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

Int J Earth Sci (Geol Rundsch) (2012) 101:463–482DOI 10.1007/s00531-011-0682-yORIGINAL PAPERCentrifuge modelling of deformation of a multi-layered sequenceover a ductile substrate: 1. Style and 4D geometry of active coverfolds during layer-parallel shorteningChris Yakymchuk • Lyal B. Harris •Laurent GodinReceived: 5 October 2010 / Accepted: 29 May 2011 / Published online: 10 July 2011Ó Springer-Verlag 2011Abstract Centrifuge analogue modelling illustrates theprogressive development of active folds in multilayersupon a ductile substrate during layer-parallel shortening.Models simulate folding of a mechanically stratified sedimentarysequence upon migmatitic gneisses in a large hotorogen, or upon a thick basal evaporite ± shale sequencein deeper levels of fold belts. The absence of a weak lowviscosityand low-density layer at the interface promotesinfolding of the cover sequence and ductile substrate,whereas a planar upper surface to the basal ductile substrateis preserved when it is present. Whilst fold style,wavelength, and deformation of the interface with theductile substrate differ depending on whether a low-viscosityand low-density layer is present at the base of thecover sequence, there is no marked systematic curvature offold axes as seen in previous sandbox models for faultbendor fault propagation folding during bulk shortening.Bulk shortening of a layered sequence with relatively thickindividual layers above a ductile substrate promotes aregular and upright train of buckle folds, whereas thinnerlayers promote a more irregular distribution of buckle foldswith variable vergence, style, and amplitude. Buckle foldsC. Yakymchuk L. GodinDepartment of Geological Sciences and Geological Engineering,Queen’s University, Kingston, ON K7L 3N6, CanadaL. B. Harris (&)Institut national de la recherche scientifique,Centre Eau Terre Environnement (INRS-ETE),490 de la Couronne, Quebec, QC G1K 9A9, Canadae-mail: lyal_harris@ete.inrs.caPresent Address:C. YakymchukDepartment of Geology, University of Maryland,College Park, MD 20742, USAabove a ductile substrate progressively develop during bulkshortening from open and upright, to angular and tight, andmay further develop into cuspate structures above relativelyweak horizons. Relatively thick weak horizonswithin the layered sequence during bulk shortening interruptregular fold patterns up structural section and allowout-of-phase folds to develop above and below the weakhorizon.Keywords Active folding Centrifuge analoguemodelling CT scanning Ductile décollement Fold belts Orogenic infrastructure and superstructure Deformation of evaporites, salt, and shaleIntroductionAimsScaled centrifuge analogue models employing new materialsand techniques are used to investigate progressivethree-dimensional (i.e. ‘‘4D’’) mechanically active, buckleand kink folding (Ramsay 1967; Ghosh and Ramberg 1968;Johns and Mosher 1996) of a multilayered sequence ofcompetent and relatively incompetent layers upon a ductilesubstrate during bulk shortening. Materials and methodologyare described for models presented herein and for thosepresented by Godin et al. (2011) describing folding withinthe underlying ductile sequence during shortening andsubsequent erosion-induced channel flow. Models illustratethe effects of (1) varying composition and thickness oflayers simulating a (meta)sedimentary sequence and (2)the presence or absence of a less-viscous and lowerdensityductile basal décollement layer. The progressivedevelopment of folds in cross section through computed123

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

466 Int J Earth Sci (Geol Rundsch) (2012) 101:463–482Fig. 1 Analogue modelconstruction and initialdimensions. A collapsing wedgecomposed of Dow Corning 3179Dilatant Compound that appliesa force on a weighted nylonplate is used to shorten themodel in the centrifuge. Anadditional low-viscosity anddensity polydimethylsiloxane(PDMS) layer at the interface ofthe layered cover sequence andductile substrate is present inmodels 54, 59, and 6180 mmCollapsing wedgeNylon plateWeightCrayola Model MagicLayered sequence200 mm15 mmDuctile substrate± Low viscositylow density layerTable 1 Rheological properties of modelling materialsModel material Prototype Effective viscosity(Pa s) aCover microlaminatesSedimentary cover sequence (large hot orogensand deeper levels of fold-thrust belts)Bulk density(kg m -3 )(\1.05)Dow Corning 3179 bDilatant Compound (DC 3179)Weak ductile cruste.g. shales4 9 10 5 1.133Crayola Model Magic (CMM) Uppermost layer – 0.504Plasticine cCompetent ductile crust8 9 10 6 –6 9 10 9 1.7e.g. quartzites or carbonatesDemco Modelling Clay (DMC)Competent ductile cruste.g. impure quartzites or marlsSimilar to plasticine 1.167Demco Modelling Clay (60%)/Crayola Model Magic(40%) (DMC-60)Weak ductile cruste.g. impure quartzites or marls– 0.900Ductile substrate Large hot orogens Fold-thrust belts (*1.05)PDMS Leucogranites Salt or gypsum 2–3 9 10 4b,d 0.979Crazy Aaron’s thinking putty (CATP) Melt-weakened lower crust Evaporites ± shales 1.5 9 10 4–5 1.05a At modelling strain ratesb Konstantinovskaya et al. (2007)c McClay (1976)d Boutelier et al. (2008)fault-susceptible layers within the microlaminates as thisstudy is concerned only with active buckle folding withoutfaults. Each individual model has slight variations in layerthickness due to imperfections in construction (Fig. 3),except for doubly-thin layers in model 61, but the rheologicaleffects are interpreted to be minor compared to themechanical contrast between the layered sequence andductile substrate. The average density of the layeredsequence ranges from 0.96 to 1.10 g cm -3 . The effectiveviscosities of the layered sequences range from 10 5 to10 6 Pa s at experimental strain rates (ca. 5.0 9 10 -4 s -1 ),based on the viscosity of layered sequences with a similarcomposition from other studies (Koyi and Skelton 2001;Harris et al. 2002; Harris and Koyi 2003).The ductile substrate is prepared by stacking layers ofCrazy Aaron Enterprise’s Thinking Putty Ò (CATP), a123

