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Structure and neotectonics of the Southern Chile forearc

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  • Earth Science

Abstract

Marine forearcs are among the geologically most complex regions on Earth. They are shaped by a variety of tectonic processes that are active on a wide range of spatial and temporal scales. The aim of this PhD thesis is to characterize the active tectonic evolution of the South Chilean marine forearc by a combined investigation of surface (swath bathymetric) and subsurface (reflection seismic) data. In particular, I will analyze how geomorphologic deformation patterns connect to individual tectonic processes. One of the major results of the study is the subdivision of the South Chilean marine forearc in the area 35°S – 40°S into four geomorphologically distinct along-slope segments. Tectonic processes such as compressional and extensional faulting as well as sediment underthrusting are identified as first order controls for this segmentation. The subdivision as well as the causal interpretation is based on a combined analysis of five reflection seismic lines and an extensive set of swath bathymetric data. Geomorphologic variations across the segment boundaries occur over remarkably short distances of some kilometres. This high level of complexity has remained unrecognized during previous studies which merely considered narrow transects normal to the plate margin along reflection seismic lines. A second focus lies on the detailed investigation of giant submarine landslides, one of the previously identified geomorphologic features, which are observed in the area of the Arauco Peninsula. The combination of reflection seismic and swath bathymetric data reveals three slope failures that range among the largest submarine slides known from active continental margins. Slope failures are likely preconditioned by the specific tectonic regime of the Arauco area where continuous high uplift rates repeatedly cause oversteepening of the continental slope. Finally, this thesis investigates the impact of the three outstanding slope failures on the long term seismotectonic evolution of the study area. The locations of the slope failures coincide with the location of a major earthquake segment boundary. The region to the south of the Arauco Peninsula was last ruptured during the Mw 9.5 Great Chile Earthquake on 22 May 1960, whereas the region to the north was affected by the Mw 8.8 Maule Earthquake on 27 February 2010. Seismic reflection data image an undisturbed and well layered sedimentary trench-fill and a continuous décollement in the areas where no slope failures are observed. However, at the exact locations of the slope failures, which coincide with the boundaries of the 1960 and 2010 earthquake ruptures, chaotic slide deposits compose the lower part of the trench-fill. At these locations no continuous décollement is documented. I suggest that the underthrusting of the highly inhomogeneous slide deposits prevents the development of a continuous décollement and thus the build-up of a thin slip zone that is continuous in space as necessary for earthquake rupture propagation. This is the first time that mass wasting is considered as a possible control on earthquake segmentation at active continental margins.

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