Himalayan metamorphic sequence as an orogenic channel: insight from Bhutan

Himalayan metamorphic sequence as an orogenic channel:insight from Bhutan

Djordje Grujic a;�, Lincoln S. Hollister b, Randall R. Parrish c

a Department of Earth Sciences, Dalhousie University, Halifax, NS, Canada B3H 3J5b Department of Geosciences, Princeton University, Princeton, NJ 08544, USA

c Department of Geology, University of Leicester and NERC Isotope Geosciences Laboratory, Keyworth, Notts NG12 5GG, UK

Received 11 October 2001; received in revised form 17 January 2002; accepted 21 January 2002

Abstract

The Bhutan Himalayas differ from the rest of the Himalayas in two major ways: (i) low-grade sedimentary rocks lieabove the Greater Himalayan Sequence as klippen (i.e. erosional remains of the South Tibetan Detachment); and (ii)an out-of-sequence thrust, the Kakhtang thrust, lies structurally above the klippen, and it doubles the exposedthickness of the Greater Himalayan Sequence. Our field observations and geochronological data constrain the mainkinematic events in the Bhutan Himalayas. Crystallisation ages of leucogranite dykes deformed by the Main CentralThrust and the South Tibetan Detachment indicate that these two structures operated together between 16 and 22Ma. The out-of-sequence Kakhtang thrust was active at 10^14 Ma and was concurrent with reactivation of the SouthTibetan Detachment. Restoration of the Bhutan Himalayas prior to the out-of-sequence thrusting shows that theGreater Himalayan Sequence was the core of a long, low-viscosity crustal channel extending under the Tibetanplateau. We propose that the gravity-driven southward extrusion of the channel material from underneath the Tibetanplateau caused the inverted metamorphic sequence in the Lesser Himalayan Sequence and in the Greater HimalayanSequence. This process also led to occurrences of present-day surface rocks that were derived from variable distancesdown dip, but from similar crustal depths. Such an exhumation pattern can explain the similar peak pressures for theGreater Himalayan Sequence along the length of the Himalayas. ß 2002 Elsevier Science B.V. All rights reserved.

Keywords: orogeny; exhumation; metamorphism; middle crust; Himalayas

1. Introduction

The tectonic setting of the Bhutan Himalayas[1^5] shares many similarities with the Himalayasof Nepal and India. These include the continuityof major tectonostratigraphic units and structures

such as the Siwalik Group, the Main BoundaryThrust (MBT), the Lesser Himalayan Sequence(LHS), the Main Central Thrust (MCT), theGreater Himalayan Sequence (GHS), and theSouth Tibetan Detachment (STD). Two majordi¡erences are outliers of low-grade sedimentaryrocks in the midst of the GHS, and a thrust lo-cated between the MCT and the STD (Fig. 1).Explanation of these phenomena accounts formost of the processes fundamental for the tecton-ics of the Himalayas.

0012-821X / 02 / $ ^ see front matter ß 2002 Elsevier Science B.V. All rights reserved.PII: S 0 0 1 2 - 8 2 1 X ( 0 2 ) 0 0 4 8 2 - X

* Corresponding author. Tel. : +1-902-494-2208;Fax: +1-902-494-6889.

E-mail address: [email protected] (D. Grujic).

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Earth and Planetary Science Letters 198 (2002) 177^191

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Two apparently incompatible kinematic eventscharacterise the deformation in the Himalayas:south-directed thrusting along the MCT, andnorth-directed normal faulting along the STD.Neither kinematic event had ideal simple sheargeometry because a signi¢cant amount of pureshear is recognised for both events [6,7]. The sim-ilar ages for the shearing along the MCT andSTD, and the apparent wedge shape of the GHSas de¢ned by these two structures (e.g. [8^10]),were crucial for the development of models forthe tectonic, metamorphic and magmatic historyof the Himalayas [11]. A common feature of thesemodels is the extrusion of a wedge (either rigid orductile) consisting of the GHS from between theMCT and the STD.

The deformation front of the Himalayas prop-agating southward is a generally accepted con-cept. However, in several places the thrustinghas shifted to the interior of the orogen, resultingin out-of-sequence structures. Some of thesethrusts have been observed only recently [2,12],and several authors (e.g. [11,13]) have suggestedthe MCT in Nepal was reactivated as an out-of-sequence structure.

Recognition of a major out-of-sequence thrustwithin the GHS, and of a greater extent of theSTD, leads us to suggest that the GHS of Bhutanoriginated as an orogenic channel that projectsfor over 200 km to the lower crust of the TibetanPlateau. The prior existence of this low-viscosity,10^15 km thick crustal channel is consistent withthe interpretation of Clark and Royden [14] thatsuch a channel exists now in the lower crust ofTibet. The south-directed return £ow in the chan-nel during extrusion resulted in the combinationof south-directed thrusting at the bottom of thechannel and apparent north-directed extension atthe top.

In this paper, we compile our ¢eld observationsfor Bhutan and integrate them with petrologicand geochronological data [15], and with resultsof geodynamic modelling [16]. Data from thesesources lead to an interpretation that a low-vis-cosity crustal channel can explain most of thegeologic features of Bhutan and, by inference,Himalayan tectonics in general.

2. Main structures of the Greater HimalayanSequence

The following section is a general summary ofthe major structures of the GHS in the BhutanHimalayas based on our prior work [2^4] and onrecent ¢eld observations (Fig. 1). New geochrono-logical data [15] provide constraints on timing ofdeformation. We refer to the crystallisation agesof leucogranite intrusions that were deformed bya given structure. The crystallisation age of de-formed intrusions constrains the maximum ageof that structure, whereas the crystallisation ageof undeformed intrusions that cut a de¢ned struc-ture provides the minimum age of the deforma-tion. We did not ¢nd any undeformed intrusionsthat had clear relationships to recognised struc-tures.

