Extension as a major tectonic process of the evolution of Southern Ecuador compressional forearc
Résumé
Extensional faults are often observed along subduction zones as a response of a margin
collapse. However, their role remain underestimated within the structural evolution of
forearc systems. In southern Ecuador, the oblique subduction of the Nazca Plate
beneath South America led to the formation of the Cretaceous-Miocene NW-SE
Chongón-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 of
the Gulf of Guayaquil.
Herein, we explore fault distribution and kinematics along the onshore-offshore Santa
Elena Peninsula using academic and industrial 2D seismic profiles calibrated with wells
and field data. Polyphase trench-parallel and NW-SE-trending, >20km-long normal
faults are observed cross-cutting the former Chongón-Colonche thrust wedge. Faults
are either steep and oceanward-dipping along the trench, or low-angle continentward-
dipping through the platform where they likely reactivate former thrusts. We
specifically observe repeated extensional pulses during Eocene, Miocene and Plio-
Pleistocene times, separated by inversion periods, and shaping the geometry of various
forearc basins. In the Late Pleistocene, normal faults are also associated with uplifted
marine terraces.
We interpret normal faulting as an overlap of long-term and short-term processes, such
as the subduction of bathymetric asperities, underplating and/or variations of
convergence velocity, and more recently the North Andean Sliver escape. NW-SE
extensional structures dissect the overall thrust wedge through the Cenozoic and are
combined with N-S to NW-SE active faults over the Quaternary. This polyphase setting
shows eastward-stacked accretionary wedges with thrusts reactivated as normal faults,
suggesting that pervasive inheritance processes take place continuously. This project is
part of the ANR MARACAS (ANR-18-CE31-0022, MARine terraces along the northern
Andean Coast as a proxy for seismic hazard ASsessment).