@article{DeyThiedeSchildgenetal.2016,
  author    = {Dey, Saptarshi and Thiede, Rasmus Christoph and Schildgen, Taylor F. and Wittmann, Hella and Bookhagen, Bodo and Scherler, Dirk and Jain, Vikrant and Strecker, Manfred},
  title     = {Climate-driven sediment aggradation and incision since the late Pleistocene in the NW Himalaya, India},
  series = {Earth \& planetary science letters},
  volume    = {449},
  journal   = {Earth \& planetary science letters},
  publisher = {Elsevier},
  address   = {Amsterdam},
  issn      = {0012-821X},
  doi       = {10.1016/j.epsl.2016.05.050},
  pages     = {321 -- 331},
  year      = {2016},
  abstract  = {Deciphering the response of sediment routing systems to climatic forcing is fundamental for understanding the impacts of climate change on landscape evolution. In the Kangra Basin (northwest Sub-Himalaya, India), upper Pleistocene to Holocene alluvial fills and fluvial terraces record periodic fluctuations of sediment supply and transport capacity on timescales of 10(3) to 10(5) yr. To evaluate the potential influence of climate change on these fluctuations, we compare the timing of aggradation and incision phases recorded within remnant alluvial fans and terraces with climate archives. New surface-exposure dating of six terrace levels with in-situ cosmogenic Be-10 indicates the onset of incision phases. Two terrace surfaces from the highest level (T1) sculpted into the oldest preserved alluvial fan (AF1) date back to 53.4 +/- 3.2 ka and 43.0 +/- 2.7 ka (1 sigma). T2 surfaces sculpted into the remnants of AF1 have exposure ages of 18.6 +/- 1.2 ka and 15.3 +/- 0.9 ka, while terraces sculpted into the upper Pleistocene-Holocene fan (AF2) provide ages of 9.3 +/- 0.4 ka (T3), 7.1 +/- 0.4 ka (T4), 5.2 +/- 0.4 ka (T5) and 3.6 +/- 0.2 ka (T6). Together with previously published OSL ages yielding the timing of aggradation, we find a correlation between variations in sediment transport with oxygen-isotope records from regions affected by the Indian Summer Monsoon. During periods of increased monsoon intensity and post-Last Glacial Maximum glacial retreat, aggradation occurred in the Kangra Basin, likely due to high sediment flux, whereas periods of weakened monsoon intensity or lower sediment supply coincide with incision. (C) 2016 Elsevier B.V. All rights reserved.},
  language  = {en}
}
@article{DeyThiedeSchildgenetal.2016,
  author    = {Dey, Saptarshi and Thiede, Rasmus Christoph and Schildgen, Taylor F. and Wittmann, Hella and Bookhagen, Bodo and Scherler, Dirk and Strecker, Manfred},
  title     = {Holocene internal shortening within the northwest Sub-Himalaya: Out-of-sequence faulting of the Jwalamukhi Thrust, India},
  series = {Tectonics},
  volume    = {35},
  journal   = {Tectonics},
  publisher = {American Geophysical Union},
  address   = {Washington},
  issn      = {0278-7407},
  doi       = {10.1002/2015TC004002},
  pages     = {2677 -- 2697},
  year      = {2016},
  abstract  = {The southernmost thrust of the Himalayan orogenic wedge that separates the foreland from the orogen, the Main Frontal Thrust, is thought to accommodate most of the ongoing crustal shortening in the Sub-Himalaya. Steepened longitudinal river profile segments, terrace offsets, and back-tilted fluvial terraces within the Kangra reentrant of the NW Sub-Himalaya suggest Holocene activity of the Jwalamukhi Thrust (JMT) and other thrust faults that may be associated with strain partitioning along the toe of the Himalayan wedge. To assess the shortening accommodated by the JMT, we combine morphometric terrain analyses with in situ Be-10-based surface-exposure dating of the deformed terraces. Incision into upper Pleistocene sediments within the Kangra Basin created two late Pleistocene terrace levels (T1 and T2). Subsequent early Holocene aggradation shortly before similar to 10ka was followed by episodic reincision, which created four cut-and-fill terrace levels, the oldest of which (T3) was formed at 10.10.9ka. A vertical offset of 445m of terrace T3 across the JMT indicates a shortening rate of 5.60.8 to 7.51.1mma(-1) over the last similar to 10ka. This result suggests that thrusting along the JMT accommodates 40-60\% of the total Sub-Himalayan shortening in the Kangra reentrant over the Holocene. We speculate that this out-of-sequence shortening may have been triggered or at least enhanced by late Pleistocene and Holocene erosion of sediments from the Kangra Basin.},
  language  = {en}
}
