10 km Minimum throw along the West Bohemian shear zone: Evidence for dramatic crustal thickening and high topography in the Bohemian Massif (European Variscides)

Abstract

The West Bohemian shear zone (WBSZ) forms a steep collapse structure along which east-side- down normal movements led to the juxtaposition of the relatively cold Cadomian basement of the Tepla-Bar- randian unit against high grade Moldanubian rocks. Syn- kinematic plutons straddle the WBSZ. The Mutenin plu- ton intruded into Moldanubian crust at a depth of 23±4 km as derived by using Al-in-hornblende barome- try. The Tepla-Barrandian Babylon pluton intruded at <12 km depth as indicated by phengite barometry and petrogenetic considerations. Both emplacement depths, together with mineral cooling ages, result in a minimum vertical displacement of 10 km between 340 and 320 Ma. This large throw could be explained by over- thickened crust that was weakened from below. The al- kaline signature of the Mutenin diorite indicates that mantle melting was important to thermally weaken the crust at 340 Ma. The cold Tepla-Barrandian upper crust sank into its weak, partly molten Moldanubian substra- tum, resulting in elevator-style movements, not only along the WBSZ, but also along the Hoher Bogen and Central Bohemian shear zone. All these ductile normal shear zones were active simultaneously during the Low- er Carboniferous and dip steeply towards the Tepla-Bar- randian unit that probably formed a highly elevated pla- teau at this time.