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_Journal of Structural Geology 33 (2011) 62e77_ _Contents lists available at ScienceDirect_ _Journal of Structural Geology_ _journal homepage: www.elsevier.com locate jsg_ _Structural evolution of fold-thrust structures in analog models deformed in a large geotechnical centrifuge_ _Todd E. Noble*, John M. Dixon_ _Experimental Tectonics Laboratory, Department of Geological Sciences, Queen’s University, Kingston, Ontario, Canada K7L 3N6_ _article info_ _Article history: Received 7 May 2010 Received in revised form 11 December 2010 Accepted 13 December 2010 Available online 21 December 2010_ _Keywords: Analogue experiments Centrifuge modeling Fault-propagation folding Distance-displacement methods Thrust sheet deformation_ _abstract_ _We investigate the structural evolution of fault-propagation folds and fold-thrust systems with scaled analog modeling carried out using the 5.5 m radius geotechnical centrifuge at C-CORE, St. John’s NL. The experiments presented here are the first of their kind, scaled ten times larger than predecessors and deformed using a custom rig with load monitoring and displacement control. Plane-layered models approximately 1 m long and representing 50 km sections are shortened horizontally under an enhanced gravity field of 160 g. The large model scale allows for a proportionally large number of bedding laminations that act as strain markers. This allows detailed analysis of strain partitioning and interplay, both at the scale of a fold-thrust system and the individual fold-thrust structure. Layer-parallel shortening (“LPS”) and rotation of fault-bounded blocks are revealed by mapping contraction fault populations and bedding-contraction fault intersection angles. Low-angle contraction faulting and LPS are found to be dominant at early stages of development and rotation of fault-bounded blocks occurs during progressive folding of the hanging-wall panel during fault-propagation folding. Displacement-distance data obtained from major thrusts in the model show relative stretch values, and consequently fault slip propagation ratios, that are similar to natural structures._ _? 2010 Elsevier Ltd. All rights reserved._ _1. Introduction_ _Reconstructing a complete multi-scale strain history of a natural fold-thrust structure based on field observations of macroscopic deformation features and microscopic fabric elements is challenging. The deformation processes of folding, faulting and penetrative deformation are competing strain accommodation mechanisms, all of which are active in varying proportions throughout the evolution of a fold-thrust structure (Jamison, 1992; Woodward, 1999). The scale of the observation is also important; for example, penetrative strain in a thrust sheet can be viewed either as a ductile process at the regional scale or brittle deformation at the hand-sample and thin-section scale (Price, 1973). Re-construction of a strain history therefore requires both an understanding of the relationship and interaction of the macroscopic folding and faulting processes, and the mesoscopic and microscopic fabric-forming processes._ _Field observations of the interaction of folding and thrusting (Willis and Willis, 1934; Dahlstrom, 1970; Williams and Chapman, 1983) have been used to develop geometric and kinematic models_ _* Corresponding author. Present address: Shell Canada Ltd., 400 4th Avenue SW, P.O. Box 100, Station M, Calgary, Alberta, Canada T2P 2H5. Tel.: ?1 403 691 3251._ _E-mail addresses: Todd.Noble@Shell.com (T.E. Noble), John.Dixon@queensu.ca (J.M. Dixon)._ _0191-8141 $ e see front matter ? 2010 Elsevier Ltd. All rights reserved. doi:10.1016 j.jsg.2010.12.007_ _for detachment and fault-propagation folding, structural styles which are characterized by different types of interaction between folding and faulting (e.g. Suppe, 1983; Jamison, 1987; Suppe and Medwedeff, 1990). Mechanisms such as brittle fracturing and penetrative deformation accommodate internal strain during the deformation and transport of thrust sheets (e.g. Reks and Gray, 1983; Wojtal, 1986; Geiser, 1988). Mesoand microscopic features such as faults, fractures, cleavage and stylolites constitute fabric elements that are useful for reconstructing strain history (e.g. Woodward et al., 1986). Overprinting and cross-cutting of structural elements observed at outcrop, hand-sample and