Oak Ridge Fault Research Articles

Tectonic controls on Quaternary landscape evolution in the Ventura basin, southern California, USA, quantified using cosmogenic isotopes and topographic analyses

AbstractThe quantification of rates for the competing forces of tectonic uplift and erosion has important implications for understanding topographic evolution. Here, we quantify the complex interplay between tectonic uplift, topographic development, and erosion recorded in the hanging walls of several active reverse faults in the Ventura basin, southern California, USA. We use cosmogenic 26Al/10Be isochron burial dating and 10Be surface exposure dating to construct a basin-wide geochronology, which includes burial dating of the Saugus Formation: an important, but poorly dated, regional Quaternary strain marker. Our ages for the top of the exposed Saugus Formation range from 0.36 +0.18/-0.22 Ma to 1.06 +0.23/-0.26 Ma, and our burial ages near the base of shallow marine deposits, which underlie the Saugus Formation, increase eastward from 0.60 +0.05/-0.06 Ma to 3.30 +0.30/-0.41 Ma. Our geochronology is used to calculate rapid long-term reverse fault slip rates of 8.6–12.6 mm yr–1 since ca. 1.0 Ma for the San Cayetano fault and 1.3–3.0 mm yr–1 since ca. 1.0 Ma for the Oak Ridge fault, which are both broadly consistent with contemporary reverse slip rates derived from mechanical models driven by global positioning system (GPS) data. We also calculate terrestrial cosmogenic nuclide (TCN)-derived, catchment-averaged erosion rates that range from 0.05–1.14 mm yr–1 and discuss the applicability of TCN-derived, catchment-averaged erosion rates in rapidly uplifting, landslide-prone landscapes. We compare patterns in erosion rates and tectonic rates to fluvial response times and geomorphic landscape parameters to show that in young, rapidly uplifting mountain belts, catchments may attain a quasi-steady-state on timescales of <105 years even if catchment-averaged erosion rates are still adjusting to tectonic forcing.

Structure of the San Fernando Valley region, California: Implications for seismic hazard and tectonic history

Industry seismic reflection data, oil test well data, interpretation of gravity and magnetic data, and seismic refraction deep-crustal profiles provide new perspectives on the subsurface geology of San Fernando Valley, home of two of the most recent damaging earthquakes in southern California. Seismic reflection data provide depths to Miocene–Quaternary horizons; beneath the base of the Late Miocene Modelo Formation are largely nonreflective rocks of the Middle Miocene Topanga and older formations. Gravity and seismic reflection data reveal the North Leadwell fault zone, a set of down-to-the-north faults that does not offset the top of the Modelo Formation; the zone strikes northwest across the valley, and may be part of the Oak Ridge fault system to the west. In the southeast part of the valley, the fault zone bounds a concealed basement high that influenced deposition of the Late Miocene Tarzana fan and may have localized damage from the 1994 Northridge earthquake. Gravity and seismic refraction data indicate that the basin underlying San Fernando Valley is asymmetric, the north part of the basin (Sylmar subbasin) reaching depths of 5–8 km. Magnetic data suggest a major boundary at or near the Verdugo fault, which likely started as a Miocene transtensional fault, and show a change in the dip sense of the fault along strike. The northwest projection of the Verdugo fault separates the Sylmar subbasin from the main San Fernando Valley and coincides with the abrupt change in structural style from the Santa Susana fault to the Sierra Madre fault. The Simi Hills bound the basin on the west and, as defined by gravity data, the boundary is linear and strikes ∼N45°E. That northeast-trending gravity gradient follows both the part of the 1971 San Fernando aftershock distribution called the Chatsworth trend and the aftershock trends of the 1994 Northridge earthquake. These data suggest that the 1971 San Fernando and 1994 Northridge earthquakes reactivated portions of Miocene normal faults.

