Annapurna detachment fault in the Greater Himalaya of central Nepal

Geological Society, London, Special Publications

doi: 10.1144/GSL.SP.1993.074.01.31p461-473.

1993, v.74;Geological Society, London, Special Publications Richard L. Brown and Jeffrey H. Nazarchuk of central NepalAnnapurna detachment fault in the Greater Himalaya

serviceEmail alerting

new articles cite this article to receive free e-mail alerts whenhereclick

requestPermission

part of this article to seek permission to re-use all orhereclick

Subscribe

Collection London, Special Publications or the Lyell

to subscribe to Geological Society,hereclick

Notes

2014© The Geological Society of London

by guest on May 7, 2014http://sp.lyellcollection.org/Downloaded from by guest on May 7, 2014http://sp.lyellcollection.org/Downloaded from

http://www.lyellcollection.org/cgi/alerts

http://www.geolsoc.org.uk/permissions

http://www.lyellcollection.org/site/subscriptions

http://sp.lyellcollection.org/


Annapurna detachment fault in the Greater Himalaya of central Nepal

R I C H A R D L. B R O W N & J E F F R E Y H. N A Z A R C H U K

Department o f Earth Sciences, Carleton University, and Ottawa-Carleton Geoscience

Centre, Ottawa, ON, Canada K1S 5B6

Abstract: Displacement on the Annapurna detachment fault (ADF) has exhumed the Greater Himalayan metamorphic sequence in central Nepal. The fault is most likely a continuation of the South Tibetan detachment system recognized elsewhere in the Himalayan orogen.

The characteristics of detachment faulting and the structural and metamorphic histories of the hanging wall and footwall in the Thakkhola region are described. Two stages of normal-sense shearing on the ADF are recognized: the first stage occurred at mid-crustal depth; the most recent detachment faulting on the system was brittle and may have been accompanied by orogen-parallel extension.

First-phase isoclinal folds in the Tibetan sedimentary sequence above the ADF reflect a southwesterly sense of shear and are refolded by spectacular northeasterly verging second-phase folds that have affected a crustal thickness of over 6 km. Third-phase southwesterly directed thrust faults and folds have disrupted the earlier structures but predate the most recent period of normal-sense shearing on the ADF.

The kinematic significance of the northeasterly verging second-phase folds is not clear; these structures may have formed during early motion on the ADF but, alternatively, may reflect older events associated with compression and crustal thickening of the orogen.

Crystalline rocks of the Greater Himalayan metamorphic sequence (known also as the Main Central Thrust Sheet or the Tibetan Slab) have been thrust southwestward along the Main Cen- tral Thrust over the lower grade rocks of the Lesser Himalayan sedimentary sequence. Within the Greater Himalayan metamorphic sequence, metamorphic grade increases structurally up-section giving an inversion of isograds (sillimanite above kyanite) in the middle to uppermost structural levels (Swapp & Hollister 1991 and references therein). This metamorphic inversion may be only an apparent one that results from a moderate-pressure, high- temperature Buchan event superimposed on an earlier high-pressure, high-temperature Barro- vian event, and not a continuous increase in metamorphic grade (Brunel & Kienast 1986; Hodges & Silverberg 1988; Hodges et al. 1988 and references therein; Swapp & Hollister 1991).

The top of the Greater Himalayan metamorphic sequence is bounded in southern Tibet by a northeasterly dipping normal-sense shear zone (Burg et al. 1984). It has recently been demonstrated that this shear zone is part of the regional South Tibetan detachment system (Burchfiel et al. 1992).

Northeasterly displacement on the South Tibetan detachment system and exhumation of the Greater Himalayan metamorphic sequence

have been linked to emplacement of Miocene leucogranites in the upper part of the sequence (Hodges et al. 1988; Hubbard & Harrison 1989). Although the age of displacement on the Main Central Thrust is not well constrained along much of its length, in eastern Nepal thrusting appears to have been coeval with leucogranite emplacement (Hubbard & Harrison 1989); thus, it appears that the Main Central Thrust Sheet was extruded southwestward relative to the overlying and underlying strata and was exhumed shortly after generation of the leucogranites. This hypothesis is interpreted by Burchfiel & Royden (1985) in terms of gravitational collapse of the overthickened orogen (see also Dalmayrac & Molnar 1981; England & McKenzie 1982, 1983; Houseman et aI. 1981). Northeasterly directed thrust faults and crustal- scale folds in the hanging wall of the South Tibetan detachment system have also been attributed by some authors to gravitational sliding or collapse associated with northeastward displacement on the detachment (Caby et al. 1983; P6cher 1991; Burchfiel et al. 1992).