Int J Earth Sci (Geol Rundsch) (2012) 101:463–482 467Log viscosity [Pa s]654-4DC 3179DC 3179 + Glow (4%)CATP silverCATP amberCATP amber + Glow (4%)PDMS-3 -2 -1 0 1Log strain rate [s-1]Layered sequenceModel60 6154 5259****5 mm = 5 kmFig. 2 Compilation of flow properties of Dow Corning 3179 dilatantcompound (DC 3179) (Poulin 2006), Crazy Aaron Thinking Putty(CATP; Poulin 2006), and polydimethylsiloxane (PDMS; Weijermars1986; Sokoutis 1987; ten Grotenhuis et al. 2002). Flow properties arealso shown for DC 3179 and CATP with an added 4% weight ofstrontium aluminate-europium powder used to produce marker layerson computed tomodensitometry scans (Poulin 2006; Konstantinovskayaet al. 2007)Ductile substrate**silicone putty with scaled densities and viscosities similarto Rhodorsil Gomme Silbion described by Weijermars(1986) and Hailemariam and Mulugeta (1998) that representductile mid-crust (Table 1). Different colours andradiometric properties (see section below on CT scanning)are used for visual contrast, but these different layersexhibit only minor rheological variations (Poulin 2006).Densities of the putties range between 1.05 and1.08 g cm -3 , which is slightly denser to within error of thedensity for the overlying layered sequence. The viscosity ofthe putties ranges from 10 4 to 10 6 Pa s at the estimatedexperimental strain rates (c. 5.0 9 10 -4 s -1 ; Fig. 2), aboutan order of magnitude less than the layered sequence,which is considered a reasonable approximation of viscositycontrasts that exists in nature between the uppercrust and a melt-weakened mid-crust (Rosenberg andHandy 2005). In models 54 and 61, a 2 mm layer ofpolydimethylsiloxane (PDMS; Weijermars 1986; Boutelieret al. 2008) is placed between the layered sequence andductile substrate lengthwise across half of the model parallelto the shortening direction to investigate mechanicaldecoupling, whereby both layers of the model deformindependently to each other, across this interface (Figs. 1,3). PDMS has been previously used to simulate granitoidintrusions (e.g. Dietl and Koyi 2008) or salt (e.g. Koyi1991; Forien and Dietl 2009) in centrifuge models incorporatingother silicone putties ± modelling clays.Materials used to emulate the mid-crust exhibit a non-Newtonian behaviour defined by the flow law:r n ¼ leLayered sequenceDMCDC 3179DMC-60CMMPlasticineRigid BaseDuctile substrateDMCCATPPDMS* 4% by weightstrontium europiumaluminatepowderFig. 3 Construction of an upper-layered sequence overlying a ductilesubstrate. The layered sequence is composed of a repetitive sequenceof microlaminates (c.f. Dixon and Summers 1985). The ductilesubstrate consists of interlayered silicone putties of different coloursthat exhibit minimal dynamic variation. Individual models showslight variations in layer thickness. An asterisk adjacent to a layerindicates that it contains 4% weight strontium europium-aluminatepowder (Ultra Green Glow in the Dark Powder TM from Glow Inc.).CATP: Crazy Aaron Thinking Putty, CMM: Crayola Ò Model Magic,DC 3179: Dow Corning Dilatant Compound 3179, DMC: Demco Òmodelling clay, DMC-60: Demco Ò modelling clay mixed with 40%Crayola Ò Model Magic, PDMS: polydimethylsiloxaneð1Þwhere l = viscosity, e = strain rate, r = stress, and n isthe power law exponent. Materials with n = 1 exhibit alinear relationship between stress and strain, or Newtonianbehaviour. Materials with n [ 1 have a variable viscositydepending on the strain rate. The effective dynamic viscosityof materials that model the ductile mid-crust isdefined here as l eff = r/e and plotted against increasingstrain rates in Fig. 2. At this strain rate, the effective viscosityof the layered sequence, ductile substrate, and123

468 Int J Earth Sci (Geol Rundsch) (2012) 101:463–482PDMS are c. 1 9 10 6–8 , 2 9 10 5 , and 4 9 10 4 Pa s,respectively.ScalingModels are scaled to simulate a simplified orogenic system.Scaling of dynamic, geometric, and rheological propertiesis achieved by scaling the gravitational stress r using theequation:r ¼ q g l ð2Þwhere q, g, and l are density, gravity, and length, respectively,and the asterisk denotes the dimensionless ratiobetween the model and the prototype (Hubbert 1937;Ramberg 1967a, b, 1973, 1981; Dixon and Summers 1985).Models are scaled to the prototype with a length scalingratio of l = 1 9 10 6 , whereby 1 mm in the model represents1 km in the prototype (the same as in many othercentrifuge experiments of fold-thrust belts, such as Dixonand Spratt 2004; Hill et al. 2010). The scaling ratios fordensities, stress, time, acceleration, velocity, and viscosityare shown in Table 2.Model limitationsThe geometries and rheological properties of the layeredsequence and the ductile substrate are necessary simplificationsof a natural complex and heterogeneous system.These models cannot account for inherited structures andstrength anisotropies, which may locally affect stress fieldsand influence the geometry of folds. The rheology of rocksis temperature, confining pressure, strain-rate, and pore andfluid pressure dependent, which is simplified here into atwo-layered model representing the upper- and mid-crustseparated by an abrupt interface. As the rheology of rocksin nature is not well constrained, materials are used thatdisplay a significant enough viscosity contrast to be a goodapproximation. Models initially contain a ductile substrate;however, in a typical orogenic setting, the lower substratewould start strong and progressively weaken aftercrustal thickening and a period of thermal incubation (e.g.Beaumont et al. 2004). Model construction, especially inrolling more competent layers, introduced some minorheterogeneities in some layers and some small bubblesremained in silicone layers despite care in their preparation.However, such small initial heterogeneities areimportant in nucleating some structures in the ductilesubstrate, although a discussion of this is beyond the scopeof the present paper.Model preparation and deformationThe PR-7000 centrifuge at INRS-ETE in Quebec City(modified for physical modelling by the Bureau of EconomicGeology, University of Texas at Austin) is capable ofsubjecting models up to 200 mm 9 80 mm in plan view toaccelerations exceeding 8,000g. Materials are rolled to adesired thickness, encased on all sides with plastic plates,and frozen to facilitate handling. The frozen layeredsequence is placed upon the frozen ductile substrate,encased on all sides and allowed to return to room temperaturebefore being placed in the centrifuge. Due to thetendency of the silicone putties to flow under normal gravityat room temperature, all sides of the model are covered in athin layer (\1 mm thick) of DMC to prevent the puttiesfrom escaping the model chamber. In preparing all models,every side of the completed model is coated in a thin layerof silicon plumbers grease and covered with a slice ofoverhead transparency plastic film to minimize boundaryeffects during shortening. The prepared model is orientedvertically in the centrifuge chamber, and a counterweight isplaced in a chamber on the opposite side of the centrifuge tomaintain balance. Each model is run for 6–8 stages, eachat c. 1,000g for 360 s with a run-up time of 60 s and aTable 2 Scaling properties of models and prototypesQuantity Model Prototype Scaling ratioLength ratio 1 mm 1 km l r = l m /l p = 1 9 10 -6Density (bulk) (kg m -3 )Layered sequence 1,050 2,700 q r = q m /q p = 0.39Ductile substrate 1,050 2,700 q r = q m /q p = 0.39Acceleration 1,000 1 a r = 1,000Velocity (mm year -1 ) 2.2 9 10 6a 58 v r = l r /t r = 3.8 9 10 4Time 360 s 0.44 My t r = l r /l r q r a r = 2.6 9 10 -11Viscosity (Pa s) b 1 9 10 5 1 9 10 19 l r = l m /l p = 1 9 10 -14Subscripts ‘m’ and ‘p’ denote model and prototype, respectivelya Calculated for 25 mm shortening each runb At experimental strain rates of c. 5 9 10 -4 s -1123

