Abstracts

Fisher et al, 2002, AGU and SCEC posters:
Marine geology of the San Pedro continental shelf and initial results from seismic reflection data obtained in Santa Barbara Channel

Abstract:
The San Pedro continental shelf is cut by several large faults; chief among them is the Palos Verdes fault, which extends for nearly 100 km across the region. Research into the earthquake hazards posed by the Palos Verdes and other offshore faults is based primarily on small-airgun, multichannel, seismic-reflection data that were migrated and converted to depth sections. These data were interpreted together with the results of modeling aeromagnetic and gravity data and tomographic analysis of LARSE deep-penetration, seismic-reflection data. Our research shows that south of the Palos Verdes Peninsula and west of the Palos Verdes fault, rocks of middle Miocene through Pliocene age, at least 2 km thick, are folded into a large anticline and syncline--the Shelf Projection anticlinorium of Nardin and Henyey [1978]--that diverge to the northwest from the Palos Verdes fault. Aeromagnetic data show a 120-nT positive anomaly over the western limb of the syncline. Modeling of this anomaly suggests the presence of a large body of probable middle Miocene basalt. Isostatic gravity values, however, do not reveal a large dense body, perhaps owing to widely spaced measurements. Even so, we propose that a mass of basalt, more competent than the encasing sedimentary rocks, controlled the location and development of sub-shelf folds and of thrust faults that deform rocks farther west, below the continental slope. This basalt, then, appears to have played an important role in establishing the earthquake potential of faults near the San Pedro shelf. During June 2002, seismic-reflection data, of several different resolutions, were collected by the USGS in the Santa Barbara Channel. These data will be used to support research into earthquake hazards and the generation of submarine landslides, like the large Goleta slide below the northwestern part of the channel. In a collaborative effort, geologists at U.C. Santa Barbara will use these data to evaluate the in teraction between climate, sedimentation and tectonics. These new seismic-reflection data confirm that rocks of probable late Pleistocene age crop out along the Mid-Channel Trend, a series of anticlines developed adjacent to the Oak Ridge fault. These rocks are attractive targets for sampling to obtain dates of fault movement, fold development and paleoclimatic events. Seismic-reflection data were collected across the active Oak Ridge reverse fault, near where it crosses the shoreline at Ventura.

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Kennedy et al, 2002, GSA poster:
Geologic map and digital data base of the Oceanside 30'x60' quadrangle, California

Abstract:
The Oceanside 1:100,000-scale quadrangle lies between 33° and 33°30'N latitude and 117° and 118°W longitude. It underlies a rapidly urbanizing part of southern California. The area is tectonically and seismically active and is dissected by four major, northwest trending, oblique right slip, Pacific/North American Plate boundary fault zones. They include the Elsinore Fault Zone in the northeastern corner of the quadrangle, the Newport-Inglewood Fault Zone in the center of the quadrangle (origin of the 1933, M=6.3, Long Beach earthquake), the Coronado Bank Fault in the near offshore region and the San Diego Trough Fault Zone in the southwestern corner of the quadrangle (origin of the 1986, ML=5.3, Oceanside earthquake). Landslides are abundant in the western and offshore parts of the quadrangle. Also, seismic hazards are numerous throughout the area. A tsunami hazard exists along the coastal margin.

The quadrangle is underlain by a thick sequence of forearc and forearc-basin Jurassic and Cretaceous (mostly low grade greenschist facies but partly unmetamorphosed) andesitic flows, sedimentary and volcaniclastic breccias and marine metasedimentary rocks that have been intruded in their older part by the southern California batholith. The batholith is Cretaceous in age and in part coeval with the forearc and forearc-basin rocks. The batholithic rocks are mostly tonalite and granodiorite with less common gabbro, diorite, monzogranite and granite. Pegmatite dikes are common in these intrusive rocks. The western part of the quadrangle is underlain by a relatively thick (>1000m) succession of Upper Cretaceous, Tertiary and Quaternary sedimentary and volcanic rocks that unconformably overlie the older plutonic and forearc basement rock sequence. These rocks consist chiefly of beds of marine, paralic, and nonmarine claystone, siltstone, sandstone and conglomerate and minor flows consisting mostly of Neogene basalt. Many cycles of uplift, erosion, subsidence and deposition since the Late Mesozoic have created the complexity of the existing stratigraphic and structural settings.

Fractured and deeply weathered bedrock associated with K/T boundary subareal extremes mantles much of the interior highlands and particularly the steep slopes adjacent to and southwest of the Elsinore Fault Zone.

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Normark et al, 2002, AGU poster:
Emplacement of the 7,500 yr B.P. Palos Verdes submarine debris avalanche, southern California

Abstract:
The Palos Verdes debris avalanche in San Pedro Basin offshore southern California is the largest volume late Quaternary mass-wasted deposit known from the inner California Borderland basins. Closely spaced, high-resolution deep-tow boomer profiles of the Palos Verdes debris avalanche collected in 1998 and 2000 show that it fills a turbidite leveed channel that extends from San Pedro Sea Valley. The bulk of the avalanche deposit, which is about 14 km long, appears to have resulted from a single failure on the adjacent slope because no internal stratigraphy was observed. Blocks as large as 40 m high have been transported onto the gently sloping basin floor. The volume of the debris avalanche deposit is between one and two cubic kilometers. The total volume of sediment that failed on the adjacent slope is difficult to estimate because of the unknown extent of a muddy debris flow on the basin floor that is coeval with the avalanche. The Palos Verdes deposit has been used to model tsunami generation in recent studies attempting to quantify local coastal hazards. Earlier studies speculated that the slope failure and the resulting deposit occurred less than a thousand years ago, well after sea level reached its present position. Radiocarbon dates from two piston-cores obtained near the distal toe of the avalanche deposit, however, indicate that the main failure occurred about 7,500 yr B.P. Subsequently, one or more smaller failures resulting in muddy debris flows have occurred as sea level rose during the mid to late Holocene.

