BehaviourProc Indian ofNatn Basement-cover Sci Acad 75 No.1 Decoupling pp. 27-40 (2009)in Compressional Deformation Regime 27

Behaviour of Basement-cover Decoupling in Compressional Deformation Regime, Northern Kumaun () Himalaya

KS VALDIYA1* and KANCHAN PANDE2 Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore–560 064 Indian Institute of Technology-Bombay, Mumbai–400 076. E-mail: [email protected]

(Received on 16 March 2009; Accepted† on 11 May 2009)

In the compressional regime related to -Asia convergence, the plane of decoupling of easily yielding sedimentary pile of the Tethys basin from its rigid basement of the Vaikrita crystalline complex behaved differently at different point of time in northern Kumaun (Uttarakhand) Himalaya. The terrain-defining Trans-Himadri Detachment Fault (T-HDF) system comprises a number of normal gravity faults that split both hangingwall and footwall rocks. The detachment related fault propagation took place during the climactic phase of the Himalayan orogeny in the Early Miocene, spreading on to larger network of faults along the basement-cover contact. Apart from the contrast in the structural architecture and attitude between hangingwall and footwall, two critical stratigraphic horizons of the footwall were attenuated and eliminated by faulting. Brecciation and mylonitization of the footwall rocks and gravity-driven northward collapse of folds in the hangingwall characterize the T-HDF all along its extent. Occurrence of large plutonic bodies of granite in the hangingwall sedimentary rocks in the Jadhganga valley made the structural design very complex due to faulting along the contacts of plutons with the host rocks and folding around the intrusive bodies. There are three spectacular and tectonically significant development invariably related to the T-HDF in the valleys of Kali, Eastern Dhauli, Gori and Western Dhauli — (i) formation of very narrow gorges with convex walls and slit canyons immediately downstream of the fault crossing, (ii) rise of footwall basement rocks higher than the hangingwall sedimentary cover, and (iii) emplacement of large thick deposits of fluviolacustrine and glaciolacustrine sediments of Late Quaternary age upstream of the T-HDF, that is, in the hangingwall. Penecontemporaneous deformation structures in these lacustrine sedimentary succession at some (including basal) levels suggest repeated reactivation of the Trans-Himadri Detachment Fault system. Key Words: Detachment Fault; Slit Canyon; Penecontemporaneous Deformation Structure; Palaeolake; Late Quaternary

Introduction Convergence of India and Asia continents Following India-Asia convergence the horizontal continuing, the rigid basement complex was squeezed shortening of the Himalayan crust manifested itself in up as a huge wedge or slab to great height, eventually compressional deformation in the manner of southward forming the Himadri, commonly known as the Great thrusting and attendant ductile deformation of rock piles. Himalaya. The loaded superincumbent Tethyan A multiplicity of planes of splitting developed as rock sedimentary cover on the steepened northern slope of masses advanced southwards in northern part of the the Himadri slid down, toppling over northwards and Kumaun (Uttarakhand) Himalaya (Fig. 1). One of the forming northward-vergent backfolds and backthrusts. planes of splitting was the plane of detachment of This plane of detachment — which is the terrain-defining sedimentary pile from its basement. The basement fault separating the domain of Himadri from the Tethys comprised high-grade metamorphic rocks intimately terrain (Fig. 2) — was first designated as the Malari associated with Cambro-Ordovician and Early Miocene Thrust Fault in the valleys of Western Dhauli and Gori granites that build the bulk of the Himadri (Great in northern Kumaun Himalaya [2–5] and later renamed Himalaya). And the sedimentary cover embraced Trans-Himadri Fault [6-8]. Neoproterozoic to Eocene sedimentary succession of the This paper describes salient structural characteristics Tethys terrain. The detachment plane behaved differently of and geomorphic development related to the formation in different sectors of the Himalayan arc, and at of what is now christened Trans-Himadri Detachment Fault different point of time during the Himalayan (T-HDF) in the valleys of Kali, Eastern Dhauli (Darma), orogeny [2-5,24,25,27,30,31,33,44]. Understandably, its Gori, Western Dhauli and Jadhganga (the major tributary inclination and attitude vary from sector to sector. of the Bhagirathi) in Uttarakhand (Figs. 1 & 2).

* Author for Correspondence E-mail: [email protected] † The March 2009 Issue of the Journal had got delayed. To avoid delay in Publication this article has been included in the present issue even though it was accepted after March 2009. 28 KS Valdiya and Kanchan Pande

5910 80o 81o Basement Complex (Vaikrita Group) Trans-Himadri Deatchment fault System JadhgangaGb R. Gb ● SONAM Main Central (Vaikrita) Thrust Kailas ● NAGA NilapaniF R. Thrust 5450 6700 NELANG Other Thrusts 6910 Mana 31o T-HDF Martoli Fm Ralam Fm 31o Saraswati R. 7770 Gb Garbyang Fm Arwa Kamet ● GOTING Shalshal 5800 6940 ● NITI ● BADCHADHURA GAMSALI ● Gb ● KEDARNATH ● BADARINATH ● LAPTHALI Mansarovar

