Ranges Of Southern California Research Articles

Ecology and Ecophysiology of a Subalpine Fellfield Community on Mount Pinos, Southern California

ABSTRACT Mount Pinos at the western margin of the Transverse Ranges of Southern California. reaches 2692 m in elevation at its peak. The broad summit of the mountain supports an area of subalpine fellfield vegetation, as well as an unusual low elevation occurrence of limber pine (Pinus flexilis). We describe the summit area of the mountain and characterize the community structure and ecophysiology of the fellfield community and associated pines. The fellfield community, dominated by low mats and cushions of Eriogonum kennedyi, Phlox diffusa, and Lupinus breweri var. bryoides, has a mean plant cover of 44%. These fellfield species have amphistomatic leaves with abundant, slender palisade cells on both sides of the mesophyll. These traits represent a strategy shared with desert plants. Amphistomaty helps to increase assimilation by maximizing stomatal conductance to CO2 during the gas phase of diffusion, and the isolateral mesophyll maximizes diffusion of CO2 by exposing a very high mesophyll membrane area ...

TOWARD A BETTER UNDERSTANDING OF THE "TRANSVERSE RANGE BREAK": LINEAGE DIVERSIFICATION IN SOUTHERN CALIFORNIA

The Transverse Ranges in southern California have been identified as having a prominent phylogeographic role. Numerous studies have identified distinct north-south and/or east-west lineage breaks involving the Transverse Ranges. However, in evaluating their findings, most authors have regarded this complex system somewhat simplistically. In this study we more deeply investigate these breaks using two approaches: first we examine the phylogeographic history of Sepedophilus castaneus (Coleoptera: Staphylinidae) and then implement a comparative phylogeography approach applying Brooks parsimony analysis to the topologies of nine additional taxa. Phylogenetic analysis, nested clade analysis, and AMOVAs for S. castaneus agree that there is a major lineage break between the eastern and western Transverse Ranges, localized between the Sierra Pelona and the San Gabriel Mountains. The comparative phylogeographic analysis supports a generally strong concordance of area relationships with geographic proximity. It is notable, however, that the Transverse Ranges as a group do not show phylogenetic cohesion, but rather they are split into three main regions: an eastern region (San Gabriel, San Bernardino, and San Jacinto Mountains), a central region (central Transverse Ranges and Sierra Pelona) that is often grouped with the Tehachapi and Sierra Nevada populations, and a western region (northwestern Transverse Ranges and Santa Ynez Mountains) that is consistently grouped with coast range areas to the north. The lineage break between east and west Transverse Ranges is attributable to the presence of a marine embayment in what is now the Santa Clara River valley 5-2.5 million years ago.

New constraints on relative motion between the Pacific Plate and Baja California microplate (Mexico) from GPS measurements

SUMMARY We present a new surface velocity field for Baja California using GPS data to test the rigidity of this microplate, calculate its motion in a global reference frame, determine its relative motion with respect to the North American and the Pacific plates, and compare those results to our estimate for Pacific‐North America motion. Determination of Pacific Plate motion is improved by the inclusion of four sites from the South Pacific Sea Level and Climate Monitoring Project. These analyses reveal that Baja California moves as a quasi-rigid block but at a slower rate in the same direction, as the Pacific Plate relative to North America. This is consistent with seismic activity along the western edge of Baja California (the Baja California shear zone), and may reflect resistance to motion of the eastern edge of the Pacific Plate caused by the ‘big bend’ of the San Andreas fault and the Transverse Ranges in southern California.

Vertical-axis rotation controlled by upper crustal stress based on force balance analysis: A case study of the western Transverse Ranges of California

This paper evaluates driving mechanisms of vertical-axis rotation using data from the western Transverse Ranges in southern California. Simple force balance considerations and comparison of torque applied to a rotating block indicate that shear forces applied to the base of the block are not strong enough to produce the motions and deformation observed at the surface. For the measured dimensions of the crustal blocks and crustal viscosities in southern California, stresses transmitted through the upper crust are one to three orders of magnitude stronger than forces generated in the ductile lower crust. These results suggest that the kinematics of crustal blocks in continental deformation zones are primarily controlled by forces within the upper crust rather than a flow field beneath.

