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Collision Induced Deformation of the Trans Himalayan Lohit Batholith, Arunachal Pradesh, India

T. K. GOSWAMI Department of Applied Geology, Dibrugarh University, Dibrugarh -786004, Email: [email protected]

Abstract

The Trans Himalayan Lohit Batholith in the eastern part of the Eastern Himalayan Syntaxis runs for about 250 kms from the Noa Dihing Valley in the southeastern part through upstream segments of Lohit and Dibang rivers and Tuting granite in the northeast probably represents the northwestern extremity of the batholith. The batholith is thrusted over the Tuting –Tidding Suture zone which represents imbricately faulted litho packages of the ophiolitic sequence and are themselves thrusted over the crystallines of the Himalayan Belt in the southwest. The Batholith represents multiple phases of intrusion history. The earlier phase of gabbro-quartz diorite sequence is followed by syn- collisional intrusive phase of tonalities or trondhjemites. The post collisional oblique convergence is probably accommodated by large scale strike slip faults in the zone of transpression and is coeval with the copious volume of leucogranite intrusion in to the batholith near Thrust. The imprints of collision induced deformation

are well preserved in the Batholith. Evidences of at least four phase of deformation (D1to D4) in the batholith represent phases of pre, syn and post collision and subsequent convergence of Indian plate with the neighboring Asian and Burmese plate. On the other hand, shear related early and late folds, asymmetry of the fold patterns and offset of veins help us to understand the sense and localization of shear in the batholith. The gradual rotation of the fold hinge lines and subsequent parallelism with the shear direction, prevalence of shear related asymmetry could be used as shear sense indicator. Two distinct sense of movement (top- to- SE and top- to- SW) may be related to an initial compression, which becomes transpressional with the rotation of the compression axis followed by the development of a major dextral strike slip fault in the region.

Keywords: Lohit batholith, deformation, transpression, stretching lineations, dextral shearing

INTRODUCTION

The spatio-temporal relationship between the granite magmatism and the synchronous deformation and metamorphism of the host rock and the magmatic body during its pre, syn or post emplacement phases is a well recognized phenomenon. The dilatational space for the emplacement of huge batholithic body is another important factor to be studied specially during the time of active deformation. A huge zone providing space for ascent and emplacement of garnitoids in a compressional or extensional setting do not seem to be a rule followed always. 20 T. K. GOSWAMI

Large shear zones in a transpressional regime or thrust zones may be equally capable of providing the space for ascent and emplacement and simultaneous deformation of the granitic body and the host (Druguet and Hutton, 1998). The Lohit plutonic complex consists of multiple igneous intrusion of batholithic dimension and thrusted over the Tidding Group of rocks along a NE dipping thrust known as the Lohit thrust (Nandy, 1976). The serpentinites in the Payapani and Mompani in the Lohit valley and Endoline Nala in the Dibang valley demonstrates the tectonic imbrications of the ultrabasic bodies. The batholith is divided in to a western belt and an eastern part separated by another regional thrust which is named as Walong Thrust (Gururajan and Chowdhury, 2007).The batholith is about 100km in thickness in the Lohit section and runs in the NW-SE from Noa Dihing valley to Dibang valley for about 250 km and further NW direction. Tuting granite and Tuting gneisses may represent the northwestern continuation of the Lohit batholith. The northwestern trend of the Mishmi crystallines, Tidding suture and Lohit Plutonic Complex is truncated by the NNE- SSW trending eastern Himalayan lithotectonic units along the Siang fracture (Nandy, 1980). Burg et al. (1998) were of the opinion that the Trans-Himalayan belt around Namche Barwe may be linked geologically with the Lohit plutonic complex. As the batholith contains signatures of different phases of deformation along with the compositional continuum from southwest to northeast, an interpretation of the kinematics in the light of collision tectonics is important and urgent. An attempt is made to describe and correlate the field structures and structural analysis of the fabrics with the possible orientation of the stress field in the zone of two river sections.

