Himalayan hinterland-verging superstructure folds related to ...

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6L. Godin et al. / Journal of Structural Geology xxx (2010) 1e14Fig. 5. Compilation of flow properties of Dow Corning 3179 dilatant compound (DC3179; Poulin, 2006), Crazy Aaron Thinking Putty (CATP; Poulin, 2006), and polydimethylsiloxane(PDMS; Weijermars, 1986; ten Grotenhuis et al., 2002).Fig. 4. (A) Model set-up showing the collapsing wedge in the hinterland that activateslayer-parallel shortening, similar to Dixon and Tirrul (1991). Models consist of a 10 mmthick brittleeductile superstructure overlying a 5 mm thick ductile infrastructure. Aweighted nylon plate is inserted between the model and the collapsing wedge toprovide an equal distribution of force across the hinterland face of the model. Theportrayed irregular erosion front was created on some models, while others containedan extra layer of polydimethylsiloxane (PDMS), a clear, low density and viscositypolymer, at the infrastructure-superstructure interface. (B) Model composition. Notethat a thin (0.5 mm) layer of PDMS was introduced in some models to mimic thepresence of migmatites and leucogranites in the immediate footwall of the SouthTibetan detachment system. All models were covered by a thin layer (1e2 mm) of lowdensityCrayola Model Magic.buffer the density contrast between the uppermost superstructureand air (Fig. 4) as no brittle, upper crustal layer was included. Theinfrastructure of the models is composed of Crazy Aaron EnterprisesThinking Putty Ò , a silicone putty (similar to Dow Corning RhodorsilGomme Ò whose composition and properties are described byWeijermars, 1986) with scaled densities and viscosities that representductile melt-weakened mid-crust. Effective viscosities ofmaterials at experimental strain rates are presented in Fig. 5. In themodels, the ductile infrastructure has approximately the samedensity (
L. Godin et al. / Journal of Structural Geology xxx (2010) 1e14 7folding; (2) presence or absence of less-viscous ductile layer at thedetachment; and (3) angle of focused erosion front.4.1. Erosion and infrastructure flow below an unfolded layeredsuperstructureThe goal of this experiment was to evaluate if underlying horizontalductile stretching flow (stretching in the direction of flow) ofthe infrastructure could create superstructural folds, as suggestedby Carosi et al. (2007) (Fig. 3c). Model 47 (95 mm length) consists ofan undeformed layered superstructure resting above a homogeneousinfrastructure (Table 2).An erosion front was cut at the foreland-end of the model beforethe initial run. This was done to impose at the onset a lateral lithostaticpressure gradient deemed essential for mid-crustal flow(i.e. channel flow; Beaumont et al., 2001). The erosion front was cutright down to the base of the infrastructure, with half the modelhaving a steep w60 e80 front and the other half with a shalloww30 e45 front (and a gentle curved transition between the twoerosion fronts e see Fig. 4). The model was run in seven stages; theerosion front angle was re-cut after each step to maintain initialconfiguration. The last two stages of this experiment were run atpeak acceleration (w1100 g) for 15 min with the same run-up anda slightly longer run-down time in an attempt to trigger superstructuralfolding.The model behaved similarly with a steep or shallow erosionfront. No deformation was recorded in the superstructure in eithercase, whereas significant horizontal stretching and vertical thinningoccurred in the infrastructure (Fig. 6). The superstructuremicrolaminates remain parallel and were not affected by eitherflow-parallel extension or shortening. Although a single erosionfront was maintained, the infrastructure unexpectedly flowedtowards the erosion front and the rigid backstop. Despite the factthat the model did not behave as originally envisaged, the resultsshow that at no time during the experiment can the infrastructuralhorizontal flow trigger superstructure folding.4.2. Infrastructural flow below already established superstructurefolds e the influence of detachment ductilityThe second experiment (Model 50) was run to assess theinfluence of the ductility of the detachment on the potential forsuperstructural fold geometry modification (Fig. 7). In this model,a 0.5-mm thick layer of Polydimethylsiloxane (PDMS), simulatingmigmatites and melt pooling in the upper part of the infrastructure,is incorporated on half the superstructure-infrastructureinterface to investigate its effects on superstructure foldgeometry and superstructure-infrastructure decoupling efficiency(Table 2).The model underwent two stages of shortening withouterosion (31% of total shortening), followed by four stages duringwhich the erosion front was created and then maintained at 40 dip (Fig. 7). The erosion front was cut down to the base of theinfrastructure.Fig. 6. Sequential photographs of Model 47. In this experiment, an erosion front cutting down to the base of the infrastructure was created during the first model run (left side) togenerate a dominant foreland-directed horizontal infrastructural flow. Half the model had a steep w60e80 and the other half a shallow w30e45 front. Since both sides hadsimilar behaviours, only the steep side is shown. At no time during the experiment did the superstructural package fold. The bulk of the deformation was entirely localised inthe infrastructure, which underwent vertical thinning and horizontal stretching, mostly towards the erosion front, but also towards the hinterland side of the model in thelater stages.Please cite this article in press as: Godin, L., et al., Himalayan hinterland-verging superstructure folds related to foreland-directed..., Journal ofStructural Geology (2010), doi:10.1016/j.jsg.2010.09.005
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