Geomorphologic impact of the subducting Nazca plate on the southern Peru (14 degree S-16 degree S)-northern Chile (17 degree S-20 degree S) continental margin

Abstract

Interpretations of SeaMARC II side-scan sonar, analog single-channel seismic reflection and 3.5 kHz profiles across the intersection of the Nazca plate with the Peru-Chile trench in the Nazca Ridge and Arica Bight-Iquique areas were used to describe the geomorphology and topography of these two sections of the Nazca-South America plate convergence. In the Nazca Ridge area, the entry of the aseismic Nazca Ridge into the southern Peru trench causes the uplift of the overriding continental margin. The uplift of the continental margin is indicated by a) shoaling of the trench axis, b) tilting of the slope terraces, c) raised marine terraces, d) deflection of submarine channels and onshore rivers, and e) an uplifted marine forearc basin. The similarity of morphologic features between the uplifted region of the continental margin and the Nazca Ridge suggests that the extent of uplifted region of the continental margin is controlled by the geometry of the subducting Nazca Ridge. In the Arica Bight-Iquique area, the subduction of the more highly faulted trough-like relief of the Nazca plate under the continental margin is interpreted to have resulted in the subsidence of this section of the overriding continental margin. The subsidence of the continental margin in this section of the convergence is demonstrated by a) deepening of the curved trench, b) depression of the slope terraces and forearc basin, c) subsidence of the upper slope ridge and platform, d) formation of the offshore and onshore semi-centripetal drainage pattern. and e) landward retreating of the coastal cliff. The mechanism for the uplift of the overriding continental margin in the Nazca Ridge area is due to the gravity isostatic buoyancy difference between the less-dense, thicker crust of the subducted Nazca Ridge and its adjacent, denser oceanic crusts. Conversely, the subsidence of the overriding continental margin in the Arica Bight-Iquique area may have resulted from the isostatic sinking of the denser crust of the subducted, trough-like structure of the oceanic plate.