Abstract The rate and location at depth of fault creep are important, but difficult to characterize, parameters needed to assess seismic hazard. Here we take advantage of the magnetic properties of serpentinite, a rock type commonly associated with fault creep, to model its depth extent along the Bartlett Springs fault zone, an important part of the San Andreas fault system north of the San Francisco Bay, California (western United States). We model aeromagnetic and gravity anomalies using geologic constraints along 14 cross sections over a distance of 120 km along the fault zone. Our results predict that the fault zone has more serpentinite at depth than inferred by geologic relationships at the surface. Existing geodetic models are inconsistent and predict different patterns of creep along the fault. Our results favor models with more extensive creep at depth. The source of the serpentinite appears to be ophiolite thrust westward and beneath the Franciscan Complex, an interpretation supported by the presence of antigorite, a high-temperature serpent ine mineral stable at depth, in fault gouge near Lake Pillsbury.
ABSTRACT The San Andreas fault system in northern California forms an 80–90 km wide zone of right‐lateral shear. Extensional tectonism within this broad shear zone is indicated by both Neogene silicic volcanic rocks that gradually young in the direction of shear propagation to the north‐west and by numerous Neogene faultbounded structural basins filled with thick non‐marine sequences. The Little Sulphur Creek basins, three well‐exposed 1·5–2 km wide pull apart basins within this shear system, have sedimentation patterns analogous to those of much larger pull‐apart basins. They were formed and subsequently deformed by east‐west extension and by north‐west to south‐east‐orientated right‐slip concurrently with basin filling. Palaeocurrent and maximum‐clast size data indicate both lateral sediment transport from fault‐bounded basin margins and longitudinal transport down the basin axes. The basins are filled primarily with coarse alluvial‐fan and streamflow deposits derived from a surrounding igneous, sedimentary, and metamorphic provenance. Two of the basins contain basin‐plain‐type lacustrine turbidites that grade laterally into distal alluvial fan, fan‐delta, and sublacustrine delta deposits. Talus deposits along the south‐west margin of the basins contain megabreccia indicative of active uplift. Structures indicative of dewatering, liquefaction, and slumping suggest penecontemporaneous tectonism.
Research Article| October 01, 1999 Middle Miocene paleotemperature anomalies within the Franciscan Complex of northern California: Thermo-tectonic responses near the Mendocino triple junction Michael B. Underwood; Michael B. Underwood 1Department of Geological Sciences, University of Missouri, Columbia, Missouri 65211 Search for other works by this author on: GSW Google Scholar Kevin L. Shelton; Kevin L. Shelton 1Department of Geological Sciences, University of Missouri, Columbia, Missouri 65211 Search for other works by this author on: GSW Google Scholar Robert J. McLaughlin; Robert J. McLaughlin 2U.S. Geological Survey, 345 Middlefield Road, M.S. 975, Menlo Park, California 94025 Search for other works by this author on: GSW Google Scholar Matthew M. Laughland; Matthew M. Laughland 3Mobil Research and Development, P.O. Box 650232, Dallas, Texas 75265-0232 Search for other works by this author on: GSW Google Scholar Richard M. Solomon Richard M. Solomon 4Geotechnology Inc., 2258 Grissom Drive, St. Louis, Missouri 63146 Search for other works by this author on: GSW Google Scholar Author and Article Information Michael B. Underwood 1Department of Geological Sciences, University of Missouri, Columbia, Missouri 65211 Kevin L. Shelton 1Department of Geological Sciences, University of Missouri, Columbia, Missouri 65211 Robert J. McLaughlin 2U.S. Geological Survey, 345 Middlefield Road, M.S. 975, Menlo Park, California 94025 Matthew M. Laughland 3Mobil Research and Development, P.O. Box 650232, Dallas, Texas 75265-0232 Richard M. Solomon 4Geotechnology Inc., 2258 Grissom Drive, St. Louis, Missouri 63146 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1999) 111 (10): 1448–1467. https://doi.org/10.1130/0016-7606(1999)111<1448:MMPAWT>2.3.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Tools Icon Tools Get Permissions Search Site Citation Michael B. Underwood, Kevin L. Shelton, Robert J. McLaughlin, Matthew M. Laughland, Richard M. Solomon; Middle Miocene paleotemperature anomalies within the Franciscan Complex of northern California: Thermo-tectonic responses near the Mendocino triple junction. GSA Bulletin 1999;; 111 (10): 1448–1467. doi: https://doi.org/10.1130/0016-7606(1999)111<1448:MMPAWT>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract This study documents three localities in the Franciscan accretionary complex of northern California, now adjacent to the San Andreas fault, that were overprinted thermally between 13.9 and 12.2 Ma: Point Delgada–Shelter Cove (King Range terrane); Bolinas Ridge (San Bruno Mountain terrane); and Mount San Bruno (San Bruno Mountain terrane). Vein