Integrated geophysical model approach for Nazca Plate subduction in Ecuador

Abstract

This research addresses 2D models of the Nazca-South America subduction zone off the coast of Ecuador using the European Improved Gravity Model of the Earth by New Techniques (EIGEN-6C4), the Earth Magnetic Anomaly Grid with a 2-arc-minute resolution model (EMAG2 V3), and the seismic catalog from the Ecuadorian Geophysical Institute of the National Polytechnic School (IGEPN). Since the Upper Cretaceous, the region has experienced several geomorphological and structural alterations within the oceanic plate (Carnegie Ridge and Grijalva Fault Zone) and continental plate (North Andean Block and South American Plate). These changes result from a complex geodynamic history involving ancient subduction zones, accretions, and roll-back stages of allochthonous oceanic terrains and the autochthonous continental coast. To ensure the accuracy of the selected gravimetric model, the EIGEN-6C4 model was statistically compared with the pure-satellite gravimetric model, GO-CONS-TIM-R6e. Several geophysical analyses, such as terrain correction, geostatistical analysis, clustering, and power spectrum, were performed to gain valuable insights into gravity and magnetic sources. Then, these results were incorporated as constraints into the forward modeling procedure, which was adjusted to hypocenters, generating four subduction profiles that were then refined by an inversion method. The 0°S profile shows a subduction angle of 15° and escalates to 35° beyond 55 km depth, with the oceanic crust thickening at the Carnegie Ridge up to 18 km. The 1°S profile displayed two inflection points implying changes in the dip angle: a transition from 14° to 7° dip angle, supporting a change to low-angle subduction, and a transition from 7° to 40°, suggesting normal subduction beneath the Andean Cordillera. In the 2°S profile, the subduction initiates with a 15° angle beneath the continental plate extending to 60 km beneath the Manabi Basin, then potentially increases to 30° beyond 120 km depth. The 3°S profile beneath the Andes revealed a stable subduction zone with a constant dip angle of 14° and flat-slab subduction. Creating a three-dimensional interpolated model between the modeled profiles as a sketch of the angle variation of the subducted slab in the Equatorian region. The small root-mean-square error indicates that the models adequately represent the data. These models provide valuable observations of the geometric variations of the subducting plate, highlighting the impact of heterogeneous physiographic elements within the oceanic crust and the presence of an ancient subduction slab derived from the Farallon Plate.