GEOLOGICAL AND STRUCTURAL SETTING

The area lying to the easternmost part of the Himalaya show a bend in their regional strike from ENE-WSW to NW-SE known as the eastern syntaxis. Eastern syntaxis represents a major antiformal structure known as the Siang antiform (Fig.1). The study area, in the eastern part of the syntaxis in the eastern Arunachal Pradesh, is a part of the Lohit plutonic complex and is considered as the SE continuation of the Gangdese belt. The geology of the eastern syntaxis is fairly discussed (Gansser, 1964; Karunakaran, 1974; Nandy 1980, 2001; Jain et al. 1974; Thakur and Jain, 1975; Acharyya, 1987 and Singh, 1993). Recently Gururajan and Chowdhury (2007, 2009) discussed the geochemistry of the Lohit Plutonic Complex. However, deformation of the batholith needs a more detailed interpretation. Moreover no attempt has been made to correlate the deformation to India-Asia collision tectonics. Signatures of deformation in the batholith indicate changing stress fields of pre to post collisional episodes between India and Asia, probably extended from the late cretaceous to the mid- tertiary geodynamic evolution of the region. One of the outstanding features of the Mishmi block of eastern Arunachal Pradesh is the wide compositional spectrum of Lohit batholith. In the Lohit and Dibang valley the Lohit batholith represents continental arc magmatism and is thrusted over the Tuting-Tidding Suture (Acharyya, 1986) along the Lohit Thrust (Nandy, 1976). The huge batholithic body is foliated and the general trend of the foliation is NW-SE with a steep dip towards NE. The COLLISION INDUCED DEFORMATION OF THE TRANS HIMALAYAN LOHIT BATHOLITH, 21 ARUNACHAL PRADESH, INDIA

Fig.1. Geological map of Arunachal Pradesh modified from Singh 1990. 1. Siwalik Belt 2. Abor volcanics 3.Gondwana belt 4.Dedga Menga Belt. 5. Belt 6.Sela Belt 7. Tuting –Tidding Belt 8.Tourmaline bearing leucogranite 9.Mishmi belt 10. Fault 11. Thrust. The extent of the investigation in the two river sections are bordered. Tuting-Tidding belt is considered as the southeast continuation of the Indus- Tsangpo Suture Zone(Thakur and Jain, 1975; Mitchel, 1981; Acharyya, 1987; Singh and Chowdhury, 1990). The Lohit batholith represents a continuum from calc- alkaline metaluminous diorite to quartz- diorite in the western part through a middle zone of metaluminous trondhjemite to peraluminous leucogranites in the eastern part. Gabbro occurs as enclaves in the quartz diorite. Amphibolite occur in the middle zone with tabular sheet like bodies of trondhjemite in the Lohit valley near Chingawanti, while in the Dibang valley they occur more eastern part near Angolian. Garnetiferous- granodiorite gneiss and sillimanite gneiss near Etalin and Yasong probably represent the host rock lithology where they show gneissic banding. Unmapable bodies of crystalline limestone and dykes of aplite and pegmatite are prevalent at many places. Leucogranites are occasionally tourmaline bearing. The diorites and quartz -diorites of the southwestern part are composed of mainly hornblende, plagioclase, quartz and biotite with sphene, apatite,zircon as the accessories. The leucogranites in the eastern part of the batholith are dominantly composed of quartz, k- feldspar, plagioclase with or without biotites. The batholith is metaluminous in the south western part while in the north eastern part it becomes peraluminous. The detail geochemistry of the batholiths is discussed in another paper. The batholith is thrusted over the suture zone rocks near Paya in the Lohit valley and at Endolene Nala in the Dibang valley. Moreover, the eastern part of the batholith has a thrust contact with the western part and this contact could be seen at Yasong in the Lohit valley. Thrust related shear of the batholith is observed at several places. This thrust is named as Walong Thrust (Gururajan and Chowdhury, 2007) with a NW-SE extension (Fig.2). 22 T. K. GOSWAMI

Fig.2. Simplified geological map of area (modified after Mishra, 2009; Gururajan and Chowdhury, 2007). The investigation is carried out in the Lohit and Dibang valleys beyond Tidding Thrust. 1. Sewak Group 2. Lalpani Group 3. Mayodiya Group 4.Tidding Formation 5. Lohit Batholith western part :(Quartz diorite, gabbro, granodiorite, trondhjemite, amphibolites, crystalline limestone, pegmatite, aplite and quartz veins) 6. Lohit batholith eastern part: Quartz diorite, garnetiferous- granodiorite- gneiss, biotite granite, leucogranite, aplite and quartz veins) 7. Alluvium 8. Thrust 9. Foliation.

