Earthquakes and tectonics of the Himalayan syntaxis: Namcha Barwa (eastern) and Nanga Parbat (western)
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PLEASE SCROLL DOWN FOR ARTICLE This article was downloaded by: [Mukhopadhyay, Basab] On: 5 May 2011 Access details: Access Details: [subscription number 937263838] Publisher Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37- 41 Mortimer Street, London W1T 3JH, UK Geomatics, Natural Hazards and Risk Publication details, including instructions for authors and subscription information: http://www.informaworld.com/smpp/title~content=t913444127 Seismotectonics at the terminal ends of the Himalayan Arc Basab Mukhopadhyay a ; Anshuman Acharyya a ; Debkumar Bhattacharyya a ; Sujit Dasgupta a ; Prabhas Pande a a Geological Survey of India, Kolkata, India First published on: 04 May 2011 To cite this Article Mukhopadhyay, Basab , Acharyya, Anshuman , Bhattacharyya, Debkumar , Dasgupta, Sujit and Pande, Prabhas(2011) 'Seismotectonics at the terminal ends of the Himalayan Arc', Geomatics, Natural Hazards and Risk,, First published on: 04 May 2011 (iFirst) To link to this Article: DOI: 10.1080/19475705.2010.536263 URL: http://dx.doi.org/10.1080/19475705.2010.536263 Full terms and conditions of use: http://www.informaworld.com/terms-and-conditions-of-access.pdf This article may be used for research, teaching and private study purposes. Any substantial or systematic reproduction, re-distribution, re-selling, loan or sub-licensing, systematic supply or distribution in any form to anyone is expressly forbidden. The publisher does not give any warranty express or implied or make any representation that the contents will be complete or accurate or up to date. The accuracy of any instructions, formulae and drug doses should be independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims, proceedings, demand or costs or damages whatsoever or howsoever caused arising directly or indirectly in connection with or arising out of the use of this material.
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PLEASE SCROLL DOWN FOR ARTICLE
This article was downloaded by: [Mukhopadhyay, Basab]On: 5 May 2011Access details: Access Details: [subscription number 937263838]Publisher Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK
Geomatics, Natural Hazards and RiskPublication details, including instructions for authors and subscription information:http://www.informaworld.com/smpp/title~content=t913444127
Seismotectonics at the terminal ends of the Himalayan ArcBasab Mukhopadhyaya; Anshuman Acharyyaa; Debkumar Bhattacharyyaa; Sujit Dasguptaa; PrabhasPandea
a Geological Survey of India, Kolkata, India
First published on: 04 May 2011
To cite this Article Mukhopadhyay, Basab , Acharyya, Anshuman , Bhattacharyya, Debkumar , Dasgupta, Sujit andPande, Prabhas(2011) 'Seismotectonics at the terminal ends of the Himalayan Arc', Geomatics, Natural Hazards andRisk,, First published on: 04 May 2011 (iFirst)To link to this Article: DOI: 10.1080/19475705.2010.536263URL: http://dx.doi.org/10.1080/19475705.2010.536263
Full terms and conditions of use: http://www.informaworld.com/terms-and-conditions-of-access.pdf
This article may be used for research, teaching and private study purposes. Any substantial orsystematic reproduction, re-distribution, re-selling, loan or sub-licensing, systematic supply ordistribution in any form to anyone is expressly forbidden.
The publisher does not give any warranty express or implied or make any representation that the contentswill be complete or accurate or up to date. The accuracy of any instructions, formulae and drug dosesshould be independently verified with primary sources. The publisher shall not be liable for any loss,actions, claims, proceedings, demand or costs or damages whatsoever or howsoever caused arising directlyor indirectly in connection with or arising out of the use of this material.
Seismotectonics at the terminal ends of the Himalayan Arc
BASAB MUKHOPADHYAY*{, ANSHUMAN ACHARYYA{,DEBKUMAR BHATTACHARYYA{, SUJIT DASGUPTA{ and
PRABHAS PANDE{{Geological Survey of India, Kolkata, India; {DDG (Retd), Geological Survey of India,
Kolkata, India
(Received 30 June 2010; in final form 27 October 2010)
The Himalayan arc has an arcuate E–W trending geometry with reversal of trendat the terminal ends – Nanga-Parbat (western) syntaxis and Namcha-Barwa
(eastern) syntaxis. Both ends are characterized by an actively deformed uplifteddome with its flanks bounded by active shear zones/faults that cause the majorityof the seismicity. Compiled map data and seismo-geological depth sections
around these two syntaxial zones have brought out active crustal structure andseismotectonic setup. The Nanga-Parbat syntaxis exhibits upward bending andsubsequent thickening of the Indian plate with the cluster of seismicity along theNNE–SSW trending Raikhot fault/Diamer shear in its western margin and a
comparatively less active Rupal–Chichi shear zone of N–S trend with diffusedseismicity towards the east. The 2005 Kashmir earthquake is spawned due tointeraction of the Main Boundary thrust and the Muzaffarabad fault. The
Namcha–Barwa syntaxis displays a fault-bounded upliftment and thickening ofthe Indian plate where Canyon thrust marks the boundary between the Indianand Eurasian plates. The occurrence of the 1950 Assam earthquake in the vicinity
of the eastern syntaxis is attributed to a regional right lateral strike-slip motion onthe causative fault plane. The seismicity in the syntaxes is primarily controlled bystrike-slip faults/shear zones along the flanks of popup antiforms.
