Late Pleistocene rates of rock uplift and faulting at the boundary between the southern Coast Ranges and the western Transverse Ranges in California from reconstruction and luminescence dating of the Orcutt Formation

The western Transverse Ranges and southern Coast Ranges of California are lithologically similar but have very different styles and rates of Quaternary deformation. The western Transverse Ranges are deformed by west-trending folds and reverse faults with fast rates of Quaternary fault slip (1–11 mm/yr) and uplift (1–7 mm/yr). The southern Coast Ranges, however, are primarily deformed by northwest-trending folds and right-lateral strike-slip faults with much slower slip rates (3 mm/yr or less) and uplift rates (<1 mm/yr). Faults and folds at the boundary between these two structural domains exhibit geometric and kinematic characteristics of both domains, but little is known about the rate of Quaternary deformation along the boundary. We used a late Pleistocene sedimentary deposit, the Orcutt Formation, as a marker to characterize deformation within the boundary zone over the past 120 k.y. The Orcutt Formation is a fluvial deposit in the Santa Maria Basin that formed during regional planation by a broad fluvial system that graded into a shoreline platform at the coast. We used post-infrared–infrared-stimulated luminescence (pIR-IRSL) dating to determine that the Orcutt Formation was deposited between 119 ± 8 and 85 ± 6 ka, coincident with oxygen isotope stages 5e-a paleo–sea-level highstands and regional depositional events. The deformed Orcutt basal surface closely follows the present-day topography of the Santa Maria Basin and is folded by northwest-trending anticlines that are a combination of fault-propagation and fault-bend-folding controlled by deeper thrust faults. Reconstructions of the Orcutt basal surface and forward modeling of balanced cross sections across the study area allowed us to mea­sure rock uplift rates and fault slip rates. Rock uplift rates at the crests of two major anticlinoria are 0.9–4.9 mm/yr, and the dip-slip rate along the blind fault system that underlies these folds is 5.6–6.7 mm/yr. These rates are similar to those reported from the Ventura area to the southeast and indicate that the relatively high rates of deformation in the western Transverse Ranges are also present along the northern boundary zone. The deformation style and rates are consistent with models that attribute shortening across the Santa Maria Basin to accommodation of clockwise rotation of the western Transverse Ranges and suggest that rotation has continued into late Quaternary time.

Effect of anisotropy, angle, and critical tensile stress and confining pressures on evaluation of shale brittleness index — Part 1: Methodology and laboratory study

Brittleness is an important evaluation parameter in shale fracturing. Current methods of brittleness evaluation can be classified into two categories: elastic parameter-based and mineral content-based methods. However, both categories neglect the effect of anisotropy on the brittleness index (BI) computation of shale resources. We have redefined a new BI by integrating failure criteria stress and anisotropy parameters estimated (BIac) from seismic waves. According to the new definition, the BI at one analysis point varies with the incident angle of the seismic wave and confining pressures. We applied the BIac method to laboratory-measured shale samples acquired from the Monterey Formation, Santa Maria Basin. We found that the delta parameter [Formula: see text] is more responsive to the BIac than the gamma [Formula: see text] and epsilon [Formula: see text] anisotropic parameters, and it indicates a good linear fit relationship with the BIac at different angles. The slope of the linear is variable with the angles, thus delta can be used to predict the BIac in the Monterey Formation, Santa Maria Basin.