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_Journal of Structural Geology 32 (2010) 28–44_ _Load and unload as interference factors on cyclical behavior and kinematics of Coulomb wedges: Insights from sandbox experiments_ _S. Bigi, L. Di Paolo, L. Vadacca, G. Gambardella Dipartimento Scienze della Terra, Sapienza Università di Roma, P.le A. Moro, 5 – 00183 Roma, Italy Dipartimento di Geologia, Università di Roma Tre, L.go S. Murialdo, 5 – 00147 Roma, Italy_ _Article info_ Article history: Received 1 September 2008; Received in revised form 13 May 2009; Accepted 29 June 2009; Available online 9 July 2009 _Keywords: Analogue model Critical taper Accretion mode Sedimentation Erosion_ _Abstract_ Modes of accretion in orogenic wedges are strongly controlled by mass balance and the efficiency of basal detachment. Sandbox models have demonstrated that fault systems grow episodically via cycles of alternating wedge thickening and lengthening. Generally, a new thrust plane generates within the footwall of the previous one, following a piggy-back mode of accretion, whereas redistribution of the mass balance in the wedge is associated to underthrusting and reactivation of previous thrusts. We present results from five sandbox experiments that model interaction between tectonic accretion, sedimentation in foredeep area, and erosion in axial zone. These experiments analyze influence of these events on cyclical behavior of a growing wedge. Modifications of initial setup were made to examine effect on wedge development by syntectonic denudation, syntectonic sedimentation, and coeval sedimentation and erosion, which markedly altered mass transfer within the wedge. Lowering surface slope by syntectonic erosion triggered synchronous accretion and underthrusting modes; in contrast, sudden syntectonic sediment load in prowedge region promoted prolonged phases of underthrusting, retarding accretion of new imbricates at prowedge toe. Whether wedge volume increased or not. High length thickness ratio (120 cm × 2.5 cm) allowed monitoring complete cycle of system recovery after each episode of sedimentation and erosion, observing how wedge reacted to modifications both altering its own cyclicity and progressively recovering it. © 2009 Elsevier Ltd. All rights reserved. _1. Introduction_ _Thin-skinned thrust belts and accretionary wedges form at convergent plate margins by horizontal shortening and vertical thickening of sedimentary strata. In response to horizontal compression, sedimentary sequences undergo thrust faulting, which is the main mechanism of deformation. At the same time, especially in continental convergence settings, both lithospheric flexure and superficial processes control kinematic behavior of orogenic wedges. Critical taper theory has proven very effective in linking force balance of a growing orogenic wedge to its bulk geometry, mechanical properties, and kinematics (Chapple, 1978; Davis et al., 1983; Dahlen, 1984; Dahlen et al., 1984). This approach has been successfully used to explain mechanics of fold-and-thrust belts, forearc wedges, and entire orogenic belts (Davis et al.,_ _* Corresponding author. Tel.: +39 06 49914922; fax: +39 06 4454729. E-mail address: sabina.bigi@uniroma1.it (S. Bigi).* _0191-8141 $ – see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jsg.2009.06.018_ _Scaled analogue experiments with granular materials (e.g., sand) link internal structural evolution of the wedge to material properties, thereby yielding a first-order approach to understanding mechanics of different deformation styles present in nature (Malavieille, 1984; Liu et al., 1992; Lallemand et al., 1994; Koyi, 1995; Gutscher et al., 1996, 1998; Wang and Davis, 1996)._ _Accretionary wedges or fold-and-thrust belts, as well as sand wedges, are described in terms of the critical angle (q), defined as the sum of dip of wedge topography (a) and dip of basal detachment (b). The theory of critical taper predicts that coefficients of friction of both wedge material (m) and basal detachment (mb) control variation of these angles. Simple variations in m mb ratio allow derived equations to describe complete sequence of faulting processes occurring at contractional margins. Moreover, according to the theory, processes of deformation progressively vary shape of wedge until it reaches critical taper value (Davis et al., 1983; Dahlen, 1984; Dahlen et al., 1984)._ _Observed variation of critical taper values in analogue models and natural wedges shows cyclical behavior characterized by discontinuous phases of alternating wedge thickening, accommodated by slip along thrust faults within the wedge, and wedge lengthening, accommodated by nucleation of new thrust faults in front of the wedge (Mulugeta and Koyi, 1992; Storti and McClay, 1995; Gutscher et al., 1996; McClay and Whitehouse, 2004; Konstantinovskaia and Malavieille, 2005; Del Castello and Cooke, 2007). These two processes, referred to as underthrusting and accretion, progressively reduce q variation until it reaches constant critical value (Mulugeta and Koyi, 1992), and in those analogue models where dip angle of basal detachment is assumed constant (b = 0), topographic slope a becomes only observed critical parameter. This makes possible observation of way in which (a) value and wedge itself react to potential interference factors affecting system._ _Ramp spacing and length of the wedge show cyclical behavior during deformation; general re-organization can be observed after modification of system._ _Syntectonic sedimentation and erosion strongly influence mechanical balance of foreland thrust-and-fold belts and accretionary prisms, consequently their a value and wedge kinematics (Davis et al., 1983; Willet et al., 1993; Beaumont et al., 1999; Storti et al., 2000; Graveleau