assemblages of quartz, carbonate, sulfide minerals, and adularia were precipitated locally in highly fractured wall rock. Vitrinite reflectance (Rm) values and illite crystallinity decrease away from the zones of metalliferous veins, where peak wall-rock temperatures, as determined from Rm, were as high as 315 °C. The δ18O values of quartz and calcite indicate that two separate types of fluid contributed to vein precipitation. Higher δ18O fluids produced widespread quartz and calcite veins that are typical of the regional paleothermal regime. The widespread veins are by-products of heat conduction and diffuse fluid flow during zeolite and prehnite-pumpellyite–grade metamorphism, and we interpret their paleofluids to have evolved through dehydration reactions and/or extensive isotopic exchange with accreted Franciscan rocks. Lower δ18O fluids, in contrast, evolved from relatively high temperature exchange between seawater (or meteoric water) and basaltic and/or sedimentary host rocks; focused flow of those fluids resulted in local deposition of the metalliferous veins. Heat sources for the three paleothermal anomalies remain uncertain and may have been unrelated to one another. Higher temperature metalliferous fluids in the King Range terrane could have advected either from a site of ridge-trench interaction north of the Mendocino fracture zone or from a "slabless window" in the wake of the northward migrating Mendocino triple junction.A separate paradox involves the amount of Quaternary offset of Franciscan basement rocks near Shelter Cove by on-land faults that some regard as the main active trace of the San Andreas plate boundary. Contouring of vitrinite reflectance values to the north of an area affected by a.d. 1906 surface rupture indicates that the maximum dextral offset within the interior of the King Range terrane is only 2.5 km. If this fault extends inland, and if it has been accommodating most of the strike-slip component of San Andreas offset at a rate of 3–4 cm/yr, then its activity began only 83–62 ka. This interpretation would also mean that a longer term trace of the San Andreas fault must be nearby, either offshore or along the northeast boundary of the King Range terrane. An offshore fault trace would be consistent with peak heating of King Range strata north of the Mendocino triple junction. Conversely, shifting the fault to the east would be compatible with a slabless window heat source and long-distance northward translation of the King Range terrane after peak heating. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
This chapter contains sections titled: Introduction Franciscan Complex Coast Range Ophiolite-Great Valley Sequence Tectonic Interpretation Road Log--Day 1, June 28, 1988-- San Francisco to Healdsburg Road Log--Day 2, June 29, 1989-- Healdsburg-Arcata
Research Article| April 01, 1986 Paleogene accretion of Upper Cretaceous oceanic limestone in northern California William V. Sliter; William V. Sliter 1U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025 Search for other works by this author on: GSW Google Scholar Robert J. McLaughlin; Robert J. McLaughlin 1U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025 Search for other works by this author on: GSW Google Scholar Gerta Keller; Gerta Keller 2Department of Geological and Geophysical Sciences, Princeton University, Princeton, New Jersey 08544 Search for other works by this author on: GSW Google Scholar William R. Evitt William R. Evitt 3Department of Geology, Stanford University, Stanford, California 94305 Search for other works by this author on: GSW Google Scholar Geology (1986) 14 (4): 350–353. https://doi.org/10.1130/0091-7613(1986)14<350:PAOUCO>2.0.CO;2 Article history first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation William V. Sliter, Robert J. McLaughlin, Gerta Keller, William R. Evitt; Paleogene accretion of Upper Cretaceous oceanic limestone in northern California. Geology 1986;; 14 (4): 350–353. doi: https://doi.org/10.1130/0091-7613(1986)14<350:PAOUCO>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Blocks of pelagic limestone, tectonically incorporated into coastal terrane melange, contain planktonic foraminifers of early Campanian to middle Maestrichtian age (82-69 Ma) typical of low latitudes between 20°N and 20°S. Carbonate concretions from the sheared terrigenous matrix of the melange yield temperate, middle-latitude planktonic foraminifers and dinoflagellates of middle to late Eocene age (49-41 Ma). The pelagic limestone and the basaltic substrate presumably formed on the eastern flank of the Pacific-Farallon spreading ridge in a low-latitude zone of high productivity and migrated northward on the Farallon plate until about 40 Ma when they initially encountered terrigenous turbidites north of lat. 30°N. Northward translation and final accretion were completed by about 25 Ma. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
Abstract This tour begins and returns to Clearlake Highlands. The route for the trip is shown in Figure 1. Note that several stops on mis trip are described in detail in the field guide that is part of the Paper by Goff and Janik (this volume).
