CONJUGATE SHEAR ZONES IN THE SOUTHERN BOHEMIAN MASSIF (AUSTRIA) - IMPLICATIONS FOR VARISCAN AND ALPINE TECTONOTHERMAL ACTIVITY

Mylonitic fabrics developed within conjugate wrench ductile shear and fault systems in the Southern Bohemian Massif display both dextral (NW -SE-trending systems) and sinistral (NE-SW-trending systems) shear sen ses. Contrasting temperature conditions of deformation can be observed in the different shear zones. Temperatures above 650 degrees C were r eached in the Pfahl shear zone. Shearing under greenschist facies cond itions took place in the Rodl and Danube shear zones. Brittle deformat ion dominated in the Vitis and Diendorf shear zones. Ar-40/Ar-39 datin g of various size fractions of muscovite formed and/or deformed during mylonitization yield ages of ca. 287 Ma (the NW-SE-trending system) a nd ca. 288-281 Ma (the NE-SW-trending system). The shear zones are int erpreted as a late Variscan conjugate system. Ar-40/Ar-39 age spectra of fine-grained newly grown sericite or rejuvenation of ages give evid ence for post-Variscan reactivation of the shear zones. Brittle deform ation within the shear zones was probably associated with maintenance of very high fluid pressure during Variscan deformation. Foreland defo rmation during the Alpine orogeny also played a significant role, lead ing not only to the development of the Ceske Budejovice Graben but als o to local reactivation of the shear zones at higher crustal levels.