ 6.0 d 3196.7 m 3197.8 m pers. comm). Samples from may be related to deformation and/or fluid-rock interactions. 1857 M 7.9 [3294.9 – 3311.8 m ]. Hole G Core Images f 3297.8 m g h e g core area are highlighted in Across the low velocity zone (LVZ) defined by borehole geophysical data, lithologies Los Angeles % of Total Sampled Core Lithostratigraphic Units the outlined box and shown ~ 48 mm/yr 3% Lithic arkose are comprised of a heterogeneous sequence of fine-grained sandstones, siltstones, 1% 3305.7 m 3312.1 m 3.3% Feldspathic Arkose below. mudstones, and shale with block-in-matrix textures and pervasively foliated fabrics. 4% Silty black shale/mudstone 3192.7 m 3193.7 m 3193.7 m 7.6% Phyllosilicate-rich melange More competent clasts within the block-in-matrix materials exhibit pinch-and-swell Study Area Location 0.7% 3192.9 m Black cataclasite to ultracataclasite shaped structures with crosscutting veins that do not extend into the surrounding Siltstone with carbonate veining j Banded siltstone i LOI Sheared black silty shale phyllosilicate-rich matrix. 14% 16.6% Massive graywacke k Shear zone with veining (Sh1) Discrete microfaults with slickenlines to cm-wide zones of cataclasite Narrow fault strands at 3192 and 3302 m bound the LVZ and correspond to sites of active casing deformation (aseismic 6% Fault rocks (FZ1 and FZ2) Serpentinite (S) 3198.7 m and/or clay gouge, pinch-and swell clasts with extensional veins, lens- creep). Here, the rock consist of ~ 2 m thick serpentinite-bearing phyllosilicate gouge with a pervasive penetrative scaly clay 3300 m 3311 m 42.5% shaped (phacoidal) clasts with apparant structural order (Sills et al., fabric and phacoidal-shaped clasts. Bounding these two active slip surfaces are highly sheared and comminuted ultrafine- 2009), scaly cleavage, localized veining, black staining (see Janssen et grained black fault rocks with abundant calcite veins parallel and oblique to foliation orientation. al., 2010), mineralization (calcite, pyrite), neomineralized clay fracture ***In ~ 41 m total core have over 65% weak minerals/lithologies coatings (see Schleicher et al., 2010) clast or block Localized shear surfaces bound multi-layered zones of medium to ultra-fine grained cataclasite in the near-fault environ- Ultracataclasite ~ Black Rock 3193.9 ment and record multiple generations of brittle deformation processes. Fault Rocks and Alteration 3193.6 m X-Ray Diffraction of 3193.9 m Fault Gouge associated with SDZ casing deformation black cataclasite fault identified by Zoback et al. (2010) cataclasite slip localization rock at 3193.6 m Deformation at high-strain rates is suggested by the presence of crack-seal veins in clasts within the block-in-matrix materi- chrysotile gouge? measured core depth. Fault Gouge associated with CDZ casing deformation identified by Zoback et al. (2010) als, the presence of porphyroclasts, and the development of S-C fabrics in the phyllosilicate-rich gouge. Across the fault(s) Note accessory minerals. block or clast Calcite-rich vein within clast and related damage zones, foliated fabrics alternating with discrete fractures suggest a mixed-mode style of deformation XRF including both ductile and brittle deformation processes during the temporal and spatial evolution of the fault zone. 3192.7 m 3196.1 m 1 mm 1 mm 100 μm Quartz Si O2 block or clast garnet? basalt lithic Magnetite block or clast Evidence for fluid-rock interaction across the fault zone is indicated by depletion of Si and enrichment of MgO, FeO, and Fe +2 Fe2 +3 O4 LOI values range Quartz Si O2 Albite Si Al CaO; with