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Covault et al, 2005, Cordilleran GSA oral presentation:
Sea-level and tectonic controls on late Quaternary sedimentation in San Diego Trough, offshore California
Abstract:
High-resolution deep-tow boomer seismic-reflection data are used to characterize late Quaternary (< 50 Ka) deposition in San Diego Trough, a structurally active basin located in the California Continental Borderland. Four modern submarine canyon-channel systems feed sediment into San Diego Trough: Newport, Oceanside, Carlsbad, and La Jolla, from north to south. Newport Canyon, which is more than 50 km north of San Diego Trough, contributes sediment longitudinally, whereas the other three systems are lateral sources. Contrary to typical depositional models in which coarse clastic supply dominates submarine fan deposition during marine lowstand, our examination of deposition in San Diego Trough during Holocene transgression reveals that two of the four canyons remain active. As sea level rose, Oceanside and Carlsbad Canyons were stranded on the outer shelf, deprived of littoral sediment. At present only Newport and La Jolla Canyons have their heads on the inner shelf and continue to feed sediment to their submarine turbidite channel extensions. Within San Diego Trough, all channels extending from these canyons have low relief levees. The juxtaposition of Newport and La Jolla Canyon sediment persisted throughout the latest Quaternary. Displacements along strike-slip faults and related pull-apart depressions and uplifted ridges considerably affected the sediment dispersal of the Newport and Carlsbad canyon-channel systems contributing sediment to San Diego Trough. Deformation approximately 40 km south of Newport Canyon ultimately led to the deflection of Newport channel 20 km to the west. This deflection resulted in Newport channel feeding northern San Diego Trough. Along the eastern side of San Diego Trough, anticlinal folding along a strand of the Coronado Bank fault resulted in blocking progradation of Carlsbad submarine fan and redirecting its sediment to the mid La Jolla Fan area. Late Quaternary deposition in San Diego Trough reveals a complex interplay of river- and littoral drift-fed canyon-channel systems prograding into an elongate structurally active deepwater basin.
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Nicholson et al, 2005, SCEC poster:
High-Resolution Stratigraphy and the Evolution of an Active Fault-Related Fold in 3D, Santa Barbara Channel, California
Abstract:
As part of a global climate study investigating the sedimentary record of Santa Barbara Basin, grids of high-resolution MMS analog, industry multichannel, and 2D high-resolution seismic reflection data (collected in separate cruises by the USGS and UCSB in 2002) are being used to correlate dated reference horizons from ODP Site 893 across the basin to the Mid-Channel Trend. The Mid-Channel Trend is an active oblique fold related to the offshore Oak Ridge fault system. Continuous late-Quaternary strata deposited in the deep paIeo-bathymetric basin were uplifted and folded across this Mid-Channel anticline. Extrapolation of available age data suggest that strata as old as ~450 ka (OIS 12) appear to be exposed at the seafloor where they are now accessible to piston coring. The mapped horizons (dated at ~110 ka and ~160 ka, and including sequence boundaries interpolated at ~320 ka, ~420 ka, and ~475 ka) provide the basis for modeling the evolution of the structure and stratigraphy in 3D, and to precisely locate suitable core sites. In late August 2005, gravity and piston cores-together with deep-towed chirp data-will be taken using the R/V Melville to survey and sample these horizons and their intervening sequences where they are expected to crop out over the Mid-Channel Trend. Subsequent analyses of the cores and chirp data will be used to verify the predicted outcrop pattern and the basin-wide sequence stratigraphic interpretation. Thus, in addition to its contributions to Quaternary climate history, this project will help document the nature, geometry and evolution of the Mid-Channel anticline, its relation to the oblique Oak Ridge fault, and the local interaction between tectonics, climate, and sea-level change. To date, our results show that the Mid-Channel Trend has propagated from east to west as previously proposed. South of Santa Barbara harbor, folding on the anticline began about 1 Ma, while 10 km farther west, folding began after ~475 ka. Our sequence of multiple mapped reference horizons documents a fairly complicated process of how slip on the deep fault system is transformed at shallow levels into fold growth as different strands and back-thrusts become active. The active offshore Oak Ridge fault is thus mostly blind, despite offsetting the unconformity created during the Last Glacial Maximum.
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Normark et al, 2005, Cordilleran GSA poster:
Natural hydrocarbon seeps on the inner shelf off Pt. Conception and western Santa Barbara Channel, California
Abstract:
A study of natural oil and gas seeps on the inner shelf of the western Santa Barbara Channel and at the southern end of the Santa Maria Basin, offshore California, was conducted in collaboration with the Minerals Management Service (MMS). The goal was to establish the geologic framework for, and to document the locations of, active seeps. The area surveyed forms a broad V-shaped swath 3 to 5-km wide both north and east of Pt. Conception. Each side of the V is about 15-km long, covering water depths between 40-150 m. Sidescan sonar with a resolution of 50-cm along-track, and 17-cm across-track, was collected throughout. For most of the survey, a chirp sonar system provided high-definition profiles of the underlying geology and seep structures. While seeps are found in areas of bare rock outcrop, eroded during the last sea-level transgression, many active seeps occur on mounds, which previous MMS work indicates are formed by tar residue and sand. These mounds range from a few meters across to large accumulations that can exceed a kilometer in width and 5 m height. We recognize them primarily just west of Pt. Conception, where they cover an irregular area of approximately 5 km2. Overall, a total of more than 100 likely active seeps were identified that are the targets for ongoing collection of gas and tar samples. This mapping activity is in support of an effort to establish chemical correlations between offshore active seeps and coastal oil residues as well as to measure the rate of natural seepage at individual sites that can be used to assess regional rates.
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Normark et al, 2005, Cordilleran GSA presentation:
Late Quaternary turbidite systems in Santa Monica Basin, offshore California
Abstract:
Santa Monica Basin is a tectonically active depression that is filling more rapidly than other offshore basins in the California Continental Borderland. Sediment accumulation rates during the last glacial maximum exceed 4m/103 yr, which is about an order of magnitude greater than for other basins. Four main submarine canyons feed sediment to the basin --- from west to east, Hueneme, Mugu, Dume and Santa Monica Canyons. An extensive multichannel and single-channel seismic-reflection profiling data set allows evaluation of the relative contribution of sediment from these four sources since about Marine Isotope Stage 12. In addition, high-resolution deep-tow boomer profiles, and recently available radiocarbon ages for sediment cores including those at ODP Site 1015, provide an opportunity to construct a detailed stratigraphic evolution of deep-water depositional systems in the basin during the last 30 ka. The dominant source for sediment in the Santa Monica Basin is the Santa Clara River and adjacent drainages, which feed both Hueneme and Mugu Canyons as well as three smaller delta-edge canyons between them. These canyons feed channel-levee complexes featuring high (>50-m relief) muddy levees and sandy floors. In contrast, Dume Canyon receives sediment from littoral drift and feeds a small channel with low (15-m relief) sandier levees; the sandy, prograding wedges of Dume Fan are generally buried by sediment coming from the Santa Clara river delta via Hueneme Canyon. Santa Monica Canyon, which is the longest canyon feeding the basin, has apparently not been a major conduit and its associated fan has the most limited expression of any of the turbidite systems in the basin. Rivers crossing the present Los Angeles Basin area appear to preferentially feed sediment farther south toward the Long Beach shelf area.
