Geomorphic evolution of Dehra Dun, NW Himalaya: Tectonics and climatic coupling

    Geomorphic evolution of Dehra Dun, NW Himalaya: Tectonics and climatic coupling Swati Sinha, Rajiv Sinha PII: DOI: Reference:

S0169-555X(16)30266-5 doi: 10.1016/j.geomorph.2016.05.002 GEOMOR 5593

To appear in:

Geomorphology

Received date: Revised date: Accepted date:

22 November 2015 1 May 2016 2 May 2016

Please cite this article as: Sinha, Swati, Sinha, Rajiv, Geomorphic evolution of Dehra Dun, NW Himalaya: Tectonics and climatic coupling, Geomorphology (2016), doi: 10.1016/j.geomorph.2016.05.002

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Geomorphic evolution of Dehra Dun, NW Himalaya: Tectonics and climatic coupling

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Department of Earth Sciences, Indian Institute of Technology Kanpur, Kanpur 208016, India

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Swati Sinha and Rajiv Sinha*

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Abstract

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The Dehra Dun is a good example of a piggyback basin formed from the growth of the Siwalik

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hills. Two large rivers, the Ganga and the Yamuna, and their tributaries deposit a significant part

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of their sediment load in the Dun before they enter the Gangetic plains. This work documents the

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geomorphic complexities and landform evolution of the Dehra Dun through geomorphic

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mapping and chronostratigraphic investigation of the incised fan sections. Lesser Himalayan

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hills, inner and outer dissected hills, isolated hills, proximal fan, distal fan, dip slope unit,

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floodplains, and terraces are the major geomorphic units identified in the area. Isolated hills of

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fan material (IHF), proximal fan (PF), and distal fan (DF) are identified as fan surfaces from

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north to south of the valley. The OSL based chronology of the fan sediments suggests that the

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IHF is the oldest fan consisting of debris flow deposits with a maximum age of ~43 ka

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coinciding with the precipitation minima. The proximal fan consisting of sheet flow deposits

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represents the second phase of aggradation between 34 and 21 ka caused by shifting of

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deposition locus downstream triggered by high sediment supply that exceeded the transport

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capacity. The distal fan was formed by braided river deposits during 20–11 ka coinciding with

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the deglacial period. The IHF, PF and DF surfaces were abandoned by distinct incision phases

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during ~40–35, ~20–17, and ~11–4 ka respectively. A minor phase of terrace deposition in

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Dehra Dun was documented during 3–2 ka. Our results thus show that the evolutionary history of

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the alluvial fans in Dehra Dun was primarily controlled by climatic forcing with tectonics

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playing a minimum role in terms of providing accommodation space and sediment production.

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Keywords: Intermontane valleys, Himalayan foreland, valley fills, fan deposits.

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Corresponding author ([email protected])

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1. Introduction Tectonic and climatic processes have interacted through tightly coupled feedbacks at variable

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spatiotemporal scales to shape mountainous landscapes along the Himalayan belt (Seeber and

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Gornitz, 1983; Ori and Friend, 1984; Lavé, and Avouac, 2000, 2001; Kirby and Whipple, 2001;

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Kumar et al., 2007; Suresh et al., 2007; Singh and Tandon, 2007; 2008). Intermontane valleys

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(Duns) are important elements of mountainous landscapes in the Himalaya that have developed

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in response to climatic fluctuations and neotectonic activities during the Quaternary period

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(Kimura, 1999; Singh et al., 2001; Dühnforth et al., 2007; Goswami and Pant, 2007; Kumar et

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al., 2007; Suresh et al., 2007; Singh and Tandon, 2008; Suresh and Kumar, 2009; Malik et al.,

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2010; Dutta et al., 2012). Alluvial fans, fluvial terraces and floodplains have been mapped as the

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main landforms in the Duns (Nossin, 1971; Nakata, 1972; Rao, 1978; Chandel and Singh, 2000;

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Singh et al., 2001; Suresh et al., 2007; Thakur et al., 2007; Singh and Tandon, 2010; Sinha et al.,

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2010; Dutta et al., 2012). The alluvial fans in the Duns occur at multiple topographic levels

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(Nakata, 1972; Suresh et al., 2002, 2007; Philip et al., 2009) and have been described as a

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piggyback system developed through multiple phases of aggradation and entrenchment during

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the late Quaternary period (Kumar et al., 2007). The development of these fans is influenced by

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tectonic and climate factors such as glacial-interglacial cycles, base-level changes, sediment

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supply from the catchment, availability of accommodation space, and degree of tectonic activity

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(Harvey, 1990, 2004; Silva et al., 1992; Ritter et al., 1995; Harvey et al., 1999a, 1999b; Harvey

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and Wells, 2003; Mather and Stokes, 2003; Mather et al., 2009; Viseras et al., 2003; Robustelli et

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al., 2009). Fluvial terraces also occur widely in Duns, and multiple levels of terraces have been

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mapped in the frontal parts of the Himalaya, e.g., three to five levels in the Yamuna valley

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(Singh et al., 2001; Dutta et al., 2012) and four levels in the Ganga valley (Sinha et al., 2010).

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Most rivers draining the fan surfaces are incised and therefore have developed rather narrow

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floodplains except in the distal parts (discussed later). In a more recent work, several piggyback

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basins along the Himalayan front were examined to explore storage and release of sediments

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from these basins and their downstream influence (Densmore et al., 2015). The suggestion has

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been made that the Duns have controlled the temporal variations in sediment flux into the

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Gangetic plains and have therefore influenced the landscape development in the plains

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(Densmore et al., 2015).

