ductile extrusion in continental collision zones...

Ductile extrusion in continental collision zones:

ambiguities in the definition of channel flow and

its identification in ancient orogens

R. R. JONES1,2, R. E. HOLDSWORTH3,

M. HAND4 & B. GOSCOMBE4,5

1Geospatial Research Ltd, Department of Earth Sciences, University of Durham,

DH1 3LE, UK (e-mail: [email protected])2e-Science Research Institute, University of Durham, DH1 3LE, UK

3Reactivation Research Group, Department of Earth Sciences, University of Durham,

DH1 3LE, UK4Continental Evolution Research Group, Geology and Geophysics,

University of Adelaide, 5005, Australia5Northern Territory Geological Survey, PO Box 8760, Alice Springs, 0871 NT, Australia

Abstract: Field characteristics of crustal extrusion zones include: high-grade metamorphismflanked by lower-grade rocks; broadly coeval flanking shear zones with opposing senses ofshear; early ductile fabrics successively overprinted by semi-brittle and brittle structures; andlocalization of strain to give a more extensive deformation history within the extrusion zone rela-tive to the flanking regions. Crustal extrusion, involving a combination of pure and simple shear, isa regular consequence of bulk orogenic thickening and contraction during continental collision.Extrusion can occur in response to different tectonic settings, and need not necessarily imply adriving force linked to mid-crustal channel flow. In most situations, field criteria alone are unlikelyto be sufficient to determine the driving causes of extrusion. This is illustrated with examples fromthe Nanga Parbat–Haramosh Massif in the Pakistan Himalaya, and the Wing Pond Shear Zone inNewfoundland.

In this paper we review channel flow and crustalextrusion in relation to field geology. Our aim isto provide a critical evaluation of whether the pre-dictions of channel flow models are testable usingfield observations, and also to consider the rel-evance of channel flow in relation to existingmodels of crustal extrusion. We illustrate theinherent difficulty and ambiguity of applying thechannel flow concept with field examples fromboth active and ancient orogens (the NangaParbat–Haramosh Massif, Pakistan Himalaya, andthe Wing Pond Shear Zone in the Appalachianorogen, Newfoundland). General aspects of oroge-nic thickening and crustal extrusion are presentedfirst, followed by more specific discussion ofchannel flow and the nature and limitations ofpossible diagnostic characteristics.

Orogenic thickening and crustal

extrusion

Field studies in orogenic belts have long shownthat deformation arising from continental collision

generally involves a combination of large-scaleoverthrusting and bulk crustal thickening. Thiscombination is sometimes considered in terms ofthe relative importance of simple shear and pureshear to the mountain-building process, althoughthese are end-member strain components and mostparts of an orogen will experience more generalshear (e.g. Sanderson 1982; Platt & Behrmann1986; Passchier 1986, 1987; subsimple shear ofSimpson & De Paor 1993). The variety and com-plexity of structures typically observed in orogenicbelts illustrate that the distribution and localizationof strain is highly heterogeneous (e.g. Jones et al.2005). Overthrusting is associated with zones ofhigh strain concentration and high vorticity thatgive the orogen marked polarity (i.e. asymmetry),recognizable in outcrop as dominantly foreland-verging structures, including shear zones with top-to-foreland shear sense. Regions of bulk crustalthickening represent a more widely distributedresponse to horizontal shortening across the oro-genic belt, and show large variations in width andin localization of strain. The boundaries to these

From: LAW, R. D., SEARLE, M. P. & GODIN, L. (eds) Channel Flow, Ductile Extrusion and Exhumation in ContinentalCollision Zones. Geological Society, London, Special Publications, 268, 201–219.0305-8719/06/$15.00 # The Geological Society of London 2006.

regions can be well-defined shear zones or diffusetransitions with gradually varying style and inten-sity of strain. Some areas of large-scale crustalthickening can involve broad zones characterizedby intense folding and widespread formation of per-vasive tectonic fabrics. Crustal shortening can alsobe localized into narrow regions of more intensedeformation (often marked by well-defined highstrain boundaries), in which shortening is matchedby thickening occurring as concentrated zones ofcrustal extrusion. Irrespective of whether strain iswidely distributed across a broad area, or localizedinto a more concentrated zone of extrusion, diag-nostic characteristics of crustal extrusion includethe uplift of high-metamorphic-grade rocks in thecore of the zone (e.g. the Kaoko Belt in Namibia;Goscombe et al. 2003a, b, 2005), and the reversalin the sense of vorticity across the region of defor-mation (e.g. southwestern Hellenides; Xypolias &Doutsos 2000; Xypolias & Koukouvelas 2001;Xypolias & Kokkalas 2006).

Numerical modelling of crustal extrusion

Regions of crustal extrusion bounded by coeval,opposing shear zones are a characteristic featureof numerical models of deformation that involvecombinations of coaxial and non-coaxial strain(e.g. Sanderson 1982; Sanderson & Marchini1984). Although strain matrix modelling isnaively simplistic in approach, and explores thekinematics of deformation without any thermal–mechanical considerations, it is computationallyefficient, and has the advantage that it can be usedto model first-order relationships between bulkfinite strain and boundary conditions. In addition,it also helps to demonstrate the innate complexityof progressive deformation in four dimensions(4D) (i.e. one temporal and three spatial dimen-sions). Strain matrix modelling has been appliedextensively to improve understanding of variousnon-coaxial, non-plane strains that typify zones oftranspressional and transtensional deformation

(e.g. Fig. 1; Tikoff & Fossen 1993; Jones et al.1997, 2004; Fossen & Tikoff 1998; De Paolaet al. 2005). The significance of transpression isthat deformation resulting from oblique plate col-lision cannot be accommodated by simple shearalone, and must involve a significant componentof crustal extrusion, so numerical modelling oftranspression can provide insight into the interplaybetween orogenic shortening, overthrusting andcrustal thickening.

Application of transpressional strain modelling toareas of oblique convergence and divergence hasshown close correlation between model predictionsand observed aspects of crustal extrusion (e.g.Robin & Cruden 1994; Teyssier et al. 1995; Jones& Tanner 1995; Holdsworth et al. 1998, and refer-ences therein). In the case of transpression (Fig. 1c,d), upward extrusion of crustal material is caused byshortening across the deformation zone, and is thusequally applicable to plane-strain coaxial shorten-ing during orthogonal collision (Fig. 1a, b). Thethermal implications of crustal thickening and extru-sion have been explored for both orthogonal andoblique shortening by Thompson et al. (1997a, b).Thermal–mechanical models (Ellis et al. 1998),representing an extra level of geological realismcompared with simplistic strain modelling, repro-duce similar combinations of large-scale overthrust-ing and broad zones of bulk crustal extrusionresulting from continental collision.

