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_Journal of Structural Geology 33 (2011) 329-342_ Contents lists available at ScienceDirect Journal of Structural Geology journal homepage: www.elsevier.com/locate/jsg Himalayan hinterland-verging superstructure folds related to foreland-directed infrastructure ductile flow: Insights from centrifuge analogue modelling Laurent Godin a,*, Chris Yakymchuk a, Lyal B. Harris b a Department of Geological Sciences and Geological Engineering, Queen’s University, Kingston, Ontario K7L 3N6, Canada b Institut national de la recherche scientifique, Centre Eau Terre Environnement, 490 de la Couronne, Quebec City, Quebec G1K 9A9, Canada Article info Article history: Received 3 January 2010; Received in revised form 29 August 2010; Accepted 13 September 2010; Available online 4 November 2010 Keywords: Centrifuge analogue modelling Channel flow Himalaya Tectonics Detachment shear zones Abstract The orogenic superstructure (SS) and infrastructure (IS) constitute two levels of a mountain belt with contrasting structural styles. In the Nepal Himalaya, N-verging back folds, which oppose the orogenic vergence, dominate the SS. Competing explanations for these folds are tested using centrifuge analogue models. Modelling suggests that SS folding occurs during bulk shortening accompanied by IS thickening before IS flow. Focused erosion then instigates IS lateral flow and stretching, decoupling of the SS, and transposition of the lower SS into a detachment zone. Decoupling at the ISeSS interface separates an SS dominated by older folds and an IS characterised by younger horizontal transposition and stretching of early folds. Extrusive ductile flow of the IS locally modifies fold vergence in the SS. The fold asymmetry is thus controlled by the efficiency of coupling between IS and SS; a low viscosity at the ISeSS interface favours complete decoupling and hinders modification of fold vergence, whereas a higher viscosity IS-SS interface favours fold vergence modification. Modelling supports a tectonic scenario in which Himalayan hinterland-verging folds are the product of early shortening of the SS followed by local modification of fold geometry when the IS subsequently stretches and flows during focused erosion and melt-enhanced IS weakening. © 2010 Elsevier Ltd. All rights reserved. 1. Introduction Continental crust undergoing regional deformation displays complex vertical strain distribution (or zonation) reflecting dominant deformation processes that vary with depth. The superstructure is typically characterised by upright folds in low metamorphic-grade rocks, indicative of bulk horizontal shortening, whereas the subjacent infrastructure contains high-metamorphic grade migmatitic rocks with gently-inclined, strongly-transposed features emblematic of horizontal stretching and shearing (Wegmann, 1935). The terms “superstructure” and “infrastructure” are used here to describe crustal-scale differences between the upper crust, which dominantly deforms by frictional-plastic (Coulomb-type) failure, and the underlying mid-crust in which the deformation is dominated by power-law creep. The superstructure-infrastructure classification has recently been revived in the literature, partly due to its similarities with and applicability to numerical models of continental collision that produce mid-to lower crustal flow (Beaumont et al., 2001, 2006; Culshaw et al., 2006). In such models, the weakened mid-crust responds either to the introduction of a lower crustal indenter or an imposed lithostatic pressure gradient by flowing laterally towards the orogenic front beneath a comparatively passive orogenic superstructure (Beaumont et al., 2001, 2006; Godin et al., 2006a, for review). In this paper, analogue centrifuge models are used to investigate contrasting deformation styles in the superstructure and infrastructure observed in continental collision zones. Models are designed to simulate the structural evolution of horizontal shortening in a superstructure-infrastructure package, followed by vertical thinning and horizontal stretching and ductile flow of the melt-weakened infrastructure due to focused erosion and a lithostatic pressure gradient in a manner akin to channel flow. Emphasis in this study is placed on examining: (1) coeval and dynamically linked deformation (coupling) between the superstructure and infrastructure; (2) the formation of drag folds along the interface between the superstructure and infrastructure during infrastructure horizontal flow; and (3) the effects of varying the slope steepness of the orogenic topographic front, which drives extrusion of the ductile infrastructure, on the development of deformation features in the superstructure. Specifically, centrifuge analogue modelling is used to test whether hinterland-verging folds can develop above a sub-horizontally stretching foreland-directed ductile-flowing infrastructure (e.g., Larson et al., 2010). 330 L. Godin et al. Journal of Structural Geology 33 (2011) 329-342 Our modelling approach, in particular, investigates hinterland-directed folds preserved in the Tethyan sedimentary sequence, the Himalayan superstructure in central Nepal, where anomalous north-verging folds of unresolved age dominate (Bordet et al., 1971; Godin et al., 1999a; Godin, 2003; Kellett and Godin, 2009). These folds are preserved in the hanging wall of a crustal-scale detachment, the South Tibetan detachment system (STDS), below which the infrastructure, the Greater Himalayan sequence, has been extensively transposed and metamorphosed (Fig. 1; Grujic et al., 1996; Vannay and Hodges, 1996; Grasemann et al., 1999; Searle and Szulc, 2005; Jessup et al., 2006; Larson and Godin, 2009). 