Mylonites and pseudotachylytes are common rocks within fault zones. Whereas in general the kinematics and deformation mechanisms of fault zones are well understood, their timing still remains poorly constrained by geochronological data. This is because the deformationally induced re-equilibration of isotopic systems is often incomplete and the extraction of strictly synkinematically grown minerals is very difficult to achieve.
We present geochronological data from mylonites and pseudotachylytes obtained by refined Rb/Sr and 40Ar/39Ar isotopic dating techniques. Rb/Sr and 40Ar/39Ar are the only available geochronological systems which meet the following prerequisites for direct dating of deformation:
a) Isotopic (re-)equilibration by dynamic recrystallization of minerals; this is probably fullfilled for both the Rb/Sr and K/Ar (40Ar/39Ar) systems since both have rather mobile parent-daughter elements; b) Substantial geochemical parent-daughter fractionation is achieved for certain minerals (e.g. micas; isochron approach); c) Sufficiently high concentrations of these elements allow the analysis of tiny sample volumes representing very thin recrystallized layers.
Prerequisite (a) is best attained on the mm-scale. Therefore, the material analysed was prepared directly from polished thick-sections (<100 mm) by drilling/cutting under the microscope. The typical weight of the analysed material is 20-100 mg. Minerals newly crystallized during deformation (e.g. between stretched minerals) can also be separated by this method. This necessary reduction in the amount of analysed material requires 1-10 ng of Sr to be precisely analysed and a significant blank reduction to <30 pg Sr. Sr TIMS analyses were done with a Ta-emitter solution (Birck, 1986) which enhances ionization of Sr significantly.
The samples derive from different faults spatially related to and partly interacting with the Periadriatic (Insubric)
fault system (Switzerland, Italy, Austria). However, their respective timing (and/or contemporaneity) has not been extensively investigated which, however, is critial for palinspastic reconstructions. Samples currently analysed were collected from the Peio, Jaufen and Canavese faults.
Preliminary Rb/Sr data from two Peio fault mylonites show practically complete isotopic re-equilibration within dynamically thoroughly recrystallized zones. Small white mica and feldspar layers extracted from very fine-grained domains yield a Rb/Sr errorchron corresponding to an age of 36.6±11.5 Ma (95% c. l., MSWD=9.1) and a two-point tie line giving 35.0±1.1 Ma (2s). A pseudotachylyte from the same fault analysed by conventional 40Ar/39Ar techniques (1.5 mm thick drill core from an injection vein lacking clasts) yields an almost plateau-like age pattern - the mean age of ~70% of 39Ar released is 36.6±0.7 Ma (95% c. l.).
A second pseudotachylyte crosscutting one of the analysed mylonites shows a more complex age pattern as expected from the occurrence of various clasts. Ages increase
regularly from 11 to c. 38-42 Ma. At the highest release temperatures, ages of ~65 Ma reflect incomplete resetting during the pseudotachylyte-forming event so that no simple age information can be inferred.
Pseudotachylytes from the other faults display significantly younger ages: One Canavese fault pseudotachylyte
(S Switzerland) yields a plateau-like 40Ar/39Ar age pattern of 16.5±1.1 Ma (95% c. l.) which is similar to that obtained from the Jaufen fault (16.0±5.5 Ma; 95% c. l.). The kinematics of the most recent movements on these faults are also similar.
The preliminary results demonstrate the effectiveness of the method for dating specific high-strain deformation events. All the analysed Peio fault samples show identical ages within error limits, suggesting a genetically linked development of both ductile mylonites and brittle pseudotachylytes. The Oligocene age of deformation obtained clearly contradicts previous Cretaceous age estimates based on limited field constraints (Werling, 1992). Dikes crosscutting the analysed mylonites provide an excellent opportunity for independent control on the deformation ages. U-Th-Pb-zircon dating of these dikes is in progress
Birck, J. L., Chem. Geol. 56, 73-83 (1986).
Werling, E., PhD thesis, ETH-Zürich, pp. 276, (1992).