In general, strain modelling usually assumes thatcrustal extrusion occurs by upward migration,deformation and erosion of the Earth’s surface,although it is also possible for crustal material tobe extruded laterally (i.e. axially, parallel to theorogenic trend; Dias & Ribeiro 1994; Jones et al.1997; Dewey et al. 1998). Note that extrusion inthis sense refers to the lateral escape of crustalblocks (e.g. Eastern Anatolia; Dewey et al. 1986).Lateral extrusion is also used in a slightly differentsense to describe outward ductile flow at lowercrustal levels, driven by lateral pressure gradients(e.g. Bird 1991; Clark & Royden 2000).

(c) (d)(a) (b)

Fig. 1. First-order kinematics of tectonically driven crustal extrusion caused by boundary displacement: (a) verticalextrusion, plane strain; (b) inclined extrusion, plane strain; (c) vertical extrusion, non-coaxial non-plane strain(transpression); (d ) inclined extrusion, non-coaxial non-plane strain (inclined transpression).

R. R. JONES ET AL.202

Evidence for crustal extrusion in central

and eastern Himalayas

Recognition of north-dipping normal faulting overlarge distances along-strike in Tibet (Burg et al.1984), led Burchfiel & Royden (1985) to suggestthat the Greater Himalayan Sequence (GHS) hasbeen extruded relative to Tibet and the LesserHimalaya. Subsequent work has recently beenextensively discussed and summarized by Searleet al. (2003), so only an overview of field-basedevidence for crustal extrusion is given here. Thepossible relationship between crustal extrusion ofthe GHS and mid-crustal channel flow (togetherwith evidence from regional geophysics) isdiscussed later.

The GHS is basally bounded by the Main CentralThrust (MCT), and separated from the overlyingTibetan crust by the South Tibetan Detachment(STD) system (Gansser 1964; Burg et al. 1984).Structural descriptions of normal displacement onthe STD are given by Burchfiel et al. (1992),Edwards et al. (1996), Grujic et al. (1996, 2002)and Law et al. (2004). Evidence for contempora-neity of movement on the MCT and STD betweenc. 22 and 17 Ma is documented by Hodges et al.(1992, 1993) and Walker et al. (1999). Detailedkinematic analyses from the MCT (Grasemannet al. 1999) and STD (Law et al. 2004; see alsoCarosi et al. 2006; Jessup et al. 2006) suggest that

finite strain in the GHS is a subsimple shear (i.e. acombination of pure shear flattening and simpleshear). Higher metamorphic grades are recordedfrom central parts of the GHS, with invertedpressure–temperature profiles across the MCT(e.g. Hodges et al. 1988, 1993; Searle & Rex1989; Vannay & Grasemann 2001). Crustal-meltMiocene leucogranites were emplaced betweenthe MCT and STD (e.g. Searle et al. 1993; Searle1999; Edwards et al. 1996; Wu et al. 1998). Not-withstanding ongoing debate regarding some ofthe field evidence (e.g. Hubbard & Harrison 1989;Harrison et al. 1999), the combined stratigraphical,structural, metamorphic and geochronologicaldata provide a strong indication that the HigherHimalayan slab has undergone southward-directedextrusion. The existence of this body of field-based evidence is independent of geodynamicinterpretations that are based on regional geophy-sics, or that link crustal extrusion to mid-crustalchannel flow beneath Tibet.

Summary: crustal extrusion

In this paper we use crustal extrusion as a general termto signify the commonly recognized thickening/extrusion of crust in response to bulk contraction. Inthis sense, the term implies no specific geodynamicprocess or driving force. The bulk geometry andkinematics of crustal extrusion can be modelled in

Fig. 2. Crustal extrusion in relation to channel flow: (a) schematic (scale-independent) depiction of typical criteria thatcharacterize regions of crustal extrusion: such criteria are consistent with crustal-scale channel flow, but may also bethe result of other driving mechanisms; (b) variation in the degree of strain localization at the margins of crustalextrusion zones: upper figures are schematic cross-sections; lower graphs show generalized variation in strainmagnitude for a typical transect across the zone; (c) schematic depiction of causative processes and metamorphic andstructural characteristics of channel flow predicted from thermal–mechanical modelling of Beaumont et al. (2001,2004) and Jamieson et al. (2002, 2004).

DUCTILE EXTRUSION AND CHANNEL FLOW 203

4D using strain matrix modelling. Diagnostic fieldcharacteristics of crustal extrusion include: meta-morphic inversion at the lower boundary of a high-grade zone flanked by lower-grade rocks; high-grade regional metamorphism, possibly with partialmelting in the core of the zone; broadly coevalshear zone margins with opposing senses of shear;early ductile fabrics progressively overprinted bylater brittle structures; and localization of strain,with some phases of deformation only seen in thezone and not in the flanking rocks (Fig. 2a).

Channel flow

Ductile flow in the lower crust

Laboratory studies of rock strength (e.g. Goetze &Evans 1979; Brace & Kohlstedt 1980), as well asdeep seismic data (e.g. Chen & Molnar 1983;Kuznir & Matthews 1988), have underpinnedsuggestions that layers of low viscosity in the lowercrust can undergo ductile flow (e.g. Kuznir &Matthews 1988; Bird 1989, 1991; Block & Royden1990; Molnar 1992; Bott 1999; see also Maggiet al. 2000, and Jackson 2002). These ductile zonesmay be weak enough to allow a degree of mechanicaldecoupling between upper and lower crust(e.g. Oldow et al. 1990; Royden 1996; Ellis et al.1998; Teyssier et al. 2002; Tikoff et al. 2002;Grocott et al. 2004; Klepeis et al. 2004). Possiblerepresentatives of lower-crustal flow in the rockrecord include large tracts of granulite and migmatiteterranes, and regions of large-scale ductile sheathfolding.

