The East Gondwana was formed by the assembly of continental fragments, namely India, Madagascar, Sri Lanka, and Antarctica, with the evolution of 2000 km long East African Orogen around 550 Ma. Lack of knowledge on the subsurface structure of the orogen led to a wide range of evolutionary models. The present study attempts to resolve various models’ ambiguity by presenting new results of the first deep seismic reflection study across the Achankovil Shear Zone, Southern Granulite Terrain, India that was evolved during the East African Orogen. The seismic study is also the first of its kind among the east Gondwana fragments. The Common reflection surface stack images of the present study reveal hitherto unknown details along with a unique south-dipping reflection fabric extending from the surface to upper mantle depth. We interpret this fabric is formed due to subduction- accretion of Madurai and Trivandrum blocks during the East African Orogen and represents a suture zone. Two deep-seated shear zones, the Achankovil and Tenmalai were also developed at both ends of collision fabric with strike-slip characteristics. Orogenic collapse and post-orogenic extension are inferred from the sub- horizontal lower-crustal reflection fabric, which is interpreted as an equilibrated younger Moho. The present study provided an opportunity to understand the formation and exhumation of granulites, which act as a window on the inaccessible lower continental crust. Gravity modelling is performed along the seismic profile. Integration of present results with the existing geological and geophysical data provided basic constraints on the structure and tectonics of east Gondwana fragments and the East African Orogen.
Fig.: Geodynamic evolution of the Achankovil Shear Zone (AKSZ), the East African Orogenic event over the Indian Shield. (a) MB and combined TB and NB are separated by an Ocean. (b) MB subducted beneath the composite TB and NB with the evolution of East African Orogen during ~550 Ma. (c) The present crustal structure of the region. MB: Madurai Block; TB: Trivandrum Block; NB: Nagercoil Block.
The unique feature of the present study is to image the subduction- collision signature represented by a dipping reflection fabric extending from the surface to a depth of 60 km, which indicates the detachment boundary is much deeper (>60 km). The orogenic event in the present study indicates the operation of thick-skinned tectonics, contrary to many Phanerozoic orogens like the Appalachian, Variscides, etc., which exhibit thin-skinned tectonics with the detachment boundary at shallow depths. The present study may be useful to understand the causes responsible for the difference in tectonic style (thick and thin) during the Proterozoic and Phanerozoic periods.
Biswajit Mandal, V. Vijaya Rao, P. Karuppannan, S. Raju, Shib Sankar Ganguli
Thick-skinned tectonics of the Achankovil Shear Zone, southern India, inferred from new deep seismic reflection image: Constraints on the East African Orogen