Western Transverse Ranges Of California Research Articles

Topographic development of a compressional mountain range, the western Transverse Ranges of California, USA, resulted from localized uplift along individual structures and regional uplift from deeper shortening

Abstract The western Transverse Ranges are a tectonically active mountain belt in southern California (USA) characterized by fast rates of shortening and rock uplift. Large drainages at the western end of this mountain belt, including the Santa Ynez River and its tributaries, transect regional west–northwest-striking reverse faults and folds. We used fluvial strath terraces within the Santa Ynez River watershed as geomorphic markers for measuring Quaternary rock uplift and deformation across these structures. Mapping, surveying, and numerical dating of these strath terraces in both hanging-wall and footwall blocks of the major reverse faults allow us to separate regional uplift from localized uplift along individual structures. Luminescence dates from 18 sites within the Santa Ynez River watershed show that the three prominent terrace levels present throughout the area formed between ca. 85 ka and 95 ka, 55 ka and 75 ka, and 30 ka and 45 ka. All three fluvial terrace straths grade into marine paleo-shore platforms along the coast that formed during sea-level highstands. The fluvial straths were formed as a result of lateral erosion during warm, dry climate intervals when vertical incision was temporarily arrested. Incision of the terraces followed during intervening periods of wet climate. Mapping and valley-long profiles of the terraces document deformation by faults and folds, and we infer minimum rock-uplift rates from the amount of incision below the terrace strath surfaces. Rock-uplift rates range from 0.3 mm/yr to 4.9 mm/yr, with faster rates in the hanging-wall blocks of the major reverse faults and slower rates in the footwall blocks. Rock-uplift rates calculated from strath terraces in the footwall blocks range from 0.3 mm/yr to 1.6 mm/yr, which indicates a regional component of uplift that results from deeper deformation. Higher rates of rock uplift in the hanging-wall blocks (0.5–4.9 mm/yr) are superposed on this regional component. Incremental rock-uplift rates calculated over three time intervals and differences in terrace deformation with age suggest that deformation rates across some structures have decreased over the past 85 k.y. We conclude that topographic growth of the western Transverse Ranges results from a combination of localized uplift along individual structures that varies both spatially and temporally and a more constant regional uplift that likely results from deeper ductile deformation or slip along detachment faults that have been inferred to underlie the area.

Geological Structure of the Sylmar Basin: Implications for Slip Distribution Along the Santa Susana/Hospital and Mission Hills Fault System in the San Fernando Valley, CA, U.S.A.

We developed a forward model using the Trishear module in MOVE to better understand the structure of the northwestern San Fernando Valley and the relationship among the Santa Susana, Hospital, Mission Hills and Northridge Hills faults. This study was motivated by the 1971 San Fernando earthquake and previous work that inferred a high slip rate on the Santa Susana fault, which is in apparent contrast to the lack of significant geomorphic expression of the fault in the Sylmar Basin region. We trenched the Mission Hills anticline from the crest to the base of slope and demonstrate that the Mission Hills anticline is an actively growing fault propagation fold. The associated thrust tip is either deeper than 15 m or sufficiently far to the south that the fault was not encountered in large diameter borings, but the minimum structural relief across the Mission Hills fault since the late Pleistocene is on the order of 37 m, suggesting a minimum uplift rate of 0.5 mm/yr. Our work presents a structural analysis that demonstrates how the Santa Susana fault system evolved in time, with the frontal thrust progressively migrating southward to the Mission Hills fault, and farther south to the Northridge Hills blind thrust. The progression of faulting towards the direction of vergence is compatible with the observed thrust front migration in the western Transverse Ranges of California, and other trust belts around the world.

