Evaluating Along-strike Variations in Surface Uplift of the Andes: Constraints from Molecular Paleoaltimetry in the Eastern Cordillera of Colombia

University Of Texas At Austin, Austin TX

Investigators

Abstract

Paleobotanical proxies for the Eastern Cordillera of Colombia suggest a rapid rise in elevation of 1.5?2.5 km over the past 6 million years. Independent paleoaltimetry proxies are needed to determine the timing and rate of uplift in the northern Andes. As in many tropical locations, the appropriate carbonate or silicate minerals needed to apply oxygen isotope paleoaltimetry are missing. Fortunately, extensive preservation of organic-rich deposits and paleosols affords an excellent opportunity to test two new paleoaltimeters. The first paleoaltimeter exploits the variation in deuterium isotopes preserved in leaf waxes to reconstruct the lapse rate of stable isotopes with elevation. The second, the MBT/CBT paleo-thermometer (paleothermometer based on the Cyclisation of Branched Tetraether (CBT) index and Methylation of Branched Tetraether (MBT) index), utilizes the proportions of distinct moites of tetraether lipids produced by bacteria in soils (which are a sensitive function of mean annual temperature) to reconstruct the temperature versus elevation lapse rate. The two independent paleoaltimeters will be applied at both high- and low-elevation sites, providing multiple records that can be compared to determine if variations are the result of surface uplift or climate change. These data will be combined with chronostratigraphic results to determine the timing and rate of surface uplift. Recent models based on oxygen isotope and paleobotanical proxies posit rapid surface uplift of approximately 2.5 km for the Altiplano plateau in the central Andes between about 10 and 6 Ma. This rapid rate of uplift has led to suggestions that punctuated uplift resulted from delamination or convective removal of lower lithosphere previously thickened by extensive horizontal shortening. Estimates of total shortening in the central Andes are about 200?350 km. In the northern Andes, paleobotanical paleoaltimetry estimates show a similarly rapid late Cenozoic rise in elevation, although crustal shortening in the Eastern Cordillera of Colombia does not exceed 60 km. Nevertheless, like the central Andes, deformation in the Eastern Cordillera was ongoing since the Eocene, which would suggest a more gradual isostatic uplift history for both the central and northern Andes. Determining whether rapid uplift occurred in the absence of extensive horizontal shortening will shed light on the ongoing debate over the mechanisms of uplift and support of elevated orogens.