Lateral displacement of crustal units relative to underlying mantle lithosphere: Example from the Bohemian Massif

5 years ago

Lateral displacement of crustal units relative to underlying mantle lithosphere: Example from the Bohemian Massif

We propose a mechanical model of deformation of the entire lithosphere of the Bohemian Massif (BM), whose core is formed by an asymmetric block of the Teplá-Barrandian (TB) unit in between the Saxothuringian (ST) and Moldanubian (MD) units. For the modelling, we have re-processed P-wave travel times recorded during the last two decades at dense networks of seismic stations installed in the BM during several passive seismic experiments. We also use previous results of anisotropic studies based on splitting of teleseismic shear waves. This allows us to refine estimates of the lithosphere thickness and delimit deep margins of the individual mantle lithosphere domains. The domains are rigid enough to preserve pre-orogenic olivine fabrics differently oriented in each of the units. Shapes and dips of the mantle boundaries, representing major zones of weakness inherited from the Variscan amalgamation of independent microplates, indicate that north-westward subductions beneath the TB unit dominated tectonic development of the core of the BM. Two mantle lithosphere domains with different fabric orientations, separated by a WSW-ENE striking shear zone, underlie the TB crust. The NW domain is the TB mantle lithosphere, while the SE domain is the MD mantle lithosphere thrust under the TB crust. Lithosphere of the north-western TB domain, compressed between early Variscan subductions of the ST continental lithosphere from the northwest and the MD continental lithosphere from the southeast, was pushed south-westward by about 50km. Though the crust of the south-westerly TB promontory is commonly attributed to the MD unit, apparently it preserves the TB mantle lithosphere. The shifted TB lithosphere provides compelling evidence in support of older views suggesting that the Zone Erbendorf-Vohenstrauss (ZEV) originally belonged to the tilted western rim of the TB unit. During the final phase of the assemblage of the BM, the rigid TB lithosphere was disrupted by the southward pushing ST lithosphere along the newly formed NW-SE striking Jáchymov Fault Zone (JFZ). This lithosphere-scale process most likely changed the tectonic regime, released subduction-related forces and started the gravity-dominated tectonics.

Publisher URL: www.sciencedirect.com/science

DOI: S1342937X17303040