BLUESCHIST BLOCKS IN THE SHALE-MATRIX MELANGE AT SAN SIMEON, CALIFORNIA: METAMORPHISM, DEFORMATION, AGE

Shale-matrix mélange is well exposed in 6+ km of nearly continuous seacliff and wave-cut benches at the classic occurrence near San Simeon. Blocks of blueschist are amongst more abundant greywacke and greenstone±chert. The cause of the mixing is again a matter of some debate. The blueschist blocks are the exotic that attract most attention and many (n=34) were studied in detail to evaluate their diversity. The Na-amphibole and lawsonite ± epidote schists were penetratively deformed by ductile folding at ~325±25°C at 5<P<9 kb (~20°C/km). Some of these mafic blocks contain phengitic mica associated with epidote that yield Ar-Ar ages (n = 3) that are 152-154 Ma. Petrographic analysis bolstered with mineral analysis reveal actinolitic cores in amphiboles and lawsonite replacement of epidote. These blueschists experienced a counterclockwise P-T path under progressively less dynamic conditions while cooling to ~225-250°C at P~5 kb (~15°C/km) as evident from the formation of lawsonite + aragonite veins. After more cooling, the fragments of blueschist became incorporated into the shale-matrix mélange. Tectonic boudinaging to create elliptical pieces was accommodated by cataclasis and chlorite + pumpellyite formation that is more intense nearer the block margins. The surrounding scaly black shale attained temperatures of 100 to 150°C while the T-depth conditions for the blocks must have been <10°C/km.

Explaining the blueschist blocks as sedimentary olistoliths requires exhumation to form a phantom source terrane and then resubduction. The simplest explanation is that these blocks were part of a metamorphic aureole that formed at the base of the ophiolitic leading edge of the North American plate during subduction initiation. Later, tectonic melange was generated when convergence caused shale-rich sediment to upwell along the top of the subduction channel. Slab(s) of blueschist detached from the aureole were boudinaged as they became mixed with fragments of greenstone and chert detached from subducting seamounts. The exposure of the blueschist along the roof of the subduction channel may have been localized to near where normal faults formed in the overriding block as Farallon plate dip flattened at 80 Ma. Blueschist block incorporation into upwelling mélange is a kind of subduction erosion akin to plucking by glaciers.