SAN RAFAEL MTS. MÉLANGE IS KEY TO SOUTHERNMOST NACIMIENTO BLOCK TECTONIC HISTORY

Based on detailed study of the San Rafael Mountains mélange (SRMM), located on the Nacimiento block in the southernmost Coast Ranges of California, an interpretation of the regional geologic history is offered as tectono-historical context, including a timeline of major events, from Early Jurassic to the present (see Franciscan mélange reflects Nacimiento block contractile deformation; CGS, 2025).

Notably, the mélange contains both Pliensbachian MORB and likely-Bajocian, marginal basin greenstone, along with both harzburgite and lherzolite serpentinized mantle peridotite. 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. In this configuration, unsubducted crustal rocks were integrated with blueschist-bearing tectonite to create in situ contractile tectonic mélange located near the outer arc high.

A terrigenous provenance review suggests the existence of two plutonic sources, with one being Jurassic and the other being Cretaceous (located in the eastern Transverse Ranges province?).

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. Accordingly, strike-slip faulting, likely related to Nacimiento-fault tectonics (75-56 Ma), placed the Coast Range ophiolite into its current coastal position at Pt. Sal. In contrast, a sheared unconformity separates the nearby Franciscan Blue Canyon mélange from overlying, Upper Cretaceous GVG (75-70 Ma). Evidently, the trace of the Coast Range fault is buried farther inboard (south, post-rotations). The GVG was eroded off the SRMM as a result of Paleocene reverse faulting, also likely related to Nacimiento-fault tectonics.

Pleistocene transform tectonics manifested as normal convergence, reflected in the reactivation of the originally head-on convergent Mesozoic structures in the SRMM, although the Little Pine (reverse) fault has a very slight, left-slip component, as indicated by the geometry and kinematics of the Loma Alta fault.