Magnetic fabric of the late Miocene extrusive rocks of the Combia Volcanic Province: Tectonic implications for the northern Andes

The accretion of the Panama–Chocó Block to the South American Plate partially drove the geological setting of the northern Andes. This event occurred in different collisional stages that are recorded in Oligocene–middle Miocene deformed rocks of the inter-Andean valley between the Western and Central Cordilleras of Colombia. However, uncertainty remains about the age of the latest accretionary phases of the Panama–Chocó Block. Poorly studied late Miocene volcanic rocks within the northern inter-Andean valley may provide key information to constrain the temporality of that final collision. Here, we study the deformational features of the ~12–6 Ma extrusive rocks of the Combia Volcanic Province located in the northwestern Andes (Colombia). We present anisotropy of magnetic susceptibility (AMS) data for pyroclastic and volcanic rocks within the Amagá Basin, an inter-Andean depression with Oligocene–middle Miocene sedimentary rocks that recorded NW–SE compression and NE–SW simple shear caused by the Panama–Chocó Block collision. We identified that the magnetic fabrics of the extrusive rocks of the Combia Volcanic Province reveal flow directions that indicate the occurrence of ancient volcanoes in the central axis of the Amagá Basin. Some of these fabrics do not contain any deformational features, whereas others record the same structural regime as the Oligocene–middle Miocene sedimentary rocks. We infer that variations in the intensity of the deformation promoted late Miocene local fault reactivations that, in contrast to the Oligocene–middle Miocene deformational events, did not affect the entire Amagá Basin. Age differences among the studied sections can also explain the different deformational patterns identified in the basin. Both interpretations suggest that the most significant collisional events of the Panama–Chocó Block occurred in the Oligocene–middle Miocene, whereas the formation of the Combia Volcanic Province may have either followed or coincided with the latest stages of the accretion.