DEFORMATION OF THE BATHOLITH

Large pluton or batholith often helps in determining the time intervened between two episodes of deformation (Davis and Reynolds, 1996, p.487). They may be emplaced before, during or after the deformation and metamorphism of the host rock and accordingly they are called as pre, syn or post kinematic. The prekinematic batholith have their foliations and lineations parallel to the country rock it intrudes indicating the fact that the intrusive and the country rock share the same phase of deformation. The degree of prekinematic deformation of the intrusive depends upon the factors like ductility contrasts or intensity of the local or regional strain. A synkinematic intrusion should have evidences of (1) intrusion of melt into synchronously developing deformed structures in the host rock (2) deformed early phases crosscut by late phases and (3) a geometric coordination between the primary magmatic fabric and deformation of the country rock (Paterson and Tobish, 1992). A postkinematic intrusion lacks the fabrics of the deformed country rock rather they may contain some inclusions of previously deformed wall rocks. Field investigations carried out in the Lohit and Dibang valleys demonstrates that Lohit batholith preserves the imprints of at least four phases of deformation. This is based on the geometry of folds, associated foliation planes and orientation of lineations. The earliest phase COLLISION INDUCED DEFORMATION OF THE TRANS HIMALAYAN LOHIT BATHOLITH, 23 ARUNACHAL PRADESH, INDIA

Fig.3. Structural elements in the Lohit batholith in (A) Dibang and (B) Lohit valley. The L1 and L2 subhorizontal lineations are shown plunging to ESE and WNW directions.

D1 responsible for generation of tight isoclinal folds (F1) and pervasive foliation S1. The second generation of deformation (D2) has resulted in the formation of open upright folds (F2), which are mesoscopic to regional in nature and are coaxial with F1. The third phase of deformation

(D3) has produced broad warp type of folds (F3) and NNE trending cross fractures. The last phase of deformation (D4) is responsible for the formation of lots of faulting and shearing in the batholith. Although both dextral and sinistral shearing in the batholith is observed, the dextral shearing in the batholith appears to be more pronounced and might be associated with the syn to post- tectonic compression that has affected the batholith. The phases of deformation are discussed briefly in the following section.

The tight isoclinal folds F1 are observed in the gabbro enclaves as well as in the quartz diorites. The F1 fold axis trend NNW with a moderate dip. The S1 foliation trends NNW-SSE and dips moderately towards NE (Figs.4.B, C). The F1 fold axis lineation L1 dips to the

NNW. The F1 folds have thicker nose and tightly appressed limbs, which are sheared out at places (Fig.5C). The second deformation is most widespread and probably coeval with the high tem- perature metamorphism of the host rock. The open asymmetrical folds are observed in the meta diorites, gabbro enclaves in the metadiorites and also in the quartz diorites. The F2 fold axis plunges moderately to the NW to NNW (Figs. 4A, B; 5.A, B). The S2 foliations are axial planar to F2 and most pervasive in the batholith, which dip at low angle to the SW direction. 24 T. K. GOSWAMI

Fig.4. Structures from the Lohit batholiths in the Lohit Valley. (A) South east directed dextral shearing near Nara. F1 rootless intrafolial folds refolded to F2 plunges gently to NE. (B) sheared basic lenses in quartz diorite. L2 fold axis lineation plunges steeply to SE direction. (C) Dextrally sheared quartz vein. The host is partly mylonitised. (D) Basic lenses and the hosts are folded and banded in appearance. Layer normal fractures demonstrate post tectonic brittle deformation. (E) Basic enclaves sisnistrally sheared. Shear related F1 and F2 folds are coaxial. (F) Walong Thrust zone. Brittle faulting reactivated with rotation of the down dip stretching lineation to sub horizontal.

The stereoplots for the F2 fold axis lineation shows maxima towards the NW segment. F2 folds show Class 2 geometry (Ramsay, 1967) on profile. It is observed that F1 and F2 folds are coaxial in nature.

The third phase of deformation has resulted broad warp type of F3 folds with fold axis plunging either to NNW to NNE. S3 foliation is infrequently developed and has a strike of NNW-SSE with low dip to the NNE direction. COLLISION INDUCED DEFORMATION OF THE TRANS HIMALAYAN LOHIT BATHOLITH, 25 ARUNACHAL PRADESH, INDIA

Fig.5. Structures in the Lohit batholith in the Lohit valley. (A) Conjugate shear in the batholith. The basic enclaves reactivated by the late SE directed dextral shearing. (B) Down dip stretching lineation boudinaged. Shear related tight isoclinal folds plunge moderately to SE direction. (C) Top- to- SW directed dextral shearing near Yasong Bridge. (D) Conjugate sense of shear and down dip stretching lineation. The basic enclaves mylonitised. (E) & (F) Reactivation of the down dip stretching lineation to sub horizontal L1 lineation plunging gently to NE.

