Channel flow, ductile extrusion and exhumation in continental collision zones: an introduction L. GODIN 1 , D. GRUJIC 2 , R. D. LAW 3 & M. P. SEARLE 4 1 Department of Geological Sciences & Geological Engineering, Queen’s University, Kingston, Ontario, K7L 3N6, Canada (e-mail: [email protected]) 2 Department of Earth Sciences, Dalhousie University, Halifax, Nova Scotia, B3H 4J1, Canada 3 Department of Geological Sciences, Virginia Tech., Blacksburg, VA 24061, USA 4 Department of Earth Sciences, Oxford University, Oxford, OX1 3PR, UK Abstract: The channel flow model aims to explain features common to metamorphic hinterlands of some collisional orogens, notably along the Himalaya – Tibet system. Channel flow describes a protracted flow of a weak, viscous crustal layer between relatively rigid yet deformable bounding crustal slabs. Once a critical low viscosity is attained (due to partial melting), the weak layer flows laterally due to a horizontal gradient in lithostatic pressure. In the Himalaya–Tibet system, this lithostatic pressure gradient is created by the high crustal thicknesses beneath the Tibetan Plateau and ‘normal’ crustal thickness in the foreland. Focused denudation can result in exhuma- tion of the channel material within a narrow, nearly symmetric zone. If channel flow is operating at the same time as focused denudation, this can result in extrusion of the mid-crust between an upper normal-sense boundary and a lower thrust-sense boundary. The bounding shear zones of the extruding channel may have opposite shear sense; the sole shear zone is always a thrust, while the roof shear zone may display normal or thrust sense, depending on the relative velocity between the upper crust and the underlying extruding material. This introductory chapter addresses the historical, theoretical, geological and modelling aspects of channel flow, emphasiz- ing its applicability to the Himalaya – Tibet orogen. Critical tests for channel flow in the Himalaya, and possible applications to other orogenic belts, are also presented. The hinterlands of collisional orogens are often characterized by highly strained, high-grade meta- morphic rocks that commonly display features con- sistent with lateral crustal flow and extrusion of material from mid-crustal depths towards the oro- genic foreland. A recent model for lateral flow of such weak mid-crustal layers has become widely known as the ‘channel flow’ model. The channel flow model has matured through efforts by several research groups and has also been applied to a variety of geodynamic settings. Thermal-mechanical modelling of collision zones, including the Himalayan – Tibetan system, has brought the concept of channel flow to the forefront of orogenic studies. Original contributors to the concept of channel flow initiated an important paradigm shift (Kuhn 1979), from geodynamic models of conti- nental crust with finite rheological layering to the more encompassing channel flow model. This time-dependent mid- to lower crustal flow process, which will be reviewed in this chapter, may progress into foreland fold-and-thrust tectonics in the upper crust, thereby providing a spatial and temporal link between the early development of a metamorphic core in the hinterland and the foreland fold-and-thrust belt at shallower structural levels. Outcomes and implications of such a viscous flowing middle to lower crust include a dynamic coupling between mid-crustal and surface processes, and limitations to accurate retro-deformation of orogens (non-restorable orogens, e.g. Jamieson et al. 2006). This Special Publication contains a selection of papers that were presented at the conference ‘Channel flow, extrusion, and exhumation of lower to mid-crust in continental collision zones’ hosted by the Geological Society of London at Burlington House, in December 2004. Because most of the ongoing debate on crustal flow focuses on the Cenozoic age Himalaya – Tibet collisional system, some of the key questions that are addressed in this volume include the following. . Does the model for channel flow in the Hima- laya – Tibet system concur with all available geological and geochronological data? From:LAW, R. D., SEARLE, M. P. & GODIN, L. (eds) Channel Flow, Ductile Extrusion and Exhumation in Continental Collision Zones. Geological Society, London, Special Publications, 268, 1–23. 0305-8719/06/$15.00 # The Geological Society of London 2006.
Channel flow, ductile extrusion and exhumation in continental collision zones: an introduction
Aug 17, 2015
The channel flow model aims to explain features common to metamorphic hinterlands
of some collisional orogens, notably along the Himalaya–Tibet system. Channel flow describes a
protracted flow of a weak, viscous crustal layer between relatively rigid yet deformable bounding
crustal slabs. Once a critical low viscosity is attained (due to partial melting), the weak layer flows
laterally due to a horizontal gradient in lithostatic pressure. In the Himalaya–Tibet system, this
lithostatic pressure gradient is created by the high crustal thicknesses beneath the Tibetan
Plateau and ‘normal’ crustal thickness in the foreland. Focused denudation can result in exhuma-
tion of the channel material within a narrow, nearly symmetric zone. If channel flow is operating at
the same time as focused denudation, this can result in extrusion of the mid-crust between an upper
normal-sense boundary and a lower thrust-sense boundary. The bounding shear zones of the
extruding channel may have opposite shear sense; the sole shear zone is always a thrust, while
the roof shear zone may display normal or thrust sense, depending on the relative velocity
between the upper crust and the underlying extruding material. This introductory chapter
addresses the historical, theoretical, geological and modelling aspects of channel flow, emphasiz-
ing its applicability to the Himalaya–Tibet orogen. Critical tests for channel flow in the Himalaya,
and possible applications to other orogenic belts, are also presented.
of some collisional orogens, notably along the Himalaya–Tibet system. Channel flow describes a
protracted flow of a weak, viscous crustal layer between relatively rigid yet deformable bounding
crustal slabs. Once a critical low viscosity is attained (due to partial melting), the weak layer flows
laterally due to a horizontal gradient in lithostatic pressure. In the Himalaya–Tibet system, this
lithostatic pressure gradient is created by the high crustal thicknesses beneath the Tibetan
Plateau and ‘normal’ crustal thickness in the foreland. Focused denudation can result in exhuma-
tion of the channel material within a narrow, nearly symmetric zone. If channel flow is operating at
the same time as focused denudation, this can result in extrusion of the mid-crust between an upper
normal-sense boundary and a lower thrust-sense boundary. The bounding shear zones of the
extruding channel may have opposite shear sense; the sole shear zone is always a thrust, while
the roof shear zone may display normal or thrust sense, depending on the relative velocity
between the upper crust and the underlying extruding material. This introductory chapter
addresses the historical, theoretical, geological and modelling aspects of channel flow, emphasiz-
ing its applicability to the Himalaya–Tibet orogen. Critical tests for channel flow in the Himalaya,
and possible applications to other orogenic belts, are also presented.