Raksas Tal TRANS HIMADRI DETACHMENT ● TOPIDHUNGA F

● Amlangla JOSHIMATH 7060 ● UKHIMATH Gb Lissar R. E. Dhauli R. Dunagiri 7200 ● MILAM ● 7880 Gb Gurla Mandakini R. AlaknandaCHAMOLI R. ● Mandhata Trishul Nandadevi MARTOLI Alaknanda R. ● ● 6850 ● MUNSIARI RILKOT RALAM ● KUTI ● 7160 Shalang R. DUGTU VAIKRITA (MC) TAKLAKOTI Nandkot PANCHCHULI ● Gb ● Gori BALING GUNJI 6910 ● 6180 LILAM ● SELA Gb ● ● TINKAR MUNSIARI ● 5770 BUDHI GARBYANG F o Pindari R. Kali R. 30 30o 20 km o Gori R. E. Dhauli R. o 80 ● DHARCHULA 81 Fig. 1: Sketch map of northern part of the Kumaun (Uttarakhand) Himalaya showing position of the Trans-Himadri Detachment Fault. Note that the Martoli and Ralam formations are attenuated and eliminated in the Kali valley. The Tethyan sedimentary succession of the hangingwall is split into imbricating thrust sheets and severed from the Gurla Mandhata dome by a tear fault along the upper reaches of the Karnali River. (Based on Heim and Gansser, 1939 [1], Valdiya, 1979 [2] and recent work by authors).

T-HDF T-HDF

Garbyang Fm Basement complex

Martoli Fm

Basement complex

A B

T-HDF

Martoli Fm

Basement complex

B

Fig. 2: Trans-Himadri Detachment Fault separates the basement complex of high-grade metamorphic rocks with their granites from the cover sediments — the Middle Ordovician Garbyang limestone (G) in the Kali valley and the Martoli flysch (M) in the Western Dhauli and the Jadhganga. Behaviour of Basement-cover Decoupling in Compressional Deformation Regime 29

Structural Characteristics the Jadhganga valley a number of faults of lesser The evolution of the T-HDF system in the Kumaun magnitude occur above and below the T-HDF. Himalaya entailed formation of a number of structural The 65–70º NNE-dipping Vaikrita rocks of the features characterizing the terrain boundary between the basement are characterized by NNE–SSW oriented Himadri and Tethys domain. strong lineation and northeastward-plunging reclined Recent studies demonstrate that the T-HDF folds with prominent axial-plane foliation inclined represents a system of gravity faults which split both downdip. The Tethyan rocks of the hangingwall, in the hangingwall and footwall rocks (Fig. 3). In the Kali contrast, exhibit disharmonic folds (Fig. 4) and NNE- valley the Budhi Schist unit of the Vaikrita (basement) vergent backfolds locally broken by backthrusts. complex is split by a fault into two slabs, one steeply Obliterating the bedding plane, the strongly developed dipping and another gently inclined [9]. This fault is axial-plane joints dip 20–35º ENE/NE/NNE and NNW. traceable along the Patang gorge, filled with stupendous Conjugate pairs of joints have complicated the structural mass of debris stretching downstream upto the Kali. architecture of the hangingwall as seen in the Chhialekh– Budhi is located on the terrace of this debris fan. Not Garbyang section in the Kali valley and between Burphu only is this terrace tilted 10º at a place, but dissected and Milam in the Gori valley. into two parts, the northern part being some 120 m above Excepting the Kali River section, in all valleys the the southern patch. Choked with colluvial debris and high-grade metamorphic rocks of the basement morainic material, the Malchhi stream in the Eastern encompass large and small bodies of leucogranite Dhauli (Darma) valley, has cut a gorge along the fault intimately associated with migmatites. In the Jadhganga below the T-HDF traceable through Baling. and Eastern Dhauli valleys mylonitization is pronounced A NNW–SSE striking tear fault (Fig. 1), partly in porphyritic granite, the intensity of cataclasis coinciding with the main fault, dextrally offsets the T- increasing progressively towards the shear zone of the HDF system in the Gori valley between Mapong and T-HDF. Locally broken augen of feldspar occur in the Martoli. Failure of the mountain slopes has given rise to chaotic matrix of practically milled or brecciated rocks huge debris fans, one of which is more than 5 km wide as seen in the Gori valley between Mapong and Martoli. at the toe and about 200 m in elevation. There are quite In the proximity of the T-HDF, granites and many shear zones in the hangingwall between Bilju and migmatites show very prominent, commonly open master Milam. The Kosa Gad in the valley of Western Dhauli, joints hading 70–65º NE/ENE/ESE (Fig. 5). In the characterized by enormous volume of colluvial-glacial Chhialekh area in the Kali valley, these joints control debris and nearly vertical scarp slope more than 1000 m the straight courses of ravines and gullies. The formation high above the stream bed, marks the fault that seems to of steep to near vertical scarp faces and pinnacled branch off from the T-HDF as clearly seen in the Gunti summits of the mountain rampart of the hangingwall in stream at Malari. Similar situation is witnessed in the the Western Dhauli are attributed to these joints. Amritganga ravine at Gamsali upstream of Malari. In

