Detailed seismic velocity structure of the Caribbean and Nazca Plates beneath Valle Medio del Magdalena region of NE Colombia

A detailed 3D velocity tomography for P and S-waves and P to S ratios (Vp/Vs) was performed for the NE margin of Colombia, called the Valle Medio del Magdalena Basin (VMM) (Middle Magdalena Valley basin). A total of 21,265 local earthquakes, were used to obtain the 3D velocity structure of that region and surroundings. Low Vp and Vs values were found beneath the VMM basin at depths up to 15 km. Moreover, high Vp/Vs values were found beneath the VMM from depths of 2-10 km. On the other hand, the subducting slab portion observed beneath the VMM showed low and high Vp/Vs regions from depths of 20-100 km. This subduction slab is interpreted as corresponding to the Caribbean (CP) and the Nazca Plate (NP). The slab seems to be segmented in two main different blocks, one exhibiting low-Vp/Vs ratios (high-Vp and Vs), at depth >80 km associated to the Caribbean Plate with a dip angle of similar to 34 degrees, and one exhibiting high-Vp/Vs ratios (low-Vp and Vs), at depths >80 km associated with an eastern and northernmost block of the Nazca Plate, with a dip angle of similar to 50 degrees. The limit of both plates is located at 6.6 degrees N. The Bucaramanga seismic nest (high-Vp and Vs, and low Vp/Vs) is interpreted as the collision of the northernmost block of the NP and the CP, in a region with mantle-rich material and high-Mg content. The limit of the northernmost block of the NP and the southern wide block of the same plate shifted 200 km to the west is the Caldas tear, which is proposed as located inside the NP, not the boundary between the CP and the NP. This tearing of the NP is associated with Quaternary volcanism lined up in an E-W direction, such as the current magmatic active Paipa-Iza volcanic Complex, and the Villamatia-Termales and Samand monogenetic volcanic fields. This finding indicates that the active volcanism in Colombia is extended to similar to 6 degrees N, one degree further north than previously assumed.