2. The superstructure-infrastructure association and the Himalayan orogen 2.1. The superstructure-infrastructure concept A common observation in orogenic belts is the change in dominant structural style from upright open folds in the superstructure to isoclinal recumbent folds in the infrastructure (Murphy, 1987; Godin, 2003; Williams and Jiang, 2005; Culshaw et al., 2006; Williams et al., 2006; Denle et al., 2009; Kellett and Godin, 2009). In this paper, the superstructure of an orogenic belt refers to the weakly metamorphosed to unmetamorphosed sedimentary sequence resting on the underlying migmatitic infrastructure. The rheology of the superstructure renders it mostly susceptible to brittle and brittle-ductile deformation. A crustal-scale detachment, usually marked by a strain gradient across which rheology and structural styles are often contrasting, separates the infrastructure from the superstructure. The infrastructure refers to the ductile, thermally-weakened middle-crust that is of a similar density and lesser viscosity than the overlying superstructure (Mecklenburgh and Rutter, 2003; Rosenberg and Handy, 2005). Examples of such infrastructure-superstructure relationships are found in several orogens, such as the Canadian Cordillera (Murphy, 1987; Glombick et al., 2006), the Western Superior Province of Canada (Culshaw et al., 2006), the French Pyrenees (Denle et al., 2009), the Oman ophiolitic belt (Searle et al., 2004), and the Australian Petermann orogen (Raimondo et al., 2009). 2.2. The Himalayan prototype The Himalayan orogen is the type example of a large hot orogen, characterised by a viscous mid-crust that contains significant in situ partial melts and where the channel flow process may have been operative (Bird, 1991; Grujic et al., 1996, 2002; Royden et al., 1997; Clark and Royden, 2000; Beaumont et al., 2001, 2006). The Himalayan orogen initiated in early Eocene times, following collision of the Indian and Eurasian plates (see Hodges, 2000 and Yin and Harrison, 2000 for reviews). The convergence culminated in closure of the Tethyan Ocean, southward imbrication of the Indian crust, and northward continental subduction of Indian lower crust and lithospheric mantle beneath Asia (Hodges, 2000; Yin and Harrison, 2000). The Himalayan orogen consists of five, broadly parallel lithotectonic belts, separated by mostly north-dipping faults. The Greater Himalayan sequence is bounded by two parallel and opposite-sense shear zones that were both broadly active during the Miocene (Fig. 1; Hubbard and Harrison, 1989; Searle and Rex, 1989; Hodges et al., 1992, 1996; chronological review in Godin et al., 2006a). The Main Central thrust (MCT) zone marks the lower boundary of the Greater Himalayan sequence, juxtaposing the metamorphic core above the underlying Lesser Himalayan sequence (Searle et al., 2008). The STDS defines the upper boundary roof fault of the Greater Himalayan sequence, placing it in tectonic contact with the overlying weak-to unmetamorphosed Tethyan sedimentary sequence (Burg and Chen, 1984; Burchel et al., 1992; Searle and Godin, 2003). The apparent coeval movement of the MCT and STDS, combined with the presence of highly sheared rocks and high grade to migmatitic rocks within the Greater Himalayan sequence, has led many workers to view the metamorphic core as a north-dipping, southward extruding slab of mid-crustal material flowing away from the thick southern edge of the Tibetan Plateau, towards the thinner foreland fold-thrust belt (Grujic et al., 1996, 2002; Law et al., 2004; Searle and Szulc, 2005; Jessup et al., 2006; Godin et al., 2006a; Cottle et al., 2007; Larson and Godin, 2009; Larson et al., 2010). 2.2.1. The Himalayan infrastructure: The Greater Himalayan sequence The Greater Himalayan sequence, where exposed at the surface, comprises greenschist to granulite facies metamorphic rocks. The structural upper half of the Greater Himalayan sequence is dominated by migmatites and discontinuous Miocene leucogranite bodies (Fig. 2). The Greater Himalayan sequence is pervasively transposed by top-to-the-s' Ключевые слова: koyi, evolution, material, decoupling, overlying superstructure, chen, godin, geophysical, society london, rapid exhumation, tibet, greater himalayan, law, anticlines, central nepal, melt, early, scale, searle, metamorphic, superstructure infrastructure, society, hanging wall, eastern nepal, ductile, core, himalayan sequence, centrifuge modelling, geological society, wa, absolute age, numerical model, parrish, layer, ma, journal structural, erosion, earth, anticlinal core, himalayan infrastructure, centrifuge, larson, experiment, collapsing wedge, elsevier, rheological contrast, himalayan superstructure, tibetan detachment, exhumation, sequence, institut national, angle, fold, high-acceleration centrifuge, ?ow, continental collision, view, gravity deformation, india, back-rotated fold, geological society london, beaumont, ductile infrastructure, early shortening, kellett, model, science, harrison, hinterland-verging fold, hanging, steep erosion, godin larson, fort, fold geometry, orogenic, signicant, horizontal, extrusion, steep, deformation, folding, shallower erosion, tectonophysics, ramberg, szulc jessup, geometry, shortening phase, upper, crustal, himalayan, ss, foreland, scaled comparison, london, rheological, nepal, shallow erosion, pdms, royden, journal structural geology, journal, south tibetan, shortening, superstructure, himalayan orogen, mm, greater himalayan sequence, stages, carosi, table, himalaya, infrastructure, central, local modication, tectonic evolution, upright fold, continental, chen burchel, channel, southward extrusion, sedimentary sequence, stretching, lateral, drag, zone, progressively tighter, structural geology, garzanti, rutter rosenberg, thrust, focused erosion, williams, crust, bulk thickening, tectonic, interface, cut, early stage, tectonics, geological map, created, structural, natural prototype, pdms layer, geological, eds, melt weakening, discussion, modelling, rock, sedimentary, grujic, fold style, godin journal, foreland erosion, plan view, stds, horizontal stretching, jiang, geology, tibetan, sequential photograph, layer-parallel shortening, haimanta group, detachment, stage, hodges, bons, jamieson, superstructure fold, experimental deformation, upper crust, tethyan sedimentary, burbank, structural evolution, nucleate fold, polydimethylsiloxane pdms, hinterland face, nature, upright, density, vertical thinning, orogen, greater, burch?el, collision, south, kellett godin, modi?cation, hollister, topographic surface, metamorphic core, special, tethyan, equal distribution, wall, thickening, hinterland, shear