Singhbhum Craton

The Singhbhum craton is located in eastern India, bounded by the Mahamadi graben and Sukinda thrust fault in the south, the Narmada-Son lineament in the west, the Indo-Gangetic Plain in the north, and the Bay of Bengal in the east. The craton has three main parts: the old Archean Singhbhum nucleus in the south, the 2.2- to 1.0-billion-year-old Singhbhum-Dhalbhum mobile belt north of this, and the Chotanagpur-Satpura belt of gneisses and granites north and west of the mobile belt.

The Singhbum craton comprises many old rocks, including the older metamorphic group that is about 3.2 billion years old. Some evidence points to the possibility of rocks as old as 3.8 billion years in the Singhbum nucleus. Most magmatic activity ended in the Singhbum nucleus by 2.7 billion years ago. These rocks are intruded by granites 2.91-2.95 billion years old and then by diabasic intrusives between 1.5 and 1.0 billion years ago.

The Singhbum craton has three major thrust belts: the Dalma thrust in the northern part of the Singhbum-Dhalbhum mobile belt, the Singhbum thrust between the mobile belt and the Singhbum nucleus, and the Sukinda thrust along the southern margin of the craton. The center of the craton is cut by a major rift valley, filled with Gondwana sediments in the Damodar Valley.

The Singhbum thrust is a 120-mile (200-km) long, bow-shaped belt along the northern side of the Singhbum nucleus. It is more than 15 miles (25 km) wide, and contains at least three main thrust slices. Seismic evidence shows that the structure penetrates the thickness of the lithosphere and is therefore interpreted to be an ancient plate boundary. Blueschist facies rocks, which are high-pressure, low-temperature metamorphic rocks characteristic of younger subduction zone settings but exceedingly rare in Pre-cambrian belts, have reportedly been found along the Singhbum thrust by Indian geologists S. N. Sarkar and A. K. Saha, but other geologists have disputed their finds.

A large part of the Singhbum nucleus consists of the Older Metamorphic Group, preserved as remnants in the intrusive Singhbum Granite complex. These rocks include mica schists, quartzites, calc-silicates, and amphibolites, along with gneissic remnants dated to be 3.8 and 3.2 billion years old. These older metasedimentary rocks are overlain by the Iron ore Group of shales, hematitic jasper with iron ore layers, mafic lavas, sandstone, and conglomerate, but it is not clear which group is older. The iron ore deposits show three periods of folding, including F1 reclined folds, and F2 and F3 upright folds that interact to form fold interference patterns.

The Singhbum-Dhalbhum mobile belt includes rocks of the Singhbum Group, including mica schists, hornblende schists, quartzose schists, granulites, chloritic schists, and amphibolites. Rocks north of the Singhbum thrust are disposed in a large anti-clinorium, then farther north they form a synclino-rium containing ophiolitic-type volcanic and plutonic rocks of the Dalma Group in its uppermost sections. The rocks show up to four folding generations including early subhorizontal recumbent folds, followed by two generations of upright folds. The fourth generation of folds is associated with the shear zones in the south, related to the formation of the large Singhbum thrust. The Chotanagpur terrain north of the mobile belt contains a large area of gneissic and granitic rocks but also includes metasedimentary rocks, gran-ulites, mafic/ultramafic schists, and anorthosites, and may represent a continental fragment or an island arc terrane.

Rocks in the Singhbum-Dhalbhum mobile belt are interpreted as a Proterozoic orogen deformed at 1.6 billion years ago. These rocks include shallow water sedimentary sequences overthrust by oceanic and ophiolitic assemblages preserved as structurally bounded mafic/ultramafic sequences during closure of a Proterozoic ocean. The ophiolite belt is succeeded southward by a flysch belt, a fold-thrust belt, then a molasse basin on the older Singhbum cratonic nucleus. The rocks were transported from north to south over the Singhbum granite, which represents the foreland to the orogen. The hinterland, or internal parts of the orogen, is in the Chotanagpur area, and the orogen represents the collision of the Chotanag-pur block with the Singhbum nucleus about 1.6 billion years ago.

Continue reading here: Aravalli Craton

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