Structural modeling of the Western Transverse Ranges: An imbricated thrust ramp architecture

AbstractActive fold-and-thrust belts can potentially accommodate large-magnitude earthquakes, so understanding the structure in such regions has both societal and scientific importance. Recent studies have provided evidence for large earthquakes in the Western Transverse Ranges of California, USA. However, the diverse set of conflicting structural models for this region highlights the lack of understanding of the subsurface geometry of faults. A more robust structural model is required to assess the seismic hazard of the Western Transverse Ranges. Toward this goal, we developed a forward structural model using Trishear in MOVE® to match the first-order structure of the Western Transverse Ranges, as inferred from surface geology, subsurface well control, and seismic stratigraphy. We incorporated the full range of geologic observations, including vertical motions from uplifted fluvial and marine terraces, as constraints on our kinematic forward modeling. Using fault-related folding methods, we predicted the geometry and sense of slip of the major faults at depth, and we used these structures to model the evolution of the Western Transverse Ranges since the late Pliocene. The model predictions are in good agreement with the observed geology. Our results suggest that the Western Transverse Ranges comprises a southward-verging imbricate thrust system, with the dominant faults dipping as a ramp to the north and steepening as they shoal from ∼16°–30° at depth to ∼45°–60° near the surface. We estimate ∼21 km of total shortening since the Pliocene in the eastern part of the region, and a decrease of total shortening west of Santa Barbara down to 7 km near Point Conception. The potential surface area of the inferred deep thrust ramp is up to 6000 km2, which is of sufficient size to host the large earthquakes inferred from paleoseismic studies in this region.

Climate-controlled landscape evolution in the Western Transverse Ranges, California: Insights from Quaternary geochronology of the Saugus Formation and strath terrace flights

The Las Posas and Ojai Valleys, located in the actively deforming Western Transverse Ranges of California, contain well-preserved flights of strath terraces and Quaternary strata (i.e., Saugus Formation) that when numerically dated elucidate the tectonic, geomorphic, and fluvial histories that sculpted the landscape since ca. 140 ka. This study includes 14 new optically stimulated luminescence and 16 new terrestrial cosmogenic nuclide ages from the late Pleistocene to Holocene that record two regional aggradation events and four intervals of strath terrace formation. Geochronologic data indicate that terrestrial Saugus strata in the Las Posas Valley (Camarillo Member) prograded over marine deposits at ca. 125 and 80 ka and are as young as 60–25 ka, which is an order of magnitude younger than the youngest Saugus strata elsewhere in Southern California. These results highlight the need for precise dating of Saugus strata where identified and utilized to assess rates of tectonic deformation. Based on its compositional character, thickness, stratigraphic relations, and inferred ages, the Camarillo member of the Saugus Formation is correlated with sediments of the Mugu aquifer identified in subcrop throughout the Ventura Basin and thus provides a new regional chronostratigraphic subsurface datum. The aggradation of these sediments and similar deposits in the study area between 13 and 4 ka is subsequent to the transition from humid to semiarid climate correlating to the end of the ultimate and penultimate glacial maximums. Aggradation is inferred to have resulted from increased sediment supply in response transient vegetative conditions and consequent hillslope destabilization. Similar to aggradational events, strath terrace cover sediments ages correlate to dry warm climate intervals, indicating straths in Southern California were cut at ca. 110–100 ka, 50–35 ka, 26–20 ka, and 15–4 ka. These results support recent mathematical and experimental models of strath formation, where increased sediment flux and decreased water discharge enhances lateral erosion rates and inhibits vertical incision. Subsequent incision and strath terrace formation is inferred to occur during intervening wet climate intervals. The correlation of strath terrace ages and aggradational events with environmental changes that are linked to global climate indicates that climate rather than tectonics exhibits first-order control of depositional, denudational, and incisional processes in the Western Transverse Ranges. Moreover, these results provide a chronostratigraphic framework that allows these landforms to be regionally correlated and used to assess rates of active tectonics where geochronologic data are unavailable.