Research Article| August 01, 1983 Post-middle Miocene accretion of Franciscan rocks, northwestern California: Discussion and reply: Reply R. J. MCLAUGHLIN R. J. MCLAUGHLIN 1U.S. Geological Survey, Menlo Park, California 94025 Search for other works by this author on: GSW Google Scholar Author and Article Information R. J. MCLAUGHLIN 1U.S. Geological Survey, Menlo Park, California 94025 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1983) 94 (8): 1029–1031. https://doi.org/10.1130/0016-7606(1983)94<1029:PMAOFR>2.0.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation R. J. MCLAUGHLIN; Post-middle Miocene accretion of Franciscan rocks, northwestern California: Discussion and reply: Reply. GSA Bulletin 1983;; 94 (8): 1029–1031. doi: https://doi.org/10.1130/0016-7606(1983)94<1029:PMAOFR>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract No Abstract Available. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
Abstract An exposure of a creeping segment of the Bartlett Springs Fault (BSF), part of the San Andreas Fault system in northern California, is a ~1.5‐m‐wide zone of serpentinite‐bearing fault gouge cutting through Late Pleistocene fluvial deposits. The fault gouge consists of porphyroclasts of antigorite serpentinite, talc, chlorite, and tremolite‐actinolite, along with some Franciscan metamorphic rocks, in a matrix of the same materials. The Mg‐mineral assemblage is stable at temperatures above 250–300 °C. The BSF gouge is interpreted to have been tectonically incorporated into the fault from depths near the base of the seismogenic zone and to have risen buoyantly to the surface where it is now undergoing right‐lateral displacement. The ultramafic‐rich composition, frictional properties, and inferred mode of emplacement of the BSF serpentinitic gouge correspond to those of the creeping traces of the San Andreas Fault identified in the SAFOD (San Andreas Fault Observatory at Depth) drill hole. This suggests a common origin for creep at both locations. A tectonic model for the source of the ultramafic‐rich materials in the BSF is proposed that potentially could explain the distribution of creep throughout the northernmost San Andreas Fault system.
In the southern San Francisco Bay region of California, oblique dextral reverse faults that verge northeastward from the San Andreas fault experienced triggered slip during the 1989 M7.1 Loma Prieta earthquake. The role of these range-front thrusts in the evolution of the San Andreas fault system and the future seismic hazard that they may pose to the urban Santa Clara Valley are poorly understood. Based on recent geologic mapping and geophysical investigations, we propose that the range-front thrust system evolved in conjunction with development of the San Andreas fault system. In the early Miocene, the region was dominated by a system of northwestwardly propagating, basin-bounding, transtensional faults. Beginning as early as middle Miocene time, however, the transtensional faulting was superseded by transpressional NE-stepping thrust and reverse faults of the range-front thrust system. Age constraints on the thrust faults indicate that the locus of contraction has focused on the Monte Vista, Shannon, and Berrocal faults since about 4.8 Ma. Fault slip and fold reconstructions suggest that crustal shortening between the San Andreas fault and the Santa Clara Valley within this time frame is ∼21%, amounting to as much as 3.2 km at a rate of 0.6 mm/yr. Rates probably have not remained constant; average rates appear to have been much lower in the past few 100 ka. The distribution of coseismic surface contraction during the Loma Prieta earthquake, active seismicity, late Pleistocene to Holocene fluvial terrace warping, and geodetic data further suggest that the active range-front thrust system includes blind thrusts. Critical unresolved issues include information on the near-surface locations of buried thrusts, the timing of recent thrust earthquake events, and their recurrence in relation to earthquakes on the San Andreas fault.