1. Introduction
The Himalayas have an E–W bow-like shape with trend reversal and higherelevations at the terminal ends: Nanga-Parbat in the west (western syntaxis) andNamcha Barwa in the east (eastern syntaxis) (figure 1). Actively deformed uplifteddomes characterize these two ends and their flanks are deformed by seismicallyactive shear zones/faults. The geodynamic interactions of these shear zones/faultsaided by deep river erosion (Indus in the west and Siang in the east) and massremoval exposes the exhumed Cenozoic metamorphic rocks, granulites and granitesoriginating from the Indian plate. The structure of the western syntaxis is expressedby a popup antiformal structure of NW vergence with high-grade gneissic rocks ofthe Nanga-Parbat–Harmosh massif at its core. The Nanga-Parbat massif issurrounded entirely by Main Mantle Thrust (MMT) and bordered in the west andeast by accreted rocks of the Mesozoic Island arc system of the Kohistan and
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Ladakh arc, and to the north by rocks of the Karakoram arc along the MainKarakoram Thrust (MKT)/Shyok suture (Naqvi 2005). The eastern syntaxis, on theother hand, a rather small but sharp syntaxial bend compared to the western one, isdefined by a popup antiformal structure verging towards the NE in the easternterminal end of the Himalayan arc. It is represented by two antiformal structures: aNE–SW-trending Namcha Barwa antiform with high-grade mobile rocks at coretowards north and a NW–SE-trending Siang window with folded low-grade meta-sedimentary and meta-volcanic rocks of Abor and Miri formations in the south.
In the last three and half decades, several scientists studied the seismotectonics ofthe western and eastern syntaxes to throw light on the contemporary geodynamics,crustal evolution and tectonics of theHimalaya (Rastogi 1974, Tandon and Srivastava1975, Kumar 1975, Chandra 1978, Molnar and Chen 1983, Baronowski et al. 1984,Mukhopadhyay 1984, Biswas and Dasgupta 1986, Coward et al. 1987, Dasgupta et al.1987, Verma and Prasad 1987, Chen and Molnar 1990, Holt et al. 1991, Nandy andDasgupta 1991, Pegler and Das 1998, Singh 2000, Purnachandra Rao et al. 2003,Kayal 2008, Tiwari et al. 2009). These studies also gave a broad regional frameworkpertaining to tectonics, relationship between active tectonic surfaces and seismicity,crustal structure and also focus on the enduring crustal evolution. Further, an accountof the seismotectonics in and around the Indian subcontinent was captured in framesby a publication of the Geological Survey of India, Seismotectonic Atlas of India andits Environs (Dasgupta et al. 2000). The exercise, as a unique comprehensive step, hasmarked the correlation between earthquake occurrences, fault plane solutions andactive seismic structures of the entire Indian subcontinent with special reference to theHimalayas. The analysis was based on geological, geophysical and seismologic datacompiled up to 1993. As a follow-up of this study, we zoomed into both the terminalends of the Himalaya for understanding the ongoing tectonics with the help ofupdated seismic data up to 2007, prepared up-to-date seismo-geological maps, andconstructed eight seismo-geological depth sections. The compiled maps and seismo-geological depth sections aimed to bring out the interaction between seismogenicsurfaces, surface and subsurface geology and crustal configuration.
2. Data
Earthquake data for the time period between 1905 and 2007 (source ISS/ISC/NEIC)with magnitude (mb�4) has been used for this study. For generating topographic
Figure 1. Location map of western and Eastern syntaxes. NP, Nanga Parbat; NB, NamchaBarwa; MBT, Main Boundary Thrust; MCT, Main Central Thrust.
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profiles on the seismo-geological depth sections, Shuttle Radar Topographic Mission(SRTM) data of 90-m resolutions have been used. The available centroid-momenttensor (CMT) solutions for the area around the western syntaxis (data period 1980–2007) and the eastern syntaxis (data period 1950–2007) have been compiled frompublished sources and the HRVD website (www.seismology.harvard.edu) (tables 1and 2).
There are 949 earthquake (mb�4.0) records in the selected catalogue of thewestern syntaxis spanning the period 1905–2007. The area around the easternsyntaxis has recorded only 191 earthquakes (mb�4.0) for the time period 1906–2007. Earthquake frequencies in different magnitude classes for both the syntaxes aresummarized in table 3. More numbers of earthquakes in the lower magnitude range(4–5.5) have been detected in the western sector in comparison to the eastern sector.The number of higher magnitude earthquakes (magnitude4 5.5) is more or less thesame in both of them. The presence of a strong seismic network in the western sectorenables detection of low-magnitude tremors. Such a network is yet to be establishedin the eastern part of the Himalayas. Furthermore, the occurrences of earthquakeswith focal depth greater than 100 km are more in the western sector compared to theeastern sector; the earthquakes in the eastern sector originate from a shallowercrustal depth (*40 km). It is known that the catalogue data of the ISS/ISC/NEIChave depth errors, and many events are located with restricted depth. The utmostcare has been taken to choose the events for constructing depth sections where eventswith restricted depths are removed from the catalogue used for further study.
