VORTICITY OF FLOW ALONG THE MOINE THRUST ZONE FROM WHITEN HEAD TO NORTHERN ASSYNT, NORTHWEST SCOTLAND

Many early studies assumed that flow in ductile shear zones is homogenous both across and along strike, and is generally dominated by simple shear. These views were often propagated by studies focused on smaller, mostly meso-scale shear zones. In the last two decades, a number of techniques have emerged for quantifying vorticity (i.e. the relative contributions of pure and simple shear) during ductile deformation. Here, we have applied vorticity analysis techniques to samples collected from the Moine thrust zone in northwest Scotland, in order to test the validity of these aforementioned assumptions.

Numerous vorticity analysis techniques are available, depending on the lithologic and microstructural characteristics of a given sample, including: Method 1A: the rigid grain orientation method of Wallis et al. (1993), Method 1B: the rigid grain net method of Jessup et al. (2007), Method 2: the oblique recrystallized grain/quartz C-axis fabric method of Wallis (1992, 1995), and Method 3: the Rxz strain ratio/quartz c-axis fabric method of Wallis (op cite).

During the summer field seasons of 2006 and 2007, ~250 samples were collected in the Moine thrust zone and overlying Moine nappe from the north Sutherland coast south to southern Assynt. Preliminary rigid grain vorticity analyses reveal mean kinematic voriticty number (Wm) values of 0.63-0.79 (40-55% pure shear) using Method 1A and 0.63-0.88 (30-55% pure shear) using Method 1B. All analyses to date were performed on sections cut parallel to lineation (often assumed to represent the transport direction) and perpendicular to foliation.

Our analyses indicate that general shear (roughly equal contributions of pure and simple shear) is dominant along the Moine thrust zone from the north coast to northern Assynt. More notable is the along strike and lithologic variation of observed Wm values, which may indicate that in large, crustal-scale shear zones, simple and pure shear components of flow are actually partitioned into discrete regions and/or into rheologically distinct horizons. This suggests that assumptions of flow homogeneity and simple shear dominance in large shear zones are likely not valid, especially on tectonic scales.