Lateral Variations in Compressional Structural Geometries Along an Active Plate Margin; The Assam-Arakan Fold-Thrust Belt of Northeast India

Abstract

Located along the northeastern collisional boundary to the Indian plate and subcontinent, the Assam-Arakan mountain range is a 1200 km long, convex eastward, north-south oriented fold-thrust belt that has undergone complex yet variable compressional deformation since the late Miocene. Within northeast India the Assam-Arakan fold-thrust belt displays three distinct structural zones, including the Upper Assam-Nagaland thrust belt to the north, the Cachar-Mizoram fold-belt to the south, and an intervening tectonic transition zone where the fold-thrust belt impinges on the east-west trending basement uplift of the Shillong Plateau. The Upper Assam-Nagaland fold-thrust belt is characterized by the presence of a frontal, emergent imbricate-fan where the thrust belt overrides the Brahmaputra foreland basin, and an internal structural zone along which multi-kilometer-thick thrust sheets and underlying duplex systems have accommodated up to 100 km of tectonic shortening. The Cachar-Mizoram sector of the fold-thrust belt is characterized by an up to 100 km wide frontal fold-train along which tight, symmetric to asymmetric detachment folds, faulted detachment folds, and/or fault-propagation folds have been mapped. The internal sectors of the Cachar-Mizoram tectonic zone include complexly deformed fold-trains and associated thrust faults that have undergone significant uplift along an underlying tectonic wedge that has been interpreted as a tectonic-scale passive-roof duplex. A structural transition zone that separates the northern and southern sectors to the fold-thrust belt has been mapped along the east-plunging nose to the south-vergent, basement-cored Shillong Anticline, where the fold-thrust belt impinges upon this basement high and along which the underlying detachment surface to the Assam-Arakan fold-thrust belt has been rotated into a sub-vertical position on the south-limb of the structure. Geological mapping further indicates that the Dauki fault, mapped along the southern flank of the Shillong Plateau is an up-turned basal detachment surface to the Assam-Arakan thrust-belt rather than a strike-slip fault, as interpreted in the past. While the Upper Assam-Nagaland sector of the fold-thrust belt and the adjacent Brahmaputra foreland basin have included numerous oil discoveries over the last 150 years, the Cachar-Mizoram sector of the orogenic belt is a less well-explored region dominated by natural gas seeps and recent gas discoveries.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018