Search

The Wheeler Ridge anticline, located in the southern San Joaquin Valley of California, USA, is a well-studied and classic example of a laterally growing fault propagation fold. New high-resolution lidar elevation data combined with nine infrared stimulated luminescence (IRSL) ages of discrete geomorphic surfaces that are bounded by prominent transverse wind and river gaps allow for investigation of tectonic topography through time. Luminescence ages from four of the six surfaces yield depositional ages that range from 32 ka to 153 ka, which are broadly consistent with a previously published soil chronosequence. Our graphical modeling indicates an average surface uplift rate of ~2.1 mm/yr and an average along-strike fold propagation rate of ~20 mm/yr. However, our probabilistic modelling and topographic analysis suggest a rate decrease of both uplift and lateral propagation toward the fault tip from ~2.4 to 0.7 mm/yr and from ~49 to 14 mm/yr, respectively. Rate decreases are not progressive but rather occur in punctuated deformational intervals across previously documented structural barriers (tear faults) resulting in a fold that is characterized by discrete segments that exhibit a systematic deformational decrease toward the east. The punctuated tectonic growth of Wheeler Ridge has also locally controlled the topographic evolution of the anticline by effecting the formational timing and position of at least seven wind and river gaps that result from multiple north-flowing antecedent streams that traverse the growing structure. We quantify the timing of wind and river gap formation, based on IRSL results and inferred incision rates, and present a model for the spatiotemporal evolution of transverse drainages and the topographic development of Wheeler Ridge. Our chronology of gap formation broadly correlates with regional Late Pleistocene dry climate intervals suggesting that both tectonics and climate were integral to the geomorphic development of the Wheeler Ridge anticline.

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.

The Camarillo fold belt in the Western Transverse Ranges poses a significant seismic hazard to nearly one million people living in Southern California, yet few published geologic or geochronological data from this fold belt exist. The Camarillo fold belt is composed of several south-verging, west-plunging anticlines that characterize the western extent of the Simi fault zone, which extends for 40 km through urbanized Ventura and Los Angeles Counties. Surface and subsurface geologic data are utilized to accurately construct five cross sections within discrete structural domains to assess the local style of deformation, and to quantify the magnitudes of fault slip, fault- and fold-related uplift, and percent shortening. Eight new optically stimulated luminescence (OSL) dates from three paleoseismic trenches and six numerical dates from a previous study were utilized to quantify the timing and rates of deformation on discrete faults and folds. The onset of deformation in the Camarillo fold belt is everywhere younger than ca. 125 ka and locally as young as ca. 25 ka. Quaternary deformation occurs on reactivated steeply dipping (70°) Miocene faults, with shortening being largely accommodated in a narrow zone (

The 1995 and 2005 landslides in the 200-m high sea cliff above the community of La Conchita, California, are known to be part of a reactivated Holocene prehistoric landslide. We propose that the prehistoric Holocene slide is part of a much larger, several hundred million cubic meter late Pleistocene slide complex composed of upper slumps and lower flows, informally termed as the Rincon Mountain megaslide. An approximate age of 30 ka for the Rincon Mountain landslide is derived based on a 25-m high fault scarp produced in earthflow deposits that overlie the megaslide deposits and a known rate of faulting (~ 0.8 m/ky). Geomorphic evidence for the megaslide includes a prominent 100-m high amphitheater-shaped head scarp, back-tilted landslide benches, hummocky topography, and numerous smaller landslides and earthflow deposits. Geologic evidence includes deposits composed of slide breccia with fragments of the late Pleistocene (45 ka) emergent marine platform and terrace deposits displaced several tens of meters. Isolated parts of the Rincon Mountain landslide are active in the La Conchita area, but no evidence exists that the entire slide mass is moving as a unit. Landslides from the 200-m high slope behind La Conchita will reoccur and future development on the proposed Rincon Mountain slide should be very carefully evaluated to avoid reducing slope stability and reactivation of the megaslide.