Convergence tectonics of the Peru Trench : 8°S-15°S latitude

Abstract

This study has examined the tectonics of convergence along the Peru Trench with the specific objective to gain insight into the sediment subduction process. Analysis of GLORIA (Geological Long Range Inclined Asdic) long-range side-scan sonar and other geophysical data show that the Peru Trench is a site of long term sediment subduction where bending-induced normal faulting of the Nazca plate plays a key role. The sediments are continually removed from the trench axis region by their inclusion in the normal fault-defined void spaces, such as grabens. In the process of subduction, the sediments are often offscraped, folded and kneaded temporarily at the base of the shoreward trench slope. The subduction complex comprising such sediments is narrower than 20 km in this region and its volume remains more or less constant due to regular removal of its contents along with the subducting Nazca plate. It is also believed that the morphology of the subducting plate is responsible for subduction erosion of the South American plate, where the horst blocks cause tectonic erosion by abrasion and the grabens provide a mechanism for removal of the eroded material. An additional finding of this study is rifting of the Nazca plate along Mendana Fracture Zone, caused by subduction-induced extensional stresses oriented parallel to the trench axis. This rifting is accompanied by organized seafloor spreading that has produced up to a 50 km width of new seafloor within the fracture zone in the last (,(TURN))3 Ma. The Mendana Spreading Center appears to be propagating seaward with respect to the South American plate at a rate of about 25 km/Ma. The inactive pre-bending features of the Nazca plate mapped in this region include a NNW-trending spreading fabric relief draped by a 100-200 m sedimentary cover, the newly discovered Viru Fracture Zone (,(TURN))100 km north of the Mendana Fracture Zone, a narrow arcuate trough subducting along the trench, and numerous submarine volcanoes. Magnetic anomalies 6C-8 have been identified north of the Mendana Fracture Zone and 13-18 south of it, indicating an age offset of (,(TURN))13 Ma across the fracture zone. A minor offset of less than 1 Ma is observed across the Viru Fracture Zone.