Three-Dimensional Gravity and Magnetic Modelling Along the Peruvian Margin

Abstract:

The gravity and magnetic models are constructed for three areas along the Peruvian margin between 7.25°S and 16.75°S and are based on the all available wide-angle seismic velocity models. The gravity and magnetic models image nearly the whole margin which has been only partly resolved with geophysical methods up to now. The continental margin is characterized by positive free-air anomalies of varying amplitudes, indicating that the margin has been shaped by the subduction of different features on the Nazca Plate. In the Yaquina Area (7.25°S to 11°S) gravity anomalies caused by the Trujillo Trough and the Mendaña Fracture Zone are successfully modelled with remarkable undulations in the layer geometry of the oceanic crust. Along the continental margin, especially in the Lima Area (10.50°S to 14.40°S), strong undulations of the lower continental crust influence the upper sedimentary layers and support the development of basins along the Peruvian margin. The theory stating that the Peruvian margin is uplifted by the subducting Nazca Ridge is supported by gravity modelling. Consequently the buoyant Nazca Ridge is, at least partly, responsible for the extended region of flat subduction. The thickened and slightly asymmetrical crust of the Nazca Ridge is envisaged in gravity modelling. In the Nazca Ridge Area (14.25°S to 16.75°S) no accretionary prism is modelled. We conclude that the ridge is eroding the continental margin; furthermore the subduction of eroded sediments is probable. Gravity modelling suggests that the Nazca Ridge has fractured the continental margin. North of the ridge, in the Lima Area, a rather uniform accretionary complex is observed. This indicates that, after the margin was eroded by the southwards moving Nazca Ridge, the prism rapidly reached its stable size. In the Yaquina Area an accretionary prism is modelled in the whole research area but local variations of its location and structure indicate the former erosive influence on the continental margin of subducting features on the Nazca Plate. The layers of the oceanic crust show increasing densities implying they possess an originally high degree of porosity before actually subducting.