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Ryan et al, 2005, SCEC poster:
Quaternary Faulting in the Inner Southern California Borderland, Offshore San Diego County, California
Abstract:
Multichannel 2D seismic reflection (MCS) data collected by Western Geophysical in 1975 and recently made publicly available combined with high-resolution MCS, single-channel Huntec and Geopulse profiles collected by the USGS in 1999 and 2000 are used to address two key questions pertaining to offshore faulting in the southern California Borderland: 1) has the Oceanside detachment fault been reactivated as a thrust fault during the Quaternary, and 2) is the Palos Verdes fault zone continuous with the Coronado Bank fault zone south of La Jolla fan valley (LJFV)? The Oceanside detachment fault is best imaged north of San Mateo Point, where it dips landward (eastward), and prominent folds deform hanging wall rocks. However, the age of these folds is unknown. In some areas they appear to be onlapped by flat-lying basin sediment. South of San Mateo point, the Oceanside detachment is not as well defined in the MCS data, however, a prominent strike-slip fault, the San Onofre-Oceanside fault (after Fischer and Mills, 1991), is observed near the base of the continental slope. Near the southern termination of this fault offshore of Carlsbad, there is another zone of folding near the base of the slope. This zone of folding is coincident with the intersection of a narrow subsurface ridge that trends at a high angle and interacts with the margin. Recent motion of the Oceanside detachment as a thrust fault therefore appears to be limited to the area between Dana and San Mateo Points, and offshore of Carlsbad.
The Coronado Bank fault zone (CBFZ) has generally been mapped as a steeply dipping, NW-trending zone consisting of multiple strands that extend from south of the border to offshore of San Mateo Point. South of LJFV, the CBFZ is primarily transtensional and appears to terminate at the LJFV in a series of horsetail splays. Whether the CBFZ continues north of LJFV is problematic. North of the LJFV, the CBFZ forms a positive flower structure that can be mapped at least as far north as Oceanside. However, north of Oceanside, the fault zone is more discontinuous than to the south and in places, there is no strong physiographic expression of faulting. This contrasts with the Palos Verdes fault zone north of Lasuen Knoll, which shows clear evidence for recent faulting. Therefore, although the northern segment of the CBFZ may connect with the Palos Verdes fault zone, it does not appear to have a similar style of deformation and suggests that some of the net slip between LJFZ and Lasuen Knoll may be transferred to faults other than the CBFZ.
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Conrad et al, 2004, SCEC poster:
Characteristics of the Southern Part of the Palos Verdes Fault from High Resolution and Multichannel Seismic Reflection Data
Abstract:
The southern part of the Palos Verdes fault system (PVF) comprises a semi-continuous series of fault segments with various deformation styles extending from the Palos Verdes Peninsula to the Coronado Bank fault zone north of La Jolla Fan Valley (LJFV). South of the Palos Verdes Peninsula, multichannel seismic (MCS) data show that the PVF consists primarily of a single, well-defined fault trace trending about N30°W, which extends from the San Pedro shelf to the southern part of Lasuen Knoll (LK). In places, the fault branches into two or three strands near the surface. It is difficult to map the PVF system south of LK where it appears to merge into a diffuse zone of transtentional faults. The transition between the PVF and the Coronado Bank fault zone (CBF) occurs between LK and LJFV. Scant data coverage precludes identification of a clear, through-going fault trace, although we are able to map several short fault segments along strike of both the PVF and CBF. A positive flower structure comprised of three closely spaced parallel fault strands are mapped for 16 km north of LJFV. These strands trend N40°W along a bathymetric ridge. LJFV appears to mark a significant structural boundary, as fault traces cannot be confidently mapped across the canyon axis into the main CBF zone.
High-resolution seismic reflection data (Huntec) are interpreted to determine the recency of faulting of the PVF-CBF fault system. The PVF does not appear to be active beneath the San Pedro shelf or along the northern and western edge of LK (although this could reflect rapid reworking of surface traces in shallow water on San Pedro shelf and obscuration by slumps and/or channel features to the south). The only evidence of active slip is found along the southwestern edge of LK just north of where the fault merges into a broad transtensional zone. Recent offset on this segment of the PVF appears to die out south of LK, perhaps being transferred to smaller faults in the extensional zone and/or an active fault strand mapped east of the main strand of the PVF, which has a more westerly trend. Another anomalous 20-km-long single active fault strand is imaged on Huntec data about 10 km southeast of the main PVF strand. This fault trends more northerly than the PVF zone, actively cutting through young basin sediments with the eastern side down. The faults imaged in the PVF-CBF transition zone immediately north of La Jolla Canyon are most likely active as they offset the seafloor.
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Fisher et al, 2004, SSA poster:
Evaluating the Tsunamigenic Potential of Submarine Landslides Below Santa Barbara Channel, Southern California
Abstract:
Seismic-reflection, multibeam bathymetric, and chronostratigraphic data reveal the generation, timing, and emplacement processes of large landslides along the north side the Santa Barbara Channel, southern California. Age control from ODP Site 893 together with seismic-reflection data indicate that over the past 250 ka landslides have occurred consistently along one segment of the shelf break. These landslides directly underlie or make up the Goleta landslide complex, the largest in the region. Thrust-faulted, Miocene and older rocks are thought to underlie the part of the shelf break that spawned the landslide complex. What appears to be a talus deposit along the downthrown (south) side of these deep faults dates from about 350-250 ka. These observations indicate that the part of the shelf break that produced the Goleta landslide complex has shed mass-wasting debris into the basin for much of the late Quaternary.
Seismic-reflection data indicate that a shelf-edge delta of probable Quaternary age was the source material for the Goleta landslide. This landslide includes three main lobes. High-resolution seismic-reflection data and age dates from ODP Site 893 reveal that undeformed shallow sediment covering the northwest lobe dates from about 5-6 ka, suggesting that material making up the toe of this lobe was emplaced before about 6 ka ago. In contrast, high-resolution seismic-reflection data over the southeastern lobe show that the shallowest sediment in the lobe is deformed, perhaps meaning that this lobe is very young.
The Goleta landslide complex does not represent a single or even several sediment failures, isolated in time, but instead the complex is made up of deposits from a sequence of mass failures that occurred over a relatively long time period (about 300 ka). For the region of the Santa Barbara Channel, the tsunami hazard from submarine landslides should be evaluated with this in mind.
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Hein et al, IGC poster:
Methanogenic carbonates collected with gas hydrate from a mud volcano offshore Southern California
Abstract:
In July 2003, a 2.1 m piston core from 813 m water depth near the summit of a mud volcano located 24 km off shore of Los Angeles, California, contained gas hydrate in its lower 0.5 m. Bivalve mollusk shells and methanogenic carbonate occur above the hydrate in olive-green silty mud. Two types of methanogenic carbonate are present: (1) massive, recrystallized nodular masses of calcite, with an outer mm-thick sugary patina; and (2) a bivalve coquina with carbonate cement. Shells of living and dead Lucinoma aequizonatum (thick walled) and Vesicomya elongata (thin walled) were recovered, both of which are characteristic of methane-seep environments. L. aequizonatum also occurs in sulfide-rich habitats. Carbon isotope values (-46 to -58‰) clearly indicate that the oxidation of biogenic methane was the source of the carbonate carbon. Carbon isotopes (-17 to -19‰) from shells of both clam species also indicate a carbon source from oxidized methane, or possibly oxidized organic matter. This is the first reported occurrence of such 12C-rich shells from living clams. Strontium isotopes indicate that the Sr in both the carbonate calcite and aragonite shells was derived from seawater. Calculated temperatures of 2.5 to 10.0° C (assuming seawater δ18O) for calcite and aragonite formation indicate that they were not in equilibrium with the bottom-water temperature of 6.5° C. Six additional sediment cores obtained within 15 m of the hydrate-bearing core contained olive-brown silty mud with variable amounts of bivalve shells and methanogenic carbonate fragments, but no gas hydrate. Methane is the dominant gas collected, with minor hydrogen sulfide and trace amounts of heavier hydrocarbon gases. Sediment gas chemistry infers that the gas hydrate is a Structure I hydrate, although the hydrate was not preserved and analyzed directly. The above characteristics indicate a cold-seep environment with methane likely venting at the seafloor. The mud volcano, 300 m in diameter at the base, exhibits no structure on either deep-tow boomer or single-channel air-gun profiles, most likely because of its high gas content and sediment deformation. This diapiric structure cuts through well-bedded sediment in an area transitional between strike-slip motion along basin-slope faults and convergent motion farther north. The source of the gas forming the hydrate appears to be collecting in beds as shallow as 200 m below the regional seafloor.