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Sorlien et al, 2002, SCEC poster:
A blind fault beneath Santa Monica Bay

Abstract:
We used industry seismic reflection data to map a blind N-dipping low-angle fault beneath central and eastern Santa Monica Bay, along 50 km of its strike. This fault could be called the tip of the Santa Monica Mountains thrust, the Shelf Projection thrust, or the San Pedro escarpment thrust. We interpret it to be a basal Miocene detachment associated with clockwise vertical-axis rotation of the Santa Monica Mountains. It is located south of and beneath the Santa Monica-Dume fault. Several contractional structures indicate that it has been reactivated, with different structural styles for each. The western part, south and southwest of Pt. Dume, has been only slightly reactivated near its upper tip by post-Miocene folding. However, its downdip projection merges with or intersects the moderately-dipping Santa Monica-Dume fault. Any active folding of the Santa Monica Mountains anticlinorium absorbs a deep thrust slip component on these faults. The central part of the fault is overlain by the 10x15 km WNW-trending Shelf Projection anticlinorium, located off of Manhattan Beach. We are investigating whether Pliocene-early Quaternary folding of this structure continues through late Quaternary time. The M5.0 1979 and 1989 earthquakes are spatially associated with this fold. The fault, or linked system of faults, then bends to the southeast, where its tip is beneath the base of San Pedro escarpment. The San Pedro escarpment is a dip slope associated with a SW-dipping fold limb, where the seafloor is almost as steep as the underlying strata. Further investigation is required to see whether this fault tip is associated with the Compton-Los Alamitos thrust of Shaw and Suppe (1996). It is possible that deformation above the Compton-Los Alamitos has changed with time as the tip of the reactivation propagated offshore. Contraction near this tip may now absorb thrust slip at the expense of the onshore fold, without need for a change in slip rate on the deep blind fault. Quantifying contraction across the San Pedro escarpment and the Shelf Projection anticlinorium is needed to evaluate rates of slip on the newly mapped blind fault.

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Bohannon, 2001, AGU poster:
Quaternary Tectonic Evolution of the Coastal Belt Southwest of Los Angeles Basin

Abstract:
Modern geologic hazards in the coastal belt southwest of Los Angeles Basin are intimately tied to its Quaternary tectonic evolution. Models describing tectonism during this period fall into at least three classes depending on what type of feature is showcased. 1.) Fold-and-thrust belt models feature blind thrusts, 2.) convergent-flake-tectonic models emphasize rigid upper-crustal blocks that interact above a mobile middle crust, and 3.) strike-slip models center on the interaction of blocks bounded by vertical faults with lateral offsets. High-resolution, multi-channel, seismic-reflection data, collected in a network of lines offshore, image numerous structures and tectonic features that have geometric characteristics that can be used to support each of the models, depending upon where one looks. Numerous folded uplifts and reverse faults are consistent with fold-and-thrust models. Some of the broad, deep basins might be best explained by convergent-flake tectonics. Complex vertical fault zones separating blocks with different seismic stratigraphy suggest strike-slip. In addition, large normal faults and deep fault-bounded basins are widespread, but are not explained well by any of the models. One aspect of local tectonic history, not considered by any models, is a major reversal of the regional physiography that occurred during the Quaternary. Los Angeles Basin (LAB), which is now sub aerial, was mid-bathyal in the Pliocene whereas Santa Monica and San Pedro (SM/SP) Basins, which are presently mid-bathyal, were shallow to sub aerial. The physiographic reversal resulted from a combination of folding and uplift in the Palos Verdes/Santa Monica areas, which impounded sediment causing LAB to fill, and extensional faulting and rapid subsidence nearby in SM/SP Basins. These seemingly opposed tectonic styles can be easily documented with seismic data, but these styles are thought to be incompatible in most models.

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Fisher et al, 2001, AGU poster:
Geology of the Continental Margin Beneath Santa Monica Bay, Southern California, from Small-Airgun Seismic-Reflection Data

