Dating fault-generated pseudotachylytes: comparison of Ar-40/Ar-39 stepwise-heating, laser ablation and Rb-Sr microsampling analyses

Abstract

Three different geochronological techniques (stepwise-heating, laser-ablation 40Ar/39Ar, Rb-Sr microsampling) have been evaluated for dating fault-generated pseudotachylytes sampled along the Periadriatic Fault System (PAF) of the Alps. Because pseudotachylytes are whole-rock systems composed of melt, clast and alteration phases, chemical control from both Ar isotopes (Cl/K, Ca/K ratios) and EMPA analyses is crucial for their discrimination. When applied to stepwise-heating 40Ar/39Ar analyses, this approach yields accurate melt-related ages, even for complex age spectra. The spatial resolution of laser-ablation 40Ar/39Ar analyses is capable of contrasting melt, clast and alteration phases in situ, provided the clasts are not too fine grained, the latter of which results in integrated "mixed" ages without geological information. Elevated Cl/K and Ca/K ratios were found to be an invaluable indicator for the presence of clast admixture or inherited 40Ar. Due to incomplete isotopic resetting during frictional melting, Rb-Sr microsampling dating did not furnish geologically meaningful ages. On the basis of isotopic disequilibria among pseudotachylyte matrix phases, and independent Rb-Sr microsampling dating of cogenetic (ultra) mylonites, the concordant 40Ar/39Ar pseudotachylyte ages are interpreted as formation ages. The investigated pseudotachylytes altogether reveal a Cretaceous to Miocene history for the entire PAF, consistent with independent geological evidence. Individual faults, however, consistently reveal narrower intervals of enhanced activity lasting a few million years. Electronic supplementary material to this paper can be obtained by using the Springer LINK server at http://dx.doi.org/ 10.1007/s00410-002-0381-6