TECTONICS OF THE SHILLONG PLATEAU AND ADJOINING REGIONS IN NORTHEAST INDIA USING FIELD AND REMOTELY SENSED DATA

Abstract

Northeast India is one of the most tectonically active regions in the world and frequently affected by minor to moderate earthquakes. In this region, the Shillong Plateau was the locale of one of the largest known earthquakes of the world of 1897. The Archean gneissic basement of the Shillong Plateau has been experiencing earth movements since Early Proterozoic. It is bounded by relatively young Himalayan and Indo-Burman fold belts and documents prolonged tectonic activity. This part of India has been subjected to the collision tectonics from the north and east since the subduction of the Indian Plate under the Tibetan and Burmese Plates respectively. The southern margin of the Shillong Plateau is demarcated by the well known E-W trending Dauki Fault, that has played an important role in its geotectonic history. Initially, eastward lateral movements of the order of 200 km or more along the Dauki Fault were postulated. However, later investigations have indicated vertical fault movements along this fault. Satellite images of this region provide a synoptic view of numerous geological structures. Remote sensing data have been integrated with geological, geomorphological, structural and seismological studies for a better understanding of various tectonic structures of the Northeast India, mainly the Shillong Plateau. The plateau is bounded by the prominent N-S trending Jamuna Fault along its western margin, E-W trending Dauki Fault and Brahmaputra lineament along the northern and southern edge of the plateau respectively and Kopili graben to the east. On satellite images, the Shillong Plateau can be seen criss-crossed by innumerable fractures with predominant trends in N-S and NE-SW. Frequency plotting of lineaments indicate that NW-SE trending lineaments become scarce gradually from west to east of the plateau. Further, joint data from the Precambrian rocks also show dominant trend in N-S and NE-SW directions. Numerous horst and graben structures could be seen on satellite images along southern margin of the plateau which have formed due to the extensional tectonics across the pre-existing N-S and NE-SW trending fractures. The N-S trending dextral Urn Ngot Fault dissects the Shillong Massif at longitude 90°E, and was active only before the Eocene. The N-S trending Dudhnai Fault, located in the western Shillong Massif, runs across the whole width of the massif and displaces the Dauki Fault. These faults are deep-seated, as indicated by the intrusion of carbonatite along these faults. Aset of N-S trending Kopili fracture lineament has been identified in the area between the Shillong and Mikir Hills Massifs showing lateral movements. The Mikir Hills Massif has shifted northward along these fractures. The Dapsi Reverse Fault and the Haflong Thrust Zone in the western part and southeastern part of the Shillong Massif are the result of NE-SW and NW-SE trending compressional stresses respectively. On satellite images, the most spectacular folds can be seen in region south of the Haflong Thrust. The limbs of these folds are curved and hinges compressed. The morphological features of the folds indicate that these could have formed due to the NW-SE trending compressive stress. In the Himalayan Foothills, the Siwalik rocks are missing at longitude SOWE. The faulted river fans adjacent to the foothills indicate recent active character of the Main Frontal Thrust (MFT). In Arunachal Himalaya, the Dafla Hills tectonic block shows southward migration due to N-S compressional stress. In the Naga Foothills, an interlocking structure has formed due to translational fault movement. Under the intense compressional tectonism, the foothill rocks exhibit migration tendency towards the Brahmaputra valley. This effect is very conspicuous in a region where the Brahmaputra Basin is bordered by the Dafla Hills tectonic block in the north and the Mikir Hills block in the south respectively. The deformation pattern and nature of migration of the Brahmaputra River suggests that the Brahmaputra Basin is getting compressed gradually. Ill The Brahmaputra River in the Northeast India flows through technically very active narrow valley, bounded by the hills and plateau. This river has conspicuously migrated northward for considerable length between Shillong and Mikir Hills Massifs. Northward shift of the Brahmaputra River is attributed to the probable southward thrusting of the Brahmaputra Basin against the Shillong Plateau. On satellite images, the E-W trending Dauki Fault Zone is, at places, represented by very high landforms, and very steep escarpment between longitudes 91° and 92°E with huge blocks of sandstones with the E-W trending vertical slip surfaces. Further, western and eastern segments of the Dauki Fault Zone show uplift and thrusting effects respectively. Two troughs could be recognized in the Bangladesh area adjacent to this zone. The various folds in sedimentary rocks just south of the DFZ are convex indicating variation in stress orientation between NW-SE to NE-SW. The satellite images also clearly depict a prominent NE-SW trending Barapani Shear Zone. Landform, developed along this shear zone, reveal structural characteristics of the shear zone and nature of shearing. This shear zone is located within the Precambrian phyllites belonging to the Shillong Group. The elongated lensshaped ridges and dragged narrow long ridges clearly indicate left-lateral slip along the shear zone. The Barapani Shear Zone had undergone reactivation due to horizontal shearing probably in Miocene as a result of intense N-S compression. Drainage system in the Shillong region is strongly influenced by structural grains and mostly fracture-controlled. Along the Barapani Shear Zone, the northeastward flow of the Wah Umiam River has been obstructed by a ridge and the river takes a U-turn. Careful examinations of fracture lineaments and various joints indicate that the region has suffered multi-directional tectonic stresses, most prominent being in NW-SE and N-S directions. Neotectonic activities are expressed through deformation of soft sediments. Present-day deformations are actively taking place in foothill zone of the Himalaya and Naga Hills and the younger sediments south of the Shillong Massif. IV The region is seismically very active and some of the earthquake potential zones have been identified viz., Tura Block, Shillong Block, Dafla Hills Block and the area between DFZ and HTZ. It is likely that the Brahmaputra Basin is thrust against the northern margin of the massif, while the rigid massif, in turn, appears to override the Bangladesh Plains at depth. This study also postulates presence of thrusts beneath the Brahmaputra Basin and the Shillong Massif. These structures are thrust southward against the northern margin of the Shillong Massif and the Bangladesh Plains respectively.