305-4 Franciscan Mélange is Key to Southwestern California’s Marginal Tectonic History

Session: Subduction Zone Processes: Insights from Geology, Geochemistry, and Petrochronology (Posters)

Presenting Author:

Author:

Abstract:

Based on a detailed study of the San Rafael Mountains mélange (SRMM), located on the Nacimiento block in the southernmost Coast Ranges of California, combined with numerous reconnaissance observations made of Mesozoic geologic features in the area, the tectonic history of the region is offered, including a timeline of major events, from Early Jurassic to the present (see Franciscan mélange reflects Nacimiento block contractile deformation; CAGS Special Report, to be published in late 2025).

Notably, the mélange contains both Pliensbachian MORB and likely-Bajocian, marginal basin greenstone, along with both harzburgite and lherzolite serpentinized mantle peridotite (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.

A terrigenous provenance review indicates the existence of two plutonic sources, with one being Jurassic and the other being Cretaceous (San Gabriel block, western 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), bearing pink granite and blueschist cobbles that also likely relate to Nacimiento-fault tectonics. Evidently, the lower GVG was eroded off and the trace of the Coast Range (thrust) fault is buried farther inboard (south, post-rotations; outboard of Catalina Island, pre-rotations). The GVG was eroded off the SRMM as a result of Paleocene reverse faulting, which was 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.

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

doi: 10.1130/abs/2025AM-8604

© Copyright 2025 The Geological Society of America (GSA), all rights reserved.