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Hopkins et al, 2004, AGU poster:
A Test for Extending the High-Resolution Global Climate Record in Santa Barbara Basin
Abstract:
ODP Site 893 in Santa Barbara Basin recovered high-resolution global climate data extending back to ~160 ka at 200 m sub-seafloor. Safety concerns though have prevented deeper drilling at this site. Santa Barbara Basin is, however, tectonically active. As a result, continuous late-Quaternary strata deposited in the deep paleo-bathymetric basin were uplifted and folded across the Mid-Channel Trend, and strata as old as ~450 ka (OIS 12) appear to be exposed at the seafloor where they are now accessible to piston coring. This project will test the accessibility along the anticline of these older stratigraphic sequences through detailed basin correlation of high-resolution seismic stratigraphy and subsequent coring. In preparation for coring in summer 2005, grids of high-resolution MMS analog, industry multichannel, and 2D USGS high-resolution seismic reflection data (collected in 2002) are being used to correlate dated reference horizons at ~120 ka, ~160 ka and ~1 Ma along with several intervening seismic sequence boundaries across the Mid-Channel Trend. Results provide the basis for modeling the structure and stratigraphy in 3D, and to precisely locate suitable sites for coring. Subsequent core analyses will be used to verify the predicted outcrop pattern and basin-wide sequence stratigraphic interpretation. Thus, in addition to its contributions to Quaternary climate history, this project will help document the nature and evolution of the Mid-Channel anticline, and the local interaction between tectonics, climate, and sea-level change. To date, our results show that the Mid-Channel Trend has propagated from east to west as previously proposed. South of Santa Barbara harbor, folding on the anticline began about 1 Ma, while 10 km farther west, folding began after ~450 ka. Furthermore, our results confirm that older strata (extending back to inferred OIS 12) of the paleo-Santa Barbara Basin have been folded, and are present at or near the seafloor.
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Lee et al, 2004, AGU poster:
Size and Age Characteristics for West Coast Tsunamigenic Landslides
Abstract:
Multibeam bathymetric imagery is now available for a number of well-defined submarine landslide deposits along the west coast of the United States. Several of these landslides are known to have caused damaging tsunamis and others are of sufficient size to have generated tsunamis when they occurred, assuming that their motion was rapid. These failures are located off Palos Verdes Peninsula, California, and within Santa Barbara Channel, California, Commencement Bay, Washington, Resurrection Bay, Alaska, and Port Valdez, Alaska. For two of these failures, ages were determined by identifying acoustic reflectors in the vicinity of the failed masses that either clearly postdate or predate the landslide events. The ages of the reflectors are determined by tracing them to the locations of nearby ODP borings or to piston cores dated using radiocarbon methods. Three of the landslides produced tsunamis during historic time (post 1750 AD) so the ages are well constrained. High-resolution subbottom reflection profiles also allow us to estimate the dimensions of the failed masses. Although the examples selected clearly do not represent all scales of tsunamigenic west coast landslides, this information is useful in providing input to statistically based models of landslide-induced tsunamis.
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Normark et al, 2004, SCEC poster:
Late Quaternary Deformation in Santa Monica Basin Deposits Adjacent to Santa Cruz-Catalina Ridge, Offshore Southern California
Abstract:
High sedimentation rates in Santa Monica Basin provide an ideal opportunity to document tectonic activity during the last 100 ka offshore Los Angeles, California. The history of sedimentation has been documented with high-resolution deep-tow boomer profiles and with both multichannel and single-channel seismic-reflection profiles, which have been used to map 15 nearly basin-wide reflecting horizons. The initial results for sediment-accumulation rates obtained from cores recovered at Ocean Drilling Program (ODP) Site 1015 on the floor of Santa Monica Basin indicated a Holocene rate of nearly 3 m/ka, which is the highest yet documented for southern California deep-water basins. A more detailed determination of the sedimentation-accumulation rate in the basin allows improved age assignments for the high-resolution seismic stratigraphy, which in turn, results in better control on the timing of deformation within the basin. New radiocarbon dates obtained from sediment recovered at ODP Site 1015 provide stratigraphic age control for the upper 12 reflectors back to 32 ka at ~100 meters below the sea floor (mbsf). Ages estimated for the three deepest reflectors (140 to 220 mbsf) are based on extrapolation of rates for the dated sequence between 75 and 100 mbsf. The bulk of the sediment fill in Santa Monica Basin is from the Santa Clara River delta. The frequency of turbidite deposits at ODP 1015 is roughly equivalent to the frequency of 100-year storm events in southern California and more work is needed to attempt using the turbidite record to develop a paleoseismic record for the basin.
The Santa Cruz-Catalina Ridge (SC-CR) forms the southwestern margin of Santa Monica Basin. Deformation of basin-fill sediment adjacent to the SC-CR is limited to the northwestern corner of the basin in the area of convergence between thrusting to the north and strike-slip along the southwestern margin of the basin. Here, turbidite deposits of Hueneme Fan show local evidence for flexure of sediment horizons as young as 6 ka with minor fault offsets as recently as 1.5 ka. Larger scale anticlinal folding (~5 km width and >100 m of relief) of the basin fill is observed for strata older than ~65 ka, and faulting within (and assumed to be subparallel to the trend of) the anticline continued until sometime between 13 and 20 ka. To the south, the flat-lying turbidite fill of the basin onlaps both the SC-CR and Redondo Knoll with no evidence for disruption or folding post 75 ka. This suggests that recent faulting along the trend of the SC-CR must occur within the ridge itself except in the northwest corner of the basin. Very limited data from the sediment fill of Catalina Basin, which is adjacent to the SC-CR on the southwest side, shows little deformation within the basin fill and only minor westward tilting during the latest Quaternary.
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Normark et al, 2004, AGU poster:
Sediment accumulation rates of late Quaternary deposits in San Pedro Basin, the Gulf of Santa Catalina, and San Diego Trough, offshore southern California
Abstract:
A multiyear program of seismic-reflection profiling and sediment coring has focused on understanding the history of late Quaternary sedimentation within the inner basins of the California Borderland. The objective of this study has been to develop a high-resolution seismic stratigraphy primarily for the Last Glacial Maximum and the Holocene that can be used to understand depositional processes, sediment budgets, and deformation within the basins. The Santa Barbara and Santa Monica Basins in the northern Borderland are closed basins and both were cored during the Ocean Drilling Program (ODP) in support of paleoclimatic studies. In contrast, the inner basins in the southern Borderland that are the focus of this report are open-ended basins that have not been the subject of scientific drilling. We present the preliminary results of sediment coring at 21 sites in San Pedro Basin, the Gulf of Santa Catalina, and San Diego Trough. Initial estimates of sediment-accumulation rates for these basins are based on 48 previously unpublished radiocarbon dates. During the Holocene, average sediment-accumulation rates are generally less than 0.5 m/ky on the basin floors where turbidite deposition locally continued at reduced rates from those of the OIS 2-lowstand interval. This rate is nearly an order of magnitude less than was documented for the Holocene by ODP coring at Site 1015 in Santa Monica Basin reflecting, in part, the loss of sediment in the non-closed basins. Background hemipelagic rates range from 0.01 to 0.1 m/ky, with the lowest rates on mid-basin highs, e.g., Lasuen Knoll. More dating is currently underway for some of the core sites to determine the rate of change in sediment accumulation during the Holocene.
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Ryan et al, 2004, SCEC poster:
New Fault Map of the Inner Southern California Borderland Offshore San Diego and Orange Counties, California
Abstract:
We present a new fault map of the offshore southern California borderlands south of Newport Beach, CA. Major faults remapped from west to east include the San Diego Trough fault (SDTF), Coronado Bank fault (CBF), a prominent fault zone located near the base of the continental slope, and offshore segments of the Newport-Inglewood (NIF) and Rose Canyon (RCF) faults. Our interpretations are based on high-resolution, multichannel (MCS), Huntec and Geopulse seismic reflection profiles collected by the USGS from 1998-2000. The Huntec and Geopulse data are used to identify the most recently active segments of the fault zones. In addition, deep penetration MCS data collected by Western Geophysical in 1974-75 and Jebco in 1988 are utilized to determine the primary fault traces within multi-stranded fault zones and the attitude of faults at depth.
