Extremely weak lower crust and the lower crustal flow induced by crustal thickening

Abstract:

Orogen building processes are typically associated with crustal thickening and an excess in Gravitational Potential Energy controlled by convergent forces and material resistance. The reduction of gravitational potential commonly occurs via erosion or internal material transfers. Among the latter, the inverse ductile thinning mechanism considered in this work refers to the lower crustal flow induced by lateral pressure gradients in association with extreme ductility of the lower crust. This mechanism has been proposed on the basis of geological evidence in the Vepor Dome, West Carpathians (Jeřábek et al., 2012) and the concept is further developed by means of numerical simulations.

The characteristic feature of the lower crustal flow in the Vepor Dome is that it occurs during nappe stacking and overall crustal thickening in the Cretaceous Central West Carpathians orogenic wedge. The along-strike lower crustal flow in the internal part of the wedge induced by thickening and associated lateral pressure gradients is accompanied by synkinematic prograde Barrovian type metamorphism. The high proportion of weak metapelites in the lower crust is suspected to promote nearly instantaneous response to the overburden. To understand dynamics of the proposed mechanism, we designed a two dimensional numerical model characterized by felsic crust with low viscosity layer overlaying elastic mantle lithosphere. In our model, we simulated PT trajectories of individual crustal levels for different rates of tectonic loading and variable initial thermal state. The results of numerical modeling were compared with PT trajectories obtained by thermodynamic modelling and geochronological data from the Vepor Dome. Our numerical simulations imply that the inverse ductile thinning is indeed conditioned by the presence of extremely weak lower crustal layer, however the simulated PT paths were only comparable to our metamorphic data if the constant loading lasted at least 30 m.y.

References:

Jeřábek, P., O. Lexa, K. Schulmann, and D. Plašienka (2012), Inverse ductile thinning via lower crustal flow and fold-induced doming in the West Carpathian Eo-Alpine collisional wedge, Tectonics, 31(5), 1–26, doi:10.1029/2012TC003097.