@phdthesis{Dey2016,
  author    = {Dey, Saptarshi},
  title     = {Tectonic and climatic control on the evolution of the Himalayan mountain front},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-103390},
  school      = {Universit{\"a}t Potsdam},
  pages     = {xii, 118},
  year      = {2016},
  abstract  = {Variations in the distribution of mass within an orogen may lead to transient sediment storage, which in turn might affect the state of stress and the level of fault activity. Distinguishing between different forcing mechanisms causing variations of sediment flux and tectonic activity, is therefore one of the most challenging tasks in understanding the spatiotemporal evolution of active mountain belts. The Himalayan mountain belt is one of the most significant Cenozoic collisional mountain belt, formed due to collision between northward-bound Indian Plate and the Eurasian Plate during the last 55-50 Ma. Ongoing convergence of these two tectonic plates is accommodated by faulting and folding within the Himalayan arc-shaped orogen and the continued lateral and vertical growth of the Tibetan Plateau and mountain belts adjacent to the plateau as well as regions farther north. Growth of the Himalayan orogen is manifested by the development of successive south-vergent thrust systems. These thrust systems divide the orogen into different morphotectonic domains. From north to south these thrusts are the Main Central Thrust (MCT), the Main Boundary Thrust (MBT) and the Main Frontal Thrust (MFT). The growing topography interacts with moisture-bearing monsoonal winds, which results in pronounced gradients in rainfall, weathering, erosion and sediment transport toward the foreland and beyond. However, a fraction of this sediment is trapped and transiently stored within the intermontane valleys or 'dun's within the lower-elevation foothills of the range. Improved understanding of the spatiotemporal evolution of these sediment archives could provide a unique opportunity to decipher the triggers of variations in sediment production, delivery and storage in an actively deforming mountain belt and support efforts to test linkages between sediment volumes in intermontane basins and changes in the shallow crustal stress field. As sediment redistribution in mountain belts on timescales of 102-104 years can effect cultural characteristics and infrastructure in the intermontane valleys and may even impact the seismotectonics of a mountain belt, there is a heightened interest in understanding sediment-routing processes and causal relationships between tectonism, climate and topography. It is here at the intersection between tectonic processes and superposed climatic and sedimentary processes in the Himalayan orogenic wedge, where my investigation is focused on. The study area is the intermontane Kangra Basin in the northwestern Sub-Himalaya, because the characteristics of the different Himalayan morphotectonic provinces are well developed, the area is part of a region strongly influenced by monsoonal forcing, and the existence of numerous fluvial terraces provides excellent strain markers to assess deformation processes within the Himalayan orogenic wedge. In addition, being located in front of the Dhauladhar Range the region is characterized by pronounced gradients in past and present-day erosion and sediment processes associated with repeatedly changing climatic conditions. In light of these conditions I analysed climate-driven late Pleistocene-Holocene sediment cycles in this tectonically active region, which may be responsible for triggering the tectonic re-organization within the Himalayan orogenic wedge, leading to out-of-sequence thrusting, at least since early Holocene. The Kangra Basin is bounded by the MBT and the Sub-Himalayan Jwalamukhi Thrust (JMT) in the north and south, respectively and transiently stores sediments derived from the Dhauladhar Range. The Basin contains ~200-m-thick conglomerates reflecting two distinct aggradation phases; following aggradation, several fluvial terraces were sculpted into these fan deposits. 