microscopic scales suggest that the strain distribution has evolved within the rocks according to the local passage of propagating fault tips, fold growth and thrust sheet transport (Dahlstrom, 1970; McConnell et al., 1997; Nicol et al., 2002; Price, 1967). Furthermore, field studies suggest that thrust sheet strain varies systematically between frontal and lateral ramps, as a function of thrust propagation and internal strain mechanisms specific to their geometries (Coward and Potts, 1983; Wibberley, 1997). In this context, dislocated folds can be interpreted as remnants of the propagation stage of a thrust fault now carried passively or modified by subsequent thrust sheet strains. Earlier strain events related to the incipient propagation of fault tips and the low-strain “process zone” deformation (Cowie and Scholz, 1992; Jolley et al., 2004) are rarely preserved, and are difficult to deconvolve from subsequent strain._ _T.E. Noble, J.M. Dixon Journal of Structural Geology 33 (2011) 62e77_ _63_ _events that are imparted on the rocks. Subsequent phases of deformation can be imparted on thrust sheets through a normal sequence of thrust development, with younger thrusts tending to form beneath and on the foreland side of older ones. The resulting older fold-thrust structures tend to be subjected to deformation as they are carried up ramps on the underlying younger thrusts._ _Since geological structures evolve on a time-scale too large for direct observation, a high-resolution forward modeling method is needed that captures incipient strain patterns and clearly demonstrates the evolution and interplay between strain mechanisms at different scales. Ideally, modeling enables prediction and explanation of strain type, intensity and distribution for fold-thrust structures that are comparable to field outcrop examples._ _Physical analog modeling is a proven tool for understanding the interplay of folding, thrusting and penetrative strain. Pioneering work by Davis et al. (1983) and Malavieille (1984) laid the foundation of modeling fold-thrust structures with simple sandbox models. Peltzer (1988) used a large geotechnical centrifuge to conduct experiments investigating the large-scale tectonics of the IndiaeEurasia collision. Dixon and Liu (1992) and Liu and Dixon (1995) used the centrifuge technique of modeling with layered visco-plastic materials to demonstrate localization of thrust ramps by buckle folding; Storti et al. (1997) studied the nucleation and displacement profiles of thrust fault-related folds in sandbox models with layered granular materials; and Adam et al. (2005) introduced high-resolution optical correlation techniques to monitor displacement fields in order to quantify the spatial and temporal patterns of strain accumulation, including folding, faulting and penetrative strain. A short-coming of sandbox models that use granular materials is the model stratigraphic sequences typically fail to simulate the inherent bedding anisotropy of natural strati?ed sedimentary rocks. Some degree of anisotropy has been added to sandbox modeling by introducing materials with different mechanical properties (Colletta et al., 1991; Teixell and Koyi, 2003). In addition, the granular materials in sandbox models do not typically reveal the small-scale brittle deformation features that contribute to the overall internal strain picture. Plasticine and silicone putty analog models also have their limitations: they tend to deform in ductile folding styles typical of structures in the middle to deep crust; the adhered nature of the layers tends to inhibit interlayer slip widely observed in natural structures; and surface processes like erosion and sedimentation are difficult to include in the modeling process._ _This paper presents experimental results of dynamically scaled analog models constructed of stiff visco-plastic materials that are deformed in a large (5.5 m radius) geotechnical centrifuge. These models are based on smaller-scale equivalents deformed in a (30 cm radius) centrifuge at the Experimental Tectonic Laboratory at Queen’s University (Dixon and Liu, 1992; Liu and Dixon, 1990, 1991, 1995). This analog modeling technique enables observation of the structural evolution of fold-thrust structures, including fold-thrust relationships and the development of deformation fabrics. With their large size, the models are constructed with a large