3. Seismotectonic model of the Himalayas
The Himalayas came into existence due to collision of the Indian shield with theEurasian/Tibetan plate, accompanied by compression and thrusting along majorfaults such as the Main Central Thrust (MCT), the Main Boundary Thrust(MBT) and the Main Frontal Thrust (MFT) (Valdiya 1976, Le Fort 1986). Thedeformation front propagated southward with the MFT as the most recentlyactivated one in the entire sequence of thrusting. Seeber, Armbruster andQuittmeyer (1981) were the first to suggest a tectonic model of the Himalayasfrom the seismic data analysis and suggested a gently dipping Indian slab, anoverriding Tethyan slab and a sedimentary wedge (figure 2). The sedimentarywedge is decoupled from the Indian and Tethyan slabs. Simultaneous activationof the MCT and the MBT is also proposed in this model. However, Ni andBarazangi (1984) argued that the MCT became dormant and the MBT wasactive. In their model, the interface between the subducting slabs and sedimentarywedge is a ‘plane of detachment’ and further north, the contact between theIndian and Tethyan plates is marked by a thrust, the Boundary Thrust (BT). Thezone between ‘plane of detachment’ and the BT roughly coincides with the hightopographic gradient between the Lesser and the Higher Himalaya and ischaracterized by the steep dip of the MCT and ramping of the Himalayan crustat the northern edge of the Indian plate (Lyon-Cean and Molnar 1983, Schellingand Arita 1991, Lave and Avouac 2000). Further north, below the higher rangeof the Himalayas, a mid-crustal reflector at a depth of *25 to *40 km has beenimaged in the INDEPTH profile (Zhao et al. 1993, Nelson et al. 1996) and wasnamed the Main Himalayan Thrust (MHT). In the extreme southern end of theHimalayas below the Ganga foredeep, the major detachment surface MFT is
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located at a shallower depth (*5 km) and extends subhorizontally northward.The MFT in the frontal belt joins the splays of MBT and MCT below the LesserHimalayas, and further connects to the MHT below the Higher Himalayas.Further, the wedge-shaped Himalayan collision boundary defines the crustal scalefault bend folds, the formation of the Lesser Himalayan Duplexes that form taperand controlled the foreland-ward propagation of the thrust sheets [Mukul (2010)from his study in the Darjiling–Sikkim–Tibet (DaSiT) Himalayan wedge].Recently, relict majorite has been identified from the eclogite rocks of Himalaya.This in turn shows that the Indian tectonic plate was forced down under theAsian plate, sinking down into the Earth’s mantle to a depth of at least 200 kms(Pandey et al., 2010). Another important aspect of Himalayan seismicity is thepresence of lower crustal earthquakes beyond the Himalayan seismic zone southof the MBT in the Ganges Alluvial plain. Two such large earthquakes thatoccurred within such a premise are 15 January 1934 Bihar earthquake (Ms 8.4),60 km south of MBT at a focal depth of *30 km (Abe 1981) and 20 August1988 Bihar–Nepal earthquake (Ms 6.6) with an estimated focal depth of *50 km
Table 3. Earthquakes for the western and eastern syntaxes are classified according to theirmagnitude and frequency.
Western syntaxis (Data period 1905–2007)Eastern syntaxis (Data period 1906–
Figure 2. Seismotectonic model of Himalaya (modified after the compilation of Kayal 2010;Kayal 2001; from Seeber et al. 1981 and Valdiya 1976). Q, Quaternary; US, MS, LS, Upper,Middle and Lower Siwaliks; IS, Indus Suture; MBT, Main Boundary Thrust; MCT, MainCentral Thrust; MFT, Main Frontal Thrust; MHT, Main Himalayan Thrust.
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(Kayal 2010). GSI (1993) interpreted the occurrence of these earthquakes due tointeraction of a long transverse fault (East Patna Fault) with Himalayantrend. The role of such transverse faults that run across the Indo-Gangeticplain and transverse to the Himalayan trend has been argued for the occurrenceof such earthquakes throughout the Himalayan arc south of the MBT(Mukhopadhyay 1984, Kayal 2001). Monsalve et al. (2006), based on precisiondigital seismic data in the Central Himalayas, has identified a bi-modal seismiczone south of MBT, one along the detachment surface and another at the deeperzone at 40–50 km level. In the light of the above data, Kayal (2010) argued thatthe deeper events of 1934 and 1988 are related to the seismic zone of 40–50 kmlevel and are not connected to the main Himalayan seismicity along the plane ofdetachment.
In recent times, due to locking of the Himalayan thrusts, the interactions betweenMFT, MBT and allied thrust planes have generated many great earthquakes (1905Kangra, 1934 Bihar, 1950 Assam, 2005 Kashmir, etc.) in the frontal belt of theHimalayas and moderate-sized events (1980 Gangtok, 1991 Uttarkashi, 1999Chamoli, etc.) in the mid-crustal ramp zone (Dasgupta et al. 2000).
4. Seismic zones and seismotectonic sections around the western syntaxis
4.1 General geology
The earthquake epicentres were plotted on a generalized tectono-geological map(figure 3) compiled from the Seismotectonic Atlas of India and its Environs(Dasgupta et al. 2000). The geological sketch map displays a complex litho-tectonicpacket that are structurally controlled and overprinted by strong discontinuities.Discontinuities like the Indus–Tsangpo Suture Zone (ISZ) and the Main MantleThrust (MMT) are present in between the Sub and Lesser Himalaya lithopacketsand the Kohistan–Ladakh arc, whereas the Main Karakoram Thrust (MKT)/Shyok Suture Zone (SSZ) is observed in between the Kohistan/Ladakh Complexand the Hindukush–Karakoram belt. The litho-units of the Karakoram beltcomprise trans-Himalayan rocks of Karakoram Group, Karakoram Granite andKarakoram metamorphic complex. The Kohistan Island arc lithopacketsrepresenting the Neo-Tethyan oceanic lithosphere of Mesozoic age demonstratesrocks of Kohistan/Ladakh granitic complex, accreted rocks of Shyok Group,dismembered ophiolite/melange zone, basic island arc type volcanics of Chilascomplex, Kamila amphibolites, etc. Towards the south, the rocks of the Kohistanisland arc are separated from the Peshawar and Kashmir basins by the MMT,while in the east Ladakh Batholith is delimited by the ISZ from the ophiolite-dotted accretionary complex (Naqvi 2005). The MMT in the northern part swervesaround the Nanga-Parbat–Harmosh massif characterized by gradual uplift andexposing granitoid and metamorphites of the Indian Plate. The Cenozoiccontinental collision (Honneger et al. 1982) between the Indian and Eurasianplates had welded the entire sequence described above and promoted intensecrustal shortening, metamorphism, crystallization and movement of the thrustsheets from north to south. The entire sequence is thrusted sequentially southwardover the lesser Himalayan rocks along the MCT, subsequently on sub-Himalayanforedeep basin filled with Siwalik molasse originated from the Himalayan uplift bythe MBT and finally to the Indo-Gangetic plane by the MFT.
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4.2 Seismo-geological sections
The earthquake epicentre plot on map (figure 3) has brought out five visible seismiczones from west to east: Kashmir–Hazara; Kohistan; Nanga-Parbat; LesserHimalayan zone; and Karakoram. These are further analysed by drawing seismo-geological cross-sections to understand the plate configuration in conjunction to theknown seismo-tectonic model of the Himalaya described in an earlier section. Theearthquake data belonging to three boxes (figure 3) are extracted to draw fourseismo-geological sections. Section lines (X–X/, Y–Y/, W–W/ and Z–Z/), 24 well-constrained fault plane solutions with beach ball diagrams are shown (table 1;figure 4).
