Young oceanic crust is initially covered by biogenic oozes that are succeeded normally by clastic deposits during subsidence and crustal transport toward a subduction zone. Diagenetic alteration of the biogenic deposits with concurrent weathering of the subjacent oceanic basalt produces a time-dependent convergence in physical properites. Deep Sea Drilling Project (DSDP) results from a vertical section in the north-western Pacific indicate that the density and sonic velocity of mature biogenic rocks are similar to the density and sonic velocity of the underlying basalt but are much higher than those of the overlying clastic sediment. The bulk of the clastic sediments accumulates near the subduction zone and thereafter undergo relatively little diagenesis. Thus, the normal depositional sequence and diagenetic evolution of deep-sea deposits can create a major discontinuity in density and strength between the clastic sediments and biogenic rocks rather than between the biogenic rocks and the basaltic oceanic crust. The relatively low density and strength of the clastic deposits encourages their deformation and accretion at shallow depths within the subduction zone, whereas the relatively high density and strength of the mature biogenic rocks may allow their selective subduction with the underlying oceanic crust.

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