Constraints on early sinistral displacements along the Great Glen Fault Zone, Scotland: Structural setting, U-Pb geochronology and emplacement of the syn-tectonic Clunes tonalite

Abstract: Current tectonic models emphasize the role of crustal-scale transcurrent faults in controlling the ascent and emplacement of plutons in orogenic belts. The Clunes Tonalite crops out adjacent to the transcurrent Great Glen Fault Zone within the Ordovician-Silurian Caledonian orogenic belt of Scotland. The tonalite is characterized by a strong fabric defined by aligned plagioclase, hornblende and biotite that is interpreted to be a magmatic state deformation fabric formed during emplacement. Field and microstructural evidence derived from the tonalite and its Moine Supergroup country rocks supports an emplacement model that links intrusion directly to early sinistral displacements along the Great Glen Fault Zone. U-Pb zircon geochronology yields a crystallization age for the Clunes Tonalite of 427.8 +/- 1.9 Ma. The age is thus interpreted to date early sinistral displacements along the Great Glen Fault Zone and is identical within error to that of the Ratagain granite emplaced during sinistral displacement along the Strathconon Fault to the NW.

Keywords: Clunes Tonalite, Great Glen Fault Zone, U-Pb, syn-tectonic processes, emplacement.

A detailed knowledge of the displacement history of major fault zones is an essential prerequisite for development of regional models for crustal deformation. There are various ways in which it is possible to place temporal constraints on fault displacements. They may be provided by the ages of pre-, syn- and post-tectonic sedimentary sequences and displaced marker units. In many cases, however, these provide inadequate resolution (>tens of millions of years). Many fault zones, particularly in basement terrains, are defined by broad mylonite belts. Deformation fabrics developed in greenschistamphibolite facies mylonites can be dated directly using isotopic methods. Synkinematic crystallization can be dated provided the closure temperature for a particular isotopic system within a specific mineral phase is higher than the synkinematic temperature (e.g. Getty & Gromet 1992). Many major faults are dominated at the present erosion surface by cataclasites developed at high levels in the crust; such rocks cannot as yet be reliably dated using isotopic methods. Consequently, establishing the absolute timing of the displacement history of such a fault can be problematic. Faults may also act as fundamental controls on the ascent and emplacement of magmas (e.g. Hutton 1988; Hutton & Reavy 1992; D'Lemos et al. 1992), and displacements can therefore be constrained indirectly by the isotopic dating of such syn-tectonic igneous rocks.

In this paper, we place constraints on the timing of early displacements along the Great Glen Fault Zone, a regional-- scale, reactivated fault that cuts across the Ordovician-Silurian Caledonian orogenic belt of Scotland (Fig. 1). This is an example of a major fault zone dominated by deformation fabrics developed in the region of the frictional-viscous transition (c. 9-15 km depth; Stewart 1997; Stewart et al. 1997, 1999, 2000; Holdsworth et al. 2001). Mylonitic rocks are rare and the protoliths and fault rocks are dominated by quartzofeldspathic mineralogies that are not suitable for direct isotopic dating. Existing stratigraphical and isotopic constraints on the timing of early displacements along the fault zone are poor. We present the results of the structural analysis and precise U-Pb zircon dating of the Clunes Tonalite that constrain the age of early sinistral displacements along the Great Glen Fault Zone, and consider the larger-scale implications of this data.

Regional geology

Seismic reflection studies show that the Great Glen Fault Zone is coincident with a subvertical structure which extends to at least 40 km depth (Hall et al. 1984) and the fault may therefore have some expression in the upper mantle. There is widespread geological and geophysical evidence for several phases of displacement and fault-related deformation affecting a broad range of strata in onshore and offshore regions. The fault zone comprises a c. 3 km wide belt of fault rocks ranging from rare mylonite and quartz blastomylonite to common cataclasite, hydrated cataclasite and phyllonite (Stewart 1997; Stewart et al. 1997, 1999, 2000; Holdsworth et al. 2001). On a regional scale, the fault zone incorporates fault-bounded units of rocks deformed at different crustal levels, a result of differential displacement and exhumation across the structure. A c. 300 m wide zone in the centre of the fault zone is thought to contain the principal displacement surfaces and separates the two regional crustal blocks that define the wall rocks to the Great Glen Fault Zone (Stewart et al. 1999). Kinematic indicators for main phase deformation fabrics consistently demonstrate a sinistral sense of displacement with a minor southeasterly component of downthrow. Microstructural analysis of these fabrics suggests that deformation occurred adjacent to the frictional-viscous transition zone, corresponding to depths of about 9-16 km (Stewart et al. 1999, 2000).