Neotectonics of the Offshore Oak Ridge Fault near Ventura, Southern California

The Oak Ridge fault is a large-offset, south-dipping reverse fault that forms the south boundary of the Ventura Basin in southern California. Previous research indicates that the Oak Ridge fault south of the town of Ventura has been inactive since 200–400 ka ago and that the fault tip is buried by ∼ 1 km of Quaternary sediment. However, very high-resolution and medium-resolution seismic reflection data presented here show a south-dipping fault, on strike with the Oak Ridge fault, that is truncated at 80 m depth by an unconformity that is probably at the base of late Pleistocene and Holocene sediment. Furthermore, if vertically aligned features in seismic reflection data are eroded remnants of fault scarps, then a subsidiary fault within the Oak Ridge system deforms the shallowest imaged sediment layers. We propose that this subsidiary fault has mainly left-slip offset. These observations of Holocene slip on the Oak Ridge fault system suggest that revision of the earthquake hazard for the densely populated Santa Clara River valley and the Oxnard coastal plain may be needed.

Comment on "Faulting Apparently Related to the 1994 Northridge, California, Earthquake and Possible Co-seismic Origin of Surface Cracks in Potrero Canyon, Los Angeles County, California," by R. D. Catchings, M. R. Goldman, W. H. K. Lee, M. J. Rymer, and D. J. Ponti

Catchings et al. (1998) hypothesized that several shallow faults (<100 m deep) and two deeper reverse faults (∼400–1500 m deep) cut through gently dipping bedding in the core of a broad syncline below Potrero Canyon based on shallow, high-resolution, seismic-reflection profiles modeled from a seismic line completed by the U.S. Geological Survey (USGS) in 1994 following the Northridge, California, earthquake. Catchings et al. (1998) then attempted to correlate the faults with ground cracking produced in Potrero Canyon by the earthquake and with the Oak Ridge fault system based on map relations and projection of the profiles into nearby published cross sections. However, in contrast to the flat structural geometry inferred at depth by Catchings et al. (1998) published maps, outcrop geology, and subsurface geologic data all indicate that the bedrock at the seismic line dips 40° to 60° northward. The subsurface structure inferred by Catchings et al. (1998) cannot be reasonably reconciled with the known surface geology, the structure shown in nearby published cross sections, or structure contours based on oil-well data. This indicates that the geologic structure at depth is not accurately represented by the seismic profile, and therefore the interpreted faults are not substantiated. In addition, field evidence and ground-rupture modeling overwhelmingly indicate that fault rupture during the Northridge earthquake ceased below a depth of 4 km and that surface cracking in Potrero Canyon was the result of strong ground motions and secondary effects rather than surface faulting. A combination of errors in the P -wave velocity model and reflection artifacts are a likely cause for the inaccurate seismic profile. Our main concerns outlined previously are discussed in detail along with apparent inconsistencies in the correlation of the seismic profile with published cross sections. Catchings et al. (1998) (referred to herein as “the authors”) stated that Potrero Canyon …

Geomorphic indicators of active fold growth: South Mountain–Oak Ridge anticline, Ventura basin, southern California

South Mountain–Oak Ridge, near Ventura, California, is an asymmetric anticlinal uplift forming at the present time above the active, buried Oak Ridge reverse fault. Shortening along the Oak Ridge fault accumulated largely in Quaternary time and is responsible for the growth and present topography of the westernmost 15 km of the ridge during the past 0.5 m.y. Tectonic geomorphic analysis using several indices of active tectonics provides information concerning fold growth. Stream-gradient indices are relatively high in the northern, eroded fold scarp of the ridge, a pattern consistent with the existence of active, rapid slip on the Oak Ridge fault. Mountain-front sinuosity along the northern slope of the anticlinal ridge roughly decreases from ∼2 to 1 toward the westernmost 10 km of observed surface folding. Valley floor width to valley height ratios along the northern flank of the ridge generally decrease westward from ∼1.5 to 0.5. Values of the hypsometric integral along the northern flank increase significantly from ∼0.35 to 0.4 (maximum ∼0.55) from east to west. Drainage density varies from ∼4 to 6 km/km2 along both flanks of the South Mountain–Oak Ridge anticline. Entrenchment of streams into the (southern) backlimb of the fold along the westernmost 9 km of the structure decreases from ∼20 m to <1 m from east to west. Apparent backlimb rotation, as measured by dip of strata along the westernmost 7 km of the fold, decreases from east to west, from ∼35° to 20°. Fold growth of the South Mountain–Oak Ridge anticline occurred during the past 0.5 Ma following deposition of the Saugus Formation. Lateral and vertical fold growth were likely produced by westward decrease in fault slip along the buried Oak Ridge fault.

