The presumed Teplá-Barrandian/Moldanubian terrane boundary in the Orlica Mountains (Sudetes, Bohemian Massif): structural and petrological characteristics

The Sudetes in southwestern Poland and northern Bohemia expose a Palaeozoic collage of the northeastermost extremity of the Variscan belt. One of presumed terrane boundaries occurs in the Orlica Mts, along the contact between the phyllite-amphibolite complex (Neoproterozoic?) of the Nové Město unit of probable Teplá-Barrandian affinities in the SW and the orthogneiss (Early Ordovician protolith) and mica-schist (Neoproterozoic?) complexes of the Orlica-Śnieżnik unit in the NE, most likely representing the Moldanubian terrane. The synmetamorphic structural evolution of both adjacent units comprised four deformation events. The Nové Město rocks have recorded an early stage of ductile, top-to-the SE thrusting (D1), which must have resulted in an early juxtaposition of the both terranes within a nappe pile. The original overthrust contact was subsequently modified by a dextral shear-dominated event (D2), which produced a 1-2 km wide shear zone at the boundary of both units, in which earlier fabric elements were overprinted and mostly obliterated. This deformation brought the lower-grade Nové Město rocks into present-day strike-slip contact with those of the regionally uplifted, higher-grade Orlica-Śnieżnik unit. Postdating this juxtaposition are E-W trending folds F3, which affect both the adjacent units. The contact shear zone also contains record of a late, top-to-the SW, semi-brittle, normal-slip displacement (D4), downthrowing the Nové Město unit. The peak, amphibolite facies metamorphic conditions were attained in the Orlica-Śnieżnik unit and in the contact shear zone prior to the D2 event. They corresponded to maximum temperatures and pressures of ca. 600 °C and 10 kbar, respectively. The D1 thrusting event in the phyllites of the Nové Město unit took place under greenschist facies conditions (ca. 350 °C and 4.5 kbar) and its effects mostly survived the later amphibolite-facies metamorphism. These structural relationships are equivalent to those observed elsewhere in the Bohemian Massif on the Teplá-Barrandian/Moldanubian terrane boundary. The characteristic features of this boundary are a high metamorphic gradient but lack of metamorphic inversion; the occurrence of mid-Carboniferous stitching plutons; and the importance of mid-Carboniferous deformation that resulted in significant downwards movement of the low-grade Teplá-Barrandian terrane relative to the uplifted high-grade and hot lower to middle crust of the Moldanubian terrane. The downthrow of the Teplá-Barrandian occurred on ductile shear zones showing either down-dip-slip kinematics or that of transfer strike-slip, as was the case with the Orlica Mts., both kinematics being associated within the same, regional-scale, linked shear zone extensional system. The extensional crustal collapse must have been preceded by a SE-directed crustal stacking achieved through overthrust emplacement of the Teplá-Barrandian on top of the Moldanubian terrane.