Coulomb theory applied to accretionary and nonaccretionary wedges: Possible causes for tectonic erosion and/or frontal accretion Serge E. Lallemand, Philippe Schnürle, Jacques Malavieille To cite this version: Serge E. Lallemand, Philippe Schnürle, Jacques Malavieille. Coulomb theory applied to accretionary and nonaccretionary wedges: Possible causes for tectonic erosion and/or frontal accretion. Journal of Geophysical Research, American Geophysical Union, 1994, 99 (B6), pp.12033-12055. hal-01261600 HAL Id: hal-01261600 https://hal.archives-ouvertes.fr/hal-01261600 Submitted on 25 Jan 2016 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 99, NO. B6, PAGES 12,033-12,055,JUNE 10, 1994 Coulomb theory applied to accretionary and nonaccretionary wedges: Possible causes for tectonic erosion and/or frontal accretion Serge E. Lallernand URA CNRS 1760 "G6ophysiqueet Tectonique",Universit6 Montpellier II, Montpellier, France Philippe Schntirle URA CNRS 1315, Laboratoirede G6ologieStructurale, Universit6 Pierre et Marie Curie, Paris, France JacquesMalavieille URA CNRS 1760 "G6ophysiqueet Tectonique",Universit• Montpellier II, Montpellier, France Abstract. Basedon observationsfrom both modemconvergent margins and sandboxmodeling, we examine the possibleconditions favoring frontal accretionand/or frontal and basal tectonic erosion.Mean characteristic parameters (!.t, !.t*• and [) areused to discussthe mechanical stability of 28 transectsacross the frontal part of convergentmargins where the Coulomb theory is applicable.Natural observations reveal that "typical accretionary wedges" are characterizedby low taperswith smoothsurface slope and subductingplate, low convergencerates and thick trench sediment,while "nonaccretionarywedges" display large taperswith irregularsurface slopes and roughsubducting plate, high convergencerates and almostno trenchfill. Sandboxexperiments were performed to illustrate the effects of seamounts/ridgesin the subductionzone on the deformationof an accretionarywedge. These experiments show that a wedge of sandis first trappedand pushedin front of the seamountwhich actsas a movingbulldozer. This is followed by a tunnelling effect of the subductingseamount through the frontal wedge material, which resultsin considerablesand reworking. At an advancedsubduction stage, the d6collementjumps back from a high level in the wedgeto its formerbasal position. We concludethat a high trench sedimentationrate relative to the convergencerate leadsto frontal accretion.In contrast,several conditionsmay favor tectonicerosion of the upperplate. First, oceanicfeatures, such as grabens, seamountsor ridges, may trap upper plate material during their subductionprocess. Second, destabilizationof the upperplate material by internalfluid overpressuringcausing hydrofracturing is probablyanother important mechanism. Introduction "critical taper" at failure for a given rock density within the wedge. These parametersare the internal and effective-basal Following work by Davis et al. [1983] on the mechanicsof friction angles and pore fluid pressurein the wedge. In this fold and thrust belts, Dahlen [1984] derived a rigorous paper we apply this theory both to accretionary and solution for the "critical taper" that a submarine sediment nonaccretionarywedges using 28 well-constrainedgeometries wedge of noncohesivefrictional material on a basal plane of of convergentmargins. weaknesswill attain, when sufficiently compressedfrom its We use the classificationof convergentmargins as defined back end. The rock is treatedas a frictionalplastic (Coulomb- by yon Huene and Scholl [1991, 1993], dependingon the type) material without cohesion.A cohesiveplastic material occurrenceor absenceof an accretionarywedge (or prism or without frictional shear strength has been previously complex)at the toe of the activemargins. Such a complexis investigated by Chapple [1978]. This material model thus generally well imaged on seismic records because of its neglectselastic strains and strainhardening and softeningand particularfold andthrust structure. The complexcontinuously the limiting condition will be given by the effective-stress- grows by incorporationof new imbricate slices of trench fill dependentinternal friction of Coulomb-Mohr[Mandl, 1988]. and oceanic material or by underplatingalong accreting Since that time, numerous authors have applied the margins (Figure 1). On the other hand, the wedge either "Coulomb theory" to accretionarywedges to estimate some maintainsits initial