Prekambryjskie struktury Bornholmu i ich odniesienia do regionu Kaszub

Zbigniew Cymerman


Precambrian structures of Bornholm and their relationships to the Kaszuby region (N Poland).
S u m m a r y. This paper summarizes new results of structural analysis of the Precambrian rocks of Bornholm Island and presents a working hypothesis on a tectonic setting of the Proterozoic rocks from the SW part of the Baltic Shield. The D1 deformation is characterized by the development of the S1 foliation and L1 mineral extensional lineation. The gneissic foliation (S1), which is mainly N–dipping with a moderate to gentle plunging L1 lineation, appears to be thrust–related in ductile shear zones. The W–E-striking shear zones are separated by domains of less strained rocks with a parallel oriented S0 magmatic and S1 metamorphic foliation. The ductile non-coaxial fabric formed during the D1 deformation is consistent with a top-to-the-S shearing. Field data suggest that the migmatization and D1 deformation occurred simultaneously during this shearing. The partial melts facilitated the tectonic transport, acting as a thin intracrustal decollement. A model of serial ductile thrusting along listric, frontal ramps is used to explain the deformation history of the Bornholm heterogeneous shear zone system. This system of shear zones was locally overprinted by the younger extensional structures developed during the D2 deformation. In the absence of reliable geochronological data, it was assumed that the crystalline rocks of Bornholm and Kaszuby and Pomerania regions (northern Poland) were subjected to similar ductile thrusting during the Gothian orogeny ca. 1.5 Ga. All data (BABEL seismic line A, structural and kinematics) suggest the existence of the relict suture zone with an accretionary active plate margin in the northern part of Poland and Bornholm.

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