Gravity and magnetic anomalies of ecuadorian margin: Implications in the deep structure of the subduction of Nazca Plate and Andes Cordillera

Abstract

To improve our knowledge of the deep structure of the Ecuadorian continental margin we combined satellite gravity and magnetic anomalies with the gravity field data obtained in land surveys, integrated as well as with seismicity and geological data. Satellite Bouguer anomalies are elongated, arched and parallel to the continental margin. The lows are related to the Andes Cordillera, while relative westward parallel highs correspond to the Cordillera Costanera and the Talara-Tumbes Cordilleras. Smooth and irregular positive anomalies characterize Oriente Basin. The oceanic Nazca Plate shows negative relative to the sounding normal oceanic crust, ENE-WSW gravity anomalies highlighting the Carnegie Ridge, surrounded by highs corresponding to standard oceanic lithosphere. New gravity field profiles at 1°S and 4°S show Bouguer anomalies comprised between -273 and + 75 mGal, giving higher resolution than satellite data. Gravity crustal models suggest that the continental crust in the Andean Cordillera is 70 km thick, while 20 to 40 km thick in the Coastal region and Oriente Basin. Seismic data show the oceanic crust, 8 km thick, extends beneath the continent to a depth of 200 km, with the highest dip of the subduction slab modelled at Profile 1ºS, which is the area of the Andes Cordillera having maximum elevation. Main sedimentary basins and intrusive bodies are related to the local low Bouguer anomalies, whereas high values correspond to allochthonous oceanic units. Reverse faults are widespread and contribute to crustal thickening. Normal faults are located in the forearc basin. Satellite magnetic data reveal anomalies related to the subducting slab below the South American margin, reaching the Andes Cordillera. The Carnegie Ridge is characterized by a roughly ENE-WSW minimum, corresponding to highly magnetized rocks located along the 1ºS profile. The E-W anomaly pattern changes sharply at the eastern edge of the seismicity related to subduction; further eastward the NW-SE anomalies are associated with the South American craton in Oriente Basin.