The geology and tectonic evolution of the San Rafael Mountains Mélange, with implications for the Mesozoic–Cenozoic marginal history of southwestern California, USA

Detailed geologic mapping, stratigraphic study, and structural observations indicate that the San Rafael Mountains Mélange (SRMM), located in the southernmost Coast Ranges of California (USA), is a first-order contractile tectonic mélange with an unconformable, intraformational, and syndeformational secondary sedimentary mélange component. Removing the overprinting effect of the tectonized sedimentary mélange and backing out the blocks of tectonite—serpentinized mantle peridotite—reunites the original stratigraphic sequence, which was dismembered and mixed after being upright folded, reverse faulted, horizontally flattened, and vertically sheared. The marginal closing of a Late Jurassic divergent double subduction zone docked far-traveled Franciscan oceanic crust with the North American plate, escaping off-scraping, underthrusting, deformation, and metamorphism. Complete eastward evacuation of the west-dipping segment of this eastward-subducting plate off-scraped Middle Jurassic high-grade blocks and coherent sheets onto the adakitic base of the Coast Range ophiolite. A trailing slab window opened at the terminal closure, generating the amphibolite source for conglomerate clasts included within the pre-mélange stratigraphic sequence. During the Late Cretaceous, this coherent stratigraphic sequence was tectonically integrated with uplifted blueschist-bearing tectonite and subsequently transformed into mélange, in situ, near the forearc high. At ca. 80 Ma, the fully evolved SRMM was thrust beneath the forearc basin along the Coast Range fault. Subsequent Nacimiento fault tectonics drove reverse faulting on the Little Pine fault, uplifting the SRMM and erosionally removing its forearc basin structural cover. Post-Eocene transpressional transform tectonics mimicked normal convergence, as reflected in the reactivation of the original, head-on convergent Mesozoic structures within the SRMM.