Deformation temperatures, vorticity of flow, and strain in the Moine thrust zone and Moine nappe: Reassessing the tectonic evolution of the Scandian forelandehinterland transition zone J. Ryan Thigpen a, * , Richard D. Law a , Geoffrey E. Lloyd b , Summer J. Brown a a Department of Geosciences, Virginia Tech, 4044 Derring Hall, Blacksburg, VA 24061, USA b School of Earth and Environment, The University, Leeds LS2 9JT, UK article info Article history: Received 18 May 2009 Received in revised form 21 April 2010 Accepted 2 May 2010 Available online 10 May 2010 Keywords: Moine Scotland Vorticity Strain Assynt Deformation temperatures abstract Examination of deformation temperature, 3-D strain and flow vorticity (W m ) in mylonites from the Assynt-Loch More region of the Moine thrust zone (MTZ) allows quantitative kinematic and thermal characterization of shearing at the base of the Scandian (435e425 Ma) orogenic wedge. Quartz micro- structures and c-axis fabric opening angles from mylonites in the immediate hangingwall and footwall to the Moine thrust suggest that deformation temperatures are highest in the eastern part of the Assynt region (including mylonites close to alkaline intrusive complexes) and decrease along strike both to the north (Stack of Glencoul e Loch More) and to the south (Knockan). Quartz c- and a-axis fabrics, together with limited 3-D strain data, indicate that deformation in both the footwall and hangingwall mylonites dominantly involved plane strain to general flattening, although domains of more constrictional flow are identified adjacent to thrust transport-parallel lineaments in the overlying Moine nappe. Rigid grain analyses indicate a remarkably constant flow vorticity for tens of kilometers along orogenic strike (40e60% pure shear) in both the hangingwall and footwall mylonites. Integration with previously reported strain and vorticity estimates from the Assynt region indicates a 50e75% sub-vertical short- ening perpendicular to gently dipping foliation, that must have been accommodated by either volume loss or extrusion of material toward the synorogenic topographic surface. Extrusion implies a causal link between upper and lower crustal processes, with significant implications for the kinematic and geometric evolution of the Scandian wedge. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction Recent integration of vorticity and strain studies indicates that ductile deformation in the hinterland regions of collisional orogenic systems (High Himalaya, Scottish Caledonides, European Alps, Greek Hellenides, Appalachian Blue Ridge) involves a significant component of pure shear deformation (Kassem and Ring, 2004; Law et al., 2004; Jessup et al., 2006; Bailey et al., 2007; Larson and Godin, 2009; Law, 2010; Thigpen et al., 2010; Xypolias et al., 2010). Kine- matic and mechanical models that attempt to explain thrust kine- matics and wedge dynamics (Chapple, 1978; Davis et al., 1983; Dahlen et al., 1984; Platt, 1986; Dahlen, 1990; Holdsworth and Grant, 1990; Willett et al., 1993) must take these pure shear components into account. Major theoretical implications of a significant pure shear contribution to ductile deformation include: 1) vertical ductile thinning contributes to synkinematic exhumation of progressively lower portions of the orogenic wedge (Feehan and Brandon, 1999); 2) under approximately plane strain isochoric conditions, vertical ductile thinning leads to significant transport- parallel lengthening of thrust sheets, driving ductile extrusion toward the synorogenic topographic surface. In the former case, vertical ductile thinning acting in conjunction with erosion and normal faulting generates rapid synkinematic exhumation which may, in turn, explain preservation of inverted metamorphic isograds observed in the structurally lower levels of many orogenic wedges (e.g. Stephenson et al., 2000; Vannay and Grasemann, 2001; Kidder and Ducea, 2006) by vertical ductile thinning of the overlying nappe pile (Ring and Kassem, 2007). Ductile extrusion toward the syn- orogenic topographic surface has kinematic, geometric, and strain rate implications for deformation occurring at upper crustal levels (Williams and Jiang, 2006) and may result in kinematic and dynamic linkage between lower crustal ductile processes and upper crustal brittle processes (Northrup, 1996). The majority of previously published vorticity studies have focused on identifying variation in vorticity and 3-D strain with change in structural depth along orogen normal sampling * Corresponding author. Tel.: þ1 5402319740; fax: þ1 5402313386. E-mail address: thigpe05@vt.edu (J.R. Thigpen). Contents lists available at ScienceDirect Journal of Structural Geology journal homepage: www.elsevier.com/locate/jsg 0191-8141/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jsg.2010.05.001 Journal of Structural Geology 32 (2010) 920e940