Numerical modelling of ductile flow in the lowercrust has shown that the flow may be driven bygravitational potential in regions of elevated topo-graphy, and could provide an effective mechanismfor the compensation of local gravity anomalies(Bott 1999), and for large-scale variations inMoho topography to be gradually smoothed overgeological time (Kuznir & Matthews 1988; Bird1991). This approach to modelling has utilizedstandard fluid dynamics equations (e.g. Turcotte &Schubert 1982, 2002), generally assuming laminarflow within a channel geometry. Similar flow con-ditions have also been used to model exhumationof material along subduction channels by reverseflow (England & Holland 1979; Mancktelow1995). Common to all the above models is that thelocation of channel flow is coincident with low-strength regions of the lower crust above theassumed rheology transition at the crust–mantleinterface.

Channel flow in the mid- and upper crust

Grujic et al. (1996) subsequently suggested thatcomparable fluid dynamics principles might be

applicable to mid- and upper-crustal levels, andused channel flow terminology in a conceptualmodel to explain field observations from Bhutan.This extended ideas developed by Nelson et al.(1995, 1996; see also Hauck et al. 1998, andAlsdorf et al. 1998), who presented geophysicaldata from the INDEPTH project, and postulatedthat mid-crustal flow is occurring under theTibetan Plateau, with the southerly continuation ofthis mid-crustal low-viscosity layer being extrudedas the GHS. An assumption of this model is thatpartial melting related to crustal thickening canradically alter the standard strength profile of thecrust, allowing weaker zones of reduced viscosityto develop at mid-crustal levels that would gene-rally be too strong for extrusive flow to occur.This concept has influenced thermal–mechanicalmodels of the Himalaya, which have incorporatedlow-strength mid-crustal rheologies in order toinduce channel flow to develop.

Thermal–mechanical models

and channel flow

Recent advances in thermal–mechanical finiteelement modelling (e.g. Ellis et al. 1998; Jamiesonet al. 2002) represent a new level of sophisticationin the analysis of progressive evolution of orogenicbelts. Thermal–mechanical models allow testablepredictions to be made in relation to geodynamics,tectonics, geomorphology, and gross structural andmetamorphic patterns. Application of this approachto lithospheric-scale modelling has been applied tothe Himalaya–Tibet orogen (e.g. Beaumont et al.2001, 2004, 2006; Jamieson et al. 2004, 2006). Inthese cases, the boundary conditions and inputparameters are carefully chosen to be geologicallyrealistic and to match actual Himalayan values asclosely as possible. Conformity between the resultsof modelling and many first-order geological andgeophysical characteristics of the Himalaya–Tibetbelt suggests that the models are well calibrated tothis specific orogen. The importance of this is thatproper calibration is essential prior to using the mod-elling to test the sensitivity of different input par-ameters in relation to resultant tectonism. In otherwords, it should not be considered a weakness of themodelling method that a model is tuned to match aspecific orogenic setting. On the contrary, this isthe basis for a better understanding of which factorsare likely to have most influence on dynamic orogenicprocesses such as orogenic thickening, crustal extru-sion and channel flow.

Implications from thermal–mechanical

models

Iterations of the thermal–mechanical modellingthat test the sensitivity of various input parameters


(Beaumont et al. 2004; Jamieson et al. 2004)emphasize that channel flow is unlikely to havedeveloped unless certain boundary conditionswere favourable. The aim of the following charac-terization of channel flow is to separate key attri-butes of the models into two main types, broadlycorresponding to ‘cause and effect’: firstly, transientfeatures relating to prerequisite tectonic and geody-namic boundary conditions shown to be importantin inducing channel flow (‘cause’); and secondly,attributes relating to the resultant channel(‘effect’), evidence for which may remain in therock record over significant periods of geologicaltime, long after orogenesis has abated (Fig. 2c).

Modelling: requisite conditions for

channel flow to occur

Large, hot, long-lived orogen. Modelling suggeststhat in order for a mid-crustal low-viscosity channelto develop, the crust must be suitably radiogenic,and large-scale crustal thickening must last longenough for partial melting to occur. An ancientorogen that was small, short-lived or had radiogeni-cally depleted crust would be unlikely to have beenable to support significant gravity-driven channelflow.

Extensive elevated plateau region. One of themain conclusions of modelling is that mid-crustalchannel flow is driven by a large horizontal gradientin lithostatic pressure relating to a wide uplandplateau region (corresponding to Tibet in the appli-cation of the model to the Himalayas), flanked by anerosional/topographic front. Indeed, the thermal–mechanical models for the Himalaya–Tibet orogenshow that there appears to be such a strong depen-dency on gravitational loading that channel flow isunable to develop unless a large plateau is present.In an ancient orogen, lack of evidence of a suffi-ciently large source of gravitational potential (suchas a plateau) will therefore make it significantlymore difficult to demonstrate conclusively thatgravity-driven channel flow has occurred. In manyold orogens it may no longer be possible to demon-strate the presence of an ancient plateau region,particularly in orogenic belts that have beenheavily reworked (e.g. Fenno-Scandian, Grenvil-lian, Grampian, Acadian, Variscan and manyothers), or those in which large lateral motionsalong strike-slip faults have dismembered much ofthe original collisional architecture (e.g. WesternCordillera, Scandian, Taconic, etc.).

Evidence of high denudation rates. Modelling hasshown that a high rate of denudation at the marginsof the elevated topographic front can have amajor effect in causing the channel to propagate(‘tunnel’) upwards from mid-crustal levels,

towards the Earth’s surface. Thus, high denudationrates are not a prerequisite for the channel todevelop at depth (e.g. fig. 6a of Beaumont et al.2004), but may be instrumental in promoting theexhumation of an active channel. In most ancientorogens, direct evidence of elevated rates of denu-dation will have disappeared, although indirectevidence may be present (e.g. from pressure–temperature–time (P-T-t) data, or from the sedi-mentary record in adjacent deposits; e.g. Galyet al. 1996).

Field characteristics of resultant channel

flow inferred from modelling

Pressure, temperature and relative displacementpaths in thermal–mechanical models allow infer-ences to be made about the resultant metamorphicand structural characteristics that should be visiblein outcrop if channel flow had developed. Herewe evaluate the degree to which specific character-istics will be useful as diagnostic attributes inancient orogens. We consider features of the mid-crustal channel, as well as upper-crustal extrusionzones where the channel reaches the Earth’ssurface.