4.2.1 Kashmir–Hazara syntaxis and Kohistan arc. The Kashmir–Hazara syntaxisand Kohistan arc (box marked as section X in figure 3) encompass both the LesserHimalayan and Kohistan arc tectonic blocks. The earthquakes in the Kashmir–Hazara syntaxis are nucleated from the movement along the MBT and recent
Figure 3. Seismo-geological map of areas in and around the western syntaxis. Generalgeology, earthquake with magnitude variation, tectonic planes and zones of earthquakeclustering are shown. SRT, Salt Range Thrust; JMT, Jwalamukhi Thrust; MBT, MainBoundary Thrust; MCT, Main Central Thrust; ISZ, Indus–Tsangpo Suture Zone; MF,Muzaffarabad Fault; MMT, Main Mantle Thrust; MFT, Main Frontal Thrust; MKT, MainKarakoram Thrust; RF, Raikhot Fault; R–CS, Rupal–Chichi Shear Zone; A, Attock; P,Poonch; An, Anantnag; L, Leh; NP, Nanga Parbat.
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reactivation of the Muzaffarabad faults, whereas earthquakes in the Kohistan arccluster lying on the top of the Kohistan/Ladakh plutonic complex (magmatic arc)are yet to be assigned to any known seismogenic surface. The seismo-geologicalsection X–X/ (figure 5(a)) along the Kashmir–Hazara and Kohistan arc hasbrought out the configuration of subducted Indian Plate beneath the EurasianPlate and the accretionary Himalayan, Kohistan and Karakoram arc litho-packages overriding it. This also intersects the visible spatial seismic clusters of theKashmir–Hazara syntaxis and Kohistan arc. The MBT and MKT are the mostactive tectonic planes along this section line showing thrust and strike-slipmovements, respectively. The Hazara–Kashmir cluster around traces of the MBTand the activated Muzaffarabad fault has four fault plane solutions (Sl. No. 16, 17,20 and 21, table 1). These solutions are remarkably alike, with high angle tensionalong the NW–SE direction and low angle compression along the NE–SWdirection. The fault plane solutions also indicate thrusting with subordinate strike-slip motion along a NW–SE fault plane dipping moderately towards SW. Thecluster with the epicentre of the Kashmir earthquake (Mw 7.7 of October 2005)and its aftershocks forms a NW–SE elliptical cloud with isoseismal lines (Mahajanet al. 2006) oriented in the same direction, indicating a close fault–thrust
Figure 4. Seismo-geological map with location of fault plane solutions. Beach ball diagramsfrom the data in table 1 are presented in the right panel. Section lines for seismo-geologicalsections are shown. DT, Drang Thrust; SRT, Salt Range Thrust; JMT, Jwalamukhi Thrust;MBT, Main Boundary Thrust; MCT, Main Central Thrust; MF, Muzaffarabad Fault; ISZ,Indus–Tsangpo Suture Zone; MMT, Main Mantle Thrust; MFT, Main Frontal Thrust;MKT, Main Karakoram Thrust; RF/DS, Raikhot Fault/Diamer Shear; R–CS, Rupal–ChichiShear Zone.
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interaction between the left lateral strike-slip Muzaffarabad fault and the splays ofMBT. The seismotectonics around this area in relation to the recent Kashmirearthquake will be elaborated in the discussion section. The spatial cluster in theKohistan magmatic arc with two fault plane solutions (solutions 4 and 5 of figure4) along the same section line (figure 5(a)) is tectonically significant. We infer ahidden thrust plane (strike NW–SE dipping moderately towards NE) to beresponsible for generating the cluster. This out of sequence thrust or reactivationof an older thrust plane is seismically active today. However, existence of thisactive fault is not marked in the compiled regional tectonic map. This section alsoexplains that basement (Indian Plate) and sedimentary cover (accretionarycomplex) participated equally to generate seismicity (thick-skinned tectonics).The seismicity is more pronounced along the detachment plane present in the plateinterface.
Figure 5. Seismo-geological sections across the western syntaxis. Position of section lines ismarked in figure 4. Note the subducting Indian plates and overriding Eurasian plates in thesections to generate the imbricate structure. Note the out-of-sequence thrust in the Kohistansector in section X–X/. The thickening of the Indian Plate in sections (b) W–W/ and (c) Y–Y/ isnoteworthy. The fault plane solution numbers of Table 1 are marked in bold letters;earthquake magnitude is annotated within brackets. NP – Nanga Parbat.
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4.2.2 Nanga-Parbat syntaxis. The cluster of seismicity around the Nanga-Parbat syntaxis (box in the middle of figure 3) manifests that earthquakes aregenerated here from the Indian Plate. The earthquakes are clustered more in thewestern part along the seismically highly active Raikhot fault and the Diamershear zone rather than in the eastern part, where it shows diffused seismicity.Seismically, the MMT is almost passive. Two sections (Y–Y/ and W–W/), onealong the axial trace and the other along the profile section of the popupantiformal structure, are constructed (figure 5(b) and (c)) to define the crustalstructure of the Nanga-Parbat region. This zone has five fault plane solutions (Sl.No. 6, 14, 15, 18 and 19 in table 1). Out of these five solutions, 6 and 19 showstrike-slip motions along the N–S to NE–SW plane, whereas 14, 15, and 18 shownormal fault movements along the NE–SW-trending moderately SE-dippingplane. The solution no. 6 at 103 km depth with a pure strike-slip solution alongthe N–S plane with vertical dipping fault plane indicates a possible adjustment ofthe crust following rapid uplifting and mass flowage. Furthermore, to compensatethe rapid vertical uplift occurring in this zone, normal (gravity) faulting atshallow crustal level (*20 km) occurred as a part of crustal adjustment.Seismicity is defined by extremely active NNE–SSW-trending Raikhot fault andDiamir shear with strike-slip sense of movement in the western side (see thecluster in the western side of Nanga-Parbat in figure 3 as discussed), and acomparatively less active Rupal–Chichi shear zone of N–S trend towards the east(figure 4). The tensional axis is subhorizontal oriented in the NW–SE direction,whereas the compressional axes vary in plunge and orientations. The thickeningof the crust *75 km compared to the surrounding regions is worth mentioning.The fault plane solutions and concentration of earthquakes along the Raikhotfault support the hypothesis of upward-directed mass flowage, uparching, rapiderosion by contemporaneous rivers and tectonic aneurysm of the Indian plate(Zeitler et al. 2001, Koons et al. 2002 and references therein).