Catastrophe, recovery and range limitation in NE Pacific kelp forests: a large-scale perspective

The 1997-98 El Nino was one of the strongest on record and resulted in widespread losses of the giant kelp Macrocystis pyrifera (Agardh) along the west coast of North America. Draw- ing on a rich history of studies that have shown abnormally large waves and warm nutrient-poor water associated with El Ninos to negatively impact giant kelp populations at some locations in south- ern and Baja California, we examined (1) how these impacts scale up when considered across the species' geographic range in the NE Pacific Ocean and (2) if these impacts are generalizable over broad spatial scales. Working at 56 sites in 14 study locations over a 3 yr period (1997 to 2000), we examined how giant kelp populations were impacted by and recovered following the 1997-98 El Nino over a ~1500 km span along the west coast of North America. Our results indicate that while nearly all giant kelp disappeared from the southern one-third of the species' range along the coast of Baja California, Mexico, and heavy losses occurred throughout the central one-third of the species' range in southern California, USA, only minor impacts were observed throughout the northern one- third of the species' range in central California. Further, although highly variable among regions, these impacts were similar and generalizable among locations within each region. Our results also suggest that, as has been observed in local-scale studies, this large-scale variability in giant kelp mortality was driven by large-scale patterns in ocean temperature (nutrient concentration) and wave intensity. Recovery following El Nino, in contrast, was variable at multiple spatial scales and although not directly tested here, presumably influenced by numerous factors such as proximity to upwelling areas, competition with other algae, grazing, and propagule availability. Further, variability in the rates of recovery among locations resulted in a generally slow recovery of giant kelp throughout most of Baja California, and residual large-scale impacts of the El Nino were still evident 2 yr after the El Nino ended. As global climate change may lead to increases in the frequency and intensity of El Ninos, our findings have broad implications for the ways in which ecosystems might be expected to respond to them and provide a measure by which their impacts to giant kelp ecosystems may be compared among events.

LEPECHINIA ROSSII (LAMIACEAE), A NARROW ENDEMIC FROM THE WESTERN TRANSVERSE RANGES OF SOUTHERN CALIFORNIA

ABSTRACT Lepechinia rossii (Lamiaceae) is described as a new species narrowly endemic to the western Transverse Ranges of southern California. It is a member of section Calycinae, which includes four additional species endemic to California and adjacent Baja California, Mexico—L. calycina, L. cardiophylla, L. fragrans, L. ganderi—and L. mexicana, an anomalous, and probably unrelated, species from central Mexico. Lepechinia rossii is most readily distinguished from other members of section Calycinae by geniculate inflorescence axes, bent at 60–90° angles relative to the subtending stems, and by large, foliaceous inflorescence bracts which are generally equaling or exceeding their adjacent flowers in length, and little reduced distally. At present, two populations are documented, one in the Liebre Mountains (Los Angeles County) and one in the Topatopa Mountains (Ventura County), both occurring in chaparral, on public lands administered by the U.S. Forest Service. Conservation concerns include habitat degrad...

Effects of bathymetric and oceanographic variations on short range high frequency acoustic propagation in shallow water

Detailed bathymetry of shoaling regions at two shallow water sites of the San Clemente Offshore Range in Southern California is combined with oceanographic data, taken with CTD, XBT, and bottom moored ADCPs during the winter and summer conditions, to evaluate the effects of environmental variability on acoustic transmission loss and ambient noise measurements made at 16 discrete frequencies ranging from 3 kHz to 30 kHz over 10 km path lengths. Over such short ranges, bottom slopes and tidal effects have the strongest influence on acoustic propagation. [Work supported by the U.S. Naval Oceanographic Office.]

Magnetostratigraphic Tests of Sequence Stratigraphic Correlations from the Southern California Paleogene

ABSTRACT Several sequence stratigraphic analyses have been conducted on the Eocene-Oligocene rocks of the San Diego region and of the western Transverse Ranges in southern California. Recently developed magnetic stratigraphy and 40Ar/39Ar dates, as well as previously neglected biostratigraphic data, provide a high-resolution chronostratigraphic framework to test correlation of these sequence stratigraphic interpretations to the global onlap-offlap chart (Haq et al. 1987, 1988; Hardenbol et al. 1998). In the San Diego area, previous stratigraphic analyses miscorrelated nearly all sequences and sequence boundaries to the global time scale; some correlations are off by as much as ten million years. In the western Transverse Ranges, three contradictory sequence stratigraphic analyses are evaluated. The revised dates of the sequence boundaries show poor correlation with the onlap-offlap chart of Haq et al. (1987, 1988) or Hardenbol et al. (1998), and some units are misdated by as much as six million years. In both regions, few sequences were correctly predicted by the onlap-offlap chart, suggesting that tectonic controls were more important than eustatic controls in this tectonically active region. The errors in so many sequence stratigraphic correlations when tested by high-resolution chronostratigraphic data should caution geologists about using sequence stratigraphic age assignments uncritically. Sequence stratigraphic correlations are only as good as the chronostratigraphy on which they are based.

