STRUCTURE AND CRUSTAL SHORTENING OF THE SUBHIMALAYAN FOLD AND THRUST BELT, WESTERN ARUNACHAL PRADESH, NE INDIA

The Subhimalayan fold and thrust belt of western Arunachal Pradesh (AP) includes Miocene and younger sedimentary rocks that comprise northward dipping thrust sheets, structurally below the Main Boundary Thrust (MBT) and above the Main Frontal Thrust (MFT). Balanced cross sections are constructed at various locations along a 100-km segment of the AP Subhimalayan fold and thrust belt. Comparison of these sections reveals structural configurations that are remarkably consistent perpendicular and parallel to the fold and thrust belt. Three regionally significant faults are developed from north to south and are the: (1) MBT, (2) Tipi Thrust and (3) MFT. Within the AP Subhimalayan zone the structures can be lumped into two tectonostratigraphic units: (1) the MFT-Tipi Thrust unit and (2) the MBT-Tipi Thrust unit. The structures of the fold and thrust belt between the MFT and Tipi Thrust in most locations consists of a northerly dipping (10° to 30°) monocline consisting of the Subansiri and Kimin Formations. This part of the fold and thrust belt is interpreted to be the hanging wall flat above the MFT. There are also large folds developed between the Tipi Thrust and MFT. The Tipi Thrust is interpreted to flatten into a decollement that is regionally extensive and located at the base-Neogene unconformity. The decollement is developed in the AP Subhimalayan zone over a distance of at least 300 km. It is likely that the same stratigraphically controlled, regional decollement exists at the base of the Siwalik or top of pre-Tertiary basement as far west as Nepal. The depth to the detachment in both Nepal and western part of AP is approximately 5 km.

An estimate of the amount of shortening within the AP Subhimalayan fold and thrust belt is obtained from restored balanced cross sections. The minimum shortening of about 6 km occurs between the MFT and Tipi Thrust and >6 km between the MBT and Tipi Thrust. This interpretation includes a dramatic four-fold stratigraphic thickening of the Dafla Formation across the Tipi Thrust. Structural thickening caused by thrust imbrication is an alternative interpretation. Given the minimum estimate of shortening of 12 km assuming northward thickening of the Dafla Formation, a slip rate of 8 mm/yr is estimated for motion on the MFT. If the Dafla Formation has a constant thickness, the slip rate could be as high as 20 mm/yr.