In this paper we document the presence of a detachment fault, the Annapurna detachment fault, in the Thakkhola region of central Nepal (Figs 1 and 2), which is presumed to be correla- tive with the South Tibetan detachment system in southern Tibet. Structures in the hanging wall

From TRELOAR, P.J. & SEARLE, M.P. (eds), Himalayan Tectonics Geological Society Special Publication, No. 74, pp. 461-473.

461

by guest on May 7, 2014http://sp.lyellcollection.org/Downloaded from


462 R.L. BROWN & J.H. NAZARCHUK

_=:- -_ 2 : : : : : ' -

8 4 o E

2 9 o N

TIBET

NEPAL -~

m TIBETAN SEDIMENTARY SEQUENCE

PLIO-PLEISTOCENE GRABEN SEDIMENTS

TETHYAN SEDIMENTS

GREATER HIMALAYAN METAMORPHIC SEQUENCE MIOCENE GRANITIC ROCKS } TIBEq-AN BASEMENT GNEISSES SLAB

LESSER HIMALAYAN SEDIMENTARY SEQUENCE

. . . . ~ - -~:---

• . O o O o : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : ~ : -

~ ~ . _ ~ - - ~ : j - : - - : - : - : - : - : - : - : - : - : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : :

10 km i i

...... 85oF_.

2B °N

Fig. 1. Geological map of central Nepal simplified and modified after Colchen et al. (1986): MCT, Main Central Thrust; ADF, Annapurna detachment fault; TNF, Thakkhola normal fault; box locates Fig. 3; A-B locates section line of Fig. 2.

A B Annapurna I S S W

N N E "fi l icho 8091 Annapurna South 7134 Nilgiri Fang 7219

T h o r ~ g , ~ e N. Tho rungse S. P u c h e n " r a I 7223 I 7647 ,

• ~ c " ~ ~ x ~> ~ l ~ o S l ; n g ~ ' ' i l ~

JOMOSOM ~ ...... 10 km 0 J I I

QUATERNARY GRABEN ~ TETHYAN SEDIMENTS SEDIMENTS

[ ] GREATER HIMALAYAN METAMORPHIC SEQUENCE

LESSER HIMALAYAN SEDIMENTARY SEQUENCE

Fig. 2. Cross section simplified and modified after Colchen et al. (1986); MCT, Main Central Thrust; ADF, Annapurna detachment fault (located as by Caby et al. 1983).

of the detachment appear to be of both extensional and compressional origins. These structures are described and their kinematic significance is discussed. The timing of orogen- parallel extension in the Thakkhola region and its possible correlation with the most recent period of displacement on the Annapurna detachment fault are considered.

Fie ld relat ions

The Greater Himalayan metamorphic

sequence (Main Central Thrust SheeO

Where the Greater Himalayan metamorphic sequence crosses the Kali Gandaki River in central Nepal the thrust sheet is only about 4 km



ANNAPURNA DETACHMENT FAULT, CENTRAL NEPAL 463

Fig. 3. Location of current study: shaded dashed line follows route of traverses; see Fig. 1 for legend; A-B locates section line of Fig. 2; MCT, Main Central Thrust; ADF, Annapurna detachment fault; TNF, Thakkhola normal fault.

thick. Its base is exposed to the east of Dana on the trail to Narcheng and along the Miristi Khola (Fig. 3), where the boundary is poorly defined by the last appearance of kyanite-bearing pelitic and quartz-feldspar gneisses, which are underlain by phyllite, minor quartzite, and calcareous schist at sub-amphibolite grade. There is clearly post-metamorphic strain in the low-grade rocks of the footwall, but the kyanite-bearing gneisses are annealed and show no sign of retrogression. At this locality it appears that the Main Central Thrust can be located at the boundary between the kyanite-bearing gneisses and the underlying lower grade schists; the most recent motion along this boundary is younger than the metamorphic culmination observed in the hanging- wall gneisses.

