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_Journal of Structural Geology 33 (2011) 192-202_ _Contents lists available at ScienceDirect_ _Journal of Structural Geology_ _journal homepage: www.elsevier.com/locate/jsg_ _Deformation partitioning in transpressional shear zones with an along-strike stretch component: An example from the Superior Boundary Zone, Manitoba, Canada_ _Yvette D. Kuiper a,*, Shoufa Lin b,c, Dazhi Jiangd a Department of Geology and Geophysics, Boston College, Chestnut Hill, MA 02467, USA b Department of Earth Sciences and Environmental Sciences, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada c School of Resource and Environmental Engineering, Hefei University of Technology, Hefei 230009, PR China d Department of Earth Sciences, University of Western Ontario, London, Ontario N6A 5B7, Canada_ _article info_ _Article history: Received 5 February 2010; Received in revised form 27 June 2010; Accepted 4 July 2010; Available online 14 July 2010_ _Keywords: High-strain zone Strain partitioning Triclinic flow Orogen-parallel stretch Lineation Trans-Hudson orogen_ _abstract_ _The partitioning of triclinic flow into domains of apparent monoclinic and apparent orthorhombic flow is described and discussed, using the Aiken River shear zone (ARSZ) as an example. The ARSZ is a 1-1.5 km wide east-west trending, dextral, north-side-up, mylonite zone within the northern part of the Superior Province in Manitoba. It displays a high along-strike stretch (>10), which is most likely indicative of an escape-tectonic setting._ _Although the central mylonite zone exhibits an apparent monoclinic fabric symmetry, the actual flow field was probably triclinic with a high simple shearing over pure shearing ratio, which resolves potential strain compatibility problems with neighbouring domains. The simple shearing-dominated zone is relatively narrow and has well-defined boundaries. An up to 20 km wide zone adjacent to the ARSZ shows an apparent orthorhombic fabric symmetry with shear zone boundary-parallel horizontal stretch and shear zone-orthogonal shortening. However, the actual flow may have been triclinic with a low simple shearing over pure shearing ratio. Either way, the pure shear component of the ARSZ is distributed over a much broader area than the simple shear component and has diffuse boundaries._ _? 2010 Elsevier Ltd. All rights reserved._ _1. Introduction_ _Shear zones may have monoclinic or triclinic symmetry. Monoclinic shear zones can be considered as special cases, where the shear direction is parallel to one of the principal axes of the pure shearing component (cf. Robin and Cruden, 1994; Jiang and Williams, 1998; Lin et al., 1998). Shear zones may be thinning (transpression), thickening (transtension) or not changing thickness._ _Triclinic transpression zones are most common, because most orogens, active convergent plate boundaries, and volcanic arcs are associated with oblique convergence between plates or blocks (Jiang et al., 2001; Jiang, 2007b, and references therein)._ _In transpression zones, the shear zone-parallel maximum stretch is usually assumed or interpreted as being vertical or down dip (Sanderson and Marchini, 1984; Tikoff and Greene, 1997; Lin et al., 1998) or oblique (Czeck and Hudleston, 2003). Shear zone boundary-parallel horizontal stretch in subvertical shear zones is not commonly reported. Similarly, orogen-parallel stretch is not common and existing estimates of finite orogen-parallel stretch are low._ _The two types of stretch are related, because crustal-scale subvertical shear zones are generally parallel to the trend of the orogen. It is commonly believed that a high shear zone boundary-parallel horizontal stretch or orogen-parallel stretch is not favourable, because it would cause space and strain compatibility problems in deformation._ _Furthermore, strain hardening would resist the shear zone-or orogen-orthogonal (pure shear) shortening that usually occurs at the same time._ _In this paper, we present evidence for a high shear zone boundary-parallel horizontal stretch in the subvertical Aiken River shear zone (ARSZ) in the Superior Boundary Zone, northeast of Thompson, Manitoba, Canada (Fig. 1). The amount of horizontal stretch along the shear zone is estimated to be in the order of 10._ _We discuss the feasibility of such high shear zone-parallel stretches for the ARSZ as well as in general. A common tectonic setting for such shear zones may be lateral extrusion zones in escape-tectonic settings._ _We also discuss strain partitioning in shear zones, using the ARSZ as an example. Transpression along this shear zone is partitioned into zones with a high and a low simple shearing over pure shearing (g3) ratio, where ‘simple shearing’ and ‘pure shearing’ indicate rates of simple shear and pure shear, respectively (Means, 1990). The pure shear component is consistently distributed over a wider area than the simple shear component (cf. Lin et al., 1998), as is indicated by rotation patterns of lineations and fold hinge lines along the ARSZ, as well as by rotation or shear zone-orthogonal shortening or map scale structures along other shear zones in the Superior Boundary Zone (cf. Kuiper et al., 2009). Furthermore, triclinic flow along the ARSZ is partitioned into apparent monoclinic and apparent orthorhombic domains._ _The nature of these domains, and whether flow fields in these domains were truly monoclinic and orthorhombic or triclinic with apparent monoclinic and orthorhombic symmetries, is discussed. It is argued that the domain with apparent monoclinic fabric symmetry is in fact a region of triclinic flow, based on strain compatibility arguments with adjacent domains._ _The apparent monoclinic fabric symmetry is a result of a high g3 ratio and low finite strain._ _The domain with apparent orthorhombic fabric may be truly