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_Journal of Structural Geology 32 (2010) 920-940_ Contents lists available at ScienceDirect Journal of Structural Geology journal homepage: www.elsevier.com/locate/jsg Deformation temperatures, vorticity of flow, and strain in the Moine thrust zone and Moine nappe: Reassessing the tectonic evolution of the Scandian foreland-hinterland transition zone J. Ryan Thigpen a,*, Richard D. Law a, Geoffrey E. Lloyd b, Summer J. Browna a Department of Geosciences, Virginia Tech, 4044 Derring Hall, Blacksburg, VA 24061, USA b School of Earth and Environment, The University of 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 (Wm) 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 (435-425 Ma) orogenic wedge. Quartz microstructures 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-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 (40-60° pure shear) in both the hangingwall and footwall mylonites. Integration with previously reported strain and vorticity estimates from the Assynt region indicates a 50-75° sub-vertical shortening 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. © 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). Kinematic and mechanical models that attempt to explain thrust kinematics 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; 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 synorogenic 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 transects. Many of these studies have almost exclusively related vorticity variation to either structural position or to rheologic partitioning controlled by lithologic variation (e.g., Sullivan, 2008). However no studies have, to date, examined whether significant variation of flow vorticity occurs along orogenic strike. In this paper we first present new vorticity and deformation temperature data obtained from Moine thrust zone (MTZ) samples collected along strike at the base of the Scandian orogenic wedge exposed in northwest Scotland (Fig. 1). The sampling traverse is approximately 30 km in length and extends from Loch Dionard southwards through the eastern part of the Assynt district to Knockan (Fig. 2). A complementary quartz petrofabric study allows the plane strain vs. non-plane strain and coaxial vs. non-coaxial components of the deformation to be qualitatively characterized. We then discuss the tectonic significance of a pure shear contribution to ductile flow, as well as the factors that may influence variation in flow both along and across orogenic strike. Variation in estimated flow vorticities is examined with respect to lithology, deformation temperature, 3-D strain type and structural position within the nappe stack, as well as with respect to footwall structural architecture. 2. Tectonic setting The Scandian orogenic wedge exposed in northwest Scotland (Fig. 1) is the composite result of at least three structural-thermal events, including the Knoydartian thermal event (820-730 Ma) and the Grampian (475-460 Ma) and Scandian (435-415 Ma) phases of the Caledonian orogeny. The mid-Neoproterozoic Knoydartian structural-thermal event is interpreted to result from crustal thickening, possibly preceded by extension (Vance et al., 1998; Cutts et al., 2009). Grampian orogenesis, which is attributed to an arc-continent collision, resulted in significant crustal thickening (<35 km; Friend et al., 2000). Later Scandian thrusting and associated penetrative deformation produced the present structural architecture. From west to east, three major ductile thrust sheets (Moine, Naver, Skinsdale) are variably deformed and metamorphosed and internally imbricated by Scandian ductile thrusts (Fig. 1; Holdsworth et al., 2001; Strachan et al., 2002). At the base of the ductile Scandian orogenic wedge, mylonites along the upper part of the MTZ represent the foreland-to-hinterland transition zone of the Scottish Caledonides. Structurally beneath the upper MTZ mylonites, numerous major and minor thrusts of the lower MTZ imbricate zone accommodate west-northwest directed brittle thrusting and folding of Archaean (Lewisian) basement and overlying Neoproterozoic (Torridonian) and Cambrian platform successions (Fig. 2). 