A detailed discussion of the fabrics in the vicinity of the Main Central Thrust is beyond the scope of this paper, but we should comment here

that kinematic indicators are evident despite considerable post-tectonic recrystallization of the gneisses; not surprisingly, they record a southwesterly direction of shearing of the Greater Himalayan metamorphic sequence relative to the underlying strata of the Lesser Himalayan sedimentary sequence.

The central part of the Greater Himalayan metamorphic sequence is not well exposed in the Kali Gandaki Valley but may be studied in detail by following the mountain trail east of Narcheng along the Ghaleti Khola (Fig. 3). Colchen et al. (1986) describe the Greater Himalayan metamorphic sequence (referred to by them as the 'Tibetan Slab') in this region as a sequence with quartz-feldspar gneisses in the lower part and calc-silicate rocks predominating in the upper part. They did not recognize any major folding or faulting within the sequence. We have found quartz-feldspar and pelitic gneisses both below




NNE j F1 SSW

10

w

, ? . : ~ ~ 5 5 5 5 2 ~ 5 f . - ~ _ : ~ _ ~ - i ~ i ~ - i ~ - ~ : - i : ~-~ " - ......................... ~ : : i : :~ i i ! i i~ :

TETHYAN [ ~ MIOCENE ~ QUARTZ-FELDSPAR I ~ CALC-SILICATE ~ LESSER HIMALAYAN SEDIMENTS GRANITIC ROCKS & PELMC GNEISSES GNEISSES SEDIMENTARY SEQUENCE

Fig. 4. Interpretation of part of the cross section shown in Fig. 2: MCT, Main Central Thrust; ADF, Annapurna detachment fault; F1, axial surface trace of first-phase southwesterly verging anticline; F2, axial surface trace of second-phase northeasterly verging antiform; solid arrows point in direction of stratigraphic tops.

and above the calc-silicate rocks; sparse minor structural features suggest that the calc-silicate rocks are cored by a second-phase antiform (Fig. 4). Fabric relationships in the hinges of minor folds suggest that kyanite-bearing assemblages predate development of the antiform.

Leucogranite sheets are present toward the top of the Greater Himalayan metamorphic sequence in the upper reaches of the Ghaleti Khola (Fig. 3) and are also exposed on the eastern slopes of the Kali Gandaki Valley in the vicinity of the village of Kalopani (Fig. 3). Migmatitic gneisses adjacent to the leucogranite sheets are kyanite-bearing, and there is no indication of replacement by sillimanite.

In the central portion of the Greater Hima- layan metamorphic sequence, hinge lines of minor folds are generally shallow-plunging, and there is no obvious stretching lineation. Leuco- granite sheets within the sequence are foliated but only become clearly lineated near the top of the sequence. The leucogranite sheets cut across earlier fabrics in the adjacent migmatitic gneisses.

Annapurna detachment fault

The detachment fault is well exposed in a relatively accessible stream gully west of the village of Dhumpu in the Thakkhola region of the Kali Gandaki River (Fig. 3). We have also crossed the detachment north of Hinko Cave along the Modi Khola (Fig. 3) and at Chame in the Marsyandi valley (Fig. 1), but time con-

straints and difficulty of access precluded detailed observations in these areas.

The following relationships (see Fig. 5) are clearly displayed in the stream gully on the west side of the Kali Gandaki River west of Dhumpu.

The fault trace is exposed within the stream bed with footwall rocks of the Greater Hima- layan metamorphic sequence occurring in the cliffs of the south wall of the gully and hanging- wall rocks of the Tibetan sedimentary sequence (Tethyan assemblage, see Gradstein et al. 1992) lying to the north of the stream (Fig. 6).

The footwall is composed of migmatitic quartz-feldspar gneiss and calc-silicate intruded by sheets of two-mica leucogranite. Foliation within the gneisses dips moderately (25-45 °) northeast generally parallel to foliation within the leucogranites. Stretching lineations are not well developed, but where observed, the lineation is down the dip of the dominant foliation.

Kinematic indicators are evident in the leucogranite sheets but are not as well displayed within the gneisses. These indicators include S-C fabrics, shear bands, rotated and non- rotated porphyroclasts with tails, asymmetric folds with sub-horizontal hinge lines, and pegmatite dyke arrays that have been extended or shortened according to their orientation with respect to the shear foliation. All these features consistently indicate that the upper member of the shear couple has been displaced to the northeast relative to the lower member (Fig. 5). These shear fabrics have been observed in the footwall from approximately 100 m below to within 5 m of the contact.