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Channelow,ductileextrusionandexhumationin continentalcollisionzones: anintroductionL.GODIN1,D.GRUJIC2,R.D.LAW3&M.P.SEARLE41DepartmentofGeologicalSciences&GeologicalEngineering,QueensUniversity,Kingston,Ontario,K7L3N6,Canada(e-mail:[email protected])2DepartmentofEarthSciences,DalhousieUniversity,Halifax,NovaScotia,B3H4J1,Canada3DepartmentofGeologicalSciences,VirginiaTech.,Blacksburg,VA24061,USA4DepartmentofEarthSciences,OxfordUniversity,Oxford,OX13PR,UKAbstract: The channel ow model aims to explain features common to metamorphic hinterlandsof some collisional orogens, notably along the HimalayaTibet system. Channel ow describes aprotracted ow of a weak, viscous crustal layer between relatively rigid yet deformable boundingcrustal slabs. Once a critical low viscosity is attained (due to partial melting), the weak layer owslaterallyduetoahorizontalgradientinlithostaticpressure.IntheHimalayaTibetsystem,thislithostatic pressure gradient is created by the high crustal thicknesses beneath the TibetanPlateau and normal crustal thickness in the foreland. Focused denudation can result in exhuma-tion of the channel material within a narrow, nearly symmetric zone. If channel owis operating atthe same time as focused denudation, this can result in extrusion of the mid-crust between an uppernormal-senseboundaryandalower thrust-senseboundary. Theboundingshear zones of theextrudingchannelmayhaveoppositeshearsense;thesoleshearzoneisalwaysathrust,whilethe roof shear zone maydisplaynormal or thrust sense, dependingonthe relative velocitybetween the upper crust and the underlying extruding material. This introductory chapteraddresses the historical, theoretical, geological and modelling aspects of channel ow, emphasiz-ing its applicability to the HimalayaTibet orogen. Critical tests for channel ow in the Himalaya,andpossible applicationstootherorogenicbelts,arealsopresented.The hinterlands of collisional orogens are oftencharacterizedbyhighlystrained, high-grademeta-morphic rocks that commonly display features con-sistent withlateral crustal owandextrusion ofmaterial frommid-crustal depthstowardstheoro-genicforeland. Arecent model forlateral owofsuchweakmid-crustal layers has becomewidelyknownas thechannel ow model. Thechannelowmodelhasmaturedthrougheffortsbyseveralresearch groups and has also been applied to avarietyof geodynamicsettings. Thermal-mechanicalmodelling of collision zones, including theHimalayanTibetan system, has brought theconcept of channel ow to the forefront of orogenicstudies. Original contributors to the concept ofchannelowinitiatedanimportantparadigmshift(Kuhn1979), fromgeodynamicmodels of conti-nental crust withniterheological layeringtothemore encompassing channel ow model. Thistime-dependent mid- to lower crustal owprocess, whichwill be reviewedin this chapter,may progress into foreland fold-and-thrust tectonicsin the upper crust, thereby providing a spatialandtemporal linkbetweentheearlydevelopmentof ametamorphic coreinthehinterlandandtheforeland fold-and-thrust belt at shallower structurallevels. Outcomes and implications of such aviscous owing middle to lower crust includea dynamic coupling between mid-crustal andsurface processes, and limitations to accurateretro-deformation of orogens (non-restorableorogens,e.g.Jamiesonetal.2006).ThisSpecial Publicationcontainsaselectionofpapers that were presented at the conferenceChannel ow, extrusion, and exhumation oflower tomid-crust incontinental collisionzoneshosted by the Geological Society of London atBurlington House, in December 2004. Because mostof the ongoing debate on crustal ow focuses on theCenozoicageHimalayaTibet collisional system,some of the keyquestions that are addressedinthisvolumeincludethefollowing.. Doesthemodel forchannel owintheHima-layaTibet systemconcur with all availablegeologicalandgeochronologicaldata?From: LAW, R. D., SEARLE, M. P. & GODIN, L. (eds) Channel Flow, Ductile Extrusion and Exhumation in ContinentalCollisionZones.GeologicalSociety, London, SpecialPublications, 268,123.0305-8719/06/$15.00 # TheGeologicalSociety ofLondon2006.. Howdothepressuretemperature-time(P-T-t)dataacross the crystallinecore of the Himalayatwiththeproposedchannelow?. Arethemicrostructural fabricdata(pureshearandsimpleshearcomponents)compatiblewithcrustal extrusion(thickeningorthinningoftheslab)?. If the channel ow model is viable for the Hima-layaTibet system, what may have initiatedchannelowandductileextrusion?. WhydidtheextrusionphaseoftheHimalayanmetamorphiccoreapparentlyceaseduringthelateMiocenePliocene?. Are some of the bounding faults of the potentialchannel still active, or were they recentlyactive?. Is the Himalayan channel ow model exportabletoothermountainranges?Thisintroductorypaperaddressesthehistorical,theoretical, geological and modelling aspects ofcrustal ow in the HimalayaTibet orogen. CriticaltestsforcrustalowintheHimalaya,andpossibleapplicationstootherorogenicbelts, arepresentedand difculties associated with applying thesetests arediscussed. Personal communicationcita-tions (pers. comm. 2004) identify commentsexpressedduringtheconference.TheHimalayaTibetanplateausystemThe HimalayaTibet system initiated in EarlyEocene times, following collision of the IndianandEurasianplates (seeHodges (2000) andYin&Harrison (2000) for reviews). The collisionresulted in closure of the Tethyan Ocean, southwardimbrication of the Indian crust, and northward con-tinental subduction of Indian lower crust and mantlebeneathAsia.ThecollisionthickenedthesouthernedgeoftheAsiancrust to70 km, andcreatedtheTibetan Plateau, the largest uplifted part of theEarths surface with an average elevation of5000 m(Fieldingetal.1994).TheHimalayanorogencoincideswiththe2500-km-longtopographicfrontatthesouthernlimitofthe Tibetan Plateau. It consists of ve broadlyparallel lithotectonic belts, separated by mostlynorth-dipping faults (Fig. 1). The Himalayan meta-morphic core, termed the Greater Himalayansequence(GHS), is boundedbytwoparallel andopposite-senseshearzonesthatwerebothbroadlyactive duringthe Miocene (Hubbard&Harrison1989; Searle &Rex 1989; Hodges et al. 1992,1996). The Main Central thrust (MCT) zonemarks the lower boundary of the GHS, juxtaposingthemetamorphiccoreabovetheunderlyingLesserHimalayan sequence. The South Tibetandetach-ment (STD) systemdenes the upper boundaryroof fault of theGHS, markingthe contact withtheoverlyingunmetamorphosedTethyansedimen-tarysequence.TheapparentcoevalmovementoftheMCTandSTD, combined with the presence of highlyshearedrocksandhighgradetomigmatiticrockswithin the GHS, has led many workers to viewtheGHSas anorth-dipping, southward-extrudingslab of mid-crustal material owing away fromthe thick southern edge of the Tibetan Plateau,towardsthethinnerforelandfold-thrustbelt.Dynamicsof channelowThe concepts of crustal extrusion and channeloworiginatedinthecontinental tectonicslitera-tureintheearly1990s. Unfortunately, thesetwoprocesses are often referred to interchangeablywithout justication. Oneof themainpoints thatemerged fromthe Burlington House conferencewas that a distinctionbetweenchannel owandcrustal extrusion must be made. Parallel versustaperingboundingwalls onchannel owand/orextrusion processes, and how these processesmay replenish over time, are two resolvableparameters that are critical for distinguishingchannel ow from extrusion. Brief denitionsandoverviewsofthetwoprocessesarepresentedbelow. Amore detailed overviewof the mech-anics of the related processes is provided byGrujic(2006).ChannelowChannel ow involves a viscous uid-lled channellyingbetweentworigidsheets. Theviscousuidbetweenthe sheets is deformedthroughinducedshear and pressure (or mean stress) gradientswithin the uid channel (Fig. 2; e.g. Batchelor2000;Turcotte&Schubert2002). Theweaklayerows laterally due to a horizontal gradient in litho-static pressure; gravity is therefore the drivingforce.Thenitedisplacementdependsonthegeo-metryof thechannel, viscosity, anddisplacementrate of the bounding plates. In situations wherethechannel wallsarenon-parallel, the(non-litho-static) pressuregradient maycausehighrates ofbuoyant returnowof thechannel material, pro-vided that the viscosity is low enough (Mancktelow1995; Gerya & Stockhert 2002). The simplest quali-tativecharacteristicof thechannel owmodel isthatthevelocityeldconsistsofahybridbetweentwoend-members:(1)Couetteow(simpleshear)between moving plates where the induced shearacross the channel produces a