S25oW N25oE F ? Gunji T-HDF Chhialekh Garbyang Kali R. Kuti R. PatangGad T 4100m Budhi T 4100m T 3100 3100 Kali R. 2100 Pindari Fm Budhi Schist Lake Deposit 2100 Garbyang Fm (Vaikrita) Shaila Fm (A) T-HDF 2 km. Nagling Malchhig Baling Dugtu N S T

3100m 3000m 2000 2000 Lake Deposit E. Dhauli R. (B) 2 km. Bilju o Martoli Gad Burphu Milum N20 W S25oE T-HDF F 4000m 4000m 3000 3000 Gori R. Budhi Marto 2000 2000 Pindari Lake Deposit (C) 2 km. Colluvial-morainic debris fan Gravity fault Joint T - THRUST

Fig. 3: Diagrammatic cross-sections depicting salient structural features related to the Trans-Himadri Detachment Fault, and formation of lakes upstream of the fault crossing. (A) Chhialekh near Garbyang, Kali valley, (B) Baling, Eastern Dhauli valley, (C) Burphu, Gori valley 30 KS Valdiya and Kanchan Pande

T-HDF

Martoli Fm

Basement complex

A

T-HDF

Martoli Fm

Basement complex B

T-HDF

Martoli Fm

C Basement complex

Fig. 4: (A) Disharmonic folding in the hangingwall rocks — the Martoli flysch — 1.5 km south of Niti in the Western Dhauli valley, (B) Subsidiary fault in the Martoli flysch intruded by granites at Nelang on the right bank of the Jadhganga, (C) Complex folding in the Martoli greywacks and phyllites at Nelang on the left bank of the Jadhganga valley Behaviour of Basement-cover Decoupling in Compressional Deformation Regime 31

Throughout the study area extending from the discernible in the Nelang–Sonam area (Fig. 6). One of Jadhganga to the Eastern Dhauli, the Tethyan succession these faults oriented east-west has right laterally offset (of the hangingwall) begins with the Martoli Fm overlain the T-HDF by about 100 m at Nelang. The anomalous successively by the Ralam and Garbyang formations. In northward plunging folds with NNE–SSW oriented axis the Gori valley the greywacke-phyllite alternation of the such as the Jadhang syncline (Fig. 6) [11] is presumably Martoli Formation is 500 to 800 m thick and the Ralam an outcome of the effect of dome-shaped pluton of conglomerate and quartzite attain a thickness of nearly granite. A major NNW–SSE trending normal fault along 500 m (Figs. 2B, 2C, 3B and 3C). In sharp contrast, 80 the Chor valley (Fig. 6) is a wrench fault, offsetting and km east of the Gori in the Kali valley, biotite- coinciding with the T-HDF. This fault extends NW across porphyroblastic calc schist of the Vaikrita is succeeded the India–Tibet border. directly by the Middle Ordovician dolomite and Geomorphic Development argillaceous limestone of the Garbyang Formation (Figs. 2A and 3A), the Martoli and the Ralam units altogether The most spectacular feature of considerable tectonic missing. This is a clear case of elimination by normal importance related to reactivation of the T–HDF is the faulting. very narrow gorge formed in the granite- and migmatite- dominant complex of every footwall. The deep defiles The granite plutons occur not only in the Vaikrita — slit canyons — and narrow gorges formed basement rocks, but also in the covering sedimentary immediately downstream of the fault crossing (Fig. 7). succession of the hangingwall. Close to the Jadhganga The change across the T-HDF is abrupt — from wide basin in the Gangotri area, granite laccoliths were valleys with gentler slopes lined with terraces of colluvial emplaced at the top of the Vaikrita succession during and/or fluvio-glacial deposits in the hangingwall to slit the onset of an extensional deformation and along canyon or narrow gorge with convex walls in the northern dextral shear zone [10]. The presence of larger footwall. The rivers break into rapids as they flow intrusive bodies in the Martoli succession in the through the narrow passages. The Western Dhauli shows Jadhganga basin (Fig. 6) complicated the tectonics of this development at Malari, some two kilometres east of the region. The contacts of the plutons with country rocks Gamsali and southwest Goting (Fig. 7). In the Gori and served as planes of dislocation, resulting in the Kali valleys the gorges are wholly obliterated or blocked development of numerous faults in the hangingwall as by stupendous fans of colluvial debris and moraines.