Subhorizontal mid-crustal channel. Post-orogenicdenudation and exhumation of mid-crustalrocks may expose evidence of the fossilizedchannel. This should contain regions of partialmelting, migmatization and elevated metamorphicgrade, flanked above and below by rocks thathave experienced lower temperature metamorph-ism. Therefore, for positive recognition of possiblechannel flow, a full section showing both upper andlower channel margins should ideally be exposed.

Modelling implies that structural evidenceshould be useful in recognizing whether channelflow may have occurred. One of the most importantdiagnostic characteristics is to demonstrate thatshear zones were active simultaneously at bothtop and base of the channel, and for the zones tohave demonstrably opposing shear sense (‘top-towards-orogenic-front’ for the basal shear zone,‘top-towards-orogenic-core’ for the upper shearzone). Furthermore, the age of shearing should bebroadly similar to the timing of migmatization inthe channel. Difficulties may arise if the channelmargins have been reworked or reactivated by sub-sequent tectonic episodes, as this may remove muchof the evidence for earlier shear during the originalchannel flow.

Model predictions suggest that recognition ofhigh strains might provide useful diagnostic infor-mation. In an idealized channel, intensity of shearstrain should be greatest at the margins of thechannel, and lower in the channel centre, thoughin reality strain is likely to be more heterogeneously


distributed. Consideration of particle path trajec-tories in thermal–mechanical models (e.g. figs 4& 6 of Jamieson et al. 2004) show that for well-developed channel flow, shear strains (g) of 102 ormore may be common along the channel margins,with g in excess of 103 also possible. Fault rocksthat have experienced such high levels of strainare generally recognizable by their ultra-myloniticfabric, even if the exact magnitude of shear strainis difficult to quantify precisely. Within thechannel there should be evidence of subsimpleshear strains (i.e. a non-coaxial shear with shorten-ing across the deformation zone; examples aregiven by Grasemann et al. 1999; Xypolias &Doutsos 2000; Vannay & Grasemann 2001;Xypolias & Koukouvelas 2001; Law et al. 2004).In such cases, the strain magnitude in the channelmargins should be lower towards the core of theorogen where the channel starts, with an increasein strain outwards towards the orogenic front (cf.‘cream-cake tectonics’ of Ramsay & Huber 1987,pp. 610–613). Clearly this diagnostic attribute isonly testable if a large extent of the mid-crustalchannel is exposed across the orogen.

Strongly localized deformation due to high shearstrain within the channel is likely to give rise tonarrow terranes (domains) characterized bycomplex structural relationships, in which there isrepeated overprinting of successive deformationalfabrics. This may give the appearance of severalphases of deformation (fabric crenulation, refoldedfolds, sheath fold development, fold and fabrictransposition), though these should span a relativelyshort time period. Because these deformational‘phases’ are caused by spatial and temporal hetero-geneity along the channel, and relate to localizedperturbations within the flow, the rocks outsidethe channel should be markedly less deformed,and some (if not most) of the ‘phases’ should berestricted to the channel, and will be largelyabsent beyond the channel margins.

Zone of crustal extrusion in the uppercrust. Modelling implies that the exposure of afossilized channel that has ‘tunnelled’ from mid-crustal to upper-crustal levels at the orogenicfront, will share some diagnostic attributes with acorresponding channel at mid-crustal levels. As atgreater crustal depths, the migmatitic channelshould be flanked above and below by rocks oflower metamorphic grade, to give characteristicinverted and right-way-up metamorphic isogradsat the base and top of the channel respectively(see section 10.1 of Jamieson et al. (2004) for dis-cussion). This metamorphic inversion is a particu-larly important diagnostic feature in the uppercrust, as there is likely to be greater contrast inmetamorphic grade between the channel and its

lower-grade flanks, with the high P-T migmatite-grade channel enveloped by markedly lowerpressure and temperature greenschist- or amphibo-lite-grade rocks typical of upper-crustal orogeniclevels. P-T-t conditions should indicate that rapidexhumation of the channel occurred synchronouslywith the timing of shear along the channel margins.

Coeval shear zones at the top and base of thechannel should once again show opposing shearsense, with thrust displacement along the lowerchannel margin and normal-sense displacement atthe upper margin. Modelling emphasizes that inorder for the channel to have propagated towardsthe surface, both margins must have experiencedvery large strain magnitudes (material in the coreof the channel must be transported large distancesduring displacement from mid- to upper crust).For flow in the channel to remain ductile at highcrustal levels, high strain rates are needed, so thatthere is insufficient time for the channel to cool totemperatures at which increase in viscosity wouldimpede flow. Thus in ancient orogens, typicalcharacteristics of upper-crustal regions of thechannel should include the presence of high strainmylonites or ultramylonites that are progressivelyoverprinted by successive down-temperature andpressure phases of semi-ductile, semi-brittle andthen brittle deformation, as material in the channelis exhumed and extruded towards the Earth’ssurface. Younger brittle structures should generallyshow the same sense of shear as the older ductilefabrics that they overprint.

In summary, thermal–mechanical modellingallows specific predictions to be made about thefield characteristics that can be expected in zonesof channel flow. However, many of these character-istics are non-unique to channel flow: they arecommon to general zones of bulk crustal extrusion,and are widely recognized in areas in which channelflow is not believed to have occurred. The signifi-cance of this in relation to active and ancientorogens is discussed in the following sections.

Discussion

Application of the channel flow concept to explainHimalayan–Tibetan orogenesis has clearly been apositive catalyst in promoting vigorous scientificdiscussion. Much of the current debate focuses onthe reliability and validity of geophysical data,and its relevance in relation to field observationsof Himalayan geology. In the following discussionwe focus on other inherent difficulties of thechannel flow concept, particularly regarding recog-nition of channel flow in ancient orogens, andwhether or not the concept is useful to field-basedanalysis of continental collision zones.


