Dynamics of the active plate boundary in southwest colombia according to recent geophysical measurements

Abstract

Deep seismic sounding refraction studies made during the Nariño Project, combined with other recent geophysical and geological data, show many features of the active boundary between the Nazca Plate and the South American Plate in Colombia. Two sea-land profiles, 600 km in length, reveal a shallow anticline about 200 km offshore, topped by a pronounced graben. In this area of lithospheric bending we have observed the beginning of a small dip in the Moho and some shallow seismicity within the uppermost mantle. The oceanic Moho can be followed by clear P n-arrivals down to depths of 50 km below the coastal sediments and further down to about 200 km by seismicity studies. The coastal plains are interpreted as overlaying a hidden trench. The West Andes, being oceanic according to geological investigations, are also found to be oceanic by their high velocity—high density crustal composition. They seem to be carried to their present position by a process of obduction and introduction, which always seems to accompany a general subductive process. The chain of andesitic volcanoes in the Central Andes in situated about 150 km off a hidden trench and 150 km above the subducted oceanic crust. The Central Andes show a continental structure with rather low crustal velocities indicating high temperatures and strong inhomogeneities, possibly caused by frictional heating and magmatic activity. In addition to subduction, lateral strike-slip displacements seem to have taken place during the last sixty million years.