Geometry and Kinematics of Quaternary Fault and Fold Growth Along the Northeastern Himalayan Front: Implications for Himalayan Drainage Development and Active Expansion of the Himalayan arc.
Understanding active mountain building can provide a guide to determine the processes responsible for the formation of older orogens. With regard to the Himalaya, early research efforts have mostly focused on Tertiary structures located within the mountain range such as the Main Central Thrust (MCT). Only minimal effort, all conducted in the western and central Himalaya, was devoted to investigation of the Main Frontal Thrust Zone (MFTZ), the youngest Himalayan fault system that is actively uplifting and expanding the Himalayan range. Our study addresses this issue by integrating field- and satellite-based structural data at several sites along the eastern Himalayan front (Arunachal) in NE India. On a regional scale, the interaction between the MFTZ and the eastern Himalayan drainage systems is most prominently displayed. Specifically, folds are aligned in an en echelon fashion at about 20-30 degrees from the main trace of the Himalayan frontal thrust that defines a continuous topographic front. The en echelon folds in map view indicate a left-slip shear across the eastern segment of the Himalayan front. All the folds are east-plunging and are associated with eastward deflection of generally south-flowing Himalayan rivers. We focused our study at one site where the Main Frontal Thrust is expressed by the Bhalukpong thrust and the growing fold is represented by the southeast-plunging ~25 km- long Balipari anticline, across which the generally south-flowing Kameng River is deflected to the east around its eastern nose. Associated with the river deflection are several levels of tilted and uplifted terraces. The Bhalukpong thrust ~10 km north of the anticline juxtaposes a recumbent fold in the Miocene Dafla Formation over Quaternary gravels. Although the thrust bounds several levels of Quaternary terraces in its hanging wall, the fault trace itself is covered by a young fluvial terrane, suggesting that the fault has not been active since its deposition. Field mapping and a total-station survey revealed three major terrace levels across the contractional structures. We also use 29 14C samples to date the terraces, among which the age of the T2 riser on the south side of the anticline is best constrained. Three samples from T2 were collected across the south flank of the anticline: two from our best total-station-survey sites at the northwest side of the fold and one at the nose of the anticline, which yield ages between 883+/- 38 yrs BP and 922 +/- 11 years BP. Abandonment of T1 above the Bhalukpong thrust was constrained by a single charcoal age at 618 +/- 37 yrs BP. With respect to end- member models for the Himalayan arc growth emphasizing cross-sectional evolution (i.e., fault-propagation folding in the western Himalayan front and fault-bend folding in the central Himalaya), our work points to the importance of lateral growth of contractional structures, alternating motion among several contractional structures in the MFTZ, and role of orogen-parallel shear at the ends of the Himalayan arc.
AGU Fall Meeting Abstracts
- Pub Date:
- December 2007
- 8104 Continental margins: convergent;
- 8107 Continental neotectonics (8002);
- 8175 Tectonics and landscape evolution