volume or consumesitself by tectonic characteristicparameters [e.g., Dahlen et al., 1984; Zhao et erosion at nonaccretingmargins (Figure 1). Both sediment al., 1986]. Three parametersare sufficient to calculate the accretionat the toe and crustconsumption further back may occursimultaneously as is documentedalong some transects of Copyright1994 by the AmericanGeophysical Union. the Japanand Peru Trenches [von Huene and Lallemand, 1990]. Several authors have demonstrated that considerable loss of Paper number 94JB00124. upper plate material has occurred along some convergent 0148- 0227/94/94 JB-00124505.00 marginsthrough tectonic erosion processes [e.g., von Huene 12,033 12,034 LALLEMANDET AL.:ACCRETION•Y ANDNONACCRETIONARY WEDGES NON-ACCRETIONARY MARGINS (21,000 KM) and Lallemand, 1990; von Huene and Scholl, 1991; Lallemand 100% SEDIMENT UNI)ERT!tRUST et al., 1992a]. For example, the Japan and Peru submarine wedgesprobably decreased their volumeby about50 % during OCEAN FLOOR the Neogene.Lallemand [1992] compiledthe subductionzones SEDIMENT where great subsidencetogether with volcanic front retreat _'C7___-_-; were documented.These "erosional"margins include northern Japan,Peru, Izu-Bonin, Mariana, Tonga and Guatemala- Costa Rica. Although the loss of upper plate material appearsquite IGNEOUSOCEAN CRUST v certain, especially in the light of the great subsidencerecorded along these margins, the mechanism responsible is still poorly known. Several explanationshave been proposedsuch as the wedging of positive relief featuresinto the subduction zones(Japan, Peru, Tonga) causingfrontal erosion[yon Huene INTERMEl)iATE-ACCRETIONARY MARGINS ( 16,000 KM ) and Lallemand, 1990; Lallemand et al., 1990], the 80% SEDIMENT UNi)ERTHRUST fragmentationof the upper plate by hydrofracturingleading to basal erosion [yon Huene and Lee, 1983; Platt, 1990] or a normal geological mode for a convergentmargin where the OCEANFLOOR ]xRWPRISM trench fill does not exceed 500 m in thickness [Le Pichon and Henry, 1992]. SEDIMENT/xCCRETIO•:::::::::.:......,.....• To investigate the possible causes of tectonic erosion and/or sediment accretion, we review some of the parameters V V V V V V V V V of active margins such as the convergence rate and the ••' IGNEOUSOCEAN CRUST occurenceof grabensor seamountson the subductingplate. We also use laboratory sandboxmodels to illustrate possible mechanismsof basal and frontal erosion of the overriding plate. TYPICAL-ACCRETIONARY MARGINS (7,000 KM) 70% SEDIMENT UNDERTHRUST The Coulomb Theory Applied to Accretionary Wedges OCEANFLOORAccRETIONAR¾ pRISM Previous Studies As a first approximation, we assume that accretionary prisms behave as homogeneous wedges formed of noncohesiveCoulomb material frictionally sliding on a rigid '•• IGNEOUSOCEANCRUVST v v v v base (subductingplate). This theory is scale-independent.The maximum depth of applicability is given by the downward increasein temperatureand correspondsto the transitionfrom Figure 1. Classificationof convergentmargins as proposed by von Huene and $choll [1993, p. 168] with their cumulative a pressure-dependent,time-independent Coulomb behavior to length of occurrence. In this paper we call "intermediate a pressure-independent, temperature-dependent plastic accretionary wedges" what von Huene and Scholl called behavior [Davis et al., 1983]. This transition occurs at "accretionarywedges with small prisms(5-40 km wide)" and variable depths, dependingon the thermal gradient, generally "typical accretionarywedges" what they called "accretionary at 15 + 5 km depth. wedgeswith largeprisms (> 40 km wide)." We use the exact solution of Dahlen [1984] for a critical after Dahlen 1984 Figure 2. Cross-sectionalsketch of a submarinenoncohesive critical wedge showing the Cartesian coordinatesx, z and the angleso•, [i, W0, andWb. Strengthin the wedgeis proportionalto the effectivestress Oz, shownschematically by the shadedarea on the right. Modified after Dahlen [ 1984]. LALLEMAND ET AL.: ACCRETIONARY AND NONACCRETIONARY WEDGES 12,035 taper. Let ct and 13be the topographicslope angle and the solutions(I.t, I.t*b = tan•*•, •.) whichaccount for thestability relatedd6collement angle (Figure2). Wband W0 are the angles of a given wedge. Furthermore,it is very difficult to measure betweenthe maximum compressivestress c h and the baseand these parameters in situ. Table 1 summarizes the few seafloor, respectively. If the wedge is uniform and measurementsthat are presently available. noncohesive,then the orientation of o•