Int J Earth Sci (Geol Rundsch) (2012) 101:463–482 469run-down time of 420 s. In between each stage, the model ispartially frozen and a vertical section is cut parallel to theshortening direction to be photographed *5 mm from eachend of the model. Models are then reassembled and allowedto return to room temperature before going through anotherstage. New slices are made at 5 mm increments towards thecentre of the model after each stage.Initial model dimensions are 170–200 mm 9 80 mm inplan view by 15–20 mm in height (Fig. 1). Models consistof a layered sequence 8–10 mm thick overlying a ductilesubstrate 5–6 mm thick. Initial layer thickness and modelgeometries for individual models are given in Table 3 andFig. 3. The model is progressively shortened in 2–4 stagesvia a collapsing wedge of DC 3179 30 mm 9 80 mm inplan view by 50 mm in height (c.f. Dixon and Summers1985; Jackson et al. 1988; Dixon and Tirrul 1991; Johnsand Mosher 1996). The wedge is confined on four facesand is free to collapse parallel to the long axis of the model.The wedge induces layer-parallel shortening by collapsingbehind the model to equilibrate radial pressure gradientswithin the wedge, consequently forcing the nylon platetowards the model (Fig. 1). A plastic plate with a weightedbase is inserted between the model and the collapsingwedge (as in Johns and Mosher 1996) to provide an equaldistribution of force throughout the back face of the model.Computed tomodensitometry (‘‘CT scanning’’)Computed tomodensitometry, also known as computedtomography or commonly as CT scanning, is used to obtain3D information of the progressive evolution of severalmodels. For a detailed explanation of the techniques ofX-ray computed tomography and geological and analoguemodelling applications, readers are referred to Duliu(1999), Ketcham and Carlson (2001), Mees et al. (2003),and Poulin (2006). CT scanning is a passive method toimage models in 3D and extract model cross sections andplan views, at various heights in the model analogous toviewing different depths or erosion levels. In addition, CTscans allow 3-dimensional reconstruction of models. CTscanning has been successfully used in sandbox studies toimage individual layers (e.g. Colletta et al. 1991; Konstantinovskayaet al. 2007), but due to problems in attenuationof X-ray energy and in resolving thin layers ofsimilar density, CT scanning has rarely been used formodels incorporating modelling clays and silicone putties.For cohesive materials in analogue models, apart for thedevelopment of CT-scanning techniques in multilayermaterials (Poulin 2006) used in our research and experimentson diapir emplacement (Harris et al. 2008), CTscanninghas been only previously used to image a singlediscrete boudinaged plasticine layers (e.g. Zulauf et al.2003; Zulauf and Zulauf 2005) or basal silicone layers in asandbox (e.g. Konstantinovskaya et al. 2007).Two centrifuge models were constructed and scannedbetween each deformation stage using a Siemens SomatonSensation medical CT-Scanner. To improve image contrast,approximately 4 wt% of strontium aluminate-europiumpowder (Ultra Green Glow in the Dark Powder TM fromGlow Inc. Ò composed of a proprietary combination ofAl 2 O 3 –SrCO 3 –Eu 203 –Dy 203 –TiO 2 ) is added to specificlayers (denoted by an asterisk in Fig. 3). In some models,the ductile substrate contains a single layer of amber CATPthat is easily imaged without the addition of the powder.This method developed by Poulin (2006) was successfullyemployed in sandbox (Konstantinovskaya et al. 2007) andcentrifuge experiments (Harris et al. 2008). The Glow in theDark powder induces a Compton scattering effect thatTable 3 Initial setup and geometry of modelsLayered sequenceModel 60 Model 61 Model 54 Model 52 Model 59Thickness (mm) 810101010Layer repetition 16× 32× 16× 16× 16×Interface– Half PDMS(2 mm)Half PDMS(2 mm)Ductile sequenceThickness (mm) 55665Composition – – – Competent layer –Initial length (mm)Shortening stagesTotal shortening (%)180232190237170342–200225Full PDMS(1 mm)170342123