Differential susceptibility to xylem cavitation among three pairs of Ceanothus species in the Transverse Mountain Ranges of southern California

Chaparral shrub communities of California are often dominated by indigenous species of the genus Ceanothus, which is further divided into two subgenera: Cerastes and Ceanothus. Cerastes is comprised of species that are non-sprouters after fire, and which have leaf morphological features suggesting strong tolerance to drought. In contrast, species in subgenus Ceanothus generally sprout after fire and have less xeromorphic leaves. We examined three pairs of Cerastes/Ceanothus species that commonly grow together in the Transverse Mountain Ranges of southern California (e.g., Santa Monica Mountains and San Gabriel Mountains). Each species pair was sequentially replaced along an elevational gradient, with C. megacarpus/C. spinosus occurring at a low elevation, coastal site; C. cuneatus/C. oliganthus at a site intermediate in elevation and distance from the ocean; and C. crassifolius/C. leucodermis at a high elevation, inland site. In general, the length of the summer drought period decreases and the amount of rainfall increases with increasing elevation and distance from the ocean. In all pairwise comparisons, the non-sprouters in subgenus Cerastes (C. megacarpus; C. cuneatus; C. crassifolius) were found to be more resistant to xylem embolism caused by water stress than species in subgenus Ceanothus (C. spinosus; C. oliganthus; C. leucodermis). The level of water stress causing 50% loss in hydraulic conductivity due to gas embolism varied between -7.1 to -10.8 MPa for subgenus Cerastes but -4.6 to -7.3 MPa for subgenus Ceanothus. As species pairs were sequentially replaced from low elevation to higher elevation, susceptibility to embolism also progressively increased. These results are consistent with the xeromorphic differences between subgenera and geographic distribution of the species.

Among-Site Variation in Riparian Vegetation of the Southern California Transverse Ranges

-Data from 37 sites in the Transverse Ranges in southern California were used to assess the influence of a variety of environmental variables on among-site variation in riparian vegetation. Ordination and regression analyses indicated that elevation, unit stream power, fire history and valley width all have significant impacts on the composition of this vegetation. TWINSPAN classification generated four species groups: a Baccharis glutinosa/Salix laevigata group that appears related to low elevations, high stream power and recent fire; a Populusfremontii/Alnus rhombifolia group associated with high stream power, but longer time since burning and higher elevations; a Salix lasiolepis/Rosa californica group typically found at high elevation sites with low stream power; and a Platanus racemosa/Adenostoma fasciculatum group that did not show a clear environmental preference.

Determination of earthquake energy release and ML using TERRAscope

We estimated the energy radiated by earthquakes in southern California using on-scale very broadband recordings from TERRAscope. The method we used involves time integration of the squared ground-motion velocity and empirical determination of the distance attenuation function and the station corrections. The time integral is typically taken over a duration of 2 min after the P-wave arrival. The attenuation curve for the energy integral we obtained is given by q(r) = cr^(−n)exp(−kr)(r^2 = Δ^2 + h_(ref)^2) with c = 0.49710, n = 1.0322, k = 0.0035 km^(−1), and h_(ref) = 8 km, where Δ is the epicentral distance. A similar method was used to determine M_L using TERRAscope data. The station corrections for M_L are determined such that the M_L values determined from TERRAscope agree with those from the traditional optical Wood-Anderson seismographs. For 1.5 6.5, M_L saturates. The ratio E_S/M_0 (M_0: seismic moment), a measure of the average stress drop, for six earthquakes, the 1989 Montebello earthquake (M_L = 4.6), the 1989 Pasadena earthquake (M_L = 4.9), the 1990 Upland earthquake (M_L = 5.2), the 1991 Sierra Madre earthquake (M_L = 5.8), the 1992 Joshua Tree earthquake (M_L = 6.1), and the 1992 Landers earthquake (M_w = 7.3), are about 10 times larger than those of the others that include the aftershocks of the 1987 Whittier Narrows earthquake, the Sierra Madre earthquake, the Joshua Tree earthquake, and the two earthquakes on the San Jacinto fault. The difference in the stress drop between the mainshock and their large aftershocks may be similar to that between earthquakes on a fault with long and short repeat times. The aftershocks, which occurred on the fault plane where the mainshock slippage occurred, had a very short time to heal, hence a low stress drop. The repeat time of the major earthquakes on the frontal fault systems in the Transverse Ranges in southern California is believed to be very long, a few thousand years. Hence, the events in the Transverse Ranges may have higher stress drops than those of the events occurring on faults with shorter repeat times, such as the San Andreas fault and the San Jacinto fault. The observation that very high stress-drop events occur in the Transverse Ranges and the Los Angeles Basin has important implications for the regional seismic potential. The occurrence of these high stress-drop events near the bottom of the seismogenic zone strongly suggests that these fault systems are capable of supporting high stress that will eventually be released in major seismic events. Characterization of earthquakes in terms of the E_S/M_0 ratio using broadband data will help delineate the spatial distribution of seismogenic stresses in the Los Angeles basin and the Transverse Ranges.

