Discriminating Mechanisms for Coarse Clastic Progradation in the Colombian Foreland Basin Using Detrital Zircon Double-Dating

Coarse, amalgamated, clastic units in distal regions of retro-arc foreland basins have been attributed to both periods of waning tectonism and active thrust-induced exhumation. In the former, coarse material deposited in the proximal basin is reworked into the distal basin during erosion-induced flexural rebound. In the latter, coarse detritus eroded during rapid hinterland exhumation floods the foreland, overwhelming proximal accommodation. We discriminate between these mechanisms using U-Pb geochronology and (U-Th)/He thermochronology applied to individual zircon grains of Maastrichtian to Neogene foreland basin strata in the Eastern Cordillera of Colombia.

In the stratigraphic record, an upsection decrease in lag time reflects accelerating exhumation in the hinterland while increasing lag time reflects erosion (recycling) of uppermost crustal material or decelerating exhumation. The detrital thermochronology history reveals three stages of acceleration in exhumation rate associated with deposition of coarse clastics. 1) A Maastrichtian–early Paleocene decrease in lag time coupled with an influx of Mesozoic detrital zircons in the foreland basin marks the onset of compressional deformation in the Central Cordilleran magmatic arc. 2) An early Eocene–late Oligocene acceleration of exhumation is associated with exhumation in the Middle Magdalena Valley and axial Eastern Cordillera. This stage is accompanied by decreasing contributions from the Middle Magdalena Valley (MMV) Upper Cretaceous strata and increasing contributions from Eastern Cordilleran Upper Cretaceous strata. This stage indicates a stripping of the MMV Upper Cretaceous strata and eastward migration of the deformation front. The arrival of western-derived detritus in the Nunchia Basin at the end of stage two indicates a removal of a drainage divide consistent with eastward migration of the flexural wave. 3) Between the late Miocene and Pliocene, another acceleration of exhumation reflects thrusting along the Servita fault. This stage indicates the removal of Eastern Cordilleran Upper Cretaceous strata and increased influx of Early Cretaceous detritus. The acceleration of exhumation shown by the decrease in lag-time in coarse clastic units, confirms that coarse-grained deposition in both proximal and distal settings correlates with rapid exhumation in the orogenic hinterland in the retro-arc foreland basin of Colombia.