From latest miocene thrusting to quaternary transpression and transtension in the Interandean Valley, Ecuador

Abstract

Field studies of folds and striated fault planes were carried out in the Interandean Valley (IV) of Ecuador, a NNE-SSW elongated depression lying between the Cordillera Occidental and the Cordillera Real. Four tectonic events were recognized: (a) at the end of Miocene the continental deposits of Chota Group were involved in cylindrical folds characterized by a WNW-ESE direction of shortening; (b) a second compressive pulse occurred during Pliocene or early Pleistocene when lava flows dated 6 Ma were affected by NNE trending right-lateral strike-slip faults and fluvial deposits of 1.7 Ma were involved in N-S trending folds; (c) lacustrine, fluvial and volcanic intermontane deposits dating up to the middle Pleistocene were deformed during late Pleistocene by a system of strike-slip and reverse faults, consistent with a N-S direction of the greatest principal stress (σ1), and by en-échelon folds with an average E-W axis, oblique to the NNE trend of the Cordilleras; (d) a last tectonic pulse involved also volcano-sedimentary deposits dating up to the Holocene, which were affected by an en-échelon NNE left-lateral normal and N-S pure normal faults responding to an E-W least principal stress (σ3). The en-échelon arrangement of the post-Miocene structures, the predominance of strike-slip and oblique-slip motions and the presence of strike-slip faulting in the Cordilleras, lead us to interpret the Plio-Quaternary tectonic events in the frame of transpressive and transtensive deformation regimes. This evolution of the deformation styles is thought to be the result of the reactivation of the early Tertiary suture zone, laying beneath the volcano-sedimentary filling of the IV, by the differential motions between the Cordillera Occidental and Cordillera Real crustal blocks during Plio-Quaternary times.