uniformvorticityacross the channel (Fig. 2A, left); and (2) Poiseuilleow(also known as pipe-ow effect) betweenL.GODINET AL. 2stationary plates in which the induced pressure gra-dient produces highest velocities in the centre of thechannel andoppositeshearsensesforthetopandbottomof the channel (e.g. Mancktelow1995;Fig.2A,right).Quantitativeanalyses of channel owbetweenmoving or stationary boundaries (e.g. Batchelor2000; Turcotte & Schubert 2002) have beenappliedtoawiderangeof geodynamicprocessesincluding: (1) asthenospheric counterow(Chase1979; Turcotte&Schubert 2002); (2) mechanicsof continental extension (Kusznir & Matthews1988; Block&Royden1990; Birger1991; Kruseet al. 1991; McKenzieet al. 2000; McKenzie&Jackson2002); (3) continental plateau formationandevolution, bothinextensionandcompression(Zhao &Morgan 1987; Block &Royden 1990;Wernicke1990; Bird1991; Fieldinget al. 1994;Clark&Royden2000;McQuarrie&Chase2000;Hodges et al. 2001; Shenet al. 2001; Husson&Sempere2003; Clarket al. 2005; Gerbault et al.2005;Medvedev&Beaumont2006);(4)tectonicsof large continent continent collision orogens(Johnston et al. 2000; Beaumont et al. 2001,2004, 2006;Grujicet al. 2002, 2004; Williams&Jiang 2005; (5) metamorphic histories in large,hot, collisional orogens (Jamieson et al. 2002,2004, 2006); (6) subduction zone owregimesunder both lithostatic and overpressured conditions(Bird1978; England&Holland1979; Shreve&Cloos 1986; Peacock 1992; Mancktelow 1995;Geryaet al. 2002; Gerya&Stockhert 2002); and(7)deformationalongpassivecontinentalmarginsin the presence of salt layers (Gemmer et al.2004; Ings et al. 2004). For all these examples,with the exception of salt tectonics, the mostlikely cause for weakening in the channel ispartial melting. Inthecaseof salt tectonics, owFig.1. Simplied geologicalmap oftheHimalayanorogen,with generalphysiographicfeaturesoftheHimalayaTibet system (inset). The Greater Himalayan sequence is bounded above by the north-dipping top-to-the-north SouthTibetan detachment system (STDs), and belowby the north-dipping top-to-the-south Main Central thrust zone (MCTz).The Himalaya is arbitrarily divided into four sections to facilitate age compilations (see Fig. 4). ITSZ: IndusTsangpoSutureZone.INTRODUCTION 3isduetotheinherent lowviscosityof salt underuppercrustalconditions.The application and signicance of channelowincontinent continent collisional settingsisbecomingprogressivelymorerened, yet remainscontroversial. Evidence for channel owand/orductile extrusion of mid-crustal rocks fromthegeologically recent HimalayaTibet orogen(Grujic et al. 1996, 2002; Searle &Szulc 2005;Carosi et al. 2006; Godinet al. 2006; Hollister&Grujic 2006; Jessupet al. 2006; Searleet al. 2006)andrelatedgeodynamicmodels(Beaumont et al.2001, 2004, 2006; Jamieson et al. 2004, 2006)are vigorously disputed (e.g. Hilley et al. 2005;Harrison 2006; Williams et al. 2006), while there arestillfewdocumentedexamplesfromolderorogens(Jamiesonetal.2004;Whiteetal.2004; Williams&Jiang 2005; Brown &Gibson 2006; Carr &Simony2006; Hatcher &Merschat 2006; Kuiperet al. 2006; Xypolias & Kokkalas 2006).ExtrusionExtrusion is dened as the exhumation process of achannel(ortheshallowerpartofit)operatingatalocalized denudation front. Channel ow and extru-sioncanoperatesimultaneously, withlateral tun-nellingoccurringat depth, whileextrusionoccursat the front of the system, at progressively shallowercrustal levels (Fig. 3). The focused denudationresults in exhumation of the channel materialwithin a narrow, nearly symmetric zone; theextruded channel is characterized by an uppernormal-sense boundary, anda lower thrust-senseboundary. We present brief reviews of the fourmajorchannelowandextrusionmodels.Figure3Apresentsaschematicoverviewofthekinematicrelationshipsbetweenchannel owandextrusion processes. The weak crustal channelow (Fig. 3A, no. 5) is localized structurallybelowthe 7508Cisotherm, where melting starts(Fig.3A,no.8).MaterialpointsaffectedbyaPoi-seuille ow within the channel (Fig. 3A, no. 7) tra-verse the rheological boundary at the tip of thechannel (Fig. 3A, no. 9), andwill continuetheirexhumation by extrusion of the palaeo channel(Fig. 3A, no. 10). It follows that the ductileoutwardchannelowconvertstoductileextrusionwhererockmotionisbalancedbysurfaceerosion(Fig. 3A, no. 13). The extrusion is aided byfocused erosion along the mountain front (e.g.Grujicetal.2002;Vannayetal.2004),therateofextrusion/exhumation being proportional to therateof owinthemid-crustal channel. Extrusionofthecrustallayerisassumedtooccuralongtwoboundingshear zones(Fig. 3A, nos1112). Thebounding shear zones may have opposite shearsense; the sole shear zone is always a thrust,whiletheroof shear zonemaydisplaynormal orthrust sense, depending on the relative velocityvelocitydistributionduetoshearingvelocitydistributionduetopressuregradientCouetteflowPoisseuilleflowcphcfhexhumationburialccrit.(A) (B)sFig.2. Schematicdiagram oftheowpatterninaviscouschannel ofwidthh.Theviscosity ofchannelmaterialislowerthantheviscosityofrocks inthehanging wallandinthefootwall(mh. mc, mf).Velocitydistributionsareshown relativetoareference frameattached tothehangingwall.Thevorticity values(rotationalcomponent oftheowprole) areschematically indicated by thewidth oftheblack bar:thewide bar segmentindicatesahigh simpleshearcomponent;thenarrow barsegmentindicatesahighpure shearcomponent. Onlytheabsolutevalueofthevorticityisindicated regardlessofwhetheritispositive (sinistral simpleshear)ornegative(dextral simple shear).(A) End-members of ow in a channel: left, Couette ow with velocity prole caused by shearing (vc: vorticity in pureCouette ow); right, Poiseuille ow with velocity prole caused by pressure gradient within the channel (vp: vorticityin pure Poiseuille ow). (B) For a given velocity of the subducting plate and channel width there is a critical viscosityofthechannelmaterialbelow which thePoiseuille owwillcounteracttheshearforcesandcausereturnow(negative velocity) and therefore exhumation of that part of the channel material. The part of the channel that remainsdominated by the induced shear (positive velocities) will continue being underplated (vs: vorticity in a hybrid channelow).FromGrujicetal.(2002), afterMancktelow(1995)andTurcotte&Schubert(2002).L.GODINET AL. 4betweentheupper crust (Fig. 3A, no. 4) andtheunderlying extrudingmaterial. This is similar totheasymmetricthrustexhumation/extrusionmodedescribedbyBeaumontetal.(2004).Theextrudingmid-crustal layercanbeslab-orwedge-shaped, depending on the parallelismofthe boundingshear zones, andadvances towardstheforeland. Asthechannel material isextruded,deformationispervasivelydistributedwithin,oratthe boundaries of the crustal layer. A concentrationof deformation along the boundaries results inextrusionof a rigidcrustal wedge (Fig. 3B; e.g.Burchel &Royden 1985; Hodges et al. 1992).This type of extrusion cannot be a long-livedgeological process, but rather is probably a transientevent (cf. Williams et al. 2006). Alternatively,deformation that is distributed throughout thewedge results in ductile extrusion (Fig. 3C;Grujic et al. 1996). The vorticityof owwithinthe extruded crust may be a perfect simpleshear (Fig. 3C), or more likely a general shearcombiningcomponents of simpleshear andpureshear (Fig. 3D; Grujic et al. 1996; Grasemannet al. 1999; Vannay & Grasemann 2001;Law et al. 2004; Jessup et al. 2006; but cf. Williamsetal.2006).During extrusion, the crustal slab or wedge coolsfrom mid-crustal ductile ow to upper crustal brittleFig.3. (A) Schematic diagram of kinematic relationship between channel ow and extrusion of a palaeo-channel. Allmaterialdepictedbelongs totheunderthrustingplate.1,Lithosphericmantle;2,lowercrust;3,mid-crust;4,uppercrust;5,weakcrustal channel;6,isotherms (takenfromBeaumontetal.2004), 7,schematicvelocity proleduringreturnchannel ow;8,7508Cisotherm structurallybelow which partialmeltstarts;9,rheologicaltipofthechannel: at lower temperatures (for a given channel width and pressure gradient) Couette ow will dominate and all thematerialwillbeunderthrust; 10,extruding crustal block( palaeo-channel):iftherheologicaltipisatsteady state,material points may move through this tip and pass from the weak crustal channel into the extruding block; 11, lowershear zone of the extruding crustal block (dominantly thrust-sense kinematics); 12, upper shear zone of the extrudingcrustal block(dominantly normal-sensekinematics); 13,focusedsurfacedenudation (controlled bysurfaceslopeand, as in the Himalaya, by orographic precipitation at the topographic front). (B)(D) Possible strain distribution in anextruding crustal block; possible end-members (inspired by Grujic et al. 1996; Grasemann et al. 1999). (B) Rigid blockwith high concentration of strain along the boundaries (Hodges et al. 1996). (C) Ductile block deforming by pervasivesimple shear.