470 Int J Earth Sci (Geol Rundsch) (2012) 101:463–482decreases the energy of reflected X-rays that mimics greaterdensity contrasts in the CT scanner but only increases thetrue density of the materials by\5% whilst the viscosity ofmaterials is minimally affected by the addition of thepowder (Fig. 2; Poulin 2006). The scanner is set up for aradial scanning pattern with an isotropic resolution of0.24 mm. Image analysis and processing is conducted usingOsiriX medical imaging software (Rosset et al. 2004). 3Dvolumes were constructed by interpolating betweenapproximately 430 individual slices using 3D volume renderingprotocols formulated in Osirix. Optimal parameterswere developed to visualize several marker layers in models;the slight radiometric density contrasts between somelayers, especially in the ductile substrate, make it extremelydifficult to clearly image some of the thin layers.Differences to previous centrifuge studiesApart from centrifuge models by Dennis and Häll (1978),Koyi (1988), and Sans and Koyi (2001) active buckledetachment-related folding above a (relatively) thick ductilesubstrate without contemporaneous faulting in eitherthe basal or cover sequence has received little attention.Most previous centrifuge models of the progressivedevelopment and 3D geometry of structures in fold andthrust belts have used simple plasticine and silicone puttymicrolaminates directly overlying a rigid base (e.g. Dixonand Tirrul 1991; Dixon 2004), basal layers of competentmodelling clay (e.g. Dixon and Spratt 2004; Hill et al.2010), or cardboard and wax (Hynes and Dixon 2005) andstudied the interaction between folding and faulting. InDixon’s (2004) centrifuge models, localized detachmenthorizons were simulated by simply lubricating a layer withpetroleum jelly. Centrifuge models that include the investigationof differences in layer thickness on fold geometryover a thin ductile layer during shortening are presented inAydemir (1998); however, these models are extremelysimple and the ductile basal layer is relatively thin; thesemodels are more applicable to upper crustal deformation atthe local scale (which was their intended purpose).Our centrifuge modelling goes beyond experiments atnormal laboratory conditions by Bucher (1956) and centrifugemodels of Dennis and Häll (1978), Koyi (1988) andSans and Koyi (2001) in studying the effects of a ductilesubstrate and the role of a low-viscosity and densitydetachment horizon for part of some models in 4D. Ourmodels differ significantly from Koyi (1988) and Sans andKoyi (2001) in that they do not address the process ofdiapirism and diapir-fold interaction as, apart from thePDMS layer used in part of some models described below,the thick ductile substrate and cover sequence are of similardensity, achieved by using lower-density modelling claymixes than in previous research. Tomodensitometry (‘‘CTscanning’’) techniques used in some of our models providefor the first time an exceptional view of the progressivedevelopment of structures in 3D.ResultsFolding of a layered sequence directly upon a ductilesubstrate (Model 60)The model consists of a finely layered sequence upon aductile and mechanically homogenous substrate. In contrastto models 61 and 54 (described below), no low-viscosity/low-densitylayer is present along the interfacebetween cover sequence and substrate. During progressiveshortening, folds in the layered cover sequence propagatefrom the advancing end wall of the model towards the fixedend (Fig. 4a–c). Folds are developed across the length ofthe model. Fold style and wavelength are variable. Abroad, rounded antiform develops in front of the advancingram (Fig. 4b, c). As folds tighten, initial box folds evolveinto more angular folds (Fig. 4b, c) through hinge migrationand axial surface rotation. Synformal axial planes areFig. 4 Model 60. Photographs of vertical sections at a distance of *5 mm from each other towards the centre of the model after each stage inthe centrifuge. See text for discussion. a Layers before shortening and addition of uppermost white layer, b 22% shortening, c 33% shortening123

Int J Earth Sci (Geol Rundsch) (2012) 101:463–482 471nearly vertical at the top of the model and graduallybecome reclined towards the ductile substrate (Fig. 4c).The folds have a dominant wavelength of 7 mm in thelayered cover sequence and 3 mm in the ductile substrate(i.e. equivalent to 7 and 3 km in nature, respectively). Theinterface separating the layered sequence and the ductilesubstrate is folded with sharp cuspate antiforms and broadopen synforms.Folding of a layered sequence over a two-layeredductile substrate (Models 61 and 54)Two models (Models 61 and 54) investigated the contrastingdeformation styles between a layered sequence and theductile substrate with and without an intermittent weakdécollement zone (Figs. 1, 3). The layered sequence containsfiner layers than other models to also investigate the influenceof layer thickness on buckle folds in the layered sequence. Alow-viscosity polymer, PDMS (2 mm thick), is incorporatedfor half the uppermost ductile substrate to investigate theeffect of a weak ductile décollement horizon. For example,the PDMS horizon simulates leucogranites emplaced duringdeformation along the interface separating the upper- andmid-crust, as documented in the Himalaya (Searle et al. 1997,2003) and modelled in Godin et al. (2011), and the Thor Odindome of the Canadian Cordillera (Vanderhaeghe 1999), or apure gypsum or salt layer between a sedimentary coversequence and an evaporitic ± shale substrate in a sedimentarysequence of a fold belt. Sequential photographs of bothhalves of the model are presented in Fig. 5, showing theevolution of the PDMS-present (left side) and PDMS-absenthalves of the model (right side).Progressive shortening of the layered sequence of Model61 produces tight to open, chevron to sharp folds of variablevergence (Fig. 5). Where the PDMS layer is present, foldsverge towards the moving end plate equivalent tohinterland-verging folds in nature, whereas folds where noPDMS layer is present are more upright and where inclined,tend to verge away from the moving end plate, i.e., equivalentto foreland verging in nature. During the shorteningstages (a, b in Fig. 5), a flat decoupling interface developsbetween the PDMS and the denser and more viscous ductilesubstrate, and the PDMS layer flows laterally to infill antiforms.In the PDMS-absent half of the model, the interfaceis folded and folds in the ductile substrate mimic those inthe layered sequence. After final shortening (Stage B inFig. 5), folds in the PDMS-absent half are generally tighter,more angular, than the more open, sub-rounded, and uprightfolds above the PDMS. The largest folds above the PDMShave greater amplitudes (19 mm) than the largest foldsdirectly above the ductile substrate (17 mm; Fig. 5). Foldsabove the PDMS-absent half of the model have a smallerdominant wavelength (9 mm) and greater variety of foldwavelengths than those above the PDMS (17 mm averagedominant wavelength). Two prominent box folds developabove the PDMS-absent half of the model at the forelandsidein stage B (Fig. 5). The PDMS layer thickens beneathantiforms in the layered sequence and thins beneath synforms(Stage B in Fig. 5). Both interfaces between thelayered sequence and ductile substrate and between theuppermost low-density and lesser viscosity (DMC) modellingclay layer display cuspate-lobate structures where thecusps point towards the more competent cover sequencesimilar to mullions formed during layer-parallel shorteningdescribed by Ramsay (1967) and Urai et al. (2001). This isseen on both sides of the model but is more prominent onthe PDMS-absent side of the model (Fig. 5).The second model that investigates the effect of a weakdécollement zone on folding in the cover sequence(Model 54) is similar to the previously described model(Model 61). This model was also prepared for CT imaging.Half of the ductile substrate is capped with 2 mm of PDMSFig. 5 Model 61. Successive photographs 5 mm from each otherfrom both ends of the model towards the centre. One half of the modelcontains a layer of PDMS at the interface (left) and one half withoutPDMS (right). a Undeformed, b, c Stage A: 14% shortening, d,e Stage B: 29% shortening123