2.1. Main Central Thrust

A number of workers (e.g. [6,17,18]) have con-cluded that the south-directed shearing is not lim-ited to a single surface of the MCT but rather isdistributed across a wide zone, commonly knownas the MCT zone. In the Bhutan Himalayas thelower boundary of the structural domain, withpervasive top-to-the-south shearing, is at least2 km below the MCT [3], and the upper boundaryof this kinematic domain is in the hanging wall ofthe STD. Even at the highest structural levels ofthe GHS, we ¢nd synmetamorphic kinematic in-dicators within the main foliation that showsouth-directed shearing. Similar to the GHS ofNepal [19], several localised zones of deformationexist within the GHS of Bhutan showing that thesouth-directed deformation was heterogeneous inspace and time. Near the MCT, the degree of¢nite deformation is the highest, indicated by pro-tomylonites in both footwall and hanging wallrocks. Thus, we follow Davidson et al. [4] in de-¢ning the MCT as a protolith boundary separat-ing LHS-derived protomylonites and the GHS-de-rived protomylonites.

In eastern Bhutan, immediately above theMCT, is a zone of garnet þ staurolite schist thatis tens of metres thick. For the next few hundredsof metres up section, kyanite is abundant. Within

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this portion of the section occur centimetre- todecimetre-sized pods and sills of leucogranitethat are concordant to the dominant foliationand are deformed within it. Kyanite occurs withinsome of these leucogranite bodies, indicating thatpressures prior to or during thrusting were greaterthan 8.5 kbar [4]. Thermobarometry for the asso-ciated schist records pressures of 10^12 kbar at750^800‡C [15]. The crystallisation ages of leuco-some within migmatites in this zone range be-

tween 16 and 18 Ma [15] (Fig. 2). We interpretthese ages to be the time of metamorphism beforeor during south-directed shearing.

Beginning some hundreds of metres above theMCT, ¢brolite appears, and sillimanite partiallyto totally pseudomorphs kyanite [3,4,15]. Melt inthis zone was present during top-to-the-southshearing as indicated by leucosome-¢lled shearbands. Within this portion of the section are mul-tiple generations of sills and dykes of leucogran-

Fig. 1. Simpli¢ed geological map of Bhutan (after [1,5,24,25] and our own observations). Arrows indicate the orientation of themean azimuth for the hanging wall movement direction (Fig. 4). All the data are from our own observations except: 1 from [9],2 from [22], and 3 from [24]. ISL, Project INDEPTH seismic lines [50]. MFT, Main Frontal Thrust. MBT, Main BoundaryThrust. MCT, Main Central Thrust. BKT, Kakhtang thrust. Klippen from east to west: SK, Sakteng. UK, Ura. BMK, BlackMountain. TCK, Tang Chu. LS, Lingshi syncline. STD, South Tibetan Detachment. YCS, Yadong Cross Structure. Tectonicmap of Asia modi¢ed from [1] shows the location of the Kingdom of Bhutan.

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D. Grujic et al. / Earth and Planetary Science Letters 198 (2002) 177^191 179

ite. A sample from the thickest of these sills (Fig.2) yielded a monazite date of 13.5 Ma. This date,together with the kinematic indicators from with-in the sills, shows that thrusting was still in prog-ress at about 14 Ma.

2.2. Metasedimentary klippen

Within the GHS of Bhutan, sedimentary rocksof greenschist facies and lower metamorphicgrade occur in several synclines (Fig. 1). Becausenorth-directed ductile shear zones sole them, theseerosional outliers, which lie on top of the GHS,are klippen associated with the STD. These unitsthus mark the southernmost extent of the STD inthe Bhutan Himalayas. Although no erosionalremnants of the STD have been observed in theHimalayas outside Bhutan, it is likely that theSTD extended farther south in all of the centralHimalayas, but now has been completely removedby erosion.

All the outliers occur as cores of open, uprightsynclines with variable axial orientation (Fig. 1).

The metasedimentary rocks belong to the Chekhaformation of assumed Late Proterozoic age [1,5].The lowest fossiliferous layers (found only in theTang Chu Klippe) are Middle to Upper Devonianlimestones [21] and were ascribed to the Tethyansequence [1]. Edwards et al. [22], in suggestingthat this Tethyan outlier in Bhutan may be anerosional remnant of the STD, proposed thatthe Devonian limestones directly overlie theSTD. We found the detachment to be situatedstructurally and stratigraphically deeper than theDevonian limestones, at the base of the Chekhaformation, where a zone of ductile deformationoccurs along the contact with the GHS, andwhere well-developed kinematic indicators (Fig.3a^d) are consistent with normal sense northeast-ward to northwestward displacement of the hang-ing wall.

The metamorphic rocks beneath the Chekhaformation consist of two-mica garnet þ sillimanitegneiss and schist, and migmatites. The rocks arecommonly highly sheared. In general, top-to-the-north-directed CP-type shear bands overprint the

Fig. 2. Cross-section A^AP (Fig. 1) of the eastern Bhutan Himalayas after Gansser [1] and our own observations. MCT, MainCentral Thrust. SK, Sakteng Klippe. KT, Kakhtang thrust. STD, South Tibetan Detachment. Ages are for the solidi¢cation ofleucogranite bodies. Deformation kinematics on respective outcrops are indicated by half-arrows. The topographic swath pro¢le(25 km wide) was constructed by Chris Duncan (University of Massachusetts, Boston, MA, USA) from DEM data.

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Fig. 3. Representative structures in the Bhutan Himalayas. (a) Leucosome-¢lled shear band in migmatites in the footwall of theChekha formation. Ura Klippe. Melt-present deformation might explain the high angle between the shear zone and the foliation[20]. (b) Two sets of shear bands in schist of Chekha formation at the base of the Ura Klippe. (c) Flanking-fold [51] in quartziteand phyllite of Chekha formation in the Tang Chu Klippe. The leucogranite dyke is boudinaged and aligned within the axial sur-face. The distribution of boudins and the fold asymmetry are consistent with top-to-the-northwest shearing. Consistent orienta-tion of £anking-folds throughout this structural zone and their association with shear bands is compatible with their developmentduring north-directed shearing. (d) Boudinaged and northward-tilted leucogranite dyke at the top of the Chekha formation in theSakteng Klippe. (e) c-type porphyroclast of staurolite suggesting top-to-the-south sense of shear. Field of view 3 mmU2 mm.Footwall of the Kakhtang thrust near the village of Kakhtang, central Bhutan. (f) c-type boudins of leucogranite dyke in migma-tites in the hanging wall of the Kakhtang thrust. Eastern Bhutan.