Abstract:
Small-airgun, multichannel-seismic-reflection (MCS) data collected in Santa Monica Bay reveal the geologic structure of the continental shelf and of the adjacent basin slope and deep-water basins. The Palos Verdes fault under the northern, shallow part of the bay cannot be identified in seismic-reflection data, even though many studies suggest its presence. This fault neither offsets the seafloor nor cuts through an undeformed sediment apron. Other major faults, such as the east-west Dume fault under the northern part of Santa Monica Bay, show evidence for recent activity. Tomographic-velocity data, derived from arrivals along the LARSE, deep-crustal MCS streamer, show that metamorphic rocks, probably the Catalina schist, underlie Santa Monica Bay and the deep-water Santa Monica basin to the south. The Santa Monica canyon marks a significant change in deformation of the lower slope of the Santa Monica Basin. North and northwest of this canyon, the slope is underlain by a little-deformed sediment apron; the main apron structures are two anticlines that extend toward Point Dume and are cored by reverse or thrust faults. Southeast of the canyon, lower-slope rocks are deformed by a complex zone of strike-slip, normal and reverse faults. The San Pedro bathymetric escarpment rises abruptly along the southeast side of Santa Monica Canyon. Structures underpinning this escarpment steepen progressively southeastward; they cut downward into basement rocks, and merge with the San Pedro Basin fault zone, which is nearly vertical and possibly strike-slip. The escarpment and its attendant structures extend for 60 km along the margin, separating the continental shelf from the mid-bathyal Santa Monica and San Pedro basins. The Santa Monica Basin, the more extensive of the two, contains about 1.5 km of turbidite fill that is deformed only near the northern basin margin. Data from ODP site 1015 indicate that this fill is most likely no older than Quaternary, and possibly no older than 600 ka. To the south, the San Pedro basin is a narrow graben that widens to the southeast. Locally near the Redondo Canyon and Palos Verdes Peninsula, graben rocks are deformed by reverse faults and folds.

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Sliter et al, 2001, AGU poster:
Does recent deformation at the base of slope provide evidence of a connection between the Newport-Inglewood and the Rose Canyon fault zones offshore southern California?

Abstract:
The possible offshore connection of the Newport-Inglewood fault zone (NIFZ) and the Rose Canyon fault zone (RCFZ) between Newport Beach and La Jolla, California is important to the assessment of earthquake hazards in southern California. One or more strands of the NIFZ head offshore near Newport Beach; the RCFZ heads offshore and offsets the Scripps submarine canyon near La Jolla. Many workers have proposed that the faults are connected by a complex zone of faulting along the continental shelf, with the main deformation occurring near the shelf edge. However, fault strands mapped on the shelf north of Oceanside do not disturb the seafloor.

The USGS collected high-resolution (35 in³ GI gun, 250 m 24-channel streamer) multichannel seismic reflection (MCS) data in 1998 and 1999, and high-resolution Geopulse (boomer) data over the shelf and slope in 2000. We observe sediments at the seafloor deformed near the base of the slope at water depths of about 700 m on MCS data between Dana Point and Oceanside. Between Oceanside and Carlsbad, at about 300 m water depth, we observe folding of the seafloor. The boomer data show recent faulting on the shelf (< 100 m water depth) associated with the Rose Canyon fault from Carlsbad to La Jolla. We interpret the base of the slope faulting to be related to a strand of the NIFZ. This strand may connect with the RCFZ by a left step near Carlsbad, as evidenced by recent folding of the seafloor.

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Bohannon and Gardner, 2000, AGU oral presentation:
Submarine Landslides of San Pedro Sea Valley, Southwest Los Angeles Basin

Abstract:
The coastal infrastructure of the southern greater Los Angeles metropolitan area would be profoundly affected by a large tsunami of local derivation. Submarine slope failures and active faults, either of which could potentially generate a large tsunami, are both known on the shelf and slope near Long Beach. This paper examines features suspected of being large slope failures on the San Pedro Escarpment and on the basin slope adjacent to the San Pedro shelf using detailed bathymetry and seismic profiles. The southeastern part of the escarpment has had a long history of slope failure. The most recent of these, the valley failure scar, is over 4.5 km long, might be less than 500 years old, and involved over 0.34 km³ of material, which now litters the adjacent basin floor. Other smaller deposits from other nearby failures are also present, as are buried wedges of allochthonous debris which indicate that slope failure has been occurring locally throughout the Holocene and much of the late Pleistocene. Slope failures have occurred in response to continual uplift of the Palos Verdes anticlinorium during that same period. A large feature on the basin slope south of the San Pedro Shelf, called the lower-slope structure, has many morphologic characteristics of a slope failure, but is probably an incipient uplift of tectonic origin. Although it has a distinct upper slope break that resembles a headwall-breakaway and it appears to have a rumpled toe that overrides basin sediment, seismic profiles indicate that it is a deeply rooted structure. The apparent breakaway is merely a sharp change in slope caused by folding and the toe is a series of normal faults with intermediate west dips. The entire structure is underlain by a shallow basement core. The valley failure scar is a likely candidate for failure-related tsunamogenisis because it started in shallow water, evolved on low-drag bedding planes, had a long slide path, and involved high-strength li thified material. The lower-slope structure may have seismic tsunomogenic potential. Since all features are pre-historic, the ancient record in and around Long Beach should be examined for evidence of local tsunami activity.