4.2.3 Lesser Himalayas and Karakoram. The seismicity around the eastern sideof the Western Himalayas (figure 3) has two seismic zones, one in the LesserHimalayas towards the south in the Jhelum re-entrant and another along theKarakoram sector in the north. The section (Z–Z/, figures 4 and 5(d)) along theLesser Himalayan and Karakoram was drawn across the Himalaya, Ladakh andKarakoram arc to define the tectonic movement along the eastern side of thewestern syntaxis in the Main Himalaya–Ladakh–Karakoram arc segment. Alongthis line, all known tectonic planes are active and manifested by seismicity ofvarious degrees. The Lesser Himalayan zone has three fault plane solutions (Sl.No. 2, 3 and 13). These solutions indicate thrusting along the plane striking NW–SE dipping at low angle towards the NE. Solutions 2 and 3 point outsouthwesterly movement of the MFT, whereas solution 13 attests the movementalong the MBT. The Karakoram sector along the section line towards the northhas five fault plane solutions (Sl. No. 7, 8, 9, 11 and 23). These solutions indicatethe strike-slip movement of the crust along the E–W to ESE–WNW striking planesdipping at moderate to high angle on either side of the Karakoram and AltynTagh Faults. This zone, by and large, demonstrates strike-slip trans-tensionaltectonism between the MKT and the Altyn Tagh Fault (ATF) in the Karakoramdomain where litho-units experience arc-parallel mass flowage towards the east(Seeber and Pecher 1998).
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5. Seismic zones and seismotectonic sections around the eastern syntaxis
5.1 General geology
The earthquake epicentres are plotted on a generalized seismo-geological map of theeastern syntaxis (figure 6), compiled from various published sources (GopendraKumar 1997, Dasgupta et al. 2000, Ding et al. 2001, Nandy 2001, Quanru et al.2006). The map demarcates six distinctive tectono-geological provinces, namely (a)Brahmaputra Valley with folded and thrusted subcrop sediments of Assam Tertiariesby Naga and Disang thrusts in the south; (b) Mishmi belt tectonites in the east; (c)Trans Himalayan Tethyan sediments with ophiolites of ISZ, Andean-type gneiss andmetamorphites (Gangdise pluton) in the north; (d) eastern Himalayan tectoniteswith MFT, MBT and MCT in the west; (e) a popup antiformal structure comprisingfolded and thrusted metamorphites with mobile high-grade rocks at the core (easternsyntaxis) near Namcha Barwa (Ding et al. 2001); and low-grade foldedmetamorphites and volcanic rocks of the Siang window in the middle. Forseismotectonic appraisal, three tectonic domains are chosen for their unique tectoniccharacters and seismicity: Mishmi belt, Eastern Himalayan tectonites and easternsyntaxis. The Mishmi belt comprises NW–SE trending high-grade rocks (Mishmi
Figure 6. Seismo-geological map of areas in and around the eastern syntaxis. Generalgeology, earthquake with magnitude variation, tectonic planes and zones of earthquakeclustering, location of fault plane solutions are shown. Beach ball diagrams from the data intable 2 are presented in the right panel. Section lines for seismo-geological sections are shown.MBT, Main Boundary Thrust; MCT, Main Central Thrust; ITS, Indus–Tsangpo Suture Zone;MFT, Main Frontal Thrust; SF, Siang Fracture; CT, Canyon Thrust; NLT, Namula Thrust;BTF, Bame Tuting Fault; NB, Namcha Barwa; D, Dibrugarh; DG, Dashing Gompa; N,Nijamghat; NL, North Lakhimpur.
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crystalline/Lohit Gneisses) in the frontal part; low-grade schist, serpentinite andlimestone of Tidding Group in the middle; and Lohit Granite–Granodiorite terraintowards the northeast (Nandy 2001). The eastern Himalayan tectonites containSiwalik molasse sediments in the south; lenses of Continental and MarineGondwana, structurally overlain by sequences of carbonates, gneisses, orthoquart-zites and low-grade schists with MBT in between; followed by patches of high-gradeschists and gneisses of Central crystallines separated from earlier units by the MCTtowards the north (Nandy 2001). The eastern Himalayan syntaxis comprises threemajor tectonic units: Gangdise magmatic belt (early Jurassic to Carboniferous meta-sedimentary and plutonic rocks), Namcha Barwa Group (Precambrian gneisses withhigh-pressure granulite lenses), and Yarlung Tsangpo shear zone (tectonic melangezone with exotic blocks of Namcha Barwa Group and Gangdise Pluton, basic andultramafic rocks with ophiolite sheets) (Quanru et al. 2006).
5.2 Seismo-geological sections
The earthquake epicentres and 17 well-constrained fault plane solutions compiledfrom published sources (table 2) are plotted on a compiled geological cum tectonicmap (figure 6). The plot brings out three visible spatial zones: Mishmi, NamchaBarwa and Eastern Himalaya. These zones are analysed further by drawing seismo-geological sections across them to unearth the crustal configuration. From thedistribution of earthquake epicentres and geological cum tectonic understanding ofthe terrain, four seismo-geological sections have been drawn along three tectonicdomains: R–R/ along Mishmi Himalaya (figure 7(a)), S–S/ and U–U/ along theNamcha–Barwa syntaxis (Figure 7(b) and (c)), and V–V/ along the easternHimalayan tectonites (Figure 7 (d)).