Ambiguity of channel flow semantics

Earlier sections of this paper illustrate that commonusage of the term ‘channel flow’ can convey a rangeof slightly different meanings, each of whichcan carry subtle implications depending uponcontext. For example, with regard to fluiddynamics, channel flow is simply a generic termthat encompasses a number of more specific typesof quantitative flow equation. In contrast, whenused with reference to outcrop observations (e.g.Grujic et al. 1996), channel flow is a conceptualterm that is consistent with crustal extrusion (imply-ing inverted metamorphism, high shear strains, andcontemporaneous thrusting and normal faulting). Inthis context, for some workers the term ‘channelflow’ may also tacitly imply that continuation ofthe channel extends from the surface down tomid-crustal levels. Finally, within the context ofthe thermal–mechanical models of Beaumontet al. (2001, 2004), channel flow is a dynamic,gravity-driven process that requires high heat flowand large gravitational potential for significantamounts of flow to occur, as well as high rates oflocalized exhumation for the channel to reach thesurface.

In relation to Himalayan geology, these semanticdiscrepancies are generally unimportant, since mostproponents of channel flow generally include dis-cussion that encompasses all the different contextsoutlined above. However, in older orogenic belts,the distinction may be much more important,since in general there will be a very differentbalance of available data upon which interpretationcan be based.

Nature of evidence in ancient orogens

The study of modern mountain belts is essential inorder to improve our understanding of processesthat produced ancient orogens. Conversely,ancient orogens can also increase our understandingof modern-day orogenic processes, because exhu-mation of ancient mountains allows us to studylarge tracts of deep levels of the orogen that arenot widely exposed in active mountain regionstoday. In modern orogens such as the Himalayan–Tibetan system, many different types of data areavailable to help constrain tectonic interpretationand to guide the choice of suitable boundary con-ditions and geodynamic input parameters to use inmodelling. Some of the most useful types of dataare transient in nature (e.g. earthquake focal mech-anisms, heat flow, bright spots on deep seismicreflection data, geodetic measurements showingrelative displacement recorded by arrays of GPSstations). Other criteria can be longer-lived (e.g.topography, and sea-floor magnetic anomalies that

record relative plate motion). However, in themajority of older orogens, these types of transientshort-lived geodynamic data provide little or noconstraint. Most ancient terranes do not havewell-preserved boundary conditions, and it can beextremely difficult to infer the nature of the far-field driving forces that controlled deformation(e.g. Dewey et al. 1986). Furthermore, factorssuggested by thermal–mechanical modelling to beprerequisite for channel flow to occur – such ashigh gravitational potential, heat flow and exhuma-tion rates – are also transient in nature, and willgenerally be of much less use in constraininggeodynamic interpretations of ancient orogens.

Thus, many of the types of data that are currentlydriving debate about channel flow in the Himalayaswill not be as useful when testing the applicabilityof the channel flow concept to ancient orogens. Insuch cases, interpretation will inevitably placegreater emphasis on field studies and analysis ofsamples collected at outcrop, and must relyheavily on a combination of critical types ofmetamorphic, structural and geochronological evi-dence. In our opinion, in order to test different geo-dynamic models as objectively as possible, it isimportant to maintain a descriptive frameworkfor making field observations that avoids recordingdata in terms of a genetic description linked to aspecific interpretation of causative processes.

Non-uniqueness of field characteristics

inferred from modelling

Predicted field characteristics of channel flowinferred from thermal–mechanical modellingshow close similarity with actual field observationsfrom the Himalayan–Tibetan system (see above).However, while field data do provide strong evi-dence that crustal extrusion has taken place in theHigh Himalaya, it is much more contentious todemonstrate incontrovertibly that extrusion at thesurface is linked to zones of channel flow beneathTibet far to the north. Even in modern orogenswhere transient geodetic and geophysical data canbe acquired, a fundamental challenge facing geo-scientists is to test the range of possible geodynamicinterpretations that are consistent with the availabledata. At the current level of understanding, thecharacteristics predicted by thermal–mechanicalmodelling, as well as actual field observations andanalyses from the Himalayas, are non-unique interms of causative geodynamic driving forces. Inparticular, regions of crustal extrusion can resultfrom horizontal forces relating to convergent platemotion, and need not imply processes driven predo-minantly by gravitational potential. This is illus-trated below with an example from the Nanga


Parbat–Haramosh Massif of northern Pakistan. Inolder orogenic systems these challenges are exacer-bated, as shown later with an example from theWing Pond Shear Zone in the Appalachian orogenof Newfoundland.

Crustal extrusion in the Nanga

Parbat–Haramosh Massif

The Nanga Parbat–Haramosh Massif (NPHM) liesat the NW end of the Himalayan arc, and is bor-dered to the west by the Hindu Kush, to thenorth by the Karakorum range, and to the SE bythe High Himalayas (Fig. 3a). As part of thelong-lived, hot Himalayan system, with the vastTibetan plateau to the north, the massif sharesmany of the geodynamic attributes that characterizeregions further to the east. The NPHM is markedby extreme topography, with over 7000 m ofrelief between the summit of Nanga Parbat(8125 m) and the base of the Indus valley. Rapiddenudation is facilitated by the enormous sedi-ment-bearing capacity of the Indus, which mayhave allowed an effective positive feedback mech-anism to develop, where increased uplift rates aredriven by rapid erosion (Zeitler et al. 2001; Koonset al. 2002).

The massif is a large asymmetric antiformaldome (Fig. 3b), forming a crustal-scale syntaxis(Fig. 3c) that marks a major change in the overalltrend of the orogenic belt (Tahirkheli & Jan 1979;Coward 1983, 1985). Formation of the syntaxishas reworked the earlier structure of the MainMantle Thrust (Butler & Prior 1988a, b), which rep-resents the tectonic plate boundary between Indiaand Asia. The age of the syntaxis post-dates peakmetamorphism, and probably initiated within thelast 10 million years (Treloar et al. 2000; Zeitleret al. 2001; Butler et al. 2002). The core of themassif consists of migmatitic crystalline basementrocks of Indian plate affinity that have beenextruded through structurally overlying units ofthe Kohistan–Ladakh Arc, which had accreted tothe SE margin of the Asian plate prior to collisionwith India from c. 55 Ma onwards (Treloar et al.1989; Treloar & Coward 1991). Active crustalextrusion is ongoing, and is associated with extre-mely rapid uplift (Zeitler 1985) and high denuda-tion rates (Bishop & Shroder 2000), suggestingexhumation rates of up to 7 mm a21 (Zeitler1985; Zeitler et al. 2001). Seismic data suggestthat the base of the upper-crustal seismogeniczone is significantly elevated beneath the syntaxis(Meltzer & Christensen 2001; Meltzer et al.2001). The NPHM is one of the most intenselystudied regions of Himalayan geology, and there

Fig. 3. Nanga Parbat–Haramosh Massif (NPHM), northern Pakistan: (a) satellite image of Himalaya, Tibet andthe Indian subcontinent (courtesy of NASA/Visible Earth); (b) cross-section showing asymmetric antiformal nature ofthe massif and amount of material removed by erosion (after Butler et al. 2002); (c) map of the massif (collated fromButler et al. 1989, 2000; Searle & Khan 1996; Pecher & Le Fort 1999; Argles 2000; Edwards et al. 2000).


is a wealth of published data that integrate a widerange of stratigraphic, metamorphic, structural, iso-topic, geochemical and geophysical evidence.Detailed descriptions of the geology of the massifare given by Khan et al. (2000) and referencestherein, as well as Zeitler et al. (2001) and Butleret al. (2002).