From the Mexican border north to the La Jolla fan valley (LJFV), the San Diego Trough fault is composed of one or two well-defined linear fault strands that cut through the center of the San Diego Trough and trend N30°W. North of the LJFV, the fault zone steps west and is composed of up to 4 fault strands. The Coronado Bank fault zone is a complex feature with multiple fault strands that can only be mapped over relatively short distances (generally less than 10 km). South of LJFV, the CBF zone is primarily transtentional; we mapped two pull-apart basins within the fault zone. The CBF is difficult to map northward across the LJFV, however, the two westernmost strands may continue north of the fan valley. The CBF zone north of the LJFV forms a positive flower structure that can be mapped at least as far north as Oceanside. An unnamed fault zone near the base of the continental slope (600-700m water depth) is imaged discontinuously from about 10 km north of LJFV to offshore of Dana Point. This fault zone changes in style of deformation along strike, with segments of the fault zone exhibiting compression as evidenced by prominent folds imaged in the deep penetration MCS data. The age of these compressional features is unclear as in places the folds are cut by younger near-vertical faults. The multi-stranded RCF and NIF generally follow the shelf break between La Jolla and Newport Beach. The transition between these fault zones is associated with a prominent active anticline at mid-slope (300-400 m) depths, suggesting a left step in slip between these two fault zones.
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Sorlein et al, 2004, SCEC poster:
The complete Palos Verdes Anticlinorium and Offshore Evidence for the Compton Blind Fault Beneath it
Abstract:
The Palos Verdes anticlinorium is part of a NW-SE-trending >130 km-long Inner Borderland contractional belt. The top of its southwest limb coincides with the shelf break, and its offshore northwest plunge is transitional between Borderland and E-W structure of the western Transverse Ranges. The Compton blind thrust ramp was imaged and inferred to explain the southwest flank of Los Angeles basin (Shaw and Suppe, 1996). A SW-directed tectonic wedge continues southwest of the ramp on published cross sections. A SW-dipping backthrust that forms the roof of this wedge was called upon to explain uplift of the onshore Palos Verdes anticlinorium. Our mapping shows that the anticlinorium continues beneath the offshore Shelf Projection for 30 km to the northwest, reaching beyond Santa Monica Canyon. The southwest limb of the anticlinorium is a continuous, linear structure for 45 km, and is responsible for the offshore San Pedro escarpment. A SSW-facing escarpment continues another 10 km south of a right step at San Pedro Sea Valley. This escarpment separates the anticlinorium from the more subdued expression of 5 km-wavelength folds farther southeast. Short-wavelength (1 to 2 km) folds, some with seafloor expression, are superimposed on the northwest 30 km of the anticlinorium. Five km wavelength folds beneath San Pedro shelf overprint the larger anticlinorium. The Palos Verdes anticlinorium, especially its southwest limb, is continuous for 45 km, with an additional 10 km of broad uplift of southern San Pedro Shelf.
We interpret the SW-dipping fold limb to be a forelimb above NE-dipping blind thrust or oblique thrust faults. These offshore faults project in 3D into the SCEC Community Fault Model (CFM) representation of the Compton Thrust ramp, at least for the northwest 30 km of the structure. Horizontal-axis rotation (progressive tilting) of an upward-widening zone absorbs thrust slip; slip on the blind faults gradually decreases updip as it is transformed into folding. Significant SW-dipping roof thrusts are not needed in this model. The NNE-dipping backlimb is also progressively tilting along at least 10 km of the northwest plunge of the anticlinorium, suggesting a listric area on the underlying fault. Activity along the northwest offshore anticlinorium is suggested by the following observations: (1) the floor of Santa Monica Canyon and early Holocene strata farther northwest are warped; (2) post-50 ka strata are deformed above the tips of the blind faults; (3) Palos Verdes Hills are uplifting; (4) the Shelf Projection has eroded, and the wavecut platform is shallower than low eustatic sealevels, suggesting rock uplift, above an inferred subsiding base level. Area balancing in cross-section yields up to 1.7 km of blind thrust slip beneath the Shelf Projection in the last ~2.5 M.Y., or more if sediment compaction and pressure solution are significant.
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Baher et al, 2003, AGU poster:
Estimating Earthquake Hazards in the San Pedro Shelf Region, Southern California
Abstract:
The San Pedro Shelf (SPS) region of the inner California Borderland offshore southern California poses a significant seismic hazard to the contiguous Los Angeles Area, as a consequence of late Cenozoic compressional reactivation of mid-Cenozoic extensional faults. The extent of the hazard, however, is poorly understood because of the complexity of fault geometries and uncertainties in earthquake locations. The major faults in the region include the Palos Verdes, THUMS Huntington Beach and the Newport-Inglewood fault zones.
We report here the analysis and interpretation of wide-angle seismic-reflection and refraction data recorded as part of the Los Angeles Region Seismic Experiment line 1 (LARSE 1), multichannel seismic (MCS) reflection data obtained by the USGS (1998-2000) and industry borehole stratigraphy. The onshore-offshore velocity model, which is based on forward modeling of the refracted P-wave arrival times, is used to depth migrate the LARSE 1 section.
Borehole stratigraphy allows correlation of the onshore and offshore velocity models because state regulations prevent collection of deep-penetration acoustic data nearshore (within 3 mi.). Our refraction study is an extension of ten Brink et al., 2000 tomographic inversion of LARSE I data. They found high velocities (>6 km/sec) at about ~3.5 km depth from the Catalina Fault (CF) to the SPS. We find these velocities, shallower (around 2 km depth) beneath the Catalina Ridge (CR) and SPS, but at a depth 2.5-3.0 km elsewhere in the study region. This change in velocity structure can provide additional constraints for the tectonic processes of this region.
The structural horizons observed in the LARSE 1 reflection data are tied to adjacent MCS lines. We find localized folding and faulting at depth (~2 km) southwest of the CR and on the SPS slope. Quasi-laminar beds, possible of pelagic origin follow the contours of earlier folded (wavelength ~1 km) and faulted Cenozoic sedimentary and volcanic rocks. Depth to basement, where observed, is approx. 1.7 km. beneath the base then shallows to approx. 1 km at the top of the SPS. This corresponds to the results obtained by Fisher et al. (in press) and Wright (1991). The pattern of faulting changes from southwest to the northeast. West of CF, faulting is confined to the pelagic and older units. Closely spaced faulting (~0.75 km) is prominent between CF and Avalon Knoll (AV), while generally more widely spaced faults (~5 km) with localized fracture zones is observed from AV to the SPS. The SPS is dominated by major faults such as the Cabrillo, Palos Verdes, THUMS Huntington Be ach and Newport-Inglewood fault zones. The Cabrillo and Palos Verdes fault are major stratigraphic discontinuity with laminar beds (~30 cm) adjacent to gently folded sediments (wavelength ~1.5 km). There is evidence of recent displacement on the Cabrillo fault.
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Baher et al, 2003, SCEC poster:
Estimating Earthquake Hazard in the San Pedro Shelf Region, Southern California
Abstract:
The San Pedro Shelf lies within the inner Continental Borderland region in southern California. This offshore region poses significant seismic hazard to contiguous Los Angeles Area, owing to late Cenozoic compressional reactivation of mid Cenozoic extensional faults. This hazard, however is poorly understood due to complexity of fault geometries and uncertainties in earthquake locations. The faults that posed the most significant hazard in the offshore include the Palos Verdes, THUMS-Huntington Beach, and Newport Inglewood fault zones. The recorded seismicity in this region is diffuse, even for those events associated with main-shock sequences, which could in part be due to asymmetrical station locations that results in large uncertainties in event locations. Initial findings show that the Palos Verdes fault consists of several strands that define the western border of the Wilmington Graben. Previous studies of this fault show a slip rate of 2.7-3.0 mm/yr that could generate a 7.0-7.2 event every 400-900 years.