(D)Ductileblockdeforming bygeneralshearwith apureshearcomponentincreasing towardsthebottomofthewedge,aswellaswith timefollowing adecelerating strainpath(Grasemannetal.1999;Vannay &Grasemann2001).INTRODUCTION 5conditions where deformation is partitioned into dis-cretefaults.Thisconceptresemblesthebuoyancy-driven extrusion of a crustal slab within asubduction zone (Chemenda et al. 1995). The differ-ences between these two models include the size ofthe extruding wedge and nature of the primarydriving forces. Analogue models of Chemendaetal.(1995)demonstratethatsyncollisionalexhu-mation of previously subducted (underthrust)crustal material can occur due to failure of the sub-ducting slab. In this model, erosional unloadingcauses the buoyant upper crust to be exhumed (at aratecomparabletothesubductionrate),producinganormal-sensemovementalongtheuppersurfaceof the slab. This model, however, regards theexhumed crustal slice as a rigid slab boundedbelow and above by well-dened thrust and normalfaults.OneofthekeyHimalayanproblemsiswhetherthe GHS represents extrusion of a completesectionofthemid-crust, withtheSTDMCTsur-faces representing potential channel-boundingstructures, or whether the GHS is simply anextrudedsegment of acoolingchannel, withtheSTDMCTsurfacesbeingmoreakintoroof andsole faults boundingathrust duplex(Yin2002).Furthermore, theGHS-boundingfaultsexposedatthetopographicsurfacecouldbeassociatedwithlate-stageexhumationoftheGHS, ratherthantheoriginal channel formed at depth beneath theTibetanPlateau(Jessupet al. 2006). Relatedpro-blemsalsoconcerntheoriginoffabricswithintheGHS; aretheyrelatedtoowduringchannelling,extrusion, or could they pre-date the Himalayanevent? Another unresolved question is whetherthe currently exposed GHS more closely resemblesanexhumedpluggedchannel, withlittleextrusionduring exhumation, or whether there was (andperhaps still is) active extrusion at the surface(e.g. Hodges et al. 2001; Wobus et al. 2003)(Beaumontetal.2004,p.26).ExhumationExhumation is dened as the displacement of rockswith respect to the topographic surface (England &Molnar 1990), andrequireseither removal of theoverburden (e.g. by erosion,normal faulting, verti-cal lithosphericthinning) or transport of materialthroughtheoverburden(e.g. bydiapirism, buoy-ancy-driven return ow in subduction zones)(see reviews by Platt 1993; Ring et al. 1999).Inthecontext ofchannel ow, exhumationofthechannel occurs by a balance between orographicallyandtopographicallyenhancedfocusederosionandextrusiononthesouthernslopesoftheHimalaya.The channels tunnelling capacity may be dra-maticallyreducedasitisdeectedupwardduringexhumationandcooling(Fig.3A;Beaumontetal.2004). Although exhumation of the GHSin theHimalaya may be associated with southward extru-sionalongcoeval STDMCTboundingfaults, itmay also be locally enhanced by post-MCTwarping of the GHS and localized erosion followingcessationofextrusion(Thiedeetal.2004;Vannayet al. 2004; Godin et al. 2006), and growth ofduplexes inthe footwall of the MCTduringthemiddleMiocene(Robinson&Pearson2006).Exhumation of tunnelling material (not to be con-fusedwithextrudedpalaeo-channel material) willonly occur if the active channel ow breaksthrough to the topographic surface. In the numericalmodelsofchannel ow, thisisdeterminedbytherheological properties of the upper-middle crust,frictional strength, and degree of advective thinningof the surface boundary layer (Beaumont et al.2004). Conceptually, athresholdexhumationratemust beachievedtokeepthechannel sufcientlyhotsothatthematerialdoesnotfreezeuntilitisclosetothetopographicsurface(Beaumont et al.2004). This situationmayhavebeenachievedinthetwoHimalayansyntaxes:theNangaParbat Haramoshmassif (e.g. Crawet al. 1994; Zeitleretal. 2001;Butleretal. 2002;Koonsetal. 2002;Joneset al. 2006) andtheNamcheBarwamassif(e.g. Burg et al. 1997, 1998; Burg 2001; Dingetal.2001).RequirementsandcharacteristicsofchannelowThefollowingisalistofgeologicalcharacteristicsof channel ow, based on eld observation andgeodynamic modelling, and eld criteria for recog-nizing an exhumed channel fromthe geologicalpast (Searle &Szulc 2005; Searle et al. 2003,2006). The criteria used to identify an activechannel arebasedongeological andgeophysicaldata (Nelson et al. 1996; S. Klemperer pers.comm. 2004; Klemperer 2006) and landscapeanalyses(Fieldingetal.1994;Clarketal.2005).(1) Acrustalpackageoflowerviscositymaterialboundedbyhigherviscosityrocks.(2) A plateau with well-dened margins (or a sig-nicant contrast in crustal thickness) toproduce a horizontal gradient in lithostaticpressure.(3) Coevalmovementonshearzoneswiththrustand normal-fault geometry that bound thechannelowzone.(4) Kinematic inversion along the roof shearzone: earlier reverse-sense motion resultingfromunderthrusting(Couetteow) followedby normal-sense shearing resulting frombackow(bydominant Poiseuille ow) inL.GODINET AL. 6thechannel, and/or bynormal-sensemotionon shear zones and brittle faults during extru-sionandexhumationofthepalaeo-channel.(5) Pervasive shearing throughout the channel andextrudedcrustalblock,althoughstrainispre-dicted to be concentrated along its boundariesdue to the owgeometry and deformationhistory.(6) Inverted and right-way-up metamorphicsequences at the base and top of the extrudingchannel,respectively.Modellingofthechannelowpredictsthefollow-ingtectonicconsequences.(1) The incubation period necessary for mid-crustal temperatures to rise, thereby increasingthe melt content for commencement of channelow, istypicallybetween10and20millionyears from the time of onset of crustal thicken-ing. This incubation period is judged necessarytoincreasethemid-crustal temperaturesuf-cientlytoproducethelowviscositynecessaryforinitiation of channel ow.(2) Melts (leucosomes) coeval with ductilechannel ow must be younger than shorteningstructuresinoverlyingrocks(uppercrust).(3) Whenactive, thechannel is predictedtobe1020 kmthick(Roydenet al. 1997; Clark& Royden 2000; Beaumont et al. 2004;Jamiesonetal.2004,2006).(4) Thereismorelateral transportofmaterialinthechannelthanvertical.(5) Pre-existing structures cannot be traced throughthechannel(fromtheuppercrust,throughthechannel and into channel footwall rocks).This has direct consequences on correlationpossibilitiesbetweenrockunitsandstructuresfrom the upper crust to the lower crust.These conditions and consequences are reviewedfortheHimalayanbelt, andcomparedwithavail-ableeldandgeochronologicaldata.ViscosityAsmall percentage of partial melt signicantlyreduces the effective viscosity of rocks (Rosenberg&Handy2005, andreferencestherein). TheGHSincludes a signicant percentage of migmatitesandsynorogenicleucogranites, whileevidenceforpartial melting is absent in both the Lesser Himala-yan sequence and the Tethyan sedimentarysequence. Atthetimeofprotractedpeaktempera-ture metamorphism(up to granulite facies) andmelt generation, the GHS was therefore weakerthan the overlying and underlying rocks by atleast one order of magnitude (Beaumont et al.2004, 2006; Hollister &Grujic 2006; Medvedev&Beaumont2006).The Lesser Himalayan sequence consists of athick package of metasediments that were deformedunder greenschist facies or lower conditions(,c. 3008C). Althoughthesetemperaturesallowfor ductile owof quartz-dominated rocks, theexpectedviscositiesarehigherthanforrockswithpartial melt (see Medvedev &Beaumont 2006).The Tethyan sedimentary sequence is generallyunmetamorphosed and only experienced greens-chist-faciesmetamorphisminanarrowzoneat itsbase(Garzanti et al. 1994; Godin2003), althoughcontact metamorphicaureoleshavebeenreportedassociated with young granites emplaced in theTethyansedimentary sequence of southernTibet(Leeetal.2000,2006).PlateauformationThe Himalayan orogen is genetically linked togrowthoftheTibetanPlateau(Hodges2000; Yin&Harrison2000). Various palaeo-elevationdatasuggest that the southern Tibetan Plateau hasexistedsince at least the mid-Miocene (Blisniuket al. 2001; Rowleyet al. 2001; Williams et al.2001;Spiceretal.2003),attaininghighelevationssimilar to the present day by 1812 Ma, andpossiblyby35Ma(Rowley&Currie2006).Geo-chronological and structural data suggest thateast west extension in the Tibetan Plateau which isbelievedto be linked to crustaloverthick-eningwas well underway by 14 Ma (Coleman &Hodges1995; Williamset al. 2001). Contrastsincrustal thicknesses (between the Indian forelandandtheTibetanPlateau)thatproducedthenecess-arygravitationalpotentialenergyforchannelow(Bird 1991) may have existed at least since theMiocene,andperhapsearlier.Creation of high topography by inationalthickening is a potential consequence of crustalow (Royden 1996; Burchel 2004). In theLongmen Shan belt of eastern Tibet, Clark &Royden(2000)andClarketal.