ORIGIN OF BLUESCHIST-BEARING MELANGE AT SAN SIMEON, CENTRAL CALIFORNIA COAST RANGES

Mud-matrix melanges at San Simeon have been variously interpreted as deformed olistostromes or as subduction-channel flow melanges. Detailed examination shows that seven types of melange can be distinguished, with transitions among them. All types contain exotic clasts of greenstone, chert, and more rarely blueschist, in addition to greywacke sandstone, and the same materials occur as blocks meters to tens of meters in diameter immersed in melange. The seven types are (1) bedded conglomerate, (2) structureless conglomerate, (3) pebbly mudstone, (4) sandy block melange, (5) broken formation, (6) mud-matrix melange without deformational fabric, (7) sheared melange. Types (1) to (3) are clearly sedimentary in origin. Types (4) and (5) were clearly formed from unconsolidated sediment. Transitional types suggest that the mud-matrix melange (6) formed by disruption of types (1) to (5). Disruption took place while the sediment was unconsolidated, either on the trench slope or at very shallow levels in the accretionary wedge. Sheared melange (7) was formed by low-temperature post-consolidational deformation of all other types, which produced shear bands and a crude fabric transitional between slaty cleavage and gouge fabric. Kinematic indicators of shear direction are rare, but assuming the fabric and shear planes are coeval, the shear direction and sense can be determined from their intersection. Most shear planes are gently dipping and have normal sense displacements. Shear directions are highly variable, with the highest concentrations between WNW and S. This suggests that the main phase of shearing took place during a phase of vertical shortening and extension, rather than during accretion. Post-accretionary dextral shearing on NNW-trending vertical planes, and sinistral shear on a variety of trends, are likely related to Neogene transform tectonics. The simplest interpretation of these relationships is that the disrupted character of the melanges formed primarily by sliding down the trench inner slope of unconsolidated sediment, including clasts and blocks of previously accreted and exhumed greenstone, chert, and blueschist. The deformational fabric is largely unrelated to the disruption, and was formed during late-stage extension in the accretionary wedge.