Convergence rates across the Ventura Basin, California

Four cross sections are balanced and retrodeformed to 250±50 ka and 975±75 ka to yield crustal shortening amounts and rates for the western Transverse Ranges of California. The cross sections compare the shortening that occurs along a transfer zone in which displacement is transferred eastward from a surface reverse fault (Red Mountain fault) to a blind thrust, to a combination of both a surface reverse fault (San Cayetano fault) and the blind thrust, and to a surface reverse fault (Modelo lobe segment of the San Cayetano fault). Deformation can be separated into three phases: (1) pre- Vaqueros (pre-late Oligocene-early Miocene) tilting in the hanging-wall block of the Oak Ridge fault that is coincident with normal faulting farther south at Big Mountain and to the east in the east Ventura basin, (2) Pliocene reverse faulting and folding, and (3) Quaternary deformation. Crustal shortening rates have increased through time. For the four cross sections, crustal shortening rates were 4±4 mm/y, 5±2 mm/y, 6±5 mm/y, and 14±6 mm/y for the interval between 250 ka and 975 ka. Since 250±50 ka, crustal shortening rates increased to 23±12 mm/y, 29±7 mm/y, 27±11 mm/y, and 25±11 mm/y. But crustal convergence rates determined by Global Positioning System (GPS) surveys, taken over an interval of 2.7 years, indicate a shortening rate of only 7±2 mm/y across the basin. The discrepancy between a rate of strain over a short, recent time period of little seismic activity, and a faster rate determined by the offset of bedrock horizons over several hundred thousand years indicates that much of the present crustal movement is being stored as elastic strain that could result in the release of energy in damaging earthquakes

IODP Expedition 334: An Investigation of the Sedimentary Record, Fluid Flow and State of Stress on Top of the Seismogenic Zone of an Erosive Subduction Margin

The Costa Rica Seismogenesis Project (CRISP) is an experiment to understand the processes that control nucleation and seismic rupture of large earthquakes at erosional subduction zones. Integrated Ocean Drililng Program (IODP) Expedition 334 by R/V JOIDES Resolution is the first step toward deep drilling through the aseismic and seismic plate boundary at the Costa Rica subduction zone offshore the Osa Peninsula where the Cocos Ridge is subducting beneath the Caribbean plate. Drilling operations included logging while drilling (LWD) at two slope sites (Sites U1378 and U1379) and coring at three slope sites (Sites U1378–1380) and at one site on the Cocos plate (Site U1381). For the first time the lithology, stratigraphy, and age of the slope and incoming sediments as well as the petrology of the subducting Cocos Ridge have been characterized at this margin. The slope sites recorded a high sediment accumulation rate of 160–1035m m.y.-1 possibly caused by on-land uplift triggered by the subduction of the Cocos Ridge. The geochemical data as well as the in situ temperature data obtained at the slope sites suggest that fluids are transported from greater depths. The geochemical profiles at Site U1381 reflect diffusional communication of a fluid with seawater-like chemistry and the igneous basement of the Cocos plate (Solomon et al., 2011; Vannucchi et al., 2012a). The present-day in situ stress orientation determined by borehole breakouts at Site U1378 in the middle slope and Site U1379 in the upper slope shows a marked change in stress state within ~12 km along the CRISP transect; that may correspond to a change from compression (middle slope) to extension (upper slope)

Southern California Earthquake Center Geologic Vertical Motion Database

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/94655/1/ggge1319.pd

Unified Structural Representation of the southern California crust and upper mantle

We present a new, 3D description of crust and upper mantle velocity structure in southern California implemented as a Unified Structural Representation (USR). The USR is comprised of detailed basin velocity descriptions that are based on tens of thousands of direct velocity (Vp, Vs) measurements and incorporates the locations and displacement of major fault zones that influence basin structure. These basin descriptions were used to developed tomographic models of crust and upper mantle velocity and density structure, which were subsequently iterated and improved using 3D waveform adjoint tomography. A geotechnical layer (GTL) based on Vs30 measurements and consistent with the underlying velocity descriptions was also developed as an optional model component. The resulting model provides a detailed description of the structure of the southern California crust and upper mantle that reflects the complex tectonic history of the region. The crust thickens eastward as Moho depth varies from 10 to 40 km reflecting the transition from oceanic to continental crust. Deep sedimentary basins and underlying areas of thin crust reflect Neogene extensional tectonics overprinted by transpressional deformation and rapid sediment deposition since the late Pliocene. To illustrate the impact of this complex structure on strong ground motion forecasting, we simulate rupture of a proposed M 7.9 earthquake source in the Western Transverse Ranges. The results show distinct basin amplification and focusing of energy that reflects crustal structure described by the USR that is not captured by simpler velocity descriptions. We anticipate that the USR will be useful for a broad range of simulation and modeling efforts, including strong ground motion forecasting, dynamic rupture simulations, and fault system modeling. The USR is available through the Southern California Earthquake Center (SCEC) website (http://www.scec.org)

