Dynamics of Caribbean and Nazca Plate Subduction Beneath Colombia from Receiver Function Analysis

Abstract:

The tectonics of northwestern South America are controlled by the complex interactions of the South American, Nazca, and Caribbean plates. In order to better understand subduction within the region, we utilize data recorded by the Colombian National Seismic Network to calculate P-to-S receiver functions at a range of frequencies across the nation of Colombia. Where the station spacing was dense enough, receiver functions were stacked using the Common Conversion Point (CCP) method in order to better image lateral changes in crustal and upper mantle structure. Along the Pacific margin of Colombia, where the Nazca plate is subducting beneath South America, the subducting slab dips too steeply to image it with receiver functions. However, layering and strong negative arrivals are observed in the crust above the subducting slab where active volcanoes are present. The presence of these arrivals is possibly indicative of slab dehydration and the presence of partial melt within the crust. In northeastern Colombia, the Caribbean plate is subducting beneath South America at an oblique angle. Along the direction of convergence, the slab extends ~500 km inland with a relatively shallow dip before steepening. Preliminary receiver function images from this region show a shallowly-dipping negative arrival, interpreted as the top of the slab. This arrival is underlain by a positive conversion, interpreted as the down-going oceanic Moho. As the dip of the seismicity associated with the subducting slab steepens, these arrivals are no longer observed within the receiver function stacks. These cross sections of the Caribbean plate subduction are consistent with the idea that phase changes within the downgoing oceanic crust and mantle are controlling the slab buoyancy and, as a result, the angle of subduction. As the receiver functions are refined and further combined with local earthquake locations, we will better be able to understand the location of earthquakes within the subducting slab and the role of slab hydration and subsequent dehydration in the region’s seismicity.