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The present study is an attempt to understand the antiquity of the preserved fluvial landforms and its response to the climate–tectonics nexus through geomorphological investigations along the Jadukata, Umpung, Umngot and Umtongoi rivers in the southern front of the Shillong plateau (SP), NE India. Sedimentological characteristics, chronological analyses and morphotectonic parameters were used to describe the spatial and temporal variability in the patterns of aggradation, landform evolution and neotectonic influences in the study. Our results indicate that valley aggradation processes occurred around the transitional zone in the southern front of the SP during the mid–late Holocene era along with a hiatus in sediment deposition after 4.3 ka. Sediment generation and aggradation is modulated by precipitation anomalies associated with the Indian Summer Monsoon (ISM) variability whereas morphometric analysis suggests that activity along the Dauki–Dapsi fault has been contributing to the uplift-related deformation. Sedimentological observation supported by optically stimulated luminescence chronology obtained on palaeoflood deposits, valley-fill fluvial terraces and alluvial fans indicate their deposition during three pluvial phases: (i) 5.3–4.3 ka, (ii) 2.4–1.0 ka and (ii) 0.7–0.3 ka. Our data indicate that valley aggradation and geomorphic processes in the southern part of SP responded to short-term changes in the ISM variability with contributions from the morphotectonic activities associated with the Dauki–Dapsi fault during the late Holocene period.

Although the M=w8.7, 1950 Assam earthquake endures as the largest continental earthquake ever recorded, its exact source and mechanism remain contentious. In this paper, we jointly analyze the spatial distributions of reappraised aftershocks and landslides, and provide new field evidence for its hitherto unknown surface rupture extent along the Mishmi and Abor Hills. Within both mountain fronts, relocated aftershocks and fresh landslide scars spread over an area of ≈330 km by 100 km. The former are more abundant in the Abor Hills while the later mostly affect the front of the Mishmi Hills. We found steep seismic scarps cutting across fluvial deposits and bounding recently uplifted terraces, some of which less than two thousand years or even a couple centuries old, at several sites along both mountain fronts. They likely attest to a minimum 200 km-long 1950 surface rupture on both the Mishmi and Main Himalayan Frontal Thrusts (MT and MFT, respectively), crossing the East Himalayan Syntaxis. At two key sites (Wakro and Pasighat), co-seismic surface throw appears to have been over twice as large on the MT as on the MFT (7.6 ± 0.2 m vs. >2.6 ± 0.1 m), in keeping with the relative, average mountain heights (3200 m vs. 1400 m), mapped landslide scar numbers (182 vs. 96), and average thrust dips (25–28° vs. 13–15°) consistent with relocated aftershocks depths. Corresponding average slip amounts at depth would have been ≈17 and ≈11 m on the MT and MFT, respectively, while surface slip at Wakro might have reached ≈34 m. Note that this amount of superficial slip would be out of reach using classic paleo-seismological trenching to reconstruct paleo-earthquake history. Most of the 1950 first arrivals fit with a composite focal mechanism co-involving the two shallow-dipping thrust planes. Their intersection lies roughly beneath the Dibang Valley, implying forced slip parallel to GPS vectors across the East Himalayan Syntaxis. Successive, near-identical, terrace uplifts at Wakro suggest near-characteristic slip during the last two surface rupturing earthquakes, while terrace boulder ages may be taken to imply bi-millennial return time for 1950-size events. As in Nepal, East-Himalayan mega-quakes are not blind and release most of the elastic, interseismic shortening that accumulates across the range. NRF (Natl Research Foundation, S’pore) MOE (Min. of Education, S’pore) Published version

The present study is undertaken in the Kulsi River valley, a tributary of the Brahmaputra River that drains through the tectonically active Shillong Plateau in northeast India. Based on the fluvial geomorphic parameters and Landsat satellite images, it has been observed that the Kulsi River migrated 0.7–2 km westward in its middle course in the past 30 years. Geomorphic parameters such as longitudinal profile analysis, stream length gradient index (SL), ratio of valley floor width to valley height (Vf), steepness index ( $$k_{s})$$ indicate that the upstream segment of the Kulsi River is tectonically more active than the downstream segment which is ascribed to the tectonic activities along the Guwahati Fault. $$^{14}\hbox {C}$$ ages obtained from the submerged tree trunks of the Chandubi Lake, which is located in the central part of the Kulsi River catchment suggests inundation (high lake levels) during 160 ± 50 AD, 970 ± 50 AD, 1190 ± 80 AD and 1520 ± 30 AD, respectively. These periods broadly coincide with the late Holocene strengthened Indian Summer Monsoon (ISM), Medieval Warm Period (MWP) and the early part of the Little Ice Age (LIA). The debris which clogged the course of the river in the vicinity of the Chandubi Lake is attributed to tectonically induced increase in sediment supply during high magnitude flooding events.

Study of quantitative morphometric parameters was taken up in four major river valleys in the southern part of Shillong Plateau using SRTM DEM in GIS. The study indicates that the region is undergoing differential uplift. This is evidenced by preferential tilting towards east, while the central part of the plateau exhibits higher rate of uplift than the eastern and western segments. We ascribed the higher rate of uplift in the central segment of Shillong Plateau to the activity along the Dapsi Thrust and Dauki Fault.