Geochronological, metamorphic and structuraldata provide strong evidence that ductile extrusionof mid-crustal material is taking place in the coreof the massif. Fission track data and muscovite,biotite and hornblende cooling ages show that thecentral parts of the NPHM have experienced veryrapid exhumation relative to surrounding regions:40Ar/39Ar mica cooling ages suggest rates ofbetween 3–4 mm a21 (Whittington 1996) and7 mm a21 (Zeitler 1985; Zeitler et al. 2001). Arange of P/T isotope data suggest that up to25 km of crust may have been eroded from thecentral parts of the massif within the last 10million years (Butler et al. 1997; Whittingtonet al. 1999). Uplift and exhumation are associatedwith partial melting and emplacement of smallgranitoid bodies. These yield zircon ages as recentas 1 Ma (Zeitler et al. 1993), and generally youngtowards the core of the massif (summarized inZeitler et al. 2001).

Structural studies around the margins of themassif have revealed a protracted kinematic his-tory in which earlier ductile fabrics are generallyoverprinted by later semi-ductile to brittle struc-tures. The age of deformation is locally well con-strained, either by direct dating of structuralfabrics, or by dating of cordierite seams and ana-tectic leucogranites that are intimately associatedwith shearing (e.g. Zeitler & Chamberlain 1991;Zeitler et al. 1993; Butler et al. 1997; Schneideret al. 1999; Whittington et al. 1999). Currentlyactive thrusting along the western margin of theNPHM is emplacing Nanga Parbat gneisses north-westwards over unconsolidated river gravels androcks of the Kohistan arc (Butler & Prior 1988a;Butler 2000). In their hanging wall, these brittlestructures carry semi-brittle and ductile fabricsthat also have the same top-to-NW kinematics,including 1–2 km of mylonitic gneiss with well-developed S-C fabrics (Butler 2000). Detailedfabric analysis comparing variation in the ellipticityand preferred orientation of augen, shows that thefinite strain in the gneiss is a subsimple shear,with a strong component of up-dip elongation(Butler et al. 2002). Boudinaged calc-silicatelayers suggest elongations of 30%. There is alsoevidence of a component of right-lateral strike-slip along the western margin (Butler et al. 1989;Butler 2000), showing that the bulk deformationof the region is transpressional. The sequence ofductile to brittle structures now visible at the

surface is likely to be very representative of compar-able structures that are currently forming at differentdepths in the massif, and which will be progressivelyexhumed as crustal extrusion continues.

The eastern margin of the NPHM is not as widelystudied or well understood as the western margin.There are local areas of top-to-NW thrusting(Argles 2000), while south of Nanga Parbat, top-to-south thrusting has been documented (Butleret al. 2000). However, most parts of the easternmargin appear to be marked by ductile fabrics ofthe Main Mantle Thrust (MMT), subsequentlysteepened during formation of the syntaxis (Butleret al. 1992; Wheeler et al. 1995), without significantoverprinting of recent brittle or semi-brittle defor-mation (Argles 2000). Although further fieldworkis clearly needed to provide additional constraintson the kinematics of the eastern margin, availableevidence suggests that later (post-MMT) strain ismore distributed than in the west, and that top-to-SE deformation was accommodated largely byupwarping of the eastern antiformal limb duringformation of the syntaxis.

Summary

There is a wealth of field data showing key charac-teristics of crustal extrusion in the NPHM, includ-ing: high-grade units in the core of the massif,with inverted metamorphism across the lowermargin; evidence of partial melting, marked byyounger ages in the centre of the zone; active defor-mation synchronous with crystallization of anatec-tic melts; ductile fabrics progressively overprintedby semi-ductile and brittle structures; and subsim-ple shear strains, with a large component of up-dipstretching. Recognizing that the prerequisite geo-dynamic conditions for channel flow are present(large, hot orogen, elevated plateau, extremedenudation), Beaumont et al. (2004) proposedthat the NPHM is another potential example ofactive channel flow. However, it remains extre-mely difficult to establish the main causativedriving force(s) of extrusion in the massif, andseveral alternative models for its developmenthave been proposed. These include the NPHM asa fault-related antiform at the lateral tip to majorHimalayan thrusts (Coward et al. 1988); a zoneof NW shortening in response to orogen-parallelextension around the Himalayan arc (Seeber &Pecher 1998); upwelling of crustal gneiss domes(Pecher & Le Fort 1999; Koons et al. 2002);crustal-scale buckling (Burg & Podladchikov2000); and north–south constriction/transpressionabove a thrust tip (Butler et al. 2000). Althoughthe far-field boundary displacements betweenIndia and Asia are well established and reasonablystraightforward (Treloar & Coward 1991), the


regional-scale tectonics of the syntaxis are highlycomplicated. Deformation in the massif is non-plane strain, is spatially very heterogeneous, andhas changed significantly with time, even overshort periods, so understanding the geodynamicsof the region requires a 4D analysis of thesystem. Although vertical forces associated withthe gravitational loading by Tibet may conceivablyhave direct influence on crustal melting and extru-sion beneath the NPHM, it remains at least asplausible that the main driving force of extrusionis the far-field tectonic force of the northwardmotion of India, and the resultant complex inter-actions that are occurring at the corner of theIndian indentor between the Himalaya, Karakorumand Hindu Kush.

