Evolution of the Manabí forearc basin in Ecuador: From the collision of Caribbean oceanic plateau to the build up of the Andes and Coastal Cordilleras

Abstract

We investigate the evolution of the onshore Manabí forearc basin in Ecuador, which has developed on accreted oceanic terranes. We use 2D seismic profiles to identify 10 seismic units, separated by 9 unconformities. These units were dated by correlation with Ricaurte-1 well and regional stratigraphy. Unit S1 represents the first depocenter developed on the oceanic plateau before accretion. The emplacement of NE-dipping oceanic plateau slices evidencing the accretionary process dated to ∼75 Ma (Unconformity U1), was followed by folds and east-dipping thrusts at the Andes foothills that were eroded by unconformity U2 (56–41.3 Ma hiatus). The large distribution of unit S3 (late Eocene-Oligocene) over the topography inherited from the collision, points to the configuration of the post-collision forearc basin. The geometry of slope front fill and the thickening of unit S4 at the foot of the Andes reveals the building of Andean reliefs during the early Miocene. Unit S5 (17.8–15.9 Ma) is fan-shaped and abuts against the Jama fault, evidencing local extensional. After unconformity U5 (∼16 Ma), differential subsidence causes the installation of a westward-prograding sedimentary platform during the middle to late Miocene (S6 to S8). Since the Pliocene, the uplift of the Coastal Cordillera is related to localized shortening across the Jama and Canandé faults. Enhanced tectonic erosion at the tip of the margin,likely related to the Carnegie ridge subduction, could have led to underplating of offscraped material beneath the Coastal Cordillera, contributing to the uplift and enlargement of the Coastal Cordillera. The Pleistocene eastward tilt of the Santo Domingo fan (unit S10) relates to the Coastal Cordillera uplift. The fan is then deformed along the Andean thrust. This study provides a remarkable example of a forearc basin-fill developed on accreted oceanic terranes and controlled by erosional and underplating subduction processes, reactivation of inherited crustal structures, and the rise of peripheral reliefs.