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Reid et al, 2000, AGU poster:
Multibeam Image and New High-Resolution Seismic Reflection Profiles Confirm Recent Deformation in the Loma Sea Valley, Offshore San Diego

Abstract:
Recently acquired multibeam images of the Loma Sea Valley, offshore San Diego, illuminate the effects of recent faulting and folding on the seafloor as evidenced by newly acquired multi-channel airgun and Huntec high-resolution seismic-reflection profiles. The Loma Sea Valley lies between the northern extension of Coronado Bank and the Point Loma peninsula of San Diego, and the valley is underlain by multi-strand faults that are considered part of the Palos Verdes Hills-Coronado Bank (PV-CB) fault system. The seismic-reflection profiles show a group of separate fault strands that cut the mid-slope region and trend north-northwest parallel to the valley axis and Coronado Bank. Some of these strands extend to and cut the sea floor, appearing as lineations in the high-resolution multibeam bathymetric data and affecting the seabed morphology, forming lows and controlling drainage. A separate fault strand on the lower slope trends parallel and adjacent to the valley axis and is evident as a lineation in the multibeam bathymetry for at least six kilometers. These fault strands appear on the seismic-reflection profiles as vertical to steeply dipping and some show complex changes in deformation style and character along strike, in some cases suggesting differential block movement along individual fault segments. This multi-strand fault zone, about 5-km wide, is subparallel to the better-known Rose Canyon Fault system that transects nearby downtown San Diego. Both the Rose Canyon fault and Loma Sea Valley segment of the PV-CB system are part of the West Coast's distributed shear fault systems making up the Pacific/North American plate boundary. These seismic-reflection profiles, along with the multibeam bathymetric data, suggest that faults within the Loma Sea Valley area may pose a hazard to western San Diego of the same magnitude as faults in the Rose Canyon system.

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Simila et al, 2000, AAPG poster:
The Los Angeles Region Seismic Experiment, Phase II (LARSE II); a survey to identify major faults and seismic hazards beneath a large metropolitan area

Abstract:
A number of institutions, including the U.S. Geological Survey and the Southern California Earthquake Center, collaborated in a seismic-imaging survey known as the Los Angeles Region Seismic Experiment, Phase II (LARSE II). This survey included an active and passive component and was concentrated along a 100 km-long corridor extending from Santa Monica Bay northward to the western Mojave Desert, crossing the Santa Monica Mountains, the San Fernando Valley (Northridge epicentral area), the Santa Susana Mountains, and the western Transverse Ranges. In the active component, 1400 seismographs were deployed at 100 m spacing along the main corridor, with shot points approximately 1000 m apart; with additional cross-lines. Chief imaging targets included the Santa Monica, San Gabriel, and San Andreas faults, blind thrust faults (e.g., Northridge fault), and the depths and shapes of the sedimentary basins in the San Fernando Valley and Santa Monica areas. Preliminary analysis indicates good data quality and energy transmission. Small shots (5-20 lbs) in the San Fernando Valley were recorded 50-80 km away in the Tehachapis.

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Holton et al, 1999, AGU poster:
Holocene deformation in the Santa Monica Basin, offshore southern California

Abstract:
The upper 200 meters of slope sediment and basin-floor turbidites in the Santa Monica Basin were surveyed by the Canadian Geological Survey in 1992 and the U.S. Geological Survey in 1998 using high resolution deep-tow boomer (~100 m penetration) and multichannel airgun seismic-reflection systems (~1000 m penetration). This cooperative study establishes a basin-wide seismic-stratigraphic framework, which can be used to establish the character of deformation, especially in the vicinity of previously mapped faults near the basin margins. Constraints on the timing of deformation are tied to recognizing disruption of key stratigraphic marker horizons within the sediment fill. We have assigned tentative age ranges to the key horizons using preliminary drilling results, from ODP Leg 167 at site 1015, that indicate an early to mid Holocene sedimentation rate approaching 3 meters per thousand years (Shipboard Scientific Party, 1997). With the high resolution (~0.5 meter) capa bilities of the deep-tow boomer system, we are able to infer dates for a suite of key reflectors in the upper 50 meters of sediment. Deformation features such as slumps, growth folds, and fault offsets that disrupt Holocene sediment are common near all margins of the Santa Monica Basin. Estimates for the time of deformation, based on the high sedimentation rate in the basin, indicate widespread tectonic activity over the last 12 ka. Some offsets on the eastern slope reach the sea floor, but timing of motion on these features is not well constrained because the rate of late Holocene slope deposition is less certain than on the basin floor.

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Marlow et al, 1999, GSA poster:
Palos Verdes Fault Complex: An Example of Fault Segmentation in Offshore Southern California

Abstract:
Recently acquired high-resolution multibeam bathymetric imagery reveals several linear ruptures in the sea floor on the upper continental slope southeast of Palos Verdes Peninsula. High-resolution multichannel and boomer seismic-reflection profiles show that these linear ruptures are the surficial expressions of Holocene faults with vertical to steep dips. One prominent fault, which lies about 10 km to the west of the mapped trace of the Palos Verdes Fault, can be traced on the multibeam imagery for 14 km between the shelf edge and the base of the continental slope. The fault trace is informally called the Avalon Knoll fault for the nearby geographic feature of that name. The reflection profiles show that the Avalon Knoll fault is part of a northwest oriented complex of faults and anticlinal uplifts that are evident as scarps and bathymetric highs on the multibeam imagery. The Palos Verdes Fault bounds this complex on the east and can be discontinuously traced on the multibeam imagery and geophysical profiles from Lasuen Knoll to the shelf edge. Folding coincident with the Palos-Verdes Fault alternates in intensity and bathymetric expression on either side of the Palos Verdes Fault between Lasuen Knoll and the shelf edge, suggesting that a scissoring of tectonic blocks occurs along the fault trace. The multibeam-bathymetric imagery allows the ready identification of previously unmapped fault traces that rupture the sea floor. The imagery also shows offset slope drainage systems that would not be evident on a grid of geophysical profiles alone. The imagery data are a valuable tool for delineating offshore earthquake hazards through recognition of the length of offset sea floor, and these offsets can be confirmed by carefully sited, high-resolution seismic-reflection profiles.

