Geomorphic evolution of Medziphema intermontane basin and ...

Zeitschrift für Geomorphologie Vol. 55,2, 247–265 Stuttgart, June 2011

Article

Geomorphic evolution of Medziphema intermontane basin and Quaternary deformation in the schuppen belt, Nagaland, NE India Imtiwapang Aier, Khayingshing Luirei, S. S. Bhakuni, Glenn T. Thong, and Girish C. Kothyari with 7 figures and 1 table

Summary. The Medziphema intermontane basin developed in the schuppen belt of Nagaland, Northeast India, has been studied to understand its tectonic and geomorphic evolution on the basis of field evidence and morphometric parameters. Major part of the basin is covered with aggradational landforms such as fan and terrace deposits. The development of this linear basin within the schuppen belt is the result of initiation of displacement of Paleogene rocks over the Neogene rocks along the Sanis-Chongliyimsen Thrust during Late Miocene and later displacement of Neogene over the Sub-Recent to Recent sediments along the Naga Thrust after the deposition of the Dihing sediments during the Lower Pleistocene. The schuppen belt is a distinct tectono-geomorphic unit, 20–25 km in width and ~ 400 km in length that is truncated at the northeast by the Mishmi Thrust and in the southwest by the Dauki Fault. The Medziphema intermontane basin, 16.5 km in length and 11 km in width, is one of the most accessible valleys of the Naga-Patkai-Mizo Hills. Late stage tectonic activities are imprinted in the Quaternary fluvial deposits in the form of truncation of terraces and tilting of fan and terrace deposits. Most of the rivers and streams draining into the Neogene hills are structurally controlled. Lateral displacement of rivers has been attributed to active strike-slip faults resulting in the formation of shutter ridges. Thrusting of the Surma sediments over the Quaternary Brahmaputra alluvium has resulted in narrowing of river channels at their exits from mountains. Strath terraces, suggesting tectonic uplift in the region, are observed along river sections within the intermontane basin.

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Introduction

The tectonic framework of Northeast India and the adjoining regions evolved due to convergence of the Indian, Eurasian and Sunda plates (Le Fort 1975, Molnar & Tapponnier 1975, Tapponnier et al. 1986), which began during the Alpine orogenesis. The convergence of three plates is responsible for the development of a tectonically complicated structural framework. The N-S compression of the IndianEurasian plates and anti-clockwise rotation of the Indian plate affect the western segment of the Indo-Myanmar Ranges (IMR) resulting in subduction of the Indian lithosphere below the Myanmar plate (Mitchell & Mckerrow 1975, Verma et al. 1976, Gupta et al. 1984, Mukhopadhyay & Dasgupta 1988, Ni et al. 1989). The subduction of Indian lithosphere is still continuing (Nandy 1976, Verma 1985, © 2011 Gebr. Borntraeger Verlagsbuchhandlung, Stuttgart, Germany