We report here the analysis and interpretation of wide-angle seismic reflection and refraction data recorded as part of the Los Angeles Region Seismic Experiment line 1 (LARSE 1), multichannel seismic (MCS) reflection data recorded by the USGS (1998-2000) and industry borehole stratigraphy. An onshore-offshore velocity was developed by forward modeling of the refracted P arrival times and this model was used to depth migrate the LARSE 1 seismic line. The structural horizons were mapped and tied to nearby marine reflection lines. Borehole stratigraphy connects the onshore to the offshore velocity models due to restrictions on data nearshore data collection. A velocity step from 3.5 to 5.0 km/sec is observed at shallow depths (1.5-2.5 km) beneath the inner borderland. This step is interpreted as the contact between Cenozoic sedimentary (and volcanic) rocks and the Catalina schist. In the contact ramps downward to the east across the Palos Verdes fault and is consistent with a 1-km down-to-east basement offset shown by Wright (1991).
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Broderick et al, 2003, SCEC poster:
Blind Thrust Faulting and Shelf-Slope Deformation in Eastern Santa Monica Bay, California
Abstract:
We used industry and USGS multichannel seismic-reflection data to extend our mapping of the Shelf Projection blind fault and associated deformed strata across the Shelf Projection anticlinorium in eastern Santa Monica Bay. The thrust-reactivated eastern segment of the blind fault, which strikes approximately N55W, is subparallel to the axial trace of the Shelf Projection anticlinorium and dips gently northeast beneath a broad shelf-edge fold. Contraction of the Shelf Projection anticlinorium results from a restraining left step between the northwest-striking, right-lateral San Pedro Basin and Palos Verdes faults, such that slip is absorbed in folding of the hanging-wall above the blind thrust. Digital structure-contour maps of the top-Miocene unconformity and an upper Pliocene "Repetto" horizon indicate at least late Pliocene deformation, while young strata (<60 ka) correlated from ODP Site 1015 are deformed in the Shelf Projection anticlinorium's south limb. The blind fault continues southeast to dip beneath the southwest-dipping fold limb of the San Pedro escarpment, offshore west of the Palos Verdes peninsula. When viewed obliquely down dip using 3D visualization, the Shelf Projection blind fault projects directly into the Compton-Los Alamitos thrust ramp beneath Los Angeles basin. If the two thrust segments are linked, then slip on the upper part of this combined fault system may be absorbed by folding of the overlaying San Pedro escarpment. Furthermore, assuming the Shelf Projection/Compton-Los Alamitos fault system is active, then earthquake hazard models for the Los Angeles metropolitan area must be reevaluated to account for increased fault surface area.
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Erohina et al, 2003, AGU poster:
Acoustic Facies of Late Quaternary Channel and Overbank Systems - Gulf of Santa Catalina, California Borderland
Abstract:
The Gulf of Santa Catalina is a deepwater (>300m) basin that lies offshore between Long Beach and Dana Point California. Deep-tow Huntec boomer data from the northern gulf cover a complex channel system consisting of a series of slope gullies euphemistically known as the Newport Canyon. The boomer data have a vertical resolution of 50 cm and the system works effectively for imaging sandy turbidite deposits. A preliminary evaluation of the seismic-reflection data, together with available multibeam bathymetry, provides a better understanding of when different parts of the Newport channel system were active.
Three distinct acoustic facies characterize the late Quaternary turbidite sequence of the Newport channel system: an acoustically transparent facies, a high-amplitude discontinuous facies, and a moderate-amplitude sinusoidal facies. The acoustically transparent facies drapes the sea floor on both overbank deposits and some channel floors. Commonly the overbank areas on the west sides of channels preserve the thickest transparent sediment cover. The seismic characteristic of this facies is consistent with a mud-rich deposit but the thickness variation appears to rule out a simple hemipelagic deposit.
The high-amplitude discontinuous facies is associated with depressions in the sea floor. These features include active channels, large-scale scours and channel remnants. The moderate-amplitude sinusoidal facies is typical for large amplitude sediment waves.
The sediment waves interpreted from the boomer data are not everywhere coincident with the scale of those visible on the multibeam bathymetry. The sediment waves migrate upslope and away from the channel axes. This wave growth pattern is probably related to flows that are substantially thicker than the channel relief and is similar to wave forms found on sediment drifts in the deep ocean.
A second channel system similar to the Newport system, associated with the San Gabriel Canyon approximately 9 km to the west, provides additional insight to the depositional processes in the Gulf of Santa Catalina. This system has been strongly affected by the Palos Verdes fault and, therefore, the channel activity is less likely to be autocyclic in nature.
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Fisher et al, 2003, AAPG oral presentation:
Geologic Structure of the San Pedro Shelf Region, Southern California
Abstract:
An integrated interpretation of marine seismic-reflection and aeromagnetic data as well as multibeam bathymetry shows the complex Pleistocene history of rocks beneath the San Pedro shelf, west of Los Angeles. Prominent structures include the nearshore Wilmington graben, the Palos Verdes Fault Zone, and numerous faults below the west part of the shelf and slope. For subsurface depths less than 2 km, the Palos Verdes Fault Zone can be divided into three segments. Under the shelf, the northwest segment includes several fault strands that dip steeply west. Under the slope, the middle fault segment comprises several normal faults, most of which dip east. Near Lasuen Knoll, the southeast fault segment, includes thrust and reverse faults, many of which dip east. Apparently fresh seafloor scarps along the Palos Verdes Fault zone near the base of this knoll indicate recent fault movement. Possible wavecut terraces and sediment core samples that contain fossils of Quaternary outer-shelf fauna indicate that this knoll was subaerial and has rapidly subsided several hundred meters. Models of aeromagnetic data measured over the west San Pedro shelf indicate a large magnetic rock body that is probably middle Miocene basalt. Sedimentary rocks over the basalt are tightly folded, whereas folds in sedimentary rocks east of the basalt have much longer wavelengths. This difference probably resulted because the basalt was more competent during deformation than the sedimentary rocks.
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Fisher et al, 2003, SCEC poster:
Geology of Submarine Landslides Below Western Santa Barbara Channel, California
Abstract:
Submarine landslides under the western Santa Barbara channel may contribute to the regional hazard from tsunamis, like the tsunami that reportedly followed the 1812 Santa Barbara earthquake. Multibeam bathymetry shows that the Goleta landslide, the largest one in the area, consists of two main lobes that extend 10-15 km southwestward across the lower slope and basin floor. High-resolution seismic-reflection data show what appear to be older mass-wasted deposits that underlie the Goleta landslide. These older deposits are late Pleistocene, on the basis of biostratigraphic data from drilling at ODP Site 146-893, which is located about 1 km southwest of the toe of the Goleta landslide. Drilling data indicate that some of the underlying mass-wasting deposits are at least as old as 164 ka, the maximum age obtained from drilled rock and sediment. Thus the Goleta landslide does not represent a single failure, isolated in time, but instead it represents the latest in a series of sediment failures.
Chirp seismic-reflection data obtained over the ODP drill site and the toe of the Goleta landslide show that a thin (0.01 s, about 8 m) sediment layer covers the landslide, and on seismic-reflection sections, this layer appears similar to the shallowest sediment drilled at the ODP site. If the two sediment layers correlate, then the landslide is covered by sediment that is between 3 ka and 6 ka old, indicating a minimum age range for the landslide. The top of oxygen-isotope stage 4 in the ODP hole corresponds in two-way traveltime with the base of a band of high-amplitude reflections. These reflections can be correlated through the basin fill that surrounds the Goleta landslide, but disrupted reflections from sediment underlying this landslide makes this correlation tenuous. High-resolution seismic reflection data show that sedimentary rocks under the shelf edge dip south toward the channel and that the headwall of the slide cuts downward into these rocks. A rotated, apparently intact block of these rocks underlies a narrow seafloor ridge that extends along most of the length of the headwall. The headwall of the Gaviota submarine landslide developed directly over a shallow anticline, and both features formed along the North Channel Fault. A series of seafloor cracks and pockmarks, evident in multibeam bathymetry, connect eastward with the headwall of the Goleta landslide and show possible future sites for submarine failures.
