In arc systems where an oceanic plate with a thick cover of sediments is subducted, the sediments are scraped off and accreted to the base of the inner trench slope. The accreted sediments form ridges, behind which younger sediments are often ponded in basins. The width of these basins progressively increases from 2 to 3 km at the base of the lower slope to 10 km near the trench slope break. Sediments within the basins increase in thickness from nearly zero in the basins now at greatest depths to several kilometres in the basins on the shallowest part of the slope. Slope basins begin to form at the base of the lower slope, where sediments accumulate between adjacent thrust faults. Addition of more accreted material at the trench causes uplift and rotation of the thrust slices and of overlying slope sediments. As deformation proceeds, motion along some of the thrusts dies out, and the inactive thrusts become buried by slope sediments, thus increasing the size of the slope basins. In orogenic belts, sedimentary rocks that were deposited on a “basement” of mélange and are now tectonically enclosed by mélange are hypothesized to be ancient slope-basin deposits.

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