Major Geological Divisions of India

The physiography and relief of India are directly governed by its deep geological structure and tectonic history. Based on lithology, structural configurations, and evolutionary processes, the Geological Survey of India (GSI) divides the Indian landmass into three distinct, macro-geological divisions. These units have evolved under different tectonic regimes, ranging from ancient stable cratonic nuclei to active, modern collision zones.

The Peninsular Shield

The Peninsular Shield represents the oldest, most stable, and rigid geotectonic block of the Indian landmass. It is a composite shield formed by the amalgamation of several ancient continental nuclei or cratons, stitched together along Proterozoic mobile belts known as Central Indian Tectonic Zones (CITZ).

Geological Composition and Crustal Evolution

• Archean Basement Complex: The core of the shield consists of highly metamorphosed crystalline rocks like the Bundelkhand Gneiss, Bengal Gneiss, and Nilgiri Granulites, dating back more than 3.5 billion years.
• Dharwar Fold Belts: These represent the oldest deformed and metamorphosed sedimentary rock systems in India, highly enriched in metallic minerals like iron, manganese, and gold.
• Proterozoic Purana Sedimentary Basins: Large synclinal sag basins like the Cuddapah and Vindhyan systems contain unmetamorphosed sediments, rich in high-quality limestone, dolomite, and building sandstones.
• Gondwana Graben Systems: Linear rift valleys formed due to crustal extension during the Paleozoic era house India’s primary Gondwana coal seams along the Damodar, Mahanadi, and Godavari basins.
• Deccan Volcanic Province: A massive flood basalt construct covering nearly 500,000 square kilometers, formed by fissure eruptions when the Indian plate passed over the Reunion Hotspot during the Late Cretaceous epoch.

Relief and Geomorphic Characteristics

• Senile Topography: The plateau displays a highly denuded, senile landscape characterized by broad, graded valleys, rounded hills, and relict features.
• Relict and Block Mountains: It features residual mountains formed by differential erosion, such as the Aravallis, Javadi, and Shevaroy hills, alongside horst and graben structures like the Vindhya and Satpura ranges.
• The Western Ghats Escarpment: Structurally, the Western Ghats are not true folded mountains but form the elevated, faulted western edge of the Gondwana block that remained tilted eastward after the separation of Madagascar.

The Extra-Peninsular Region (The Himalayan Mountain System)

In complete contrast to the stable Peninsular Shield, the Extra-Peninsular region is a young, flexible, and tectonically unstable structural unit. Its evolution is entirely attributed to the ongoing continent-to-continent collision between the Indian and Eurasian plates.

Stratigraphic Architecture and Tectonic Wedges

• The Tethyan Sedimentary Sequence: The northernmost zone contains highly fossiliferous marine sedimentary sequences ranging from the Cambrian to Eocene periods, representing the scraped-up floor of the ancient Tethys Ocean.
• Crystalline Core Zone: The Higher Himalayas consist of high-grade metamorphic rocks like schists and gneisses interspersed with sheet-like granitic intrusions of Neogene age.
• The Siwalik Foreland Basin: The outermost zone comprises thick sequences of freshwater molasses deposits, rich in mammalian fossils, representing the youngest structural addition to the mountain chain.

Major Structural Thrusts and Sutures

• Indus-Tsangpo Suture Zone (ITSZ): The definitive tectonic boundary marking the subduction zone where the Neo-Tethys ocean closed and the Indian plate locked with the Eurasian plate.
• Main Central Thrust (MCT): A major north-dipping ductile shear zone that brings the high-grade metamorphic rocks of the Higher Himalayas over the Lesser Himalayas.
• Main Boundary Thrust (MBT): The tectonic interface separating the sedimentary sequences of the Lesser Himalayas from the younger sub-Himalayan Siwalik range.
• Main Frontal Thrust (MFT): The active structural blind thrust where the Siwalik hills abruptly terminate and override the Quaternary alluvium of the Indo-Gangetic plains.

Relief and Geomorphic Characteristics

• Juvenile Topography: It features an youth-stage landscape characterized by lofty peaks, steep-sided V-shaped valleys, deep gorges, hanging valleys, and active glacial landforms.
• Exogenic Instability: Due to high relief and active tectonic uplift, this division experiences rapid rates of erosion, massive mass-wasting events, and high vulnerability to landslides.

The Indo-Gangetic-Brahmaputra Plain

The Indo-Gangetic-Brahmaputra plain is the youngest geological division of India, originating during the upper Tertiary and Quaternary periods. It acts as an intermediate structural transition zone between the rigid Peninsular Shield to the south and the shifting Himalayan thrust belt to the north.

Tectonic Origin as a Foredeep

• Flexural Sag Mechanism: As the Indian plate pushed beneath the Eurasian plate, the northern edge of the Peninsular shield suffered a downward flexure, creating a massive asymmetric structural trough or foredeep.
• Aggradational Deposition: This deep trough, originally acting as a marine gulf, was systematically filled over millions of years by immense volumes of detrital sediments brought down by the cascading Himalayan rivers and northward-flowing Peninsular streams.
• Sedimentary Depth: The thickness of the alluvial deposit varies across the plain, reaching its maximum structural depth of over 4,000 to 6,000 meters in the northern parts closest to the Main Frontal Thrust.

Geomorphic Lithology and Subdivisions

• Bhabar Belt: A narrow, porous tract running parallel to the Siwalik foothills composed of un-sorted boulders and coarse gravel. Rivers sinking into this gravel bed often disappear underground.
• Terai Belt: A marshy, damp, and heavily vegetated zone located south of the Bhabar, where the underground streams re-emerge at the surface due to a sharp reduction in slope porosity.
• Bhangar Allvium: The older, pleistocene-age alluvial terraces situated well above the active floodplains. It is characterized by calcareous clay sheets and nodular kankar accretions.
• Khadar Allvium: The newer, holocene-age fertile sandy-loam deposits found within the active flood channels. It is renewed annually by seasonal river floods.

Comparative Matrix of the Major Geological Divisions

Geotectonic Trivia for Civil Services Examination

• The Delhi Ridge: The rocky protrusion extending up to Delhi is an underground extension of the ancient Aravalli Craton, part of the Peninsular Shield, which remains buried beneath the Indo-Gangetic alluvium further east.
• The Shillong and Karbi-Anglong Plateaus: Despite their geographical location in Northeastern India adjacent to the Himalayas, the Meghalaya Plateau and the Karbi-Anglong hills are structurally detached fragments of the Peninsular Shield. They were separated from the Chhota Nagpur Plateau by the tectonic subsidence of the Malda Gap during the late Tertiary period.
• The Hidden Ridge: The Sub-Alluvial Munger-Saharsa Ridge acts as a structural basement high beneath the Ganga plain, significantly altering the sediment thickness and local hydrological flow paths in North Bihar.