5.2.1 Mishmi. Earthquakes in Mishmi Himalaya along the zone between Pochufault and Mishmi thrust (figure 5) are analysed. The seismogeological section (R–R/)(figure 7(a)) defines the tectonic behaviour of two contrasting tectonogens ofMishmi Himalaya and rocks of the Eurasian/Tibetan Plate (Gangdise) fusedtogether in an imbricate zone. The earthquakes are concentrated at the interfacebetween them. Mishmi thrust acts as a floor thrust/detachment plane in thisimbricate zone. The great Assam earthquake (M 8.7) of 1950 with focal depth ofaround 40 km occurred along the plate interface between the Indian and EurasianPlates and has two contrasting fault plane solutions: strike-slip (Ben-Menahem et al.1974) and thrust (Chen and Molnar 1977). From fault plane solution constructedfrom amplitude inversion and aided by geological evidences, Armijo et al. (1989)suggested that right lateral movement on the Po Qu fault (Po Chu fault) generatedthe Assam earthquake. They also suggested the possible connection of AssamEarthquake with the Sagaing right-lateral strike-slip fault further east in Burma.They further argued that a discontinuation of ophiolite in the syntaxis zone is thegeological evidence of strike-slip movement on the faults that wrap around thesyntaxis, and presently the strike-slip motion accommodates the plate movement.These evidences led us to believe that this great earthquake occurred by a strike-slipmechanism and may not be categorized as a thrust event along the plane ofdetachment. The crustal thickness estimated in this region ranges from 50 to 65 km(Holt and Wallace 1990) and the Assam earthquake occurred at the lower crust or atplane of detachment placed between crust and upper mantle. Further, the terrain
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surrounding the study area towards east within eastern Tibet witness right lateralstrike-slip motion on N–S to NW–SE trending fault planes. Contemporary GPSmeasurements carried out by the Geological Survey of India towards north of Lohitthrust demonstrate consistent movement vectors directing towards south (DuttaGupta, T. 2010, personal communication). Buying this logic, a right lateral strike-slip mechanism of Assam earthquake is likely (figure 7(a)). The conclusion is furtherstrengthened by the occurrences of two earthquakes with strike-slip solutions(solutions 8 and 9 of figure 6) at a later date (1983, 1984). These events occurred dueto adjustment of lithosphere following the Great Assam Earthquake and alsoindicate right lateral strike-slip motion of the lithosphere north of Lohit Thrust. Theseismic activity is concentrated in the frontal part only within the zones betweenLohit and Mishmi thrusts, whereas the area east of them is characteristicallyaseismic.
5.2.2 Namcha–Barwa syntaxis. Earthquakes in the Namcha–Barwa syntaxis aremainly concentrated along two bounding strike slip fault planes, namely SiangFracture and Yiemla fault (figure 6). The sections along the Namcha–Barwa antiform
Figure 7. Seismo-geological sections across the eastern syntaxis. Position of section lines ismarked in figure 5. Note the imbricate structure generated by subducting Indian plates andoverriding Eurasian plates in the sections. The thickening of the Indian Plate in sections (b) S–S/ and (c) U–U/ is noteworthy. The fault plane solution numbers of table 2 are marked in boldletters; earthquake magnitude annotated within brackets.
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(S–S/ and U–U/, figure 7(b) and (c)) demonstrated the relationship between uparchingand thickening of the Indian Plate and seismic activity associated with the Namulaand Canyon thrusts. The movement of these two active thrust planes is well-supported by two strike-slip faults: Yiemla fault in the NW and Siang Fracture in theSE. The upward movement of this fault-bounded lithopackage contributes inuparching of the Indian Plate, formation of domal structure and thickening of thecrust. The brittle crust is available up to a depth of 50 km where Moho (MohorovicicDiscontinuity) has been uparched (Ren and Shen 2008). Low P and S wave velocitypatches in tomographic sections (Ren and Shen 2008) may be related to mass flowageof mobile rocks and creation of small magmatic pockets by possible decompressionalmelting. Po Chu fault has two fault plane solutions (nos. 5 and 16; figures 6 and 7(b)).Solution No. 16 shows the right lateral strike-slip motion along this fault. From thestudy of fault plane solutions, it is difficult to ascertain the relationship betweenearthquakes and their causative fault planes. It is therefore prudent to outline theoverall tectonism that operates in this part. Both thrust as well as strike-slipmovements are operative in equal proportion. Bounding strike-slip faults like Yiemlaand Siang Fractures are extremely active presently to generate bulk of the seismicity.The Canyon thrust marks the boundary between the Indian and Eurasian plates.
5.2.3 Eastern Himalaya. The earthquakes in eastern Himalayan tectonites areconcentrated in the area between MFT and MCT (figure 6). The V–V/ section (figure7(d)) along Arunachal Himalaya illustrates an imbricate structure in the Himalayancollisional front. Earthquake occurrences are more prominent in the lesserHimalayan domain predominantly by thrust movements along the MBT, MFTand sympathetic thrust planes. There are four fault plane solutions, namely 2, 3, 4and 13 (table 2; figures 5 and 6(d)). Fault plane solutions 2, 3 and 4 are associatedwith MBT and sympathetic thrust planes. The solutions show movement alongthrust planes trending E–W dipping *108 towards north. The thrust planeorientation changes to NE–SW with a higher dip of 268 directed towards NW(fault plane solution No. 13). MCT and ITS are aseismic in nature. This scenariochanges further west (not in the study area) where N–S trending strike-slip transversefaults instead of prominent Himalayan thrusts control the moderate level ofseismicity (Mukhopadhyay 1984, Kayal et al. 1993).
