Summary of Accretionary Processes, Deep Sea Drilling Project Leg 66: Offscraping, Underplating, and Deformation of the Slope Apron
Initial Reports of the Deep Sea Drilling Project
Leg 66 results indicate that structural packages of trench sediment are scraped off at the toe of the trench slope and comprise a zone of landward-dipping reflectors increasing in age (Quaternary-Miocene) upslope. These reflectors are presumably bedding surfaces and their apparent stratigraphic inversion most simply explained by thrust faulting subparallel to bedding. The seismic reflection data suggest that offscraped, thrust-bound packages of trench sediment are about 1 km in structural
... in structural thickness and up to 8 km in lateral extent down-dip. Imbricate wedging of these structural packages results in their modest landward tilt close to the base of the trench slope. Landward of the zone of active offscraping and tilting the near uniform dip of the structural packages, coupled with their long-term uplift, suggests a subjacent process of mass addition or underplating beneath the external "rind" of older offscraped deposits. Trench deposits with possible additions of oceanic plate pelagic and hemipelagic deposits probably constitute the bulk of the underplated materials, a lithologic suite more diverse than the offscraped trench sediments. The lack of coherent reflections from the underplated terrane argues for complex deformation relative to the offscraped trench deposits. Depth of occurrence and thermal gradients predict a maximum of zeolite facies metamorphism for the offscraped deposits and zeolite to prehnite-pumpellite facies for the underplated deposits. Geologic contrasts between the offscraped and underplated terranes provide a basis for identifying possible ancient analogs in the Kodiak Islands of Alaska. The rate of tilting of surface sediment decreases with distance from the base of the slope and with distance above the downgoing slab, indicating an extreme concentration of deformation at the base of the trench slope. Hence sediments accumulated on the lowermost slope are less likely to be structurally differentiable from the underlying offscraped rocks than those deposited farther upslope.