STRUCTURAL GEOMETRY AND KINEMATIC EVOLUTION OF FAR-EASTERN LESSER HIMALAYAN FOLD THRUST BELT: INSIGHTS FROM SIANG WINDOW, ARUNACHAL PRADESH, INDIA

The Lesser Himalayan sequence (LHS), bounded by the Main Central thrust (MCT) in the north and the Main Boundary thrust (MBT) in the south, accommodates the highest orogenic shortening (~52-70%) and records the highest lateral variation in its distribution along the Himalayan fold thrust belt (FTB). The Siang valley in the far-eastern Arunachal Himalaya exposes the LHS rocks that is repeated by a series of imbricate thrusts forming one of the largest windows in the Himalayan FTB. The Pelling-Munsiari thrust (PT) brings the hanging wall Paro-Lalpani Group of the LHS over the Daling Group of the LHS, and is the northernmost regional thrust in this region. The MCT lies farther north of the geographic border. The next major footwall thrust, the Ramgarh thrust (RT), separates the Daling Group in its hanging wall from the carbonates and quartzites of the Buxa Group in the footwall. The RT defines the Siang window. Farther south, the Bomdo thrust (BT) carries the Buxa carbonates of the LHS over the Abor Volcanics. Structurally underlying the BT, at least 6 imbricates repeat the LHS. In the frontal segment, the Main Boundary thrust (MBT) separates the LHS in the hanging wall from the Siwalik Group and the Main Frontal thrust (MFT) juxtaposes the Siwaliks over the Quaternary sediments. The PT and the RT are folded along 14 ^o, 054^o and 5^o, 045^o, respectively. The BT forms a doubly plunging culmination along 26^o,279^o and 12^o,120^o . The frontal MBT and the MFT sheets are folded as fault-bend antiforms along 5^o,314^o and 11^o,313^o , respectively.

We constructed a regional balanced cross-section along the inferred regional transport direction (342^o-162^o). The most prominent structure is the Lesser Himalayan duplex (LHD) comprising at least 12 LHS horses. The basal decellement, the Main Himalayan thrust (MHT), is the floor thrust of the LHD and the RT is its roof thrust. These horses are constrained by the presence of fault rocks, structural discordance, stratigraphic relationships, and estimated thicknesses of the thrust sheets. The MHT lies ~15 km below the LHD and ramps to ~7km in the foreland, as constrained by the restoration of the Siwalik Group. Preliminary estimate of total minimum shortening from 2D-move is ~354 km (~72%), without incorporating slip on the MCT. The highest shortening is accommodated by the LHS (~308 km, ~73%).