Tectonic accretion versus erosion along the southern Chile trench: Oblique subduction and margin segmentation

The southernmost tip of South America is an active continental margin where oblique convergence between plates, transcurrent motion, and tectonic rotation on land make the geodynamic setting more complex than that of the central Andes. A multichannel seismic data set has been used in conjunction with multibeam and altimetry data to clarify the regional architecture of the continental margin from 50°S to 57°S. Despite the thick sedimentary section in the trench and the slow plate convergence rate peculiar of typical accretionary margins, seismic reflection profiles image widely varying frontal wedge morphologies, different rates of accretion, a high degree of structural diversity, and different modes of continental building (offscraping, underplating, tectonic erosion). Correlation between structural parameters (depth of the décollement level, width of the wedge, accretionary rates, Moho depth) suggests large scale structural control on margin geometry and structural diversity. The transition from tectonic accretion to erosion and general structural variations do not show any gradual trend, but rather, they occur along oblique structural trends that produce a tectonic segmentation of the margin. This is mainly related to tectonic processes on the overriding plate (block rotations along strike slip faults), while local disturbances on the incoming plate (seamounts, fracture zones, and oceanic fabric relative orientation) add further structural complexity. Altimetry data, in conjunction with structural analysis, suggest that large scale tectonic variations and structural development are related to trench parallel gravity anomaly variations and, ultimately, to basal friction on the plate interface. Strong negative gravity anomalies are associated with sedimentary basins, wide wedges, and accretionary domains, while positive gravity anomalies mainly refer to transverse structural highs and narrow wedges.