Centrifuge modelling of deformation of a multi-layered sequence ...

More documents

Recommendations

Info

$Fayek & Kyser 2000 uraninite fractionations.pdf - Queen's University$

464 Int J Earth Sci (Geol Rundsch) (2012) 101:463–482tomodensitometry (CT scanning) and changes in fold orientationand geometry in 3D are presented. Models simulateand help resolve problems in our understanding offolds in (1) (meta)sedimentary sequences that constitute asuperstructure upon ductile migmatitic gneisses and (2)within layered sedimentary sequences upon a ductile substratesuch as shales ± evaporites in deeper levels of foldand thrust belts.Orogenic infrastructure and superstructurein large hot orogensThe concept of superstructure and infrastructure (Wegmann1935; Fyson 1971; Murphy 1987; Culshaw et al. 2006) isuseful in distinguishing components of an orogenic belt,especially large hot orogens (Beaumont et al. 2006) that,whilst deformed together, display contrasting rheologiesand structural styles and orientation. The superstructuregenerally comprises a weakly metamorphosed to unmetamorphosedsequence that acts as an orogenic lid to theunderlying ductile infrastructure. The superstructure mayundergo brittle, brittle-ductile, and ductile deformationdepending on constituent rock types and is dominated byactive, upright to inclined open to close buckle and/orrounded to angular kink-style folds. The infrastructurecommonly comprises ductile partially molten middle crustthat is of a similar density but of lesser viscosity than theoverlying superstructure (Rushmer 2001; Mecklenburghand Rutter 2003). The infrastructure is typically dominatedby a shallowly dipping to horizontal foliation and recumbentfolds with variable axes indicative of penetrativeductile flow (Roger et al. 2004; Harrowfield and Wilson2005; Williams and Jiang 2005). Separation of the superstructurefrom the infrastructure by an essentially planarcrustal-scale décollement is described in the Himalayas(Hodges 2000; Godin 2003; Searle et al. 2003), the westernSuperior Province of Canada (Culshaw et al. 2006), theNorth American Cordillera (Murphy 1987; Vanderhaeghe1999; Brown and Gibson 2006), and the inner Piedmont ofthe Appalachians (Hatcher and Merschat 2006). In somecases, however, the superstructure may be infolded into theinfrastructure (e.g. Harris et al. 1987; Chen 1998; Aspleret al. 2002).Modelling presented in this paper illustrates a mechanicallyanalogous process to folding of a sedimentarysuperstructural sequence following its tectonic juxtapositionupon a ductile infrastructure of migmatitic gneissesand granitoids during an initial period of layer-parallelshortening. Results have implications for understanding thegeometry and progressive development of structures inorogenic systems with a thermally weakened mid-crust,especially how the geometry and style of active folds in thesuperstructure vary depending on the presence or absenceof a low-viscosity and density layer upon the ductile substrateand the factors that promote preservation of a flatbasal ductile décollement instead of folding of the superstructurewithin the infrastructure. Our modelling approachwas initially applied to the Himalayan infrastructure–superstructure association in Godin et al. (2011).Active folding of layers above a ductile décollementin fold and thrust beltsModels also simulate an analogous situation in many foldbelts where a sedimentary sequence of mechanicallyactive, competent layers (such as quartzite or limestone)interbedded with thin incompetent shale horizons overlie athicker shale or evaporitic ± shale sequence that constitutesa ductile substrate. Whereas sandbox experimentsprovide valuable information of structures in the uppercrust where layers deform as Coulomb wedges (Davis et al.1983), for fault-bend folding (Suppe 1983; Suppe et al.2004) and fault-propagation folding (Mitra 1990), ourmodels simulate deeper levels of fold belts and/or whereactive buckle folding has taken place due to mechanicaldifferences between layers that deform in a ductile manner.In select models, a low-density and low-viscosity layer isincorporated at the base of half of the cover sequence thatis analogous to a horizon of salt and/or pure gypsum. Thepresence or absence of this layer is used to test its effectson the regional curvature of fold axes and the developmentof décollement or detachment controlled salients as proposedby Macedo and Marshak (1999), Affolter and Gratier(2004), and Sussman et al. (2004).Why centrifuge modelling?Apart from 3D models of fold development (e.g. Kaus andSchmalholz 2006), most numerical models, especially ofregional tectonic processes such as channel flow (e.g.Beaumont et al. 2001, 2004, 2006; Jamieson et al. 2004;Culshaw et al. 2006) and deformation of thin-skinned foldand-thrustbelts (e.g. Stockmal 2007; Simpson 2009),provide only 2D, cross-sectional views. Despite thedevelopment of 3D modelling codes such as Ellipsis 3D(O’Neill et al. 2006) and Underworld (Humble 2008) thatcan simulate ductile flow, folding, and shearing at highstrains, numerical models generally lack the resolution toimage discrete structures in fine detail and, whilst providinga valuable overview, commonly do not portray subtle,detailed features akin to those that may be observed in thefield or from high-resolution seismic data. Numericalmodels examining the interplay between folding andfaulting of a competent sequence over a weak décollementin upper-crustal layers have demonstrated the conditionsnecessary for buckle folding to dominate but have not123