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Greene et al, 2003, AGU poster:
Dating One Slide Event of the Complex Compound Goleta Submarine Landslide, Santa Barbara Basin, California USA
Abstract:
Dating of submarine sediment slumps and slides is fraught with problems, especially those taking place during the past several thousand years. However, we need to develop accurate dating techniques so as to improve our ability to assess the periodicity of, and the actual risk from, these events. The large (130 km2) Goleta landslide located off Coal Oil Point near the town of Goleta, California measures 14.6-km long, extends from 90 m to nearly 574 m below sea level, and is 10.5 km wide. The slide is complex, and includes both buried slump blocks and mudflows, with a surface expression of three distinct segments of failures. Each segment is composed of a distinct head scarp, down-dropped head block, and a slide debris lobe. We estimate that at least 1.75 km3 of surface material represents the most recent displaced events of this slide.
Dating of the more recent failures of this slide has been speculative and based on modeling and estimated sediment cover on one of the head scars. The most recent event is thought to be the cause of the reported tsunamis of 1812. However, sophisticated dating of the slide has yet to be done. Recent interpretation of newly acquired seismic reflection profiles indicates that the head slump block of the eastern segment of the slide dammed sediment behind it after it came to rest at the base of the head scarp. We calculated that about 64 m of sediment was dammed behind the block, based on a measured two-way travel time through the sediment pond of 0.075 sec. and an assumed velocity of ~1680 m/sec. Using a sedimentation rate of 1.73 m/Ka to 2.5 m/Ka, the range of sedimentation rates determined by other investigators for the mid- to lower shelf areas of the western Santa Barbara Basin, the time required for deposition would be 37-25.6 Ka. Therefore, if the sedimentation rates are correct and that the ponded sediment represent nothing but normal sedimentation devoid of sloughing or debris flows down the slope, the maximum age of when the block came to rest in its present position would be 37-25.6 Ka. However, given the tectonic dynamics of the area and the instability of the slope in this part of the Santa Barbara Basin, the likelihood of deposits derived from mass movement filling the depression behind the slump block is high, thus suggesting a possible younger age for the failure.
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Hart et al, 2003, SCEC poster:
New Access to Proprietary Marine Seismic Reflection Data along the U.S. West Coast
Abstract:
High-quality seismic reflection data are essential to geological investigation of the offshore. Over the past thirty years the USGS has acquired approximately 12,000 km of 2D data off the west coast of the United States. During this same period the petroleum exploration industry acquired probably 30 to 50 times that amount. To date, these data have been proprietary and available for publishable research only with significant restrictions. Although the commercial value of these data has diminished as a result of technological advances and offshore development moratoria, these data continue to have great relevance value to current and future scientific research efforts. The value and risk of loss of these data was recently the subject of a National Research Council report "Geoscience Data and Collections: National sources in Peril" by the Committee on the Preservation of Geoscience Data and Collections, Committee on Earth Resources.
Recently, two companies with perhaps the largest holdings of proprietary data off the US west coast, WesternGeco and ChevronTexaco, have offered to transfer more than 250,000 km to the USGS for the purpose of making those data publicly available for research and educational purposes. Extension of these offers to include data offshore Alaska and the east coast of the U.S. is under discussion. The data being offered are for the most part conventional 2D multichannel airgun seismic reflection acquired and processed according to standards of the mid 1970's and early 1980's. The data commonly were acquired using a 5-to-25 element tuned airgun source totaling 1,000 to 3,000 cubic inches; 24-fold to 48-fold stacking; 25-to-50 meter CDP interval; 6 second recording with 4 msec sampling; processed through stack and post-stack migration. The surveys were generally limited to the continental shelf, extending up to 150 miles offshore southern California. line spacing is dense off southern California, and becomes sparser to the north.
Cataloging, documenting, reformatting, and providing access to these data will require a multi-year effort. Tens of thousands of digital 9-track tapes will be transferred to modern media, and auxiliary data (such as observer logs and processed film records) will be scanned. The available processed data (post-stack and migration) for all the data sets will be reformatted and cataloged on-line. However, a much larger quantity of data (25- to 50-times greater) exists in the form of original field recordings (pre-stack) for each data set. While these pre-stack field data are also extremely valuable, the resources required to capture and subsequently reprocess these data are correspondingly large. A prioritization strategy for salvage of the pre-stack data must be developed.
The USGS has several partners in this effort, including NSF (Earthscope), Institute for Crustal Studies at UC Santa Barbara, the IRIS Data Management Center, University of Southern California, and Minerals Management Service. The ultimate objective is the creation of a comprehensive geophysical data repository accessible on-line, with a graphical search and retrieval interface that would be available to teachers and researchers.
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Lee et al, 2003 AGU poster:
Ages of Potentially Tsunamigenic Landslides in Southern California
Abstract:
The innermost basin slopes of the Southern California Borderland contain numerous large-scale submarine landslides. Many of these are of sufficient size to have generated tsunamis when they occurred, assuming that the imaged deposits represent a single failure and that their motion was rapid. We have obtained high resolution subbottom profiles of five well-defined slope failures. These failures are located off the Palos Verdes Peninsula, within Santa Monica Bay, and within the Santa Barbara Channel. A landslide motion and tsunami generation model for one of the landslides shows that it likely generated a tsunami with a source height of at least 8 m. For four of the failures, ages were determined by identifying acoustic reflectors in the vicinity of the failed masses that either clearly postdate or predate the landslide events. The ages of the reflectors are determined by using seismic-reflection profiles to correlate with dated sections at ODP sites or by directly da ting the slide masses with piston cores. In the case of the most recent failure in the Santa Barbara Channel, post-failure sediment deposited on the scar of the landslide was sampled completely with a gravity corer, and the age was estimated based on a representative sedimentation rate. Our assessment shows that three of the four older landslides are approximately 10,000 years old and that the Palos Verdes event was about 7,500 years ago. However, the most recent failure may be as young as 200 years. Such a landslide might have generated a tsunami such as was observed by early Spanish settlers during the 1812 earthquake.
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Normark, 2003, GAC invited keynote address:
Geohazards of the southern California offshore area: out of sight, out of mind?
Abstract:
The California Borderland offshore southern California is a tectonically active area of basins and ridges that exceeds 150-km in width and includes part of the wide distributed boundary between the Pacific and North American plates. Intensive, on-land earthquake hazard research over the last few decades has shown that only about 80% of the strain associated with plate movement is recorded in onshore areas. Since 1997, the Coastal and Marine Geology Program of the USGS has been conducting marine surveys to assess geohazards of the inner Borderland basins. The seaward limit of the survey is roughly estimated to include faults that could cause damage to the adjacent onshore infrastructure at earthquake magnitudes of 6.5 and higher. The study extends from the city of Santa Barbara in the north to the US-Mexican border south of San Diego, along the most densely populated coastal corridor on the west coast that represents a major component of the earthquake risk for the en tire United States.
The first phase of the geohazard research involved imaging the offshore structures to determine the length and sense of motion on active faults and folds using high-resolution multichannel and boomer seismic-reflection profiles. The techniques available for this phase of the research were restricted by State and Federal regulations and resulted in cruise operations that involved more marine mammal observers than geoscientists on board the ships and, in addition, precluded work inside the three-mile limit (albeit closest to the populace at risk). The current phase of the geohazard assessment is to determine the recency of deformation and, where possible, recurrence intervals on active faults. This activity involves establishing a detailed sequence stratigraphy for the last 10 ka using piston cores and the two existing ODP boreholes. The final phase of the geohazard evaluation will be a collaboration with the academic community to deploy an offshore strain monitoring program that is needed to fully evaluate the geohazards within the inner Borderland.
