Age constraints on faulting and fault reactivation: a multi-chronological approach

Movement within the Earth's upper crust is commonly accommodated by faults or shear zones, ranging in scale from micro-displacements to regional tectonic lineaments. Since faults are active on different time scales and can be repeatedly reactivated, their displacement chronology is difficult to reconstruct. This study represents a multi-geochronological approach to unravel the evolution of an intracontinental fault zone locality along the Danube Fault, central Europe. At the investigated fault locality, ancient motion has produced a cataclastic deformation zone in which the cataclastic material was subjected to hydrothermal alteration and K-feldspar was almost completely replaced by illite and other phyllosilicates. Five different geochronological techniques (zircon Pb-evaporation, K-Ar and Rb-Sr illite, apatite fission track and fluorite (U-Th)/He) have been applied to explore the temporal fault activity. The upper time limit for initiation of faulting is constrained by the crystallization age of the primary rock type (known as Kristallgranit) at 325 +/- A 7 Ma, whereas the K-Ar and Rb-Sr ages of two illite fractions < 2 mu m (266-255 Ma) are interpreted to date fluid infiltration events during the final stage of the cataclastic deformation period. During this time, the Kristallgranit was already at or near the Earth's surface as indicated by the sedimentary record and thermal modelling results of apatite fission track data. (U-Th)/He thermochronology of two single fluorite grains from a fluorite-quartz vein within the fault zone yield Cretaceous ages that clearly postdate their Late-Variscan mineralization age. We propose that later reactivation of the fault caused loss of helium in the fluorites. This assertion is supported by geological evidence, i.e. offsets of Jurassic and Cretaceous sediments along the fault and apatite fission track thermal modelling results are consistent with the prevalence of elevated temperatures (50-80A degrees C) in the fault zone during the Cretaceous.