MELANGE AT CONVERGENT PLATE MARGINS: THE FRANCISCAN COMPLEX OF CALIFORNIA

Melanges are mapable bodies of fragmented and mixed blocks in a matrix. Melanges with a matrix of shale or serpentinite are commonly associated with subduction zone settings and of debated origin. Processes proposed include chaotic sedimentary mass flows (olistostromal), tectonic faulting and/or flowage, and diapirism. The most problematic melanges are the extensive shale-matrix terranes found in California, Alaska, Japan, and Indonesia. These mixed terranes are very poorly exposed where weakly lithified. The California example, the Central Belt of the Franciscan Complex, is especially noted for the inclusion of "exotic" blocks of garnet-bearing blueschist and rare eclogite. Field relationships, best seen in coastal exposures near San Simeon, reveal ductile flowage was concentrated in the shale matrix with all block types displaying gradations of pinch-and-swell to complete separation into elliptical boudins. There is a remarkable dichotomy in grain-scale deformation mechanisms operative during boudinaging. Masses of graywacke distorted primarily by particulate flow. Clastic quartz grains float in a clay-sized groundmass generated from the destruction of lithic and feldspar grains. In striking contrast, mafic greenstone and blueschist distorted via intense cataclasis concentrated along the block margins and along small shear zones that penetrate into the interiors. These microtextures indicate most of the deformation occurred while the graywacke blocks and shale matrix were essentially unlithified: fluid contents were high (20%±) and fluid pressures were at or very near lithostatic values. Concurrent with boudinaging, the mafic blocks were altering to low-temperature (<150°C) mineral assemblages. The formation of the Franciscan melange belt and similar terranes is best explained as due to tectonically-driven flowage and upwelling of thick piles of shale-rich sediment along the top of subduction channel shear zones. The distinctive microtextures would be obliterated with heating to temperatures above about 250°C. It is likely that some, and perhaps many, schist terranes containing lenticular masses of various lithologies were once Franciscan-like melanges metamorphosed during subduction-terminating collisional tectonism.