DEVELOPMENT OF THE ACCRETIONARY PRISM ALONG PERU AND MATERIAL FLUX AFTER SUBDUCTION OF NAZCA RIDGE

Results from Ocean Drilling Program Leg 112 indicated tectonic erosion of the Peruvian convergent margin during subduction of the Nazca Ridg e followed by renewed accretion against the erosional scar. Seismic im ages and swath mapping morphology show a regional geology shaped as su bduction of Nazca Ridge migrated 800 km along the Peru Trench. The acc retionary prism is imaged at two stages of development since upper Mio cene time with depth-migrated seismic records in which the ''back stop '' is well defined. From these data, material flux can be quantified. After the ridge crest subducted and accretion dominated over erosion, the prism grew rapidly to 10- to 15-km width. During rapid growth, the margin taper was large and about 60 percent of the sediment supply ac creted. As the prism growth slowed, only 30 percent accreted despite a n increased trench sediment supply. Since the convergence rate changed little, the inverse relation between sediment supply and prism growth rate suggests that other processes allow most of the sediment input t o be subducted. The decreased prism growth may be partially controlled by varying the structure of the back stop. The force required to acti vate faults cutting across the back stop is probably greater than the force allowed by basal friction across the plate boundary. Thus the sh ear stress transmitted across the plate boundary is insufficient to dr ive ''out-of-sequence'' thrust faults that would thicken the back stop or raise it beyond a limited height. A constant back stop height limi ts accretion, and the subduction window appears to open and accommodat e increased trench sediment beneath the back stop rather than inducing a transport of sediment over the top of the back stop. Nazca Ridge su bduction affected the convergent margin mass flux for about 8 Ma. Mass balancing indicates an order of magnitude increase in the current sed iment volume subducted in the north compared to that near the ridge cr est. When growth of the accretionary prism slowed, a greater terrigeno us input through the trench was accommodated by sediment subduction. T he resulting range of material flux is estimated to increase the sedim ent interlayer between the plates about 500 m which may affect couplin g.