| Summary: | The rheological behavior of the lithosphere beneath the Northwestern (NW) Andes (Colombian) remains uncertain under a complex geological setting. Here, we analyze the earthquake distribution and the stress drop on the western and the eastern of the transition area between the Upper Magdalena Valley (UMV) and the Middle Magdalena Valley (MMV), a zone related to a plate tectonic transition from steeper to flatter subduction style slabs (Nazca and Caribbean plates). Our results consistently explain the tectonic and geodynamic context of the study area supported in two rheological profiles, which reveal the brittle/ductile transition of the lithospheric crust and mantle and allow to constrain the lithospheric rheological thickness. The subduction phenomenon of the Nazca Plate under the South American Plate is observed in the western zone. In the eastern region, we find a remarkable aseismic zone under the Eastern Cordillera below 20-km-deep, which made more difficultan accurate estimation of the crust/mantle boundary. This seismic particularity leads us to support the hypothesis of a delamination process due to the tectonic overload generated by the Eastern Cordillera, and that promotes thermal anomalies. Our results also suggest that the earthquake dataset correlated with rheological estimations may offer a consistent interpretation of the mechanical behavior of the lithosphere.
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