Sediment yield from the tectonically active semiarid Western Transverse Ranges of California

Sediment yields from the world's rivers are generally highest from steep drainage basins with weak lithology, active tectonics, or severe land-use impacts. Here, we evaluate sediment yields from the Western Transverse Ranges of California in an attempt to explain why they are two- to tenfold greater than the surrounding areas of California. We found that suspended-sediment yields across the gauged basins of the Western Transverse Range during 1969–1999 varied by approximately an order of magnitude (740–5300 t/km2/yr). Similarly, fine-sediment concentrations for normalized discharge rates varied by almost two orders of magnitude (e.g., 1.3–110 g/L for the mean annual flood) for 11 previously unmonitored drainages of the Santa Ynez Mountains. Areas with high sediment yields consistently have weakly consolidated bedrock (Quaternary-Pliocene marine formations) and are associated with the highest rates of tectonic uplift of the region (>5 mm/yr). These regions are important to the sediment discharge budgets, because ~50% of the total suspended-sediment discharge from the Western Transverse Range is estimated to be generated within these regions, even though they represent only ~10% of the total watershed area. Previous estimates of suspended-sediment discharge from the Ventura River have likely been underestimated by ~50% because the gauging station is located immediately upstream of a high sediment yield region. We also found a significant and positive correlation between sediment yield and the percentage of a watershed with grassland and agricultural land use. These results suggest that there is adequate variation within the lithology, tectonics, and land use of the broader Western Transverse Range geologic province to induce large variations in sediment yield at the local scale.

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.

Dating alluvial deposits with optically stimulated luminescence, AMS 14C and cosmogenic techniques, western Transverse Ranges, California, USA

In an effort to better understand chronology of alluvial episodes in Cuyama Valley in the western Transverse Ranges of California, USA, we employed optically stimulated luminescence, radiocarbon and cosmogenic radionuclide surface exposure dating methods. Twenty-one optical dates ranging from 0.01 to ∼27 ka were obtained from exposures of late-Holocene axial-fluvial deposits, Pleistocene–Holocene alluvial-fan deposits, and axial-fluvial sands interbedded within a late Pleistocene alluvial fan. These were cross-checked with 37 AMS radiocarbon dates from charcoal and wood from within a and five 10Be surface exposure dates from boulders on alluvial-fan surfaces. The OSL results show generally good stratigraphic consistency, logical comparison with the radiocarbon and cosmogenic data, and appear to be the best method for accurate dating within deposits of this nature because suitable material is fairly easy to find in these environments. The radiocarbon data contained numerous “detrital ages”, but well-bedded lenses of apparently in situ or minimally transported charcoal provide reliable age estimates for the associated alluvium. Radiocarbon dating of detrital charcoal in the older alluvial fan deposits was problematic. Our cosmogenic surface-exposure dating was consistent stratigraphically and with our other data, but we were unable to determine its accuracy due to the limited number of samples and the possibility of inherited radionuclides and post-depositional erosion. In light of our results, we suggest that OSL dating using the latest analytical techniques combined with rigorous methods for estimation of paleodase is reliable and of increasing utility in otherwise difficult-to-date coarse alluvial environments in the southwestern United States and elsewhere.

The effects of buoyant crust on the gravitational instability of thickened mantle lithosphere at zones of intracontinental convergence

Using both numerical experiments and approximate linear theory, we examine the stability of a 2-D, three-layered viscous system that represents crust over mantle lithosphere, overlying asthenosphere. If horizontal shortening of lithosphere occurs, as in mountain building, the development of gravitational instability may follow one of two distinct paths. Localized downwelling occurs beneath the region of horizontal shortening and crustal thickening, if the layer representing crust has a large viscosity coefficient, is relatively dense or is thin, relative to the underlying layer that represents mantle lithosphere. Conversely, for a low viscosity ratio or buoyant crust, downwelling flow develops on the flanks of the zone of convergence, and the mantle lithosphere layer thins beneath the central region. Thinning in such a setting would facilitate volcanism and high-temperature metamorphism during the development of a mountain belt. A series of numerical experiments with different density and viscosity structures, including runs with non-Newtonian viscosity, show that this general description is insensitive to minor differences in the depth distributions of density and viscosity. The tendency of structures with relatively thick crustal layers (or relatively thin lithospheric mantle) to favour paired, marginal downwellings may account for sustained volcanism during intracontinental deformation where lithosphere initially is warm, thin and weak. The western part of the Tien Shan in Asia and the region including and surrounding the Alboran Sea illustrate some of the features associated with paired, marginal downwellings. The Western Transverse Ranges of California and the Southern Alps of New Zealand share features associated with single downwelling flow beneath the axes of the belts.