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Bhattacharjee 1991). This has led to major seismic events in this region and its surroundings. The region has experienced more than 1,600 earthquakes of magnitude greater than 4.5 since 1968 (Kent & Dasgupta 2004). The Shillong earthquake of 1897 and the Great Assam earthquake of 1950 (Oldham 1898, Khattri et al. 1983) are classic examples of major earthquakes. Present day tectonic activity of the IMR is manifested in the major earthquake of 26th December, 2006 (M 9.3), which occurred at the interface of the Indian and Myanmar plates (Banerjee et al. 2005, Pollitz et al. 2008). The seismic hazard map of India (Bureau of Indian Standards 2002) categorizes this region at the highest level of seismic hazard potential while the Global Seismic Hazard Assessment Programme (Bhatia et al. 1999) classifies the northeastern region at high seismic risk. The western limit of the IMR, defined by closely spaced imbricate thrust slices, has been designated the ‘Belt of Schuppen’ (Mathur & Evans 1964). This is a prominent narrow NE-SW trending morphotectonic unit, about 20–25 km wide and 400 km long comprising two major thrusts, viz., the Haflong/Naga Thrust and the Disang Thrust and two minor thrusts, namely the Sanis-Chongliyimsen and Piphema thrusts. With the help of aerial photographs and seismic data, Srinivasan (2007) prepared a photo-geological map of this schuppen belt showing three SE-dipping intra-schuppen belt sub-thrusts and proposed a seismic gap within the schuppen belt in which he identified probable areas of future major shocks parallel to the trend of the belt. Seismological studies carried out by Thingbaijam et al. (2008) and Angelier & Baruah (2009) also suggest that the region is tectonically active. GPS measurements along some fault planes in this region have indicated movements of as much as 36 mm/year (Sahu et al. 2006). To understand such a geologically complex terrain seismological, paleoseimological and morphotectonic investigations have been carried out in the Shillong Plateau and Arunachal Himalaya (Kayal 1987, Kayal & De 1991, Sukhija et al. 1999, Rajendran et al. 2004, Bilham & England 2001, Jade et al. 2007, Yin et al. 2010). Various site-specific morphometric parameters have been dealt with in different parts of the world to understand the evolution of landforms, soils and deposits formed by active tectonics (Bull & Mcfadden 1977, Yeats 1986, Schumm 1986, Keller 1986, Cox 1994, Azor et al. 2002) while such approaches are lacking from the study area. Various aspects of geology of the region have been dealt with by workers of the Geological Survey of India, Oil and Natural Gas Corporation (Mallet 1876, Evans 1964, Acharyya et al. 1986, Nandy 1986, Awasthi & Mehrotra 1993, Kayal 2003), Directorate of Geology and Mining, Nagaland and universities and institutions of the region, but a morphotectonic approach to understand the tectono-geomorphological evolution of the schuppen belt and Medziphema intermontane basin is lacking. The present study is carried out to identify active faults and folds and their implication on the development of landforms and drainage pattern through detailed fieldwork, drainage analysis and calculation of geomorphic indices. 2

Methods

IRS-1D (PAN+LISS 3 merged) data acquired on 16th November 2002, Survey of India (SoI) toposheets (1:50,000 scale) and a Digital Elevation Model (DEM) have been used to demarcate lineaments and determine drainage patterns and morpho-

Geomorphic evolution of Medziphema intermontane basin

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metric features. The data generated are substantiated by extensive fieldwork in the identified thrust zones. Morphometric parameters such as mountain front sinuosity index (Bull 1977, Bull & Mcfadden 1977 and Bull 1978), transverse topographic symmetry factors after Cox (1994), and valley floor width to height ratio (Keller & Pinter 1996) have been calculated from the SoI toposheets. Mountain front sinuosity is an index that reflects the balance between erosional forces that tend to cut the embayments along mountain fronts and neotectonic forces trying to create straight mountain fronts coinciding with active range bounding faults (Bull 1978). Transverse topographic symmetry factors have been used to detect tectonic tilting transverse to flow at drainage basin. Ratio of valley floor width to valley height is calculated after Keller & Pinter (1996) to determine the relative upliftment of the area which has broad-floored canyons and V-shaped valleys. 3

Geology

The region surrounding the study area may be broadly divided into the Paleogene and Neogene lithotectonic units (table 1) located at the SE and NW, respectively (fig. 1A). These major units are affected by a number of structural features including SE dipping thrust sheets. From west to east the Naga, Sanis-Chongliyimsen, Piphema and Disang thrusts are encountered. The Neogene rocks making up high linear ridges are thrust over the Brahmaputra alluvium along the NE-SW trending frontal Naga Thrust, whereas Paleogene rocks are thrust over the Neogene along the NE-SW trending Sanis-Chongliyimsen Thrust. The hinterland Disang Thrust separates the schuppen belt from the intermediate hills of the Inner Fold Belt of Nagaland. The Oligocene unconformity between Paleogene and Neogene may be related to the early stages of the Himalayan uplift or coincident with the MidOligocene global-eustatic lowstand, or both (Vail et al. 1977, Kent & Dasgupta 2004). The schuppen belt is bounded by two major thrusts viz., the Naga and Disang thrusts. The NE-SW trending Naga Thrust separates the upthrusted Neogene sandstones from the horizontally stratified Quaternary alluvium. As a result of imbrications, tectonic blocks of older rocks are found enmeshed within incompetent crushed rocks in the thrust zone. This suggests that the thrust is steep-angled and deep seated as it has brought up older rocks. Srinivasan (2007) described these slivers of rocks as tectonic mélange. The Sanis-Chongliyimsen Thrust marks the boundary between the Paleogene and Neogene rocks. The Disang Thrust, bearing the same trend as that of the Naga Thrust, delimits the hanging wall of the Disang (Upper Cretaceous-Eocene) from the footwall of the Barail (Oligocene). East of the schuppen belt, gravity faults have developed between Peducha and Zubza villages in the hanging wall of the hinterland Disang Thrust (fig. 1B). This thrust is responsible for a large landslide zone with repeated generation of debris along the National Highway 39 between the two villages (Bhattacharjee et al. 1998), suggesting reactivation of imbricate thrusts at the base of the hanging wall of the Disang Thrust. During the Plio-Pleistocene, fault bounding normal faults were reactivated and inverted by compression associated with the formation of imbricate structures propagating from the southeast (Kent & Dasgupta 2004).