A Re-Evaluation of the Horkelia bolanderi (Rosaceae) Complex, with the New Species Horkelia yadonii

The Horkelia bolanderi complex is re-evaluated, based on morphological and field studies. A new species, Horkelia yadonii, is recognized from the South Coast Ranges of California. It is a potentially threatened species growing in dry streambeds and meadows in chaparral and foothill pine woodlands. Horkelia tenuiloba is restricted to Marin, Sonoma, and adjacent Mendocino counties, where it is also a rare species occurring in moist openings in the chaparral. Horkelia clevelandzi is treated as a distinct species, which occurs in open forests in the mountains of San Diego County and Baja California. Horkelia bolanderi subsp. parryi becomes H. rydbergii, a relatively common species growing at the edge of meadows and drainages in the Transverse Ranges of southern California. Horkelia bolanderi is limited to Lake and adjacent Colusa counties, where it is a rare plant at the margins of vernal wet areas. Collections previously identified as members of the H. bolanderi complex from Santa Cruz and San Mateo counties are assigned to H. marinensis, a possible allopoly- ploid with H. cuneata subsp. sericea as the other parent. A key and map are included. Horkelia Cham. & Schldl. is a close relative of Potentilla L. that is centered in California. In general, members of Horkelia are perennial herbs with a distinct resinous fragrance from abun- dant glands. Leaves, which are primarily basal, are pinnately compound with indistinct ter- minal leaflets. In this regard they resemble many species of the related Ivesia Torrey & A. Gray, from which they differ in their characteristic flower structure. The hypanthium of Horkelia forms a flat-bottomed cup, generally about half as deep as wide, and the stamens are always 10 in number. In most species, the filaments are somewhat flattened and form a tube around the styles in early anthesis; anthers dehisce along their inner face. Petals are white (to pinkish or cream-colored) and usually ? oblanceolate-el- liptic. Sepals, which alternate with smaller bractlets, are often sharply reflexed at anthesis and purple-flecked. In contrast, Ivesia is more variable, with the hypanthium often shallow and seldom flat-bot- tomed. The five to twenty stamens generally have slender, spreading filaments and anthers that dehisce laterally. The linear to obovate pet- als are white to yellow, and the sepals are gen- erally spreading at anthesis and seldom purple- flecked.

Magnetotelluric evidence for a brittle‐ductile transition, peninsular ranges batholith, southern California?

A magnetotelluric traverse of the Peninsular Ranges in southern California has revealed a pervasive zone of lower resistivity beginning at a uniform depth of 10 km and extending to depths of 60–90 km. Resistivities above 10 km depth are similar to those found in batholiths; very high values correspond to outcrops of crystalline basement. Because seismicity below 11–12 km is sparse, others have concluded that the brittle‐ductile transition is shallow beneath the range. The zone of low resistivity in the lower crust corresponds well to the ductile region, and we conclude that the lower values are caused by fluids trapped below the transition. Because the range has experienced vertical uplifts during the Pliocene era and the top of the low resistivity zone is flat, the present brittle‐ductile transition must have been formed in the last 5 M.y. A possible source for the fluids is the rift to the east in the Salton Trough.

