Stratigraphy and Structure of Siwaliks in Arunachal Pradesh: a Reappraisal through Remote Sensing Techniques

The Siwalik belt in Arunachal Pradesh, which extends from Bhutan border in the west to Pasighat in the east over a distance of about 350 km, was studied in detail using recent aerial photos and IRS-1C LISS 3 digital data products for understanding its stratigraphy and structure. Critical field checks were subsequently carried out.

The Siwalik belt, which is roughly 20 km wide, is exposed between Brahmaputra alluvium in the south and pre-Tertiary rocks in the north. Main Boundary Fault (MBF) defines the northern limit of Siwalik sedimentaries.

Two major en echelon E-W to ENE-WSW trending, northerly dipping faults (Fault-1, 2) have divided the Siwalik belt into three tectonic blocks (Tectonic Block-1, 2 and 3). Fault-1 branches out from MBF in the west and extends through Siwalik belt to Subansiri River in the middle part of the study area. Here Fault-2 in the north, branches out from MBF and extends through Siwalik belt to Nari in the eastern part.

The poorly cemented pebble/cobble beds and soft, immature sandstones/sand rocks of Tectonic Block-1 (south of Fault-1) are considered to represent the youngest part of Siwalik Group and they were folded to NE plunging (5° to 10°) open, mega anticline. The well-bedded sandstones of Tectonic BJock-2 (north of Fault-1 or south of Fault-2) and thickly bedded sandstones with huge pebble bed horizons of Tectonic Block-3 (north of Fault-2) reveal gentle to moderate (10° to 30°) homoclinal dip towards north to NW.

Two contradicting models are proposed to explain the tectonic setup of Siwalik belt. In the first model, MBF, which is dipping at -60° due north at the surface level, is considered to become gentle at depth and towards north. Fault-1 and Fault-2 are considered as diverging splay thrusts from the main thrust, MBF. In the second model, it is proposed that MBF, Fault-1 and Fault-2 extend downwards as near-vertical faults, even though they reveal moderate northerly dip at the surface level. This second model, which involves reactivation of basement weaker zones and differential vertical movement in a non-compressive environment, explains in a convincing way the overall tectonic setup of Siwalik belt in Arunachal Pradesh.