number of thin, mechanically passive laminae “bedding” which act as strain markers. The strain markers record mesoscopic deformation fabrics that reveal where and when strain accommodation mechanisms are active within fold-thrust structures. Also, these markers record the evolution of fold-thrust relationships during the deformation, and allow a comparison to geometrical and kinematic models previously derived from natural analog structures. We demonstrate that these models are scaled representations of earlier centrifuge experiments as well as natural prototype systems; passive bedding laminae reveal complex strain accommodation mechanisms and fold-thrust relationships unseen in previous centrifuge models; and model fold-thrust relationships_ _are consistent with published folding and fault-propagation concepts formed from observation of natural structures._ _2. Experimental setup and procedure_ _2.1. Modeling apparatus_ _The experimental work reported here was carried out at the CCORE facility in St. John’s, NL, Canada that houses an Accutronic 680-2 centrifuge (Fig. 1). This machine can accommodate a payload package measuring up to 1.1 m (h) by 1.4 m (l) by 1.1 m (w), with a load capacity of 130 g-tonnes at a maximum acceleration of 200 g that is attained at 189 RPM._ Ключевые слова: internal, fold, core, thrust, model demonstrates, ramp angle, rheological behavior, model deformed, entire forelimb, nicol, buckle, journal structural geology, evolution, silicone, loading platen, suppe, characteristic velocity, fault-propagation folding, competent, subsequent strain, block, thickness, mcclay, hanging-wall panel, slope change, incompetent unit, mechanism, leading edge, fold-thrust structure, development, horizontally shortened, fold growth, model, chapman, folding, stress, jamison, displacement, sheet, bulk, brittle, strain, strain accommodated, geology, willis, rheology, competent beam, wall, structural, ramp, geometric analysis, large number, hanging, internal strain, strain marker, fault-bounded, c-core experiment, lps, thickening, wojtal, fault-propagation, sequential development, tectonics, homogeneous lps, dixon, journal structural, kinematic model, canada, structural dip, centrifuge modeling, woodward, tectonophysics, penetrative strain, subsequently develop, structural geology, ms, hanging wall, shortening, williams, table, carried passively, prototype, university, regular spacing, mesoscopic subfabrics, relative, major thrust, stratigraphic sequence, mature structure, summers, ramberg number, stage, strain rate, rate, liu dixon, granular material, centrifuge, scale, slip, incompetent, model ratio, deformation mechanism, fold thrust, fold-thrust, geological, earth, dixon journal, fault, experiment, angle, bef, queens university, contraction, ratio, foreland, bedding lamination, deformed, brittle deformation, strain partitioning, time ratio, bulletin, iii, stress ratio, overthrust, displacement-distance diagram, contraction fault, thrust propagation, centrifuge experiment, queen, relationship, silicone putty, prototype rock, stretch, liu, analog, acceleration ratio, american, thinning, mitra, unit, cowie scholz, dixon journal structural, journal, accommodated, material, lamina, thrust belt, progressive folding, koyi, progressive evolution, natural, fault slip, putty, plasticine, thrust structure, large, mcconnell, cut, fault-bounded block, mm, model material, gray, london, competent unit, fault propagation, queens centrifuge, c-core, deformation, length, thrust sheet, inter-limb angle, rotation, model fold-thrust, overthrust structure, price, layer, noble, natural structure, stratigraphic, relative displacement, dixon journal structural geology, thrust ramp, panel, fault plane, elsevier, petroleum, natural prototype, fault-propagation fold, geometric, thrust fault, footwall panel, relative stretch, bedding, ramberg, duplex structure, c-core centrifuge, dixon summers, dahlstrom, transverse proles, initially planar, geometry, horizontal shortening, noble dixon, box, experimental, total shortening, theoretical geometry, eld study, forelimb, gravity, rheological data, fold thrust structure, modeling, reversible sheeter, geometric model, sandbox model, hanging-wall, rock, earths crust, fault nucleates, vertical thickening, continued, structure, structural evolution, test package, propagation, buckle fold, deformation fabric, early stage