IODP expedition 334: An investigation of the sedimentary record, fluid flow and state of stress on top of the seismogenic zone of an erosive subduction margin

金沢大学理工研究域地球社会基盤学

The geotectonics and geotechnics of Traveston Crossing Dam foundation

Traveston Crossing Dam is proposed for construction at AMTD 207.6 km on the Mary River about 25 km upstream of Gympie in South East Queensland. The Mary Valley at the damsite is located in a zone of complex geology resulting from formation in a tectonic accretionery wedge setting. This has been responsible for a complex current geological setting which has required a range of geological/geotechnical investigation and interpretation techniques to develop a model on which to base the dam's preliminary design. This paper describes the tectonic history and the innovative techniques used in developing the geological model for the dam foundation. The investigation involved the following specific investigative techniques; aerial photograph interpretation, geological mapping, geotechnical drilling including water pressure testing, seismic refraction profiling, downhole geophysical logging, excavation and geological mapping of large excavations, and hydrogeological investigation involving investigative drilling and pumping tests. A Vulcan 3D computerised geological model was constructed using borehole data, seismic refraction interpretation and downhole geophysics interpretation. The geological model has been used in the development of the preliminary design and confirms that the foundations are suitable for the proposed structure

plate1-XVI.jpg

The Upper Ojai Valley is a tectonic depression between opposing reverse faults, Its northern border is formed by the active, north-dipping San Cayetano fault with 6.0 km of dip-slip displacement in the Silverthread oil field and 2.6 km displacement west of Sisar Creek; the fault dies out farther west in Ojai Valley. The southern border is formed by the late Quaternary Sisar-Big Canyon-Lion fault set which dips south and merges into the Sisar decollement within the south-dipping, ductile Rincon Formation. Folds with north-dipping axial planes, including the Lion Mountain anticline and Reeves syncline, are middle Pleistocene or older and are related to movement on a frontal strand of the San Cayetano fault. In late Quaternary time, the Sulphur Mountain anticlinorium and the Big Canyon syncline began forming as fault-propagation folds, followed closely by the ramping of the south-dipping faults to the surface over the Saugus Formation. To the east, the San Cayetano fault locally overrides and folds the south-dipping faults. Cross-section balancing shows that the Miocene and younger rocks above the Sisar decollement are shortened 6.7km more than the more, competent rocks below. A solution to this bed-length problem is that the decollement becomes a ramp and merges at depth with the steeply south-dipping Oak Ridge fault. This implies that the Sisar, Big Canyon, and Lion faults are frontal thrusts to the Oak Ridge fault. The total horizontal shortening since Pliocene time is 14.5km. Recently-drilled wells in the Chaffee Canyon oil field, Ventura County, California, reveal that the Wiley Canyon producing anticline formed in the Pleistocene prior to much of the displacement on the Oak Ridge fault. The east and west plunge in part predates deposition of the Vaqueros Formation of early Miocene age. The dip on the south strand of the Oak Ridge fault increases eastward across the field from 70-75° to 83-85°; farther east, the fault plane is overturned and dips north. The Torrey fault can be traced northwest under a landslide east of Wiley Canyon anticline but not farther northwest

Plate_III.jpg

Four cross sections are balanced and retrodeformed to 250±50 ka and 975±75 ka to yield crustal shortening amounts and rates for the western Transverse Ranges of California. The cross sections compare the shortening that occurs along a transfer zone in which displacement is transferred eastward from a surface reverse fault (Red Mountain fault) to a blind thrust, to a combination of both a surface reverse fault (San Cayetano fault) and the blind thrust, and to a surface reverse fault (Modelo lobe segment of the San Cayetano fault). Deformation can be separated into three phases: (1) pre- Vaqueros (pre-late Oligocene-early Miocene) tilting in the hanging-wall block of the Oak Ridge fault that is coincident with normal faulting farther south at Big Mountain and to the east in the east Ventura basin, (2) Pliocene reverse faulting and folding, and (3) Quaternary deformation. Crustal shortening rates have increased through time. For the four cross sections, crustal shortening rates were 4±4 mm/y, 5±2 mm/y, 6±5 mm/y, and 14±6 mm/y for the interval between 250 ka and 975 ka. Since 250±50 ka, crustal shortening rates increased to 23±12 mm/y, 29±7 mm/y, 27±11 mm/y, and 25±11 mm/y. But crustal convergence rates determined by Global Positioning System (GPS) surveys, taken over an interval of 2.7 years, indicate a shortening rate of only 7±2 mm/y across the basin. The discrepancy between a rate of strain over a short, recent time period of little seismic activity, and a faster rate determined by the offset of bedrock horizons over several hundred thousand years indicates that much of the present crustal movement is being stored as elastic strain that could result in the release of energy in damaging earthquakes