Tham khảo tài liệu 'earth as an evolving planetary system part 3', khoa học tự nhiên, địa lý phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 80 Tectonic Settings Figure Franciscan melange near San Simeon California. Large fragments of greenstone metabasalt left and graywacke center are enclosed in a sheared matrix of serpentine and chlorite. Courtesy of Darrel Cowan. shear zone subparallel to a subducting slab. Fragments of oceanic crust and trench sediment are scraped off the descending plate and accreted to the overriding plate. Gravitational slumping may produce olistostromes or debris flows on oversteepened trench walls or along the margins of a forearc basin. Debris flows in which clay minerals and water form a single fluid possessing cohesion are probably the most important transport mechanism of olistostromes. Arc Systems 81 Forearc Basins Forearc basins are marine depositional basins on the trench side of arcs Fig. and they vary in size and abundance with the evolutionary stage of an arc. In continentalmargin arcs such as the Sunda arc in Indonesia forearc basins can be up to 700 km in strike length. They overlie the accretionary prism which may be exposed as oceanic hills within and between forearc basins. Sediments in forearc basins which are chiefly turbidites with sources in the adjacent arc system can be many kilometers in thickness. Hemipelagic sediments are also of importance in some basins such as in the Mariana arc. Olistostromes can form in forearc basins by sliding and slumping from locally steepened slopes. Forearc-basin clastic sediments may record progressive unroofing of adjoining arcs as shown by the Great Valley Sequence Jurassic-Cretaceous in California Dickinson and Seely 1986 . Early sediments in this sequence are chiefly volcanic detritus from active volcanics and later sediments reflect progressive unroofing of the Sierra Nevada batholith. Volcanism is rare in modern forearc regions and neither volcanic nor intrusive rocks are common in older forearc successions. Arcs Volcanic arcs range from entirely subaerial such as the Andean and Middle America arcs to mostly
