13-3 Franciscan Mélange Reflects Contractile Deformation

Session: Toe to Toe: Cordilleran Systems from Trench to Retroarc Domains

Presenting Author:

Author:

Abstract:

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 included with blueschist blocks following their contractional uplift from great depth, to create in situ tectonic mélange, located near the outer arc high. Circa 78 Ma, fully evolved SRMM was thrust beneath the forearc basin, Great Valley Group, along the Coast Range fault in place of the forearc basement, Coast Range ophiolite.

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, MA, 1992) and along adjacent Santa Cruz Creek (Wahl, A.D.; U.C. Santa Barbara, MA, 1995; USGS Bulletin 1995-W, 1998).

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 as part of a Special Report that is scheduled for publication by the CAGS in late 2025 (see Franciscan mélange reflects Nacimiento block contractile deformation).

The revisit focused primarily on documenting as follows: the upright folding and reverse faulting of the greenstone blocks; the envelopment of greenstone, chert, and blueschist blocks by tectonite; and the components of the sedimentary mélange, including its intercalated siliciclastic- and serpentinite-mudstone matrix. Research further included a petrographic review of the graywacke. New observations expanded understanding of the significance of the sedimentary mélange, and the structural order found previously, expressed as concordantly deformed and aligned greenstone blocks, integrated with subvertical sheets of homogeneous tectonite.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

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