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

More documents

Recommendations

Info

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

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
Page 1 and 2: Int J Earth Sci (Geol Rundsch) (201
Page 5: Int J Earth Sci (Geol Rundsch) (201

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

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?