"~o, TIBETAN N SEDIMENTARY

~ , ~ ~ ~ SEQUENCE

\

~ , ' , \ f e ............

GREATER ...... ~ \ , H MALAYAN .

METAMORPHIC ~" SEQUENCE ............

Fig. 5. Schematic diagram (not to scale) of view looking west in the Dhumpu region west of the Kali Gandaki River: ADF, Annapurna detachment fault; arrows indicate sense of shear within footwall rocks; lightly shaded bands in footwall illustrate sheared leucogranite; two orientations of lines within leucogranites illustrate S-C fabrics; magnifying glass illustrates pressure shadows on rotated and unrotated porphyroclasts; SO, orientation of bedding; S1, $2, and $3, first-, second-, and third-phase cleavages; F2 and F3, second- and third-phase folds; darkly shaded lenses above ADF illustrate boudinaged quartz-feldspar dykes. See text for explanation, Fig. 6 for view of ADF, and Figs 7-13 for illustration of structural features in hanging wall.

Fig. 6. View to the west of the Annapurna detachment fault south of Larjung, near Dhumpu along the Kali Gandaki River. Dotted line shows the contact between kyanite-grade migmatite and leucogranite of the Greater Himalayan metamorphic sequence in the footwail and biotite-grade psammitic rocks and micaceous schist of the Tibetan sedimentary sequence in the hanging wall. See Fig. 5 for details of contact relationships.




Across this width of the shear zone there is no sign of retrogression of the peak mineral assemblages. Within pelitic layers of the migmatite, the assemblages include kyanite, garnet, biotite, muscovite, plagioclase and quartz. The 5-10 m thick contact zone is sheared and brecciated; chloritic shear zones occur in pelitic lenses and crushed pegmatite.

Above the brittle zone is an abrupt contact with flaggy fine-grained psammitic rocks, micaceous schists, and phyllites, which grade up- wards into fine sandstones and calcareous schists, which have been correlated with the Annapurna Formation by Colchen et al. (1986). The hanging-wall rocks have been metamor- phosed to biotite grade. Fabrics within these hanging-wall rocks do not penetrate the footwall, and they clearly predate the most recent normal-sense motion on the detachment.

Structures in the hanging wall o f the

Annapurna detachment fault

Three phases of deformation are readily identi- fied in the strata of the hanging wall.

Cleavage parallel to bedding (S 1) is generally present and is defined primarily by preferred orientation or flattening of detrital grains; locally, minor isoclinal folds (F1) have been

observed with this cleavage cutting their hinges (Fig. 7). This phase has been recognized by earlier workers (see Le Fort 1975 and references therein) and is illustrated on a crustal scale in cross sections through the Annapurna massif by Colchen et al. (1986; see Fig. 2). A major isoclinal nappe is shown closing in the summit of Fang (Varashikhar) (Fig. 2), which according to stratigraphical superposition originated as a south-southwesterly verging anticline (Fig. 4). This is clear since the sequence is right way up on the upper northeast flank of the F1 closure. (Note that shearing on the Annapurna detachment fault has cut out the complementary F1 syncline.)

A second cleavage ($2) oblique to bedding is generally the most obvious planar fabric in the hanging-wall rocks adjacent to the Annapurna detachment fault (Fig. 5). This cleavage is defined by the preferred orientation of micas in pelitic strata and is a spaced cleavage in the more sandy layers (Fig. 8). Folds with this cleavage parallel to their axial surfaces occur as megascopic tight to open folds in the metasediments directly overlying the detachment and continue up through the Tibetan sedimentary sequence as parasitic structures to major northeasterly verging folds (Figs 9 and 10). A spectacular example of these F2 folds may be viewed in the west face of the Nilgiri peaks (Colchen et al. 1986; Figs 2

Fig. 7. Photograph shows an F1 isocline outlined by original layering (SO). At this location (south of Larjung), SO, S1, and $2 are all sub-parallel.




Fig. 8. Photo shows decimetre-scale sandstone and pelite with a well developed spaced axial-plane cleavage in the core of a northeasterly verging F2 fold (in the vicinity of Marpha).

