EXPOSURES AT BLIND AND NORTH SHELL BEACHES, SONOMA COUNTY COAST, CALIFORNIA: INSIGHT INTO MÉLANGE-FORMING PROCESSES AND SUBDUCTION SLIP ACCOMMODATION

Seacliffs and wave-cut platforms at Blind and north Shell Beach, Sonoma County coast, California, south of the Russian River, provide superb exposures of Franciscan Complex siliciclastic matrix mélange and bedded (block-free) rocks. Muddy to sandy mélange matrix and associated bedded rocks have undergone prehnite-pumpellyite facies metamorphism. Most blocks-in-mélange have prehnite-pumpellyite facies metamorphic assemblages but other blocks include those of lawsonite-blueschist facies, and high-grade blocks up to 100 m in size with garnet amphibolite, amphibolite, and eclogite assemblages. The largest blocks, up to 700 m, are metasandstones with lawsonite-blueschist assemblages and a strong stretching lineation. Pervasively foliated zones are localized in muddier matrix and may range in thickness up to about 100 m. Bedded sandstones lack a penetrative cleavage in most of their volume, but are imbricated, with local development of block-in-matrix textures (sandstone-in-mudstone). Mélange matrix ranges from undeformed conglomerate (mud matrix, sand matrix, clast-supported), to strongly foliated matrix with minor lenses of little-deformed matrix. Clast populations in undeformed conglomerates resemble the mélange block inventory, including the range of metamorphic clasts. Matrix surrounding a 60-m garnet-diopside-amphibolite block and a 30-m prehnite-pumpellyite facies pillow basalt block show localization of deformation around block margin with little-deformed matrix preserved in contact with the blocks in reentrants. Mélange and interbedded block-free horizons are folded in tight to overturned folds. The geologic relationships suggest localization of subduction slip during accretion in discrete horizons ranging from 100-m-thick shear zones to hairline faults that imbricate internally undeformed sandstone beds. Deformation associated with subduction slip during accretion at prehnite-pumpellyite facies conditions was overprinted on a clastic trench-fill package that included sandstone and mudstone without blocks and interbedded olistostromal (sedimentary mélanges) horizons that include blocks of higher-metamorphic grade than the matrix derived from previously exhumed rocks. Superimposed deformation included development of a second-stage block in matrix fabric.