analogue benchmarks of shortening and extension schlisch/a45_2006_jslsp_  · analogue

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Analogue benchmarks of shortening and extension experiments

GUIDO SCHREURS1, SUSANNE J. H. BUITER2, DAVID BOUTELIER3,

GIACOMO CORTI4, ELISABETTA COSTA5, ALEXANDER CRUDEN3,

JEAN-MARC DANIEL6, SILVAN HOTH7, HEMIN KOYI8, NINA KUKOWSKI7,

JO LOHRMANN7, ANTONIO RAVAGLIA9, ROY W. SCHLISCHE10,

MARTHA OLIVER WITHJACK10, YASUHIRO YAMADA11, CRISTIAN CAVOZZI5,

CHIARA DELVENTISETTE12, JENNIFER A. ELDER BRADY10,

ARNE HOFFMANN-ROTHE7, JEAN-MARIE MENGUS6,

DOMENICO MONTANARI12 & FARAMARZ NILFOROUSHAN8

1Institute of Geological Sciences, University of Bern, Baltzerstrasse 1-3, CH-3012 Bern,

Switzerland (e-mail: schreurs@geo.unibe.ch)2Centre for Geodynamics, Geological Survey of Norway, 7491 Trondheim, Norway

3Department of Geology, University of Toronto, 22 Russell St., Toronto,

Ontario M5S 3B1, Canada4CNR-Istituto di Geoscienze e Georisorse, Sezione di Firenze, via G. La Pira 4,

I-50121 Firenze, Italy5Dipartimento di Scienze della Terra, Universita di Parma, Parco Area delle Scienze 157/A,

I-43100 Parma, Italy6Institut Francais du Petrole, 1 et 4 avenue de Bois Preau, F-92500 Rueil Malmaison, France

7GeoForschungsZentrum Potsdam, Telegrafenberg, D-14473 Potsdam, Germany8Hans Ramberg Tectonic Laboratory, Department of Earth Sciences, Uppsala University,

Villavagen 16, S-75326 Uppsala, Sweden9Dipartimento di Scienze della Terra, Universita di Pavia, via Ferrata 1, I-27100 Pavia, Italy

10Department of Geological Sciences, Rutgers University, Piscataway, NJ, 08854, USA11Department of Civil and Earth Resources Engineering, Kyoto University,

Kyoto 606-5801 Japan12Dipartimento di Scienze della Terra, Universita di Firenze, via G. LaPira 4, 1, I-50121

Firenze, Italy

Abstract: We report a direct comparison of scaled analogue experiments to test thereproducibility of model results among ten different experimental modelling laboratories.We present results for two experiments: a brittle thrust wedge experiment and a brittle-viscous extension experiment. The experimental set-up, the model construction technique,the viscous material and the base and wall properties were prescribed. However, each lab-oratory used its own frictional analogue material and experimental apparatus. Comparisonof results for the shortening experiment highlights large differences in model evolutionthat may have resulted from (1) differences in boundary conditions (indenter or basal-pullmodels), (2) differences in model widths, (3) location of observation (for example, sidewallversus centre of model), (4) material properties, (5) base and sidewall frictional properties,and (6) differences in set-up technique of individual experimenters. Six laboratories carriedout the shortening experiment with a mobile wall. The overall evolution of their models isbroadly similar, with the development of a thrust wedge characterized by forward thrustpropagation and by back thrusting. However, significant variations are observed inspacing between thrusts, their dip angles, number of forward thrusts and back thrusts, andsurface slopes. The structural evolution of the brittle-viscous extension experiments issimilar to a high degree. Faulting initiates in the brittle layers above the viscous layer inclose vicinity to the basal velocity discontinuity. Measurements of fault dip angles and

From: BUITER, S. J. H. & SCHREURS, G. (eds) 2006. Analogue and Numerical Modelling of Crustal-Scale Processes.Geological Society, London, Special Publications, 253, 127.0305-8719/06/$15.00 # The Geological Society of London 2006.

fault spacing vary among laboratories. Comparison of experimental results indicates anencouraging overall agreement in model evolution, but also highlights important variationsin the geometry and evolution of the resulting structures that may be induced by differencesin modelling materials, model dimensions, experimental set-ups and observation location.

Geoscientists have used scaled experimentalmodels (from here on also referred to as analoguemodels) for more than a century to gain insightinto the kinematic and dynamic evolution of geo-logical structures (e.g., Cadell 1888; Ramberg1981; Koyi 1997). The ability to observestructures while they form is both attractive andinsightful, and the ease with which modelexperiments can be performed has certainlycontributed to their widespread application.Although many experiments have investigatedsimilar geological processes, a direct comparisonof experiments using the same prescribed set-uphas until now been lacking. Testing the reprodu-cibility of model results among different labora-tories is important as it allows for documentationof the degree to which variations in modelapparatus, modelling materials and experimentaltechniques can affect the outcome of exper-iments. Assessing the reliability of analoguemodels also has implications for comparisonsbetween structures formed in experimentalmodels, results of numerical simulations andnatural field examples.

