CONSTRAINING BASEMENT UPLIFT IN THE NORTHERN ANDES USING DETRITAL ZIRCON PROVENANCE ANALYSIS: U-PB GEOCHRONOLOGICAL RECORD OF EXHUMATION OF THE GARZóN MASSIF, COLOMBIA

The Garzón massif is a major topographic barrier (2500 m), forms an extreme large climatic rain shadow, and represents the largest exposure of Precambrian basement in the northern Andes. However, its history of uplift-induced exhumation and relationship to the structural evolution of the broader Eastern Cordillera remain unclear. The Eastern Cordillera has undergone major east-west shortening since the Oligocene, with significant fold-thrust deformation and topographic development occurring during the late Miocene. On the basis of extensive, coarse-grained deposits of late Miocene-Pliocene age, previous studies have inferred uplift of the Garzón massif during the late Miocene, coincident with the rapid rise in topography elsewhere in the Eastern Cordillera. We present new detrital zircon provenance data (10 sandstone samples from middle to upper Miocene clastic fill) that indicates exhumation began at approximately 11 Ma, much later than the Oligocene initiation of shortening observed elsewhere in the Eastern Cordillera, but prior to the main phase of Eastern Cordillera uplift. The Garzón massif currently separates the elevated Upper Magdalena Valley hinterland basin from the lowland Llanos/Putumayo foreland basin, and may be associated with regional basement uplift of foreland arches (such as the Vaupés swell) that separate the Amazon and Orinoco drainage networks. As a result, improved constraints on the timing of uplift-induced exhumation of the Garzón massif will provide critical information on the depositional history and topographic isolation of these economically important basins. At face value, our results suggest that initial uplift of the Garzón massif in the late Miocene (~11 Ma) may be decoupled from documented earlier (pre-late Miocene) phases of deformation in the broader Eastern Cordillera.