Northeast India is geologically a very complex region since it represents the dynamic frontal part of the Indian plate on the one hand and the relatively stable central (Chinese) plate on the other. in between these two plates there lay the organically active bed of the Tethyan Geosyncline with its foredeep skirting the dynamic Indian plate.
The Geological history of the region directly or indirectly involves almost all the major geological ages from the Precambrian to the recent. The shilling Plateau and the Karbi Hills being part of the Gondwanaland are Archaean or Pre-Cambrian in age.
In its northward movement, a part of Gondwana massif (also termed as the Indian plate) exerted pressure on the geologically Tethyan belt, which is pressed by the plate in the south and west and by Chinese plate in the north and northeast took a syntaxial bend along what is today Dibang- Lohit Hills.
The whole geosyncline belt representing the Arunachal Himalayan and quaternary in the course of Himalayan orogeny in the late Tertiary and Quaternary periods. Thus, geological bends ranging from Pre-Cambrian to the recent periods can be found scattered in the region.
Geologically Northeast Indian region may be divided into the following main divisions:
The overthrust eastern Himalaya,
The overthrust Naga Hills including the Patkai-Kohima synclinorium,
The foreland spur between the eastern Himalaya and Naga Hills extending north-eastward of the Shillong plateau into the Upper Assam,
The Bengal basin and Surma Valley,
The monoclinic strip between the Shillong Plateau and the Surma Valley associated with the great Dauki fault.
The arcuate Arakan-Yoma belt extending in an approximate north-south direction along the Indo-Burma border.
Northeast India Geological Regions
From the above list, it is clear that the Northeast India region is divided into six regions which are broadly discussed below
1. Eastern Himalaya
The general structure of the Himalayan foothills is a very complex one, but in this range, some of the thrustings took place during the Miocene period. The Himalayan Tectonic belt is sub-divisible into a tectonic zone constituted of Precambrian to Tertiary Metasediments.
2. The Naga Hills
The Naga Hill’s includes a few low ranges of hills extending in the plains of Upper Assam, also included in this range is a part of the North Cachar Hills. The Naga overthrusts form a complex pattern, near the higher ones override the lower ones, and the frontal thrust which is generally known as the Naga thrust is found to be composed of four different thrusts.
The Disang thrust constitutes the uppermost and it overrides all the lower thrusts in the North Cachar Hills thereby resting directly on the Foreland Spur.
3. The Patkai-Kohima synclinorium
This is found to be fairly broad and is broken up by strike-slip faults. Further, the Disang series underlies the Barail series on either side of this syncline-rium, and in the region where the Kohima synclinorium approaches the eastern end of the Surma Valley is the Surma series.
The Patkai Range has several sharp anticlines with overthrust faults on the Assam side and has also pronounced thrust in the Disang rocks. The Patkai synclinorium merges into the Kohima synclinorium to the SSW and finally merger into the eastern end of the Surma Valley imperceptibly,
4. The Shillong Plateau and Mikir Hills
The Shillong series is mainly of sedimentary origin. It is mainly composed of quartzites, conglomerates, phyllites, etc. with occasional carbonaceous slates and banded ferruginous rocks. Similar is the case in Simsang Valley in the Garo hills and also in parts of the Mikir hills.
The Shillong series which are intruded by the Khasi greenstones have been folded up and are therefore presumably of Archaean age. It shows some gradation towards gneisses near the junction with granitic rocks and distinctly is of Mylliem Granite. Again, the Upper Assam self and Upper Assam depression to the northeast of the Mikir Hills shows the development of marine sediments ranging in age from late Cretaceous to Pleistocene and these being covered by a Veneer of younger and older alluvium of the Brahmaputra.
5. The Surma Valley
The Surma Valley and the Bengal Basin is relatively simple in structure and seems to have been covered by a layer of alluvium. The eastern part of Surma Valley, i.e., lachar and Tripura has been folded thereby forming broad flat symmetrical synclines and narrow anticlines and this part has a great resemblance with the Kohima synclinorium.
The southern extension of the Kohima synclinorium, as well as the eastern extension of the Surma Valley, passes into Manipur. In between the western part of the Surma Valley and the Shillong, plateau lays a narrow strip of southerly dipping beds called the Dauki fault zone.
A number of studies have proved that the Dauki fault is a tear- fault along which the Surma Valley region has moved westwards relative to the Shillong plateau by not less than 200 km. The Bengal Basin which is to the south of the Shillong plateau is overlain by Mesozoic and Tertiary sediment and these are largely covered by the Ganges-Brahmaputra alluvium.
6. The Arakan Yoma Belt
It extends in an approximately north-south direction along the Indo-Burmese border and is constituted of Mesozoic and Tertiary sediments and having a thickness of 13 km. These sediments have been severely folded and faulted and include narrow belts of incomplete dismembered ophiolites emplaced parallel to the regional strike.
On the western side of the main Arakan Yoma folded zone is the Frontal fore deep folded zone which is developed in the southern Surma Valley - Tripura sector. In the northern sector, a ’Belt of Schupoen' about 20 km wide has been recognized and beyond this belt is the Patkai-Kohima Synclinorium.