472 Int J Earth Sci (Geol Rundsch) (2012) 101:463–482Fig. 6 Photograph of Model 54 after 42% shortening. One half of themodel contains a layer of PDMS at the interface between the layeredsequence and ductile substrate, and the other half has no PDMS alongthis interfaceand the other half is covered with yellow CATP (Fig. 3).Oblique model photographs and 3D model reconstructionsfrom CT images are shown in Figs. 6 and 7, respectively.During bulk shortening (stages A to C in Fig. 7), folds in thelayered sequence progressively propagate from theadvancing endplate towards the fixed end (equivalent toforeland propagation). Folds are non-cylindrical and differin style depending on the nature of the substrate. Initial foldsabove half of the model with a PDMS substrate have smalleramplitudes than folds on the opposite side with no PDMS inStage A (Fig. 7b, c). Initially round folds tighten andbecome sub-angular towards the hinterland end of themodel, and new angular folds nucleate in the foreland duringstage B (Fig. 7d, e). Rounded open folds above the PDMSlayer laterally change to sub-angular, tighter folds towardsthe PDMS-absent half of the model (Fig. 7, stage B). Duringthe final shortening stage (stage C in Fig. 7), all folds progressivelytighten and are sub-angular to angular. Fold axialtrace across the top of the model is slightly arcuate and moreso above the PDMS (Fig. 7f, g). Importantly, however, thereis no simple constant sense of curvature of fold axes acrossthe PDMS-present and PDMS-absent halves of the model,i.e., folds may curve across the interface in both senses.Folds are doubly plunging and conical. A single angular foldabove the PDMS-absent half of the model graduallybecomes rounded and bifurcates into two angular foldsabove the PDMS transition zone (Fig. 7f, g). After the finalshortening stage (stage C in Fig. 7), the interface betweenthe PDMS and the ductile substrate remains relatively planarcompared to undulating interface without the PDMS.Folding of thicker cover sequence layers over a ductilesubstrate with a thin competent layer (Model 52)This model investigates the influence of layer thickness onthe development of buckle folds in the layered sequencewhere individual thicknesses of cover sequence layers aretwice those of Model 61 (previously described) butconstructed of the same materials (Fig. 3) and where thereis a single competent layer within the ductile substrate.This model lacks a Crayola Model Magic cover (Fig. 3).Bulk shortening produces a consistent train of open andupright folds in the cover sequence with an averagewavelength of 9 mm (Fig. 8a, b). Folds initiate as open andround folds and progressively tighten into sub-angularfolds at the interface with the ductile substrate and becomebox folds towards top of the model (Fig. 8b). Folds in thelayered sequence initiate and remain upright through thebulk shortening stages (Fig. 8a, b). The interface betweenthe layered sequence and the ductile substrate is folded bynumerous antiformal cuspate structures (Fig. 8b). Folds inthe layered cover sequence are independent to those of thecompetent layer in the middle of the ductile substrate andsome are completely out of phase (i.e. synform beneathantiform), although wavelengths are similar.Folding of two competent layer packages separatedby a ductile layer in the cover sequence (Model 59)This model differs from previously described models inthat the cover sequence is composed of two laminatedpackages dominated by modelling clay with thin interbeddedsilicone layers separated by a 2-mm thick siliconelayer (with a thin internal modelling clay marker). Thislayer acts as an internal ductile décollement horizon withinthe cover sequence as seen in the vertical slice of the modelafter the shortening (Figs. 9, 10a). A thin (\1 mm) PDMSlayer is present beneath the layered sequence across theentire model (Fig. 3). 3D images of the bulk shorteningstages and cross-sectional images from both sides of themodel are presented in Fig. 10 (best illustrating the 3Dgeometry of fold hinges) and in Fig. 11 where fold profilesin modelling clay layers are best illustrated.During Stage A (Figs. 10b, 11a, b), initial folds of DMClayers adjacent to the advancing end wall (equivalent to thehinterland of a fold belt) are upright and angular. Fold axesare straight and continuous across the model during StageA (Fig. 10b). Folds formed in Stage A increase in amplitudeduring Stage B (Figs. 10c, d, 11a, b), and folds of thethick silicone layer develop a cuspate geometry withthickened hinges and thinned limbs. The development ofthese folds is accompanied by the nucleation of smallamplitude and open folds at the end of the model farthestfrom the advancing end wall (equivalent to the foreland ofa fold belt) during Stage B (Figs. 10c, d, 11c, d). Fold axesat the ‘‘foreland’’ end of the model are slightly arcuate butremain rectilinear at the ‘‘hinterland’’ end of the model(Fig. 10c, d). During Stage C (Figs. 10e, f and 11e, f),cuspate folds in the thick silicone layer become moredeveloped and folds of the DMC layers begin to develop acuspate geometry (Fig. 11e). Folds at the ‘‘hinterland’’ end123

Int J Earth Sci (Geol Rundsch) (2012) 101:463–482 473Fig. 7 Model 54. SequentialCT images showing each stageof model development:a undeformed, b and c Stage A(19% shortening), d and e StageB (31% shortening), f andg Stage C (42% shortening).Oblique images show surfacedeformation features and crosssections along the PDMSpresent(b, d, f) and PDMSabsent(c, e, g) halves of themodelof the model increase in amplitude during Stage C andbecome more angular (Fig. 11e, f), whilst folds at the‘‘foreland’’ end of the model show a relatively smallincrease in amplitude. Fold hinges in the hinterland remainlinear and continuous (Fig. 10e, f) in contrast to arcuatefold hinges that develop at the foreland end of the modelduring Stage C (Fig. 10e, f). Prominent cuspate structuresdevelop at the interface between the layered sequence andductile substrate during Stage C (Fig. 11e, f).Fold style changes upwards across the ductile décollementhorizon in the layered sequence. Synforms in thelower layered sequence (beneath décollement) graduallytighten upwards reaching a maximum tightness directlybeneath the décollement (Fig. 9). Directly above thedécollement, in the same synform structure, folds are lesstight and more open and progressively tighten upwards(Fig. 9). The opposite pattern exists in antiforms withtighter folds directly above the décollement that can befollowed into more open folds directly beneath thedécollement (Fig. 9). The upper layered sequence alsocontains several minor folds that cannot be traced acrossthe décollement into the lower layered sequence (Fig. 9).Some smaller folds are slightly out of phase with theirequivalents across the décollement (Fig. 9).DiscussionLayer-parallel shortening of a mechanically anisotropiclayered sequence overlying a ductile substrate produces123

