Rb-Sr mineral ages have often been used as time-markers in the thermal (cooling) history of metamorphic terrains. However, further extrapolation of the data to the timing of tectonic processes is hindered by assumptions about how the thermal regime responds to deformation. Dating the generation of white mica strain fabrics is a powerful technique for constraining the timing of tectonic processes directly. Since white micas commonly define deformation fabrics in medium-grade metamorphic rocks, their ages can be directly related to structural geometries with regional context. This technique has been applied to major shear zones in the Alps and Himalaya and has distinguished shear events with characteristic geometries and regional significance to a resolution of ~1Ma.
The requirements for successful Rb-Sr shear fabric dating are:
i) Crystallisation at lower temperatures than the effective diffusion (closure) temperature, in this case ~500-550°C. This avoids the effects of post-shear diffusive resetting during cooling.
ii) Thorough replacement or recrystallisation of earlier white mica generations. This is assessed by detailed mineral chemistry characterisation, since mica chemistry is sensistive to P-T conditions and will reflect the change in prevailing conditions between microstructurally-distinguished fabric-forming events.
iii) The initial 87Sr/86Sr must be estimated from a low-Rb/Sr phase that represents the Sr reservoir with which the micas equilibrated during recrystallisation or growth. Calcite and feldspar are typically used.
iv) An understanding of the tectonic significance of fabrics with specific orientations and kinematics, derived from detailed field structural studies.
The Entrelor Shear Zone of the Western Italian Alps is a layer-contractional structure that carries basement and cover of the stacked European continental margin, as well as previously overthrust oceanic units, back towards the east. It was active at greenschist facies conditions and is geometrically linked with a broad zone of extensional shear at the same crustal levels that may have exhumed eclogites. Detailed structural analysis reveals the development of pervasive mylonitic fabrics in the Entrelor thrust zone, but not all the micas crystallised during this stage. Some samples contain remnant micas from an earlier fabric generation whose chemistry reflects slightly higher pressures. White mica separates from samples dominated (>85%) by the later generation yield Rb-Sr ages at 34±1Ma. The samples with an appreciable proportion of earlier micas yield ages of 35-37 Ma. Crystal chemistry is less well-defined in the extensional shear zone, but white micas consistently yield Rb-Sr ages at 38±1Ma. The two shear zones were clearly not contemporaneous; an early fabric generated in the extensional event at ~38Ma was reworked by the backthrusting at ~34Ma.
These results provide important new constraints on an orogenic segment where the kinematics of shear systems are crucial to understanding geodynamic processes, but where timing cannot always be resolved by field relations. A new study in the Valtournenche area is further constraining the role and timing of different shear systems in thickening and extension of the Alpine crust.
The Zanskar Shear Zone of NW India is part of the South Tibetan Detachment System, a zone of normal shear that on the basis of field relations is considered contemporaneous with forethrusting, high-grade metamorphism in the footwall and intrusion of crustal melt granites. Precise ages of shear activity and thermal history in the footwall are required to assess thermo-tectonic models of the crust in this orogen. Available data from chlorite-grade shear zone rocks suggest that white mica crystallised during deformation around 22Ma. This contrasts with white mica cooling ages in the deeper, hotter parts of the footwall at 18Ma. Biotites from two footwall sections with contrasting metamorphic grade yield Rb-Sr ages of ~15Ma, indicating a common cooling history 3my after normal shear. New data from low-grade mylonitic rocks in the shear zone will place further constraints on the timing of normal faulting in two key crustal sections, while data from higher grade footwall rocks will constrain the cooling rates engendered by denudation on the Zanskar Shear Zone. They may also yield important information regarding the timing of peak metamorphic temperatures recorded by mineral assemblages, which may significantly pre-date the Himlayan orogeny itself.
Carefully selected specimens from well-characterised shear zones can be dated by Rb-Sr in white micas to yield direct age constraints on tectonic processes, which in favourable circumstances can be resolved to the limit of analytical precision of the technique.