It is observed that in eastern part of the Lohit and Dibang valley the F3 folds are replaced by coplanar NW-SE trending dextral shear zones.The stretching lineations in the shear zones become sub vertical, associated with vertical foliations trending NW-SE. Therefore a spatial clockwise rotation of structures from NW or NNW to SE is observed specially along the

Walong Thrust zone. Therefore the D2 deformation is corelatable with a compression dominated transpression which has changed to a wrench dominated transpression. This event demonstrates a separate phase of deformation. 26 T. K. GOSWAMI

Fig.6. Structures in the Lohit batholith in the Dibang valley. (A) SE directed dextral shearing in the diorite near Amboli. The shear related F2 folds show curvilinearity of the fold axis. (B) Isoclinal F2 folds with limbs asymmetrically rotated and dismembered. (C) Garnetiferous- granodiorite gneiss shows pretectonic deformation in the basic enclave near Etalin. (D) Southwest directed dextral shearing in the amphibolites and trondhjemite interlayering. Asymmetric boudins in the trondhjemite sheet indicate shear related extension. (E) F2 fold axis in the basic enclave plunging gently to NNW. (F) Subhorizontal

F2 fold axis plunging to SE direction in the banded gneiss.

STRUCTURAL ANALYSIS

The field structural relations studied in the Lohit batholith both in Lohit and Dibang

Valley demonstrates that the batholith is deformed and the planar form surface S1 is developed. The dominant foliation trends NW-SE with moderate to steep dip towards NE (Fig.3A,B). Whether this fabric is prekinematic and associated with the deformation and metamorphism of the host rock is yet to be confirmed. The L1fold axis plunges moderately to NW or SE direction. The most dominant deformation of the batholith is responsible for the formation of the S2 syntectonic foliation trending NNW-SSE. The L2 fold axis plunges moderately due NE. The F2 folds on profile have Class-1C and Class- COLLISION INDUCED DEFORMATION OF THE TRANS HIMALAYAN LOHIT BATHOLITH, 27 ARUNACHAL PRADESH, INDIA

2 geometry. The asymmetric isoclinal F2 folds are parallel to the shear directions and have highly attenuated and sheared out limbs. The F3 folds are also related to the shear in the batholith and demonstrates a synkinematic episode of ductility for transposing limbs of F2 folds to open asymmetric F3 folds. In the Walong thrust zone the top- to- SW directed asymmetric shear folds are developed (Gururajan and Chowdhury, 2003). The down dip stretching lineations are frequently observed in the fault zone with a very high plunge to the SE direction. The stretching lineations become boudinaged and dismembered. The gradual rotation of these stretching lineations to the subhorizontal lineation with gentle plunge to the ENE to NE direction is associated with the reactivation of the Walong Thrust fault to dextral strike slip fault. Therefore thrusting and reactivated strike slip shifting of the thrust zone at Walong are associated with the fourth phase of deformation of the batholith (Fig.3). Thus, batholith preserved the post kinematic fabrics related to post India-Asia collisional tectonics. The Walong Thrust represents a broad shear zone covering both Lohit and Dibang valley demonstrating a change of the stress field from compression dominated transpression to the wrench dominated transpression. The down dip stretching lineations in the pure shear reactivation of the wrench dominated transpression is definitely a late phenomenon which has further transposed the down dip lineation to subhorizontal nature.

SHEAR RELATED STRUCTURES

Shear related structures are best demonstrated in the Walong Thrust zone. Shear related folds may predate, contemporaneous or post date the shearing event (Carreras et al. 2005). The asymmetry in the fold pattern could be regarded as a kinematic indicator (Simpson and Schmidt, 1983; Choukroune et al. 1987; Hanmar and Passchier, 1991; Passchier and Trouw, 1996, p.110). The shear related folds of the present area could be divided into two types: 1) the shear related early folds and (2) shear related late folds (Figs.4.A, B, C; 5.B, C; 6.F). Moreover, the passive shearing of the pre-existing folds is also observed. Due to the passive shearing the pre-existing folds might have developed tight folds and may become progressively unfolded with the development of sigmoidal structures with ‘S’ or ‘Z’ asymmetries. Fold hinge line rotated towards the shear direction. On the other hand, despite the initial orientation of the pre-existing folds they will come to lie in close parallelism with the shear direction. In Fig.6D, the rigidly folded amphibolite layer is preserved in pinch and swell structure in the shear zone interior. Shear related early folds present a continuous spectrum from sigmoidal shaped deflection (Fig.5 E &F) and on the other end shearing of the pre existing surfaces lead to the growth instabilities which resulted in compression (folds/crenulations) or extensional structures (boudins). Late shear folds developed on the surfaces closely parallel to the shear zone boundary and have affected the newly formed foliation. Synthetic folds are common. The prevalence of shear related asymmetry could be used as the shear sense indicator (Carreras et al. 2005). In the present area dominant top- to- SE movement is observed whereas top-to-SW movement is seen at places. The two distinct shear direction may be related to an initial 28 T. K. GOSWAMI compression-related transpression, which become wrench dominated transpression followed by the development of a major dextral strike slip fault in the region. There is a remarkable parallelism between the predominant orientation of the lensoid magmatic bodies and the dykes and the trend of the axial planes of the folds developed synmagmatically. This may be due to the fact that the rotation of the short limb of the folds led to layer perpendicular stretching parallel to the axial planes and the dilatation holes are produced. As a result some of the dykes appear to be formed in the array of boudins with the continuity of the layers in the inter-boudinal zones can be readily observed (Fig.6D).

