Nature of the Naga Thrust and Fold Belt, Assam Shelf, India: Insights from cross-sectional balancing

Abstract

The Naga Thrust and Fold Belt of the Assam Shelf, India, is one of the most prolific onshore hydrocarbon provinces in the world remaining underexplored due to limited conceptualization and testing of structural concepts. We have restored two interpretations of a profile across the Jaipore Anticline, the hanging wall anticline of the Naga Thrust, constrained by well-tops and inversion-based imaging. The first is in line with the fault-propagation fold model, commonly applied to the Naga Thrust, but does not agree with dips of the ramp reflections. The second adequately honors the ramp reflections and requires presence of two decollements - shallower in the Upper Barail group (Naga Thrust) and deeper in the Jaintia group (Bally Thrust). The deeper thrust is blind and from inversion of pre-existing growth structures. In both interpretations, the hanging wall exhibits extensive second - order deformations from an antithetic thrust, accommodating over 50% of the total shortening and the footwall is characterized by duplex structures resulting from under-thrusting and triangle zone formation. Results do not refute the existing fault-propagation fold model of the Naga Thrust. Rather, it proposes that there is a change in structural style and kinematics along the thrust belt, which separates the location of our profile from the type-locate of the fault-propagation fold model. A variety of structural prospects emerge from the dual-decollement model, which may provide a new direction to exploration in the Naga Thrust and Fold Belt.