BASIN FORMATION AND TECTONIC INVERSION WITHIN THE INNER CALIFORNIA CONTINENTAL BORDERLAND

Contact of the East Pacific Rise with the Farallon subduction zone offshore southern California created the California Continental Borderland. Pacific Plate movement away from the continental margin combined with capture of the Monterey Microplate to produce oblique extension and rift the accretionary wedge and forearc basin terranes from the continental margin. An inboard array of oblique-slip basins records the Inner Borderland Rift evolution. Pacific oceanic crustal magnetic anomalies provide timing for initiation of Borderland basin formation. Large-scale segmentation of Borderland structure correlates to a segmented East Pacific Rise during the progressive southward propagation of the Rivera Triple junction.

Initial linear rift basins formed along the rift breakaway above east-dipping detachments. The widening rift accommodated a northwest-trending, right-stepping, dextral transform fault system aligned with the direction of relative plate motion. Magmatism focused along the edges of the growing rift above a growing slab gap. Pull-apart basins opened in major fault step-overs and accumulated thick sequences of sediments that record basin evolution. Vertical-axis block rotations of rifted terranes formed the Western Transverse Ranges, producing triangular basins at their edges and progressively blocking northwest transport of the Inner Borderland region. As transpression builds, tectonic inversion occurs in many basins which provided excellent traps for petroleum resources documented with numerous wells that define lithology and timing of significant sequences. Also, changes in the PAC-NAM relative motion direction produce transpression along the initial northwest-trending transform faults and invert basins into restraining bend pop-up structures. Continued activity of Borderland fault systems show combinations of strike-slip with extension along coast-parallel breakaway faults, contraction at restraining bends and step-overs, and dextral shear parallel to the modern PAC-NAM relative plate motion on major transform faults. Basement-involved transpression produced linear restraining fault ridges while north-trending pull-aparts with axial volcanoes resemble nascent or fossil spreading centers. Igneous and metamorphic rocks are exposed on islands and ridges while airborne ash deposits exist within the basin sediments.