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Fig. 1. A. Geological map of the study area (Modified after Geological Survey of India, 1983). B. Regional tectonic map.

Geomorphic evolution of Medziphema intermontane basin Table 1.

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Stratigraphic Succession of parts of Nagaland.

Age

Group

Lithoformations Outer and Intermediate Hills

Recent-Pleistocene

Dihing

Alluvium and High level terraces Boulder beds

Unconformity Mio-Pliocene

Dupi Tila

Namsang

Unconformity Tipam Miocene Surma

Girujan Clay Tipam Sandstone Bhuban

Unconformity Oligocene

Barail

Upper Cretaceous-Eocene

Disang

Renji Jenam Laisong

Modified after Mathur & Evans (1964).

3.1 Geomorphology Towards the northeast corner of Arunachal Pradesh, the NE-SW trending Himalayan range along the Siang antiform turns NW-SE in the Mishmi Hills and thereafter NE-SW along the IMR. The Patkai-Naga-Manipur-Mizo Hills form the western limit of the IMR. The area may be divided into the Brahmaputra plains, the frontal hills of the Patkai-Naga-Manipur-Mizo Hills and the intermontane valleys of the schuppen belt. These intermontane valleys are distinct geomorphic units bounded by the Naga Thrust in the NW and the Sanis-Chongliyimsen Thrust in the SE. An abrupt rise of ~ 524 m in elevation is noted from the low-lying Brahmaputra plain (~ 184 m) to the Naga Hills (~ 708 m) across the mountain front defined by the Naga Thrust. This tectono-geomorphic boundary marks the edge along which Neogene rocks have been thrust over the Brahmaputra alluvium (fig. 2). Between the Naga and SanisChongliyimsen thrusts a number of intermontane valleys are developed. These are similar to those of the Doons developed in the Indian and Nepal Sub-Himalayas (Medlicott 1864, Karunakaran & Rao 1979, Raiverman et al. 1983). These intermontane valleys are discontinuous but are linearly aligned with NE-SW trends. The Medziphema intermontane basin is the largest in terms of area and the most accessible. The size of the valley decreases towards the NNE. The maximum length of this basin is ~ 16.5 km and width is 11.5 km. Fan and terrace deposits that make up the sediment-filled valley rest over the Neogene rocks. The Medziphema intermontane basin is separated from the Brahmaputra alluvium by Neogene ridges. These ridges are highly dissected due to high erodibility of the poorly cemented and ill sorted

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Digital Elevation Model and cross-sections along study area.

Dihing and Namsang sediments similar to the highly dissected Siwalik Hills in the Himalayan region. Three levels of terraces are developed along the Diphupani and Dzumha river valleys and two levels of fans deposited at the footwall of the SanisChongliyimsen Thrust. Most of the fans are high angled (~ 15°) while one is inclined ~ 8°. Erosional surfaces in the form of escarpments of about 15 m are observed along the channels of Diphupani and Dzumha river valleys. Pedimented Dihing rocks are exposed along Diphupani and Jharnapani rivers in the form of strath terraces.