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Normark et al, 1999, AGU poster:
Evidence for a buried gas-hydrate mound in turbidite fill of the Gulf of Santa Catalina, offshore southern California

Abstract:
The possible presence of a buried gas-hydrate mound in turbidite deposits offshore Oceanside, California is indicated by an anomalous column of concave-upward reflections capped by a convex-upward reflection that has been observed in two multichannel seismic-reflection profiles across the basin floor in 850 m water depth. The column of anomalous reflections is about one kilometer wide and extends from about 45 m below the sea floor (mbsf) in the otherwise flat-lying sediment to a depth of at least 200 mbsf. This distinct column of convex/concave reflections generally resembles velocity-amplitude structures (VAMPs) described from the deep Bering Sea (Scholl and Cooper, 1978). In the Bering Sea examples, VAMPs are caused by a localized gas-hydrate layer, which causes a velocity pull-up near the base of the hydrate stability zone. The hydrate overlies a column of free gas that results in a velocity push-down (i.e., concave-upward reflections). The reflection geometry of the acoustic anomaly observed offshore southern California cannot be interpreted as a VAMP because: (1) the relief (in two-way travel time) of the concave-upward reflections is too great to result from a low-velocity free-gas zone; and (2) based on nearby observations of heat flow and bottom-water temperature, the base of the methane gas-hydrate stability zone is at least 700 mbsf. The southern California feature occurs above a diapiric structure that intrudes the flat-lying basin-plain turbidites. A probable explanation for this acoustic anomaly is a buried mound of sediment containing gas hydrate and, possibly, authigenic carbonate. The convex-upward reflection is the upper surface of this buried mound, which formed by gas/fluid migration above the diapir, and the concave reflections might record collapse of the diapiric crestal zone. Similar, but less striking, examples of possible lens-shaped gas-hydrate accumulations are observed on nearby multichannel seismic-reflecti on profiles.

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Bohannon, 1998, GSA oral presentation:
Tertiary tectonic evolution of the inner continental borderland, California

Abstract:
The inner continental borderland offshore of Los Angeles and San Diego has been likened to large metamorphic core-complex, based on data and interpretations derived from multi-channel seismic-reflection, seismic-refraction, and regional geologic studies. Neogene sedimentary rocks overlie a basement of Catalina Schist that has been intruded by Miocene plutons throughout this unique belt. The belt of schist is separated on its west side from the gently deformed late Cretaceous and Paleogene sedimentary rocks of the Nicolas fore-arc belt by faults with steep west dips and pronounced normal separations. On its east side the schist belt is bounded by a large detachment fault that dips gently to the east beneath the west edge of the Peninsular Ranges belt at the coastline near Oceanside. The Catalina Schist was uplifted from middle crustal depths and exposed during a major event of extensional tectonism that started in the early Miocene in conjunction with about 10° of clockwise rotation of the western Transverse Ranges belt. Part of the uplift of the Catalina Schist could have occurred on the detachment fault, but mostly it is thought to have occurred on the steep faults that bound the west edge of the schist belt. A large amount of uplift is required and it probably involved strong footwall flexural deformation in the wake of the translating and rotating western Transverse Ranges and Nicolas fore-arc belts. Extension, accompanied by probable large amounts of right slip, continued in the borderland region during and after the middle Miocene. The later stage of extension was accompanied by rapid clockwise rotation of the western Transverse Ranges of at least 90°. Most of the borderland, including the belt of schist that was uplifted in the early Miocene, was further deformed into numerous basins and ridges during this stage of oblique extension. The primary driving force for the deformation is thought to have been derived from the rapid northwest motion of the Pacific plate after it had become coupled to the Farallon plate system which had previously been subducted beneath the borderland.

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Bohannon et al, 1998, AGU poster:
Seismic hazard potential of offshore Los Angeles Basin based on high-resolution, multibeam bathymetry and close-spaced, seismic-reflection profiles

