Collisional Response of the Middle to Upper-lower Crust: A Himalayan Analogue in West Greenland

University Of Texas At Austin, Austin TX

Investigators

Abstract

The collision of two continental plates causes a wide range of spectacular geological phenomena including mountain building, earthquakes and magmatism. Much of the understanding of continent-continent collisional tectonics stems from the on-going Himalayan-Tibetan orogenic system, the best known example of this fundamental process. The 4-D dynamics and architecture of mountains and high plateaus are the consequence of a complex interplay between the upper and middle/lower crust, which deform primarily by brittle and ductile mechanisms, respectively. However, the role of crust below the brittle-ductile transition in the Himalayan-Tibetan system necessarily relies mainly on observations collected in the upper part of the brittle realm. Despite these limitations, models invoking large-scale ductile flow in a restricted channel of melt-weakened material in the mid-crust and its control on mountain building, are based heavily on this region. A complete, well-exposed, deeply-eroded ancient mountain belt that exposes middle/deep crust would permit direct observation of rocks that experienced the processes speculated about from studies of higher level orogens and numerical models. An international research team composed of geologists from the University of Texas and the Danish Lithosphere Centre, believe that such an area exists in West Greenland and we are testing a hypothesis that that two Precambrian (ca. 1.8 billion years) orogens identified in West Greenland (Rinkian Belt and Nagssugtoqidian Orogen) represent a single, +1100 km wide, asymmetric collisional orogen. Due to the appropriate levels of post-collisional exhumation, this region (with 100% exposure in an alpine setting) will serve as a unique site to directly observe the response of the middle and shallow-lower crust to continent-continent collision. As such, this accessible region will serve as a deep analogue to the Himalayan-Tibean orogenic system. These process-oriented objectives require that the style(s), timing, duration and diachroneity of deformation and metamorphism be characterized across this +1100 km orogen. Over the past three summers, field work in West Greenland by our research group (funded by Carlsberg and NERC) builds upon previous mapping and research programs to characterize the structural evolution of the region. This NSF grant supports an extensive U-Pb geochronology program that will place critical absolute time constrains on the evolution of this belt. Just as the Himalayas serves as our best example of continent-continent collision today, the West Greenland is envisioned as being a world-class, benchmark site for studying processes occurring at depth in collisional tectonic settings. A better understanding of deformation style(s) and feed back of these crustal levels will improve our understanding of topography, earthquakes, ground motion and deformation in this tectonic setting. Additionally, formulation of an integrated tectonic model for the Paleoproterozoic of West Greenland will provide the last major piece of the puzzle to better understand the growth and stabilization of Paleoproterozoic NE Laurentia. This will represent the culmination of over a decade of international research that has included groups in the USA, Canada and Europe.