THERMOCHRONOLOGICAL AND STRUCTURAL CONSTRAINTS on THE ANDEAN THRUST KINEMATIC HISTORY OF THE MIDDLE MAGDALENA VALLEY BASIN, COLOMBIA

This study presents a structural model for part of the eastern flank of the Middle Magdalena Valley basin and western boundary of the Eastern Cordillera of Colombia. In this region, the La Salina fault is the most conspicuous feature. In order to constrain the exhumation pattern and sequence of faulting, we present thermochronological and structural analysis incorporating data from both the hanging-wall and footwall blocks of this west-vergent fault.

Whereas the footwall of the La Salina fault is characterized by a series of tight, fault-related folds, the hanging-wall displays a broad gentle syncline. Kinematic evaluations suggest a possible system of west-vergent passive-roof duplexes linked with an east-vergent roof thrust. The set of horses for this thrust system reveals a significant lateral variability in displacement and size, providing a characteristic style for the structures in the footwall province.

Apatite fission track (AFT) and U-Th/He thermochronology as well as vitrinite reflectance (Ro) data allow us to identify initial exhumation of the hanging wall of La Salina fault during early Miocene (~25-20 Ma). Samples in the hanging wall at similar stratigraphic levels for both flanks of the Armas syncline show an earlier, slower phase of exhumation (42-38 Ma) for the western flank in contrast with rapid Late Miocene exhumation (~10 Ma) for the eastern flank. This pattern can be attributed to initial cooling along a frontal thrust followed by generation of a duplex roof.

We modeled AFT ages, track lengths, kinetic data and U-Th/He ages (using the kinetic annealing model of Ketcham et al. 2007 implemented in HeFTy software) to obtain cooling histories from partially reset and non-reset (detrital) samples. This modeling elucidates a late Miocene (12-5 Ma) exhumation event, presumably related to deformation in the footwall of the La Salina fault. Additionally, model results suggest some lateral differences in exhumation ages (earlier in southern parts of the thrust system), in accordance with the greater thickness of tectonic wedges in the south.

Finally, we infer a final cooling event (< 5 Ma) associated with out-of-sequence reactivation of an older fault on the basis of kinematic analyses and irregular crosscutting relationships of the La Salina fault with some footwall structures.