FOREARC BASIN FORMATION IN THE TECTONIC WAKE OF A COLLISION-DRIVEN, COASTWISE MIGRATING CRUSTAL BLOCK: AN EXAMPLE FROM THE NORTH ANDEAN BLOCK AND THE GULF OF GUAYAQUIL-TUMBES BASIN (ECUADOR-PERU BORDER AREA)

Two tectonic regimes showing different styles and ages controlled the evolution of the southern Ecuador and northern Peru continental margin and shelf. The ~N-S extensional regime along the shelf area is related to North Andean block escape tectonics, whereas the E-W extensional regime along the continental margin results from tectonic erosion along the subduction channel. Strain rotation takes place along a major N-S trending transfer system roughly located at the continental shelf-break. Trench-parallel extensional strain resulting from the northward drift of the North Andean block controlled the tectonic evolution of the Gulf of Guayaquil-Tumbes basin along the shelf area during the late Pliocene-Quaternary (1.8 Ma). The master detachment fault controlling basin evolution may represent the shallower expression of a reactivated obduction megathrust. Contrary, the continental slope is characterized by a series of seaward dipping normal faults predominantly active during Mio-Pliocene times. This system represents the northward prolongation of the well documented subduction-erosion regime off northern Peru. The stress regime segmentation along the forearc area (from subduction erosion-related along the continental slope to NAB drift-related along the shelf area) is probably the signature of the across-strike segmentation of plate mechanical coupling. A feedback relationship between: 1) tectonic processes at the shelf and continental slope areas, 2) sediment input to the trench and 3) the regime of the subduction appear as first order factors in controlling the segmentation of deformation along the forearc. We suggest that tectonic escape systems are highly sensitive to local plate coupling variations, and that these variations are key factors in locating the zones where subsidence occurs at the trailing tail. The Gulf of Guayaquil-Tumbes basin is not a classical pull-apart basin, and exemplifies a particular type of basin basically controlled by (1) detachments probably extending downward across the brittle crust, and (2) the plate coupling along the subduction decollement, which controls the inward segmentation of deformation.