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dominant foliation that contains synmetamorphictop-to-the-south shear sense indicators. Leucog-ranite sills and dykes, which are boudinaged con-sistent with top-to-the-north shearing, intrudedthis zone. Melt was present during the deforma-tion as indicated by leucosome-¢lled boudin necksand shear bands (Fig. 3a). In shear bands, and in

fault surfaces in gneiss and leucogranite, ¢broliticsillimanite grew parallel to the north-directed dis-placement direction. This grain growth impliesthat these rocks were at temperatures within the¢eld of stability of sillimanite during the north-directed shearing. In addition, the brittle^ductilebehaviour of the rocks at those temperatures sug-

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gests high pore £uid pressure (exerted by melt)had signi¢cantly reduced the e¡ective stress. Thepore £uid pressure lowers the di¡erential stressnecessary to cause failure and permits fractureat depths where the rocks otherwise would eitherbe stable or deform ductilely (e.g. [20]).

The Chekha formation comprises phyllite, micaschists, hornblende schists, quartzite, limestone,and quartz conglomerate [1,5]. Quartzite and gar-net+staurolite+muscovite schists represent thelowermost units of the Chekha formation. Withinthe Chekha formation the metamorphic graderapidly decreases upward. At the base of theunit, tectonic foliation is pervasive, whereas atthe higher structural levels, the dominant planarfabric is sedimentary bedding.

The rocks at the base of the Chekha formationare sheared although the deformation rapidly de-creases upward. The stretching lineation associ-ated with north-directed shearing is northwest-to northeast-plunging (Figs. 1 and 4). In the low-ermost units of the Chekha formation, CP-typeshear bands with top-to-the-north shearing over-print the earlier tectonic foliation that containstop-to-the-south shear sense indicators. In places,conjugate shear bands indicate shortening perpen-dicular to the foliation, i.e. a pure shear compo-nent of the deformation. However, where bothsets are present in a single thin section the top-to-the north shear bands are generally the moststrongly developed (Fig. 3b). Sillimanite grainsare aligned parallel to the main foliation and aredrawn into both sets of shear bands but still ap-

pear non-altered, indicating that penetrative top-to-the-north deformation occurred close to peakmetamorphic temperatures. At the base of theformation, asymmetric gently inclined folds occur.Their shape and locally sheared-o¡ short limbssuggest north-directed shearing (Fig. 3c). Internalboudins in the main foliation contain only quartzin the boudin necks, indicating greenschist faciesconditions during the deformation, a situationthat is consistent with the observed deformationmechanisms in quartz. Accordingly, the temper-ature of deformation in the Chekha formationwas signi¢cantly lower than that in the underlyingGHS.

In several places, leucogranite dykes intrude theChekha formation. Originally sub-perpendicularto the bedding, the dykes are tilted to the northand are boudinaged. At the base of the Chekhaformation the dykes are sheared and highly ro-tated (Fig. 3c). Higher up the dykes are deformedmostly brittle, and only gently tilted northwards(Fig. 3d). All the observed leucogranite bodiesthat cut the basal shear zone are deformed;thus, the crystallisation age of 17^22 Ma [15] fora leucogranite dyke from the Sakteng Klippe (Fig.2) is the maximum age of north-directed shearing.

2.3. South Tibetan Detachment

Similar structural relations to those in the klip-pen are observed in western Bhutan in the Lingshisyncline (or ‘Lingshi basin’ of Gansser [1] ; Fig. 1).There, the Chekha formation lies at the base of

Fig. 4. Lineations related to the top-to-the-north shearing at the base of the Chekha formation. (a) Sakteng klippe; (b) Uraklippe; (c) Tang Chu klippe. (d) Lingshi syncline. Black circles: stretching lineation (measured and constructed from shearbands); lozenges: mineral lineation; crosses: ¢brous minerals; large circle: mean orientation of the lineation. Lower hemisphere,equal area projections; numbers of measurements are indicated. Location of the respective studied area is indicated in Fig. 1 byan arrow showing the trend of the hanging wall movement direction.

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Palaeozoic to Mesozoic sediments of the Tethyansequence which are to the east and west in theimmediate hanging wall of the STD [9,22^25].The main foliation, in both footwall and hangingwall rocks, contains top-to-the-south shear sense

indicators and is overprinted by CP-type shearbands and folds consistently indicating top-to-the-northwest shearing. The north-directed shear-ing is distributed in the footwall for up to 3^4 km,and it a¡ects the lower part of the hanging wall

Fig. 5. (a) Cross-section of the Bhutan Himalayas along 90‡E longitude (Fig. 1). Structures at the surface after Gansser [1] andour own observations; structures at depth are based on the Project INDEPTH seismic data interpretations [30,31]. KT, Kakh-tang thrust. LS, Lingshi syncline. MBT, Main Boundary Thrust. MCT, Main Central Thrust. MHT, Main Himalayan Thrust.STD, South Tibetan Detachment. STFZ, South Tibetan Fault Zone. TCK, Tang Chu Klippe. Topographic swath pro¢le (25 kmwide) was constructed by Chris Duncan (University of Massachusetts, Boston, MA, USA) from DEM data. (b) Retro-deformedsection. The MHT was maintained at the present attitude as inferred from the INDEPTH seismic data. Dk, STD beneath theTang Chu Klippe; Dl, STD beneath the Lingshi syncline, ck and cl are the respective cut-o¡ lines between the two segments ofthe STD and the Kakhtang thrust. The cut-o¡ lines were constructed by projecting the STD along the known folds. The un-folded MCT and STD were projected downward from their respective southernmost outcrops (i.e. from their pin-lines). Theeroded crustal thickness is estimated to be approximately 10^15 km because the rocks below the MCT and above the STD atthe surface were at greenschist facies conditions when they last operated, and because the last recorded pressures of the high-grade rocks in the core of the GHS correspond to a depth of 10^15 km [15].