Fig. 9. Looking east from Marpha at a northeasterly verging F2 anticline.

and 4). At the Dhumpu locality it is clear that $2 is truncated by the brittle displacement of the Annapurna detachment fault (Fig. 5); its continuation eastward into the slopes of the Nilgiri and Annapurna peaks implies that the major northeasterly verging nappe would also be truncated as shown in Figs 2 and 4.

A third phase of deformation has folded and sheared the pre-existing structures (Figs 11 and 12). The third-phase microscopic to mesoscopic folds consistently verge to the southwest and appear to be coeval with southwesterly directed thrust faults that locally disrupt the Tibetan sedimentary sequence in the Thakkhola region.



468 R.L. B R O W N & J.H. N A Z A R C H U K

Fig. 10. View to the east of a megascopic F2 syncline (Jomosom and the Kali Gandaki River in the foreground). Photograph shows a lower right-way-up long limb and an upper overturned short limb on the northeasterly verging Nilgiri structure.

Fig. 11. F3 folds with a shallow northeasterly dipping axial plane (south of Larjung).



A N N A P U R N A D E T A C H M E N T FAULT, CENTRAL NEPAL 469

Fig. 12. Looking east at an overturned limb on a northeasterly verging F2 fold along the east bank of the Kali Gandaki River across from Marpha. Photograph shows F1 isoclinal hooks (SO + S1) sub-parallel to $2 and subsequent refolding by southwesterly verging F3 folds. This photograph is a close-up of the overturned limb on the fold in Fig. 9.

Fig. 13. View to the west of a boudinaged, tourmaline-rich, quartz-plagioclase pegmatite dyke in biotite-grade psammitic rocks above the Annapurna detachment fault. The extended dyke dips more steeply than the prominent $2 cleavage and is sub-parallel to the axial planes of F3 folds; this indicates southwestward thrusting.




Southwesterly verging third-phase folds observed at the Dhumpu locality are illustrated in Fig. 5.

A number of northeasterly dipping quartz- feldspar pegmatite dykes occur within a few metres of the Annapurna detachment fault and higher in the Tibetan sedimentary sequence. These pegmatite dykes are sub-parallel to the axial surfaces of F3 minor folds, dip more steeply than $2, and are boudinaged or otherwise extended (Figs 5 and 13). Above the Annapurna detachment fault, high angles are observed between layering (SO + S1), $2, and weakly boudinaged dykes. Towards the crush zone of the Annapurna detachment fault these fabric elements and dykes are transposed into nearly parallel orientations, and the dykes are progressively more boudinaged and extended (Fig. 5). The importance of these observations lies in the recognition that both the F3 folds and the dykes are younger than the second phase of deformation and older than the brittle extensional shear fabric in the hanging wall of the Annapurna detachment fault.

The youngest sense of shear observed in the footwall rocks below the brittle zone indicates a northeast normal sense of shearing. The most recent fabric in the hanging wall above the brittle zone indicates that a period of southwesterly directed shearing affected these rocks prior to the most recent period of normal faulting on the Annapurna detachment fault.

Thakkho la graben

The youngest structures recognized in the hanging-wall rocks are sub-vertical northeast- trending fractures and normal faults. These features are undoubtedly contemporaneous with formation of the Thakkhola graben, which is topographically expressed by the broad valley of the upper reaches of the Kali Gandaki River (Bordet et al. 1971; Colchen et al. 1986; Figs 1 and 3). The Thakkhola graben is bounded on its western side by the Thakkhola normal fault (TNF on Figs 1, 3), which in the Mustang area north of Jomosom (Fig. 1) is estimated to have a down-to-the-east vertical component of displacement of approximately 3000 m (Bordet et al. 1971). The eastern margin is less well defined and consists of a series of en echelon faults with small displacement, suggesting a half-graben geometry. Displacement on the fault system decreases southward, and northeast-trending faults that have been mapped south of the Annapurna detachment fault within the Greater Himalayan metamorphic sequence display only minor offsets (Colchen et al. 1986).