Map restoration of folded and faulted late Cenozoic strata across the Oak Ridge fault, onshore and offshore Ventura basin, California

The Ventura basin lies within the east-west–trending active fold-and-thrust belt of the western Transverse Ranges, California. This basin has been the site of significant earthquakes on structures within it and bordering it. The purpose of our study is to identify the main structures in the basin and its borders and to quantify their rate of deformation. Our study includes the onshore and offshore Ventura basin, the arcuate basin-bounding Oak Ridge reverse fault, and the Oxnard shelf to the south. Shortening, fault-slip, and crustal-block motions were studied using a three-dimensional map-restoration technique. Structure-contour maps on the 6 Ma surface and other horizons were digitized and restored to the initial horizontal state by unfolding them using the computer program UNFOLD and then fitting the unfolded surfaces across faults. Comparing the restored and present configuration allows us to estimate total net finite displacements relative to a fixed horizontal reference line. Average post–5 Ma shortening rates estimated from our restoration are slower than both post–1 Ma rates and present rates determined by global positioning systems. Most shortening due to folding in the onshore basin is post–1 Ma, although slip on the Oak Ridge fault has occurred both before and after 1 Ma. Displacement due to faulting and folding includes left-lateral strike-slip motion on the northeast-southwest coastal segment of the Oak Ridge fault and associated clockwise rotation of the adjacent Ventura basin. The Oxnard shelf is bordered to the south by mountains and islands that have been previously interpreted as folds above thrust-fault ramps. This onshore-offshore block moves as one continuous thrust sheet. Similarly, beyond the well-studied onshore fault, a kinematically continuous offshore Oak Ridge–Mid-Channel left-oblique fault system is interpreted to continue at least an additional 100 km westward beneath the Santa Barbara Channel.

Map restoration of folded and faulted late Cenozoic strata across the Oak Ridge fault, onshore and offshore Ventura basin, California

Map restoration of folded and faulted late Cenozoic strata across the Oak Ridge fault, onshore and offshore Ventura basin, California

Mechanical modeling of compressional basins: Origin and interaction of faults, erosion, and subsidence in the Ventura basin, California

Using geophysical observations and some basic geological ideas on the present structure of the Ventura basin, we model its possible mechanical evolution during the last 4 million years. We use a two‐dimensional finite element model with frictional and elastoviscoplastic rheologies that allow strain localization. This allows us to study the mechanical origin of the San Cayetano thrust fault (SCF) and its interaction with the Oak Ridge fault (ORF). The results of our modeling indicate that the geometry and activity of a shear zone which mimics the San Cayetano fault are conditioned both by the preexistence of the Oak Ridge fault and by the shape of the brittle‐ductile transition. Furthermore, the presence of a décollement level is shown to favor the development of the San Cayetano fault shear zone as a low‐angle structure. The value of the effective friction on the Oak Ridge fault is evaluated from this model. Possible values of the viscosity of the sediments are also estimated; it is shown that low sediment viscosity may completely hinder the activity of such a fault at the surface, while slip at depth continues. The stratigraphic profile of the basin is also shown to depend on the relative strengths of the ORF and SCF, which also provide an explanation for the shortening and rapid subsidence of the basin.