Abstract:
Young structures and mass flows on the offshore margin of Los Angeles Basin might produce nearshore earthquakes or tsunamis that could pose a hazard to the nearby urban corridor. These features are delineated on detailed bathymetry, backscatter maps, and high-resolution seismic-reflection profiles. Simrad EM1000 and EM 300 high-frequency, multi-beam data provide new base maps for much of the area. A Huntec deep-tow boomer obtained high-resolution (0.4m), vertical-incidence, sub-bottom profiles to depths corresponding to 25-125 milliseconds (TWTT). A 24 channel-seismic-reflection system acquired data to depths corresponding to at least 1 second (TWTT) using a 35 cu. in. gas-injector airgun. Close line spacing for the seismic-reflection data enabled tracing structures through the area. Numerous large folds that appear to be actively growing are aligned parallel to the base of the slope that forms the west-southwest edge of the Santa Monica shelf. These folds, particula rly the anticlinal structures, are bathymetrically expressed in the seafloor relief and their growth history is delineated by flanking unconformities and stratigraphic pinch-outs. Many of the anticlines are bounded on at least one side by short fault segments that offset some of the youngest reflectors. Some of the flanking faults break through to the seafloor. The steep west-southwest flank of the Palos Verdes uplift has been the source of numerous, large mass flow deposits that cover extensive areas in San Pedro Basin. Many of these mass flow deposits are probably Holocene in age and if they prove to have occurred catastrophically, they may have generated tsunamis. Our data suggest the strong possibility for continued mass wasting on these steep slopes. Numerous northwest-oriented active fault traces, including the previously documented Palos Verdes fault, are evident in our data on the Long Beach shelf and slope. These faults are associated with the growth and development of a large complex of submarine canyons at the shelf edge, the Lasuen Knoll anticline, and numerous unnamed ridges. Although seismicity is relatively low in this part of the California Continental Borderland, the number and variety of active structures that are evident in our dataset suggest that this region might have a high potential for seismic hazards.

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Clarke et al, 1998, GSA poster:
Kinematics of the Palos Verdes fault zone in Los Angeles Harbor, California, from multidisciplinary studies

Abstract:
Recent high and very-high resolution seismic-reflection profiling in the Los Angeles Harbor main channel provides evidence of a structural transition along the Palos Verdes fault zone, from a regime of right-oblique slip (SW-side up) that extends southeastward from the Palos Verdes Hills, to predominantly right slip to the south in the Los Angeles outer harbor and on the San Pedro shelf. In the Southwest Slip, at the northern end of our study area, a reflector tentatively correlated to the unconformity between the Lakewood and San Pedro Formations (estimated age 400-650ka), shows a vertical separation of about 40m across a narrow, sharply defined zone. This implies a minimum Quaternary vertical separation rate of about 0.06-0.10 mm/yr, which is only about 20-25% of the uplift rate estimated from marine terraces in adjacent San Pedro. At Vincent Thomas Bridge, only 800m to the south, vertical separation of the Lakewood-San Pedro contact is reduced to ~18-21m across a ~400m wide fault zone. Recent strike-slip activity appears confined to the westernmost part of the zone, whereas most vertical offset of the Lakewood-San Pedro contact occurs to the east. Although not resolved in the seismic records, logs of boreholes recently drilled beneath the bridge suggest that as much as 10m of vertical separation may have occurred in Holocene time. In these newly drilled holes, paleontological analysis, combined with ¹⁴C and amino-acid dating of recently acquired samples will establish stratigraphic control across the fault zone, enhance interpretations of the seismic records, and provide detailed constraints on timing and strain partitioning along the Palos Verdes fault in the Los Angeles Harbor.

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Kennedy et al, 1998, GSA poster:
Holocene faulting on the Rose Canyon fault zone, San Diego Bay, California

Abstract:
An important charge of the Southern California Areal Mapping Project (SCAMP) is geologic mapping and assessment of geologic hazards of the southern California continental margin. The recently completed multidisciplinary SCAMP study of the Rose Canyon fault zone, in San Diego Bay, has proven to be important in evaluating the safety of the Coronado Bridge.Sub-bottom seismic reflection profiling surveys controlled by precision GPS navigation, coupled with coring, radiocarbon dating, aminostratigraphy and molluscan biostratigraphy have been used to unravel the geometry and Holocene/Pleistocene fault history of a portion of the Rose Canyon fault zone in the vicinity of the Coronado Bridge.

Sub-bottom seismic reflection profiling surveys controlled by precision GPS navigation, coupled with coring, radiocarbon dating, aminostratigraphy and molluscan biostratigraphy have been used to unravel the geometry and Holocene/Pleistocene fault history of a portion of the Rose Canyon fault zone in the vicinity of the Coronado Bridge.

A network of approximately 500 line-km of closely spaced (~50 m), orthogonally positioned (sub-meter accuracy), high- and very-high resolution reflection seismic lines were used to develop a detailed model of the fault geometry of the central portion of San Diego Bay. Molluscan shell material collected from a series of coreholes drilled adjacent to the youngest observed faults were dated and used to establish the Holocene/Pleistocene boundary in the vicinity of the Coronado Bridge. The Holocene sediments consist of a fine-grained, estuarine sequence that is approximately 6,500 yrs old at its base. The youngest dated materials that are faulted are 4435+/-115 yrs, although faults occur higher in the section in stratigraphically younger deposits.

The Holocene fault geometry of the bay is characterized by small-scale detachment faults in a predominately strike-slip environment. Four prominent Holocene northeast-trending, normal faults that are an integral part of the detachment fault system are of special concern in that they underlie piers 1, 5, 14 and 17 of the Coronado Bridge.