3. Structural geometry of the Moine thrust zone and Moine nappe The MTZ, which accommodates Scandian (435-420 Ma) convergence and shortening along numerous thrusts that become progressively more brittle to the west, can be subdivided into two major components; the lower and upper MTZ, respectively. In the Assynt district, the lower MTZ is 10-15 km wide and contains the Sole, Glencoul and Ben More thrusts (Fig. 2; Peach et al., 1888, 1907) that accommodate brittle thrusting and folding between the Scandian foreland and the upper MTZ ductile thrusts. The Sole thrust (Fig. 2) represents the structurally lower limit of major Scandian deformation (Knipe, 1990; Mendum et al., 2009). The Sole and related thrusts are overlain by the Glencoul (20-25 km displacement) and Ben More (w28 km displacement) thrust sheets, and their associated smaller thrusts and duplexes (Peach et al., 1888, 1907; Elliott and Johnson, 1980; Fig. 2a). Along strike to the north of Assynt, the upper MTZ mylonites are floored by the Upper Arnaboll-Creag na Faoilin (UA-CNF) thrust in the southern part of the Eriboll region (Holdsworth et al., 2006, 2007; Thigpen, 2009) and the Arnaboll thrust in northern Eriboll (British Geological Survey, 2002). Although currently controversial, we suggest that the UA-CNF thrust sheet (or its lateral equivalent {s}) may extend southward along strike from Eriboll through the Loch More area into the eastern part of the Assynt district. Traced up structural section toward the east, this structure m' Ключевые слова: tectonic signicance, foliation, assynt district, searle, variation, structural depth, geological survey, parallel, mylonites, recrystallized, society london, deformation temperature, shear, data, top-to-the-west-northwest sense, plastic deformation, mtz, thigpen, variable abundance, survey, strain rock, royal society, moine thrust, plane strain, british, exhumation, strain, recrystallized optical, geologic map, thigpen journal, dahlen, eds, quartzite, strike, heat transfer, upper, mylonitic cambrian, butler, geological, mylonitic, publications, quartz, angle, wedge, pure shear, tectonics, geological society london, fabric, extrusion, oriented sub-parallel, general shear, fabric diagram, sheet, geophysical, nan, study, special publications, university, strachan, quartzofeldspathic psammite, moine nappe, thinning, knockan crag, general attening, northern assynt, mechanical model, geological society, rock, thrusting, sole thrust, recrystallized grain, earth, south, glencoul, recrystallization, salient-recess pair, general, c-axis fabric, structural, thrust, cambrian quartzite, lewisian, ebsd, holdsworth, vertical thinning, nc nc, johnson, dynamic recrystallization, northwest scotland, leading edge, geology, tectonic evolution, rigid, review, eld relationship, thesis, hydrolytic weakening, vorticity, axial ratio, structural geology, a-axis fabric, strain symmetry, deformation, thrust zone, constrictional, area, ductile thinning, opening-angle thermometer, microstructural evidence, fabric development, signicant component, integrated strain, dominantly composed, cambrian, simple shear, decompression event, krabbendam, quartz c-axis, thermal, footwall, lister, british geological, northern, c-axis, orogenic, assynt region, williams, thrust sheet, a-axis, opening angle, moine, stipp, british geological survey, vertical, location, lewisian lewisian, geological structure, cut perpendicular, stack glencoul, ductile moine, lithologic variation, nappe, point maximum, penetrative deformation, optical technique, quartz axis, cross-girdle fabric, zone, scotland, law, lateral equivalent, axis fabric, cheer, sample, optical analysis, sgr, journal structural, structural architecture, material, strain analysis, london, phyllosilicate matrix, axis, pure, northrup, thrust plane, grain, analysis, temperature, fold-and-thrust belt, volume loss, ductile, ductile deformation, crustal process, inverted metamorphism, phyllosilicate lath, loch, gbm, rotate innitely, mountain building, scandian, preferred orientation, vorticity estimate, knipe, thesis university, special, naver nappe, signicant implication, strain rate, recrystallization regime, northern scotland, metamorphism, ?ow, channel, strine, assynt, society, crystallographic fabric, optical, vorticity analysis, journal structural geology, stack, plane, opening, journal, rigid grain, structural position, ebsd analysis, structural evolution, condition, nc, estimate, lewisian gneiss, thrust belt, wm, bailey