Paleomagnetic definition of crustal fragmentation and Quaternary block rotations in the east Ventura Basin and San Fernando valley, southern California

Paleomagnetic studies of the Pliocene‐Quaternary Saugus Formation in the eastern part of the western Transverse Ranges of California show that the crust is fragmented into small domains, tens of kilometers in linear dimension, identified by rotation of reverse‐fault blocks. In an area approximately 35 × 25 km in the San Fernando valley and east Ventura Basin we identified four distinct domains. Two domains, southwest of and adjacent to the San Gabriel fault, are rotated clockwise: (1) The Magic Mountain domain, R = 30° ± 5° and (2) the Merrick syncline domain, R = 34° ± 6°. The Magic Mountain domain has rotated since 1 Ma. Both rotated sections occur in hanging walls of active reverse faults, the Santa Susana and San Fernando faults, respectively. Structural data suggest that the fault tip of the Santa Susana fault is the rotation pivot of the Magic Mountain domain. Two additional blocks are unrotated: (1) the Van Norman Lake domain, directly south of the Santa Susana fault, and (2) the Soledad Canyon domain, immediately across the San Gabriel fault from the Magic Mountain domain, suggesting that the San Gabriel fault might be a domain boundary. Our results suggest that part of the clockwise rotation of some Miocene and older rocks in this area might have occurred in the Quaternary. The Plio‐Pleistocene fragmentation and clockwise rotations continue at present, based on geodetic data, and represent crustal response to diffuse, oblique dextral shearing within the San Andreas fault system.

Convergence rates across a displacement transfer zone in the western Transverse Ranges, Ventura basin, California

Three cross sections are balanced and retrodeformed to the top of the Pleistocene Saugus Formation and to a horizon close to the base of the Jaramillo subchron to yield crustal shortening and shortening rates for the western Transverse Ranges of California. The cross sections compare the shortening that occurs along a transfer zone in which displacement is transferred eastward from a surface reverse fault (Red Mountain fault) to a blind thrust and from that to a combination of both a surface reverse fault (San Cayetano fault) and the blind thrust. Using an age of 250±50 ka for the top of the Saugus Formation based on amino‐acid racemization of fossil mollusks, crustal shortening rates appear to have abruptly accelerated through time from 3+4/−3 to 5±4 mm/yr between 250 and 975 ka to 20±6 to 28±1 mm/yr since 250±50 ka. Using an age of 500 ka for the top of the Saugus Formation based on paleomagnetic stratigraphy, there is little increase in deformation rates through time, ranging from 5±5 to 8±6 mm/yr between 975–500 ka and from 10±3 to 14±1 mm/yr since 500 ka. Crustal convergence rates determined by Global Positioning System surveys, taken over 4.6 years, indicate a shortening rate of 7–10 mm/yr across the basin. This is consistent with the slower rates of deformation calculated using an age of 500 ka for the top of the Saugus Formation.

Fluid‐pressure induced seismicity at regional scales

The role of high fluid pressure as a seismogenic agent has been the subject of intense study (Hubert and Rubey, 1959; Hanshaw and Bredehoeft, 1968; Healy and Rubey, 1968; Simpson, 1976; Walder and Nur, 1984; Sibson, 1990). Of particular interest is the so‐called fault‐valve mechanism (Sibson, 1976; Sibson, 1990) a hypothesis whereby fluid pressure rises (as a result of tectonic compression and pore volume reduction) until crustal failure occurs, triggering seismic activity and upward fluid discharge. Sealing and healing of the rock matrix (Richter and Simmons, 1977; Sprunt and Nur, 1979; Angevine et al, 1982) following coseismic stress drop facilitates reaccumulation of fluid pressure, initiating another loading cycle. The fault‐valve mechanism is entertained as a plausible explanation for present‐day seismic activity in the western Transverse Ranges of California. We provide a quantitative test of the fault‐valve hypothesis that uses geologic data and rates of active tectonics for a cross‐section through an active fold‐and‐thrust belt on the flank of a developing mountain range. Rates of fluid pressure buildup and average recurrence times of large earthquakes in the fold‐and‐thrust belt are estimated to be on the order of 104Pa/yrand hundreds of years, respectively.