STRUCTURAL MODELING

It is common that stretching lineations are associated with horizontal simple shear movements. Therefore stretching lineaions are generally parallel to the thrust transport direction. However, if the simple shear is associated with transpressional shear zone, the horizontal lineations will become sub vertical to vertical on a co-planar sub vertical or vertical foliation associated with pure shear. Further dextral shearing may produce sigmoidal foliation planes, Z-folds and asymmetric pressure shadows on the stretching lineation on a horizontal plane (Fig. 5.B-F). A model of Tikoff and Greene (1997) (Fig.7) is the basis for the model proposed for the Walong thrust zone for the formation of the subvertical down- dip stretching lineation ( Fig.7). The reactivation of the Walong Thrust as dextral strike slip fault or a wrench dominated transpressional shear zone produce horizontal lineation on a simple shear transpression are illustrated in the photographs (Figs.4F, and 5A) (Robin and Crudon, 1994).

CONCLUSIONS

• Lohit batholith preserves the imprints of deformation developed during the pre, syn and post collisional phases between the Indian and Tibetian plate. The pre and syn magmatic deformation of earlier basic phases could be studied from the basic enclaves in the later intrusives. • The garnetiferous -granodiorite gneiss and garnetiferous sillimanite gneiss in the northeastern part of the batholith indicates that the grade of metamorphism in the host rock increases from south west to the north east. • The early north south compression become transpresional and accordingly three generations of folding in the batholiths are observed which also indicate synmgamatic

deformation up to the D3 phase. This is followed by a ductile-brittle phase and compression dominated transpresion become wrench dominated. These changes are demonstrated by rotation of the down dip stretching lineation in the batholith to subhorizontal. • The rotation of the stretching lineation from subhorizontal to vertical down dip could be explained by model proposed where the dextral shear in a simple shear dominated transpression rotates the horizontal lineation to vertical in a pure shear domain. The COLLISION INDUCED DEFORMATION OF THE TRANS HIMALAYAN LOHIT BATHOLITH, 29 ARUNACHAL PRADESH, INDIA

rotation of the compression direction should be related to the clockwise rotation of the Indian plate with the development of large scale dextral strike slip faults in the region. The reactivation of the Walong Thrust as dextral strike slip fault may be triggered by Saggaing fault in the eastern Myanmar and it is possible that Walong Thrust is a splay of the Saggaing fault.

Fig.7. Model for down dip stretching lineation in a pure shear wrench dominated transpression. Basis: Tikoff and Greene, 1997. (a) and (b). XY foliation plane in both horizontal and vertical view. X becomes less than Y and the ellipsoid is shown only with the Y direction. With further rotation X become equal to Y in the XY plane and the ellipticity of the ellipse reduces. (c) The horizontal stretching lineation in a simple shear domain parallel to the long axis of the finite strain ellipsoid in the direction of tectonic transport. The dextral shear component is shown by small half arrows while the movement direction is shown by the long black arrows. (d) Wrench dominated part: the â angle increases and the rotation stretching lineations. (e) X is parallel to the vertical stretching lineation and orthogonal to the thrust transport direction.

· ACKNOWLEDGEMENTS

The author is grateful to Department of Science and Technology, New , for financial assistance (Project No.ESS/16/242/2005/SIANG-LOHIT/06). The author sincerely thanks Prof. A.K. Jain, Dept. of Earth Sciences, I.I.T., Roorkee for his comments and suggestions which helped improvement of the manuscript considerably. 30 T. K. GOSWAMI

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