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, reverse faulted, and vertically sheared. A new model is presented to explain how unsubducted rocks were deformed by head-on, near-surface, crustal shortening mechanisms and included with blueschist blocks following their contractional uplift from great depth.

Notably, the mélange contains both Pliensbachian MORB and likely-Bajocian, marginal basin greenstone, along with both harzburgite and lherzolite tectonite. The totality of the findings suggest that Bajocian oceanic crust formed in the marginal basin of an east-dipping suprasubduction-zone that formed within rifted Pliensbachian abyssal crust. After a series of major boundary condition changes, normal convergence became fully reestablished during Kimmeridgian time. This configuration allowed for unsubducted Franciscan mélange rocks to be deformed by contractile mechanisms and included with blueschist blocks 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 (GVG), along the Coast Range fault in place of the forearc basement, Coast Range ophiolite. In contrast, the southern extension of the SRMM, the Blue Canyon mélange – part of the clockwise-rotated western Transverse Ranges province, formerly located outboard of Santa Catalina Island – rests unconformably below uppermost Great Valley Group as a result of Nacimiento fault tectonics, leaving the Lower GVG preserved on under-thrusted Franciscan mélange farther west, post rotation. The GVG was eroded off the SRMM as a result of latest Cretaceous-Paleocene reverse faulting, which was also likely related to Nacimiento-fault tectonics.