Jadhang Gad

Gb 31o 31o 15’ NAKURCHE 15’

Master joint Jadhganga R. F Jadhang Syncline Gb 5259 CHANGDUM

SONAM Char Gad Gb Kung Dum Gb

T-HDF A 5837 Chaling Gad 5957 NELANG F Master joint F Nilapani Gad DUMKU F

T-HDF 6209 5km o Jadhganga R. 79 T-HDF

Basement Conglomerate (Vaikrita) Martoli Fm Ralam Conglomerate Ralam Quartzite Gb Garbyang Fm Granite Pluton F Fault Synclinal axis F B Fig. 6: Sketch map of a part of the Jadhganga basin in the Nelang– Fig. 5: Prominent master joints in the granite in the basement Sonam area showing anomalous northward plunging syncline and Vaikrita complex of the footwall in the proximity of the faults of E–W trending and NNW–SSE oriented wrench faults. (Based on the T-HDF system south of Malari in the Western Dhauli valley. Bassi and Datta, 1987) [11]. 32 KS Valdiya and Kanchan Pande

A B

C

D

Fig. 7: Slit canyons and very narrow gorges with convex walls in the footwall immediately downstream of the crossing of the Trans- Himadri Detachment Fault. (A) Gori valley, downstream of Rilkot, (B) Western Dhauli, south of Malari, (C) Western Dhauli, west of Malari, (D) Western Dhauli about 2 km east of Gamsali (It is 15–25 m wide defile) Behaviour of Basement-cover Decoupling in Compressional Deformation Regime 33

Another geomorphic feature that carries significant — subsequent to ponding of the river due to movements implication is the NW–SE to WNW–ESE oriented on the T-HDF. In our opinion it was the uplift of the escarpment on the southerly face of the hangingwall, footwall along the T-HF that was responsible for the exposing Tethyan rocks – mostly Martoli flysch injected ponding of the Kali and formation of the Garbyang lake by granite. The foot of these scarps are characterized by more than twenty thousand years ago [17,18]. large and small cones of debris derived partly from the In the Eastern Dhauli valley enormous volume of shattered and brecciated rocks of the fault zone and partly moraine as well as colluvial debris brought by Machhi from the moraines in the hangingwall. The debris cones ravine (that has carved out its course along the T-HDF) and fans could have caused damming of rivers, reducing built a 500 m high dam. Behind this dam was formed their gradient and allowing aggradation. However, the the Baling Lake, which stretched 6 km upto Dugtu (Fig. primary cause of river ponding must have been uplift of 3B). The thick deposit consisting mainly of gravely, the footwall along reactivated T-HDF which not only pebbly and granular silt, with intercalation of clay and caused blockage following uplift of downstream block mud in the distal part, is mantled by muddy and clayey but also triggered massive landslides. The mountain patches exhibiting bog-like conditions. ranges of the footwall rise more than 1500–1600 m higher than the hangingwall rocks in the Eastern Dhauli The Gori valley in the 7 km reach between Martoli implying uplift of the downstream blocks. Satellite and Bilju is lined by three terraces of fluvial sediments, imagery of the Gori valley shows that in contrast to predominantly gravels (Fig. 3C). The oldest (highest) spread and splaying out of drainage lines in the terrace at Burphu (30º22’N:80º11’E) embodies 20-25 hangingwall, the streams and gullies take straight lines m thick clay and clay-silt alternation over a stretch of in the footwall domain [12], corroborating the surmise nearly one kilometre, representing a lacustrine regime that the footwall rocks rose up with respect to the (Fig. 8B). A tributary stream Shalang Gad was also hangingwall sedimentary pile. ponded upstream of the fault crossing. The lacustrine sediment is characterized by contorted bedding, flame River Blockage and Palaeolakes structure (Fig. 12B), and normal faults typical of In the Kali valley Garbyang (30º6’N:80º50’E) is situated seismites [20]. It may be emphasized that layers with on a terrace of a lake deposit (Figs. 8A and 9). Repeated deformation structures alternate with undeformed beds. slumping triggered by undercutting of the river gave rise Luminiscence dating of quartz grains in the lake to a number of irregular terraces. The 100- to 160-m sediments revealed that tectonic movements occurred thick deposit comprises dominant silt-clay alternations in the interval 16–11 Ka. (Fig. 12A) with numerous lenses or intercalations of In the Western Dhauli no palaeolake deposits are pebble and gravel. The fluviolacustrine deposit is capped recognizable between Malari and Niti, despite the river by 3–4 m thick fluvial gravel, and the basal 5-m thick entering slit canyons at the points of fault crossing and horizon is characterized by penecontemporaneous the fluvial deposits lining the valley. However, at Goting deformation structures such as convolutional folds and (30º5’N:80º50’E) a 15–25 m thick lacustrine sequence flame structure. The origin of the lake — that stretched consists of ash grey and brownish red clays with more than 11 km from the foot of Chhialekh to Gunji intercalations of morainic material, the ash grey clay (Figs. 3A and 9) — is attributed to damming of the Kali forming the top of the succession (Fig. 10). It is overlain by moraines that descended from the Api and Nampa by 8–10 m thick gravel cover. The silt-clay alternation glaciers in Nepal and building nearly 400 m high has been described as varvite (with dropstone) [21,19]. Chhialekh dam [1, 13]. The colluvial debris dam of The penecontemporaneous deformation features (Fig. considerable height and dimension and resulting 12B), including flame structure and convolutional ball blockade that lasted more than 20 ka imply that it was seem to have been formed due to tectonic activity on the work of tectonic movements on the T-HD. The the T-HDF. The tectonic movements might have been alternation of silt and clay and mud has been interpreted accompanied by earthquakes. Some of these features can as varvite [1,13,14,15]. The occurrence of lenses and therefore be described as seismites. Radiocarbon date beds of sands and sandy gravels at various levels in the of the organic matter in the 12-m section yielded dates mud sequence indicates influence of fluvial regime of 40 Ka and 20 Ka, suggesting the life span of the lake throughout the life of the lake. [19]. Magnetic susceptibility study shows six major Thermoluminiscence dating of fine silt collected peaks at 39 Ka, 33 Ka, 29 Ka, 26 Ka, 24 Ka and 23 Ka. from varve and rhythmite yielded ages of 20±3 Ka, Regional Perspective 18±3 Ka and 13±2 Ka, suggesting that the penecontemporaneous deformation occurred sometime Across the Kali River the T-HDF system extends in the temporal interval 20 to 17 Ka and to 13 Ka [16] eastwards in Nepal (Fig. 13). Known as the Annapurna 34 KS Valdiya and Kanchan Pande

