Recent petrological, structural and geochronological studies of the
eastern margin of the Bohemian Massif (Czech Republic) suggest a
conceptual geodynamical model to explain exhumation of lower crustal (20
kbar, 800 degrees C) felsic rocks. The model involves indentation of a
weak orogenic lower crust by an adjacent rigid mantle lithosphere,
resulting in crustal-scale buckling of the weak orogenic lower/middle
crust interface followed by extrusion of a ductile nappe over the rigid
promontory. The hypothesis has been investigated using both analogue and
numerical models. Analogue experiments using a three layer sand-silicone
setup were carried out in Rennes laboratory (France). Results show that
the most important features of the conceptual model can be reproduced:
extrusion of lowermost silicone over the indenter and flow of horizontal
viscous channel underneath a rigid lid above the actively progressing
promontory. Furthermore, experimental results show that a plateau
develops above the channelling lower crust. Two sets of sandbox-scale
numerical simulations were performed. The first set of experiments is
designed to study the influence of viscosity stratification within the
crust on the extrusion process. A second set of experiments were
performed in order to quantify the influence of the viscosity and the
geometry of the indentor. Non-dimensional scaling laws were derived to
predict the maximum extrusion rates associated with the indentation
mechanism. Such laws enable the computation vertical extrusion rates
that are in good agreement with natural exhumation rates inferred from
petrological data. Finally, we discuss the potential positive feedback
of Rayleigh Taylor instability on vertical extrusion for the case of
Eastern Bohemian Massif. (C) 2010 Elsevier B.V. All rights reserved.