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rocks (e.g. [9,19,26] and this paper). Several leu-cogranite plutons intrude the shear zone (Fig. 1),and many dykes cut the base of the Chekha For-mation. All the observed dykes are folded or bou-dinaged in a style consistent with north-directedshearing. Accordingly, we interpret the contactbetween the GHS and the Chekha formation asa segment of the STD. These lithological andstructural similarities permit correlation of theSTD in the northern Himalayas with the shearzones at the base of the klippen in the heart ofthe GHS.

The top of the GHS and the base of the Che-kha formation show, therefore, alternating top-to-the-south and top-to-the-north shearing events. Inthe Annapurna range, also, south-directed thrust-ing occurred along the STD zone before and afterthe north-directed shearing [27].

2.4. Kakhtang thrust

We observed the Kakhtang thrust [1,2,4] at sev-eral locations and are thus able to extrapolate italong the whole length of the GHS in Bhutan(Fig. 1) demonstrating that this is an importantstructure. The hanging wall of this south-directedthrust is dominated by sillimanite-bearing gneissand schist and garnet^sillimanite biotite gneisslargely devoid of muscovite [1,28]. The mineralassemblages indicate that the rocks above theKakhtang thrust attained conditions above thesecond sillimanite isograd [2]. Although migma-tites are common throughout the GHS, they aremore abundant in the hanging wall of the Kakh-tang thrust. In general, the hanging wall showshigher peak temperatures than the footwall [2,4](Fig. 3e,f). The Kakhtang thrust cuts the penetra-tive fabrics associated with the south-directed

shearing within the MCT zone, and cuts acrossthe metamorphic isograds [4] ; thus, it post-datestheir formation. Because the Kakhtang thrust isstructurally above the klippen (Figs. 1 and 5a), italso post-dates the north-directed shearing at theSTD. Numerous leucogranite dykes intrude theKakhtang thrust and are deformed with it (Fig.3f). Monazite from a deformed pod of leucogran-ite has a U^Pb age of 14^15 Ma [15]. This max-imum age of shearing supports the interpretationthat the Kakhtang thrust is younger than north-directed shearing at the base of the klippen.

According to the above observations, theKakhtang thrust has brought to the surface aportion of the GHS that formed internally tothat portion now exposed above the MCT. TheKakhtang thrust is an out-of-sequence thrust rel-ative to the MCT and, as such, should sole outinto the basal detachment ^ the Main HimalayanThrust (MHT) [29]. However, we suggest that theKakhtang thrust does not sole out into the MHT,but rather merges with the STD at a depth of c.25 km (Fig. 5a). This branching of the STD atdepth can be seen on the Project INDEPTH seis-mic data [30,31]. The STD has been folded priorto activity on the Kakhtang thrust [32]. As such,the STD could not be easily reactivated duringcontinued north^south shortening, and we suggesta new fault broke out from a large syncline (Fig.6c). The hanging wall of the Kakhtang thrust thusincludes the upper structural level of the GHS(Fig. 1) and the southern Tethyan zone, togetherwith an inactive branch of the STD between them(Fig. 6d). The segment of the STD, north of thebranch line and connected to the Kakhtang thrustat depth, was likely reactivated as a south-di-rected detachment (Fig. 6d). Accordingly, thethrusting along the Kakhtang thrust might be a

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Fig. 6. Interpretative, schematic, evolutionary cross-sections of the Bhutan Himalayas along 90‡E longitude. (a) Continuousunderthrusting of the Indian crust (until Early Miocene) because of the dominant induced shear. (b) Southward £ow of the mid-crustal channel (Early and Middle Miocene). The change in velocity pro¢le occurs when the channel material becomes buoyantand less viscous because of heating [16,40]. Poiseuille £ow will become dominant and will cause £ow opposite to the movementof the subducting plate (Fig. 7b). (c) Folding of the channel. (d) Out-of-sequence thrusting along the Kakhtang thrust, and possi-ble doming and exhumation of the north Himalayan gneiss domes (Middle and Late Miocene) [16,40]. (e) Thrusting at the fron-tal parts, normal faulting at the hinterland; at depth, the hypothesised recent crustal channel oozes southward (Pliocene^recent).GHS, Greater Himalayan Sequence. GKT, Gyirong^Kangmar Thrust [33]. MBT, Main Boundary Thrust. MCT, Main CentralThrust. MHT, Main Himalayan Thrust. STD, South Tibetan Detachment. STFZ, South Tibetan Fault Zone.

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consequence of the same tectonic process thatcaused the formation of the north Himalayanantiform, and the exhumation of the north Hima-layan gneiss domes in the Middle Miocene

[29,31,33] (Fig. 6d). In this way, the Kakhtangthrust is the resurfaced normal fault detachmentof the gneiss domes. Beaumont et al. [16] proposethat various tectonic processes may cause doming

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of a crustal channel (Fig. 6c), and extension andthinning of the upper crust, therefore creatingstructures such as the north Himalayan gneissdomes located between the denudation front andthe suture.

The magnitude of the displacement along theKakhtang thrust was estimated by measuringthe displacement of cut-o¡ lines of the STDagainst the Kakhtang thrust (Fig. 5a). Consciousof unavoidable error in this reconstruction, result-ing from the irregular shape and the variable ori-entations of the folds a¡ecting the STD, we esti-mate the throw of the Kakhtang thrust to be onthe order of 10^20 km. Even this small displace-ment was su⁄cient to cause the thickening of theGHS in Bhutan, both in the map view [34] and inthe cross-section.

3. Shape of the GHS

Traditional interpretations show the GHS as anorogenic wedge £anked below by the north-dip-ping MCT, and above by the STD and the steepernorth-dipping normal faults of the South TibetanFault Zone (STFZ) [3,6,8,10,31]. In contrast, theretro-deformed section (Fig. 5b) indicates that theGHS of Bhutan had a signi¢cantly di¡erent shapeprior to the out-of-sequence Kakhtang thrust andthe upright folding. This retro-deformed section isnot a balanced cross-section because of signi¢cantvolume changes and distributed heterogeneousgeneral shear in such high-grade rocks. Moreover,the Himalayan shear zones and faults recordchanges in kinematics, show signi¢cant strike-par-allel components of displacement along them [35^37], and belong to the class of stretching faults[38].