Int J Earth Sci (Geol Rundsch) (2012) 101:463–482 465addressed the significance of infolding (Simpson 2009) andfactors controlling whether detachment or infolding willoccur. Despite advances in numerical modelling andinherent limitations of analogue models (described below),scaled analogue models therefore still provide a bettermeans to study the coeval progressive evolution of structuresat upper- and mid-crustal levels in fine detail andin 3D.There has been extensive 1g 1 physical modelling offault-related folding of upper-crustal sedimentary strataabove a ductile substrate or ductile detachment zone representinga salt or shale horizon in nature (e.g. Turrini et al.2001; Bahroudi and Koyi 2003; Luján et al. 2003, 2006;Sokoutis et al. 2005) and during channel flow (Mukherjeeand Koyi 2009). Some sandbox experiments (e.g. Lujánet al. 2003, 2006; Nilforoushan and Koyi 2007; Vidal-Royoet al. 2009) illustrate the effects of localized ductiledécollements. Fewer experiments, however, investigateactive buckle folding and whilst highly valuable, generallydate to before advances in model visualization throughmodern computing. For example, Bucher (1956) usedstitching wax (a resin with a plasticiser that deforms as anviscoelastic material) layers in petrolatum grease and Blayet al. (1977) used gelatine multilayers on a thin, lubricatedbase layer deformed by gravity gliding and (in separateexperiments) by lateral compression at 1g to simulate folddevelopment and geometry above a ductile substrate.Centrifuge modelling is, however, considered as a bettermeans to simulate active, buckle folding of a multi-layeredsequence over a ductile substrate as it:1. Enables viscous and plastic materials to be used toreplicate ductile deformation incorporating activemechanical anisotropies at different levels in the crust(Ramberg 1973, 1981; Dixon and Summers 1985; Hillet al. 2010);2. Achieves dynamic scaling of models to account forbody forces (Hubbert 1937; Ramberg 1967a, b, 1973,1981; Weijermars and Schmeling 1986; developed inthe ‘Scaling’ section below), which are especiallyimportant when a low-density and viscosity layer isincluded in models;3. Allows the use of more viscous modelling materialsthat are easier to manipulate to scale deformation ofthe upper crust and middle crust than those that areused in normal gravity experiments (Ramberg 1967a,b, 1981; Weijermars and Schmeling 1986).A detailed explanation of the centrifuge method isprovided by Ramberg (1981) and Dixon and Summers(1985).1 Where g is the local acceleration due to gravity, i.e., 1g experimentsare carried out under normal gravity.Model preparation and scalingMaterialsModels are constructed to replicate a natural prototypeconsisting of a simplified two-layered crustal profile of anupper-layered undeformed flat-lying sedimentary rocksequence resting upon a ductile substrate (Fig. 1). Modellingmaterials (Table 1) have densities and viscositiesscaled to simulate the upper crust and mid crust to lowercrust when accelerated at ca. 1,000g and are similar tothose used in previous centrifuge modelling studies (Dixonand Summers 1983, 1985; Jackson et al. 1988; Dixon andTirrul 1991; Koyi and Skelton 2001; Koyi and Harris 2001;Harris and Koyi 2003; Dixon 2004; Corti 2004; Hill et al.2010; Godin et al. 2011). Low-density Crayola Ò modellingclay is used to decrease the density of the upper-crustallayered sequence with a minimal effect on the effectiveviscosity to achieve scaled densities to the prototype.Introduction of this low-density material avoids the use ofbarium sulphate and acids, commonly used to increase thedensity of silicone putties but which can significantlychange the rheology of the materials (ten Grotenhuis et al.2002). Viscosity curves for materials used to construct theductile substrate are shown in Fig. 2. Columns depictingthe composition and geometry of individual models areshown in Fig. 3.Microlaminates composed of Demco Ò modelling clay,hereafter referred to as ‘DMC’, Dow Corning DilatantCompound 3179 (DC 3179) alone or mixed with the lowdensityCrayola Ò Model Magic (CMM) simulate a successionof competent, but ductile clastic and carbonaterocks interbedded with incompetent pelitic and semi-peliticrocks. The DMC used in this study is rheologically similarto plasticine (McClay 1976; Sofuoglu and Rasty 2000;Zulauf and Zulauf 2004) that is used to construct microlaminatesin other centrifuge studies (e.g. Dixon andSummers 1985; Dixon and Tirrul 1991; Hill et al. 2010).Microlaminates are prepared using the rolling and stackingmethod employed by Dixon and Summers (1985), but anelectric dough break was used for rolling. To decrease thedensity of the microlaminates as required for scaling, thethickest layers of DMC are thoroughly mixed with CMM ata 40:60 weight ratio (DMC-60). A thin layer (1–2 mm) of alow-density CMM is placed at the surface (Fig. 1) toeliminate an otherwise too great a contrast between the topof the layered sequence and air, which may affect thegeometry of folds in upper microlaminates. This study doesnot include upper-crustal brittle structures due to difficultiesin incorporating cohesive layers that undergo faultingin the centrifuge (where layers are placed vertically precludinguse of unconsolidated, e.g. granular materials) andhave the required density. No attempt is made to introduce123
Page 1: Int J Earth Sci (Geol Rundsch) (201
Page 5 and 6: Int J Earth Sci (Geol Rundsch) (201

Centrifuge modelling of deformation of a multi-layered sequence ...

Create successful ePaper yourself

Delete template?

Save as template?