10Be CRN surface exposure dating of these terrace levels provides an age of 53.4±3.2 ka for the highest-preserved terrace (AF1); subsequently, this surface was incised until ~15 ka, when the second fan (AF2) began to form. AF2 fan aggradation was superseded by episodic Holocene incision, creating at least four terrace levels. We find a correlation between variations in sediment transport and ∂18O records from regions affected by the Indian Summer Monsoon (ISM). During strengthened ISMs sand post-LGM glacial retreat, aggradation occurred in the Kangra Basin, likely due to high sediment flux, whereas periods of a weakened ISM coupled with lower sediment supply coincided with renewed re-incision. However, the evolution of fluvial terraces along Sub-Himalayan streams in the Kangra sector is also forced by tectonic processes. Back-tilted, folded terraces clearly document tectonic activity of the JMT. Offset of one of the terrace levels indicates a shortening rate of 5.6±0.8 to 7.5±1.0 mm.a-1 over the last ~10 ka. Importantly, my study reveals that late Pleistocene/Holocene out-of-sequence thrusting accommodates 40-60\% of the total 14±2 mm.a-1 shortening partitioned throughout the Sub-Himalaya. Importantly, the JMT records shortening at a lower rate over longer timescales hints towards out-of-sequence activity within the Sub-Himalaya. Re-activation of the JMT could be related to changes in the tectonic stress field caused by large-scale sediment removal from the basin. I speculate that the deformation processes of the Sub-Himalaya behave according to the predictions of critical wedge model and assume the following: While >200m of sediment aggradation would trigger foreland-ward propagation of the deformation front, re-incision and removal of most of the stored sediments (nearly 80-85\% of the optimum basin-fill) would again create a sub-critical condition of the wedge taper and trigger the retreat of the deformation front. While tectonism is responsible for the longer-term processes of erosion associated with steepening hillslopes, sediment cycles in this environment are mainly the result of climatic forcing. My new 10Be cosmogenic nuclide exposure dates and a synopsis of previous studies show the late Pleistocene to Holocene alluvial fills and fluvial terraces studied here record periodic fluctuations of sediment supply and transport capacity on timescales of 1000-100000 years. To further evaluate the potential influence of climate change on these fluctuations, I compared the timing of aggradation and incision phases recorded within remnant alluvial fans and terraces with continental climate archives such as speleothems in neighboring regions affected by monsoonal precipitation. Together with previously published OSL ages yielding the timing of aggradation, I find a correlation between variations in sediment transport with oxygen-isotope records from regions affected by the Indian Summer Monsoon (ISM). Accordingly, during periods of increased monsoon intensity (transitions from dry and cold to wet and warm periods - MIS4 to MIS3 and MIS2 to MIS1) (MIS=marine isotope stage) and post-Last Glacial Maximum glacial retreat, aggradation occurred in the Kangra Basin, likely due to high sediment flux. Conversely, periods of weakened monsoon intensity or lower sediment supply coincide with re-incision of the existing basin-fill. Finally, my study entails part of a low-temperature thermochronology study to assess the youngest exhumation history of the Dhauladhar Range. Zircon helium (ZHe) ages and existing low-temperature data sets (ZHe, apatite fission track (AFT)) across this range, together with 3D thermokinematic modeling (PECUBE) reveals constraints on exhumation and activity of the range-bounding Main Boundary Thrust (MBT) since at least mid-Miocene time. The modeling results indicate mean slip rates on the MBT-fault ramp of ~2 - 3 mm.a-1 since its activation. This has lead to the growth of the >5-km-high frontal Dhauladhar Range and continuous deep-seated exhumation and erosion. The obtained results also provide interesting constraints of deformation patterns and their variation along strike. The results point towards the absence of the time-transient 'mid-crustal ramp' in the basal decollement and duplexing of the Lesser Himalayan sequence, unlike the nearby regions or even the central Nepal domain. A fraction of convergence (~10-15\%) is accommodated along the deep-seated MBT-ramp, most likely merging into the MHT. This finding is crucial for a rigorous assessment of the overall level of tectonic activity in the Himalayan morphotectonic provinces as it contradicts recently-published geodetic shortening estimates. In these studies, it has been proposed that the total Himalayan shortening in the NW Himalaya is accommodated within the Sub-Himalaya whereas no tectonic activity is assigned to the MBT.},
  language  = {en}
}