Terra Nova, 22, 274–282, 2010 Abstract Fission track and (U–Th)/He thermochronology are useful tools to reconstruct distinct thermal events along fault zones, provided that heat advection brings about substantial track annealing and resetting. In a case study, apatite fission track and (U–Th)/He ages from drill cores transecting the Lavanttal fault system (Eastern Alps) revealed rejuvenation of ages in fault‐related rocks. Heat transfer via hydrothermal fluids was sufficient to partly reset apatite fission track ages producing a younger age population, reduce track lengths slightly and rejuvenate (U–Th)/He ages in fault cores. Hydrothermal fluid activity can be bracketed between the age of the slightly reset fission tracks and the (U–Th)/He ages, between Late Miocene and Early Pliocene times. Additional thermal history modelling of two host rock samples suggests that the young apatite fission track population slightly coincides with a last exhumation pulse of the adjacent Koralm Complex during Late Miocene times.

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The Lavanttal Fault Zone (LFZ) is generally considered to be related to Miocene orogen-parallel escape tectonics in the Eastern Alps. By applying thermochronological methods with retention temperatures ranging from *450 to *40°C we have investigated the thermochronological evolution of the LFZ and the adjacent Koralm Complex (Eastern Alps). 40 Ar/ 39 Ar dating on white mica and zircon fission track (ZFT) thermochronology were carried out on host rocks (HRs) and fault-related rocks (cataclasites and fault gouges) directly adjacent to the unfaulted protolith. These data are interpreted together with recently published apatite fission track (AFT) and apatite (U-Th)/He ages. Sample material was taken from three drill cores transecting the LFZ. Ar release spectra in cataclastic shear zones partly show strongly rejuvenated incremental ages, indicating lattice distortion during cataclastic shearing or hydrothermal alteration. Integrated plateau ages from fault rocks (*76 Ma) are in parts slightly younger than plateau ages from HRs ([80 Ma). Incremental ages from fault rock samples are in part highly reduced (*43 Ma). ZFT ages within fault gouges (*65 Ma) are slightly reduced compared to the ages from HRs, and fission tracks show reduced lengths. Combining these results with AFT and apatite (U-Th)/He ages from fault rocks of the same fault zone allows the recognition of distinct faulting events along the LFZ from Miocene to Pliocene times. Contemporaneous with this faulting, the Koralm Complex experienced accelerated cooling in Late Miocene times. Late-Cretaceous to Palaeogene movement on the LFZ cannot be clearly proven. 40 Ar/ 39 Ar muscovite and ZFT ages were probably partly thermally affected along the LFZ during Miocene times.

This study is concerned with a newly recognized structure at the southwestern border of the Tauern Window and the neighbouring Austroalpine basement rocks, the Lappach Structure. Structural and isotopic investigations show that this structure is the result of sinistral transpression and backthrusting along the Austroalpine–Penninic contact. Deformation partitioning and differential exhumation of crustal wedges is documented by a succession of ductile and brittle deformation stages. Two stages are distinguished: (1) up-doming of Penninic units and associated advective heat transfer caused a strong temperature variation with the highest temperatures in central portions of the Tauern Window. Coeval transpression with distributed sinistral shear formed high-temperature, partly annealed fabrics in central portions and lower-temperature fabrics with strong crystallographic preferred orientation of quartz along the Tauern Window margin. Southward decrease of temperatures was matched by increases in stress and deformation intensity. (2) Progressive cooling was accompanied by shear localization, deformation partitioning and fluid infiltration. Overall sinistral shear resolved in a discrete strike slip–fault and south vergent folds with associated thrusts defining a backthrust zone along the southern Tauern margin. Southwards extrusion disturbed previously established palaeo-isogrades and juxtaposed rocks from greater depths against lower-grade metamorphic units. Fluids penetrated faults, reduced shear strength and contributed to shear localization.

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