Our evaluation to date suggests that the area west of Los Angeles shows the most recent and recurring deformation where strike-slip shearing common in the southern part of the study area transforms into compressional structures offshore Malibu and Santa Barbara to the northwest. The largest submarine landslide in the area lies offshore from the Los Angeles/Long Beach harbors and occurred 7500 ka B.P., much older than previously thought.
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Normark and McGann, 2003, SCEC poster:
Developing a High-Resolution Stratigraphic Framework for Estimating Age of Fault Movement and Landslides in the California Continental Borderland
Abstract:
One goal of the offshore earthquake hazard study in the California Borderland is to date the recency of movement on faults and folds and age of major landslide failures. The approach has been to identify active structures and landslides using high-resolution Huntec boomer or chirp-sonar data to image the structural detail (to 50 cm resolution) in the upper 20 to 80 m of sediment. These high-resolution profiles are then used to identify sites that can be sampled using standard (wire-line) piston coring to obtain material for AMS radiocarbon dating. This approach to deciphering fault history has been euphemistically referred to as "acoustic trenching."
The initial attempts in our study focused on the Santa Monica Basin and the Gulf of Santa Catalina using cores obtained in 1998 and 1999, respectively. A total of 18 cores were recovered from 17 sites. In addition, for Santa Monica Basin, 11 radiocarbon dates were obtained from ODP Site 1015 that provide age control for a basin-wide stratigraphic framework based on Huntec boomer records. Preliminary results from the dating document Holocene displacement as young as 3 ka on base-of-slope faults along the southwestern margin of Santa Monica Basin. In addition, two small landslides at the base of slope in the eastern end of the basin both occurred about 10 ka. To the south in San Pedro Basin, the Palos Verdes debris avalanche is the largest Holocene mass failure recognized to date in the inner Borderland and is dated at about 7.5 ka. South of the Long Beach shelf, attempts to date recent offset on the Palos Verdes fault system have been inconclusive as a result of mas s-wasted deposits of older sediment covering the youngest offset horizons.
Additional sampling, including 27 piston cores, in July 2003 will be used to extend the analysis to other basins of the inner Borderland.
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Normark et al, 2003, SCEC poster:
Hydrate Discovery in Santa Monica Basin Offshore California
Abstract:
During a sediment-sampling cruise in late July 2003, a short piston core from near the summit of a mud diapir in Santa Monica Basin recovered gas hydrate at a water depth of 813 m. The discovery core was 2.1 m long and apparently stopped in the hydrate as evidenced by chunks of ice at the bottom of the core. Violent degassing of the core section between 162 and 212 cm resulted in spontaneous extrusion of the sample in the ship laboratory. Fresh mussel shells recovered from the top of the core indicate that the diapir is a site of active methane venting. The existence of hydrate at such a shallow depth in the sediment was unexpected and a decision was made to return to the site later in the cruise. Subsequent sampling with piston, gravity, and box corers (the latter included a Benthos (TM) bottom-trip camera) failed to sample more hydrate but did recover additional cold-seep fauna as well as fragments of indurated sediment. GPS positioning of the vessel allowed placement of all sample sites within a 15 m radius of the discovery core location, i.e., within about a 700 m2 area) near the summit of the diapir. Within that area, the character of core samples varied from only rock fragments in one core to over 2.4 m of sediment in another core.
The mud diapir is about 300 m in diameter at the base and narrows to less than 100 m across the gently sloping summit. The diapir is internally structureless on both high resolution deep-tow boomer and single-channel air-gun profiles (most likely as a result of the gas content and sediment deformation) and has extruded through well-bedded sediment on the lower slope of the basin producing as much as 30 m of bathymetric relief. The diapir is located about 24 km west-southwest of Redondo Beach, in an area where strike-slip motion along the San Pedro Basin fault zone to the south is replaced by convergent motion to the north. The intruded turbidite and lower slope sediment is correlated with latest Pleistocene and Holocene age deposits drilled at ODP Site 1015. The source horizon for the gas in the hydrate may be as shallow as 200 m below the regional seafloor based on the presence of a strong and irregular reflection interval observed on a 40 cu. in. sleeve-gun profile.
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Normark et al, 2003, AGU poster:
Methane Hydrate Recovered From A Mud Volcano in Santa Monica Basin, Offshore Southern California
Abstract:
In July 2003, a short (2.1 m), piston core from the summit of a mud volcano recovered methane hydrate at a water depth of 813 m in Santa Monica Basin. The discovery core penetrated into in the hydrate as evidenced by chunks of ice and violent degassing of the core section between 162 and 212 cm depth. The core consists of shell hash and carbonate clasts (to 7-cm long) in silty mud. The methanogenic carbonates are of two types: massive, recrystallized nodular masses with an outer mm-thick sugary patina and a bivalve coquina with carbonate cement. Living clams including the genus Vesicomya, commonly found at cold-seep sites elsewhere, were recovered from the top of the core. Further sampling attempts using piston, gravity, and box corers, all of which were obtained within 15 m of the discovery core, recovered olive-brown silty mud with variable amounts of whole and fragmented bivalve shells and methanogenic carbonate fragments characteristic of cold-seep environments. Gases collected in cores adjacent to the discovery core contain elevated amounts of methane and trace amounts of heavier hydrocarbon gases, indicating some component from thermogenic sources. Hydrogen sulfide was also detected in these sediment samples. Vertical channels in one core may have served as fluid pathways. The existence of hydrate at such a shallow depth in the sediment was unexpected, however, the presence of Vesicomya and hydrogen sulfide indicate that the mud volcano is a site of active methane venting.
The mud volcano, which is about 24 km west-southwest of Redondo Beach, is about 300 m in diameter at the base. No internal structure is resolved on either high resolution deep-tow boomer or single-channel air-gun profiles, most likely as a result of the gas content and sediment deformation. The diapiric structure has ascended through well-bedded sediment on the lower slope of the basin, producing as much as 30 m of bathymetric relief. It is located in an area where strike-slip motion along the San Pedro Basin fault zone to the south is replaced by convergent motion to the north. The source horizon for the gas in the hydrate is unknown but appears to be collecting in beds as shallow as 200 m below the regional seafloor based on the presence of a strong and irregular reflection interval.
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Piper et al, 2003, AGU poster:
Variations in accumulation rate of late Quaternary turbidite deposits in Santa Monica Basin, offshore southern California
Abstract:
The Santa Monica Basin off southern California provides an ideal setting to study the effects of sea-level change and turbidity current initiation styles on turbidite depositional processes along a narrow continental margin. It is the perfect sink: all sediment delivered through multiple submarine canyons along its northern margin remains within the basin, except for quantifiable advection of shelf mud to the south. High-resolution deep-tow boomer data, which provides stratigraphic detail (to 50-cm resolution) for the upper 20 to 80 m of basin fill, permits mapping of a series of reflectors across the basin floor and onto the lower basin slopes. Age control for the upper basin fill at ODP Site 1015 is provided by 11 new radiocarbon dates, back to 31 ka (radiocarbon years) at 98 m. The average sediment-accumulation rate during the Holocene of 2.4 m/ka is about half the peak lowstand accumulation rate of 4.7 m/ka. During oxygen isotope stage 3, the accumulation rate wa s 2.5 m/ka (based on two dates), comparable to the Holocene rate. Additional dates from samples taken with conventional piston cores on the basin slopes confirm age assignments and stratigraphic correlation based on the boomer data. Basin floor reflectors in the turbidite fill can be traced from ODP Site 1015 to as much as 40 m above the floor of the basin at both the western and southeastern margins. Intervals of thick sands on the basin floor have correlative fine-grained turbidites on the basin margins, whereas intervals with only thin sands on the basin floor lack identifiable turbidites. Volume estimates of sediment accumulation can thus be made for individual time slices, allowing identification of periods of shelf trapping (e.g., 7.5-4 ka), preferential mid-fan deposition from high-density turbidity currents (e.g., 4-1.5 ka) and preferential basin-plain deposition from thick turbulent flows (e.g., 1.5-0 ka).