After retro-deforming the Kakhtang thrust, wejoined into a continuous shear zone the segmentsof the STD in the footwall and in the hangingwall of the Kakhtang thrust, i.e. the STD beneaththe klippen and the STD beneath the Lingshi syn-cline (Dk and Dl respectively in Fig. 5). In thismanner, we traced the STD downdip for over 140km (Fig. 5b), from the southern edge of the klip-pen to its northernmost outcrop in Tibet. Coun-terpart estimates of the structural overlap in the

Rongbuk Valley [9,26,39] are c. 35^40 km. Thisdistance does not constrain the displacementalong the STD, as there are no matching rocksfrom its footwall to the hanging wall.

The section (Fig. 5b) shows that the GHS was ashallowly north-dipping mid-crustal layer 10^15km thick and extending for over 200 km beneathTibet. The GHS was the central part of a zone ofpervasively sheared rocks. The rocks at the bot-tom of this zone underwent continuous top-to-the-south shearing; concurrently, the rocks atthe top experienced alternating south- andnorth-directed shearing. The metamorphic ¢eldgradient in most of this zone is inverted, but itis right way up in the uppermost part. Thermal^mechanical numerical modelling by Beaumontand co-workers [16,40] shows that an invertedthermal gradient is possible in similar tectonic set-tings. The thermal inversion implies that the rocksin the zone were weaker than the rocks above andbelow. Melt-present deformation over an ex-tended time documents the relatively lowerstrength of the rocks in the GHS.

4. Orogenic channel

Several lines of evidence suggest that a low-vis-cosity middle crustal channel [41,42] presently ex-ists beneath the Tibetan plateau. The evidenceincludes the Project INDEPTH geophysical ob-servation, which indicate that the middle crustin Tibet contains £uids and may be partially mol-ten [29,30,43]. In addition, topographic analysisof the eastern plateau margin [14] suggests thatthe evolution of the large-scale morphology canbe explained by slow oozing of the low-viscositymaterial in the mid-crustal layer. The lateral pres-sure gradient caused by topography [14] governsthe £ow pattern within the assumed 15 km thickchannel. Geodynamic modelling [16,40] indicatesthat a mid- to lower-crustal low-viscosity horizonwith sub-horizontal £ow in the opposite directionto the subduction might characterise a widerrange of convergent orogens (and di¡erent stagesof their tectonic evolution). We propose that theGHS is the core of such a mid-crustal channelthat was active during Early to Middle Miocene.

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The two end-members of the velocity ¢eld in aviscous channel bounded by two rigid plates [44]are the induced shear applied at the boundaries(Fig. 7a) and the Poiseuille £ow (a.k.a. the pipe-£ow e¡ect) within it (Fig. 7a). The former has auniform vorticity across the channel. The latterhas a parabolic velocity pro¢le, with zero veloc-ities against the walls and the highest velocity inthe centre of the channel (Fig. 7a). The combinedresult has zero vorticity at the centre of the chan-nel and has opposite signs against the walls. Theshear velocity is a function of the convergencerate between two bounding plates, whereas thePoiseuille £ow is a function of the viscosity andof the pressure gradient along the channel (e.g.[14,45,46]). The resultant £ow velocity pro¢lewill be the hybrid of these two end-members[46] and will, therefore, depend on the combined£ow factors (Fig. 7b).

The viscosity of the rock in the channel is likelyto change signi¢cantly over short geological timeintervals [16,40]. Change in viscosity could be theresult of a metamorphic reaction leading, for ex-ample, to melting [2,4,15]. The metamorphismcould be a result of increase of temperaturecaused by the radioactive heating within the de-forming orogen [47,48]. A critical change of vis-

cosity within the channel will trigger changes inthe £ow pattern. Cool, high-viscosity material willbe underplated (Fig. 6a) because of the dominantinduced shear. Hot, low-viscosity and buoyantmaterial will be exhumed (Fig. 6b), because Poi-seuille £ow may become dominant [46] and cause£ow opposite to the movement of the subductingplate (Fig. 7b). Melt weakening is required to ini-tiate and sustain channel £ow [16,40], and thepresence of melt during both south- and north-

C

Fig. 7. Schematic diagram of the £ow pattern in an orogenicchannel with width h. The viscosity of channel material islower than the viscosity of rocks in hanging wall and in thefootwall (Wh sWc 6Wf ). Velocity distributions are shown rela-tive to a reference frame attached to the hanging wall. Thevorticity values (rotational component of the £ow pro¢le)are schematically indicated by the width of the black bar.Only the absolute value of the vorticity MgM is indicated re-gardless of whether it is positive (sinistral simple shear) ornegative (dextral simple shear). (a) End-members of £ow in achannel. Right, velocity pro¢le caused by shearing. Left, ve-locity pro¢le caused by pressure gradient within the channel.After [45]. (b) For a given velocity of the subducting plateand channel width there is a critical viscosity of the channelmaterial below which the Poiseuille £ow will counteract theshear forces and cause return £ow (negative velocity, U3)and therefore exhumation of that part of the channel materi-al. That part of the channel that remains dominated by theinduced shear (positive velocities, Uþ) will continue being un-derplated. Based on [46]. (c) A hypothetical velocity pro¢lein a Himalayan-type orogenic channel. Darker grey shadesdepict higher viscosities. Major structures like the MCT andthe STD are located within the channel.

EPSL 6129 26-4-02


directed shearing in the GHS shows that the high-grade metamorphism and anatexis was an integralpart of the exhumation process of the GHS. Thevariation in crustal thickness between plateau andforeland [16,40] provided the driving force tocause south-directed £ow in the channel, whereasthe coupling between surface denudation focusedon the plateau £ank and the channel £ow led tothe exhumation of the channel [16,40].

Channel £ow operates below the brittle^ductiletransition. When the rocks in the channel havecooled below the temperature for ductile £ow,the southward ductile extrusion for these rocksceased. The exhumation of the rocks now on thesurface continued (in concert with surface denu-dation) by thrusting along discrete faults: alongthe MBT, and out-of-sequence strands of theMCT (e.g. [12]).

