QUATERNARY TECTONIC HISTORY OF THE GULF OF GUAYAQUIL-TUMBES BASIN AS THE SIGNATURE OF THE NORTH ANDEAN BLOCK TECTONIC ESCAPE

Industrial multichannel seismic and well data of the southern Ecuadorian and northern Peruvian continental margin and shelf document the existence of two major zones showing two different tectonic regimes. It includes the continental margin and the shelf areas. From north to south, the Domito fault system, the Banco Peru, and the Talara fault define the limit between these two tectonic domains. An ~E-W directed extensive strain characterizes the continental margin during the past 10-15 Myr. We assume that this extensional tectonic regime is related to tectonic erosion working at depth along the upper-lower plate boundary. In contrast, the shelf zone shows no important deformation from 5.2 to 1.6-1.8 Ma. In this area, 3-4 km of sediment accumulated along depocenters during the Lower Pleistocene. Three major detachment faults, the Posorja, Jambelí and Tumbes detachment systems (PDS, JDS and TDS, respectively) controlled the subsidence of those depocenters including the Esperanza, Jambelí and Tumbes basins. These basins recorded the major subsidence phase of the southern Ecuadorian and northern Peruvian forearcs for at least the past 2 Myr. They resulted from the N-S extensional strain produced by the northward tectonic escape of the North Andean Block (NAB) during Pleistocene times. The total lengthening of a complete N-S transect between the PDS (to the north) and the TDS (to the South) ranges between 13.5 and 20 km. This lengthening is coherent with the documented NAB migration rates combined with an Early Pleistocene age for GG area main opening pulse. We consider that the abrupt increase of the GG area subsidence during Quaternary is related to a major acceleration of the NAB northward drifting. Considering that the rate and obliquity of the eastward convergence between the Nazca and South America plates remain constant during the last 5 Ma, the subduction of the Carnegie ridge at the Pliocene-Lower Pleistocene limit is proposed to be the main source of interplate coupling increase along the Ecuadorian margin. We assume that the subduction of the leading edge (or the subduction of a high) of the Carnegie ridge is at the origin of the interplate-coupling increase, which in turn produced the acceleration of the NAB northward drifting and the major periods of subsidence in its southernmost limits.