We present the results of two analoguebenchmarks: a shortening experiment and anextension experiment. Ten modelling labora-tories participated in these experiments. Theexperimental set-up, the construction technique,the viscous material (employed in the extensionexperiment) and the material covering wallsand base were prescribed. Since each laboratoryused its own frictional analogue material (sand,microbeads, wet clay) and experimental appar-atus, our study is not a benchmark in its strictestsense. Our aims are to (1) indicate the degree ofreproducibility of model results between differ-ent laboratories, (2) highlight the influence ofmaterial properties, model boundary conditionsand experimental set-up, and (3) use our resultsto suggest further benchmark experiments.

The two experimental set-ups were designedto simulate upper-crustal processes, and can beplaced among a wide range of laboratory exper-iments studying contractional and extensionaldeformation. Numerous studies document exper-iments of thrust wedges under normal gravityconditions (e.g., Davis et al. 1983; Malavieille1984; Mulugeta & Koyi 1987, 1992; Mulugeta1988; Colletta et al. 1991; Lallemand et al.1994; Koyi 1995; Storti & McClay 1995;

Mugnier et al. 1997; Gutscher et al. 1996,1998; Storti et al. 2000; Schreurs et al. 2001;Turrini et al. 2001; Kukowski et al. 2002;Costa et al. 2004; Hoth et al. 2005). Deformationin these experiments occurs either by displacinga back wall or by pulling a basal sheet below afixed wall. Such indenter-type or basal-pull-typemodels are not entirely equivalent, as we willdiscuss below. Parameters investigated in pub-lished thrust wedge experiments include basalfriction, the presence of decollement layers,dip of the initial basal surface, surface slope,and the effects of erosion. In general, suchmodels display a series of forward-propagating,in-sequence thrusts, whereby changes in, forexample, basal friction or erosion may causerenewed hinterland activity. Many authorshave investigated brittle-viscous extension in anormal gravity field (e.g., Tron & Brun 1991;Vendeville & Jackson 1992a, b; Brun et al.1994; Keep & McClay 1997; Brun 1999; Gartrell2001; Bahroudi et al. 2003). In these models, aviscous layer typically overlies a velocity dis-continuity at the base of the model and is itselfoverlain by brittle materials. In all experimentsthe viscous basal layer exerts a strong controlon fault nucleation and location.

In a companion paper, Buiter et al. (2006)present numerical equivalents of both analogueexperiments and compare the results qualitat-ively and quantitatively. This study shows thatnumerical models employing different tech-niques (finite-element, finite-difference anddistinct element methods) are able to reproduceresults of analogue models successfully. Thecombination of analogue and numericalmodelling methods may help establish therobust features of models of crustal-scale pro-cesses (e.g., Ellis et al. 2004; Pysklywec &Cruden 2004; Panien et al. 2006).

Methods

Material properties

We focus on experiments performed in a naturalgravity field. In such experiments dry sand or wetclay are commonly used to simulate brittle rocksin the upper crust, and viscous materials to simu-late salt or lower crustal rocks. In the shorteningexperiment, the models consisted of granular

G. SCHREURS ET AL.2

materials (sand and glass microbeads) only,whereas for the extension experiment bothbrittle and viscous materials were used. All lab-oratories used the same polydimethylsiloxane(PDMS) as viscous material. This is a linearviscous material (silicone polymer) with a vis-cosity of 5 104 Pas at room temperature andat strain rates below 3 1023 s21 (Weijermars1986; see also Ten Grotenhuis et al. 2002;Cruden et al. 2006). In contrast, each laboratoryused its own brittle materials, which generallydiffered in density, frictional properties, grainsizes and grain shape (Table 1). Several labora-tories measured the frictional properties of theirgranular materials using either a Hubbert-typeshear box (Hubbert 1951), a ring-shear tester(Schulze 1994) or a Casagrande shear box(Casagrande 1932). Errors in ring-shearmeasurements are usually small (less than 1%).Measurements in Hubbert-type shear boxes areless precise, because of the larger uncertaintyassociated with the determination of the exacttiming of the onset of shearing. As theCasagrande shear box measurements in thisstudy employed higher normal loads than occurin the experiments, the measured values forcohesion are not considered representative forthe experimental conditions.

Although experimental modellers have oftenassumed that their brittle materials deformaccording to the Coulomb failure criterion withconstant frictional properties, several studiesdemonstrate that

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