All of the sheared and metamorphosed rocks inthe GHS, in the upper part of the LHS, and in thebase of the Tethyan sequence de¢ne the Mioceneage channel. The boundaries of the channel arefabric boundaries: the structurally lowest andhighest levels at which ductile £ow occurs (Fig.7c). Major structures like the MCT and STDare located within the channel (Fig. 7c). Thesestructures are also discontinuities in particle dis-placement paths, according to the proposal thatthe rocks beneath the MCT and above the STDhave had much shorter paths than those withinthe GHS [16]. The MCT and the STD are, there-fore, protolith boundaries within the channel sep-arating rocks of di¡erent palaeogeographic prov-enances, and di¡erent P^T^t paths.

The recognition that the GHS made up thecentral part of a ductile channel in the middlecrust, as opposed to having the shape of a wedge,is essential for interpreting the peak pressuresfound within the GHS. Conventional studies,based on the concept of extrusion of a rigid wedge(or ‘slab’), cannot provide a consistent geodynam-ic model to explain how the rocks of the GHSattained the same burial depths of 30^35 km,and to explain the subsequent decompression athigh temperature [4,15,49]. Petrological investiga-tions in the Himalayas suggest increasing meta-morphic temperatures at constant, or decreasing,pressures towards the higher structural levels [11].

To account for the lack of a signi¢cantly invertedpeak pressure gradient across the GHS, Vannayand Grasemann [7] proposed that a general £owwith considerable pure shear component, ratherthan a dominant simple shear, was responsiblefor the extrusion of the GHS. In the Bhutan Hi-malayas [3], we observe a component of shorten-ing perpendicular to the GHS (i.e. general £ow)consistent with the observations in the northwest-ern Himalayas [6,7]. General £ow is implicit in thePoiseuille £ow, and therefore in the channel £ow.An additional pure shear component might becaused by thinning of the channel as it ‘tunnels’through the mid-crustal levels [16,40].

In a horizontal channel, pressure di¡erences be-tween bottom and top will be in a range corre-sponding to the channel thickness (i.e. 2^4 kbar).These small di¡erences will likely be obscured bythe measurement errors. Lack of pressure di¡er-ences across the GHS argues in favour of a shal-low dipping to the horizontal channel. The iden-tical pressures of the rocks now on the surfaceindicate the mean crustal level of the channel dur-ing the Miocene, rather than a particular level inthe crust at the onset of the exhumation [7].Therefore, the pressure registered in the rocks isnot indicative of the path of a particle to the sur-face. Peak pressure indicates only the verticalcomponent of it ^ the exhumation; the horizontaldisplacement to reach the surface might be muchgreater. Therefore, the amount of denudation(both tectonic and surface) can vary signi¢cantlyalong the strike of the Himalayas, and even alonga north^south pro¢le. Assuming that the orogenicchannel was at a uniform crustal depth, the re-markable similarity of pressures along the Hima-layas is not unreasonable.

5. Conclusions

Klippen of metasedimentary rocks atop theGHS demonstrate the large surface extent of theSTD in Bhutan. The STD is not a consequence ofa north^south crustal extension. Instead, we arguethat the normal fault movement along the STDresults from relative £ow of the material in thefootwall. Accordingly, the structural overlap

EPSL 6129 26-4-02


along the STD is not indicative of the displace-ment along this shear zone, nor is the metamor-phic di¡erence between the footwall and hangingwall, because of the pressure^temperature condi-tions in the GHS [16].

During the Early to Middle Miocene, the GHSof Bhutan was a core of a mid-crustal, low-viscos-ity channel. The exhumation, from depths corre-sponding to 10^12 kbar to depths correspondingto 6^4 kbar at elevated temperatures [4,15], tookplace by south-directed return £ow in the channeland concomitant surface denudation. The changefrom burial to exhumation by extrusion is de-picted in the transient kinematic character of theSTD. Once the rocks in the channel have cooledbelow the temperature for the ductile £ow, thesouthward ductile extrusion for these rocksceased.

The exposed rocks in the hanging wall of theKakhtang thrust formed in a more internal partof the channel than the rocks in the footwall. Thismay explain why eclogites have been reportedonly in the upper part of the GHS [11]. Thrustingthat caused relative southward movement of theTethyan sediments along the combined STD^Kakhtang thrust could also be coupled with theformation of the north Himalayan gneiss domes.Out-of-sequence thrusting is, in addition, coevalwith normal faulting along the South TibetanFault zone (the brittle faults overprinting the duc-tile STD; Fig. 6d).

We propose that the pressures registered in therocks of the metamorphic sequence are not themeasure of the amount of denudation. As therocks may have begun their exhumation at greatlyvariable horizontal distances along the channel,they have undergone di¡erent amounts of denu-dation. Nevertheless, because rocks now on thesurface originate from a relatively thin, sub-hori-zontal channel, their peak pressures are within asmall range.

Presence of low-viscosity material in the hotcrust, as has been inferred for Tibet [29], suggeststhat southward-directed ductile £ow in the chan-nel is probably now operating at depths beneaththe Himalayas and southern Tibet, in a mannersimilar to what Clark and Royden [14] proposedfor the eastern margin of Tibet.

Acknowledgements

This paper was conceived at the Department ofGeosciences, Princeton University, while D.G.was a visiting fellow. During that time, discus-sions with Chris Andronicos, Tony Dahlen, andChris Daniel helped with the development of theideas expressed here. Reviews by Bernhard Gra-semann and Michael Edwards are greatly appre-ciated. Chris Beaumont and Peter De Celles madevery helpful suggestions on an earlier version ofthe manuscript. Discussions in the ¢eld withKeith Klepeis, Chris Daniel, and Chris Duncanare greatly acknowledged. Chris Duncan con-structed the topographic cross-sections fromDEM data. Financial support for the ¢eldworkwas provided to D.G. by the Barth and KarlKappeler funds from ETH-Zurich, and by theDeutsche Forschungsgemeinschaft (Projekt GR1662/2-1), to L.S.H. by NSF Grant EAR9406253 and for ¢eld and analytical support toR.R.P. from the Natural Environment ResearchCouncil (UK).[AC]

References

[1] A. Gansser, Geology of the Bhutan Himalaya, Birkha«userVerlag, Basel, 1983, 181 pp.

