Material Transport within Accretionary Prisms and the "Knocker" Problem

Observations of high-pressure metamorphic rocks occurring as blocks (knockers) and as thrust sheets intercalated into non-metamorphosed rocks in subduction complexes have led to models of material transport within accretionary prisms having large-scale differential vertical transport. Many field geologists favor the rise of deep-seated material to the surface on the trench slope, followed by gravitational transport back to the zone where non-metamorphosed sediments are being incorporated into the accretionary prism. Unfortunately this scenario is not observed in contemporary examples of the type of arc systems assumed in these models. In active arcs, deformation is concentrated near the base of the trench slope, whereas blueschist formation occurs deep beneath the trench slope break and forearc basin. A cover of slope sediment, moderately deformed at best, increases in thickness away from the trench and effectively isolates the accreted material from shallow-level gravitational transport. The mixing of incompatible lithologies in subduction complexes might better be explained by processes associated with non-typical convergent boundaries. One geological agent that may play an important role is strike-slip faulting along trends sub-parallel to the arc. This process could tectonically mix lithologies along the fault zones in several ways. It could also expose high-pressure metamorphics on the trench slope by removal of that part of the accretionary prism closer to the trench. Sedimentary transport of this material to the base of the slope where continuing or renewed subduction occurred could cause the incorporation of blueschist facies blocks into non-metamorphosed melange.