Fig. 8: (A) Garbyang Lake is represented by the nearly 160 m thick deposits of silt-clay alternation with profuse intercalations and lenses of fluvial gravel, (B) Deposits in the Burphu Lake rest on a ground moraine (slightly darker in shade) (Photos: Courtesy Dr Navin Juyal).

Detachment in the Kali Gandaki basin, this normal Detachment [27,28]. In Bhutan the DontoLa Detachment ductile fault is associated with gravity-driven north- [29] represents the eastern extremity of the T-HDF. vergent large folds in the hangingwall [22], and Northwest of the Jadhganga valley (Figs. 1 and 13) characterized by eastward extension of the hangingwall the T-HDF has been described as the Tethys Thrust terrane [23,24]. In the Sagarmatha (Everest) massif in [30,31] and further northwest as the Zanskar Shear Zone northeastern Nepal where the Great Himalayan [32,33]. In the 2.25 to 6.75 km wide Zanskar Shear Zone crystalline complex is split by a number of gently dipping the displacement took place along closely spaced thrusts, the uppermost 5–15ºN dipping plane, and infinitesimal shear planes, which dip 30–40º NE (Herren, traceable north to southern Tibet is known as the South 1987). There was earlier top-SW sense of overthrust Tibet Detachment [25,26] or as the Qomolongma shearing, later superposed by NE–SW oriented extension Behaviour of Basement-cover Decoupling in Compressional Deformation Regime 35

Fig. 9: Extent of the Garbyang Lake of the past in the Kali valley. The column shows the lithology of the palaeolake. Note the penecontemporaneous deformation features occur in the lower part. (Based on Heim and Gansser, 1939 [1]; Chamyal and Juyal, 2005 [15]).

Fig. 10: Diagrammatic sketches show the location and sediment succession in the Goting palaeolake upstream of blockage in the Western Dhauli valley. (The location of Goting palaeolake is after Pant et al. (1998) [19]. 36 KS Valdiya and Kanchan Pande

Fig. 11: Deposits of the Goting palaeolake in the Western Dhauli valley. Upper: Behind the debris dam lie lacustrine sediments at the far end, Lower: Relicts of the Goting lake sediments. (Photos: Courtesy Dr Navin Juyal).

Fig. 12: (A) Rhythmite of the Garbyang palaeolake in the Kali valley, showing alternation of silt and clays, (B) Penecontemporaneous deformation features in the Goting sediment. Note that deformation features are confined to certain units (beds) which alternate with undeformed layers.(Photos: Courtesy Dr Navin Juyal) Behaviour of Basement-cover Decoupling in Compressional Deformation Regime 37

Himadri (Great Himalayan) Basement Complex

Sindhu Trans-Himadri Fault System Jhelam ZSZ Srinagar Main Central Thrust ITSZ Main Boundary Fault Chenab TT Himalayan Frontal Fault

Badarinath T-HDF Kailas Satluj R. MBT HFT MCT TETHYS ITSZ MBT Lhasa HFT ADF HIMALAYA Tsangpo R. Ganga

Thakholi ITSZ Kali MCT STDS Subansir Sagarmatha GLD

200 km Rapti Kathmandu Thimpo Gandak MBT MCT HFT Tista HFT Kosi Brahmaputra R.

