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Geodynamical model of the episodic evolution of prowedge Moldanubain structures associated with
Brunovistulian foreland underthrusting
Martin RACEK, Pavla ŠTÍPSKÁ, Karel SCHULMANN
Institute of Petrology and Structural Geology, Charles University,Albertov 6, 128 43 Prague, Czech Republic
We investigate an orogenic fabric along the E-W cross-section running from the
Raabs lower crustal unit in the west to the easterly situated Moravian zone. The traditional
cross-sections show the eastward thrusting of the Raabs lower crust over the Varied group
metasediments, which appear in form of a tectonic window (Fuchs, 1976, Matte 1991). In
this concept the granulite sheet at the eastern border of the Moldanubian zone adjacent to
Moravian zone is interpreted as a termination of the antiform stack of the Moldanubian
thrust sheet.
The structural study shows that the migmatitic and isoclinally folded fabric of the
Raabs unit is dipping to the west under shallow to intermediate angles. The boundary with
the westerly Varied group is marked by a sheet of garnetiferous amphibolites. The
underlying Varied group is strongly deformed in its western part. The deformation gradient
is decreasing towards the east from uniformly reworked, shallowly dipping rocks to steeply
folded sequences on hectometric scale. The steep fabrics in folded marbles and schists
were later reworked by flat folds and shear zones. Further to the east, the Varied group
overlies westerly dipping migmatites, which gradually pass into granulite facies rocks at the
contact with the Moravian zone. This boundary is marked by a major structural change
between compressional structures in the Varied group and gravitational flat folding
developed in the lower crust. The extensional structures clearly rework the early
compressional structures.
This tectonic scheme is interptreted as thrusting of retrograde and highly anatectic
granulite Raabs unit over the middle crust (ky-sill-grt) metasedimentary Varied group. The
limit between the Varied group and the underlying lower crust sheet is characterized by
transition from compressional style to gravitational late-metamorphic sliding to the east.
The NE-SW trending lineations and fold hinges indicate that the whole tectonic evolution
was associated with oblique convergence.
This orogenic structure is consistent with a structural and numerical model applied to
the tectonic evolution of the Pyrenees collisional crustal wedge (Beauomont et al., 2000). In
agreement with their numerical model P3, which involves inherited weak lower crustal
horizon, we place the first pro-wedge thrust at the bottom of the Raabs unit. This thrusting
is connected with extreme deformation of the eastern part of the Varied group and steep
folding of the remote eastern part of this unit. Subsequently, a new pro-wedge thrust
developed in the lower crust level and the whole sequence has thrust over the
Brunovistulian foreland. This thrusting was accompanied with gravitational sliding of
overlying Varied group and vertical gravitational collapse of steep fabrics.
References:
FUCHS, G. 1976. Zur Entwicklung der Böhmishe Masse. Jahrb. Geol. B.-A. 119: 45-61.
MATTE, P. 1991. Accretionary history and crustal evolution of the Variscan belt in Western
Europe. Tectonophysics 196: 309-337.
BEAUMONT, C., MUÑOZ, J. A., HAMILTON, J., FULLSACK, P. 2000. Factors
controlling the Alpine evolution of the central Pyrenees inferred from a comparison of
observations and geodynamical models. Journal of geophysical research 105 (B4): 81218145.