Role of the Bundelkhand Granite Massif and the Son-Narmada Megafault in Precambrian Crustal Evolution and Tectonism in Central and Western India

A unified history of crustal evolution and tectonism in the Shield area in central and adjoining part of western India has been traced to pivotal roles played by the emplacement of granites in Bundelkhand and sinistral Strike-Slip along the Son-Narmada megafault. The Bundelkhand massif is made up largely of intrusive granites that exhibit outward forces of intrusion. Similar patterns of collinear indentations in older Precambrain rocks up to 400 km in the south and 500 km in the west of Bundelkhand mark the late Archaean to early Proterozoic massif as the source of tectonic drives in the contemporary surrounding terrains. Two syntaxes have been designated in these two directions. The Mahakoshal belt of metasediments in the Son-Narmada valley is extendable to the Pre-Bundelkhand Madia Formation in southeast Bundelkhand, the Hindoli, Aravali and Champaner Groups of western India. The above and the Sausar and Sakoli Groups, affected by the syntaxial movements are, therefore, Pre-Proterozoic in age. The Bundelkhand massif is rimmed by a belt of high gravity, arguably generated by the squeezing of the Pre-Granite Madla-MahakoshaI-Hind011 crust, supplemented by the basic residues of fractionation that generated the large batholith, and by ultrabasic rocks displaced by the development of the 40 km deep ischolar_main of the massif. This belt eventually became the site of the Bijawar-Vindhyan combined basin, that snuggly fits around the Bundelkhand massif. A small and shallow Bijawar-Vindhyan structural basin over a moderate high gravity zone occurs inside the Bundelkhand massif. This points to transgression of the Vindhyan sea during the Kalrnur period Signatures of use in thermal gradient and floundering of the granitic crust have been recognized in this place. Void generated by bimodal volcanism in the adjacent area could also have contributed to the collapse. Close relation between high gravity zone and basin limits been observed in some other parts of the region also. There are signs of thinning of static crust also in the northern marginal parts of the massif, indicating that the massif does not extend much beyond its exposed limits. There are major E-W shears in the massif, along some of which layered ultramafics, usually with magnetite have welled up Such magnetite with thin quartz veins along shear zones and showing penecontemporaneous deformation were earlier considered as BIF. The kimberlite plugs of Panna have also a similar origin, with several favourable factors combining for emplacement of the diamond-bearing plugs. The remarkable parallelism among the NE-SW trending series of shear zones in Bundelkhand, the Chambal river lineament, the Great Boundary Fault of Rajasthan, the linear narrow basins in eastern BGC and the straight margins of the Delhi basin, all 350 to 400 km long, over an E-W spread of 800 km could be the result of drags of the suspected early Proterozoic sinistral megafault along the Son-Narmada Lineament. In Bundelkhand, these drags are numerous and are occupied by very long quartz veins, not seen in any other part of the Indian Shield NW-SE trending en echelon fractures, occupied by swarms of dolerites were secondary drags of the above NE-SW primary drags. There are felsic dykes in the southern part of the massif with anomalously high MgO. The high gravity rim around Bundelkhand did not develope to the northeast of the massif due to the above southwesterly drag. Much of the structural deformation, and Malam volcanism respectively in east and west-Central Rajasthan is attributed to similar drags and their westward swipe Subduction tectonism, and the future status of CIS, SNL, GBF, Jahazpur Thrust and PDZ are considered unnecessary in the tectonic evolution of the region CIS has been redefined Evidence of growth force of coarse granite is also provided by the Malanjkhand Granite in southeastern central India, where deformation and fluid movement are localized on its dorder with the Chilpi Group, resulting in a large copper deposit. This growth force caused drags forming the acrurate Central India Shear to its north. The Sausar Group has been traced to Itarsi on the Narmada valley and the group possibly merges with the Mahakoshal Group. The Betul belt shows volcanism and basin formation over high gravity zone Manifestations of the SNL drags are staggered over time and space, without resurgence of the fault. The westerly younging of formations and there deformation in Rajasthan has been explained, and southwesterly younging in fomations and granite emplacement along the Delhi axis has been noted Syntaxia1 force to the west of Bundelkhand and the drag along Delhi east border are Time-Overlapping and together, these produced several complex structures in central Rajasthan. Greatly delayed manifestation of force of batholithic granite emplacement is a distinct possibility.