Pliocene-Quaternary tectono-sedimentary evolution of the Guayaquil-Tumbes forearc basin (Ecuador-Peru)
Résumé
Forearc systems develop largely offshore above subduction zones between the trench
and the volcanic arc. The associated forearc basins are generally parallel to the trench
and involve vertical movements (subsidence vs. uplift), influencing the dynamics of the
wedge. The objectof the study is the Guayaquil-Tumbes forearc basin belonging to the
Ecuadorian-Peruvian subduction zone. This forearc basin has peculiar characteristics,
developing on the southernmost segment of a great active continental strike-slip fault,
the Dolores-Guayaquil Fault, delimiting two crustal domains, the North Andean and
Inca slivers. The divergent motions of the two slivers generate extension in the upper
plate whose interaction with the strike-slip tectonics and subduction remains poorly
constrained. Two major rivers, Chira and Tumbes, draining the Andean reliefs towards
the Pacific, generate huge Pliocene-Quaternary silico-clastic sedimentary filling trapped
in the basin, having an important impact on the growth of the wedge and fault activity.
The objective of this study is to constrain the structural architecture of the basin by
determining the nature, geometry and age of the major structures that controlled its
Pliocene-Quaternary development. This study is carried out through the interpretation
of offshore seismic profiles correlated with well data. The results show that the
structure of the basin is dominated by gravitational instabilities (classical raft tectonic)
detached northwestward and westward, and active through the Pliocene- Quaternary
times. The syn-tectonic sediments and erosional surfaces indicate that these
instabilities are progressively younger toward the south. We propose that this
gravitational sliding has been sustained by high sedimentation rates and by the modern
propagation of the Dolores-Guayaquil Fault at depth. This major fault is interpreted as
an immature strike-slip system that progressively develops towards the trench.