[2] S.M. Swapp, L.S. Hollister, Inverted metamorphism with-in the Tibetan slab of Bhutan: evidence for a tectonicallytransported heat-source, Can. Mineral. 29 (1991) 1019^1041.

[3] D. Grujic, M. Casey, C. Davidson, L.S. Hollister, R.Ku«ndig, T. Pavlis, S. Schmid, Ductile extrusion of theHigher Himalayan Crystalline in Bhutan: evidence fromquartz microfabrics, Tectonophysics 260 (1996) 21^43.

[4] C. Davidson, D. Grujic, L.S. Hollister, S.M. Schmid,Metamorphic reactions related to decompression and syn-kinematic intrusion of leucogranite, High HimalayanCrystallines, Bhutan, J. Metamorph. Geol. 15 (1997)593^612.

[5] O.N. Bhargava, The Bhutan Himalaya: A Geological Ac-count, Geological Survey of India, Spec. Publ. 39, 1995,245 pp.

[6] B. Grasemann, H. Fritz, J.-C. Vannay, Quantitative kine-matic £ow analysis from the Main Central Thrust Zone(NW-Himalaya, India): implications for a deceleratingstrain path and the extrusion of orogenic wedges,J. Struct. Geol. 21 (1999) 837^853.

[7] J.-C. Vannay, B. Grasemann, Himalayan inverted meta-morphism and syn-convergence extension as a conse-

EPSL 6129 26-4-02


quence of a general shear extrusion, Geol. Mag. 138(2001) 253^276.

[8] B.C. Burch¢el, L.H. Royden, North-south extension with-in the convergent Himalayan region, Geology 13 (1985)679^682.

[9] B.C. Burch¢el, Z. Chen, K.V. Hodges, Y. Liu, L.H. Roy-den, C. Deng, J. Xu, The South Tibetan DetachmentSystem, Himalayan Orogen: Extension Contemporaneouswith and parallel to Shortening in a Collisional MountainBelt, Geol. Soc. Am. Spec. Pap. 269, 1992, 41 pp.

[10] K.V. Hodges, B.C. Burch¢el, L.H. Royden, Z. Chen, Y.Liu, The metamorphic signature of contemporaneous ex-tension and shortening in the central Himalayan Orogen;data from the Nyalam transect, southern Tibet, J. Meta-morph. Geol. 11 (1993) 721^737.

[11] K.V. Hodges, Tectonics of the Himalaya and southernTibet from two perspectives, Geol. Soc. Am. Bull. 112(2000) 324^350.

[12] T. Ahmad, N. Harris, M. Bickle, H. Chapman, J. Bun-bury, C. Prince, Isotopic constraints on the structural re-lationship between the Lesser Himalayan series and theHigh Himalayan Crystalline Series, Garhwal Himalaya,Geol. Soc. Am. Bull. 112 (2000) 467^477.

[13] T.M. Harrison, M. Grove, O.M. Lovera, E.J. Catlos, J.D’Andrea, The origin of Himalayan anatexis and invertedmetamorphism: Models and constraints, J. Asian EarthSci. 17 (1999) 755^772.

[14] M.C. Clark, L.H. Royden, Topographic ooze building theeastern margin of Tibet by lower crustal £ow, Geology 28(2000) 703^706.

[15] C.G. Daniel, L.S. Hollister, R.R. Parrish, D. Grujic, Ex-trusion of the Main Central Thrust zone from lower crus-tal depths, eastern Bhutan Himalaya, J. Metamorph.Geol. (2001) in press.

[16] C. Beaumont, R.A. Jamieson, M.H. Nguyen, B. Lee, Hi-malayan tectonics explained by extrusion of a low-viscos-ity crustal channel coupled to focused surface denudation,Nature 414 (2001) 738^742.

[17] M. Brunel, Ductile thrusting in the Himalayas: shearsense criteria and stretching lineations, Tectonics 5(1986) 247^265.

[18] A.K. Jain, R.M. Manickavasagam, Inverted metamor-phism in the intracontinental ductile shear zone duringHimalayan collision tectonics, Geology 21 (1993) 407^410.

[19] J.-P. Brun, J.-P. Burg, C.G. Ming, Strain trajectoriesabove the Main Central Thrust (Himalaya) in southernTibet, Nature 313 (1985) 388^390.

[20] C. Davidson, S. Schmid, L.S. Hollister, Role of melt dur-ing deformation in the deep crust, Terra Nova 6 (1994)133^142.

[21] G. Termier, A. Gansser, Les se¤ries de¤voniennes du TangChu (Himalaya du Bhoutan), Ecl. Geol. Helv. 67 (1974)587^596.

[22] M.A. Edwards, W.S.F. Kidd, J. Li, Y. Yue, M. Clark,Multi-stage development of the southern Tibet detach-

ment system near Khula Kangri. New data from GontoLa, Tectonophysics 260 (1996) 1^19.

[23] J.-P. Burg, M. Brunel, D. Gapais, G.M. Chen, G.H. Liu,Deformation of leucogranites of the crystalline Main Cen-tral Sheet in southern Tibet (China), J. Struct. Geol. 6(1984) 535^542.

[24] C. Wu, K.D. Nelson, G. Wortman, S.D. Samson, Y. Yue,J. Li, W.S.F. Kidd, M.A. Edwards, Yadong cross struc-ture and South Tibetan detachment in the east centralHimalaya (89‡^90‡E), Tectonics 17 (1998) 28^45.

[25] M.A. Edwards, A. Pe“cher, W.S.F. Kidd, B.C. Burch¢el,L.H. Royden, Southern Tibet Detachment System atKhula Kangri, eastern Himalaya: a large-area, shallowdetachment stretching into Bhutan?, J. Geol. 107 (1999)623^631.

[26] R. Carosi, B. Lombardo, G. Molli, G. Musumeci, P.C.Pertusati, The south Tibetan detachment system in theRongbuk valley, Everest region. Deformation featuresand geological implications, J. Asian Earth Sci. 16(1998) 299^311.

[27] K.V. Hodges, R.R. Parrish, M.P. Searle, Tectonic evolu-tion of the central Annapurna Range, Nepalese Hima-layas, Tectonics 15 (1996) 1264^1291.

[28] A. Gansser, Geology of the Himalayas, Wiley-Inter-science, New York, 1964, 289 pp.