Upper Cretaceous and Lower Eocene Conglomerates of Western Transverse Ranges: Evidence for Tectonic Rotation: ABSTRACT

Stratigraphic and paleomagnetic studies have suggested that the western Transverse Ranges (WTR) microplate is allochthonous, and may have experienced translational and rotational motions. Present paleocurrent directions from the Upper Cretaceous Jalama Formation of the Santa Ynez Mountains are north-directed; these forearc sediments (Great Valley sequence) contain magmatic arc-derived conglomerate clasts from the Peninsular Ranges in southern California. Paleocurrents in the lower Eocene Juncal and Cozy Dell Formations are south-directed. This juxtaposition is best explained by 90/degrees/ or more of clockwise rotation of the WTR microplate, so that Upper Cretaceous forearc sediments sourced from the Peninsular Ranges magmatic arc were deposited by west-directed currents. Eocene sediments were derived from an uplifted portion of the western basin margin and deposited by east-directed currents. Franciscan olistoliths in the Upper Cretaceous sediments indicate deposition adjacent to an accretionary wedge; conglomeratic clasts recycled from the Upper Cretaceous sequence, and radiolarian cherts and ophiolitic boulders in the Eocene strata indicate derivation from an outer accretionary ridge.

Petrology of Holocene Sand, Peninsular Ranges, California and Baja Norte, Mexico: Implications for Provenance-Discrimination Models

ABSTRACT The Peninsular Ranges of southern California and Baja California Norte, Mexico, represent an uplifted and variably dissected Mesozoic magmatic arc. They are composed mostly of Cretaceous plutonic rocks and Mesozoic greenschist to amphibolite facies metasedimentary and metavolcanic rocks. In order to evaluate the reliability of the diamond-shaped QnuQuQp provenance-discrimination diagram developed by Basu et al. (1975) and the QFL provenance-discrimination diagram developed by Dickinson et al. (1983), we collected 62 samples of Holocene sand from four drainage basins along the west side of the Peninsular Ranges. The QFL composition of each specimen was determined by the Gazzi-Dickinson point-counting method; the relative percentages of med um-grained fragments of Qnu, Qu, and Qp were determined by counting 100 grains in each specimen. The results of our study indicate that Holocene sands derived from the Peninsular Ranges have the following average petrological characteristics: 1) Sands derived mostly or exclusively from plutonic rocks are composed of 16-46 percent Q, 54-83 percent F, and 0.2-0.5 percent L. The medium-grained quartz population produced by erosion of mostly or exclusively plutonic rocks consists of 61-77 percent Qnu, 4-17 percent Qu, and 13-27 percent Qp. 2) Sands originating from the erosion of metasedimentary rocks are composed of 63 percent Q, 13 percent F, and 24 percent L. The medium-grained quartz population derived from a metasedimentary source is made up of 25 percent Qnu, 9 percent Qu, and 66 percent Qp. 3) Sands produced by the erosion of mostly or exclusively metavolcanic rocks are composed of 19 percent Q, 39-44 percent F, and 38-43 percent L. The medium-grained quartz population originating from a mostly or exclusively metavolcanic source consists of 48-74 percent Qnu, 6 percent Qu, and 20-46 percent Qp. Petrological data obtained during this study indicate that the QFL provenance-discrimination model generally yields a correct interpretation of tectonic setting when applied to Holocene sand from the Peninsular Ranges. Data, however, also show that local source areas produce sands that, when interpreted on the basis of their position on the QFL provenance-discrimination diagram, yield an erroneous interpretation of tectonic setting. Data obtained from counting varieties of medium-grained quartz show that detrital quartz populations produced by plutonic rocks are discriminated from those derived from metasedimentary rocks by the QnuQuQp provenance-discrimination diagram. In contrast, quartz populations originating from a mostly or exclusively metavolcanic source are difficult to interpret on the QnuQuQp diagram. Therefore, we recommend that the QFL provenance-discrimination model be used with caution when sand or sandstone samples are from a limited geographic area and that the QnuQuQp provenance-discrimination diagram be used with prudence when metavolcanics are suspected in the provenance.