SW HIMADRI Trans-Himadri NE Detachment F. Indus-Tsangpo Suture HFT MBT Almora MCT Kiogarh Raksas tal Kailas 0 0 1000m LESSER HIMALAYA TETHYS HIMALAYA TIBET 1000 m 50 km

Fig. 13: Regional extent of the Trans-Himadri Detachment Fault (T-HDF) which has been described as the Tethyan Thrust (TT) in the eastern Himachal Pradesh [30,31], the Zanskar Shear Zone (ZSH) in western Himachal Pradesh [32,33], the Annapurna Detachment (AD) in western Nepal [22], South Tibetan Detachment (STD) in eastern Nepal [25,26] and the DontoLa Detachment (DT) in northwestern Bhutan [29]. The rectangle shows the area studied in the central sector of the Himalayan arc. along the ZSH within the domain of overall overthrusting Origin of Trans-Himadri Fault System geometry [34,35,36]. In the Zanskar belt the south- When the movements resulting from India–Asia verging folds are believed to be contemporaneous with convergence were blocked or had slowed down on the the SW-directed thrusting during the Late Oligocene– Indus–Tsangpo Suture, it was but natural that the thick Early Miocene time [37]. The NE-directed backfolds are easily-yielding Tethyan sedimentary pile was detached the result of gravitation collapse of the thrust stack [38]. from its rigid foundation of the Vaikrita complex of high- In the Chandra valley the formation of the shear zone is grade metamorphic rocks and granites (Figs. 13 and 14). attributed to a later phase of deformation [39]. In the The decoupling-related fault propagation started possibly Kishtwar region in southeastern Kashmir, the quite before the main phase of the Himalayan orogeny northeasterly dipping normal fault of the T-HDF system in the Early Miocene time. And it spread on to the larger was severely deformed in the interval 22–16 Ma [40] fault network along the basement-cover contact, giving even as the Haimanta (a” Martoli + Ralam formations rise to splaying faults associated with the main in Kumaun) of the hangingwall was thrust 30 km detachment [18]. The end result was a system of shear southwards along the Chamba Thrust [41,42]. faults — the Trans-Himadri Detachment Fault System It is evident from the brief account of the easterly (T-HDFS). and westerly lateral extent of the T-HDF that after the Taking in conjunction with the conspicuous compressional deformation that culminated in the schuppen zone in the upper reaches of the Kali and formation and evolution of SW-directed folds and thrusts, Darma (Eastern Dhauli) rivers (Fig. 1), the formation there was dominant dip-slip movement related to normal and evolution of the Trans-Himadri Detachment Fault gravity faulting in the Kumaun and eastern Himachal system seems to have been controlled by the Gurla Himalaya and large-scale strike-slip dextral movements Mandhata dome located south of Kailas–Mansarovar superposed on earlier dip-slip movement in western area (Figs. 1 and 14). The Gurla Mandhata represents Himachal Pradesh and Nepal, thus bringing about the granite-dominated top of the basement complex that extension and resultant tectonic extrusion of the buckled up when India collided with Asia. The Indian hangingwall (Tethyan terrain). The Trans-Himadri plate being nearly 20% less dense than the mantle, and Detachment Fault System thus marks a zone of more therefore comparatively buoyant, there was resistance than one phase and kind of deformation. 38 KS Valdiya and Kanchan Pande

