Inheritance processes of active extensional deformation in ocean-continent subduction zones: the example of the northern Andes compressional margin in Ecuador

Over the last 23 Myr, the roughly east-directed subduction of the Nazca Plate beneath South America led to the formation of several mountain ranges associated with the overall northern Andes evolution. Along the active southwestern Ecuadorian margin, the compressional setting involves the Cretaceous-Miocene Chongón-Colonche / Santa Elena terranes, overlain by recent sedimentary basins. This geological setting, generally interpreted as an onshore-offshore forearc system, evolves in close relation with the active tectonic escape of the North Andean Sliver and the opening of the Gulf of Guayaquil. This region is characterised by a widespread extensional deformation in the upper plate that overprints moderate subduction and crustal earthquakes.

To better document such extensional processes, we specifically explore the offshore shelf and the littoral area of the Santa Elena Peninsula using academic and industrial 2D seismic profiles calibrated with local wells and field observations. We document a trench-parallel fault network, composed of >20km-long normal faults that take place on top of the former Chongón-Colonche accretionary wedge. These faults are linearly-steep along the trench, and are listric toward the continent where they clearly control fault-block rotation. They separate flexural basins developing on the platform ahead the Chongón-Colonche Cordillera, and are associated with immerged terraces most likely formed during the Last Glacial Maximum. They also may link to further onshore marine terraces developing since the Pleistocene across the coastline.

These observations suggest a peculiar dismantlement of the margin, mainly affected by tectonic erosion involving reactivation of former compressional features. Normal faults are specifically interpreted as a regional syn-orogenic collapse of the Chongón-Colonche Cordillera, which may result from transecting subducting ridges, fracture zones and seamounts controlling, at least partially, the geometry and the nature of the deformation along the southwestern Ecuadorian margin. This deformation pattern is likely linked to a weak interseismic coupling along the subduction interface to which the active opening of the Gulf of Guayaquil overlaps. This project is funded by the project ANR MARACAS ANR-18-CE31-0022 (MARine terraces along the northern Andean Coast as a proxy for seismic hazard ASsessment).