In the upper reaches of the Kali Gandaki River (Fig. 1), Pliocene-Quaternary graben-fill clastic rocks rest unconformably on the down- faulted Mesozoic strata; the trace of the Thakkhola normal fault lies along the western edge of the graben where upturned Quaternary sediments lie in its footwall and Silurian- Devonian strata form the hanging wall. Post- glacial sediments do not appear to have been disrupted by the normal faulting (Colchen et al. 1986).

Interpretation

The A n n a p u r n a detachment faul t

Structural and metamorphic relationships described above demonstrate the existence of a major normal fault at the top of the Greater Himalayan metamorphic sequence in the Thakkhola region. The presence of kyanite- bearing migmatites in the footwall within tens of metres of sub-amphibolite-grade metasediments suggests a vertical component of displacement of at least 10km. Since observed stretching lineations are down dip, there appears to be no indication of a strike-slip component (although see P6cher (1991) for discussion of possibly regional strike-slip motions).

Generally, in single-stage normal-sense shearing and exhumation, hot middle or lower crustal rocks of the footwall are progressively brought to higher structural levels and are juxtaposed with cooler upper crustal rocks of the hanging wall. Thus the resulting shear zone may exhibit a ductile mylonitic footwall and a brittle hanging wall. In the contact zone the mylonitic rocks are generally retrograded and brecciated (see Coney 1980).

The absence of a prominent mylonite zone on the footwall of the Annapurna detachment fault requires explanation. The retrograded and brecciated zone is underlain by foliated migmatite and leucogranite with no evidence of extensive mylonitization. However, in outcrop and thin section there is ample evidence of ductile extensional shearing followed by annealing recrystallization. This annealing occurred while the rocks were in the kyanite field and after generation of the leucogranites. These observations are readily explained if a two-stage model of extensional shearing is envisaged. In the initial stage the footwall rocks were in the ductile field at middle crustal depth; extension occurred during generation of leucogranitic melts; this was followed by a period of static annealing. A second stage of extension after




cooling of the footwall gave rise to the brittle fabrics.

With the exception of high-angle faults associated with the Thakkhola graben, fabrics of the hanging wall are older than the most recent normal displacement on the Annapurna detachment fault. The third-phase folds were formed during southwesterly directed shearing and generation of a suite of granitic dykes. Attempts at U-Pb dating of these dykes have been un- successful, but it is clear that a period of compressional strain occurred after the generation of northeasterly verging phase-two structures and before the most recent extensional shearing on the Annapurna detachment fault.

The northeasterly verging folds

The northeasterly verging folds (F2) in the hanging wall of the Annapurna detachment fault predate southwesterly verging folds (F3), emplacement of a suite of granitic dykes, and the most recent normal displacement on the detachment. The second-phase structures, which include the northeasterly verging Nilgiri nappe, have a well developed axial-plane cleavage that is in part defined by the preferred orientation of metamorphic minerals such as biotite, indicating that the folds developed during metamorphism of the hanging-wall strata. This metamorphic event is undated but must be older than the compressional F3 events, which are post- metamorphic.

It has recently been proposed that structures similar to the Nilgiri nappe elsewhere in the Himalayan orogen are a result of crustal-scale normal-sense shearing related to the South Tibetan detachment system (Burchfiel et al. 1992). If this interpretation were accepted for the Thakkhola region, a two-stage model of extension with an intervening period of compression (F3) would be required. It is indeed tempting to suggest that the Nilgiri nappe and associated structures formed during the early ductile phase of extension on the Annapurna detachment fault when both hanging wall and footwall were at middle crustal depth. However, there is no direct evidence in support of an extensional origin for these northeasterly directed structures, and alternatively, they could have been generated during an early period of compression and associated crustal thickening. The resolution of this ambiguity must await additional data; particularly required is a better understanding of the regional significance of vergence reversals elsewhere in the orogen.

Significance of F3 deformation in the Tibetan sedimentary sequence

The southwesterly verging F3 structures and pegmatite dykes in the Tibetan sedimentary sequence clearly overprint and are younger than the F2 Nilgiri structure and associated $2 cleavage. All have been cut by the most recent brittle motion on the Annapurna detachment fault.

If ultimately it is shown that the Nilgiri structure is not genetically related to top-to-the- northeast normal sense of shear between the Greater Himalayan metamorphic sequence and the Tibetan sedimentary sequence (Burchfiel et al. 1992), then possibly all three phases of folding predate formation of the Annapurna detachment fault.