Variable Preservation of Middle Tertiary, Coarse-Grained, Nearshore to Outer-Shelf Storm Deposits in Southern California

ABSTRACT The middle Tertiary (Oligocene-Early Miocene) strata of the San Emigdio Range in southern California were deposited along a shoreline oriented north-northeast to south-southwest, adjacent to a tectonically active highland that formed the southern margin of the San Joaquin Basin. Abundant storm deposits (sandstone and conglomerate) are preserved in the San Emigdio, Pleito, and lower Temblor Formations. Extensive lateral exposures allow recognition of fair-weather and storm-dominated facies in contemporaneous nearshore, inner-shelf, and outer-shelf deposits. The nearshore deposits consist of coarse-grained progradational sequences that are similar to those described by Clifton (1981) in Miocene rocks of the nearby Caliente Range. Each complete sequence consists of, in ascending order, a basal transgressive lag, followed by inner-shelf, surf-zone, longshore-trough, and beach-foreshore facies. The inner-shelf deposits consist of fair- and foul-weather deposits. The former deposits comprise massive, locally pebbly, very fine-grained sandstone, whereas the latter deposits consist of alternating intervals of cross-stratified (landward-oriented) conglomerate and hummocky cross-stratified fine-grained sandstone. The outer-shelf deposits are composed of massive, very fine-grained to silty sandstone, and sequences of mollusk beds and hummocky cros -stratified sandstone. The massive outer-shelf sandstones were deposited during periods of dominantly fair weather. The sequences of hummocky cross-stratified sandstone were deposited during and immediately after storms. The outer-shelf storm deposits apparently were preserved on an event-by-event basis, whereas the inner-shelf storm deposits were preserved in amalgamated packages. Unmodified storm deposits are rare in the nearshore deposits, owing to the reworking capabilities of high-energy nearshore processes. Reconstructions of wave conditions based on Airy wave theory, paleogeographic constraints, and storm facies suggest that peak storm waves were more than 5 m high, with periods of 7-9 seconds. The study area was situated at the end of a long fetch, in a favorable position to receive the full brunt of northwesterly wind-driven waves.

Syntectonic Hydrocarbon Migration and Accumulation in Miley Reservoir, Rincon Field, Ventura County, California: ABSTRACT

The Miley reservoir of the Rincon field is located in the Central Transverse Ranges of southern California on a structural high that borders the Santa Barbara Channel. The east-west-trending Rincon and Ventura anticlines are part of a major oil-productive trend containing the Rincon, San Miguelito, and Ventura Avenue fields, which have estimated ultimate recovery of 1.7 billion BOE. Hydrocarbon accumulations in the multiple and stacked reservoirs within these three fields are controlled by the complex interplay of late Pleistocene folding and reverse fault development. The detailed interpretation reported here combines reservoir performance data with subsurface structural geology and sequential tectonic development to provide a new understanding of the relationship of migration barriers to oil accumulation and production. The Miley reservoir is an axial- and fault-controlled accumulation on the eastern terminus of the Rincon anticline. It is located in a structural saddle formed by the doubly plunging Rincon and Ventura anticlinal trend. Three operative trapping mechanisms confine oil pools: (1) axial accumulations associated with reverse fault closures; (2) traps on the hanging wall of dip-slip reverse faults; and (3) a permeability barrier developed in response to flexural slip folding. Oil trapped within the Rincon-Miley reservoir was primarily generated beneath the Santa more » Barbara Channel and migrated up the south flank of the anticlinal trend. Four stages of structural development and hydrocarbon migration, encompassing the last 700,000 years, have implications for the enhanced development of reservoirs on this anticlinal trend. « less

Stress patterns in an interplate shear zone: an effective anisotropic model and implications for the transverse ranges, California