Incipient Indus-Tsangpo Suture N S MCT T-HF 0

Tibet 50 km

100 km

NE S-W Himadri Kailas T-HF I-TS MCT 0 0 Tibet

50 km 50 km

N-verging folds Himadri T-HT NE (Nanda Devi) SW MCT

0 0

1000 m 1000 m Lesser Himalayan schuppen zone

Fig. 14: Diagrammatic cross-sections explain the stages of the formation of the Trans-Himadri Detachment Fault System in relation to the formation of other terrane-defining thrusts. Detached from the Tethyan sedimentary cover, the basement complex (Vaikrita) was squeezed up, forming the high Great Himalayan or Himadri ranges. (Modified after Valdiya, 1988) [7]. to its sliding under the Asian plate. Consequently the the crustal block coupled with strike-slip displacement leading edge of the Indian plate buckled up, forming in the eastern part. Most workers believe that the T-HDF domal upwarp all along the collision zone [5,6,7,8]. The system and Main Central Thrust movements were domal structures are discernible from Nimaling in contemporaneous, the tectonic movement taking place Ladakh to Kangmar in northeastern Nepal. over a period 22 Ma to 19 Ma and the detachment The crustal-scale resistance that caused the doming commencing around 21.5 Ma [43]. up of the frontal part of the Indian plate must have been High stress manifested also in the strike-slip responsible for the development of a pack of imbricating displacement along wrench faults represented by the thrust sheets (schuppen zone) and the T-HDF system in NW–SE oriented Humla Karnali Fault framing the Gurla northeastern Kumaun (Fig. 1). As India continued to Mandhata. Right-lateral movement is apparent in the advance northwards, the stress buildup in the Gurla Gori valley between Rilkot and Milam (Figs. 1 and 13) Mandhata dome manifested itself in the reactivation of and possibly in the Saraswati valley between Badarinath the thrusts of the schuppen zone, more so along the plane and Mana Pass. The squeezing up of the Vaikrita complex of separating the Tethyan sedimentary cover from the (making the Great Himalaya) must have caused crystalline basement. The T-HDF developed along this reactivation of the T-HDF. The reactivation of the T- plane between the rigid basement complex and the HDF system time and time again is evidenced by ponding sedimentary cover during the period when large-scale of rivers earlier and bursting later of natural dams crustal thickening took place. As the deformation front and attendant draining out of the lakes. propagated southwards, other detachment planes and Penecontemporanceous deformation structures at some shear zones were formed, culminating in the evolution levels of the lacustrine sediments column bear eloquent of the schuppen zone of Main Central Thrust during the testimony to the activeness of the T-HDF system. climactic phase of the Himalayan orogeny 21 to 22 A significant development was the right-lateral million years ago. There was simultaneous extension of displacement of the T-HDF — by an E–W tear fault in Behaviour of Basement-cover Decoupling in Compressional Deformation Regime 39 the Jadhganga valley, by a NW–SE oriented wrench fault 12. Gupta RP, Fritz H and Bojar AV On the nature of the South in the Chor Gad (a tributary of the Jadhganga (Fig. 6), Tibetan Detachment Zone, Kumaun Himalaya Intern Jour Remote Sens (2005) 1-4 and by the NNW–SSE trending fault in the Gori valley 13. Gansser A Geology of the Interscience, New York, where the two faults coincide (Fig. 1). The tear faults 289 testify to the tectonic resurgence of the Tethyan terrane 14. Kumar Rajesh and Singh GS Geology and geomorphology in the post-climactic phase of Himalayan orogeny, as of Quaternaries along Kailash–Mansarovar route, upper Kali also witnessed in the Lesser Himalayan and Siwalik Valley, district, Uttaranchal Spl Pub Geol Survey terranes [2,3,4,7]. India 65 (2001) 109–116 15. Chamyal LS and Juyal N Climatic events in southern Thar Acknowledgements Desert margin and higher central Himalaya during the Last Glacial Stage: Possible linkages Himalayan Geol 26 (2005) The first author is profoundly grateful to the Indian 241–252 National Science Academy, New Delhi for providing 16. Juyal N, Pant RK, Basavaiah N, Yadava MG, Saini NK and extremely generous financial support for his project Singhvi AK Climate and seismicity in the higher Central under the INSA Golden Jubilee Research Professorship. Himalaya during 20–10 Ka: evidence from the Garbyang The Jawaharlal Nehru Centre for Advanced Scientific basin, Uttaranchal, India Palaeogeogr Palaeoclimat Research extended all possible facilities and help. Palaeoecol 213 (2004) 315–330 17. Valdiya KS Reactivation of terrane-defining boundary thrusts The paper has greatly benefited from critical reviews in central sector of the Himalaya: Implications Curr Sci 81 by Professor Sandeep Singh (IIT, Roorkee) and Professor (2001) 1418–1431 Navin Juyal (PRL, Ahmadabad). The authors thank them 18. Valdiya KS Trans-Himadri Fault: Tectonics of a detachment for very valuable suggestions for improvement. system in central sector of Himalaya, India Jour Geol Soc India 65 (2005) 537–552 The authors are very grateful to Shri BD Patni and 19. Pant RK, Juyal N, Rautela P, Yadav MG and Sangode SJ Dr Rajeev Upadhyay for excellently organizing the field Climate instability during last glacial stage: Evidence from trips in the valleys of Eastern Dhauli and Jadhganga varve deposits at Goting, district Chamoli, Garhwal Himalaya respectively. Curr Sci 75 (1998) 850–855 20. Pant RK, Juyal N, Basavaiah N and Singhvi AK Late References Quaternary glaciation and seismicity in the Higher central 1. Heim A and Gannser A Central Himalaya Soc Helv Sci Nat Himalaya: evidence from Shalong basin (Goriganga) 73 (1939) 245 Uttaranchal Curr Sci 90 (2006) 1500–1505 2. Valdiya KS Outline of the structure of the Kumaun Himalaya 21. Sastri MVA, Chandra A and Mamgain VD Note on Jour Geol Soc India 20 (1979) 145–157 Pleistocene varved clays and associated microfauna from Niti 3. Valdiya KS Himalayan transverse faults and folds and their area of Kumaun Himalaya Indian Minerals 24 (1970) 131– parallelism with subsurface structures of North Indian plains. 138 Tectonophysics 32 (1976) 353–386 22. Brown RL and Nazarchuk JH Annapurna detachment fault 4. Valdiya KS Tectonics of the central sector of the Himalaya. in the Greater Himalayan of Central Nepal In: PJ Treloar In: HK Gupta and FM Delany (Eds.) Zagros–Hindukush– and MP Searle (Eds.) Himalayan Tectonics Geol Soc America, Himalaya; Geodynamic Evolution American Geophysical Boulder (1993) 461–473 Union, Washington, (1981) 87–111 23. Pecher A The contact between the Higher Himalayan 5. Valdiya KS Aspects of Tectonics: Focus on Southcentral Asia Crystallines and the Tibetan sedimentary series: Miocene Tata McGraw Hill, New Delhi (1984) 319 large-scale dextral shearing. Tectonics 10 (1991) 587–599 6. Valdiya KS Trans-Himadri Thrust and domal upwarps 24. Hodges KV, Parrish RR, Housch TB, Lux DR, Burchfiel BC, immediately south of the collision zone and tectonic Royden LH and Chen Z Simultaneous Miocene extension implications, Curr Sci 56 (1987) 200–209 and shortening in the Himalayan orogen Science 258 (1992) 1466–1470 7. Valdiya KS Tectonics and evolution of the central sector of the Himalaya Phil Trans Royal Soc London A236 (1988) 151– 25. Burchfiel BC and Royden LH North–South extension within 175 the convergent Himalayan region Geology 13 (1985) 679– 682 8. Valdiya KS Trans-Himadri intracrustal fault and basement upwarps south of Indus-Tsangpo Suture Zone In: LL 26. Royden LH and Burchfiel BC Thin-skinned N–S extension Malinconico and RJ Lillie (Eds.) Tectonics of Western within the convergent Hiamalayan region: Gravitational Himalaya Geological Society of America, (1989) 153–168 collapse of topographic front In: MP Coward et al. (Eds.), Continental Extension Tectonics Geol Soc Amer Washington 9. Powar KB Petrology and structure of the Central Crystalline (1987) 611–612 Zone NE Kumaun Himalayan Geology 2 (1972) 36–46 27. Searle MP Extensional and compressional faults in the 10. Scaillet B, Pecher A, Rochette P and Champenois M The Everest–Lhotse massif, Khumbu Himalaya, Nepal Jour Geol Gangotri Granite (Garhwal Himalaya): Laccolithic Soc London 156 (1999) 227–240 emplacement in an extending collisional belt Jour Geophys Res 100 (1995) 585–607 28. Searle MP, Simpson RL, Law RD, Parrish RP and Waters DJ The structural geometry, metamorphic and magmatic 11. Bassi UK and Datta BM Geology of a part of the Jadhganga evolution of the Everest massif, High Himalaya of Nepal– valley, Kumaun Himalaya Bull Indian Geologists Assoc 20 South Tibet Jour Geol Soc London 160 (2003) 345–366 (1987) 71–76 40 KS Valdiya and Kanchan Pande

