Development of fold styles in the strongly arcuate Cuddapah fold-thrust belt, India: new insights from analogue models

Many arcuate orogenic belts display complex patterns of fold structures. However, the kinematics of their development is poorly understood. This study aims to explore the origin of complex fold structures in the Proterozoic mobile belt of Cuddapah in peninsular India. This is a N-S trending thin-skinned fold-thrust belt, showing a spectacular crescent shape with westward convexity. The tectonic reconstructions suggest that the Cuddapah fold-thrust belts (CFTB) evolved through westward indentation of the westerly convex Nellore-Khammam schist belts (NKSB) during the collision between the Indian shield and the Antarctic block, interpreted as a consequence of the Rodinia Supercontinental event in Neoproterozoic time. The westward convergence resulted in the development of highly deformed Nallamalai fold belt (NFB) in the eastern part of CFTB, consisting of orogen-scale overturned folds with arcuate trends as well as higher order west-verging F₁ folds on a wide range of spatial scales. Inclined to recumbent F₁ folds are coaxially refolded by orogen-parallel F₂ folds in outcrop scale. However, the superposition of steeply inclined E-W trending cross-folds (F₃) that produced culminations and depressions on F₁ and F₂ indicates that the belt underwent orogen-parallel shortening in the course of its evolution. We performed laboratory experiments on a scaled representative CFTB model to understand such spatiotemporal evolution of the strain field in the CFTB. Two sets of experiments were run with a constant (2 cm/yr) and a temporally varying (2 cm/yr in Stage I to 1 cm/yr in Stage II) convergence velocity. The experimental results show that the CFTB during the initial periods of shortening undergoes mainly flattening deformations with maximum horizontal instantaneous stretching axes (ISA_Hmax), describing an arcuate trajectory in the eastern part that conforms to the indentor shape. In the later periods of the experimental runs (albeit at different times in the two experiments), the arcuate shape of the fixed wall causes a SW and NE-directed upper crustal flows from the elevated NE and SW parts of the CFTB, respectively. The flow convergence causes the strain field in the central part of the CFTB to develop a horizontal constriction. This kinematic transition leads to superposition of cross-folds on orogen-parallel folds, manifested in culmination (domes) and depression (basins) structures.