RECONSTRUCTING THE SAN RAFAEL MTS. MÉLANGE, SANTA BARBARA COUNTY, CALIFORNIA

A sedimentary origin for many mid-Cretaceous Franciscan mélanges has become well-established. This study of the San Rafael Mountains mélange (SRMM), located in the southernmost Coast Ranges of California, offers an alternative interpretation to the widely adopted, trench-olistostrome, extensional tectonics model: based on detailed geologic mapping and structural observations, the SRMM is a first-order, contractile tectonic mélange, with a subsequent sedimentary component that was tectonized. Removing the overprinting effect of the tectonized sedimentary mélange and backing out the blocks of tectonite (serpentinized mantle peridotite) help to reunite the original stratigraphy, which was dismembered after being horizontally shortened, upright folded, and vertically sheared. A new model is presented to explain how unsubducted rocks were deformed by head-on, near-surface, reverse-fault mechanisms and integrated with contractionally uplifted blueschist-bearing tectonite to create in situ tectonic mélange located near the outer arc high.

The mélange was previously mapped at 1:10,000 and described in detail where it is particularly well-exposed in Redrock Canyon (Warner, M.C., U.C. Santa Barbara Master’s thesis, 1992) and along adjacent Santa Cruz Creek (Wahl, A.D., U.C. Santa Barbara Master’s thesis, 1995; USGS Bulletin 1995-W, 1998; see San Marco Pass, San Rafael Mts. 7.5’ quads).

The mapped area was revisited by the presenting author from 2018 to 2021. As a result, map unit descriptions and correlations were updated, along with various contact relationships. A single, synthesized geologic map was produced, incorporating new findings and mapping with the earlier findings and mapping of Warner and Wahl. This map will be included in a special report to be published by the CGS in early 2025 (Franciscan mélange reflects Nacimiento block contractile deformation).

The revisit focused primarily on documenting as follows: the folding and faulting of greenstone blocks; the envelopment of greenstone, chert, and blueschist blocks by tectonite; and the components of the sedimentary mélange. Research included a petrographic review of the graywacke. New observations expanded understanding of the significance of the sedimentary mélange and the structural order found previously in the mélange.