significant clay alteration and/or growth of neo-mineralized vein fillings and fracture surface coatings. Shear local- X-Ray Diffraction of ( Na , Ca ) Al ( Si , Al )3 O8 Magnetite Sepiolite from 5-24% suggesting Fe +2 Fe2 +3 O4 black cataclasite fault Mg4 Si6 O15 ( O H )2 ·6 H2 O Chlorite-serpentine ization may decrease porosity and inhibit fluid flow whereas fracturing may locally facilitate fluid migration and/or chemical Kaolinite Carbon Values rock at 3193.9 m ( Mg , Al )6 ( Si , Al )4 O10 ( O H )8 Al2 Si2 O5 ( O H )4 significant hydration Kamacite? alteration within the fault zone. Results reveal the complex internal structure and fluid-rock interactions within the San An- Andradite measured core depth. ( Fe , Ni ) 3197.7 m 3297.9 m Ca3 Fe2 ( Si1.58 Ti1.42 O12 ) dreas Fault at shallow crustal levels and provide a geologic context, which can be used for further core-based studies and 1 mm 1 mm Palygorskite Mg Al Si4 O10 ( O H ) ·4 H2 O experimental analyses. reworked altered serpentinite clast cataclasite Summary Results: Trace Elements The penetrative scaly fabric within the phyllosilicate-rich block-in- matrix sequences is ubiquitous throughout the low velocity zone and SiK 100um AlK 100um 3297.8 m surrounds the main active slip surfaces and associated serpentinite- 200x kV:10.0 Tilt:0 200x kV:10.0 Tilt:0 X-Ray Diffraction of bearing clay fault gouges. 3297.7 m serpentinite-bearing S Ca II. Geologic and Geophysical Setting 1 mm 3297.8 m The penetrative and highly sheared nature of the fabric at the meso- fault gouge at scale, consisting of anastomosing slip surfaces weaving around foliated gouge altered clasts garnet porphyroclast 3297.8 m measure phacoidal shaped lozenges with striated and/or polished slip surfaces Surface Geology Borehole Geometry core depth. Note extends down to the microscale. Borehole Velocity Logs accessory minerals. Tu Some of these thin, anastomosing surfaces, may accommodate active KJf 36°02’30” & ! Tm 3000 m MD slip along the fault, while others may record continuous deformation 3 Tm Serp. Vs processes related to aseismic creep (Faulkner et al., 2003). Lithology Based on Cuttings 2.5 3298.4 m Saponite KJf 2 1 mm 500 μm Ca0.2 Mg3 ( Si , Al )4 O10 ( O H )2 ·4 H2 O QTp Development of this fabric in the phyllosilicate-rich rocks may also Buzzard Canyon Fault SAFSan Andreas Fault S K 100um CaK 100um Tm Southwest zone of Casing Clinochrysotile 200x kV:10.0 Tilt:0 200x kV:10.0 Tilt:0 SAFOD BCF Mg3 ( Si2-x O5 ) ( O H )4-4x enhance fluid-rock interactions and the rate of neomineralized growth Tu Qls 5 Kgv? 0 85° Q/T Vp Photomicrographs of samples from within the Te Tm 4 Deformation (SDZ) Ankerite by creating pathways for fluid flow (Schleichler et al., 2010) and might SEM and EDAX Images from the black cataclasite rock bounding sediments Ca ( Mg0.67 Fe0.33 +2 ) ( C O3 )2 Qal Tu 3 ~ 3192 m SDZ and CDZ regions of casing deformation highlight also help to focus or channelize fluids. The mixed fault gouge and Quartz the southwestern margin of the SDZ at 3193.9 m measured core Qal 500 Central zone of Casing related damage zone regions comprised of block-in-matrix materials San Andreas the nature of foliated fault gouge, melange texture, Si O2 Fault Zone TMT Qls Salinian Nontronite of varying strength may reflect heterogeneous continuous- depth granite Deformation (CDZ) areas of slip localization, and distinct mineral ( Na , Ca )0.3 Fe2 ( Si , Al )4 O10 ( O H )2 ·x H2 O KJf/Qls ~ 3302 m discontinuous deformation as described by Fagargeng and Sibson 36° 1000 QTp assemblages.
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