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

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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
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