6. Discussion
Augusto Gansser (1993), in his seminal lecture given at the 8th HimalayaKarakorum Tibet Workshop on the topic ‘The Himalayas seen from Bhutan’,narrated his understanding on the cause and disposition of the syntaxes as:
The western (Kashmir) Himalayas are thrust from the NE with a particularlywide belt of Siwaliks. The overall encroachment of the border ranges on to thenorthwestern spur of the Indian plate is, however, considerably smallercompared to the very narrow northeastern spur of the Assam basin. Forhundreds of kilometers the respective border ranges have overthrusted thenarrow northeastern spur of the Indian plate, which at present seems tobecome more narrow and shallower at its E end. The East-Himalayan geologycorroborates the great mobility of the border ranges dominated by largeoverthrusts on an apparently stable and fixed Indian plate.
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The present authors feel the same way and made a comparison of seismotectonicbehaviour in the areas around the western and eastern syntaxes. Despite itscontrasting tectonic setting, the terminal ends of the Himalayas are characterizedby a similar type of compressional tectonism and resulting popup antiformalstructures. The Nanga-Parbat antiform in the western syntaxis and Namcha Barwaantiform in the eastern syntaxis are the structural and topographic expression ofarc-parallel shortening at a rate of *12 mm/year that compensates for arc-parallelextension in Southern Tibet (Seeber and Pecher 1998). On the basis of the presentstudy, 3D cartoons – one along the western syntaxis (figure 8(a)) and another
Figure 8. Schematic 3D cartoons for two syntaxes. (a) Western syntaxis (modified afterSeeber and Pecher 1998). (b) Eastern syntaxis. Note the folding of the thrust planes in both thesyntaxes. SRT, Salt Range Thrust; MMT, Main Mantle Thrust; JMT, Jwalamukhi Thrust;JF, Jhelum Fault; MBT, Main Boundary Thrust; MFT, Main Frontal Thrust; RF, RaikhotFault; RCS, Rupal–Chichi Shear; MKT, Main Karakoram Thrust; MF, Muzaffarabad Fault;NP, Nanga Parbat; NB, Namcha Barwa; CT, Canyon Thrust; NLT, Namula Thrust; ITS,Indus–Tsangpo Suture; YF, Yiemla Fault; SF, Siang Fracture; MT, Mishmi Thrust; LT,Lohit Thrust.
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along the eastern syntaxis (figure 8(b)) have been drawn. The popup antiformalstructures on both these syntaxial ends of the Himalayas have folded the thrustplanes and resulted in shear zones on the flanks. Seismically active bounding faults/shear zones like Raikhot fault/Diamer Shear and Rupal–Chichi shear in thewestern syntaxis and Yiemla Fault and Siang Fracture in the eastern syntaxis withprimarily strike-slip movements have modified the existing fold–thrust systems. Thepopup structures will be modified by erosion-controlled tectonic upliftment(tectonic aneurysm) along the bounded shear zones developed at the flanks ofthese antiforms.
The crustal structure of the western syntaxis is complex. Pegler and Das (1998)have determined the shape of the seismic zone in the Pamir–Hindukush region (30–428 N; 68–788 E) with relocated hypocentral data. The study pointed out that theseismic zone under the Hindukush follows the classical pattern of subducting slabcontrolled by gravity. The Pamir region follows the trend of contorted seismic zoneslab deformation that indicates that the slab is deformed due to flow of the uppermantle. Such a structure is prominent in all our seismic sections (figure 5) drawnacross the western syntaxis. Further, the occurrence of the Kashmir earthquake (Mw7.7 of October 2005) has unfolded the seismogenic characters and stress releasepattern of this region, particularly in the part of the Kashmir–Hazara syntaxis. TheKashmir earthquake occurred in the Indus–Kohistan seismic zone accompanied by arupture length of 75 km that cut across the Kashmir–Hazara syntaxis andreactivated the Muzaffarabad fault (Avouac et al. 2006) with a net-slip of 5.4 malong the fault strand (Kondo et al. 2008). Towards the north, the rupture coincideswith the MBT and abruptly terminates at the hairpin turn of the MBT showingstrong structural control. Focal depths of the aftershocks indicate that the seismicactivity is confined to a narrow depth zone between 5 and 10 km. This distribution isindicative of extension of the Indus–Kohistan seismic zone in the Hazara–Kashmirsyntaxial area and activation of more than one fault (MonaLisa 2009). The columbstress mapping indicates increase of stress in the northwest of the rupture along thetrend of the Indus–Kohistan seismic zone (Parsons et al. 2006). A similar stressincrease along the southeast of the rupture near Kashmir Basin indicates seismicvulnerability of the Kashmir basin, where faults participated in the large earthquakesof 1555 and 1885 (Bilham 2004). Similarly, along the Muzaffarabad fault strandearthquakes that occurred during 500 and 2200 yr. B.P. were also identified (Kondoet al. 2008). These indicate the recurrence pattern of large earthquakes in this zone.Moreover, the high gravity value in the epicentral block indicates thrustingaccompanied by mass movement of high-density rocks along the syntaxial bend ofthe MBT (Tiwari et al. 2009). The tectonic loading of the high-density Muzaffarabadwedge thrust between the wedge top and the descending Indian lithosphere coupledwith continued flexure tectonics and block rotation provoked this earthquake (Khanet al. 2010). They also opined that the western limb of the buckled unit (containingboth competent and incompetent rocks) of the Kashmir–Hazara syntaxis gave rise tothe development of new thrust and associated oblique slip in the inner arc of thecompetent rock unit spawned the earthquake. Thus formation and reactivation ofnew faults/thrusts in this region is common, spawning big earthquakes today andalso in the historical past. Accordingly, the similarity of genesis between the Kangraearthquake of 1905 which occurred further east of the study area and the Kashmirearthquake of 2005 by reactivation of local fault planes are speculated (Hussain et al.2009).
