Etude géodynamique d'un bassin intramontagneux tertiaire sur décrochements dans les Andes du Sud de l'Equateur : l'exemple du bassin de Cuenca

Geodynamic study of a Tertiary intermontane basin on strike-slip faults in the Andes of Southern Ecuador : the Cuenca Basin. The Cuenca sedimentary basin (south Ecuador) is an Oligocene-Miocene to Pliocene intermontane continental basin which activity began after the collision and accretion of exotic terranes with oceanic origm. late Cretaceous-Paleogene dated. The Cuenca basln shows the main charactenstics of a stnke-slip basin an elongated form (100 km x 30 km) along N 20 to N 40 and N 170 to N-S faults an asyrnetrical section due to the preponderant activity of the eastern margin a minimal rate of sedimentatlon roughly estlmated at 200 m/my which charactenses an important subsidence .’ high abrupt facies changes of alluvial fans to lacustrine turbidites : an axial infllllng subparaiel to the major margin faults wlth a south to north constant polarity a S-E to N-W migration of the depocenters . abundant kilometric scale SynsedJmentary de formations The sequential analysis of the filling shows a cyclrcal evolution in two megasequences The first one, thinning and fining upward sequence. characterises the basin opening. the second one. thickenlng and coarsening upward sequence. characterises its closiny The basrn activity JS also characterized by an Intermittent volcanic activity Aiter the Saraguro Formation deposits (Late Oligocene). the basin is initially formed by a probable transpression. During the deposJtion of the BiblJan Formation, the opening took place concurrently with horst and half-grabens development, parallel to the N 20 to N 40 faults These faults relay the N 170 to N-S faults which move as dextral. Two svnsedimentary folded deformations which folow distinct mam shortening directions the first one N 60 and the second one E-W affect the last deposits of the basin. They induce movements respectively dextral and dextral-reverse along the N 20 to N 40 and N 170 to N-S faults, giving rise to the basin closing. The structural and cedimentologic characters of the Cuenca basin, just as the synsedimentary tectonics, characterize the stnke-slip faulting of the basin during a tectonic continuum since late Oligocene to Pliocene age. during roughly 24 my.