Abstract

Recycled Franciscan detritus has been confirmed in the Franciscan Complex west of the Salinian block in the California Coast Ranges. A mass of Late Cretaceous sandstone and conglomerate-breccia, the Las Tablas unit, locally contains blocky clasts of Franciscan greenstone, chert, ultramafic rocks, glaucophane-lawsonite schist, and graywacke, together with larger amounts of non-Franciscan material. The Las Tablas unit is enveloped in a Franciscan tectonic mélange, and it contains metamorphic pumpellyite, suggesting the probable onset of blueschist-facies metamorphism.

The Franciscan clasts in the Las Tablas unit represent former components of an early mélange related to subduction. The heterogeneous mélange accumulated as an elongate pile of “scrapings” along the inner wall of a trench. During Late Cretaceous time, submarine debris flows of Franciscan mélange material were shed from the pile into the trench, arc-trench gap, or smaller intervening basins to form the Las Tablas conglomerate-breccia. Eventually the Las Tablas unit was “cannibalized” by continued subduction. The unit was technically engulfed in a Late Cretaceous or early Tertiary neo-Franciscan mélange and was carried to a depth sufficient to induce the development of pumpellyite.

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