Faulting apparently related to the 1994 Northridge, California, earthquake and possible co-seismic origin of surface cracks in Potrero Canyon, Los Angeles County, California

AbstractApparent southward-dipping, reverse-fault zones are imaged to depths of about 1.5 km beneath Potrero Canyon, Los Angeles County, California. Based on their orientation and projection to the surface, we suggest that the imaged fault zones are extensions of the Oak Ridge fault. Geologic mapping by others and correlations with seismicity studies suggest that the Oak Ridge fault is the causative fault of the 17 January 1994 Northridge earthquake (Northridge fault). Our seismically imaged faults may be among several faults that collectively comprise the Northridge thrust fault system. Unusually strong shaking in Potrero Canyon during the Northridge earthquake may have resulted from focusing of seismic energy or co-seismic movement along existing, related shallow-depth faults. The strong shaking produced ground-surface cracks and sand blows distributed along the length of the canyon. Seismic reflection and refraction images show that shallow-depth faults may underlie some of the observed surface cracks. The relationship between observed surface cracks and imaged faults indicates that some of the surface cracks may have developed from nontectonic alluvial movement, but others may be fault related. Immediately beneath the surface cracks, P-wave velocities are unusually low (&amp;lt;400 m/sec), and there are velocity anomalies consistent with a seismic reflection image of shallow faulting to depths of at least 100 m. On the basis of velocity data, we suggest that unconsolidated soils (&amp;lt;800 m/sec) extend to depths of about 15 to 20 m beneath our datum (&amp;lt;25 m below ground surface). The underlying rocks range in velocity from about 1000 to 5000 m/sec in the upper 100 m. This study illustrates the utility of high-resolution seismic imaging in assessing local and regional seismic hazards.

Abstract: North-Vergent Thick-Skinned or South-Vergent Thin Skinned Oak Ridge Fault: A View from the Coast

Abstract Two contrasting views of the Oak Ridge fault in the Ventura basin have emerged: a thick-skinned view based on onshore subsurface oil-well data and a thin-skinned view based on interpretation of seismic profiles and dipmeter data from the Santa Barbara Channel. The offshore Oak Ridge structure is interpreted as an active axial surface above the ramp of a blind, south-vergent thrust; growth triangles in the kink band south of this surface contain parallel, north-dipping strata in which the kink band tapers toward the surface. In contrast, dips of growth strata onshore near the coast at Montalvo steepen with depth, evidence of progressive limb rotation and a steeply-dipping, large-displacement Oak Ridge fault. The onshore Oak Ridge fault is northward-vergent. West of Oak Ridge, the post-Saugus displacement is transferred via the ductile Rincon Formation to folding of the Ventura-Rincon anticline, also a north-vergent structure onshore and offshore. An onshore seismic line along a cross section documented by electric logs shows a narrow panel of north dips that could be the onshore termination of an offshore kink band mapped in the Santa Barbara Channel. North dips at greater depth in this seismic line do not correspond to well data and are artifacts of processing. Farther east, north dips are possible only if the kink band turns to the northeast, parallel to the Montalvo fault. The onshore-offshore controversy will only be resolved by comparing seismic profiles in the Santa Barbara Channel with subsurface well profiles in the same transect using well-log correlations as well as dipmeter data.

Abstract: Map Restoration of a Quaternary Horizon across the Oak Ridge Fault, Offshore and Onshore Ventura Basin, California

Abstract: Map Restoration of a Quaternary Horizon across the Oak Ridge Fault, Offshore and Onshore Ventura Basin, California

Deformation rates across the Placerita (Northridge Mw = 6.7 aftershock zone) and Hopper Canyon segments of the western transverse ranges deformation belt