(2005)suggestthattopography is generated when lower crustal owbut-tresses against cold, stronger crust (e.g. Sichuanbasin). The resistance to lateral owinates thelower weak crustal zone, and supports a high topogra-phy above, and possibly generates ramping up (extru-sion)andeventualexhumationoflowercrust.Thisprocess could partly address concerns about thesupport of the plateaus highelevation, if Tibetis underlain by a weak, low-viscosity mid-crust(S. Lamb, pers. comm. 2004). A similar lithosphericstrength contrast to the Longmen Shan could exist onthe southern edge of the Himalaya, where the south-owing weak mid-crust buttresses against cold,strongIndianlithosphere, favouringextrusionandinational supportof thesouthernTibetan Plateau(e.g. Hodges et al. 2001).INTRODUCTION 7Coevalchannel-boundingstructuresThe MCTandSTDzones include multiple faultstrandsthat operatedat different timesandunderdifferentmechanicalconditions(ductiletobrittle).Thebroadlycoeval activityof theMCTandtheSTDover extended geological time (fromc. 25Ma to 5 Ma) is documented by various sets of geo-chronological data. Figure 4 presents a compilationof interpreted age(s) of motion on the variousstrandsoftheMCTandSTD, alongthelengthofthe Himalayan belt as taken directly from the avail-ableliterature.Werefertothese variousstrandsaslowerMCT(MCT1)andupperMCT(MCTu),andlower STD(STD1) and upper STD(STDu) toavoidconfusionwith past terminology. Amajorlimitation tocompiling such diversedata (Table1)is the range of different approaches utilized bydifferentauthorstoconstraineitheramaximumorminimumage of motion, oneither the upper orlower strand of each fault system(fromindirect geo-chronological toolsmonazite crystal ages or peakmetamorphic agesto eld relationships, e.g. pre-or post-kinematicintrusions). Weemphasizethatno attempt has been made in this compilation to cri-tically assess the validity of different approachestaken by different authors in different areas.Thecompileddataindicatethat theMCTuandSTDlweremostlyactivebetween2514Maand2412 Ma, respectively (Fig. 4). The activityalongthehigherSTDuapparentlystartedlaterandlastedlonger(c. 19Mato14Ma, andperhapsisstill active today, e.g. Hurtadoet al. 2001) thanalong the more ductile lower STDl. The structurallylowest MCT1appears as the youngest structure(c. 15 Ma to 0.7 Ma). Combined, the availabledataindicatesimultaneousor overlappingperiodsof thrust- and normal-sense ductile shearingbetween c. 24 Ma and 12 Ma. However, withtime, the position of the active faults movedtowards upper and lower structural levels, andbecame more diachronous and possibly less dynami-cally linked (e.g. Godin et al. 2006). Since the earlyrecognitionoftheSTD, thecoeval activityonthetwobounding(andinnermost) shear zones(MCTuandSTDl), andits implicationfor exhumationofthe metamorphic core of the Himalaya, has beensuggested(Burchel &Royden, 1985; Hubbard&Harrison 1989; Searle & Rex, 1989; Burcheletal.1992;Hodgesetal.1992,1996;Grujicetal.1996; Grasemann et al. 1999); however, theproposeddrivingforcesandkinematicdetailsvarybetween authors.The late Miocene to recent activity along theMCTloverlaps with activity along the two in-sequence external thrust faults, the Main Boundarythrust(MBT)andMainFrontalthrust(MFT), andmay represent on-going exhumation of themodern cryptic (hypothetical) channel (Hodgeset al. 2004). In the context of proposed exhumationbycombinedchannel owandextrusion, acorre-sponding active zone of normal faulting at ahigher structural level is required. The data arescarce but there are indications of neotectonicfaultingalongthenorthernboundaryof theGHS(Hodges et al. 2001, 2004; Hurtadoet al. 2001;Wiesmayretal.2002).Theyoungingofstructuresaway from the core of the orogen may suggest pro-gressivewideningofthechannelasitpassesfromthechannelowtoextrusionmodeofexhumation(Searle&Godin2003;Searleetal.2003,2006).KinematicinversionsStudies have shown that deformation along the STDis distributed in the adjacent footwall and/orhangingwall forupto34 km, ratherthanbeingrestricted to a single fault plane. Most of thesestudies indicate an overprint of top-to-the-north(normal sense) shearing on an older top-to-the-south thrusting within the STD system (Burget al. 1984; Brun et al. 1985; Kundig 1989;Burchel et al. 1992; Vannay &Hodges 1996;Carosi et al. 1998; Godin et al. 1999a, 2001;Grujic et al. 2002; Wiesmayr & Grasemann2002). Basedoneldevidence for a reversal inshear sense during motion along the STD, itseems likely that the return ow of the metamorphiccore (relative tothe underthrustingIndianplate)developed late in the channel owhistory. Thekinematic history of the STD is further complicatedby overprinting top-to-the-south shearing (e.g.Godin et al. 1999a; Godin 2003). Some of thislatestageoverprintingmayrelatetonorth-dippingthrustfaultsintheTethyansedimentarysequence,between the suture zone and the STD (e.g.Ratschbacheretal. 1994). Geodynamicmodelling(Beaumont et al. 2004)supportsthispossibilityifthe weak channel overburden fails and glidestowards the foreland causing relative thrustingalongtheupperboundaryofthechannel.InternaldeformationwithinthechannelOn regional cross-sections and maps, the MCT andSTD are often depicted as sharp boundaries;however, both are broad ductile shear zones.Althoughmosteld-andlaboratory-basedinvesti-gationsagreethat thereisabroadzoneof defor-mationadjacenttotheMCTandSTD,pervasivelydistributed ductile shear throughout the GHS isalso documented (e.g. Jain &Manickavasagam1993; Grujicet al. 1996; Grasemannet al. 1999;Jessupetal.2006).Thekinematicsof deformationconsistently indicate top-to-the-south shearing intheLesserHimalayansequenceandinmostoftheL.GODINET AL. 830252015105 0 30252015105 030252015105 030252015105 0303030252525202020151515101010555000303030252525202020151515101010555000303030252525202020151515101010555000303030252525202020151515101010555000Quat.PlioceneM i o c e n eOligocene L a t eM i d .E a r l yQuat.PlioceneM i o c e n eOligocene L a t eM i d .E a r l yQuat.PlioceneM i o c e n eOligocene L a t eM i d .E a r l yQuat.PlioceneM i o c e n eOligocene L a t eM i d .E a r l yWesternHimalayaCentral-WestHimalayaCentral-EastHimalayaEasternHimalayaSTD-Upper STD-Lower MCT-Upper MCT-LowerA g e ( M a )1234567891011121314151617181920212223242526272829303132333435403938373641424344455649474648535251505455575859606162636465MinimumtomaximumageMaximumageMinimumageReferencenumber(seeTable1)10AbsoluteageBestfitageFig.4.CompilationofinterpretedagesofmotionontheMainCentralthrust(MCT)andSouthTibetandetachment(STD)systems.MCT-Lowerreferstothemostlybrittle,structurallylowerfaultintheMCTzone.LocalnamesincludeMCT-1,Ramgarhthrust,andMunsiarithrust.MCT-Upperreferstothemostlyductile,synmetamorphic,structurallyhigherfaultintheMCTzone.LocalnamesincludeMCT-2,Vaikritathrust,MahabharatthrustandChomrongthrust.STD-Lowerreferstothemostlyductile,synmetamorphic,structurallylowerfaultintheSTDsystem.LocalnamesincludeZanskardetachment,Sangladetachment,Annapurnadetachment,Deuralidetachment,Chamedetachment,LhotsedetachmentandZhergerLadetachment.STD-Upperreferstothemostlybrittle,post-metamorphic,structurallyhigherfaultintheSTDsystem.LocalnamesincludeJhaladetachment,Macchapuchharedetachment,Phudetachment,andQomolangmadetachment.SeeTable1forthecompletelistofdataandreferences.ThefourgeographicalareasrefertothesubdivisionspresentedinFigure1.Thethickdashedlinesrepresentbest-tagesformotiononthefaults.INTRODUCTION 9Table1. Compilationof interpretedagesof faultmotionon the MainCentralthrustsystemandSouthTibetandetachmentsystem,basedongeochronologicaldataStructure1Location2Age3Minerals4System5Reference6WesternHimalayaSTDuZanskar ,18Mato16Ma Ms,Bt Rb-Sr 1.Inger(1998)STDlSutlej 23Mato17Ma Mz,Ms Th-Pb,Ar 2.Vannayetal.(2004)STDlZanskar 23Mato20Ma Ms,Bt,Xe,Mz,ZrAr,U-Pb 3.Walkeretal.(1999)STDlZanskar c.22.2Mato19.8Ma Mz,Ms U-Pb,Ar 4.De`zesetal.(1999)STDlGarhwal 23to21Ma Mz,Ms U-Pb,Ar 5.Searleetal.