Strong lateral variations in geological structure within a transcurrent interplate deformation boundary have a substantial influence upon the way in which ambient stress is related to the relief of regional stress within the boundary zone. Much of the crustal deformational structure in southern California and environs consists of a conjugate wrench fault system. The Quaternary fault system consists of a series of parallel and sub-parallel strike-slip faults that are causally related to the horizontal interplate shearing. A prominent crustal structural inhomogeneity is the Transverse Ranges, where fault orientation is east-west, transverse to the dominant northwesterly trend. We investigate some of the consequences of this transverse inhomogeneity on the overall stress and strain field in the southern California region. The activity of the strike-slip (or wrench) system to the south and north of the Transverse Ranges suggests a mechanical model consisting of weak zones with a relatively strong degree of orientation. An effective anisotropy model is constructed based on: (1) a two-component laminate model consisting of competent unfaulted rock adjacent to incompetent faulted rock; (2) theoretical results for the weakening of a plate due to a doubly periodic array of cracks; and (3) finite element treatment of a checkerboard array of cracks. The fundamental parameter for weakening is = 1 - where is a non-dimensional form of Biot’s slide modulus. In the limit of 1 the crust becomes extremely weak and anisotropic, and as A -> 0 the condition of a strong, isotropic crust is recovered. The components of the stiffness (or compliance) matrix are directly related to the mechanical properties of a finite width fault zone, or to the average fault spacing and asperity density within a particular geological province, or both. An elastic plate model that incorporates the stress-strain channelling caused by multiple, oriented fault systems is constructed. The plate is assumed to be stressed by pure shearing forces maintained at infinity. The ambient field then corresponds to the north-south compressions, east-west extensions tectonic regime that dominates North-American-Pacific interplate shear along the San Andreas fault, California. Embedded within the plate is an elliptical inclusion in which multiple fault stress channelling also occurs. The inclusion thus mimics the misaligned structure of the Transverse Ranges in southern California. The boundary value problem associated with the model is treated both analytically and with finite element computations. The simple model predicts (i) the enhanced seismic energy release associated with the Transverse Ranges; and (ii) the clockwise rigid rotation indicated by a palaeomagnetic studies. The relatively simple nature of the model helps to isolate those features of the southern California tectonic stress regime that might be attributed to the transverse orientation of the Transverse Ranges. Stress channelled into the crosscutting tectonic structure from the ambient interplate field is significant. Contradirectionality alone cannot provide an explanation for the enhanced north-south compressive stress relative to east-west extension.

Continental Slope Deposits from a Late Cretaceous, Tectonically Active Margin, Southern California

ABSTRACT Extensive, active-margin continental-slope deposits are not commonly preserved in the rock record. The middle and upper portions of the Holz Shale (Ladd Formation) in Black Star Canyon, northern Santa Aria Mountains, represent such a sequence where shale and mudstone strata are dissected by large, coarse-grained channel-fill deposits. Channels preserve evidence of filling primarily by conglomeratic debris flows, high-density turbidites, and classic Bouma low-density turbidites; slumping and traction-current mechanisms were less important. Associated with channels are pebbly mudstones, poorly developed levee facies, and submarine chute and gully deposits. Interbedded turbidites, contourites, and hemipelagic sediments dominate interchannel strata. Hemipelagic sediments exhibit sedimento ogic fabrics that range from biologically dominated (homogeneous, bioturbated) to physically dominated (fine-scale, planar-laminated, anaerobic). This variation in fabric as well as associated digenetic information indicates that the anaerobic zone, although generally at some depth below the sediment-water interface, at times migrated up into the overlying water column. Foraminiferal assemblages within hemipelagic sediments are dominated by agglutinated forms which indicate deposition at bathyal depths. Macroinvertebrate evidence includes 1) the interchannel paleocommunity, dominated by the bivalve Inoceramus and the deposit-feeding trace fossil Chondrites and 2) the submarine-channel paleocommunity, comprised mainly of the trace fossils Thalassinoides and Ophiomorpha. Previous studies have demonstrated that these Late Cretaceous active-margin environments included a narrow continental shelf. Abundance of terrestrial plant material, paucity of displaced shelf faunas, well-rounded conglomerate clasts, and the coarse-grained texture of these deposits suggest that one or more of the Holz Shale submarine channels were receiving sediment directly from terrestrial environments. Paleogeographic reconstructions have postulated Cenozoic lateral and rotational displacement of associated Upper Cretaceous strata in the Transverse Ranges of southern California. Paleocurrent and other sedimentologic analyses of Holz Shale strata support these reconstructions and suggest that a submarine-fan complex, which was fed by Holz Shale submarine channels, is presently located to the north in the Simi Hills and Santa Monica Mountains.

The impact, control, and mitigation of off-road vehicle activity in Hungry Valley, California

Hungry Valley, and arid basin situated within the Transverse Ranges of Southern California, has been utilized by off-road vehicles (ORVs) since 1971. Loss of vegetation, severe soil erosion and gullying, alluvial fan formation, and increased sediment discharge are the direct results of ORV activity in the valley. These hydrologic-geologic effects have been documented and monitored utilizing sequential aerial photographs, ground surveys, and sediment transport measurements. Despite the increased usage of ORVs on public lands, and the well-documented short-term physical environmental impacts, little data exists on erosion or sediment transport rates in disturbed vs. adjacent undisturbed areas, the ability of natural vegetation to re-establish itself, or the feasibility of site rehabilitation techniques or methods.