ABSTRACT The Monterey Formation of Central and Southern California has produced billions of barrels of oil since the early 1900s. The Monterey Formation in the Santa Maria Basin is a tectonically fractured reservoir, meaning that the fractures formed through natural geologic processes; they are not human-generated artifacts. Open natural fractures provide the effective porosity for oil storage and the permeability pathways through which oil flows from rocks to wells. Monterey strata are notable for a diverse range of lithologies characterized by contrasts in texture and composition. Not all Monterey rock types contain natural fractures. Structural geologists applied the concepts of mechanical stratigraphy to the Monterey Formation to explain fracture variability. Hard rocks, including chert, porcelanite, and dolostone, contain extensive open-fracture systems, while softer lithologies like siliceous mudstone and organic-rich mudstone have few or no open fractures. However, the words “hard and soft” or “strong and weak” are inexact and subject to interpretation. This report constrains these qualitative descriptions by using engineering-geology data to associate rock properties with quantitative measurements of rock mechanical strength.

The majority of the San Andreas fault zone is convergently oblique to relative plate motion. The commonness of transpression makes it significant for understanding deformation of the continental lithosphere. We have quantified the distribution of transpressional deformation along the San Andreas fault zone with respect to variations in boundary conditions along its length and distance from the fault zone itself. Rock uplift was used as a proxy for transpressional deformation. The pattern of exhumation along the fault was synthesized based on previously determined apatite fission-track and (U-Th)/He ages from 210 locations within 40 km of the fault trace. Patterns of mean elevation and slope in swaths along the fault were used as rough proxies of surface uplift and erosion. Relatively higher exhumation rates and mean elevations occur most commonly along the most oblique sections of the fault, such as in the Transverse Ranges. The highest rates of exhumation (>0.5 mm/yr) and highest and steepest topography also occur almost exclusively in the near field (i.e., within ∼10 km) of the fault trace. These trends are consistent with the strain-partitioning model of transpression, in which distributed deformation is concentrated in the fault zone and the degree of partitioning between simple and pure shear is a function of obliquity. However, the pattern of rock uplift also exhibits considerable variability. Neither the degree of obliquity nor the distance to the fault trace is enough to predict where high exhumation or mean elevation will occur. This suggests that heterogeneity in boundary conditions, including mechanical weaknesses and variations in erodibility, is equally important for controlling the pattern of transpressional deformation.