orthorhombic or triclinic with a low g3 ratio._ _2. Geological background_ _The Superior Boundary Zone exists between the Archean Superior Province to the southeast and the Paleoproterozoic Trans-Hudson Orogen to the northwest._ _The Trans-Hudson Orogen is an amphibolite grade Paleoproterozoic belt that consists of Paleoproterozoic volcanic arc and passive margin sedimentary rocks, which record Paleoproterozoic continent collision and orogenesis (Machado, 1990; Ansdell, 2005; Corrigan et al., 2005). The Superior Province consists of Eoarchean to Neoarchean terranes that were amalgamated in the Neoarchean (Percival, 2007)._ _The Pikwitonei Granulite Domain represents a mid-to deep crustal segment of the Superior Province. It consists of predominantly tonalitic and granodioritic gneisses that underwent granulite facies metamorphism and retrograde amphibolite grade metamorphism in the Neoarchean (Mezger et al., 1990; B?hm et al., 1999, 2007)._ _The Split Lake Block consists of >2708 Ma gneisses and can be correlated with the Pikwitonei Granulite Domain (Corkery, 1985; B?hm et al., 1999; Kuiper et al., 2003, 2004a,b). It is bounded by the Assean Lake and Aiken River shear zones (Fig. 1)._ _It consists of predominantly tonalitic and granodioritic gneisses with lower volumes of anorthosite, gabbroic and granitic gneisses, mafic granulite, layered amphibolite and pelitic schist and gneiss (Haugh, 1969; Corkery, 1985; Hartlaub et al., 2004)._ _The Split Lake Block underwent granulite facies metamorphism and later amphibolite facies metamorphism, both in the Neoarchean (Corkery, 1985; B?hm et al., 1999, 2007; Downey et al., 2009)._ _The Assean Lake Complex (Fig. 1) consists of granitic and tonalitic gneiss, metasedimentary rocks and layered amphibolite that are older than 3.0 Ga material (B?hm et al., 2000, 2003, 2007; Hartlaub et al., 2006). The peak of metamorphism reached amphibolite facies._ _The Assean Lake Complex was deformed and metamorphosed during the Paleoproterozoic Trans-Hudson orogeny, but evidence for earlier events in the Late Archean and earliest Paleoproterozoic exist (B?hm et al., 1999, 2003)._ _The Thompson Nickel Belt (Fig. 1) consists of Neoarchean gneisses (Machado et al., 1990; B?hm et al., 2007) and Paleoproterozoic supracrustal (Zwanzig, 2005; Zwanzig et al., 2007) and ultramafic (Hulbert et al., 2005) rocks, and is characterized by retrograde metamorphism._ _Shear zones between the crustal blocks, described in more detail in Kuiper et al. (in review), display localized mylonite zones but also show evidence for shortening across the shear zone, within the shear zone as well as in adjacent domains._ _Below, the ARSZ and adjacent structural domains are described and discussed in detail._ _3. Aiken River shear zone and Split Lake Block_ _The geology related to the ARSZ has been divided into structural domains as follows (numbers correspond with domain numbers indicated in Fig. 2): (1) northern Pikwitonei Granulite Domain, (2) northern Pikwitonei Granulite Domain with refolded folds, (3) mylonite, (4) transitional zone, (5) southern Split Lake Block, and (6) central Split Lake Block._ _The structural domains are described below, from north to south (domains 6 to 1) and are summarized in Table 1._ _3.1. Central Split Lake Block (domain 6)_ _The main structural pattern of rocks in the studied part of the central Split Lake Block is dominated by generally weakly deformed rocks (Fig. 3a) and generally moderately to steeply SE-plunging open to tight folds._ Ключевые слова: river shear, oblique convergence, west, boundary-parallel, assean lake, tight fold, limb, mylonite zone, component, evidence, journal structural geology, eds, jiang, sheath fold, split, tight, orientation close, horizontal orientation, distributed, simple, earth, van kranendonk, hinge, river, granulite, shortening direction, north-side-up, plunge, fabric attractor, arsz, manitoba, maximum, adjacent domain, deformation, lateral extrusion, symmetry, general orientation, stretching, shearing, lin, pure shearing, jiang williams, escape-tectonic setting, g 3, percival, parallel stretch, rock, schmid jerbek, lineation rotate, path, zwanzig, block, lake, boundary-parallel horizontal, scale, shear direction, dextral north-side-up, subhorizontal orientation, paleoproterozoic, superior province, orthorhombic, zone domain, aiken, high strain, granodioritic gneiss, tectonophysics, abstract, price, lie, ?ow, folded, ansdell, nite stretch, shear zone, foliation, fold limb, result, mylonite, aiken river, kuiper, rotate, canada, geological, structural, economic, transpression zone, structural geology, hulbert, dextral, direction parallel, gondwana, burntwood river, actual, shoufa lin, fold hinge, williams lin, pure, shear band, tectonic, moderately, triclinic, boundary, shear zone boundary, apparent, structural domain, direction, lineation, elsevier, viscous, selected point, structural data, apparent monoclinic, deformation partitioning, liu, southeast, axial planar, orogen-parallel stretch, fabric, zone boundary, lower-mid crust, lake block, journal, corkery, simple shearing, rotation path, transpression, geology, simple shear, shear, mezger, orogen, model, machado, hinge lineation, stretch, fold, horizontal stretch, nite strain, strain ellipsoid, folds, superior, sheath, journal structural, province, haugh, earth sciences, horizontal, domain, stretching lineation, rotation pattern, high, orientation, monoclinic, high-strain zone, heaman, triclinic transpression, subhorizontal, steeper orientation, maximum stretch, parallel, precambrian geology, greene lin, price lin, assean, split lake, b?hm, consistent, rotation, channel, zone, precambrian, zone boundary-parallel, ratio, shortening, strain, wa, movement, north, williams, stretching direction, responsible, concept, shearing component, pure shear, model presented