474 Int J Earth Sci (Geol Rundsch) (2012) 101:463–482Fig. 8 Model 52. A singlecompetent layer is placed in thecentre of the ductile substrate.See text for discussion.a Undeformed, b 10%shortening, c 25% shorteningFig. 9 Model 59. a Photographof a slice through Model 59after 42% shortening. Theinterface between the ductilesubstrate and layered sequencecontains a 0.5 mm thick layer ofpolydimethylsiloxane across theentire model. A thick layer ofDC 3179 with thin internalmarker layers of plasticinerepresents a ductile décollementwithin the layered sequence.b Sketch of photographhighlighting key shorteningfeaturesactive, buckle folds with different geometries that vary as afunction of the thickness of layers in the upper sequence,the presence or absence of a low-viscosity and densityductile layer at the cover sequence–substrate interface, andthe presence of internal thick ductile horizons.Layer thicknessThe mechanical properties of the layered sequence have afundamental impact on fold styles and geometries producedduring buckle folding where the dominant wavelengthof folds in a layered sequence is controlled by thethickness of individual layers, the space between competentand incompetent layers, ease of slip between layers,the rheology of those layers, and the total bulk shorteningimposed on the model (e.g. Biot 1961; Ramberg 1967;Smith 1977, 1979). Models 52 (Fig. 8), 60 (Fig. 4), and 61(Fig. 5) are each composed of the same materials (but ofdifferent colours) in the same proportions and experienced25–37% bulk shortening but contain layers differing inthickness. Although the dominant wavelength betweenindividual models is similar (10–20 mm), the thinnestlayeredmodels show the greatest variety of fold styles,amplitudes, and wavelengths (Fig. 5). In general, thickerlayers promote a more regular fold train with a relativelymore consistent wavelength and amplitude (Fig. 8).Presence/absence of a weak ductile layerThe presence of a weak layer (PDMS) between the layeredsequence and ductile substrate promotes different foldstyles and geometries than the half of the same model123

Int J Earth Sci (Geol Rundsch) (2012) 101:463–482 475Fig. 10 Model 59. SequentialCT images showing oblique 3Dviews of both sides of the modelduring each stage of modeldevelopment: a undeformed,b Stage A (12% shortening)c and d Stage B (26%shortening), e and f Stage C(42% shortening)Fig. 11 Model 59. SequentialCT images showing 3D viewsof sections of both sides of themodel: a and b Stage A (12%shortening) c and d Stage B(26% shortening), e and f StageC (42% shortening)where it is absent. The same bulk shortening is accommodatedby folds with different styles and geometriesdepending on the viscosity and density of the mediumbeneath the interface. Folds above weak interface aretypically upright, round, and have a large amplitude(Fig. 5), whilst folds above a stronger interface are moreangular, tighter, and have a smaller amplitude (Fig. 5).Larger amplitude folds above the PDMS are likely theresult of decoupling where synforms sink into the lessdenseand less-viscous PDMS (Fig. 5), which flows laterallyinto anticlinal cores. PDMS produces a planar interfaceat the top of the ductile substrate and acts as an interfacethat promotes detachment folding of the layered sequence(Fig. 5). Folds in the cover sequence without a PDMS base123

476 Int J Earth Sci (Geol Rundsch) (2012) 101:463–482do not develop large amplitudes; rather a greater number oftight and angular folds form to accommodate the totalshortening and the layered sequence infolds along with theductile substrate (Figs. 4, 5). Despite these differences, foldhinges are similar in orientation in both halves of modelswith and without a PDMS horizon.Internal thick ductile horizonAn internal thick ductile horizon in the layered sequencecan decouple folds across it (Model 59; Fig. 9). Withoutsuch a ductile horizon, synforms become tighter and antiformsbecome more open moving upwards to the surfacesuch as in Model 52 (Fig. 8) to maintain space compatibility.The presence of a thick ductile horizon allowstighter antiforms and more open synforms to developabove the horizon than relatively open antiforms and tightsynforms below it (Fig. 9). The horizon itself is relativelymobile compared to the upper and lower bounding units,which allows it to form prominent cuspate structures in thehinges of folds (Fig. 9). This horizon can effectivelyinterrupt the fold style that typically develops in singlefolds throughout the layered sequence. The development ofminor folds above the ductile horizon that are not presentbelow it and local phase shifts in fold trains across thehorizon indicate a significant degree of decoupling ofshortening features across the horizon.Comparisons with previous modelsThe change from box folds with conjugate axial surfaces toupright chevron folds during progressive deformationproduced in our models also occurred in 1g experiments ofalternating lubricated ‘‘soft’’ and ‘‘stiff’’ plasticine multilayersembedded in slabs of a putty-plasticine mixture(Fowler and Winsor 1996). In some of their models, sliphas occurred along the basal contact that acts as a décollement,whereas the contact is folded in their other models;similar folds are produced in both cases. Fowler andWinsor (1996) provide a geometrical analysis of this processand compare the results to natural folds in turbidites incentral Victoria, Australia. Propagation of folds fromclosest to the moving end wall towards the fixed wall (i.e.foreland-propagating folds) occurred in all our modelssimilar to numerical models of Stockmal et al. (2007). Thisalso appears to be the case in centrifuge models of Sans andKoyi (2001), although is not discussed by these authors. Incomparison, whilst foreland propagation occurred in one ofthe models involving lateral compression of a thicksequence of ductile layers above a décollement zone at1g by Blay et al. (1977), folds developed diachronouslyin different parts of their models, and widely spacedindividual folds developed from furthest to the advancingend wall to progressively closer to the moving end wall(i.e. opposite to our models) in models with a large lengthto-thicknessratio. Non-sequential development of structuresalso occurred in thin sandbox models above a basalductile décollement by Costa and Vendeville (2002). Furtherexperiments are therefore required to establish whetherthe thickness of the cover sequence exerts a control on thesequence of fold formation in centrifuge models.Despite the lower density compared to the coversequence, no diapirs were developed in our models incorporatinga PDMS layer at the interface between coversequence and ductile substrate. As explained by Sans andKoyi (2001), the density inversion was not sufficient toovercome the shear strength of the modelling clay andsilicone layers of the cover sequence and no faults, boudins,or fractures developed in the cover sequence to focusdiapiric rise of the PDMS. Flow of the ductile substrateduring subsequent stages of differential erosion of ourmodels is discussed in a subsequent paper.Whereas most inclined folds in our experiments vergetowards the fixed end wall (equivalent to foreland-vergingfolds in nature), fold vergence in some models is variable,and folds developed above a relatively thick PDMS layer(Fig. 5) verge towards the moving end wall (equivalent tohinterland-verging folds in nature; further explored in Godinet al. 2011). Similar differences in fold vergence to ourmodels have been obtained in sandbox studies of thrustrelatedfaults of low metamorphic grade sedimentary rocksabove a salt horizon (Costa and Vendeville 2002), centrifugemodels of layered plasticine and silicones above asilicone putty base (Sans and Koyi 2001; Sans 2003), faultrelatedfolds in vice-models of a simplified lithosphericprofile with brittle upper crust overlying a weak ductilemid-crust and lower crust (Cruden et al. 2006), andnumerical models of a competent layered sequence above aductile detachment (Simpson 2009). Differences in foldasymmetry observed in the same model depending onwhether PDMS layer is present or absent agree with thetheoretical analysis by Pfaff and Johnson (1989) that suggeststhat fold asymmetry is determined largely by thebehaviour of layer contacts, especially the resistance to slip.The upper layered sequence in these previous models isconstructed to accommodate shortening via buckle foldingbut several key comparisons of deformation patterns can berelated to systems that deform via thrust-related faulting.The susceptibility of a material to faulting requires a lowelastic shear modulus, a high-viscosity décollement, and arelatively thick layered sequence compared to the ductilesubstrate (Simpson 2009). A brittle-ductile material shortenedover weak detachments (salt) promotes folding,whereas strong detachments (such as shale) promotethrusting of the upper-layered sequence (Sans 2003;123