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Ryan et al, 2003, AAPG oral presentation:
Recent faulting in the Gulf of Santa Catalina from San Diego to Dana Point, California
Abstract:
The inner continental borderland offshore of southern California is complexly deformed by a number of faults that generally trend in a northwest direction. We have analyzed multichannel (MCS) and high resolution seismic-reflection data recently acquired by the USGS, combined with JEBCO and Western Geophysical industry MCS data, to compile a revised fault map for the area from offshore San Diego to Dana Point. The major fault zones we mapped include 1) the Coronado Bank fault zone and its northward extension and possible connection to the Palos Verdes Fault, 2) an unnamed fault zone at the base of the slope that extends discontinuously from La Jolla Canyon to near Dana Point, and 3) the offshore connection of the Rose Canyon fault zone (mapped onshore at La Jolla Canyon) with the Newport Inglewood fault zone, which trends offshore at Newport Beach. All of the fault zones we mapped are multi-stranded, discontinuous with respect to offset of the seafloor, and show signi ficant along-strike variability. The number of fault strands, width of the fault zone and the sense of offset on an individual strand commonly change along strike. As imaged on the high-resolution data, there is a spatially coherent pattern of recent fault slip on parallel faults. The orientation, distribution, and style of deformation of the various fault splays that compose the major fault zones suggest that the faults may be interconnected, resulting in a complex pattern of slip transfer between individual strands.
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Ryan et al, 2003, SCEC poster:
Revisiting the Offshore Connection Between the Newport-Inglewood and Rose Canyon Fault Zones
Abstract:
We have interpreted very high-resolution seismic-reflection data collected by the U.S. Geological Survey in 2000 to reexamine the relationship between the Newport-Inglewood (NI) and Rose Canyon (RC) fault zones (FZ) in the offshore between Newport and La Jolla submarine canyons. The NI-RCFZ is multistranded and generally tends follow the shelf break at about 100 m water depth. We have divided the offshore portion of the fault zones into 3 distinct segments: from Newport Beach south to Dana Point, from Dana Point to Carlsbad Canyon, and between Carlsbad and La Jolla Canyons. The 'North Branch' strand of the NIFZ can be traced from on land at Newport Beach to almost as far south as Dana Point. On the shelf, this strand is generally buried by sediment of probable Holocene age. However, another sub-parallel fault strand is discontinuously imaged seaward of the shelf break and appears, in places, to offset gullies imaged on sidescan sonar data. It is difficult to trace fa ult strands across Dana Point in part because there is limited data in this area. Shelf sediment thins across Dana Point, which is the location where the San Joaquin Hills anticline plunges offshore (Grant et al., 1999).
From Dana Point south, the NIFZ is composed of up to 4 strands that are mapped near the shelf break. South of San Mateo Point, which lies about 12 km southeast of Dana Point, some of the strands appear to deform the sea floor. In addition, on many profiles shallow reflections are obscured in the vicinity of the fault zone, suggesting the presence of gas. Immediately north of La Jolla Canyon, the RCFZ follows the shelf break, which has a more northerly trend than the shelf break north of Carlsbad Canyon. We have mapped at bast 5 fault strands of the RCFZ; these strands commonly appear as terrace-like features. Locally, there are pronounced bathymetric features associated with the RCFZ. The transition between the NI and RC fault zones occurs in the region between Del Mar and Oceanside. This segment shows the most significant deformation of the sea floor, particularly near Carlsbad Canyon at mid-slope water depths of about 300 m. Here, a prominent anticline is mapped that offsets the sea floor about 60 m; its relief at the sea floor decreases to the northwest and southeast, but it is still observable over a distance of almost 15 km. The RCFZ merges into the more westerly trending transition zone at Carlsbad Canyon. The transition zone is linearly aligned with the segment of the NIFZ north of Dana Point, which suggests that the NI and RCFZ are connected at depth.
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Sliter and Ryan, 2003, AGU poster:
Possible Connections Between the Coronado Bank Fault Zone and the Newport-Inglewood, Rose Canyon, and Palos Verdes Fault Zones Offshore San Diego County, California
Abstract:
High-resolution multichannel seismic-reflection and deep-tow Huntec data collected by the USGS were interpreted to map the Coronado Bank fault zone (CBFZ) offshore San Diego County, California. The CBFZ is comprised of several major strands (eastern, central, western) that change in both orientation and degree of deformation along strike. Between Coronado Bank and San Diego, the CBFZ trends N25W and occupies a narrow 7 km zone. Immediately north of La Jolla submarine canyon (LJSC), the easternmost strand changes orientation to almost due north and appears to be offset in a right-lateral sense across the canyon axis. The strand merges with a prominent fault that follows the base of the continental slope in about 600 m water depth. The central portion of the CBFZ is mapped as a negative flower structure and deforms seafloor sediment as far north as 15 km north of LJSC. Farther north, this structure is buried by more than 400 m of basin sediment. Along the eastern edge of the Coronado Bank, the western portion of the CBFZ is characterized by high angle normal faults that dip to the east. North of the Coronado Bank, the western segment follows the western edge of a basement high; it cuts through horizontal basin reflectors and in places deforms the seafloor. We mapped an additional splay of the CBFZ that trends N40W; it is only observed north and west of LJSC.
Although the predominant trend of the CBFZ is about N40W, along strike deviations from this orientation of some of the strands indicate that these strands connect with other offshore fault zones in the area. Based on the limited data available, the trend of the CBFZ south of Coronado Bank suggests that it might connect with the Rose Canyon fault zone (RCFZ) that has been mapped in San Diego Bay. North of Coronado Bank, the CBFZ is a much broader fault zone (about 25 km wide) composed of diverging fault strands. The westernmost strand may merge with the western strand of the Palos Verdes fault zone (PVFZ) south of Lasuen Knoll. The eastern strand trends toward the Newport-Inglewood fault zone (NIFZ) as imaged offshore near Dana Point. These connections suggest that the CBFZ is linked at depth with other prominent fault zones to the north (PVFZ and NIFZ) as well as to the south (RCFZ).
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Sorlien et al, 2003, AAPG oral presentation:
Structure and kinematics beneath Santa Monica Bay, California
Abstract:
West-striking seafloor faults and underlying blind thrust faults separate the Santa Monica Mountains (north) from northwest-striking right-lateral faults beneath Santa Monica Bay to the south. The moderately north-dipping Santa Monica-Dume fault (SMDF) is continuous for 75 km and links to the Malibu Coast fault. We model 4 to 7 km of post-~4 Ma left-lateral slip on the SMDF. An east-west-elongate basin extends 75 km offshore south of the SMDF. Assuming an unconformity beneath lower Pliocene strata formed near sea level, it has since subsided between 1.5 and 4 km. This foreland subsidence is synchronous with folding and basin inversion north of the SMDF. A blind fault dips north beneath this basin and the SMDF, and is interpreted as a Miocene detachment. This structure, which we call the Shelf Projection blind fault, has been mapped along 50 km of its strike using seismic reflection data. A thrust-reactivated segment of this fault located offshore of Manhattan Beach i s overlain by the 20 by 20 km Shelf Projection anticlinorium. Folding is post-Miocene, and the south limb of this anticlinorium deforms the youngest strata. This contraction is due to a left restraining step between the NW-striking right-lateral Palos Verdes and San Pedro basin faults. Assuming a local 40-50 degree north-northeast dip beneath its imaged uppermost part, the Shelf Projection blind fault projects into nodal planes of the 1979 and 1989 M5 reverse-slip earthquakes. It continues southeast beneath a broad southwest-dipping fold limb along the San Pedro escarpment.
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