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River and stream patterns indicate influence of structures of the underlying bedrocks. During the early stages stream channel configurations are defined by the slope of the landscape; at later stages they will be dependent upon the underlying bedrock structure (Twidale 2004, Stokes et al. 2008). The drainage network of the Medziphema intermontane basin is derived from SoI toposheets (1:50,000 scale). Rivers and streams draining in the present area are divided into two domains, those in the schuppen belt and in the streams of the Kohima-Patkai Synclinoria, which are separated by the Disang Thrust. In the Kohima-Patkai Synclinoria the drainage pattern is broadly dendritic; structural influence on drainage is not as conspicuous as that observed in the schuppen belt. In the frontal part of the schuppen belt most streams are structurally controlled (fig. 3) and show deviation from the regional drainage pattern of SE-NW to NE-SW. The Der Ker, Kukhipani and smaller tributaries of the Diphu River and the Zameha, a tributary of the Dzüza River, show trellis patterns with similar NE-SW trends. Flow patterns of these streams are parallel to the regional strike of the rock beds and thrust trends. Within the intermontane valley the drainage pattern is dentritic and anastomizing. The Diphupani and Jharnapani are meandering streams that are also characteristically braided. Most streams flowing along the strike ridges are of first to second orders while the main streams follow the trend of the regional strike. In the intermontane basin and the hanging wall of the Sanis-Chongliyimsen Thrust, streams and rivers characterized by higher order, are dentritic in nature. The hanging wall of the Naga Thrust is characterized by badland topography. Lateral displacement of the Diphupani and Dzüza rivers is observed near Kukidolong and north of Khabvüma, respectively and has resulted in the formation of shutter ridges. Between Kukidolong and Chumukedima, the Diphupani valley becomes very narrow with entrenched meandering before debouching into the wide Brahmaputra plain. Wind gaps are also observed along the valley of Kukhipani. Entrenched meandering of streams and rivers are observed along the frontal ridges. 3.2 Morphotectonics Various morphometric analyses have been imparted on sections that have been identified as thrust areas with the help of DEM, satellite imagery, and toposheet followed by fieldwork. The frontal part of the NNE-SSW trending Naga Hills takes a local swing where it becomes almost N-S near Hovishe and the trend continues for about 17 km and then takes its regional NNE-SSW trend. At this locality the schuppen belt is less than 10 km in width. Mountain front sinuosity (Smf) has been calculated from southwest of Chumukedima to Muhumi village. It is defined as the ratio of the length of the mountain front along the foot of the mountain at the pronounced break (Lmf) to the straight-line length of the mountain front (Ls). The area has been divided into three sections on the basis of rock/sediments forming the footwall of the Naga Thrust. In the mid-section the Naga Thrust rides over the Namsang and has a Smf value of 2.04 (B), which is relatively higher than the other two sections where Surma sediments are thrust over Recent to Sub-Recent alluvium. These sections have S mf values of 1.89 (A) and 1.65 (C). Ratio of valley floor width to valley height for the Dzüza River, Laluki Jan, Chonga Jan, Diphu River, and Soja Ker have values of 0.525, 1.75, 0.547, 0.498, and 0.804, respectively. Drainage basin asymmetry of the Jharnapani and Diphupani indicates basin migration towards northeast and southwestward, respectively.

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Drainage map showing structural control along mountain front.

3.3 Terraces and fans The Medziphema intermontane valley resulted from faster uplift of the mountain front along the Naga Thrust than the hinterland causing a depression where sediments are deposited either in the form of fans or terraces. Recent sediments filling the valley are derived from the hinterland and reworking of the Dihing Formation. Along