Crustal extrusion in the Wing Pond

Shear Zone, Newfoundland

The Gander Zone in the Appalachian orogenic beltof northeastern Newfoundland (Fig. 4a) representspart of a Gondwanan-derived continental fragmentthought to have been accreted to Laurentia duringthe closure of the Lower Palaeozoic IapetusOcean (e.g. Williams et al. 1988; Soper et al.1992). The Iapetus suture in Newfoundland lieswithin the Dunnage Zone, the eastern part of whichwas obducted onto the Gander Zone during theArenig with the boundary marked by an allochtho-nous unit of melanges (Gander River Complex,Fig. 4a) emplaced initially on eastward-directedthrusts. The present-day eastern margin of theNE Gander Zone is the Dover Fault Zone, a majorreactivated brittle–ductile structure (Blackwood& Kennedy 1975; Holdsworth 1994). TheAvalon Zone to the east comprises a characteristicGondwanan Neoproterozoic tectonostratigraphyand Palaeozoic cover (O’Brien et al. 1983).

The metasedimentary rocks forming most of theGander Zone in NE Newfoundland (the ‘GanderGroup’) are a thick sequence of variably deformedand metamorphosed clastic metasediments andgneisses (psammites, pelites) intruded by numerousgranites (Fig. 4a; Blackwood 1977; Hanmer 1981;O’Neill 1991; Holdsworth 1994). The deforma-tional and metamorphic character of the GanderZone is highly heterogeneous. Two regional ‘flatbelts’ are recognized, characterized mainly bygreenschist-facies (chlorite–white mica) assem-blages which are deformed by generally eastward-verging recumbent folds locally termed ‘F2’ (e.g.Fig. 5a; Kennedy & McGonigal 1972) thoughtto have formed during overthrusting of theDunnage Zone in the Arenig (c. 480 Ma). Theseare post-dated by two 10–25km wide, high-straintranspressional steep belts (Fig. 4a) of similar

age, both of which are characterized by significantlyelevated P-T conditions (D’Lemos et al. 1997; King1997). The easternmost of these, here termed theHare Bay Gneiss Shear Zone (HBGSZ), is charac-terized by migmatization and syntectonic graniteemplacement during sinistral shear thought tobe related to initial docking of the Gander andAvalon terranes during the Silurian (e.g.Holdsworth 1994). Peak metamorphism, derivedfrom mineral reactions, cordierite geobarometryand hornblende–plagioclase geothermometry inthe HBGSZ was at high temperature–low pressureconditions (c. 7408C at 4.5 kbar; D’Lemos et al.1997). Further to the west, the Wing Pond ShearZone (WPSZ) (O’Neill 1991) is a structurallymore complex region of focused metamorphismand high strain up to 10 km across (Fig. 4b); itsage relative to the HBGSZ is uncertain, althoughexisting geological constraints suggest that theyare broadly contemporaneous.

The WPSZ is well exposed in roadside exposuresalong the Trans Canada Highway north and east ofthe later Gander Lake granite (Fig. 4b). Movingeastwards from Benton, early recumbent foldsand associated greenschist-facies fabrics of thewestern Gander Zone are progressively overprintedand transposed by upright ‘F3’ folds and an associ-ated steeply dipping, NE–SW trending ‘S3’ fabric(Fig. 5b; O’Neill 1991). S3 is a prograde foliationdefined by increasing proportions of biotite and isassociated with low-P prograde metamorphismresulting in formation of cordierite- and andalu-site-bearing assemblages as the intensity of S3increases. In places andalusite and cordierite areoverprinted by S3 (Fig. 6a, b), whereas in otherinstances porphyroblast growth occurred late- orpost-S3 (Fig. 6c). For the general range of bulkcompositions in the Gander Group, the growth ofcordierite- and/or andalusite-bearing assemblagesimplies prograde metamorphism at �3 kbar(Fig. 6f).

Further to the east, local ‘F4’ folds appear,tighten and are themselves transposed withinplanar, steeply dipping high-strain zones (Fig. 5c),with the preceding structural complexity onlybeing apparent in local low strain windows associ-ated with meso-scale F4 fold hinge zones (e.g.Fig. 5d). These planar high-strain panels definethe structural geometry of the WPSZ andrecord sinistral transport parallel to a low-anglesouthward-plunging lineation. The high-strainshear fabric is associated with development ofkyanite–staurolite + garnet-bearing assemblagesthat formed at the expense of syn-S3 andalusite(Fig. 6d). The kyanite-bearing assemblagesformed at around 5 kbar, 6008C (Fig. 6f) and areoverprinted by sillimanite-bearing fabric (Fig. 6e)in the core of the WPSZ.


Along the eastern margin of the shear zone, localF5 folds appear and the high-strain fabric becomesmylonitic. It is characterized by greenschist-faciesassemblages, and appears to correspond to theeastern margin of the WPSZ which is exposedalong-strike, SW of the Deadman’s Bay Granite(Fig. 4a). The entire WPSZ domain is characterizedby mainly steeply plunging mineral-stretchinglineations, although there are local high-strainzones with shallow-plunging lineations. The F3–5folds are geometrically identical sheath folds andappear to reflect progressive deformation andshear localization within the WPSZ. The easternmargin of the WPSZ is flanked by a system ofolder shallow-dipping fabrics that resemble theflat-lying system west of the shear zone. Viewed

in this context the WPSZ has a simple geometrywith a steeply dipping core flanked by domains ofprogressively lower strain.

Summary

The geometry of the WPSZ and its flanking struc-ture is mirrored by both the kinematic and meta-morphic character of the structure. The westernmargin of the shear zone records sinistral kin-ematics (Fig. 5e) with a component of east-updisplacement, while the eastern margin of theshear zone records west-up displacement withdextral component of movement (Fig. 5f). Thegradual increase in strain towards the shear zonecore suggests that this is a zone of diffuse crustal

Fig. 4. Channelized extrusion in the Wing Pond Shear Zone (WPSZ). (a) Regional tectonostratigraphic map(DBG, Deadman’s Bay Granite; HBGSZ, Hare Bay Gneiss Shear Zone, which contains numerous sheeted granites).(b) Combined metamorphic/structural map. High grade core of the zone is marked by biotite, andalusite and kyaniteisograds. Structural transect along the Trans Canadian Highway shows the increase in structural complexity andlocalization of strain towards the centre of the zone.


