Rheological and Mechanical Characteristics of an Exhumed Caledonian Shear Zone, NW Scotland
Alexander Lusk, University of Southern California
John Platt, University of Southern California
The Scandian Thrust Zone (STZ), located in NW Scotland, formed as a result of the closing of the Iapetus Ocean and docking of various terranes (Scandian phase of the Caledonian Orogeny, ca. 445-420 Ma). As defined here, the STZ comprises three major west-directed foreland-propagating faults; from east to west these are the Naver, Ben Hope (BHT), and Moine Thrusts (MT). Presently, the north-south striking STZ is exposed for >200 km along strike, and Scandian deformation can be traced up to 40 km eastward from the MT towards the hinterland. The thrust system is thought to have been exhumed while still active, resulting in the exposure of deep structural levels of the STZ.
We present temperature and stress data across the Scandian Thrust Zone from two separate transects, each spanning from the footwall of the MT to the hanging wall of the BHT, taken parallel to the direction of thrust transport (WNW). In addition, we have identified distinct microstructural domains, differentiated based on quartz deformation mechanism, rheological behavior, and lithology. We use the empirically derived piezometer for the dynamically recrystallized grain size of quartz to calculate the magnitude of differential stress across the transects. Stresses generally decrease eastward and structurally up from the MT, where ultramylonites have an average grain size of 14.7±5.8 μm, corresponding to a maximum differential stress of 60 +33/-10 MPa, to a minimum differential stress of <10 MPa at the top of the Moine Nappe. Quartz grain sizes ≤30 μm extend to a structural distance of 300 m above the MT, where quartz recrystallizes by both bulge nucleation (BLG) and subgrain rotation (SGR). At grain sizes >30 μm (structural distance >300 m above the MT) quartz recrystallization is dominated by SGR as well as grain boundary migration. Temperatures of deformation are calculated based on the titanium-in-quartz thermometer (TitaniQ). Titanium content in dynamically recrystallized quartz, although low, reveals a general trend of increasing eastward and structurally up from the MT, indicating an increase in deformation temperature. These data suggest that there is an inverted thermal gradient preserved not only between separate thrust sheets but also within the Moine Nappe. Microstructural characteristics and styles of recrystallization further support the temperature and stress data.