Downloaded from lithosphere.gsapubs.org on August 2, 2011 Lithosphere Arkosic rocks from the San Andreas Fault Observatory at Depth (SAFOD) borehole, central California: Implications for the structure and tectonics of the San Andreas fault zone Sarah Draper Springer, James P. Evans, John I. Garver, David Kirschner and Susanne U. Janecke Lithosphere 2009;1;206-226 doi: 10.1130/L13.1 Email alerting services click www.gsapubs.org/cgi/alerts to receive free e-mail alerts when new articles cite this article Subscribe click www.gsapubs.org/subscriptions/ to subscribe to Lithosphere Permission request click http://www.geosociety.org/pubs/copyrt.htm#gsa to contact GSA Copyright not claimed on content prepared wholly by U.S. government employees within scope of their employment. 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Notes © 2009 Geological Society of America Downloaded from lithosphere.gsapubs.org on August 2, 2011 Arkosic rocks from the San Andreas Fault Observatory at Depth (SAFOD) borehole, central California: Implications for the structure and tectonics of the San Andreas fault zone Sarah Draper Springer,1* James P. Evans,1 John I. Garver,2 David Kirschner,3 and Susanne U. Janecke1 1DEPARTMENT OF GEOLOGY, UTAH STATE UNIVERSITY, LOGAN, UTAH 84322-4505, USA 2DEPARTMENT OF GEOLOGY, UNION COLLEGE, SCHENECTADY, NEW YORK 12308-3107, USA 3EARTH AND ATMOSPHERIC SCIENCES, SAINT LOUIS UNIVERSITY, 3507 LACLEDE AVENUE, SAINT LOUIS, MISSOURI 63103, USA ABSTRACT The San Andreas Fault Observatory at Depth (SAFOD) drill hole encountered indurated, high-seismic-velocity arkosic sedimentary rocks west of the active trace of the San Andreas fault in central California. The arkosic rocks are juxtaposed against granitic rocks of the Salin- ian block to the southwest and against fi ne-grained Great Valley Group and Jurassic Franciscan rocks to the northeast. We identify three distinct lithologic units using cuttings, core petrography, electrical resistivity image logs, zircon fi ssion-track analyses, and borehole-based geophysical logs. The upper arkose occurs from 1920 to 2530 m measured depth (mmd) in the borehole and is composed of fi ve structural blocks defi ned by bedding orientations, wireline log character, physical properties, and lithologic characteristics. A clay-rich zone between 2530 and 2680 mmd is characterized by low Vp and an enlarged borehole. The lower arkose lies between 2680 and 3150 mmd. Fission-track detrital zircon cooling ages are between 64 and 70 Ma, appear to belong to a single population, and indicate a latest Cretaceous to Paleogene maximum depositional age. We interpret these Paleocene–Eocene strata to have been deposited in a proximal submarine fan setting shed from a Salinian source block, and they correlate with units to the southeast, along the western and southern edge of the San Joaquin Basin, and with arkosic conglomerates to the northwest. The arkosic section constitutes a deformed fault-bounded block between the modern strand of the San Andreas fault to the northeast and the Buzzard Canyon fault to the southwest. Signifi cant amounts of slip appear to have been accommodated on both strands of the fault at this latitude. LITHOSPHERE; v. 1; no. 4; p. 206–226, Data Repository 2009160. doi: 10.1130/L13.1 INTRODUCTION dicted the borehole would encounter Salinian 3157 and 3410 mmd appear to be a complex granitic rocks on the southwestern side juxta- zone of low-seismic-velocity rocks that are the The primary component of the San Andreas posed against a Franciscan complex northeast subject of other studies (Hickman et al., 2008). Fault Observatory at Depth (SAFOD) is a of a nearly vertical San Andreas fault (Hickman The presence of arkosic sedimentary rocks 3-km-deep deviated borehole that crosses the et al., 2004; McPhee et al., 2004). A vertical on the southwestern side of the fault and the San Andreas fault zone in central California pilot hole drilled in 2002 went through a 1 km presence of Great Valley sedimentary rocks (Hickman et al., 2004, 2005) (Fig. 1). Numer- thick Cenozoic sedimentary section overlying northeast of the fault, and the apparent juxta- ous questions about fault behavior, structure, Salinian granite, consistent with the geologic position of Salinian granitic rocks against the composition, physical properties, and fl uid-rock model. In 2004 and 2005, phases 1 and 2 of the arkosic rocks, pose several geological and tec- interactions in fault zones are being addressed main SAFOD