Int J Earth Sci (Geol Rundsch) (2012) 101:463–482 477Simpson 2009). Previous analogue models of folding of acompetent sequence above a ductile substrate have focusedon thrust-related folds above a weak décollement (Costaand Vendeville 2002; Sans 2003). During the initialshortening stages of models presented here, folds propagatesuccessively towards the foreland akin to foreland-propagatingthrust systems (e.g. Chapple 1978). The rate offoreland propagation of the deformation front is more rapidwith a low basal friction in analogue sandbox models(Cotton and Koyi 2000). The low basal friction beneath thelayered sequence of the models presented here likelyinduces the rapid propagation of the deformation front asfolds nucleate throughout the model after the initial stageof shortening. Therefore, the low basal friction enhancesrapid propagation of buckle folds similar to fault-relatedfolds (Cotton and Koyi 2000). Although highly curved foldhinges and marked abrupt differences in fold geometryform across frictional and ductile décollement in sandboxmodels (e.g. Cotton and Koyi 2000; Luján et al. 2003),such consistent marked differences are not seen in ourmodels. Although in the initial stage of fold developmentcurved fold axes are formed due to differences in foldwavelength, no consistent sense of curvature across theinterface between décollements is apparent in our modelsand other folds develop across the entire width of ourmodel (e.g. Fig. 7).Comparison with natural examplesFold style and vergenceModels presented here are reminiscent of many geologicalsettings that contain a layered competent sequence over aductile substrate including (but not limited to): forelandfold and thrust belts above a weak (evaporitic) décollement(Fischer and Jackson 1999; McQuarrie 2004; Latta andAnastasio 2007) and a superstructure above a ductileinfrastructure (Harrowfield and Wilson 2005; Godin et al.2011). Folds developed in our models without PDMS aresimilar in style to those in the Nuncios Fold Complexdescribed by Wilkerson et al. (2007) where redbeds andwackestones separated by shales form upright box foldswhich change along the fold hinge into inclined folds. Theinclined folds overlie and are cored by a thick sequence ofshales, mudstones, gypsum, anhydrite, and halite (Wilkersonet al. 2007). Evaporite successions in foreland fold andthrust belts often display folds in the overlying competentsequence that are symmetric with no preferred vergence.Latta and Anastasio (2007) explained these fold patterns bythe evacuation of evaporates from beneath synclines in theoverlying competent sequence and flow into the core ofanticlines in the Sierra Madre Oriental foreland ascommonly interpreted in evaporite-based fold and thrustbelts (Mitra 2003; Sans 2003). Synforms sinking into aweak substrate can reach depths beneath their regional,initial stratigraphic level during regional shortening (e.g.Sans 2003). This evacuation of weak material from downgoingsynforms into the cores of antiforms is apparent inModel 61 (Fig. 5b). Similarities with migration of PDMSinto fold hinges in 1g sandbox experiments simulatingfolding above salt and evaporite horizons (e.g. Bahroudiand Koyi 2003; Sans 2003; Schreurs et al. 2002) suggestthe applicability of this process to evaporitic layers. Thepresent geometry of these folds is thought to originate fromthe progressive rotation of fold limbs around pinned antiformaland synformal hinges (Latta and Anastasio 2007).Conversely, Homza and Wallace (1995) suggest detachmentfolds can also form through migrating hingesdepending on initial detachment depths. The progressivetightening of folds around early-nucleated hinges in Model54 (Fig. 7) suggests that folds develop with a fixed hingeline as proposed by Latta and Anastasio (2007).Infolding versus ductile substrate flowInfolding of the layered sequence into the ductile substrateproduced in some models is reminiscent of cover–basementor superstructure–infrastructure infolding duringregional shortening in the Needle Mountains of Colorado(Harris et al. 1987), the Canadian Cordillera (Ross et al.1985), Archaean greenstone belts in South Africa (Kistersand Anhaeusser 1995), and in the central Hearne domain ofthe Trans-Hudson orogen (Aspler et al. 2002). At theinterface between the cover and basement, infolding canproduce a cuspate-lobate geometry (Harris et al. 1987;Ross et al. 1985; Kisters and Anhaeusser 1995) similar tomullions (Ramsay 1967; Urai et al. 2001; Krayenbuhl andSteck 2009). Cuspate-lobate structures observed in modelsare consistent with observations of Ramsay (1967) for theirsignificant viscosity contrast between layers. Infolding inthe central Hearne domain produced open and concentricfolds and locally box folds that are interpreted to haveformed when a minimum viscosity contrast between unitsexists (Aspler et al. 2002). Model 61 shows distinct antiformalcuspate structures at the hinterland end of the model(Stage A in Fig. 5) accompanied by concentric and openfolds at the foreland end of the model directly above theductile substrate (no PDMS). The concentric and openfolds in the foreland progressively tighten and locallydevelop into cuspate structures during further shortening(Stage B in Fig. 5). Folds above the PDMS are generallyopen and broad, and cuspate geometries only locallydevelop at the hinterland end of the model (Fig. 5d). Thissuggests that cuspate structures may evolve from initiallybroad concentric folds and that both can develop when123

