Multi-Stage Deformation in the Nallamalai Fold Belt, Cuddapah Basin, South India - Implications for Mesoproterozoic Tectonism Along Southeastern Margin of India

Abstract

The common occurrence of small-scale refolded folds, deformed lineations and overprinting of foliation in the northern Nallamalai Fold Belt (NFB) within the intracratonic Cuddapah Basin, South India, suggest three phases of deformation, D 1, D 2 and D 3 during the late Paleoproterozoic to Neoproterozoic. D 1 structures are represented by tight to isoclinal folds, a slaty cleavage and local development of mylonites. D 2 structures include NE trending tight to open folds (F 2) with variable plunge indicating control of large domal structures and a steep crenulation cleavage developed in phyllites and other schistose rocks. The above structures are overprinted by E-W trending D 3 folds and cleavage, which affect Mesoproterozoic rocks of the Srisailam Formation (uppermost Nallamalai Group) in the E-W trending Vami Konda range and metasediments of the Palnad area, thought to be equivalent of the Neoproterozoic Kurnool Group. Taking into account the ages of intrusive Vellaturu Granite (∼1575 Ma) which is post-D 2 deformation in the NFB and the Chelima Lamproite (∼1400 Ma) which intrudes the folded Nallamalai rocks, we suggest that the earlier episodes of multiple deformation in the NFB represent either late Paleoproterozoic or early Mesoproterozoic NW-SE compression in this part of the East Gondwana. The E-W fold-thrust structures affecting the Srisailam Quartzite and adjacent metasediments in the Palnad area represent late Mesoproterozoic and/or Neoproterozoic event. The granite gneisses and schists of the Nellore Schist Belt, bordering the NFB are affected by both early and later events and thrust over rocks of the Nallamalai Group. We propose that early deformation in the NFB lying southwest of the Eastern Ghats and roughly parallel to the present southeastern coast of India, was unrelated to, and occurred prior to the main deformation in the Eastern Ghats, which is generally correlated with the ∼1000 Ma old Grenville event. Alternatively, in view of the complex, protracted evolution of the Eastern Ghats and published geochronological data supporting pre-Grenvillian tectonism particularly in the Western Charnockite Zone of the Eastern Ghats, D 1 and D 2 deformation in the NFB may be linked to this older (1600-1400 Ma) tectonic disturbance in the Eastern Ghats.