Kinematic evolution of Andean fold-thrust structures along the boundary between the Eastern Cordillera and Middle Magdalena Valley basin, Colombia

Abstract

Surface and subsurface data support a kinematic reconstruction of Cenozoic fold-thrust deformation along the Eastern Cordillera-Magdalena Valley transition in Colombia. The La Salina fault (LSF) marks the boundary between west-vergent Eastern Cordillera structures and hinterland deposits of the Middle Magdalena Valley basin. Apatite fission track and (U-Th)/He thermochronological results for the west-directed LSF reveal initial hanging wall exhumation during middle Eocene-early Oligocene (45-30 Ma) shortening, renewed exhumation in the early middle Miocene (18-12 Ma), and accelerated late Miocene-Pliocene (12-3 Ma) exhumation. Vitrinite reflectance data suggest maximum burial of 4-6 km, helping constrain Cenozoic basin architecture. Mapping of the LSF reveals hanging wall Cretaceous-Eocene rocks in a broad anticline-syncline pair with limited faulting and footwall Eocene-Quaternary basin fill in a complex series of tight thrust-related folds. Limited displacement along the westernmost (frontal) thrust suggests that shortening is largely accommodated by east-directed thrusting within a broader triangle zone of a passive-roof duplex (and probable minor strike-slip deformation). In the preferred kinematic restoration, the most recent phase of shortening to transpressional deformation represents out-of-sequence reactivation of the LSF consistent with irregular crosscutting relationships among footwall structures. Earliest exhumation by 45-30 Ma in the Eastern Cordillera fold-thrust belt is correlated with increased sedimentary lithic fragments and high compositional maturity in sandstones of the adjacent Magdalena Valley basin. Exhumation since ∼15 Ma coincided with decreased compositional maturity and elevated accumulation rates for the Real Group. The compositional provenance shifts are attributed to westward advance of fold-thrust deformation into the proximal (eastern) segments of the Magdalena Valley basin. Copyright 2012 by the American Geophysical Union.