and Dominguez, 2008). Using sandbox modeling, several authors have focused analysis on erosion located at top of double-verging wedges (Munoz, 1992; McClay et al., 1999; McClay and Whitehouse, 2004), syntectonic sedimentation (Storti et al., 1995; Mugnier et al., 1997), and interaction between tectonics, erosion, and sedimentation in forearc region of accretionary wedges or foreland thrust belts (Malavieille et al., 1993; Larroque et al., 1995; Graveleau and Dominguez, 2008)._ _We used experimental models to study synchronous sedimentation and erosion with shortening of sand wedges, analyzing interaction between fold-and-thrust belt construction and foredeep basin development. Experimental apparatus has general configuration similar to one used to simulate double-verging wedge characterized by reversal kinematics occurring in two stages (Liu et al., 1992; Storti et al., 1995, 2000; Persson and Sokoutis, 2002)._ _Focus was mostly on second stage of wedge evolution, characterized by propagation of thrusting in prowedge (sensu Storti et al., 2000). Length of experimental apparatus allows simulation of asymmetric wedge where prowedge area is more developed than retrowedge. This configuration is effective for analysis of cyclical behavior of critical parameters because it allows observation of whole recovery process after every episode of erosion and sedimentation._ _Axial and retrowedge zones considered as equivalents of inner zones of chain, where contractional deformation acts mainly before prowedge development and subsequent deformation results from general mass and force balance. Resulting strongly asymmetric wedges remind natural examples such as Apennines and Carpathians in continental settings (Doglioni et al., 1999) or Banda Arc and Barbados in oceanic settings (Westbrook et al., 1988; Bigi et al., 2003; Lenci and Doglioni, 2007), characterized by well-developed fold-and-thrust belts with respect to double-verging inner part of chain. Origin of double-verging inner part of natural wedges largely debated and interpreted in different ways; west-dipping subduction of continental or oceanic crust, continental collision, inversion of intraplate extensional basin, as well as switch of subduction polarity are among most common proposed models (e.g., Doglioni et al., 1999; Frizon De Lamotte, 2005; Munoz, 1992). Whether or not double-verging inner area is genetically connected to subduction (as was case in our models), initial configuration adopted allows verification if, when and how retrowedge area undergoes deformation connected to evolution of fold-and-thrust belt in prowedge (i.e., uplift, erosion, out-of-sequence thrusting, extension)._ _Localization of sedimentation in external part of developing prowedge simulates role of sedimentation in foredeep domain although experiments cannot account for isostatic subsidence and its mechanical effects. Foredeep basins considered critical features in evolution of fold-and-thrust-belts because they are sites of repeated events of syntectonic loading on sole thrust of orogen and still undeformed foreland (De Celles and Giles, 1996). First experiment E1 performed using base configuration repeated in all other experiments with_ Ключевые слова: mcclay, sixth thrust, evolution, detachment, thrusting, malavieille, westbrook, coeval sedimentation, underthrusting, tectonophysics, mode, surface slope, strain partitioning, critical taper, thrust sheet, wedge reached, tectonics erosion, shortening, previous experiment, total, numerical model, geology, lp lr, base model, experimental apparatus, journal structural, result, koyi, deformation rig, bigi, del castello, growing wedge, accretionary prism, syntectonic sediment, analogue model, davis, basal detachment, length, friction, variation, ramp formed, sequence, ?nal, basin, bulletin, active, slot, pack, asymmetric wedge, taper, ramp, elsevier, surface, balance, journal, lr, regular mode, vertical, parameter, thrust fault, prowedge, behavior, mass, castello, undeformed foreland, cyclical, thrust tectonics, wa, thickness, cyclicity, experiment, slope, stage, zone, ?rst, fold-and-thrust belt, cm, drafting, critical, fold, thrust wedge, mechanical balance, bigi journal, wedge accretion, doglioni, material, inactive thrust, mulugeta, total volume, vertical load, york, displacement, height, total shortening, subduction, subduction slot, retrowedge area, retrowedge, rt, teixell, axial zone, deformation increment, basal, ap, kinematics, strength, accretionary wedge, supercritical condition, earth, willet, belt, sedimentationerosion episode, activity, box, total mass, wedge, imposed, regular cyclicity, area, structural geology, experimental, sand, prowedge area, rst stage, frontal, fault, ramp spacing, strength governing, axial, mugnier, thrust plane, syntectonic, model, cyclical behavior, wedge started, sequential evolution, journal geophysical, axial area, thrust belt, accretion, topographic slope, synchronous thrusting, pre-deformation sequence, doi, thrust, forced redistribution, episode, hardy, selected parameter, sedimentary, mass balance, platt, syntectonic sedimentation, foredeep domain, sediment, main thrust, load, reached, basal coefcient, tectonics, surface taper, frontal accretion, sand pack, great, rst episode, critical height, boyer, evolutionary stage, american association, orogenic wedge, main, sciences, previous thrust, erosion, experiment ended, curve displacement, analogue, rst experiment, dahlen dahlen, journal structural geology, observed, geometry, koyi gutscher, sedimentation, height axial, event, stepped outward, nal geometry, accretionary, previous, hall london, figs, foreland, toe, storti, critical angle, del, sedimentation erosion, review, angle, deformation, force balance, dahlen, structural, order, geophysical, lp