29. Grujic D, Hollister LS and Parrish RR Himalayan 37. Baud A, Gaetani M, Garzanti E, Fors E, Nicora A and Tintori metamorphic sequence as an orogenic channel: Insight from A Geological observations in southeastern Zanskar and Bhutan Earth & Planet Sci Lett 198 (2002) 177–191 adjacent Lahul area Eclogae Geol Helv 77 (1984) 171–197 30. Sinha Anshu K Geology of Higher Himalaya, John Wiley, 38. Wyss M, Hermann J and Steck A Structural and metamorphic Chichester (1989) 236 evolution of the northeast Himachal Himalaya, NW India 31. Paul SK and Paul R Northeast–southwest extensional Tethyan Eclogae Geol Helv 92 (1999) 3–44 Shear Zone within compressional regime of the Himalaya, 39. Vanny JC and Steck A Tectonic evolution of High Himalaya Lahaul–Spiti In: AK Jain and RM Manickavasagam (Eds.) in Upper Lahaul (NW Himalaya) India Tectonics 14 (1995) Geodynamics of the NW Himalaya Gondwana Research 253–263 Group, Trivandrum (1999) 135–144 40. Stephenson BJ, Searle MP, Waters DJ and Rex DC Structure 32. Searle MP Structural evolution and sequence of thrusting in of the Main Central Thrust Zone and extrusion of the High Himalaya between Spiti River and Tso Morari, NW Himalayan deep crustal wedge, Kishtwar–Zanskar Himalaya Himalaya. Schweizeriche Mineralogische Petrographische, Jour Geol Soc London 158 (2001) 637–652 Mitteilungen 79 (1986) 419–430 41. Kesar Singh, Reverse and oblique-slip movement along the 33. Herren E Zanskar Shear Zone: Northeast–southwest Chamba Thrust, NW Himalaya: Implication for tectonic extension within the Higher Himalaya, Ladakh Geology 15 evolution Jour Himalayan Geol 4 (1993) 143–148 (1987) 409–413 42. Thakur VC Structure of the Chamba nappe and position of 34. Patel RC, Singh S, Asokan A, Manickavasam RM and Jain the Main Central Thrust in Kashmir Himalaya Jour Asian AK Extensional tectonics in the collisional Zanskar Earth Sci 16 (1998) 269–282 Himalayan belt In: PJ Treloar and MP Searle (Eds.), 43. Hodges KV Tectonism of the Himalaya and southern Tibet Himalayan Tectonics Geol Soc London (1993) 445–459 from two perspectives GSA Bulletin 112 (2000) 324–350 35. Jain AK and Manickavasagam RM Inverted metamorphism 44. Yin A Origin of regionally rooted low-angle normal faults – in the intracontinental ductile shear zone during Himalayan a mechanical model and its tectonic implications Tectonics 8 collision tectonics Geology 21 (1993) 407–410 (1989) 469–482 36. Jain AK, Singh S and Manickavasagam RM Himalayan 45. Harrison TM, Copeland P, Kidd WSF and Yin An Raising Collision Tectonics Gondwana Research Group, Trivandrum Tibet Science 255 (1992) 1663–1670 (2002) 114