Abstract Aftershocks of the 1994 Northridge (Mw = 6.7) earthquake provide insights into the geometry of the seismic source faults in the San Fernando Valley and the east Ventura Basin and allow the calculation of deformation rates for the region. The Northridge thrust and Santa Susana faults dip in opposite directions, with the Northridge thrust entirely beneath the Santa Susana fault. These opposing reverse faults interact, resulting in a folded active Santa Susana fault and uplift in the footwall block of that reverse fault. Two balanced cross sections suggest thick-skinned deformation of the western Transverse Ranges. The western section, across the Modelo lobe segment of the north-dipping San Cayetano fault and the easternmost surface trace of the south-dipping Oak Ridge fault, is west of any aftershocks of the Northridge earthquake and has been termed the Hopper Canyon segment of the deformation belt. Structural modeling predicts a dip of 46° S on the Oak Ridge fault at seismogenic depths. Horizontal shortening rates are calculated by adding the products of the dip-slip displacements and the cosines of the dips of both faults. The eastern cross section shows the Northridge mainshock, with a 45° south-dipping nodal plane at a depth of 18 km. Aftershocks reach a depth of 20 km. In a thin-skinned paradigm, a hinge should occur at the surface near the Santa Monica Mountains due to rocks moving from a decollement at the brittle-plastic transition and changing dip as they move up the ramp. No hinge of that magnitude occurs there. Calculation of horizontal short-ening rates across this part of the western Transverse Ranges must take into account the displacement on both the Northridge thrust (eastern extension of the Oak Ridge fault) and the Santa Susana fault (Placerita segment). Horizontal shortening rates are 8.2 ± 2.4 mm/yr across the Modelo lobe segment of the San Cayetano fault and the Oak Ridge fault and 5.7 ± 2.5 mm/yr across the Northridge thrust and the Santa Susana fault. These rates are consistent with those based on tectonic geodesy using GPS. Dip-slip displacement rates on the faults are 1.7 mm/yr for the Northridge thrust since 2.3 Ma, 4.1 ± 0.4 mm/yr for the Oak Ridge fault since 500 ka, 5.9+3.9/−3.8 mm/yr for the Santa Susana fault since 600 to 2300 ka, and 7.4 ± 3.0 mm/yr for the Modelo lobe segment of the San Cayetano fault since 500 ka. This indicates that the slip rates on the north-dipping, the eastern San Cayetano, and the Santa Susana faults are comparable, but the slip rate on the south-dipping faults decreases eastward; the slip rate on the Oak Ridge fault in the Ventura basin is more than double that of the Northridge thrust.

The Oak Ridge fault system and the 1994 Northridge earthquake

THE 17 January 1994 Northridge earthquake (moment magnitude M w = 6.7) in the San Fernando Valley in southern California illuminated a hitherto unrecognized blind reverse fault with a moderate southward dip 1. This fault lies beneath the active north-dipping Santa Susana fault system and uplifted both the footwall and the hanging wall of the Santa Susana fault during the earthquake1. Here we argue that footwall uplift on the Santa Susana fault before the earthquake could have been used to identify the Northridge blind thrust as an active fault. Moreover, we propose that the Northridge earthquake occurred on a continuation of the Oak Ridge fault system, which reaches the surface in the Ventura basin to the west. Slip rates on the western part of this fault system 2, 3 are nearly three times larger than on the Northridge blind thrust 4, increasing the probability of a N or t bridge-sized earthquake in the heavily populated Ventura basin.

Late Quaternary history of the Ventura mainland shelf, California

The Ventura mainland shelf off California provides an interesting example of sedimentary response to tectonic activity and eustatic sea-level fluctuations during the late Quaternary. A network of 370 km of high-resolution seismic-reflection profiles provides the basis for the study. The high local rates of uplift, as well as local basin subsidence, have resulted in two distinct styles of sedimentation. A late Pleistocene angular unconformity separates folded Plio-Pleistocene strata from undeformed upper Pleistocene and Holocene strata on the northern shelf. In contrast, the southern shelf is characterized by a thick section of stacked, progradational deltaic sequences containing a well-preserved record of late Quaternary sedimentation. Sediment overlying the unconformity north of the Pitas Point fault ranges from 10 to 40 m in thickness, whereas the equivalent sequence in the adjacent southern shelf is 80–85 m thick. The distribution of upper Pleistocene and Holocene deposits is controlled by structural features including the Pitas Point and Oak Ridge faults, and the Ventura Avenue anticline, as well as proximity to fluvial input from the Santa Clara and Ventura Rivers. Within upper Pleistocene deposits, a pair of buried marine terraces occur at depths of 33 and 50 m below present sea level (b.p.s.l.). Terrace deposits consist of a basal, coarse-grained, transgressive deposit which is transitional into a finer grained marine facies. These deposits are associated with a period of aggradation during minor late Wisconsinan sea-level cycles. Deltaic deposits beneath the southern shelf 4 km north of Hueneme submarine canyon suggest that the Santa Clara River debouched into the sea at this location during the late Pleistocene. The Santa Clara River shifted southward during the late Wisconsinan and cut a large channel, now buried, near Hueneme canyon. This channel indicates that relative sea level was at least 113 m b.p.s.l. during the late Wisconsinan. A smaller channel indicates a fall in sea level to 46 m b.p.s.l. during the Holocene. The Oak Ridge fault displays a general increase in offset seaward across the shelf, with a maximum vertical displacement of 17 m on the Saugus/upper Pleistocene contact. Displacements on the Pitas Point fault increase shoreward, attaining as much as 40 m of vertical displacement of the late Pleistocene unconformity. Nearshore, the Pitas Point fault vertically displaces the seafloor by 4 m.

