DEFORMATION OF BLOCKS AND MATRIX IN FRANCISCAN MELANGE, SAN SIMEON, CALIFORNIA: IMPLICATIONS FOR MELANGE GENESIS

Superior exposure of chaotically-bedded, blueschist-bearing Franciscan subduction melange exists along 4 km of seacliffs south of San Simeon, CA. The emplacement mechanism of the melange, its chaotic fabric, and the inclusion of blocks of variegated lithologies are alternately ascribed to dominantly sedimentary or tectonic processes. The melange "matrix" is a fine-grained (<20 micron) black metapelite, and bears a scaly foliation that dips at ~60° to the northeast and is parallel to the margins of elongate melange blocks. The foliation is defined by a pervasive anastomosing scaly cleavage. The cleavage is a result of late-stage dewatering, and foliation formation is coeval with final compaction of the melange. Greywacke blocks are lenticular, and display pinch-and-swell and boudinage geometries and high (4-8) aspect ratios from outcrop- to thin-section- scale. The greywacke is 50-70% clay-sized material ("groundmass", comprising detrital matrix and authigenic clays). Detrital quartz, plagioclase, and rare k-feldspar are rounded and optically appear relatively internally unstrained. SEM-CL analysis reveals moderate fracturing in quartz grains. Greenstone and blueschist blocks are similarly pinched-and-swollen and boudinaged, and locally possess very high (7-10) aspect ratios. Each contains small (1 mm - 3 cm) pieces of relatively undeformed rock set within fine-grained (<0.5 mm) cataclastic material. This material is concentrated along intra-block fractures, block edges, and necks, and is more pervasive in smaller blocks and in those with higher aspect ratios. Observations are consistent with a tectonically-driven process of melange formation. The majority of strain is accommodated by ductile flow in the pelitic matrix, separating and dispersing included blocks. Internal block deformation is taken up by particulate flow in the greywacke groundmass and by cataclastic flow in greenstone and blueschist, both encouraged by fluid influx into the blocks and by maintenance of high fluid pressure through the deformational history. Cataclasis is concentrated near block margins due to shear traction induced by the extensive flow of the incompetent matrix.