Abstract

Lag time is the difference between the closure age of a thermochronologic system and the depositional age of host strata. Lag-time analysis of sedimentary basin fill provides insight into the exhumation history of adjacent eroded orogens. In a case study of the Paleogene Floresta basin in the Eastern Cordillera fold-thrust belt of Colombia, variations in lag time reflect changes in both sediment source areas and exhumation patterns. However, near-zero lag times can be produced by either syndepositional volcanism or rapid exhumation. We applied U-Pb geochronology and (U-Th)/He (ZHe) thermochronology to individual zircon grains and identified zircons of volcanic origin as those for which the U-Pb age is within the 2σ uncertainty of their ZHe age. Consistent discrimination of young ZHe ages as the products of either rapid exhumation or volcanism reveals three stages in the history of the northern Andean hinterland. (1) Early to late Paleocene: The appearance of syndepositional and Mesozoic volcanic zircons marks the initial influx of magmatic arc detritus. (2) Middle to late Eocene: Near-zero lag times point to rapid, regionally extensive exhumation attributable to thrust-induced uplift of the Magdalena Valley basement. (3) Late Eocene to late Oligocene: Increased lag time is interpreted as recycling of shallowly buried foreland-basin strata possibly due to movement on basin-bounding thrust systems. The presence of volcanic zircons with ZHe ages younger than or indistinguishable from the youngest exhumationally cooled zircons underscores the need for double dating to reliably identify volcanic influence in detrital thermochronology datasets. These data highlight the utility of double-dated ZHe results for extracting tectonic histories and reliably excluding volcanic zircons from lag-time analysis.

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