Crustal structure along the Peruvian Margin from wide angle seismic data

Within the GEOPECO project (Geophysical Experiments at the Peruvian Continental Margin - investigations of tectonics, mechanics, gas hydrates and fluid transport) seismic refraction and reflection data were acquired during RV 'Sonne' cruise SO 146 along with bathymetric and gravimetric mapping, sea-floor sampling, observation of the ocean floor and heat flow measurements. The objectives were a quantitative characterization of the structures and geodynamics of the Peruvian section of the Andean subduction zone and the associated gas hydrate systems in regions with differing tectonic development. The oceanic Nazca Plate, which is approximately 28 to 38 million years new at the Peruvian trench, is subducting under the South American Plate. The Peruvian Continental Margin has been influenced over the last 8 million years by collision with the Nazca Ridge, a 400 km long and 50 km wide basement high. Collision migrated progressively from north to south, is presently in the area of 15°S and has influenced the area to the north in several ways. Six wide angle seismic profiles, each approximately 100nm long, were shot with three 32 liter Bolt-airguns over 9 to 14 OBH/S instruments at the Peruvian Margin. During the cruise a total amount of 127 OBH/S were successfully deployed showing high quality data. Forward modeling was performed to characterize the structure and the velocities of the different stages of the evolution of the margin after collision with the Nazca Ridge. The coincident reflection seismic profiles were used to constrain the structure and thickness of the upper layers. The resulting crustal cross sections reveal a rough surface and a thin sediment layer of the subducting oceanic Nazca Plate. The crust thickens beneath the Nazca Ridge. Its thickness also varies north and south of Mendana Fracture Zone (MFZ), which separates younger (~25 Ma old) from older (~35 Ma old) oceanic crust at about 11°S. There is no accretionary wedge where Nazca Ridge currently subducts. 3 Ma after the ridge has passed, a new accretionary prism is already set up with a width of 20 to 30 km and 4 to 5 km thickness which does not further increase in size as revealed by the profiles recorded further north of Nazca Ridge. This indicates that current subduction along the Peruvian Margin is non-accreting. The slope angle of the accretionary prism increases south of MFZ, whereas the profile north of MFZ shows a smaller slope angle. As the subducting Nazca Plate dips at about 6° on all profiles north of Nazca Ridge, the resulting taper is 12° to 17°, indicative of high basal friction and non-accretionary subduction. The horst and graben like structure and rough topography of the oceanic plate also substantiates non-accretionary even erosional subduction for the graben structures are filled with sediment before subduction. Two cross profiles from Lima Basin reveal the crustal structure of the continental slope. Lima Basin is some 80 km wide (along dip) and its thickness varies from 1 to 3 km below sea floor. Furthermore it shows an asymmetric shape and is divided into two parts by a basement high at the landward termination