BALANCED CROSS SECTIONS AND STRUCTURAL EVOLUTION OF NAGA SCHUPPEN BELT, NE INDIA

Abstract

Assam-Arakan Fold-Thrust Belt (AA-FTB) and Naga-FTB in NE India evolved due to India-Burma plate convergence. The Tertiary sediments are deformed by faulting and fault related folding, akin to thin skinned tectonic. Jaipur anticline (JA) and its northeastern extension, Digboi anticline (DA) are ramp anticlines associated with frontal most Naga Thrust (NT) of the Naga-FTB. Surface trace of Naga Thrust is convex towards foreland representing a salient, Jaipur Salient. Digboi anticline is modelled as simple step fault-propagation fold with high angle breakthrough. Jaipur anticline is modelled as simple step fault-propagation fold with synclinal breakthrough. Seismic reflection profiles constrain the branch line of Naga Thrust from detachment surface and depth of detachment. In Digboi anticline, the detachment depth is higher at 5.2 km and in Jaipur anticline, detachment depth is at 4.2 km. Total displacement and shortening in Digboi anticline are less compared to Jaipur anticline. As a result, in between Digboi and Jaipur anticline, Assam Rail Tear Fault is formed. It acts as transfer zone and offsets axial traces of both the anticlines. Due to decrease of detachment depth and increase of displacement from Digboi to Jaipur anticline, the anticlinal axial surface is gradually translated above topography in southwest sector of Jaipur anticline. Jaipur/ Digboi anticline plunges towards NE. Progressive unroofing of anticline takes place from NE to SW. In Digboi anticline, petroleum system (source, reservoir and seal) is at subsurface at anticlinal closure. Thus, potential trap form at Digboi anticline and hydrocarbon field has been discovered. In Jaipur anticline, due to progressive unroofing of anticline, reservoir and source rocks are exposed at the surface. Regional and local seals and anticlinal closure is eroded off. As a result, no hydrocarbon field has been discovered at Jaipur anticline but surface exposure of reservoir and source rocks result in oil seepages. Seismic profiles suggest buried structure in the footwall of Naga Thrust. It is modelled as buried anticline (Kusijan Anticline, KA) at the tip of blind thrust (Kusijan Thrust, KjT). Kusijan anticline is cored with source, reservoir and seal rocks; and is a producing structure. The regional section (Margherita section) extends from foreland most Naga Thrust to hinterland most Zungki Thrust. The structural geometry portrays a series of anticlines and synclines associated with thrust ramps and buried horses and duplex. All the thrusts step up from upper detachment surface, except for hinterland most Zungki Thrust, and verges towards foreland. The kinematic evolution of Margherita section involves forward breaking insequence thrusting followed by out-of-sequence thrusting in back breaking sequence. During out-of-sequence thrusting, new ramps are formed and earlier ramps are reactivated breaching duplex and anticlinal structures. Total shortening accommodated in Margherita section is 56.5%. The restored length of the section is stretch to 213 km towards hinterland. Thus, it indicates that the rocks in the deformed section of Margherita Transect are far travelled rocks, translated from deeper and interior of hinterland.