Seismic imaging of the Ecuadorian forearc and arc from joint ambient noise, local, and teleseismic tomography: catching the Nazca slab in the act of flattening

SUMMARY The Ecuadorian Andes are a complex region characterized by accreted oceanic terranes driven by the ongoing subduction of the oceanic Nazca plate beneath South America. Present-day tectonics in Ecuador are linked to the downgoing plate geometry featuring the subduction of the aseismic, oceanic Carnegie Ridge, which is currently entering the trench. Using seismic tomography, we jointly invert arrival times of P and S waves from local and teleseismic earthquakes with surface wave dispersion curves to image the structure of the forearc and magmatic arc of the Ecuadorian Andes. Our data set includes > 100 000 traveltimes recorded at 294 stations across Ecuador. Our images show the basement of the central forearc is composed of accreted oceanic terranes with high elastic wave speeds. Inboard of the Carnegie Ridge, the westernmost forearc and coastal cordilleras display relatively low Vp and Vs and high Vp/Vs values, which we attribute to the increased hydration and fracturing of the overriding plate due to the subduction of the thick oceanic crust of the Carnegie Ridge. We additionally image across-arc differences in magmatic architecture. The frontal volcanic arc overlies accreted terranes and is characterized by low velocities and high Vp/Vs indicative of partial melt reservoirs which are limited to the upper crust. In contrast, the main arc displays regions of partial melt across a wider range of depths. The Subandean zone of Ecuador has two active volcanoes built on continental crust suggesting the arc is expanding eastwards. The mid to lower crust does not show indications of being modified from the magmatic process. We infer that the slab is in the process of flattening as a consequence of early-stage subduction of the buoyant Carnegie Ridge.