Vikash Tripathy

Two major structural events characterize the tectonic evolution of the northern part of the Nallamalai Fold Belt (NFB): (1) the Late Paleoproterozoic or Early Mesoproterozoic Dn1 compression due to the thrusting of granitic gneiss and Nellore Schist Belt (NSB) over the eastern part of the NFB possibly related to the Columbia supercontinental activities; and (2) the Late Neoproterozoic-Early Cambrian Dn2 deformation in the NFB due to its overthrust movement on the western and northern part of the undeformed Cuddapah basin (i.e., the Kurnool and the Palnad sub-basins). The late structures with NW to N vergent hanging wall lithounits (NFB) are represented by several map-scale imbricate thrust sheets, flat and ramp structures, asymmetrical to overturned folds, and structures associated with nappe. The nature of deformation in the footwall (Kurnool Group in Palnad sub-basin) and the hanging wall (NFB) structures are also elaborated in view of the possible regional tectonics. Progressive oroclinal model is also proposed here to explain the changes in the structural trend of NFB from NE to E. The variations of the NFB structures in conjunction with the regional structures connotes of thin-skinned deformation during Pan-African period, which are examined to constrain the possible plate kinematics related to the convergence of East Gondwana fragments.

Fault-slip analysis along Gani-Kalva and Kona faults were performed.Extensional to transtensive regime dominated the post-lower Cuddapah or Kurnool sub-basin opening.Late compressional to transpressive/pure strike-slip regimes dominated the post-Kurnool coeval to the Pan-African event.Multiple fault-slips suggest Cuddapah basin evolution punctuated through various deformational eventsThe sedimentary basins are often influenced by adjoining mobile belts as is the case of the Proterozoic Cuddapah basin in southern India, lying in proximity of the Eastern Ghats granulite belt (EGB) and the Southern granulite terrain (SGT). Two major faults – Gani-Kalva and Kona faults – in the western part of the Cuddapah basin have been examined in this work to comment on the changes in paleostress conditions affecting the basin evolution punctuated by several stages of extensional and compressional regimes in the time interval 1.9 Ga to 0.5 Ga. The tensor solutions were obtained for the mesoscale faults and fault striae using the improved Right Dihedron method followed by the Rotational Optimization method (TENSOR, Delvaux and Sperner, 2003). Combining the paleostress field reconstruction and other structural/stratigraphic attributes we propose a possible scenario of punctuated evolution of the Cuddapah basin and its inversion. The stress regimes are tentatively correlated with multiple stages of basin opening and deformation of basin in-fill, representing the pre-Grenvillian through the Pan-African orogeny. The paleostress conditions derived from various stratigraphic horizons provide an opportunity to comment on the changes in tectonic stresses including several stages of extensional and compressive regimes in the Cuddapah basin.The tensor solutions were obtained for the mesoscale faults and fault striae in the neighbourhood of above two fault lines using the improved Right Dihedron method followed by the Rotational Optimization method (TENSOR, Delvaux and Sperner, 2003). The most prominent stress states recorded around the two fault lines can be classified into those affecting 1) the Paleoproterozoic lower Cuddapah successions, 2) the younger Kurnool Group and 3) both the groups. Integrating the stress states of the fault-slip analysis obtained from different unconformity bounded rock groups of lower Cuddapah and other geologic data, we suggest extensional to transtensive/pure strike-slip regime to be active during the first phase of basin opening and c. 1.89 Ga mafic igneous activity. Late compressional to transpressive/pure strike-slip regimes are well recorded from the younger Kurnool Group representing late Neoproterozoic deformations. The tensor solutions from different unconformity bound stratigraphic horizons constrain different states of deformation. We relate the extensional to transtensive/pure strike-slip regimes from lower Cuddapah to the ongoing basin opening events and the associated Palaeoproterozoic igneous activities. The late Neoproterozoic to early Paloezoic compressional to transpressive/strike-slip regime present the possible effects of compressional activities happening around the cratonic margin. The present analysis lends further support to the idea that estimation of paleostress states from fault-slip data even in regions of relatively weak deformation in continental interior are important while assessing influence of plate margin stresses on the continental interior.

Recent works suggest Proterozoic plate convergence along the southeastern margin of India which led to amalgamation of the high grade Eastern Ghats belt (EGB) and adjoining fold-and-thrust belts to the East Dhrawar craton. Two major thrusts namely the Vellikonda thrust at the western margin of the Nellore Schist belt (NSB) and the Maidukuru thrust at the western margin of the Nallamalai fold belt (NFB) accommodate significant upper crustal shortening, which is indicated by juxtaposition of geological terranes with distinct tectonostratigraphy, varying deformation intensity, structural styles and metamorphic grade. Kinematic analysis of structures and fabric of the fault zone rocks in these intracontinental thrust zones and the hanging wall and footwall rocks suggest spatially heterogeneous partitioning of strain into various combinations of E-W shortening, top-to-west shear on stratum parallel subhorizontal detachments or on easterly dipping thrusts, and a strike slip component. Although relatively less prominent than the other two components of the strain triangle, non-orthogonal slickenfibres associated with flexural slip folds and mylonitic foliation-stretching lineation orientation geometry within the arcuate NSB and NFB indicate left lateral strike slip subparallel to the overall N-S trend. On the whole an inclined transpression is inferred to have controlled the spatially heterogeneous development of thrust related fabric in the terrane between the Eastern Ghats belt south of the Godavari graben and the East Dharwar craton.