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the valleys of the Jharnapani and Diphupani three levels of terraces are developed. Fluvial terraces can be observed on either side of the Jharnapani. In Kukidolong and Jhanapani areas three distinct levels of fluvial terraces are observed (T 1, T2, and T3) at 250 m, 252 m and 258 m, respectively, while the present river bed is at an altitude of 244 m. The terraces comprise mainly of sub-rounded to well rounded clasts of Barail, Surma, and Tipam sandstones in a matrix of coarse sand; they are devoid of large boulders. At Razephema three levels of terraces are developed – T1 at 238 m, T2 at 240 m, and T3 at 248 m. These terraces are composed mainly of well rounded Barail sandstones in a matrix of coarse sand and rock fragments. Fans are broadly divided into two categories – the older and younger fans. The older fans are exposed east and southeast of Medziphema where the sediments are highly oxidized and at elevations of around 410 m. The younger fans around Khaibung, Zhuikhu, and Tsuma are at lower elevations of about 390 msl; they are comparatively lesser oxidized than those around Medziphema. Besides the accumulation landforms, erosional landforms in the form of strath terraces are also observed along the Jharnapani, Diphupani, and Dzümha River. Demarcation of individual fans and terraces is not possible due to extensive interference of anthropogenic activity and vegetation. At the Chumukedima section of the Diphu River, Neogene rocks can be seen overriding Dihing deposits along the Naga Thrust. 3.4 Quaternary deformation The Quaternary sediments lying within the intermontane valleys have been affected by recurring neotectonic movements along thrusts and faults. At Kukidolong, along the northeastern channel of the Jharnapani, T3 is tilted 40° ENE (fig. 4A) while in the southwestern channel T1 is tilted 30° ENE. Deformation in T2 cannot be ascertained due to lack of good exposures. Near Razephema along the Diphupani, T1 rests over inclined Barail sandstone beds. T1 and a strath terrace have been truncated by normal faulting (fig. 4B). T2 is not affected by faulting but shows warping of sediments (fig. 4C). At Molvom, clast-rich T1 is tilted 40° ESE. At Sirhima, terrace T1 has been truncated by normal faulting. Fans developed around Medziphema show almost horizontal to slight back tilting due to the possible presence of a thrust fault at the front. Blocking of the Diphupani is evident from thick lacustrine deposits at Gaili, near Molvom where the mud deposit measures about 3 m in thickness (fig. 5A). Strath terraces exposed along the Diphupani and Jharnapani indicate neotectonic uplift in the area. Demarcation of individual fans and terraces is difficult due to extensive interference of anthropogenic activity and vegetation. However, some traverses were made with a GPS and some data collected from which boundaries of the fans in the area have been reconstructed. An active fault scarp is observed (fig. 5B). Right-lateral movement along this fault has resulted in the development of a 2.5 m high fault scarp and lateral displacement of Quaternary sediments. 3.5 Tectono-geomorphic evolution of Medziphema intermontane basin Initiation of the Disang Thrust began during Late Miocene and appears to have developed with a lower detachment in the lower part of the Barail Group and an upper detachment in the Girujan (Kent & Dasgupta 2004). Initiation of displacement along

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Fig. 4. A. Tilting of fluvial deposit, B. Terrace and bedrock truncated by normal faulting, C. Warping of Quaternary deposits.


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Fig. 5. A. Mud deposit near Molvom, B. Satellite image showing Quaternary sediments affected by strike-slip faulting.

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the Naga Thrust began during Early Pleistocene that coincident with the last phase of the Himalayan Orogeny in Late Lower Pleistocene when sedimentation in the Arakan-Assam basin came to an end (Gansser 1964, Nandy 1986). During this terminal phase of the Himalayan orogeny the region was subjected to two sets of compressional forces, one directed N-S and the other SE-NW that resulted in uplift and folding giving rise to the Arakan-Yoma ranges (Roy & Kacker 1986). Prior to initiation of the frontal Naga Thrust, the Sanis-Chongliyimsen Thrust represented the mountain front where sedimentation of the Dihing Formation took place over the Namsang or older successions that are derived from the Arakan-Yoma ranges. The continued SE-NE oriented compressional force propagated the thrusts northwestward that resulted in the formation of the intermontane valleys within the schuppen belt. At present this relict mountain front, the Sanis-Chongliyimsen Thrust, represents the boundary between the Recent to Sub-Recent sediments and the Paleogene rocks of the Barail Group. Another implication of tectonic activity of the Naga Thrust is the remobilized hydrocarbons in the oil fields at Digboi (Kent & Dasgupta 2004). 4

