MAJOR STRIKE-SLIP FAULTS IN THE VARISCIDES: KINEMATICS AND THERMAL EFFECTS

Important Variscan strike-slip faults parallel the SW margin of Baltica, while others follow the structural trend of the orogen. Dextral transverse faults compensate for NW/SE collisional shortening between the Avalonian part of Laurussia, the Armorican microplates and Gondwana, which can be shown to amount to c. 2.000 km. Strike-slip is partitioned into the Thor suture, the Elbe-Odra, the Intrasudetic and the Bristol Channel-Bray fault zones. In the segment between the Elba-Odra and Intrasudetic fault zones, the Variscan terranes are rotated clockwise through c. 90°, thus forming the Bohemian Arc. The Bristol Channel-Bray fault zones excises, in SW-England, the broad clastic shelf at the southern margin of Avalonia, which constitutes the main part of the Rheno-Hercynian belt in central Europe. Orogen-parallel strike-slip faults started to operate already around 380 Ma, when the Variscan microplates were still situated S of Baltica. They have severely overprinted the original suture zones, and partly effected important palaeogeographic breaks (e.g., S margin of the Mid-German Crystalline High). Many Variscan strike-slip faults, irrespective of their orientation, have served as conduits for melts and heat during and after continental collision. Increasing evidence suggests several thermal peaks between c. 350 and 270 Ma, which are documented in mafic and felsic magmatism and also in widespread LP/HT metamorphic rocks. Metamorphism was probably caused by the ascent, along fault zones, of overheated melts created by basaltic underplating. Since part of these activities clearly predate crustal stackingin the relevant areas, they cannot result from Variscan collision. Instead, they probably relate to lithospheric thinning at the western tip of the opening Tethys ocean, when Gondwana moved westwards along EW-trending faults.

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Franke (2000) Geol.Soc.London Spec.Publ. 179: 35-62

Franke & Zelazniewicz (2000) Geol.Soc.London Spec.Publ. 179: 63-86

Franke & Zelazniewicz (2002) Geol.Soc.Spec.Publ. 201: 279-294

Franke et al. (2011) Int.J.Earth Sci. 100(4): 753 ff.