New structural and geomorphological observations at the transition from shallow to steep subducting plate along the Caldas Tear in the Colombian Cordilleras

In the northern Andes of Colombia, seismic and tomographic studies evidence a slab relay between the gently dipping northern Bucaramanga segment and the southern steeply inclined Cauca segment. These studies indicate a clear-cut change in the sub-continental slab constellation and evidence of a lithospheric tear. Superficial structural and geomorphic changes above this relay zone depend on the observation scale and are best developed in the Eastern Cordillera of Colombia, a bi-vergent mountain belt decoupled from the Andean main trunk, which evolved by the inversion of an Early Cretaceous foreland basin. At the transition from steep to flat slab segments, this mountain belt doubles its width and attains considerable heights by a combination of exhumation and surface uplift. The external part of the tear coincides with the cratonward-oriented deformation front of the Eastern Cordillera. Here, faults are related to tight, basement-involved folding, suggesting a high geothermal gradient for this ductile deformation style. The youngest structures are represented by domes aligned parallel to the structural trend. These domes overprint existing folds and suggest by their size and association to circular normal faults growing and decaying evolutionary stages, demonstrating a southward propagation of a thermal mantle anomaly. In the western Central Cordillera, transversely oriented monogenetic volcanic fields similarly show a recent southward propagation of volcanic activity. A southward shift of the slab tear is further evidenced by transversely oriented fold trains. This scenario of a southward migrating slab tear encourages us to undertake a morphological study to evaluate the maturity of drainage and landscape evolution and to corroborate an N-S younging of recent tectonic activity. A reference for recent tectonic activity is the volcanic center of Paipa, centered along a fissure-like caldera and aligned transversely to the regional fold trend, indicating axial extension. Additionally, a subvolcanic rhyolitic stock has refolded a flank of an existing fold diapirically. These crustal mobilizations occurred during and after the late Miocene folding (implying NW-SE shortening) and before a second fold phase related to E-W shortening during the Cordillera’s final uplift stage. We use a novel analysis to identify the degree of directional organization of small river segments compared to the local structural trend. The reorganization of drainage indicates ongoing folding and tilting of hanging wall blocks related to major reverse faults and regional tilting of the cordillera’s axial depression. The capture of previously established longitudinal river valleys of the high plains documents the dynamics of transverse drainage during the final uplift and exhumation of the Cordillera’s eastern flank. The deviation of transverse rivers along frontal folds helps identify actively forming structures. Conversely, the weakly preferred drainage orientation in the axial depression north of the tear suggests a more mature landscape. With these geomorphological criteria, we aim to discuss the dynamics of the Caldas Tear and its southward propagation. Did the shallowing of the subducting plate occur gradually, involving lithospheric bending, or was it caused by the migration of a tear that swept through the Cordilleran realm until reaching its present southern position?