Quaternary tectonics in the central Interandean Valley, Ecuador: Fault-propagation folds, transfer faults and the Cotopaxi Volcano

Abstract

We studied the Quaternary tectonics of the central Interandean Valley (IV, Ecuador), around the active Cotopaxi volcano, by field geological–structural survey, analysis of seismicity, precise levelling of river terraces and numerical modelling. North of the volcano, there are main Quaternary west-dipping reverse faults located along the western side of the valley. At the Cotopaxi foothills, we found NNE-SSW-striking, vertical, right-lateral oblique strike-slip faults and E-W-striking normal faults, which offset Pleistocene deposits. South of the volcano, there are several Quaternary folds that show minor normal faults at the hinge zone linked to local extension at the extrados. The core of the folds sometimes shows reverse faults. Most of the folds have flexure geometry whereas the Guambalo fold shows a double flexure resembling a huge box fold. By numerical modelling we computed the best solution that fits the observed geometry as fault-propagation folds. The fold system shows a double vergence linked to reverse faults dipping to the west along the western side of the IV and to the east along the eastern side. Computed stress tensors by striated fault inversion indicate a Quaternary E-W-trending horizontal greatest principal stress (σ1) and a vertical least principal stress (σ3) south of Cotopaxi, whereas at the volcano σ1 is ENE-WSW and σ3 is also horizontal. At the Yanayacu flexure we measured an increment of deformation along late Quaternary river deposits, suggesting a potentially very recent activity. These results are consistent with the distribution of crustal seismicity. We consider a larger Quaternary tectonic shortening south of the volcano than to the north of it, with the transcurrent faults acting as transfer structures. Magma emplacement at Cotopaxi in this compressional setting has been favoured by the horizontal σ3 linked to the transfer fault zone.