Analog experiments and mechanical analysis applied to the Alaskan Accretionary Wedge Marc-André Gutscher, Nina Kukowski, Jacques Malavieille, Serge Lallemand To cite this version: Marc-André Gutscher, Nina Kukowski, Jacques Malavieille, Serge Lallemand. Analog experiments and mechanical analysis applied to the Alaskan Accretionary Wedge. Journal of Geophysical Research, American Geophysical Union, 1998, 103 (B5), pp.10161-10176. hal-01261538 HAL Id: hal-01261538 https://hal.archives-ouvertes.fr/hal-01261538 Submitted on 26 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. 103, NO. B5, PAGES 10,161-10,176,MAY 10, 1998 Episodic imbricate thrusting and underthrusting' Analogexperiments and mechanicalanalysis applied to the Alaskan Accretionary Wedge Marc-Andrd Gu•scher • and Nina Kukowski GEOMAR, Kiel, Germany JacquesMalavieille and SergeLallemand Laboratoire de G•ophysique et Tectonique, Universit• de Montpellier II, Montpellier, France Abstract. Seismic reflection profiles from the sediment rich Alaska subduction zone image short, frontally accreted, imbricate thrust slices and repeated se- quencesof long, underthrust sheets. Rapid landward increasesin wedgethickness, backthrusting,and uplift of the forearc are observed,suggesting underthrusting beneaththe wedge.These features and a widely varyingfrontal wedgemorphology are interpreted to be caused by different modes of accretion active concurrently along the trench at different locations. Episodicwedge growth is observedin high basal friction experiments using sand as an analog material. Two phasesof an accretionarycycle can be distinguished:frontal accretionof short imbricate thrust slices,alternating with underthrustingof long, undeformedsheets. The phase is shownexperimentally to dependupon the surfaceslope of the wedge. Mechanical analysis of the forces at work predicts these two modes of deformation due to the varying frictional forcesand yield strengthsfor a temporally varying wedge geometry.Maximum length of thrust slicesis calculatedfor experimentalconditions and confirmedby the observations.For a steepfrontal slope (at the upper limit of the Mohr-Coulombtaper stability field) the overburdenis too great to permit underthrusting,and failure occurs repeatedly at the wedgefront producing short imbricate slices. The wedgegrows forward, lowering the surfaceangle to the minimum critical taper. For a shallowfrontal slope the reducedoverburden along an active roof thrust permits sustainedunderthrusting, causingfrontal erosion and backthrusting,steepening the wedgeand thus completingthe cycle. 1. Introduction configurationat the Alaska convergentmargin showsa high degreeof lateral variation, and frontal slopesvary Seismicreflection profiling of convergentmargins has from less than 20 to over 150 within a few tens of kilo- recordeda high degreeof structural diversityin accre- meters. tionarywedges where deep sea sediments are imbricated An episodic variation in frontal configuration has against and subducted beneath the overriding plate been observedin a high basal friction analog experi- [Westbrooket al., 1988; Moore et al., 1990; Moore et mentsimulating accretionary wedge growth [Gutschef al., 1991;Shipley et al., 1992]. The causesfor structural et al., 1996]. Two distinct modesof deformation,as- diversityare not fully understoodbecause the most de- sociated with imbricate thrusting and underthrusting, formedportions of the wedgeare often poorly resolved. occurreddespite a constant thicknessof incomingsed- Furthermore, it is unclear whether wedge growth oc- iment and an unchangingbasal friction. Since direct curs by steady state processesor in episodicfashion, observationof the developmentof a submarine accre- alternatingwith periodsof erosion.The frontal wedge tionary wedgeover geologictimescales is not possible, analog modelingis a useful tool permitting observation Now at Laboratoire de Gdophysiqueet Tectonique,Uni- of the completeevolution of a model thrust wedgeunder versitd de Montpellier II, Montpellier, France. controlled boundary conditions. The objectivesof this study are threefold: (1) to Copyright 1998 by the American GeophysicalUnion. quantify the conditions controlling episodic accretion Paper number 97JB03541. in analogthrust wedges,(2) to providea mechanical 0148-0227/ 98 / 97J B- 03541 $09.00 explanation for the two distinct modes of deformation 10,161 10,162 GUTSCHERET AL.