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Impact of Surface Processes on the Growth of Orogenic Wedges: Insights from Analog Models and Case Studies1 J

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Impact of Surface Processes on the Growth of Orogenic Wedges: Insights from Analog Models and Case Studies1 J ISSN 00168521, Geotectonics, 2010, Vol. 44, No. 6, pp. 541–558. © Pleiades Publishing, Inc., 2010. Impact of Surface Processes on the Growth of Orogenic Wedges: 1 Insights from Analog Models and Case Studies J. Malavieillea and E. Konstantinovskayab a Université Montpellier 2, CNRS UMR 5243, Lab. Géosciences Montpellier, 34095 Montpellier cedex 5, France, and International Laboratory, (LIA) “ADEPT”, CNRSNSC, FranceTaiwan email: [email protected] b Institut National de la Recherche Scientifique, Centre Eau, Terre et Environnement (INRSETE), 490 rue de la Couronne, Quebec City, Quebec, Canada G1K 9A9 email: [email protected] Received March 23, 2010 Abstract—Interaction between surface processes and deep tectonic processes plays a key role in the structural evolution, kinematics and exhumation of rocks in orogenic wedges. The deformation patterns observed in analogue models applied to natural cases of present active or ancient mountain belts reflect several first order processes that result of these interactions. Internal strain partitioning due to mechanical behaviour of a thrust wedge has a strong impact on the vertical component of displacement of tectonic units that in return favour erosion in domains of important uplift. Such strain partitioning is first controlled by tectonic processes, but surface processes exert a strong feed back on wedge dynamics. Indeed, material transfer in thrust wedges not only depends on its internal dynamics, it is also influenced by climate controlled surface processes involving erosion and sedimentation. Effects of erosion are multiple: they allow long term localization of deformed domains, they favour important exhumation above areas of deep underplating and combined with sedimen tation in the foreland they contribute to maintain the wedge in a critical state for long time periods. The sim ple models illustrate well how mountain belts structure, kinematics of tectonic units and exhumation are determined by these complex interactions. DOI: 10.1134/S0016852110060075 1 INTRODUCTION ies chosen in various settings characterizing first order tectonic processes (Taiwan, Alps and Variscan belt) are The geologic history of orogenic wedges records then discussed in the light of the experiments. both the main phases of tectonic evolution and the coupled influence of deep geological (rheology and kinematics, metamorphism, magmatism) and surface GROWTH OF OROGENIC WEDGES processes (climate dependent erosion–sedimenta DURING CONTINENTAL SUBDUCTION tion) active along convergent margins. In recent years more attention has been paid on the mechanical and Orogenic wedges develop in subduction settings thermormechanical aspects of mountain building due to plate convergence involving large shortening offering a better understanding of the behaviour and and deformation of the crust (Fig. 1a). Two main sub deformation of the continental lithosphere in subduc duction settings characterize mountain building. The tion settings. Today, the major role of surface processes first one, oceanic subduction, concerns Andean type is highlighted in numerous studies dealing with the mountain belts formed by subduction of an oceanic evolution of orogens at different time and space scales. plate below a deforming continental margin upper For example, the role of erosion and sedimentation on plate. Continental subduction, the second one, con fault growth, exhumation processes and deformation cerns most of other mountain belts. This paper focuses history of accretionary orogens is widely studied on orogenic wedges of the second type, either due to through geological, experimental and numerical subduction of a continental margin under a continen approaches [e.g. 6, 14, 60, 79, 90, 93]. Here, insights tal plate following an Andean type oceanic subduc from simple sandbox models are used to show how the tion, or due to subduction of a continental margin interactions between surface processes and the under an intraoceanic volcanic arc (oceanic lithos mechanical behavior of the orogenic wedge influence phere upperplate) following intraoceanic subduc its structures, kinematics of deformation, exhumation tion [64, 66]. Continental subduction occurs after clo mechanisms, and global evolution. Several case stud sure of an oceanic domain that can be wide or narrow depending on the geodynamic setting. Subduction of 1 The article is published in the original. the lithospheric mantle induces deformation of the 541 542 MALAVIEILLE, KONSTANTINOVSKAYA (a) orogenic wedge 0 input continental crust 30 S U.P. = Upper Plate U.P. lithospheric mantle km L.P. = Lower Plate t outpu L.P. Svelocity