Kinematic data from the internal zones of the Western Alps indicate both top-to-SE and top-to-NW shearing during syn-kinematic greenschist facies recrystallisation. Rb/Sr data from white micas from different kinematic domains record a range of ages that do not represent closure through a single thermal event but reflect the variable timing of syn-kinematic mica recrystallisation at temperatures between 300-450°C. The data indicate an initial phase of accretion and foreland-directed thrusting at c. 60 Ma followed by almost complete reworking of thrust-related deformation by SE-directed shearing. This deformation is localised within oceanic units of the Combin Zone and the base of the overlying Austroalpine basement and forms a regional scale shear zone that can be traced for almost 50 km perpendicular to strike. The timing of deformation in this shear zone spans 9 Ma from 45-36 Ma. The SE-directed shear leads to local structures that cut upwards in the transport direction with respect to tectonic stratigraphy, and such structures have been interpreted in the past as backthrusts in response to ongoing Alpine convergence. However, on a regional scale the top-to-SE deformation is related to crustal extension, not shortening, and is coincident with exhumation of eclogites in its footwall. During this extension phase, deformation within the shear zone migrated both spatially and temporally giving rise to domains of older shear zone fabrics intercalated with zones of localised reworking. Top-NW kinematics preserved within the Combin Zone show a range of ages. The oldest (48 Ma) may reflect the final stages of emplacement of Austroalpine Units above Piemonte oceanic rocks prior to the onset of extension. However, much of the top-to-NW deformation took place over the period of extension and may reflect either continuing or episodic convergence or tectonic thinning of the shear zone. 40 Ar/ 39 Ar data from the region are complicated due to the widespread occurrence of excess 40 Ar in eclogite facies micas and partial Ar loss during Alpine heating. Reliable ages from both eclogite and greenschist facies micas indicate cooling ages in different tectonic units of between 32-40 Ma. These ages are slightly younger than Rb/Sr deformation ages and suggest that cooling below c.350°C occurred after juxtaposition of the units by SE-directed extensional deformation.Our data indicate a complex kinematic history involving both crustal shortening and extension within the internal zones of the Alpine orogen. To constrain the palaeogeographic and geodynamic evolution of the Alps

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Rb-Sr dating of sub-closure temperature, syn-deformationly crystallized white micas from the Moine mylonites (Knockan and Dundonnell), has yielded ages which vary from 437 Ma to 408 Ma. Morphological and major element analyses of the micas indicate that all the micas within the analysed samples were (re)crystallized during the Moine Thrust shearing. The ages yielded are therefore interpreted as marking the end of crystal plastic deformation associated with shearing on the Moine Thrust. The variation in ages between samples is significantly greater than the individual analytical errors and may be a product of strain localization within the shear zone. Alternatively it may indicate flow of strontium-rich fluids derived from outside the shear zone implying that feldspars may act as potentially unreliable initial isotopic reservoirs even in very high strain samples within the greenschist-facies shear zones. However, the Rb-Sr age are internally consistent and compatible broadly with existing geological and geochronological data for the region suggesting that even if the feldspars have not acted as perfect initial isotopic reservoirs the resulting errors were minimal. The general cessation of ductile deformation at c. 430 Ma probably dates the general initiation of imbrication within footwall to the Moine Thrust. The age of 430 Ma is therefore likely to be a time of significant brittle displacement on the underlying Ben More Thrust.K-Ar dating of the (re)crystallized Moine mylonite micas yielded anomalously old ages in comparison to Rb-Sr values as a result of incorporation of excess Ar during shearing. The degree of incorporated excess Ar decreases away from the base of the shear zone into the hanging wall, becoming indiscernible at c. 1.5 km structurally above the base of the shear zone. The distance (c. 1.5 km) may represent the distance that fluid can have flowed out of the shear zone during deformation.

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