- EPISODICIMBRICATE THRUSTING in terms of the body and boundaryforces, and (3) to In the Eastern Aleutian Trench, 45 Ma old oceanic apply theseresults to the Alaska accretionarywedge. crust of the Pacific plate is subducting beneath the southernAlaskan margin at a rate of 5.7 cm/yr [DeMets 2. Tectonic Setting of the Alaska et al., 1990](Figure 1) ). The basal 500-600m sec- Convergent Margin tion of deep sea sediments,representing the Surveyor The Alaska convergentmargin offers a particularly Fan, is overlain by a 1400 m sequenceof alternating good study area since it has been investigatedby deep hemipelagicsediments and turbiditictrench fill [Kven- seadrilling [Kulmet al., 1973]and multichannel seismic voldenand yon Huene, 1985; yon Huene, 1989; Moore et reflectionprofiles [Kvenvolden and yon Huene, 1985; al., 1991].In lineEDGE-302, the frontal8 km of the ac- Moore et al., 1991], supplementedby depth-velocity cretionarywedge consist of three short imbricateslices controlfrom wide angleseismic data Iron Huene and (Figure2a), with a conjugateforethrust and backthrust Flueh, 1994; Ye et al., 1997]as well as high-resolution set defininga "pop-up"type of structure.The shallow swathmapping bathymetry [yon Huene and Flueh, 1994; 20 frontal slopeincreases to 5- 80 at the third major Friihn• 1995].The wedgeis markedby a largequantity thrust fault, locally reaching15 ø. At a distanceof 12- of incomingsediment (2--3 km) and is classedas a typ- 15 km from the deformationfront, the seismicsignature ical accretionarywedge [yon Huene and Scholl, 1991; losesits characterand the arcwarddipping and subhor- Lallemandet al., 1994]. izontal reflectorscan no longerbe assignedto any par- ,, o NORTH AMERICAN PLATE • .KSSD 2 \ \ \ \ \ \ \ ß KSSD 1 DSDP 182 PACIFIC ß• / / / PLATE •.g-• DSDP178 o ' ' 56* 208* 212' Figure 1. Alaskalocation map, with multichannelseismic lines (solid lines are presentedin text, dashedlines are discussed,but not displayed),borehole locations (small, filled triangles)[Kulm et al., 1973;Kvenvolden and yonHuene, 1985]and bathymetry(depth in m) sources:TOPEX global2 arcminbathymetry [Smith and Sandwell,1994; 1997],high resolutionswath mapping bathymetryNE of line 71 Iron Hueneand Flueh, 1994]' GUTSCHER ET AL.: EPISODIC IMBRICATE THRUSTING 10,163 a) NW EDGEline-302 SE [km] .......... '" 40 [km] 20 0 b) NW line71 SE . [km] __•5 . ' ' ' ' I ' ' ' ' I ' ' ' ' I ' ' ' ' I .... I ' ' ' ' I ' ' ' ' I .... I .... I ' ' ' '1'' • •1 •9 40 [km] 20 0 O) NW line 63 SE fore arc basin [km] ½•:•:•:•:•k.::½•;-'•;-':.-'::• ........ -- • -5 •--•i-•::•:•:•-"•:::.......... ••,• •_ ...•------- •---• •---'-• -----•- • •---•" -• 40 [km] 20 0 Figure 2. Interpretativeline drawingof seismicreflection profiles (pre stack depth migration, VE - 1.5). Heavy lines indicatefaults or erosionalunconformities, dashed where uncertain, oceanicbasement, dark shadedregion, backstop, light shadedregion. (a) Line EDGE-302, one horizonwith a clearseismic signature is tracedas a shadedline, (b) Line 71, notethe longlayered sheetsbetween km 20 and 40, and (c) Line 63, note the longsheets and the 2 km deepfore arc basin at rear. ticular stratigraphic horizon Surface morphology and all accreted and those below are all transported farther dipping zonesof high reflectivity,however, suggest four arcward below the wedge. to seven more major thrust slices truncated at 6 km depth by a midlevel detachment. At a distance of 40- 60 km from the deformation front, a 0.5-1.5 km thick 3. Experimental Modeling sequenceof slope sedimentsis marked by strong land- The growth of accretionary wedges and fold-and- ward vergentfolding and shortening.At depth, a I km thrust belts has been the subject of numerous analog thick sectionof layered reflectorsis imaged above the modelingstudies [Davis et al., 1983; Malavieille, 1984; subductingoceanic crust and beneath the backstop. Mulugeta, 1988; Malavieille et al., 1991; Liu et al., 1992; Two parallel seismiclines to the SW, lines 71 and 63, Lallemand et al., 1992; Malavieille et al., 1993; Lalle- displaylong (10-20 km) repeatingsequences of reflectors mand et al., 1994; Kukowski et al., 1994; Larroque et al., (Figures2b and 2c) 30 km from the deformationfront. 1995; Wangand Davis, 1996]. Thesestudies have con- The overlyingridge in line 63 (at 35 km) shallowsto firmed the applicability of critical wedgetheory to mod- 2.5 km depth and bounds a 2 km deep forearc basin. eling deformationin the brittle, upper portions of sub- In both this basin and the I km deep basin along strike marine accretionarywedges [Davis et al., 1983; Dahlen at the rear of line 71 (at 35 km) folding,tilting,