478 Int J Earth Sci (Geol Rundsch) (2012) 101:463–482significant viscosity contrasts exist between the basementand cover. In addition, it suggests that the absence of aweak detachment zone promotes infolding. The centralHearne domain may therefore represent a juvenile stage inthe evolution of cuspate infolding.In contrast to infolding of the cover and basement, somecover sequences that contain upright folds are separatedfrom a penetratively deformed ductile basement by ashallow-dipping décollement. For example, in the SongpanGarzê fold belt of eastern Tibet upright buckle folds of aPalaeozoic sedimentary succession are separated from apenetratively deformed and locally migmatized granitoidbasement across a basal décollement of high-temperature,mid-pressure metamorphosed Devonian-Silurian slates(Calassou 1994; Xu et al. 1992; Roger et al. 2004; Harrowfieldand Wilson 2005). Folds in the cover sequenceshow axial planes orthogonal to the basal décollementwhere sheath folds indicate penetrative high shear strainductile deformation that also locally effects the basement(Roger et al. 2004: Harrowfield and Wilson 2005). Harrowfieldand Wilson (2005) suggest that this represents thetransition from a pure-shear dominated cover sequence to asimple-shear dominated ductile substrate. Models 61 and54 preserve a remarkably planar detachment between thedetachment zone (PDMS) and the underlying ductilesubstrate beneath upright folds in the cover sequence,analogous to the proposed development of the Songpan-Garzê fold belt (Harrowfield and Wilson 2005). This suggeststhe presence of a weak viscous detachment zone ofsufficient thickness will promote a planar interface betweenthe basement and the overlying basal décollement andcover sequence.Detachment foldingDetachment folding has been extensively documentedwhen a competent sequence is folded over a less-viscousductile substrate (e.g. Costa and Vendville 2002). One ofthe best examples of detachment folding is found in theJura Mountains of Switzerland where a rigid basement isseparated from Mesozoic sediments by a Triassic evaporitelayer (Fig. 12a; Buxtorf 1916). Detachment folds have alsobeen documented in the Tian Shan region of the TibetanPlateau, where Indo-Asian convergence has deformedTertiary terrestrial sediments above a basal evaporativehorizon into box-like to upright isoclinal folds (Fig. 12b;Scharer et al. 2004). Structures akin to detachment folds ina layered sedimentary sequence develop when a layer ofPDMS is present at the interface in our models (Fig. 12c).Detachment folding has been divided by Mitra (2003) intoFig. 12 Comparison of models to natural examples. a Detachmentfolds from the Jura mountains (modified from Mitra 2003 afterBuxtorf 1916). b Detachment folds from Tian Shan, China (modifiedfrom Scharer et al. 2004). c Detachment folds across the ductileinterface of Model 61. d Close-up photographs of the layeredsequence from Model 61 showing the transition from a disharmonicbox fold to a ‘lift-off’ fold (Mitra 2003) e from stages A and B,respectively. f Lift-off fold of the Weissenstein anticline in the Juramountains modified from Mitra (2003) after Buxtorf (1916)123

Int J Earth Sci (Geol Rundsch) (2012) 101:463–482 479two end members, suggested to represent the progressiveevolution of detachment folds: disharmonic detachmentfolds and lift-off folds, based on the fold patterns observedin the Jura Mountains. Lift-off folds have a parallelgeometry of the outer units and isoclinal folding of thedetachment between the upper sequence and less competentsubstrate in the core of the anticline where disharmonicfolds have a parallel geometry of the outer arc and nonparallelgeometries in the inner arc (Mitra 2003). Figure 12shows close-up photographs of Model 61 and the evolutionfrom a box fold (Fig. 12d) into a lift-off structure(Fig. 12e) compared with the Weissenstein anticline fromthe Jura Mountains (Fig. 12f). Mitra (2003) predicts thatincreased bulk shortening will progressively produce openbroadfolds, then disharmonic folds, and finally lift-offfolds. No single fold depicts all these stages, but examplesof all of these stages in Model 61 support the sequence ofdetachment fold formation proposed by Mitra (2003).ConclusionsCentrifuge analogue models display features that simulatethe development of first-order active folds in ductile layerswhere (1) a metamorphic basement and an overlying sedimentarycarapace constituting a sedimentary superstructureand metamorphic infrastructure is deformed throughlayer-parallel shortening in large hot orogens and in someArchaean granite-greenstone belts and (2) structures duringactive folding (in the absence of faulting) of a layeredsedimentary sequence upon a thick ductile substrate in foldbelts. CT scanning of models using newly developedtechniques and materials has proven to be an excellentmeans to examine the 4D development of structures withinmodels that has not been possible in previous studies.Viscosity contrasts across the basal décollement controlif detachment folding develops above a flat interface or ifinfolding dominates the cover–basement geometry. A weakbasal décollement promotes detachment folding, whereas arelatively stronger detachment promotes infolding. A weakdetachment also promotes a broad and open fold train incontrast to a more irregular sequence of folds above a moreviscous ductile substrate. Contrary to sandbox models,whilst folds may differ, no consistent curvature of fold axesacross the interface of a more ductile décollement isobserved.A layered sequence with thicker individual layers promotesa regular upright fold train with a consistent foldamplitude and wavelength during bulk shortening, whereasthinner individual layers show a more irregular fold distributionwith a greater variety of fold styles, vergences,amplitudes, and wavelengths. Folds above a weak ductiledécollement display either a slight hinterland vergence orno discernable preferred fold asymmetry. A stronger basaldécollement promotes foreland-verging folds. Cuspatestructures can develop within a weak horizon bounded bytwo stronger horizons and evolve from open and upright, totighter and angular, and finally through lateral materialmigration from the limbs into the hinges of individualfolds. Folds of a competent horizon within the basal ductilesubstrate differ and may be out of phase with folds in thecover sequence.Acknowledgments Acknowledgment is made to the Donors of theAmerican Chemical Society Petroleum Research Fund for funding CTscanning and centrifuge modelling research at INRS-ETE and toNSERC for Discovery grants to L. Harris and L. Godin. Modellingwas undertaken by C. Yakymchuk whilst recipient of an NSERCUSRA Summer Research Scholarship. The laboratory for physical,numerical, and geophysical simulations at INRS-ETE was funded byCFI and MELS-Q grants to L. Harris with contributions from INRS-ETE, Applied Geodynamics Laboratory of the Bureau of EconomicGeology (University of Texas at Austin, who donated the centrifuge),Sun Microsystems, Seismic Microtechnology, and Norsar. CT scanningwas undertaken by L.-F. Daigle in the Quebec MultidisciplinaryScanography Laboratory at INRS-ETE. Effective viscosity measurementswere undertaken by J. Poulin and E. 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