(1999)STDlZanskar c.23Mato20Ma Ms,Bt Ar 6.Vanceetal.(1998)STDlZanskar c.26Mato18Ma Ms,Bt Rb-Sr 7.Inger(1998)STDlGarhwal ,21.9Ma Mz Th-Pb 8.Harrisonetal.(1997a)STDlZanskar 21Mato19.5Ma Mz U-Pb 9.Noble&Searle(1995)MCTuSutlej 23Mato17Ma Mz,Ms Th-Pb,Ar 10.Vannayetal.(2004)MCTuGarhwal c.5.9Ma Mz Th-Pb 11.Catlosetal.(2002)MCTuWestNepal 22Mato15Ma Ms Ar 12.DeCellesetal.(2001)MCTlSutlej c.6to0.7Ma Ms,Ap,Zr Ar,FT 13.Vannayetal.(2004)MCTlWestNepal 15Mato10Ma Ms Ar 14.DeCellesetal.(2001)Central-WestHimalayaSTDuNar 19Mato16Ma Ms,Bt,Hbl Ar 15.Godinetal.(2006)STDuNar ,19Ma Mz Th-Pb 16.Searle&Godin(2003)STDuKaliGandaki ,17.2ka Terr14C 17.Hurtadoetal.(2001)STDuLangtang ,17.3Ma Mz,Xe U-Pb 18.Searleetal.(1997)STDuAnnapurna c.18.5Ma Zr U-Pb 19.Hodgesetal.(1996)STDuKaliGandaki 15Mato13Ma Ms Ar 20. Vannay&Hodges(1996)STDuManaslu 19Mato16Ma Bt,Ms Ar 21.Guillotetal.(1994)STDlKaliGandaki c.22.5Ma Mz U-Pb 22.Godinetal.(2001)STDlManaslu .22.9Ma Mz Th-Pb 23.Harrisonetal.(1999b)STDlMarsyandi 22Mato18Ma Mz U-Pb 24. Coleman(1998),Coleman&Hodges(1998)STDlAnnapurna 22.5Mato18.5Ma Zr,Mz,Xy U-Pb 25.Hodgesetal.(1996)STDlManaslu .22Ma Hbl Ar 26.Guillotetal.(1994)STDlManaslu .20Ma Ms Ar 27.Copelandetal.(1990)MCTuLangtang 16Mato13Ma Mz Th-Pb 28.Kohnetal.(2004)MCTuKathmandu 22Mato14Ma Mz,Zr U-Pb 29.Johnsonetal.(2001)MCTuMarsyandi 22to18Ma Mz U-Pb 30.Coleman(1998)MCTuKathmandu 21Mato14Ma Ms,Bt Rb-Sr 31. Johnson&Rogers(1997)MCTuAnnapurna c.22.5Ma Mz,Zr U-Pb 32.Hodgesetal.(1996)MCTuKaliGandaki .15Ma Ms Ar 33. Vannay&Hodges(1996)MCTuKaliGandaki c.22Ma Mz,Th U-Pb 34.Nazarchuk(1993)MCTuLangtang .5.8Ma Ms Ar 35.Macfarlane(1993)MCTlLangtang c.9Ma Ms Ar 36.Kohnetal.(2004)MCTlMarsyandi c.13.3Ma Mz U-Pb 37.Catlosetal.(2001)MCTlKathmandu c.17.5Ma Ms,Bt Rb-Sr 38. Johnson&Rogers(1997)MCTlMarsyandi c.16Ma Mz Th-Pb 39.Harrisonetal.(1997b)MCTlLangtang ,9to7Ma;c.2.3Ma Ms Ar 40. Macfarlaneetal.(1992,Macfarlane(1993)Central-EastHimalayaSTDuEverest ,16Ma Mz U-Pb 41.Searleetal.(2003)STDuEverest c.17Ma Mz Th-Pb 42. Murphy&Harrison(1999)STDuEverest c.16Ma Xe,Mz,Zr U-Pb 43.Hodgesetal.(1998)STDuEverest 22Mato19Ma Ti,Xe,Hbl U-Pb,Ar 44.Hodgesetal.(1992)(Continued)L.GODINET AL. 10GHS, with top-to-the-north shearing only appearinginthetop-most part of theGHS, near andwithintheSTDsystem. Thelocationofthistransitioninshear sensehas yet tobedocumented. Fieldandmicrostructural data indicate that this pervasiveductile deformation is characterized by hetero-geneous general non-coaxial ow (componentsof both simple and pure shear) rather than byideal simple shear. For example, quartz petrofabricdata (Boullier & Bouchez 1978; Brunel 1980, 1983;Bouchez&Pecher1981; Burget al. 1984; Greco1989; Grujicet al. 1996; Grasemannet al. 1999;Bhattacharya &Weber 2004; Lawet al. 2004)consistentlyindicate a component of pure shear.Williamsetal. (2006), however, presentanoppo-site interpretation of these data based on straincompatibilityandmechanicstheory.Quantitativevorticityanalyses withintheGHSdocument a progressively increasing componentof simple shear tracedupwardtowards the STDandoverlyingshearedTethyansedimentaryrocks(Lawet al. 2004; Jessupet al. 2006), a generalshear deformation within the core of the GHS(Carosi et al. 1999a, b, 2006; Grujicet al. 2002;Lawet al. 2004; Vannay et al. 2004; see alsoFig. 3C), andanincreasingpureshearcomponenttraced downward towards the underlying MCTzone(Grasemannet al. 1999; Jessupet al. 2006;but cf. Bhattacharya&Weber2004). Macro-andmicrostructural fabric data (especially conjugateshear bands, porphyroclast inclusion trails, and cre-nulation cleavage at various stages of development)also suggest a strong component of shorteningacrossthefoliationinadditiontofoliation-parallelshearing (e.g. Carosi etal. 1999a, b, 2006;Grujicetal. 2002;Lawetal. 2004;Vannayetal.2004). The structural data indicate that ductiledeformationispervasivelydistributedthroughtheentire GHS, in the top part of the Lesser Himalayansequence,andatthebaseoftheTethyansedimen-tarysequence. Adirect implicationofthegeneralowmodel is that the boundingsurfaces of theTable1. ContinuedStructure1Location2Age3Minerals4System5Reference6STDuNyalam ,16.8Ma Mz U-Pb 45.Schareretal.(1986)STDlEverest c.21u2Ma Mz,Xe U-Pb 46.Viskupicetal.(2005)STDlEverest 18Mato17Ma Mz,Xe U-Pb 47.Searleetal.(2003)STDlEverest ,20.5Ma Mz,Xe,U U-Pb 48.Simpsonetal.(2000)STDuMakalu ,21.9Ma Mz U-Pb 49.Scharer(1984)MCTuEverest c.21u2Ma Mz,Xe U-Pb 50.Viskupicetal.(2005)MCTuDudhKosi c.25to23Ma Mz Th-Pb 51.Catlosetal.(2002)MCTuEverest c.21Ma Hbl Ar 52. Hubbard&Harrison(1989)MCTuEverest 23Mato20Ma Hbl,Bt Ar 53.Hubbard(1989)EasternHimalayaSTDuSikkim c.14l5Ma Mz,Zr Th-Pb 54.Catlosetal.(2004)STDuKhulaKangri ,12.5Ma Mz Th-Pb 55. Edwards&Harrison(1997)STDlWagyeLa c.12Ma Mz U-Pb 56.Wuetal.(1998)STDlSikkim 23Mato16Ma Grt Sm-Nd 57.Harrisetal.(2004)STDlSikkim c.17Ma Mz,Zr Th-Pb 58.Catlosetal.(2004)MCTuSikkim c.22Ma Mz Th-Pb 59.Catlosetal.(2004)MCTuSikkim 23Mato16Ma Grt Sm-Nd 60.Harrisetal.(2004)MCTuBhutan c.22Ma;18Mato13Ma Mz,Xe U-Pb 61.Danieletal.(2003)MCTuBhutan c.113.5Ma Mz U-Pb 62.Grujicetal.(2002)MCTuBhutan .14Ma Ms Ar 63.Stuwe&Foster(2001)MCTlSikkim 15Mato10Ma Mz Th-Pb 64.Catlosetal.(2004)MCTlBhutan ,11Ma Ms Ar 65.Stuwe&Foster(2001)1MCTl, Lower MCT; (and/or) mostly brittle, post-metamorphic; local names include MCT-1, Ramgarh, Munsiari. MCTu, Upper MCT;(and/or)ductile, synmetamorphic, synmagmatic; local namesincludeMCT-2, Vaikrita, Mahabharat, Chomrong. STDl, LowerSTD;(and/or)ductile, synmetamorphic, synmagmatic; local namesincludeZanskar, Sangla, Annapurna, Deurali, Chame, Lhotse, ZhergerLa.STDu,UpperSTD;(and/or)mostlybrittle,post-metamorphic;localnamesincludeJhala,Macchupuchare,Phu,Qomolangma2SeeFigure1forlocation.3Compilationof directgeochronological results only;includesage constraints based oncross-cuttingstructures/intrusion relationships.4Ap, apatite; Bt, biotite; Grt, garnet; Hbl, hornblende; Ms, muscovite; Mz, monazite; Terr, terraces; Th, thorite; Ti, titanite; U, uraninite;Xe,xenotime;Zr,zircon.5Ar,40Ar/39Ar thermochronology;14C, carbon 14; FT; ssion track geochronology; Rb-Sr, Sm-Nd, Th-Pb: thorium-lead ion microprobe(208Pb/232Thage);U-Pb,U-(Th)-Pbgeochronology.6ReferencenumbersrefertorespectiveagerangebaronFigure4.INTRODUCTION 11crystallinecore(i.e. MCTandSTDshear zones)must therefore be stretching faults (Means1989) accommodatingtransport-parallel pervasivestretching of the crystalline core during internalow (Grasemann et al. 1999; Vannay & Grasemann2001;Lawetal.2004).MetamorphiccharacteristicsOne of the most intriguing phenomena of theHimalaya is the inverted metamorphic sequencepresentinboththeLesserHimalayansequenceandGHS(seereviewsbyHodges2000). Atthetopofthe GHS and at the base of the Tethyansedimentarysequence, astronglyattenuated, right-way-updecreaseinmetamorphicgradeispresent.Models for inverted metamorphisminclude: (1)overthrustingof hot material (hot ironeffect; LeFort 1975); (2) imbricate thrusting (Brunel &Kienast1986;Harrisonetal.1997b,1998,1999a);(3) foldingof isograds (Searle &Rex1989); (4)transposition of a normally zoned metamorphicsequenceduetoeitherlocalizedsimpleshearalongthe base of the GHS (Jain &Manickavasagam1993; Hubbard1996), heterogeneous simplesheardistributedacross the Lesser HimalayansequenceandGHS(Grujicetal.1996;Jamiesonetal.1996;Searleet al. 1999) or general shear of previouslyforeland-dipping isograds (Vannay &Grasemann2001); and(5)shearheating(Englandet al. 1992;Harrisonet al. 1998;Catloset al. 2004). Themeta-morphic isograds can be deformed passively accord-ing to various kinematic models that are compatiblewith either extrusion or channel ow, or both(e.g. Searleetal. 1988, 1999; Searle&Rex1989;Jain & Manickavasagam 1993; Grujic et al.1996, 2002; Hubbard1996; Jamiesonet al. 1996;Davidsonetal.1997;Danieletal. 