Western transverse ranges crustal structure

A seismic P wave velocity and bulk density model of the crust of the western Transverse Ranges of California along a north‐south profile near the city of Santa Barbara shows south to north crustal thickening and appreciable lateral inhomogeneity within the crust. The seismic model is developed by ray tracing based on data from a land‐sea refraction experiment, Pn arrivals from regional earthquakes, and near‐vertical arrivals from teleseisms. The density model is based on published gravity data (Bouguer on land and free air at sea). The model extends to the known oceanic lithospheric structure west of the southern California Borderland, providing a boundary condition for modeling crustal thickness. The Borderland crust south of the western Transverse Ranges is 23 km thick, and the Coast Ranges crust to the north is 31 km thick. A low‐density, low‐velocity feature associated with deep Neogene sediment in the Santa Barbara Channel extends to depths greater than 10 km. A local high‐density, high‐velocity feature is associated with exposures of Mesozoic mafic crystalline rock on Santa Cruz Island. Low‐density and velocity material is resolvable to 5‐km depth south of the East Santa Cruz Basin fault.

Neogene Drape Folding Over Pre-Neogene Flexural-Slip Movements in Western Transverse Ranges, California: ABSTRACT

In several locations in the western Transverse Ranges of California are folded Neogene sedimentary sequences that unconformably overlie homoclinal sequences of pre-Neogene rocks. To accomplish the folding of the rocks above the unconformity without apparent deformation of those below the unconformity, a mechanism other than simple crustal shortening is required. It is proposed that differential flexural slip along bedding planes in the limbs of large-amplitude pre-Neogene folds produced drape folds of small amplitude in the unconformably overlying Neogene rocks. This drape mechanism implies that the Neogene rocks were folded while they were still in the soft-sediment stage and that they were lengthened parallel to bedding during the process. Procedures that use the length of folded beds to determine the amount of crustal shortening, therefore, may indicate a greater amount of crustal shortening than actually occurred. End_of_Article - Last_Page 665------------

San Luis Obispo Transform Fault and Middle Miocene rotation of the Western Transverse Ranges, California

A fault‐disrupted, linear belt of lower middle Miocene pyroclastic and volcanic rocks was erupted 15–17 m.y. B.P. along a trend 140 to 200 km long in the southern Coast Ranges and along the northern flank of the western Transverse Ranges of California. Offset segments of the belt are coincident with the Oceanic‐West Huasna, Santa Maria River‐Little Pine, and Lompoc‐Solvang fault zones. The volcanic rocks thin eastward and also vary in thickness along strike, the latter reflecting the undulation of ‘porpoising’ highs and lows of basins. To account for the linear distribution of over 3000 km2 of bimodal volcanic rock types, it is proposed that they were intruded, extruded, and ejected into a continental submarine margin environment along a feature here named the San Luis Obispo transform (SLOT). The fault was near the mid‐Miocene continental margin and evidently joined the early mid‐Miocene Mendocino triple junction with the crest of the East Pacific Rise. Subduction of the Farallon Plate had stopped along the adjacent continental margin before Miocene magmatism in this region. The inferred position of the East Pacific Rise crest was too far to the south, and the inferred position of the Mendocino triple junction was too far to the north between 15 and 20 m.y. B.P. to have been directly related to the Miocene volcanic event near 35°N latitude at that time. A part of the western Transverse Ranges was transported northwestward immediately following Miocene volcanism. Earlier movement is possible, but evidence is inconclusive. Microplate transport was concurrent with the development of pull‐apart structures and the ≃12‐ to ≃16‐m.y.‐old period of volcanism along the southern margin of the microplate within the Los Angeles basin. Variable paleomagnetic directions that display clockwise deflections in the Oligocene (e.g., Morro Rock‐Islay Hill Complex) and Miocene igneous rocks (e.g., western Transverse Ranges) in the vicinity of SLOT can be accounted for by extension and rotational tectonics during pull‐apart development, rotation of blocks between faults during the northwestward transport of the western Transverse Ranges along SLOT, and by rotation and possible northward translation of a part of the western Transverse Ranges.