Geopotential evidence of a missing lithospheric root beneath the eastern Indian shield: An integrated approach

The eastern Indian shield consists of Archaean Singhbhum Craton and Proterozoic Chhotanagpur Gneissic Complex (CGC) sandwiching the Singhbhum Mobile Belt (SMB). Since the cratonization of the Singhbhum Craton (SC) in Archaean, the growth of the eastern Indian shield took place in time and space through tectono-magmatic processes. The stability of cold and thick lithosphere is fundamental to long-term survival of cratons, whereas the geophysical studies have detected the lithosphere-asthenosphere boundary (LAB) under the eastern Indian shield at depths too shallow to be called stable. The terrestrial Bouguer gravity anomaly, and satellite-based free-air anomaly, geoid undulation, and elevation data are analyzed to ascertain the 2D lithospheric density structure across the region. Density model in this study illustrates that the density inhomogeneity exists in the crust across the three tectonic domains of the eastern Indian shield. The derived crustal model shows an upper and lower crustal density variation from 2740 to 2770 kg/m3, and from 2930 to 2940 kg/m3, respectively, and a reasonably smooth Moho at 37–41 km depth. Towards the north, the Moho undulates from 40 to 43 km under the foreland Ganga Basin (GB), whereas in the south, it varies from 38 to 30 km under the Eastern Ghats Mobile Belt (EGMB) and lastly moves to ~20 km in the Bay of Bengal (BOB). In the southern part of the Singhbhum Craton, an undissipated lithospheric mantle root is found at a depth of ~150 km. Otherwise, the LAB shallows to ~132 km in the northern Singhbhum Craton and Singhbhum Mobile Belt and then thickens to about 135–140 km depth beneath the Chhotanagpur Gneissic Complex. The foreland Ganga basin toward the extreme north is characterized by a more in-depth LAB lying at a depth of over 200 km. The LAB, in the Bay of Bengal, is at a depth of 112–125 km, except for the Kolkata coast (135 km). Moderate crustal density difference in various crustal domains, as well as an almost smooth crust- mantle boundary at 37–40 km depth, suggests the effect of substantial mafic–ultramafic crustal intrusion and together with the thin (135–140 km) lithosphere reinforces the evidence of thermo-chemical processes that controlled the lithospheric modification in the eastern Indian shield.

Fig.: Schematic cross-section showing interpretation of the lithospheric structure of the eastern Indian shield from geopotential and geological studies. The upper panel shows the general geology, and the lower panel represents the interpreted lithospheric density model.

  • In the Bay of Bengal, the ~120 km  thick lithospheric configuration is quasi-continental.
  • The thinner lithosphere (135 km), together with the crustal offset along the Sukinda thrust fault buttresses the continental collision of Eastern Ghats Mobile Belt with the Singhbhum Craton.
  • The thinning of the Singhbhum lithosphere (~135 km) is likely to be associated with, or reflection of, the thermo-chemical erosion related to the widespread re-fertilization soon after the late Archaean time.
  • The Singhbhum Mobile Belt, having shallow LAB (~138 km) and relatively denser crust, separates the domains of Singhbhum Craton and Chhotanagpur Gneissic Complex.
  • No density inhomogeneity despite the sudden change in lithospheric thickness (~140 km) across the Damodar valley envisions the Chhotanagpur Gneissic Complex as a single lithospheric block.
  • The thickest lithosphere (~200 km) beneath the foreland Ganga Basin is associated with the active tectonics between the Indian and Eurasian plates.

 

A.P. Singh , Niraj Kumar, B. Nageswara Rao, V.M. Tiwari

Geopotential evidence of a missing lithospheric root beneath the eastern Indian shield: An integrated approach

doi.org/10.1016/j.precamres.2021.106116