Discussion

The Medziphema intermontane basin, bounded by the Naga Thrust in the northwest and Sanis-Chongliyimsen Thrust in the southeast is one of many basins formed within the schuppen belt. Trending NNE-SSW, this basin is about 16.5 km in length and 11 km in width; it is drained transversely by two streams, the Diphupani and Jharnapani. Structural analysis suggests an origin of the valley similar with the Doons of Nepal and Western Himalaya. In the Nepal and Western Himalaya the Doons are bounded in the north and south by the Main Boundary Thrust (MBT) and Himalayan Frontal Thrust respectively, while in the present area a similar pattern is observed as the basin is bounded by two thrusts, the Naga and Sanis-Chongliyimsen thrusts. Roy & Kacker (1986) recognized three phases of deformation including the oldest syncollisional and two post-collisional. During the last phase of the Himalayan Orogeny in the late Lower Pleistocene the sediments of the Assam-Arakan were uplifted and folded resulting in formation of the Arakan-Yoma ranges. During this orogeny the compressional force was directed SE which led to the development of imbricate thrust sheets designated the ‘schuppen belt’. Kent & Dasgupta (2004) suggested three phases of displacement along the Naga Thrust, the first and second during Mid-Pleistocene, resulting in the development of two unconformities. The third and most recent displacement occurred during the Holocene that caused deformation of the younger unconformity. This third activity may be related with thrusting of the Neogene rocks over the Sub-Recent to Recent alluvium of the Brahmaputra plain along the Naga Thrust. The formation of the Medziphema intermontane basin is related to the Naga Thrust, which caused the Neogene rocks to be thrust over the Recent alluvium of Upper Assam. The Lower Pleistocene Dihing sediments forming the core of the valley have gentler dips compared to the older sequences suggesting that the initiation of the Naga Thrust started prior to deposition of the Dihing sediments. Development of the Medziphema intermontane basin may be related to the two later phases of deformation. Roy & Kacker (1986) opine that the last phase of deformation occurred during Pliocene; the forces of these deformations are adjusted along the thrusts of the schuppen belt as dextral and sinistral strike-slip faults. Drainage patterns


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along the frontal part of the Neogene hills and the strike-slip movement of the Quaternary deposits southwest of Heningkunglwa are signatures of the Pliocene phase. In the southeastern and central parts of the Medziphema intermontane basin there is no indication of structural influence on drainage pattern while the northwest and frontal parts show structural control of drainage. Strike-slip faults and the Naga Thrust have reoriented most of the streams and rivers. Lateral displacement of the Jharnapani and Diphupani are observed near and towards northeast of Kukidolong. The Kukhipani and Zameha stream flow towards the northeast following a strike-slip sense and lithological contact of the rock beds. The ratio of valley floor width to valley height for streams and rivers has been calculated just at their exit; values range from 0.498 to 1.75. Laluki Jan exiting into Namsang terrain has higher values (1.75) than the others (Dzuza River, Chonga Jan, Diphu River, and Soja Ker) which have values ⬍ 1.0. In the mid-section of the mountain front, the Naga Thrust overrides Namsang sediments while in the other two sections this thrust overrides Recent to Sub-Recent alluvium. The mid-section has a Smf value of 2.04 while the other sections have 1.89 and 1.65. Smf values fall under three categories; those between 1.0 and 1.6 are categorized as active mountain fronts, 1.4 and 3 as less active and that between 1.8 and ⬎ 5 as inactive (Keller & Pinter 1996). Valley width ratios and Smf values indicate that the bounding Naga Thrust is active. NE and SW migration of the Jharnapani and Diphupani respectively, indicate a growing anticline within the Medziphema intermontane basin. Strath terraces are observed along the Jharnapani and Diphupani (fig. 6A). Thrusting of the Surma over Recent fluvial deposits is observed near Chumukedima and Molvom (fig. 6B). In the northeastern Himalayan wedge the geological convergence rate of 16–22 mm/yr is comparable with GPS-derived convergence rates of 15–20 mm/yr (Mukul 2010). Srinivasan (2007) has marked four NE-SW trending, parallel tectonic zones along which are aligned major earthquakes (M ⬎ or = 7) (fig. 7). Kayal (2003) has emphasized the presence of deep-seated, hidden strike-slip transverse seismogenic faults in the northeastern Himalayan region. Along this zone the present surface traces of lineaments, the East Boundary Thrust (EBT), Disang Thrust (DT), and hinge line of the Kohima synclinorium (KS) take a swing and become parallel to one another. The schuppen belt bounded by the Disang Thrust on the south and Naga Thrust (NT) on the north is truncated by the NW-SE trending Mishmi Thrust (MT) towards the northeast in the Arunachal Himalaya. The Disang Thrust is an extension of the Dauki Fault (DF) along the Indo-Bangladesh border. To the north of the Brahmaputra River valley lies the Main Boundary Thrust (MBT); the Main Central Thrust (MCT) is noted further north, and towards northeast lies the Mishmi Thrust (MT). Therefore the present authors have added one more transverse N-S trending linear zone indicating a probable active lineament (fig. 7). Moreover, two major earthquakes fall on this zone. The role of the active N-S trending Sagaing Fault in the seismicity of the region cannot be ruled out (Valdiya 1976, Srinivasan 2007). The Manabum anticline (MA) might be a continuation of the major Siang antiform/window (Surendra Singh 1993, Srinivasan 2007). The rising eastern Himalayan syntaxis is characterized by high denudation rates estimated to be ~ 10 mm/yr over the last 4 Ma (Burg et al. 1997), and to be 2–5 mm/yr during late Tertiary-Recent times in and around the core region (Craw et al. 2005). Thus the Recent upliftment of the core region of the eastern Himalayan syntaxis may also be continuing in the frontal part of NE Himalaya.

