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

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

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
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