Late Quaternary tectonic landforms and fluvial aggradation in the Saryu River valley: Central Kumaun Himalaya

• Phases of aggradation/incision in response to climate and tectonic activity during the late Quaternary have been studied.
• The oldest aggradational landforms preserved south of the MCT are dated between 32-49 ka.
• The period between 10-5 ka suggests lowering of fluvial discharge and sediment transport capacity.
• Major phases of bedrock incision have been dated during ~ 49 ka, 32 ka, 16 ka, 12 ka, 8 ka and around 5 ka.
• The period between 9 and 5 ka shows relatively enhanced surface uplift (2-5 mm/year).

The present study has been carried out with special emphasis on the aggradational landforms to explain the spatial and temporal variability in phases of aggradation/incision in response to tectonic activity during the late Quaternary in the Saryu River valley in central Kumaun Himalaya. The valley has preserved cut-and-fill terraces with thick alluvial cover, debris flow terraces, and bedrock strath terraces that provide signatures of tectonic activity and climate. Morphostratigraphy of the terraces reveals that the oldest landforms preserved south of the Main Central Thrust, the fluvial modified debris flow terraces, were developed between 30 and 45 ka. The major phase of valley fill is dated between 14 and 22 ka. The youngest phase of aggradation is dated at early and mid-Holocene (9–3 ka). Following this, several phases of accelerated incision/erosion owing to an increase in uplift rate occurred, as evident from the strath terraces. Seven major phases of bedrock incision/uplift have been estimated during 44 ka (3.34 mm/year), 35 ka (1.84 mm/year), 15 ka (0.91 mm/year), 14 ka (0.83 mm/year), 9 ka (1.75 mm/year), 7 ka (5.38 mm/year), and around 3 ka (4.4 mm/year) from the strath terraces near major thrusts. We postulate that between 9 and 3 ka the terrain witnessed relatively enhanced surface uplift (2–5 mm/year).