Tectono-sedimentary evolution of marine slope basins in the Chilean forearc (33°30′-36°50′S): Insights into their link with the subduction process

This study addresses the nature of the link between plate convergence and the tectono-sedimentary development of forearc marine basins off the coast of the central Chile accretionary margin, between 33°30′ and 36°50′S, based on the analysis of high resolution seismic reflection profiles. Asymmetric half-graben basins characterize the middle and upper slopes and are mostly controlled by bordering faults governing differential uplift and subsidence of the upper part of the slope. The architecture of the sedimentary infill evidences three main distinctive seismic sequences, identified as basal pre-kinematic, intermediate syn-kinematic, and upper post-kinematic units. Although the absolute thickness of each of these sequences varies from basin to basin, the intermediate, syn-kinematic sequence is commonly thicker, associated with high sedimentation rates coeval with slip on bordering faults. Age estimations on boundaries of these units evidence first-order cycles which can be linked with global climate fluctuations. The dynamics of the deposition and deformation of the different sequences suggest alternating episodes of extension, relative stability and compression (and/or transpression). Positive flower structures evidence current transpressional activity associated with tectonic inversion and differential uplift of the accretionary prism. The evolution of depositional and deformational episodes is strongly controlled by tectonic events induced by subduction and the large-scale mass transfer processes. The seismic lines allow the identification of at least two mechanisms for slope basin formation, one of them (more local) associated with seamounts subduction or its proximity. A second mechanism (more widespread), would be controlled by basal accretion of large volumes of underplated sediment underneath the buttress, which in turn drives deformation and tilting of the upper part of the slope. This process is strongly connected with the available volume of sediments at the trench and the basal properties of the prism. Climatic fluctuations and currently active canyons are an important supply of sediments to the system. The study of seismic sequences thus provides a good record of the interplay between plate boundary deformation and marine basin evolution along the accretionary part of the Chilean margin.