THE POLYGENETIC HEAVEN’S BEACH MELANGE, FRANCISCAN COMPLEX, CALIFORNIA, USA (Invited Presentation)

The Franciscan Complex, Heaven’s Beach Melange is exposed in coastal cliffs of western Sonoma County, California. The mélange consists of small blocks to megablocks of diverse rock types in a sheared matrix of (meta)sandstone and meta-mudrock. Blocks are diverse in composition, size, shape, nature of contact, and metamorphic grade. Individual blocks/ megablocks range from < ¼ m to >500 m. The block/megablock rocks include clastic rocks of submarine fan facies A to G; “high-grade” glaucophane and hornblende schists, gneisses, and breccias; serpentinites; blueschist and prehnite-pumpellyite facies metsandstones/meta-mudrocks; low-grade metabasites; low- to high-grade metacherts; and radiolarian cherts. Facies A massive submarine channel olistostromal mélange with a muddy sandstone matrix containing recycled blocks of glaucophane schist, metabasite, metawacke, and chert, and Facies A conglomerate with glaucophane schist boulders, occur within a megablock and reflect recycling of older Franciscan detritus. Four discontinuous zones of serpentinite ± mudrock-matrix melange with local high grade blocks form discrete mélange layers. The layers are discontinuous and preliminary structural data suggest N-plunging open folding, so the four zones may represent two horizons. The structural fabric of the mélange is a shear-fracture (SF) fabric with weighted average trend of N51°W and vertical dip. The strike of the SF-fabric parallels the dominant N52°W strike of bedding (ave. dip = 70°NE) within megablocks. Clearly, precursor sedimentary olistostromal and tectonic mélange layers, contributed rocks to the final mélange composition. Locally, sandstones-mudrock sequences within some blocks display sandstone dikes, reflecting early fluid escape from the poorly consolidated sediments that are now metawackes. Some megablocks are olistoliths that slid or were carried into the fan channels from older, upslope exposures of Franciscan rocks. The intra-block glaucophane schist and other clasts have primary contacts with matrix and display angular to rounded forms. In contrast, blocks to megablocks, some of elliptical form, are bounded by sheared and slickensided contacts with the SF- fabric metasandstone-meta-mudrock matrix that overprints earlier sedimentary fabrics at megablock margins.