Chronostratigraphic Calibration of Late Neogene, Northeast Pacific Planktic Foraminiferal Events: Geological Applications to the Pliocene-Pleistocene of the Ventura Basin, California: ABSTRACT

An integrated chronostratigraphy of late Neogene active margin basins of the northeastern Pacific Ocean is developed by calibrating the temperature of north Pacific planktic foraminiferal evolutionary datums and paleoclimatically controlled coiling shifts of Neogloboquadrina pachyderma with paleomagnetic, radiometric, and isotopic stratigraphies. This regional chronostratigraphic framework is then adapted to individual sedimentary basins. Ongoing work in the Ventura basin illustrates such an application. Planktic foraminiferal events at Balcom, Wheeler, and Santa Paula canyons are calibrated against paleomagnetic stratigraphy and radiometrically dated ash layers. This calibration agrees with and supplements previously derived regional calibrations. In particular, several coiling shifts of N. pachyderma and the last occurrence datums of Neogloboquadrina asanoi and Neogloboquadrina humerosa appear most useful in this area. These events provide high-resolution chronostratigraphic control between approximately 2.6 and 0.7 Ma in the central Ventura basin. Such chronostratigraphic control is an integral part of geohistory analysis, an application of which is presented to illustrate the utility of the integrated chronostratigraphic framework. Geohistory analysis of the Balcom and Wheeler canyon sections is used to reconstruct stratigraphic evolution north and south of the Oakridge Fault during deposition of the Pico Formation. Both sections show rapid subsidence during the Repettian, slower subsidence or possible upliftmore » during the Venturian, and renewed subsidence during the Wheelerian. These histories do differ in several respects and a comparison of subsidence, uplift, and rock accumulation rates indicates that the fault was a normal fault, down to the north, through most of its Pliocene-Pleistocene history, being reactivated as a high-angle reverse fault only during the late Pleistocene.« less

Late Quaternary slip rate on the Oak Ridge fault, Transverse Ranges, California: Implications for seismic risk

The slip rate on the Oak Ridge fault at South Mountain in the densely populated lower Santa Clara Valley, California, is estimated as 5.9–12.5 mm/yr since the end of Saugus deposition at 0.4–0.2 Ma. A displacement of 3 m per event, assumed on the basis of surface rupture on the 1952 Kern County, 1971 San Fernando, and 1978 Tabas‐e‐Golshan earthquakes, gives an average recurrence interval of 250–500 years, with the major uncertainty being the age of the top of the Saugus. Displacement of 2375–2490 m of the top of the Saugus at South Mountain includes piercing‐point displacement and distributed displacement from drag folds near the fault; both are the near‐surface expression of faulting at potential main shock depths beneath well control. Slip rates for the nearby San Cayetano and Red Mountain faults are less well constrained because post‐Miocene strata are largely absent in their hanging‐wall blocks, but available evidence suggests that their slip rates are in the same range as that for the Oak Ridge fault. The lower Santa Clara Valley has not had a large, damaging earthquake in 200 years of record keeping, although the December 21, 1812, tsunami‐producing earthquake could have occurred on the offshore continuation of the Oak Ridge or Red Mountain fault. A trench on the Harmon alluvial fan near the Ventura County Government Center revealed evidence of cracks filled with sediments from below, suggesting liquefaction during a Holocene earthquake. Earthquake repeat times measured in hundreds of years have been reported by others for the 1971 San Fernando fault and a small fault near the Red Mountain fault, and comparable repeat times are suggested for a normal fault within the hanging‐wall block of the Oak Ridge fault. These are consistent with the average recurrence interval calculated for the Oak Ridge fault, suggesting that a destructive earthquake may strike the lower Santa Clara Valley in the near future.