discontinuity (b) rigid buttress sedimentation glass sidewall deformable U.P. erosion surface plastic sand cake sheet S 10 cm décollement layer rigid base L.P. engine 3 m Fig. 1. (a) Kinematic setting of continental subduction and (b) Schematic setting used for analogue modeling of thrust wedges. Backstop geometries and rheologies can be modified. Dotted line represents the chosen erosion surface. Sedimentation (when integrated) is performed by sprinkling sand. continental crust and controls the structural asymme of the continental crust, they can be activated (simul try of the mountain belt [33, 68]. This is well illustrated taneously or not) during the evolution of the orogen. today by various geophysical data (wide angle seismic Such a layering can be lithologic (e.g., basement cover transects and seismic tomography) from different interface, or weak layers in a sedimentary sequence), mountain chains [5, 10, 98]. They clearly show the rheologic (e.g., thermomechanical changes during subduction of the lithospheric mantle and suggest that subduction or fluids pressure changes) or inherited it could drag the continental crust or part of it. from the early tectonic history (e.g., the structural her The study of oceanic accretionary wedges has itage of an extended margin prior to continental sub played a great role in the understanding of mountain duction). During mountain building, these weak building processes. What do we learn from oceanic zones have a major impact on the mechanical behavior accretion? Two major tectonic processes act along of the wedge [16, 87] as they constitute potential déc subduction zones: tectonic erosion (when material is ollement zones. How and where these décollements removed from the upperplate margin and dragged develop and how they influence the mechanics and through the subduction channel) and accretion (when structural evolution of the orogenic wedge are major material is removed from the subducting lowerplate questions [90]. and accreted to the upperplate) [e.g., 56]. During Since the fundating works by Davis, Dahlen, and continental accretion, the whole or only part of the Suppe in the 1980s [30, 31], mountain belts have been incoming rock sequences is incorporated to the wedge often considered by geologists as crustal scale accre depending on the location of the décollements that tionary wedges [62, 78, 92] which deformation mech allow crustal material to be detached from the sub anisms can be satisfactorily described by a simple ducting plate. The part which is not involved in wedge Coulomb behaviour. The Coulomb theory gives a sim growth is dragged deeper into the mantle. At lithos ple mechanical setting allowing the definition of dif pheric scale, oceanic and continental subduction have ferent tectonic regimes depending on wedge stability: been described by a simple setting [e.g. 62] that was critical, undercritical, overcritical [e.g. 25]. Then, it used as a first order kinematic boundary condition for has been shown that orogens commonly adopt a dis many modeling approaches (Fig. 1). The location of a tinct geometry with a lowtapered prowedge facing velocity discontinuity (the “S point” in numerical the subducting plate, and a hightapered retrowedge models, e.g., [9]) determines the amount of accreted on the internal side [62]. This concept of doubly ver material (input) vs. subducted material (output) and gent accretionary wedge widely explored in the 1990s controls which part of the continental crust is sub [8, 35, 97] is still explored now [e.g., 74, 77, 81]. Ero ducted with the mantle (the whole crust or part of sion has rapidly been added as a major parameter to uppercrust). In a thrust wedge, several kinematic sin the theory [26–29] because it exerts a significant con gularities exist mainly due to the mechanical layering trol on wedge mechanics. Removing material from the GEOTECTONICS Vol. 44 No. 6 2010 IMPACT OF SURFACE PROCESSES ON THE GROWTH OF OROGENIC WEDGES 543 wedge surface induces a continuous deformation of and geophysics that are able to produce a global view the wedge changing the way for the critical state to be at a lithospheric scale, but this image is a “snap shot” maintained. If the tectonic (shortening) and climatic in time. conditions (erosional potential] remain stable, the The classical sandbox devices used (Fig. 1b) are wedge reach a dynamic steady state [6, 27, 44, 47, 95, made by a flat basal plate bound by two lateral glass 96] in which the incoming fluxes (accreted material) walls (see detail in: [14, 50, 51, 62]). A motor pulls a are compensated by the outgoing fluxes (material plastic sheet with a surface on which basal friction can removed by erosion). According to these models, the be chosen. A polished plastic film produces low basal velocity field of the crust and, hence, the exhumation friction and a rough plastic sheet surface simulates a paths of rock particles,
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