hole were drilled, and at 1920 m tonic questions regarding fault zone structure in this project (i.e., Boness and Zoback, 2006; measured depth (mmd—the distance measured and the tectonic evolution of the San Andreas Solum et al., 2006; Erzinger et al., 2006; Hole along the borehole), the borehole encoun- fault in this area. These questions include the et al., 2006; Schleicher et al., 2006; Bradbury tered an abrupt change from Salinian grano- following: What is the origin of the arkosic sedi- et al., 2007). diorite into arkosic sedimentary rocks where mentary rocks—are they a part of the Salinian A comprehensive suite of geophysical the borehole is deviated northeast (Bradbury block? Have these rocks been transported a long data and geological studies (see Geophysical et al., 2007). This section of arkose is present distance along older strands of the San Andreas Research Letters special volume 31, numbers from 1920 mmd to 3157 mmd along the track fault? The offset of the Salinian block is distrib- 12 and 15, 2004; Dibblee, 1971; Sims, 1990; of the borehole. At 3157 mmd the borehole uted over several different parallel strike-slip Page et al., 1998; Thayer and Arrowsmith, encountered fi ne-grained sandstones and mud- fault systems (Graham, 1978; Dickinson and 2005; M. Rymer, 2005, personal commun.) was stones, and at 3360 mmd it encountered rocks Butler, 1998; Whidden et al., 1998; Dickinson used to develop a subsurface model that pre- that are now identifi ed as Cretaceous Great Val- et al., 2005), which accounts for the ~500 km ley Group (Evans et al., 2005) based on their offset from the Sierra Nevada plutonic complex. *Present address: Chevron International Explora- texture, composition, and the presence of Late In the vicinity of the SAFOD site the Buzzard tion and Production, 1500 Louisiana Street, Houston, Cretaceous Great Valley fossils (K. McDougall, Canyon fault is subparallel to the San Andreas Texas 77002, USA 2006, written commun.). The rocks between fault, having an unknown amount of slip 206 For permission to copy, contact [email protected] | © 2009 Geological Society of America Downloaded from lithosphere.gsapubs.org on August 2, 2011 Arkosic rocks from the SAFOD borehole | RESEARCH Tu (Rymer et al., 2003; Thayer and Arrowsmith, KJf 36°02′30″ KJf 2005). The Buzzard Canyon fault exposed at the Tm surface has been correlated to the fault that sepa- Tm rates the granodiorite from the arkosic rocks in the SAFOD borehole (Bradbury et al., 2007). KJf Arrowsmith (2007) suggests that several faults QTp lie southwest of the San Andreas fault, and these Tm faults may have accommodated a signifi cant Tu Qls amount of the total slip across the San Andreas Kgv? fault plate boundary in central California. Te Tm This paper provides the fi rst detailed lithologic Qal Tu characterization of the arkosic rocks as a fi rst San Tu Qal step toward understanding their geologic history Andreas Tu and answering the questions posed above. The Fault Tm Qls Zone composition of the arkosic rocks is evaluated to Tsm KJf/Qls interpret provenance, depositional environment, 36° TMT Kp and diagenetic history. The observations of com- QTp position are then used to identify surface units KJf that may be equivalent to the SAFOD arkose BCFZ Te section. These possible correlations constrain QTp Kp the geometry and evolution of the San Andreas Tu QTp fault in the area of the SAFOD site. Tm Qal Tu Tvr SAFOD Te Methods drill holes Qal Qls QTp We use fi ve data sets to characterize and Qf interpret the arkosic sedimentary rocks: drill N GHF cuttings, spot and sidewall core, borehole geo- QTp Tsm Tu Qf physical logs, electrical image logs, and zircon Qal fi ssion-track analyses. Drill cuttings are silt- and Kg Qal 35°57′30″ sand-sized rock fragments created by rotary drill Tu Kg bit action on the rock at the bottom of the hole 1 km Qal QTp that circulate to the surface in the drilling mud, Tsm where they emerge as a mix of engineered drill- Qal ing mud and rock fragments. Cuttings composi- 120°33′30″ 120°30′ tion was examined as a function of depth along MAP LEGEND the borehole to identify major changes in wall SF = San Francisco Qal Alluvium rock composition. The spot cores range from LA = Los Angeles Qal centimeters to meters long, are 10–15 cm in SAF = San Andreas fault system Qls Landslide deposits = SAFOD site diameter, and were collected using a rotary cor- QfTt Alluvial fan = Salinian Block Quaternary ing tool bit at the end of the drilling string.