Oak Ridge fault, Ventura fold belt, and the Sisar decollement, Ventura basin, California

Research Article| December 01, 1988 Oak Ridge fault, Ventura fold belt, and the Sisar decollement, Ventura basin, California Robert S. Yeats; Robert S. Yeats 1Department of Geology, Oregon State University, Corvallis, Oregon 97331-5506 Search for other works by this author on: GSW Google Scholar Gary J. Huftile; Gary J. Huftile 1Department of Geology, Oregon State University, Corvallis, Oregon 97331-5506 Search for other works by this author on: GSW Google Scholar F. Bryan Grigsby F. Bryan Grigsby 1Department of Geology, Oregon State University, Corvallis, Oregon 97331-5506 Search for other works by this author on: GSW Google Scholar Author and Article Information Robert S. Yeats 1Department of Geology, Oregon State University, Corvallis, Oregon 97331-5506 Gary J. Huftile 1Department of Geology, Oregon State University, Corvallis, Oregon 97331-5506 F. Bryan Grigsby 1Department of Geology, Oregon State University, Corvallis, Oregon 97331-5506 Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1988) 16 (12): 1112–1116. https://doi.org/10.1130/0091-7613(1988)016<1112:ORFVFB>2.3.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Robert S. Yeats, Gary J. Huftile, F. Bryan Grigsby; Oak Ridge fault, Ventura fold belt, and the Sisar decollement, Ventura basin, California. Geology 1988;; 16 (12): 1112–1116. doi: https://doi.org/10.1130/0091-7613(1988)016<1112:ORFVFB>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The rootless Ventura Avenue, San Miguelito, and Rincon anti-clines (Ventura fold belt) in Pliocene-Pleistocene turbidites are fault-propagation folds related to south-dipping reverse faults rising from a decollement in Miocene shale. To the east, the Sulphur Mountain anti-clinorium overlies and is cut by the Sisar, Big Canyon, and Lion south-dipping thrusts that merge downward into the Sisar decollement in lower Miocene shale. Shortening of the Miocene and younger sequence is ∼3 km greater than that of underlying competent Paleogene strata in the Ventura fold belt and ∼7 km greater farther east at Sulphur Mountain. Cross-section balancing requires that this difference be taken up by the Paleogene sequence at the Oak Ridge fault to the south. Convergence is northeast to north-northeast on the basis of earthquake focal mechanisms, borehole breakouts, and piercing-point offset of the South Mountain seaknoll by the Oak Ridge fault. A northeast-trending line connecting the west end of Oak Ridge and the east end of the Sisar fault separates an eastern domain where late Quaternary displacement is taken up entirely on the Oak Ridge fault and a western domain where displacement is transferred to the Sisar decollement and its overlying rootless folds. This implies that (1) the Oak Ridge fault near the coast presents as much seismic risk as it does farther east, despite negligible near-surface late Quaternary movement; (2) ground-rupture hazard is high for the Sisar fault set in the upper Ojai Valley; and (3) the decollement itself could produce an earthquake analogous to the 1987 Whittier Narrows event in Los Angeles. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Differential Vertical Tectonics: Insights from Models Composed of Sandstone and Limestone Deformed at Confining Pressure: ABSTRACT

