Superimposed Compressional and Extensional Tectonics in the Karkonosze-Izera Block, Ne Bohemian Massif

Abstract

The metamorphic envelope of the Karkonosze granite in the Karkonosze-Izera block of the western Sudetes is subdivided into three main tectonic units. Structurally upwards these are: (1) Izera-Karkonosze (gneisses and mica schists), (2) South Karkonosze (phyllites and metabasites), and (3) Leszczyniec (metabasites and gneisses) units. All three units experienced contrasting metamorphic paths. Three ductile deformation events (D1, D2, D3) have been documented in the area. Foliation S1 and mineral lineation L1 are the dominant structures in the block except for the mica schists of the Izera-Karkonosze unit. In the mica schists, the D1 fabric is almost entirely obliterated by the D2 structures. Deformation D3 affected only the eastern margin of the block and produced local folding of the earlier structures. The evolution of the Karkonosze-Izera block involved two main stages. The D1 compressional event was associated with overthrusting and juxtaposition of the major tectonic units. The direction of tectonic transport was to NW in the two lower (Izera-Karkonosze and South Karkonosze) units and, probably, to NNE in the uppermost (Leszczyniec) unit. The D1 event was pre-dated by the high pressure/low temperature metamorphism of the South Karkonosze and Leszczyniec units and resulted in their tectonic uplift. The D2 extensional event involved ESE-directed collapse on the eastern margin of the block related to WNW-ESE extension. The progressive extension promoted the uplift and exhumation of the lowermost Izera-Karkonosze unit and emplacement of the Karkonosze pluton. The ductile shearing in the metamorphic complexes was accompanied by brittle faulting at a higher intersection level to the east. The normal faults controlled the subsidence of the Intra-Sudetic basin.