extrusion (cf. Fig. 2b), rather than a zone of lowstrain flanked by high strain margins. There is alsoan apparent progressive increase in metamorphicpressure towards the central region of the shearzone that is compatible with the kinematics,

suggesting relative upward displacement of theshear zone core. On the flanks, andalusite-bearingassemblages formed at pressure of c. 3 kbarduring progressive ‘D3’ deformation, while inthe central region kyanite–staurolite-bearing

Fig. 5. Field evidence for channelized extrusion from the WPSZ. (a) Flat-lying low-grade psammites and pelites withtight ESE verging and facing fold pair (local F2), typical of the flat belts outside the WPSZ. Northern shore of GanderLake. Notebook is 190 cm high. (b) Steeply dipping S3–S4 fabric typical of the WPSZ from the shores of WingPond. Note the abundant quartz segregations indicative of the higher metamorphic grades in these rocks. (c) Localintensification of S4–S5 fabric (in area to the left of the person) in the core region of the WPSZ; Square Pond quarry(east), just north of the Trans Canada Highway. (d) Typical lower strain augen in F4 fold-hinge zone from MintBrook revealing underlying structural complexity in WPSZ. Close F4 folds refold tight F3 folds whichthemselves refold a spaced S2 solution fabric. Lens cap 55 mm diameter. (e) Plan view of sinistral asymmetricboudins of quartz segregations from a high strain zone close to the NW margin of the WPSZ, next to the TransCanada Highway. ( f ) Plan view of dextral asymmetric boudins of quartz segregations from the SE margin of theWPSZ near Gambo.


Fig. 6. Petrological evidence for channelized extrusion from the WPSZ (long dimension of allphotomicrographs ¼ 7 mm). (a) Cordierite (now retrogressed) overprinted by S3 crenulations. (b) Partiallyretrogressed andalusite wrapped by S3 biotite–muscovite-bearing foliation. (c) S3 crenulations overgrown byandalusite. (d) Andalusite replaced by kyanite–staurolite-bearing ‘D4’ assemblage within the centre of the WPSZ.(e) Sillimanite-bearing ‘D4’ assemblage overprinting syn-D3 andalusite partially replaced by kyanite.( f ) Metamorphic evolution of rocks in the WPSZ system depicted on a generalized KFMASH P-T pseudosection(þ muscovite-qtz) adapted from Reche et al. (1998) applicable to the aluminous metapelitic compositions that occurwithin the Gander Zone metasediments. On the flanks of the shear zone andalusite-bearing assemblages formed duringprogressive ‘D3’ deformation. Cordierite growth is generally later than andalusite, but still locally shows synkinematicrelationships with ‘S3’. In the core of the shear zone, increasing pressure is implied by the replacement of andalusite bykyanite and staurolite. Subsequent growth of sillimanite is interpreted to reflect decompression associated withexhumation that differentially juxtaposed the now-exposed relatively high-P central region of the shear zone against thelower pressure flanking domains.


assemblages formed at around 5 kbar on an apparentclockwise P-T evolution (Fig. 6f).

Conclusions

Intense study of active orogenic belts such as theHimalayan–Tibetan system emphasizes theinherent difficulty in deriving unique geodynamicinterpretations based on available geodetic, geophy-sical and field-based evidence. In studies of ancientorogenic systems, transient geodetic and geophysi-cal data are generally not available or are lessuseful, placing greater reliance on field-based meta-morphic, structural and geochronological evidence.

Thermal–mechanical modelling can be extre-mely useful in testing the potential influence ofvarious geodynamic boundary conditions, althoughmany of the field characteristics predicted frommodelling are common to general zones of crustalextrusion arising in other tectonic regimes, includ-ing transpression zones. Field examples fromNanga Parbat in northern Pakistan and the WingPond Shear Zone in Newfoundland display strongfield evidence of significant crustal extrusion;however, there is a lack of evidence to show thatextrusion is gravitationally driven or linked to sub-horizontal mid-crustal channel flow. The maindriving force of crustal extrusion in these areas isdifficult to determine with certainty, but is at leastas likely to be tectonically rather than gravitation-ally driven.

Because of the inherent difficulties in identifyingthe driving mechanisms of lithospheric-scalegeodynamic processes, we believe that a clear dis-tinction should be made between primary obser-vations that describe field characteristics of crustalextrusion (Fig. 2a,b), and geodynamic interpret-ation that tries to relate the data to specific large-scale processes or driving mechanisms (Fig. 2c).In this respect, existing usage of the term ‘channelflow’ is already ambiguous, since it is used var-iously to describe any or all of the following: (i) ageneral type of flow condition (in relation to fluiddynamics); (ii) gravitationally driven mid-crustalflow, which if aided by high denudation might pro-pagate upwards to cause crustal extrusion at thesurface (in the context of recent thermal–mechanical models); and (iii) generalized bulkcrustal extrusion, which may extend downwardsto connect to subhorizontal mid-crustal zones (inthe context of a conceptual explanation for Himala-yan field observations such as inverted metamorph-ism, high shear strains, and contemporaneousthrusting and normal faulting). Consequently, ourrecommendation is that the terminology usedfor field description and geometric/kinematicinterpretation in orogenic belts should include

terms such as ‘crustal extrusion’, ‘ductile extrusion’or ‘channelized extrusion’. Such terms should beconsidered as broadly synonymous, and refer tozones that show inverted metamorphism, coevalmarginal shear with opposing sense etc., withoutany implication of the far-field driving mechanism.Particularly in older orogenic belts, care should betaken to explain clearly the exact meaning of theterm ‘channel flow’, to discuss any geodynamicimplications in relation to the context in which theterm is used, and to separate primary field obser-vations from subsequent geodynamic interpretation.

R. Law is thanked for a detailed and extremely constructivereview of the paper, and extensive editorial advice. U. Ringreviewed an earlier version of the manuscript. R.R.J. thanksNPHM co-workers C. Bond, R. Butler, M. Casey, G. Lloyd,P. McDade and Z. Shipton, as well as M. Bott for input onchannel flow. R.E.H. thanks R. D’Lemos and T. Kingfor their many discussions concerning the geology of theWing Pond Shear Zone. The invaluable logistical supportof S. O’Brien, P. O’Neill and F. Blackwood of the Depart-ment of Mines and Energy is gratefully acknowledged,together with the assistance of E. Saunders duringfieldwork in Newfoundland.

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