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The tectonic aneurysm (Zeitler et al. 2001) in the western syntaxis arises from thegeodynamic interaction between localized erosion, topographic stresses, rock uplift,thermal weakening of lithosphere and deformation (Koons et al. 2002). In NangaParbat, the convected materials from the core of the massif undergo rapid upliftmentand decompression as a result of rapid erosional exhumation. The extreme erosion isgenerated by the hyperactive Indus River that has generated deep-cut valleys aroundthe syntaxis. The uparching and thickening of the Indian plate as shown in thesection (figure 5(b)) further corroborated the results generated by the micro-seismicsurveys conducted by Meltzer et al. (2001). They have also found a bow-like upwardshape of seismicity following the antiformal arch at a shallow crustal level. The MTsurveys (Park and Mackie 1997, 2000) conducted in this region have also pointed outa resistive zone at around 50 km depth below the massif, and another zone situatedin top 1 km below the surface limit is quite conductive. Similarly, the zone aroundthe Raikhot fault from the surface to a depth of 10 km is also conductive. Theresistive zone has been interpreted as dehydrated zone with insufficient inter-connected fluid phase, whereas the conductive zone is hydrated by percolating waterfrom the top and possible pore pressure perturbations in interconnected fluid zones.Seismic tomography conducted in this region has predicted a lower-velocity lozengeof crust extended at a depth below the core of the massif and indicates rapidexhumation, high thermal gradient, crustal fluid flow and pockets of decompressionmelting, and subsequent generation of young metamorphic and igneous ages of therocks (Meltzer et al. 2001, Zeitler et al. 2001, Zeitler and Chamberlain 2001). Zeitlerand Chamberlain (2001) have also suggested that the present movement of NangaParbat massif is developed above a dry, hot and weak crust.
Similarly, the Namcha Barwa massif in the eastern syntaxis also has dynamicinteraction guided by local erosion by Tsangpo/Siang river, rock uplift, thermalweakening of lithosphere and deformation (Finnegan et al. 2008). This is similar tothe dynamism of the Indian Plate to form uparch antiformal domal structure as hasbeen noticed in the Nanga Parbat syntaxis. The thickening of the crust is alsonoteworthy in sections (figures 7(b) and (c)). The exhumation is caused by erosionalong the Siang River with crustal scale folding at a rate of*10 mm/year (Burg et al.1998). Tomographic surveys conducted by Ren and Shen (2008) have indicated lowP and S wave velocity patches within the uplifted rocks of this syntaxis. Thesepatches may be correlated with mass flowage of mobile rocks and creation of smallmagmatic pockets by possible decompressional melting. Geological (Tapponnieret al. 2001) and geodetic studies (Zhang et al. 2004) indicate that the crustal blockmoves clockwise around the eastern syntaxis. This mass flowage (crustal flow) ischaracterized by a reduced seismic velocity and elevated electrical conductivity. Ineastern Tibet, magneto-telluric data imaged two major zones/channels of highelectrical conductivity at a depth of 20–40 km. This indicates high fluid content in themid crust. The fluid could be derived from prograde metamorphism in a thickenedcrust or from under-plating. One of the conductors terminates near the Canyonthrust in the Yarlung–Zangbo suture (also known as ITS), indicating a shear zone(Bai et al. 2010). This shearing in the eastern part of the Himalayas may be requiredto maintain regions with interconnected fluid, lower the crustal strength and permitrapid deformation and mass flowage. Crustal thickening and deformation alongeastern margin mark the present-day tectonics of the Tibetan Plateau, except nearthe eastern syntaxis of the Himalaya, where rapid clockwise flow around thesyntaxis, not rigid-body movement, occurs (Zhang et al. 2004).
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7. Conclusion
The seismicity maps and their correlation to crustal scale thrusts and faults for thewestern and eastern syntaxes are presented in this study. Zones of earthquakeoccurrences in different sectors were studied in maps and depth sections. These havebrought out the imbricate nature of the plate boundaries where the sedimentarywedge and Eurasian plate override the subducted Indian plate according the knowntectonic model of the Himalayas.
The following inferences can be drawn from this study. (a) The earthquake clusterwith epicentre of the Kashmir earthquake (Mw 7.7 of October 2005) and theiraftershocks in the western syntaxis show a close fault–thrust interaction between left-lateral strike-slip Muzaffarabad fault and splays of MBT. (b) The spatial cluster inthe Kohistan arc is related to a hidden out-of-sequence thrust or reactivation of anolder thrust. (c) The seismicity in Nanga Parbat cluster is controlled by seismicactivities related to the Raikhot fault and Diamir Shear with strike-slip sense ofmovement present in the western side of the antiform. The thickening of Indian platein this zone is noteworthy. (d) The Kashmir earthquake (Mw 7.7 of October 2005) isspawned due to interaction of the MBT and the Muzaffarabad fault. The tectonicloading, coupled with continued flexure tectonics and block rotation, development ofnew thrust and associated oblique slip in the inner arc of the competent rock unit ofKashmir–Hazara syntaxis provoked this earthquake (Khan et al. 2010). (e) Theseismo-geological section along Mishmi Himalaya in the eastern syntaxis hasbrought out the imbricate nature of the Himalaya and Gangdise lithopackages withMishmi thrust as a major plane of detachment. The Assam earthquake (Mw 8.7,1950) occurred along the plate interface between the subducted Indian andoverriding the Eurasian Plate by strike-slip mechanism. (f) The Namcha–Barwasyntaxis indicates fault-bound upliftment of the Indian Plate to form a domalstructure. The canyon thrust marks the boundary between the Indian and Eurasianplates. The upliftment is guided by mass flowage, tectonic aneurysm and formationof magmatic pockets by possible decompressional melting. (g) The section alongArunachal Himalaya has brought out the imbricate structure of the Himalayanrocks with MBT being the active seismogenic surface at present. (h) Clockwise blockrotation of Indian plates both in the western and eastern syntaxes are inferred. (i)The three-dimensional structures brought out by the present study have resulted inan understanding that the seismicity in the western and eastern syntaxes isprimarily controlled by the strike-slip faults that present in the flanks of the popupantiforms.
Acknowledgements
We express our thanks to Prof. R.P. Singh, Editor in Chief, for his thoughtfulcomments on an earlier version of the manuscript. We are grateful to the eruditeanonymous reviewers whose suggestions have helped to improve the scientificcontent of the paper immensely.
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