Recent and active tectonics of the North-Eastern Ecuadorian Andes

The north-eastern part of the Ecuadorian Andes, comprising the Interandean Valley, the Cordillera Real and part of the Sub-Andean Zone, is a region of diffuse seismicity with events reaching up to Ms = 7.0. The neotectonics of the region east of the Interandean Valley is analysed here by the integration of mesotectonic, microtectonic and seismological data. The structural setting is characterized by a predominance of N-S and NNE-SSW thrusts in the eastern region; whereas in the western region several NNE-SSW trending right-lateral strike-slip faults are present, the longest of which (the Cayambe-Chingual Fault) has a length of more than 70 km. Right-lateral faults trending NNE-SSW and some oblique thrust faults cut volcanic rocks of Late Pleistocene age and all the other fault sets. These faults offset streams of post-glacial age, are erosionally immature and have triangular facets. These data argue for a recent age for these faults. The Quatenary state of stress computed from microstructural data has a WSW-ENE greatest principal stress which is consistent with published focal mechanism solutions in this region. These consist of a combination of thrusting and right-lateral strike-slip motions along NNE-SSW striking planes. A kinematic model of the upper crust, in which westward dipping oblique thrusts and strike-slip faults coexist and converge at depth, accounts both for the observed seismicity and for the field measurements. These structures are part of a broader fault zone with a predominant right-lateral component of motion which extends along the eastern margin of the Andean Block, from the Gulf of Guayaquil, in southern Ecuador, to northern Colombia. © 1992.