The still highly disputable terrane boundaries in the Sudetic
segment of the Variscan belt mostly
seem to follow major strike-slip faults and shear zones. Their
kinematics, expected to place important constraints
on the regional structural models, is discussed in some detail. The most
conspicuous is the
WNW–ESE Intra-Sudetic Fault Zone, separating several different
structural units of the West Sudetes. It
showed ductile dextral activity and, probably, displacement magnitude of
the
order of tens to hundreds kilometres,
during late Devonian(?) to early Carboniferous times. In the late Carboniferous
(to early Permian?),
the sense of motion on the Intra-Sudetic Fault was reversed in a semi-brittle
to brittle regime, with the left-lateral
offset on the fault amounting to single kilometres. The north–south
trending Niemcza and north-east–southwest Skrzynka shear zones
are left-lateral, ductile features in the eastern part of the West Sudetes.
Similarly oriented (northeast–southwest to NNE–SSW) regional
size shear zones of as yet undetermined
kinematics were discovered in boreholes under Cenozoic cover in the
eastern part of the Sudetic foreland
(the Niedźwiedź and Nysa-Brzeg shear zones). One of these
is expected to represent the northern continuation
of the major Stare Mesto Shear Zone in the Czech Republic, separating
the geologically different units
of the West and East Sudetes. The Rudawy Janowickie Metamorphic Unit, assumed
in some reconstructions
to comprise a mostly strike-slip terrane boundary, is characterized by
ductile fabric developed in a thrusting
regime, modified by a superimposed normal-slip extensional deformation.
Thrusting-related deformational
fabric was locally reoriented prior to the extensional event and shows
present-day strike-slip kinematics in
one of the sub-units. The Sudetic Boundary Fault, although prominent in
the
recent structure and topography
of the region, was not active as a Variscan strike-slip fault zone. The
reported data emphasize the importance
of syn-orogenic strike-slip tectonics in the Sudetes. The recognized
shear sense is compatible with a
strike-slip model of the northeast margin of the Bohemian Massif, in which
the Kaczawa and Góry Sowie
Units underwent late Devonian–early Carboniferous southeastward
long-distance displacement along the
Intra-Sudetic Fault Zone from their hypothetical original position within
the Northern Phyllite Zone and the
Mid-German Crystalline High of the German Variscides, respectively, and
were
juxtaposed with units of
different provenance southwest of the fault. The Intra-Sudetic Fault Zone,
together with the Elbe Fault Zone
further south, were subsequently cut in the east and their eastern segments
were displaced and removed by
the younger, early to late Carboniferous, NNE–SSW trending, transpressional
Moldanubian–Stare Mesto Shear Zone.
A Variscan continental collision of the West Sudetes and the Brunovistulian terrane: a contribution from structural and metamorphic record of the Stronie Formation, the Orlica-Śnieżnik Dome, SW Poland