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Fig. 6. A. Strath terrace, B. Surma sediments of Neogene overriding Recent alluvium along Naga Thrust at Chumukedima.

5

Conclusions

Analyses of data obtained from the field and supplemented by that generated from satellite imagery, DEM and morphometric parameters suggest that the schuppen belt is tectonically active. Initiation of imbricate thrusting of the schuppen belt started during Late Miocene when the Namsang and Dihing sediments were laid over older


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Fig. 7. Simplified seismicity map of northeast India (after Srinivasan 2007); inset showing probable active lineament.

sequences on the footwall of the Sanis-Chongliyimsen Thrust. Dihing sediments occupying the core of the Medziphema intermontane basin dip 15° SE, suggesting major upliftment of the frontal part of the schuppen belt after the Lower Pleistocene along the Naga Thrust. In the frontal part of the Naga Hills the Surma are thrust over either the Namsang or Recent to Sub-Recent alluvium along the Naga Thrust. Faulting and tilting of Recent fluvial deposits and lateral displacement of Recent deposits and rivers and streams indicate that the thrusts and faults of the schuppen belt are tectonically active. Morphometric data corroborate field evidence. Drainage patterns, particularly along the frontal hills show structural control of their morphology by the underlying rock beds. Evolution of the Medziphema intermontane basin has been attributed to the successive activation and abandonment of thrusts in the schuppen belt. Thrusting of Neogene rocks over the Quaternary Brahmaputra alluvium along the Naga Thrust has uplifted the frontal range resulting in the development of the intermontane valley. Upliftment along the Naga Thrust has tilted the Early Pleistocene Dihing rocks occupying the core of the intermontane valley towards the SE.

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Acknowledgements GTT is thankful to Department of Science and Technology (DST), Govt. of India for financial assistance through grant ESS/16/249(4)/2005. KL and SSB are grateful to the Director, Wadia Institute of Himalayan Geology Dehradun for kind permission to publish the paper. The authors are grateful to Dr. N. S. Virdi and Dr. V. Srinivasan for review and constructive criticisms and Dr. Temsulemba Walling (PDF) and Mr. Supongtemjen (SRF), Nagaland University for help with fieldwork.

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Yin, A., Dubey, C. S., Kelty, T. K., Webb, A. A. G., Harrison, T. M., Chou, C. Y. & Célérier, J. (2010): Geologic correlation of the Himalayan orogen and Indian craton: Part 2. Structural geology, geochronology, and tectonic evolution of the Eastern Himalaya. – Geol. Soc. Am. Bull. 122: 360–395. Manuscript received: August 2010; Revised version accepted: December 2010. Addresses of the authors: Imtiwapang Aier, Glenn T. Thong, Department of Geology, Nagaland University, Kohima-797001, India; Khayingshing Luirei, S. S. Bhakuni, Wadia Institute of Himalayan Geology, Dehradun-248001, India; Girish C. Kothyari, Institute of Seismological Research, Gandhinagar-382009, India. E-Mail: [email protected]