ALBERT

Description: We present two regional, lithospheric cross sections that illustrate eastward- and southeastward-dipping, subducted slabs to depths of 315 km beneath the surface of Colombia in northwestern South America. These cross-sectional interpretations are based on relocated earthquake hypocentral solutions, models supported on gravity and magnetic regional data, and coda-Q ( Q c ) tomography. The method of tomographic imaging based on spatial inversion of the coda wave has advantages of providing information on the lateral variations of the anelastic properties and thermal structure of the lithospheric system. Mapping of earthquake-defined Benioff zones combined with tomographic imaging reveals the presence of an ~240 km long east–west-striking slab tear, named here the Caldas tear. The proposed Caldas tear separates a zone of shallow, 20°–30°-dipping, southeastward subduction in the area of Colombia adjacent to Panama and the Caribbean Sea, which is not associated with subduction-related volcanism, from an area of steeper, 30°–40°-dipping, slab adjacent to the eastern Pacific Ocean that is associated with an active north–south chain of active arc volcanoes. We propose that the Caldas slab tear separating these two distinct subducted slabs originally formed as the southern boundary of the Panama indenter, an extinct island arc that began subducting beneath northwestern South America about 12 Ma. The area south of the Panama indenter is Miocene oceanic crust of the Nazca plate, which subducts eastward beneath northwestern South America at normal angles and melts to form a north–south-trending active volcanic arc. In addition to the formation of the Caldas tear, we propose that impedance of the thicker crustal area of the Panama arc-indenter over the past 12 Ma may have led to down-dip break-off of previously subducted oceanic crust that is marked by an extremely concentrated and active earthquake swarm of intermediate-depth earthquakes beneath east-central Colombia.