Surface processes during active orogenesis: uplift and erosion of the Sierra Nevada de Santa Marta (Colombia) at multiple time scales

Abstract

Mountain building in orogenic belts is the result of tectonic driving mechanisms, acting over a lithosphere with an intrinsic structure and rheology, modified by climate. Although theoretical models have explored the coupling and interaction between tectonics and climate, deconvolving the degree of control exerted by each one on mountain building has proven difficult. The motivation for this proposal is to identify the principal controls on late Cenozoic uplift and exhumation on the highest coastal mountain range on Earth, the Sierra Nevada de Santa Marta in northern Colombia, where the topographic and structural reliefs exceed ~ 9 km and ~ 15 km, respectively. Oblique convergence and dextral shearing along the Caribbean-South American plate boundary have shaped the northern Andes since Late Cretaceous. Although recent investigations have revealed key episodes concerning the kinematics of early orogenic stages along this margin, the timing and associated driving mechanisms for recent uplift of the SNSM remain elusive. Sparse but relative old (> 15Ma) cooling ages and a positive gravity anomaly in the SNSM suggest that the range is in a stage of denudational immaturity, that is, recent crustal stacking and rock uplift exceed the magnitude of exhumation, portraying a case of non-steady state topography likely due to a young pulse of orogenesis. This disequilibrium poses a fundamental question about the driving mechanism for recent rock uplift, since recent studies suggest that mountain building in the SNSM may have initiated in the Paleocene. Recent rock uplift of the SNSM has modified the local climate and may have conditioned basin subsidence, burial of organic matter, and the long-term evolution of petroleum systems in adjacent basins. Preliminary investigations by PI Parra show that expected recent erosion rates must be high to account for the local topographic relief and hillslopes. These rates cannot be sustained backwards in time for more than ca. 2 Ma, given that available, spatially restricted cooling ages support a magnitude of exhumation of only < 2 km. In this context, we aim at characterizing erosion and denudation rates over different timescales, including (i) recent rates using sediment discharge gauges, (ii) Quaternary rates employing cosmogenic radionuclides (CRN), and (iii) erosion rates associated with even longer time scales using low-temperature thermochronometry. The proposed CRN and thermochronologic analysis of 27 sediment samples from modern-river catchments and morphometric analyses will help determine the erosional history, the potential driving mechanisms, and the effect on petroleum systems in adjacent basins. In addition, understanding the time of orogenesis in this environment is important because it will allow deciphering the role of tectonics and climatic variability - between the wetter northern and the drier southern flanks - in mountain building. Finally, by understanding the potential variations in exhumation rates through time, we will be able to assess the response time of surface processes to tectonic forcing. From an institutional perspective, a fundamental part of the project is the establishment of a state-of-the-art thermochonometry lab at the IEE-USP. This laboratory will complement the research facilities of the Geosciences institutes at USP and will be a key element of the new Oil and Gas research division. This project constitutes also a foundation for establishing a new research group on Active Orogenesis and Surface Processes at USP, a research line that has received less attention in Brazilian geosciences and that has been the focus of PI Parra during the last 10 years. Finally, the project aims also to promote international scientific collaboration and exchange, at the level of researchers, between USP and partner universities in Germany and USA. (AU)