Long-term rift evolution

Abstract

Rift systems represent one of the most remarkable geomorphological features on Earth. Hydrocarbon resources can be found within deep rift basins, whereas rift flanks form high morphological ridges influencing climate, vegetation, and fauna (Rosendahl 1987; Ziegler 1992; Ruppel 1995; Ziegler and Cloetingh 2004; Sepulchre et al. 2006; Trauth et al. 2007). Rift systems are the perfect laboratory to understand how our planet is changing dynamically through time and how mantle processes, volcanism, rock and surface uplift, erosion and sedimentation, climate, and fauna are ultimately linked. This special issue arises from a session at the EGU 2008 and presents a broad range of papers on long-term rift evolution spanning from the Asthenosphere through the Lithosphere all the way to climate-related processes. Seven papers focus on the evolution of the Albertine Rift System situated in the western branch of the East African Rift System and are the outcome of a DFG (Deutsche Forschungsgemeinschaft) funded Forschergruppe ‘‘RiftLink’’. Another six contributions focus on the Cheb basin, the Brazilian continental margin, the East Barents Sea Basin, the East African Rift System or on rifting processes in general. The contributions consider mantle to climate models: contributions by Semprich et al., Wallner and Schmeling, and Stamps et al. present mantle and lithospheric modelling studies of extensional environments; contributions by Babuska et al., Wolbern et al., and Link et al. treat mantle-crust relations and interactions; contributions by Bauer et al., Franco-Magalhaes et al., Sachau and Koehn, Koehn et al., and Roller et al. study crustal tectonics, rock and surface uplift histories and sedimentary environments; and the contributions by Brachert et al. and Kaspar et al. investigate climate proxies and climate models. Semprich et al. show with a systematic series of density diagrams, which are based on thermodynamic calculations that density changes are crucial for isostatic models in geodynamics. These authors illustrate that buried metapelitic rocks rich in Fe and Mg experience much larger density changes than dry basalt indicating that metamorphism of a hydrous lower crust leads to significant subsidence. The implications of these calculations are discussed in detail for different geodynamic settings. Wallner and Schmeling present a geodynamic modelling study of rift-induced delamination of Mantle Lithosphere (RID) in order to explain the extreme elevation of the Rwenzori Mountains in the western branch of the East African Rift System. These authors argue that the rift systems that surround the mountains trigger the delamination and that they are reducing the viscosity and strength of the lower crust around the horst. In the model, the weakened zone becomes unstable and a block of mantle material and lower crust sinks into the Asthenosphere leading to isostatic uplift of the overlying continental crust. A variation of modelling parameters is presented in order to illustrate under what conditions delamination takes place. Stamps et al. present a modelling study in order to determine if gravitational stresses are large enough to break the continental lithosphere in East Africa. They use a thin sheet approach to compute vertically averaged deviatoric F. U. Bauer U. A. Glasmacher Institute of Earth Sciences, University of Heidelberg, Im Neuenheimber Feld 234, 69120 Heidelberg, Germany e-mail: Friederike.Bauer@geow.uni-heidelberg.de