If the Nilgiri nappe is considered the result of early crustal thickening and as a structurally higher metamorphic equivalent to the Barrovian event in the Greater Himalayan metamorphic sequence, then the F3 compressional structures and suite of granitic dykes in the Tibetan sedimentary sequence may be documenting a post- Barrovian, southwestward thrusting event that occurred before ductile extensional movement on the Annapurna detachment fault.

Elsewhere in the Himalaya, field relations as well as pressure-temperature paths from garnet- bearing pelitic rocks in the Greater Himalayan metamorphic sequence suggest that the Buchan metamorphism overprinting the earlier Barro- vian event was associated with the generation of leucogranites and a tectonic reburial of 5-7 km (Hodges & Silverberg 1988; Hodges et al. 1988). This reburial was found to be widespread, but the lack of appropriate structures from detailed mapping in the Tibetan sedimentary sequence hindered explanation. It is suggested here that the F3 compressional structures and suite of granitic dykes in the hanging wall of the Anna- purna detachment fault may be documenting the southwestward thrusting event that caused the post-Barrovian tectonic reburial observed in pressure-temperature paths and are possibly related to the Buchan metamorphism and leucogranite generation.

Thakkhola graben

The northeast-trending extensional structure is well developed in the hanging wall of the Annapurna detachment fault but cannot be traced into the footwall. This geometry suggests a kinematic link between the graben and detachment faulting. Could it be that orogen-parallel extension in the Tibetan sedimentary sequence occurred at the same time as northeastward




displacement of the assemblage on the Anna- purna detachment fault? This suggestion lacks proof at the moment , but if correct, such three- dimensional extension might account for the weak development of stretching lineations in the shear zone of the Annapurna detachment fault and in the Tibetan sedimentary sequence of the hanging wall. Since evidence of orogen-parallel extension has not been recorded in the Greater Himalayan metamorphic sequence, it appears that in this part of the Himalaya the extension has been restricted to upper crustal levels.

Conclus ions

The Annapurna detachment fault in the Thakkhola region of Nepal is a discrete northeasterly dipping normal-sense shear zone; it is correlated with the South Tibetan detachment system.

Displacement across the Annapurna detachment fault has juxtaposed mid-crustal rocks of the Greater Himalayan metamorphic sequence in the footwall with upper crustal rocks of the Tibetan sedimentary sequence in the hanging wall.

Early motion on the Annapurna detachment fault was ductile and occurred at mid-crustal levels. Later motion developed a zone of brittle strain that is superimposed on the earlier ductile fabrics.

In the hanging wall of the Annapurna detachment fault, three phases of deformation are recognized: a first phase of mesoscopic to megascopic southwesterly verging isoclines with an associated layer-parallel cleavage, a second phase of northeasterly verging tight to open megascopic folds with an axial-plane cleavage, and a third phase of post-metamorphic southwesterly verging microscopic to mesoscopic folds. The third-phase compressional structures are associated with a suite of granitic dykes, and both are younger than and superimposed on the first- and second-phase structures.

The kinematic significance of the northeasterly verging phase-two Nilgiri nappe and associated structures is not clear; these structures may have formed during the early stage of extension on the Annapurna detachment fault, but alternatively, they may reflect a period of Himalayan crustal thickening.

The most recent period of motion on the Annapurna detachment fault may have been contemporaneous with formation of the Thakkhola graben. The graben is an orogen- parallel structure in the Tibetan sedimentary sequence that was active as recently as the Quaternary.

This research was initially carried out as part of the 1991 international Lost Ocean Expedition, under the leadership of F. Gradstein. R.L.B. returned for a second field season in 1992. We would like to thank the Sherpa Society for their able logistical support. The final draft of this paper has benefitted from construc- tive reviews by J.P. Burg, L.H. Royden, and P.J. Treloar. We thank L. Hardy for assistance in manu- script preparation and production of figures. This research has been funded by Research Grant 2693 from the National Sciences and Engineering Research Council of Canada to R.L.B.

References

BORDET, P., COLCHEN, M., KRUMMENACHER, D., LE FORT, P., MOUTERDE, R. & REMY, M. 1971. Recherches gdologiques dans L'Himalaya du Ndpal, rdgion de la Thakkhola. Editions du Cen- tre National de la Recherche Scientifique, Paris, France.