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Normark et al, 1998, AAPG poster
Depositional architecture and outcrop-scale acoustic facies analysis for Hueneme and Dume fans, Santa Monica Basin, California

Abstract:
Fifteen widely correlatable key seismic reflectors provide a stratigraphic framework for interpreting Quaternary turbidite systems in Santa Monica Basin. Eight acoustic facies are distinguished with deep-tow boomer seismic-reflection profiles, which have a vertical resolution of a few tens of centimeters and acoustic penetration of 20 to 50 m. Acoustic facies, which are correlated with available core data, and reflector geometry have been integrated to define six architectural elements together with a number of subelements that are of a scale that can be recognized in outcrops of ancient turbidites on land. On Hueneme Fan, two distinct overbank elements record construction and partial erosion of laterally confined secondary levees. Near the termination of fan channels on the upper midfan, sandy facies form compensation cycles. The middle fan comprises three main subelements: sandy channel-fill, low-gradient sandy lobes, and a scoured-lobe subelement formed of alternating sand and mud filling erosional depressions. The site of thickest lobe sediment accumulation shifts from one part of the fan surface to another through time, i.e., compensation cycles. The lower fan comprises main sheet-like alternations (locally gently lensing) of sand and mud. The Santa Clara River delta is the dominant source for Hueneme Fan, for which changes in facies distribution reflect variations in the rate at which sea level rose since the last glacial maximum. The smaller Dume Fan, which is composed of coarser sediment supplied through littoral drift, has steeper gradients, less prominent levees, and displays fewer subelements than Hueneme Fan.

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ten Brink et al, 1998, AGU:
The thermal structure of California

Abstract:
The thermal and magmatic history of coastal California is generally interpreted as a consequence of asthenospheric upwelling beneath part of the forearc where the subducted lithosphere was removed in the wake of the northward migration of the Mendocino Triple Junction (MTJ). We suggest that, with the exception of the Inner California Borderland (ICB), the magmatic and thermal history of coastal California does not require asthenospheric upwelling and can simply be explained by thermal re-equilibration following cessation of subduction of young oceanic lithosphere. Geothermal gradients in the forearc and underlying oceanic crust are suppressed during subduction because of the cooling effect of the downgoing slab. When subduction stops, the geothermal gradient returns to normal bringing the temperatures in the now-fossil ocean crust to 600-750°C for parameters typical to coastal California (20-30 km thick forearc, 10-4 Ma subducting slab, 15° subduction angle, and 40 km/my subduction rate). The recovery of the thermal gradient results in a rapid increase of surface heat flow to 60-77 mW/m2, which is consistent with observations. The recovery of the thermal gradient can also generate some partial melting in the fossil ocean crust at 25-35 km depths. Amphibolite dehydration-melting can take place at temperatures as low as 630°C and pressures of 8 kbar (~25 km). This partial melting of the ocean crust could produce the minor calcalkaline Neogene volcanism in coastal California which followed the northward migration of the MTJ. The isotopic and trace-element signatures of the volcanic rocks are indicative of crustal anatexis. A recent seismic survey close to the MTJ indicates the presence of melt lenses at the base of a high velocity crustal layer, which is consistent with crustal anatexis occurring presently. Basal layers of high crustal velocities, interpreted as fossil oceanic crust, are observed under northern and ce ntral California, but not under the ICB. The ICB underwent considerable extension in the mid-Miocene. The heat flow in the ICB is indeed still high (67-89 mW/m2), consistent with high subcrustal temperatures. Volcanism here is more voluminous, its composition is indicative of a greater melting depth, and its isotopic and trace-element contents are closer to that of MORB. The ICB therefore is suggested to be the only coastal California region underlain by asthenospheric upwelling following the cessation of subduction. A consequence of the proposed thermal structure is a progressive shallowing of the brittle-ductile transition (except for the ICB) from near the base of the forearc at the end of subduction to 1315 km depth at present, which is consistent with the observed epicentral depths.

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ten Brink et al, 1998, AGU oral presentation:
Crustal structure of the Inner California Borderland: Evidence for modern deformation and for a Miocene metamorphic core complex

Abstract:
Wide-angle seismic reflection and refraction data along two lines crossing the Inner California Borderland (ICB) indicate a seismic velocity structure which is remarkably similar to that of metamorphic core complexes in the Basin and Range Province. The lines collected as part of the 1994 LARSE experiment trend roughly north-south from San Clemente Ridge to Long Beach and Santa Monica, respectively. The data support Crouch and Suppe's (1993) model for the origin of the ICB as an autochthonous highly-extended region, which was possibly formed during the Miocene by the rotation of the Transverse Ranges and by the rifting along what was then the North American-Pacific plate boundary. Lateral velocity heterogeneities in the vicinity of known faults are confined to the upper 5 km of the crust. The lower crustal velocity is laterally homogenous within the limits of our resolution and possibly consists entirely of quartz-rich Catalina Schist. The Moho is subhorizontal and i s located at a depth of 19-22 km. Thickening of the crust occurs beneath the continental shelves of Santa Monica Bay and Long Beach and marks the northern and eastern boundaries of the ICB. These data, and coincident MCS reflection data, also indicate modern compressional or transpressional deformation between the coast and the southwestern edge of Catalina Ridge. Both the Palos Verdes and the San Pedro Basin faults appear to change their sense of motion between the two profiles. The Palos Verdes fault terminates at a shallow depth (2-3 km) against a basement (?) high, whereas the San Pedro Basin Fault extends to a depth of at least 5 km within the crust. Finally, our data provide no evidence for underplated oceanic crust under the ICB, contrary to seismic observations from other parts of coastal California. The absence of this layer may be a local phenomenon caused by a slab gap under the ICB or representative of southern California as a whole. We construct a hypothetical g eothermal gradient for the ICB, which is in agreement with geochemical constraints, present heat flow, and the origin of the province as a metamorphic core complex. We use it to show that the shallow depth of earthquake epicenters beneath the ICB relative to onshore southern California is due to the felsic crustal composition, and not to the thermal gradient.