2003). Coupledthermal mechanical nite element modelling(Jamiesonetal.2004)hasbeensuccessfulinrepli-catingthedistributionofthemetamorphicisogradsand P-T-t data obtained through eld and laboratorystudies, although it failed to predict the timing of thelow temperature metamorphic overprint. Othermodelsproposeaspecicstyleof thrustingalongthe base of the GHSas an alternative model toexplainboththedistributionofmetamorphiczonesand the timing of metamorphism(e.g. Harrisonet al. 1998;Catlos et al. 2004).LateralversusverticaltransportofmaterialIntegration of geobarometry and thermochronologycandeducetheamountandtimingofexhumation:morespecically,therateofverticaldisplacementof rocks within the crust. Only the verticalcomponent ofexhumationcanbeestimatedusingthese techniques. Along low-angle shear zoneslike the STDandMCT, however, the horizontalcomponent ofdisplacement ispredominant. Someinvestigationsusethejumpinpressures,estimatedbymetamorphic assemblages across the STD, toestimate the horizontal component of displace-ment (Searle et al. 2002, 2003). Displacementestimates based on temperatures inferred frommetamorphic assemblages, however, involveassumptions about the shape of the isotherms,whichmaychangeduringtheexhumationprocess.SimpliedrestorationoftheGHS(e.g. INDEPTHdata; Nelson et al. 1996; Hauck et al. 1998) indicatethat the GHSmay extend down-dip for at least200 km, andpossiblyupto400 km(Grujicet al.2002). Exhumation from mid-crustal levels at3540 km(as suggestedbypressures at peakT;see Hodges (2000) for summary of data, andHollister & Grujic (2006) for interpretation)indicates that lateral displacement rates in theGHSarevetotentimeslarger thantheverticaldisplacement rates. These values ought to becompared with inferred surface denudation rates(e.g. Thiede et al. 2004; Vannay et al. 2004;Grujicet al. 2005), andestimationof shorteningor displacements across the MCT and STD.Conventional cross-section (usually line-length)restorationtechniques are usedtoestimate thesevalues (e.g. Schelling &Arita 1991; DeCelleset al. 2002; Searle et al. 2003). However, ifdeformation is pervasive through the GHS andthere is an inversion of the displacement alongthe STD, no single value can fully describe the dis-placement along the shear zone. Displacementsacross theGHSrelativetotheLesser Himalayansequence are also expected to progressivelyincrease towards the core, and progressivelydecreaseupwardtowardstheSTD, whichiscom-patiblewiththecalculations of particledisplace-mentpathsforvariouspointswithinamodelGHS(Jamiesonetal.2004,2006).DiscontinuityofprotolithsacrossthechannelAccordingtotheabovediscussion,thelargestrateof particledisplacement changeoccursacrosstheSTDandMCT(e.g. Davidsonet al. 1997). MostdetritalzirconandisotopicstudiessuggestthattheLesser Himalayan sequence and GHS metasedi-ments may have different protolith ages. ZirconandNdmodel agesandthe1Ndvaluessuggest aLateArcheantoPalaeoproterozoicsourcefor themetasedimentsof theLesser Himalayansequenceversus a Meso- to Neoproterozoic source forL.GODINET AL. 12the GHS (Parrish &Hodges 1996; Whittingtonet al. 1999; Ahmadet al. 2000; DeCelles et al.2000, 2004; Miller et al. 2001; Robinson et al.2001; Argles et al. 2003; Martin et al. 2005;Richardset al. 2005; but cf. Myrowet al. 2003),althoughstructural restorationsuggests otherwise(e.g. Walker et al. 2001). Thelithotectonicunits,separatedbytherst-ordershearzones, mayhavedistinct palaeo-geographic origins; however, thisdoesnotnecessarilymeantheybelongtodifferenttectonic plates. Similar results are obtainedby numerical modelling and particle tracking(Jamiesonet al. 2006), whichsuggest that fromthe base to the top of the GHS, the protolithsshould have a progressively more distal origin(with respect to the pre-collision plate margin),while the opposite situation is predicted for theLesserHimalayansequence.AlthoughdifferentprotolithoriginsfortheGHSandthe Lesser Himalayansequence might exist,a similar interpretation cannot be applied to theGHS and the Tethyan sedimentary sequence.Channel owmodelspredict that theSTDshouldbe the locus for large relative particle displacement,implyinga different originfor the GHSandtheTethyan sedimentary sequence (Jamieson et al.2006). Recent structural restorations andisotopicstudies, however, propose the lower Tethyansedimentarysequenceas apotential protolithforsome of the GHS(Vannay &Grasemann 2001;Argleset al. 2003; Gehrelset al. 2003; Searle&Godin 2003; Gleeson &Godin 2006; Richardset al. 2005). The STDis generally interpretedas either a decollement surface (stretchingfault), wherethethickpileof continental marginrocks (Tethyan sedimentary sequence) has beendecoupled without much internal disturbance tothestratigraphy,orapassiveroofthrustwithintheMCTsystem, with a hanging-wall at footwallatgeometry(Searleetal.1988;Yin2002).The GHS is dominated by three lithologicunits, which maintain their respective structural pos-itions for over a thousandkilometres along-strike(Gansser 1964; Le Fort 1975). Recent detailedmapping across the GHS locally reveals a morecomplexdistributionof,andvariationwithin,theseunits (Searle &Godin 2003; Searle et al. 2003;Gleeson &Godin 2006). Nonetheless, the rst-order lateral continuityof theGHSunitsindicatesan apparent lack of internal stratigraphic disturbance.Thishasbeenhighlightedasapossiblepitfall forthe channel owmodel (Harrison 2006). Modelresults indicate, however, that the channel mayverywell maintaininternal stratigraphy, as longas the deformation is concentrated along the bound-aries and owis planar along the length of thechannel(Jamiesonetal. 2006).TimingofmeltingandshorteningstructuresThe channel ow model assumes that melts(leucosomes and granites) will substantiallyreduce the viscosity of a crustal layer (i.e.channel). It alsopredicts that thesemelts shouldbe younger than shortening structures found inthe upper plateshorteningstructures that wouldhave created the necessary crustal thickeningand ensuing heating to partially melt and lowerthe viscosity of the underlying mid-crust. TheTethyansedimentarysequenceis theupper plateintheHimalaya.Leucosome and leucogranite bodies occurwithinall units of theGHS(Dietrich&Gansser1981; LeFort et al. 1987; Burchel et al. 1992;Guillot et al. 1993; Hodgeset al. 1996; Hollister&Grujic 2006). Most UThPb ages for themelts inthecentral Himalayarangefrom2322Ma (Harrison et al. 1995; Hodges et al. 1996;Coleman 1998; Searle et al. 1999; Godin et al.2001; Daniel et al. 2003; Harris et al. 2004)to 1312 Ma (Edwards &Harrison 1997; Wuet al. 1998; Zhang et al. 2004). However, evidencefor leucosome melt production during theOligocene also exists (Coleman 1998; Thimmet al. 1999; Godinet al. 2001). NorthHimalayangranitesfoundinsouthernTibetrangeincrystalli-zation age between 28 Ma and 9 Ma (Schareret al. 1986; Harrisonet al. 1997a; Zhanget al.2004; Aoya et al. 2005). Syntectonic (synchannel?)granites yield ages of 23.1 + 0.8 (Lee et al. 2006).Some North Himalayan granites, however, yieldzircon and monazite crystallization ages of14.2 + 0.2Ma and14.5 + 0.1Ma, respectively,indicatingthat vertical thinningandsubhorizontalstretching had ceased by the middle Miocene(Aoyaetal.2005;Leeetal.2006).Several phases of deformationarerecordedbythe overlying Tethyan sedimentary sequence(Steck et al. 1993; Wiesmayr & Grasemann2002; Godin2003). Althoughtheabsoluteage(s)of thedominant shorteningstructuresisdisputed,most authors agree that signicant thickeningoftheTethyansedimentarysequenceoccurredpriorto the Miocene, most likely in the Oligoceneor even before (Hodges et al. 1996; Vannay &Hodges 1996; Godin et al. 1999b, 2001; Wiesmayr&Grasemann2002; Godin2003;Searle&Godin2003). Some of these shortening features areinterpretedtobecoeval withhigh-pressuremeta-morphism in the GHS (Eohimalayan phase;Hodges2000), associatedwithearlyburial of theGHS beneath a thickening overlying Tethyansedimentarysequence(Godinet al. 1999b, 2001;Godin2003).INTRODUCTION 13Channelthicknessandlate-stagemodicationsDuring periods of active channel ow, modelspredict that the channel should be 10 to 20 kmthick(Roydenetal. 1997;Clark&Royden2000;Beaumont et al. 2004; Jamieson et al. 2004,2006). Thestructural thicknessoftheGHSvariesconsiderably,from23 kmintheAnnapurnaarea(Searle&Godin2003; Godinet al. 2006), upto30 kmin the Everest area (Searle et al. 2003,2006; Jessupet al. 2006), andevenmoreintheBhutanHimalaya(Grujicet al. 2002). Substantialpost-channel, post-extrusion modications havealteredtheoriginal geometryofthechannel. Out-of-sequencethrustssuchastheKakhtangthrustortheKalopanishearzone(Grujicetal.1996,2002;Vannay &Hodges 1996), and large amplitudefoldingoftheGHS(Johnsonetal. 