BRUNEL, M. • KIENAST, J.-R. 1986. Etude petro- structurale des chevauchements ductiles hima- layens sur la transversale de l'Everest-Makulu (Nepal oriental). Canadian Journal of Earth Sci- ences, 23, 1117-1137.

BURCHEIEL, B.C. & ROYDEN, L.H. 1985. North-south extension within the convergent Himalayan region. Geology, 13,679-682. , CHEN, Z., HODGES, K.V., LIu, Y., ROYDEN, L.H., DENG, C. & Xu, J. 1992. The south Tibetan detachment system, Himalayan orogen: extension comtemporaneous with and parallel to shortening in a collisional mountain belt. Geological Society of America, Special Papers, 269.

BURG, J.P., BRUNEL, M., GAPAIS, D., CHEN, G.M. & Lit;, G.H. 1984. Deformation of leucogranites of the crystalline Main Central Sheet in southern Tibet (China). Journal of Structural Geology, 6, 535-542.

CABY, R., P~CHER, A. & LE FORT, P. 1983. Le grand chevauchement central himalayen: nouvelles donn6es sur ie m6tamorphisme inverse h la base de ia Dalle du Tibet. Revue de gdologie dynami- queet de gdographie physique, 24, 89-100.

COLCHEN, M., LE FORT, P. & PECHER, A. 1986. Geological researches in the Nepal's Himalaya: Annapurna-Manaslu-Ganesh Himal. Editions du Centre National de la Recherche Scientifique, Paris, France.

CONEY, P. 1980. Cordilleran metamorphic core complexes: an overview. In: CRITrENDEN, M.D. (ed.) Cordilleran Metamorphic Core Complexes. Geo- logical Society of America, Memoirs, 153, 7-31.

DALMAYRAC, B. & MOLNAR, P. 1981. Parallel thrust and normal faulting in Peru and constraints on the state of stress. Earth and Planetary Science Letters, 55,473-481.

ENGLAND, P.C. & MCKENZIE, D. 1982. A thin viscous sheet model for continuous deformation. Royal Astronomical Society Geophysical Journal, 70, 295-321.

- - & - - 1983. Correction to 'A thin viscous sheet model for continuous deformation'. RoyalAstro-




nomical Society Geophysical Journal, 73, 523- 532.

GRADSrEIN, F.M., VON RAD, U., GmLING, M.R., JANSA, L.F., KAMtNSKI, M.A., KR1STrANSEN, I-L., OGG, J.G., ROHL, U., SARa'I, M., THUROW, J.W., WESXERMANN, G.E.G. & WIEDMANN, J. 1992. The Mesozoic continental margin of central Nepal. Geologisches Jahrbuch, Reihe B, Heft 77.

HODGES, K.V. & S~LVERBERG, D.S. 1988. Thermal evolution of the Greater Himalaya, Garhwal, India. Tectonics, 7,583-600.

- - , HUBBARD, M.S. & SILVERBERG, D.S. 1988. Metamorphic constraints on the thermal evolution of the central Himalayan Orogen. Philo- sophical Transactions of the Royal Society of London, A326, 257-280.

HOUSEMAN, G.A., MCKENZIE, D.P. & MOLNAR, P. 1981. Convective instability of a thickened boundary layer and its relevance for the thermal

evolution of continental convergent belts. Journal of Geophysical Research, 86, 6115-6132.

HVBBARD, M.S. & HARRISON, M.T. 1989. 4°Ar/39Ar age constraints on deformation and metamorphism in the Main Central Thrust zone and Tibetan Slab, eastern Nepal Himalaya. Tectonics, 8,865-880.

LE FORT, P. 1975. Himalayas: the collided range. Present knowledge of the continental arc. Ameri- can Journal of Science, 275-A, 1-44.

P~CHER, A. 1991. The contact between the Higher Himalaya crystallines and the Tibetan sedimentary series: Miocene large-scale dextral shearing. Tectonics, 10,587-598.

SWAPP, S.M. & HOLLISTER, L.S. 1991. Inverted metamorphism within the Tibetan Slab of Bhutan: evidence for a tectonically transported heat source. Canadian Mineralogist, 29, 1019-1041.



Documents

Annapurna detachment fault in the Greater Himalaya of central Nepal