[29] K.D. Nelson, W. Zhao, L.D. Brown, J. Kuo, J. Che, X.Liu, S.L. Klemperer, Y. Makovsky, R. Meissner, J. Me-chie, R. Kind, F. Wenzel, J. Ni, J. Nabelek, L. Chen, H.Tan, W. Wie, A.G. Jones, J. Booker, M. Unsworth,W.S.F. Kidd, M. Hauck, D. Alsdorf, A. Ross, M. Cogan,C. Wu, E.A. Sandvol, M. Edwards, Partially molten mid-dle crust beneath southern Tibet; synthesis of Project IN-DEPTH results, Science 274 (1996) 1684^1688.

[30] Y. Makovsky, S.L. Klemperer, H. Liyan, L. Deyuan,Structural elements of the southern Tethyan Himalayacrust from wide-angle seismic data, Tectonics 15 (1996)997^1005.

[31] M.L. Hauck, K.D. Nelson, L.D. Brown, W. Zhao, A.R.Ross, Crustal structure of the Himalayan orogen at 90‡east longitude from Project INDEPTH deep re£ectionpro¢les, Tectonics 17 (1998) 481^500.

[32] A. McKinney, K. Klepeis, D. Grujic, L.S. Hollister,Structural and kinematic evolution of eastern Bhutan:interplay between thrust faulting and normal faulting inan orogenic wedge, in: 36th Annual Meeting of theNortheast Section of the Geological Society of Americain Burlington, VT, March 12^14, 2001, Program withAbstracts, 2001.

[33] J. Lee, B.R. Hacker, W.S. Dinklage, Y. Wang, P. Gans,A. Calvert, J.L. Wan, W. Chen, A.E. Blythe, W. McClel-land, Evolution of the Kangmar Dome, southern Tibet:Structural, petrologic, and thermochronologic constraints,Tectonics 19 (2000) 872^895.

[34] M.A. Edwards, T.M. Harrison, When did the roof col-lapse? Late Miocene north-south extension in the highHimalaya revealed by Th-Pb monazite dating of the Khu-la Kangri granite, Geology 25 (1997) 543^546.

EPSL 6129 26-4-02


[35] E.A. Stutz, A. Steck, La terminaison occidentale du Cris-tallin du Tso Morari (Haut Himalaya; Ladakh me¤ridion-al, Inde), Ecl. Geol. Helv. 79 (1986) 253^269.

[36] A. Pe“cher, J.-L. Bouchez, P. Le Fort, Miocene dextralshearing between Himalaya and Tibet, Geology 19(1991) 683^685.

[37] M.E. Coleman, Orogen-parallel and orogen-perpendicularextension in the central Nepalese Himalayas, Geol. Soc.Am. Bull. 108 (1996) 1594^1607.

[38] W.D. Means, Stretching faults, Geology 17 (1989) 893^896.

[39] M.P. Searle, Extensional and compressional faults in theEverest-Lhotse massif, Khumbu Himalaya, Nepal, J. Geol.Soc. London 156 (1999) 227^240.

[40] C. Beaumont, R.A. Jamieson, M.H. Nguyen, B. Lee,Mid-crustal channel £ow in large hot orogens: resultsfrom coupled thermal-mechanical models, in: F. Cook,P. Erdmer (Eds.), Slave-Northern Cordillera LithosphericEvolution (SNORCLE) and Cordilleran Tectonics Work-shop Transect Meeting, Lithoprobe Report 79, 2001,pp. 112^170.

[41] W.L. Zhao, W.J.P. Morgan, Injection of Indian crust intoTibetan lower crust; a two-dimensional ¢nite elementmodel study, Tectonics 6 (1987) 489^504.

[42] P. Bird, Lateral extrusion of lower crust from under hightopography, in the isostatic limit, J. Geophys. Res. 96(1991) 10275^10286.

[43] L. Chen, J.R. Booker, A.G. Jones, N. Wu, M.J. Uns-worth, W. Wie, H. Tan, Electrically conductive crust insouthern Tibet from INDEPTH magnetotelluric survey-ing, Science 274 (1996) 1694^1696.

[44] D.L. Turcotte, G. Schubert, Geodynamics. Application of

Continuum physics to Geological Problems, John Wileyand Sons, Chichester, 1982, 450 pp.

[45] P.C. England, T.J.B. Holland, Archimedes and theTauern eclogites: the role of buoyancy in the preservationof exotic eclogite blocks, Earth Planet. Sci. Lett. 44 (1979)287^294.

[46] N.S. Mancktelow, Nonlithostatic pressure during sedi-ment subduction and the development and exhumationof high pressure metamorphic rocks, J. Geophys. Res.100 (1995) 571^583.

[47] R.A. Jamieson, C. Beaumont, P. Fullsack, B. Lee, Barro-vian regional metamorphism: where’s the heat?, in: P.J.Treloar, P.J. O’Brien (Eds.), What Drives Metamorphismand Metamorphic Reactions?, Geol. Soc. London Spec.Publ. 138 (1998) 23^51.

[48] A.D. Huerta, L.H. Royden, K.V. Hodges, The e¡ects ofaccretion, erosion and radiogenic heat on the metamor-phic evolution of collisional orogens, J. Metamorph.Geol. 17 (1999) 349^366.

[49] J. Ganguly, S. Dasgupta, W. Cheng, S. Neogi, Exhuma-tion history of a section of the Sikkim Himalayas, India;records in the metamorphic mineral equilibria and com-positional zoning of garnet, Earth Planet. Sci. Lett. 183(2000) 471^486.

[50] D. Alsdorf, Y. Makovsky, W. Zhao, L. Brown, K.D.Nelson, S. Klemperer, M. Hauck, A. Ross, M. Cogan,M. Clark, J. Che, J. Kuo, INDEPTH (InternationalDeep Pro¢ling of Tibet and the Himalaya) multichannelseismic re£ection data: Description and availability,J. Geophys. Res. 103 (1998) 26993^26999.

[51] C.W. Passchier, Flanking structures, J. Struct. Geol. 23(2001) 951^962.

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Himalayan metamorphic sequence as an orogenic channel: insight from Bhutan