Control of Cenozoic intra-plate volcanic phenomena by healed boundaries of ancient micro-continents distinguished by different orientations of seismic anisotropy of mantle lithosphere

Category Geophysic
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Babuska, Vladislav; Plomerova, Jaroslava
Holding Date 16 September 2008

While in the French Massif Central (MC) seismic tomography revealed a mantle plume (Granet et al., PEPI 1995), in the western Bohemian Massif (BM) we did not find a similar plume-like body down to depth of 250 km (Plomerova et al., GJI 2007). Though probably of different origin (mantle plume vs. asthenosphere upwelling), the Cenozoic volcanism in both regions finds major conduits of magmas to the surface along mantle boundaries of ancient plates, which we map by lateral variations of seismic anisotropy of the mantle lithosphere determined from body-wave anisotropic parameters (spatial variations of P-wave delay times and the shear-wave splitting). A suture in the deep lithosphere limits the thick lithosphere of the Limousin (western MC) characterized by a consistent anisotropic pattern with high velocities dipping to the west.
The eastern MC comprises two different domains with thinned lithosphere and the high velocities dipping systematically to the east. The suture between the western and eastern MC parallels the major crustal boundary, the late Variscan Sillon Houiller (SH) transfer fault in the south and the Tauve-Aigueperse fault (TA) in the north. An offset of the crustal and mantle parts of the same suture indicates a westward decollement of the rigid upper crust from the mantle lithosphere. We suggest that the mantle suture, hidden beneath an allochthonous crust, was reactivated during the Cenozoic extension of the weakened and thinned lithosphere in the south of the eastern MC and predestined a space for the major volcanism, Mont Dore and Cantal (Babuska et al., Tectonics 2002).
Three major mantle domains of the western BM, characterised by different orientations of large-scale olivine fabric, correspond to the major tectonic units - Saxothuringian (ST), Moldanubian (MD) and the Teplá-Barrandian (TB). The three mantle domains most probably represent the originally separated micro-plates assembled during the Variscan orogeny. The Eger (Ohøe) Rift (ER), an easternmost segment of the European Cenozoic rift system, developed above the ST-TBU mantle boundary. We suggest that locations of the Quaternary volcanoes, crustal earthquake swarms and escape centers of exhalations of CO2 and 3He of mantle origin are predestined by a "triple junction" of three mantle lithospheres. Cenozoic extension reactivated the junctions and thinned the crust and mantle lithosphere. The boundaries (transitions) of three mantle domains provided open pathways for Quaternary volcanism and the ascent of 3He- and CO2-rich fluids released from the asthenosphere (Babuska et al., J.Geodyn., 2007).