Rock models (12 × 3 × 3 cm, 4 × 1 × 1 in.) composed of a precut forcing block of sandstone beneath a 1-cm thick, multilithologic layered veneer of sandstone and limestone or of uncemented sand, have been deformed room-dry, at 100-MPa confining pressure, room temperature, and a strain rate of 10-4 sec-1. Upon frictional sliding along the lubricated precut (inclined at 30° to 90° to the intact veneer) the forcing blocks deform the layered veneer into a faulted drape fold. Thin sections cut parallel to the dip and to the strike of the precut and major fault in the veneer are studied to provide detailed maps of the induced deformation (microfractures, faults, intragranular strains, bedding thickness changes, brittle versus ductile behavior, and nature of the folding and hinge formation). In addition, dynamic fabric analyses of faults, microfractures, and calcite twin lamellae yield stress trajectory diagrams that serve to test the applicability of corresponding numerical or analytical solutions. Insights gained from the models primarily deal with (a) brittle or ductile behavior of the limestone depending upon location within the faulted drape fold; (b) nature of the deformation concentrated in the leading edge of the upthrown forcing block; (c) configuration and sequence of faulting associated with the major upthrust in the veneer; (d) location and significance of associated precursive microfracturing; (e) cataclastic thinning of veneers to the point where discrete layers are tectonically eliminated in the major fault zones; (f) hinge formation within the veneer; and (g) the role of bedding-plane slip in the folding process. Deformation features in these models are used to interpret the geometry and genesis of the large scale structures at the Clark's Fork Corner of the Beartooth Block and at Rattlesnake Mountain near Cody, Wyoming; at Colorado National Monument near Grand Junction, Colorado; at the Yampa drape fold, Dinosaur National Monument; and in the subsurface along the Oak Ridge fault, Ventura basin, California. Collectively, these features are criteria for, and hence help define, the differential-vertical-tectonic style. End_of_Article - Last_Page 1337------------

Active fault hazard in southern California: Ground rupture versus seismic shaking

There are exceptions to the common assumption that surface rupture by faulting is always accompanied by a damaging earthquake. Near Ventura, California, faults of the Oak View—Ojai area and Orcutt and Timber Canyons displace late Pleistocene and/or Holocene alluvial materials and soils by several tens of metres. The movement is almost exclusively along bedding planes in response to flexural slip during folding. Therefore these faults constitute a ground-rupture hazard. However, because they do not extend downward to rocks of high shear strength which could store large amounts of elastic strain energy, they do not constitute a seismic-shaking hazard. In contrast, the Oak Ridge fault near the coast does not cut strata shallower than 1,250 to 1,500 m below the surface, although pressure ridges attest to recent activity. Similarly, the Newport-Inglewood fault between Long Beach and Inglewood oil fields in the Los Angeles basin is expressed at the surface as a line of anticlinal hills rather than a throughgoing fault. Historical faulting at Inglewood and possibly at Rosecrans and Dominguez oil fields may be related to oil-field development rather than earthquake-related displacement. Those portions of the Oak Ridge and Newport-Inglewood faults pose seismic-shaking hazard but not ground-rupture potential from earthquakes alone. For planning purposes, it is necessary to distinguish faults with only ground-rupture potential or with only seismic shaking potential from the well-known faults with potential for both.

Structure of San Cayetano and Oak Ridge Thrust Faults, East-Central Ventura Basin, California: ABSTRACT

The central Ventura basin, containing at least 20,000 ft (6,000 m) of Pliocene and Pleistocene sedimentary rocks, has been long recognized as bounded by thrust faults--the north-dipping San Cayetano fault (SCF) on the north and the south-dipping Oak Ridge fault (ORF) on the south. Field investigations and synthesis of available surface and subsurface data show that the three strands of the SCF, here named the Main, Goodenough, and Piru, join at depth to form a single fault plane. The SCF shows a 30,000-ft (9,000 m) maximum separation in the Fillmore area where a possible structural downstep is stepped to the left along the Goodenough strand. The SCF loses the separation progressively eastward and within about 14 mi (22 km) the fault apparently disappears in the north flan of the Santa Clara Valley syncline. The ORF, exposed within the southeastern part of the study area, also loses its separation eastward and disappears along the axis of the syncline. The Main strand of the SCF was initiated during the deposition of the Pico. West of Hopper Canyon, the fault involves movement during and after the deposition of the Pleistocene Saugus Formation. The SCF cuts late Quaternary deposits and should be considered as potentially active. Most of the folds in the upper (north) block, and all the folds in the lower (south) block of the SCF, were contemporaneous with fault movement. End_of_Article - Last_Page 844------------