PhD. thesis - Univerzita Karlova

and deformation of lithospheric structure described above. A numerical approach that enables
us to model the deformation in a weak zone (Gemer Unit) surrounded by a rigid (Vepor Unit) or
free boundaries is presented here. This approach is based on the thin viscous sheet
approximation for modeling of the deformation. The Gemer Unit is assumed to be a horizontal
weak tabular domain subjected to flow with no tractions at top and bottom surfaces. We also
assume that vertical gradients of the horizontal velocity are negligible, which allows us to
integrate the equations of motion over the vertical dimension and to work with vertical averages
of stress and strain rate. The details are presented in appendix of Chapter 1. Using the
assumption of Newtonian behaviour (linear relation between stress and strain rate), the
procedure leads to a system of elliptic partial differential equations for two horizontal velocity
components. The system is solved by the finite element method with boundary conditions,
which represent the geological situation. In each time increment, boundaries are rearranged
simultaneously with repeating solution of the governing equations for the velocity. At each time
step we evaluate instantaneous strain rate and finite strain in a net of points in the domain. These
numerical exercises significantly contribute to understanding of reasons of local changes in
deformational regime due to deformation and help us to explain the origin of major structural
features in studied area.
During field studies in the West Carpathians we encountered a considerable problem
related to Cretaceous extensional tectonics affecting the Vepor crystalline
basement. The previous studies have resulted in the generally accepted model of post-orogenic
or orogen-parallel extension. These studies are supported by geochronology, petrology, and
particularly by interpretations of kinematic indicators namely, shear-bands. Our field revision
points out that extensional tectonics was in fact the first Alpine deformation in the studied area
and that it pre-dates the compressional stage of Alpine tectonic evolution. This causes that in
many cases we deal with oblique sections across small-scale folds or crenulation cleavage,
which are likely to be misinterpreted as shear-bands. In Chapter 2 we demonstrate that:
1) distinguishing between compressional and extensional crenulation cleavages is not always
an easy task, 2) unless this is fully appreciated there is a great danger of misinterpretation when
the shear bands are used as kinematic indicators, 3) the compressional and extensional
crenulation cleavages can appear identical when seen on flat outcrop surfaces or in thin
sections. This work aims to alert that because the shear bands are such noticeable structures and
their kinematic significance is straightforward, without detailed knowledge of 3D geometry of