2001;Gleeson&Godin2006; Godinet al. 2006) mayaccountfor some of the observed thickness variation. Alter-natively, various instabilities and failure of theuppercrustmayinducelocal accretionofchannelmaterial and sequential development of domes(i.e. spatio-temporal variations of channel thick-ness), both along-strike and down-dip (e.g.Beaumont et al. 2004). Some out-of-sequencethrustingmaybetheresultofsuchpulsedchannelowand related doming and extrusion (Grujicetal.2004;Hollister&Grujic2006).In the Kali Gandaki (Annapurna area) andeasternBhutan, theGHSisasthinas3 km, whilepreservingits typical apparent internal stratigra-phy andmetamorphiczoning. Whether this is areectionof alateral variationinthecomponentof coaxial (pureshear) deformationandthinningduring the channelling and/or extrusion phaseremainsunclear. Thinsegmentsof theGHSmayrepresent themost proximal partsof thechannel,whilethethickersegmentsaremoredistalpartsofthepalaeo-channel. Inthisalternateinterpretation,variationinthickness of theGHSat thepresent-day topographic front could reect along-strikevariation in the foreland-directed advance of achannelowregime.ChallengesandunresolvedissuesThe challenge to testing the applicability of thechannel ow model in the HimalayaTibetsystem lies within the Earth scientists abilityto accurately interpret deformation paths andpalaeo-isothermal structuresrecordedbyexhumedmetamorphic rocks that exhibit nite strainandmetamorphiceldgradients. Limitedsubsur-facegeophysical coverageoftheHimalayaTibetsystemmakes correlationof surcial datawithaputative channel at mid-crustal depths tentative.The INDEPTHprogramlaid the foundation forimagingcriticalmid-tolowercrustalfeaturesthatthe channel owhypothesis relies on. Unfortu-nately, data for the Himalaya are limited to asingle transect. Higher resolution and moreextensiveseismicsurveysmayhelpresolvelong-standing criticismof the channel ow models.For example, Harrison(2006) highlights the riskof generalizing the bright spots low velocityzones to the entire southern Tibetan Plateau,becausetheseismiclinewasrunwithinanactivegraben, where intrusions might be locally con-trolledbyextension,andnotcrustal-scalemelting.Assessing the applicability of the model to older,more deeply exhumed orogens will also prove to bechallenging because of overprinting deformationand thermal events common to these systems.In older orogenic systems, probably the mostimportant limitation to the application of thechannel owconcept istheabsenceofcontrol onpalaeo-horizontal and palaeo-vertical directions.The channel owmodel in the HimalayaTibetsystem is based on the concept of a lateral lithostaticpressure gradient acting as the driver for ow.Lateral variations in crustal thicknesses at thetime of orogenesis in older systems are simplyunknown. Furthermore, as pointed out by Joneset al. (2006), many of the structural/kinematic indi-cators of a channelized ow could also be compati-ble with tectonically rather than gravitationallydriven systems (e.g. transpressional strike-sliptectonics).Potentially the most important limitation totesting the channel owmodel is the fact thatmanyof theinitial parameters usedinnumericalmodels such as those presented by Beaumontet al. (2001, 2004) are takendirectlyfromeldand geochronological data collected in theHimalayaTibet system. Someparticipantsat theBurlingtonHouseconferencearguedthat it there-fore follows that assessing the channel owmodel, and testing its applicability against eldconstraints, becomes an inherently circularargument. In contrast, other participants (e.g.D. Grujicpers. comm. 2004) arguedthat oneofthemajor strengths of therigorouslyconstructedthermal mechanical generic models is theirability to test a wide range of potentially importantparameters, andtherebyidentifythecombinationsof parameters that produce results which mostclosely resemble a given orogenic system. Yetotherparticipantsarguedthatwheremodelresultsaresensitivetoawiderangeofpotentialboundaryconditions and input parameters, it is inherently dif-cult todeterminewhicharethemost importantparameters. In contrast, Jones et al. (2006)have argued that choosing geologically realisticboundary conditions and input parameters forL.GODINET AL. 14thermal mechanicalmodelsisacriticallyimport-ant rst step in ensuring that models are well-calibratedtoaspecicorogen. Theyfurtherarguethat tuning amodel tomatchaspecicorogenshould not be regarded as a weakness of the model-ling method, but is the basis for a better understand-ingof whichfactors arelikelytohavethemostinuence onorogenic processes suchas channelowandcrustalextrusion.ConcludingremarksTheproposedchannel owmodel explains manyfeaturespertainingtothegeodynamicevolutionoftheHimalayaTibetanPlateausystem, aswell asother older orogenic systems. It reconciles theapparent coeval nature of the MCT and STDfaults andkinematic inversions at the topof theGHS,leadingtosouthwardextrusionandexhuma-tionof thecrystallinecoreof theHimalayafrombeneath the Tibetan Plateau. In addition, it providesanalternativeandquantitativeexplanationof theinverted metamorphic sequence at the orogenscale, and effectively couples the tectonic andsurfaceprocesses. Theproposalthatthemiddleorlower crust acts as a ductile, partially moltenchannel owingout frombeneathareas of over-thickened crust (such as the Tibetan Plateau)towards the topographic surface at the plateaumarginsremainscontroversial,however,bothwithrespect to the HimalayaTibet systemand particu-larly older, less well documented orogenic systems.The channel owmodel nonetheless presents anexciting new conceptual framework for understand-ing the geodynamic evolution of crystalline cores oforogenicbelts, andmaybecomethesourcefor aparadigmshiftincontinentaltectonicsstudies.Thefollowing26papers inthis Special Publi-cationarearrangedintofour maingroups. Intherst group of papers this brief introduction tochannel owand ductile extrusion processes ispairedwithamorein-depthreviewbyGrujicofchannelowprocessesassociatedwithcontinentalcollisional tectonics. In the second group ofpapers detailed overviews are given by Klempererand Hodges of the geophysical and geologicaldatabases fromwhich the concepts of channelow and ductile extrusion in the HimalayaTibetanPlateausystemoriginallydeveloped.Different aspects of the modelling of channelowandductile extrusionprocesses are coveredin the third group of papers. Coupled thermal mechanical nite element models are presentedin papers by Beaumont et al., Medvedev &Beaumont andJamiesonet al., whiletheeffectsof volumechangeonorogenicextrusionarecon-sidered by Grasemann et al. In the last twopapers in this group, problems associated withidentifyingchannel owandductileextrusioninolderorogensarediscussedbyJonesetal., whilelinkages between owat different crustal levels(infrastructureandsuprastructure) andconstraintsontheefciencyofductileextrusionprocessesareexploredbyWilliams etal.Thefourthandlargest groupofpapersiscom-posed of a series of predominantly eld-basedcase studies providing geological constraints onchannel owandductileextrusionasanorogenicprocess. Thislast groupof papersisdividedintosubsections on the HimalayaTibetan Plateausystem, the Hellenic and Appalachian orogenicbelts, andtheCanadianCordillera. TheHimalayasubsectionbeginswithawide-rangingcritiquebyHarrison of the applicability of channel owmodels totheHimalayaTibetanPlateausystem.Subsequent papers in the Himalaya subsectionfocusdominantlyonspeciceldareaswithintheLesser andGreater HimalayaandarearrangedinorderofgeographiclocationstartingwithwesternNepal (Robinson & Pearson) and then progressingeastwardsthroughtheAnnapurnaregionofcentralNepal(Godinetal., Scaillet&Searle, Annen&Scaillet) and the NyalamEverest regions ofTibet and eastern Nepal (Wang et al., Searleet al., Jessup et al.) to the Bhutan Himalaya(Hollister & Grujic, Carosi et al.). TheHimalayaTibetan Plateau subsection concludeswith papers by Lee et al. and Aoya et al. ongneiss domes exposed to the north of the Himalayaandtheirimplicationsformid-crustalowbeneathsouthernTibet.Geological evidence for and against channel owandductileextrusioninolderorogenicsystemsisdiscussedintheremainingtwosubsectionsofthisvolume. Xypolias&Kokkalaspresent integratedstrain and vorticity data indicating ductile extrusionof mid-crustal quartz-rich units in the Hellenides ofGreece, whileHatcher&Merschatpresent eldevidence in support of channel ow operating paral-lel to orogenic strike in the Appalachian Inner Pied-mont, USA. Arguments for (Brown &Gibson,Kuiper et al.) and against (Carr &Simony)channel ow in the crystalline interior of the Cana-dian Cordillera are presented in the nal threepapersofthevolume.Theauthorswishtowarmlythankallparticipantsofthe2004 Burlington House conference for fruitful discussionsduringandafter the meeting. We thankC. Beaumont,R. L. Brown, R. A. Jamieson, M. Jessup, K. Larson,S. Medvedev,R.A. PriceandP.F.Williamsfordiscus-sion, andK. Larson, M. Jessup, andChief Editor J. P.Turner for their detailedreviews of earlier versions ofthis manuscript. 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