Extension as a major tectonic process of the evolution of Southern Ecuador compressional forearc

Description

Extensional faults are often observed along subduction zones as a response of a margincollapse. However, their role remain underestimated within the structural evolution offorearc systems. In southern Ecuador, the oblique subduction of the Nazca Platebeneath South America led to the formation of the Cretaceous-Miocene NW-SEChongón-Colonche Cordillera surrounded by Eocene-to-recent sedimentary basins.This region, generally interpreted as an onshore-offshore compressional forearc,interferes with the active tectonic escape of the North Andean Sliver and the opening ofthe Gulf of Guayaquil.Herein, we explore fault distribution and kinematics along the onshore-offshore SantaElena Peninsula using academic and industrial 2D seismic profiles calibrated with wellsand field data. Polyphase trench-parallel and NW-SE-trending, >20km-long normalfaults are observed cross-cutting the former Chongón-Colonche thrust wedge. Faultsare either steep and oceanward-dipping along the trench, or low-angle continentward-dipping through the platform where they likely reactivate former thrusts. Wespecifically observe repeated extensional pulses during Eocene, Miocene and Plio-Pleistocene times, separated by inversion periods, and shaping the geometry of variousforearc basins. In the Late Pleistocene, normal faults are also associated with upliftedmarine terraces.We interpret normal faulting as an overlap of long-term and short-term processes, suchas the subduction of bathymetric asperities, underplating and/or variations ofconvergence velocity, and more recently the North Andean Sliver escape. NW-SEextensional structures dissect the overall thrust wedge through the Cenozoic and arecombined with N-S to NW-SE active faults over the Quaternary. This polyphase settingshows eastward-stacked accretionary wedges with thrusts reactivated as normal faults,suggesting that pervasive inheritance processes take place continuously. This project ispart of the ANR MARACAS (ANR-18-CE31-0022, MARine terraces along the northernAndean Coast as a proxy for seismic hazard ASsessment).

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