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Clarke et al, 1997, GSA poster:
Seismic-reflection study of the Palos Verdes fault zone in the vicinity of the Los Angeles Harbor, California

Abstract:
Approximately 150 line-km of digitally-recorded, single-channel 350 Joule ORE Geopulse data and 50 line-km of 24 channel MCS data using a 16 in³ airgun source were collected from the San Pedro and Long Beach Channels, and from the Los Angeles outer harbor during mid 1996. Data collection was conducted jointly by the California Division of Mines and Geology and the U.S. Geological Survey, and was funded by the California Department of Transportation to study the Palos Verdes fault zone pursuant to seismic retrofit of the Vincent Thomas Bridge. Single-channel data provide 1-2 m resolution at shallow depths beneath the floor of the channels, whereas the MCS data provide 2-10 m resolution to maximum depths of about 500 milliseconds (~450 m).

Maximum age of faulting in the vicinity of the Vincent Thomas Bridge was largely constrained by disruption of the contact (ca. 600-650 ka) between the Lakewood and San Pedro Formations. One kilometer north-northwest of the bridge, the fault is a discrete rupture that cuts Holocene strata and extends to the channel floor. Beneath the bridge, the fault appears as a 400 m-wide zone of disruption containing multiple traces that closely approach or cut the channel floor. This may reflect bifurcation or a right-step of the fault zone. Data from this area are consistent with an upwardly anastomosing fault that may be resolved at depths greater than we have imaged as a negative flower structure resulting from divergent wrench faulting. In the Los Angeles outer harbor, faults in our date approach, but do not cut, strata assigned by McNeilan and others (1996) to the 8-10 ka "transgressive marine section", and would thus be assigned a late Quaternary age.

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Kennedy and Clarke, 1997, GSA poster:
An analysis of the Rose Canyon fault zone in San Diego Bay, California

Abstract:
Approximately 350 line-km of digitally-recorded, single-channel 350 Joule ORE Geopulse date and 130 line-km of 24 channel MCS data using a 16 in³ airgun source were collected in San Diego Bay during early-mid 1996. Data collection was conducted jointly by the California Division of Mines and Geology and the U.S. Geological Survey, and was funded by the California Department of Transportation to study the Rose Canyon fault zone pursuant to seismic retrofit of the Coronado Bridge. Single-channel data provide 1-2 m resolution to depths of about 120 milliseconds (~110 m) beneath the floor of the bay, whereas the MCS data provide 2-10 m resolution to depths of about 500 ms (~450 m). Together, these data have enabled us to examine the architecture of faulting in the bay in great detail.

San Diego Bay in the vicinity of the Coronado Bridge is characterized by small-scale detachment faulting in a predominantly strike-slip environment. A series of northeast-trending normal faults together form a graben that is centered along the axis of the bay. These normal faults appear to have formed in a tensional environment that is bounded by north-to-northwest-trending right-lateral strike-slip faults of the greater Rose Canyon fault zone that lie along both the eastern and western margins of the bay. Major faults mapped in San Diego Bay extend upward to near the floor of the bay (to within about 5-15 ms of the seafloor) in youthful water-saturated sediment. As the Mount Soledad strand of the Rose Canyon fault zone is known to cut Holocene deposits onland in the San Diego area, and based on the close upward proximity of faults mapped in this study to the bay floor in the vicinity of the Coronado Bridge, we consider these faults to be of probable Holocene age.

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Klitgord and Brocher, 1996, AGU:
Oblique-slip deformation in the San Pedro Basin offshore Southern California region

Abstract:
Multichannel seismic-reflection profiles acquired offshore Los Angeles during the 1994 LARSE experiment and in 1990 with the RV LEE imaged the zone of active deformation within the San Pedro Basin and along the eastern edge of Santa Monica Basin. This zone of deformation is bound on the west by the San Pedro Basin fault, an oblique-slip fault with a significant normal dip-slip component (over 500m) down thrown to the east. The San Pedro Basin fault coincides with the western limit of a dense distribution of small to moderate magnitude (Mw 3-5) earthquakes that extends east across the Palos Verdes and Newport-Inglewood fault zones. Redondo and Avalon knolls are located on the footwall (west side) of the San Pedro Basin fault and are overlain by thin (<500m), moderately deformed Pleistocene to Recent turbidites. In contrast, a thicker section (up to 1000m) of more intensely folded and faulted Pleistocene to Recent turbidites are found on the hanging wall to the east . Isopach and fault-offset (vertical) records for the Pleistocene to Recent strata in the deep water east of the San Pedro Basin fault are used to investigate how deformation has been partitioned across this part of the oblique-slip fault system. At present the most intense folding and dip-slip faulting are located about 6 km east of the San Pedro Basin fault in a zone a few km wide. A narrower 1-km wide zone of chaotic faulting but only minimal strata offset is found about 6 km further east near the shelf edge and may coincide with a region dominated by strike-slip faulting. This change in deformation character across the basin is interpreted to indicate that the proportion of dip-slip to strike-slip faulting increases to the west across the oblique-slip fault zone.

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