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Despite several generations of geomorphic mapping and landform evolution studies in the Duns

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in the NW Himalaya, we note significant gaps in terms of terminologies, field validation, and

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controlling factors. For example, the term ‘piedmont’ was used to describe the depositional

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surfaces by Singh et al. (2001), Thakur and Pandey (2004) and Thakur et al. (2007) but they do

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not ascribe the processes that might have shaped them. Generally, the terminology ‘piedmont’ is

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used for erosional and depositional landforms formed adjacent to a mountain front (Leece, 1990;

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Goudie, 2004). Chandel and Singh (2000) described the post-Siwalik deposits in Dehra Dun as

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'doon piedmont' but without much process interpretation of the associated geomorphic elements.

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Further, a suggestion was made that the geomorphic evolution of the Dun involved a number of

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depositional and erosional phases superimposed on each other and regulated by climatic changes

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and tectonics (Nakata, 1972), but very limited information is available in terms of field evidence

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and chronological data. Age data on different geomorphic surfaces in the Duns has been limited,

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and therefore a sound chronostratigraphic framework of fans is lacking. In summary, the

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understanding of the evolution of landforms and their spatial relationships is fragmentary.

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We present here a detailed geomorphology of the Dehra Dun based on mapping from high-

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resolution satellite images and digital elevation models (DEM) coupled with field validation.

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Stratigraphic framework is based on several lithologs from incised fan sections and a sound

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chronological data that provides an insight to the depositional history of the Dun.

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2. Study area and previous work

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The Dehra Dun is one of the largest intermontane valleys between the Mussoorie Ranges of the

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Lesser Himalaya to the north and the Siwalik Ranges to the south (Fig. 1). The Siwalik Ranges

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are mainly a group of rocks and categorized as lower (mudstone), middle (sandstone), and upper

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(conglomerate) units. The Dehra Dun is bounded by major faults from all sides; the Main

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Boundary Thrust (MBT) to its north, the Himalayan Frontal Thrust (HFT) to its south, the

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Yamuna Tear Fault (YTF) to the west, and the Ganga Tear Fault (GTF) to the east, making it a

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structurally isolated block (Fig. 1). The Ganga and Yamuna rivers break through the Siwalik

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Ranges along the transverse faults, the GTF, and the YTF, respectively (Karunakaran and Rao,

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1979; Raiverman et al., 1994). The NW-SE trending intermontane valley is ~80 km long and ~25

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km wide. It is an asymmetrical valley with a gentler (0-5˚) SW slope and a steeper (0-10˚) NE

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slope. Alluvial fans, hillocks, river terraces, and floodplains are the major geomorphic units in

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this region, and the valley fills have been described as 'Dun gravels' (Nossin, 1971; Nakata,

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1972; Singh et al., 2001; Thakur and Pandey, 2004; Thakur et al., 2007). The study area is

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confined to the western part of the Dehra Dun between latitudes 30˚20′ to 30˚35′ N and

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longitudes 77˚40′ to 78˚05′ E bounded by the Tons and Yamuna rivers to the east and west,

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respectively (Fig. 1). Several ephemeral streams namely, the Suarna, Sitla Rao, Koti, Mauti,

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Chor Khala, and Nimi, are flowing in this area. These streams and several other smaller rivulets

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flow SW and join the axial Asan river that in turn flows NW and meets the Yamuna River. The

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Siwalik hills to the north of the valley are highly dissected and disjointed and are separated by

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topographical breaks that are marked by sharp turns of the streams flowing through the area

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(Thakur et al., 2007). The southern part of the valley is the dip-controlled slope of Siwalik

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Ranges and mainly composed of upper Siwalik conglomerates and the boulders and gravels of

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quartzite and sandstone derived from the middle Siwalik hills (Thakur et al., 2007).

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Table 1 shows the various geomorphic elements in the Dehra Dun area proposed by various

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workers. Nossin (1971) mapped several large-sized fans descending from the foothills of the

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Mussoorie ranges. Nakata (1972, 1975) identified four fill surfaces and interpreted them to have

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resulted from differential uplift within the valley. Chandel and Singh (2000) mapped Siwalik

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hills and post-Siwalik deposits as piedmonts that were further divided into high-level terraces,

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and upper and lower doon piedmonts. Alluvial fans are the major geomorphological units

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identified by Singh et al. (2001) and Thakur et al. (2007) in the Dehra Dun. The Main Boundary

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thrust is the major structure that separates the Siwaliks and the Lesser Himalaya. Other major

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structural features documented in this area include the Santaurgarh thrust (Raiverman et al.,

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1983), the Bhauwala thrust (Singh, 1998), the Asan and Tons faults (Thakur and Pandey, 2004),

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and the Majahaun fault (Thakur et al., 2007).

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Fan deposits in Dehra Dun mainly consist of boulders, cobbles, and pebbles with a sandy and

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silty matrix (Nossin, 1971; Thakur, 1995). Previous workers (Singh et al., 2001; Thakur et al.,

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2007) have suggested four major depositional units in the valley based on OSL ages. Unit A is

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the oldest with ages varying from 40.3 ± 3.9 to 33.6 ± 4.7 ka and mainly consisting of poorly to

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fairly consolidated gravels with interlayered mudstone and sand beds (Thakur et al., 2007). Unit

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B of unconsolidated massive gravels overlies unconformably on Unit A and yields maximum

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and minimum OSL ages of 29.4 ± 1.7 ka (Singh et al., 2001) and 20.5 ± 1.8 ka (Thakur et al.,

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2007), respectively. Unit C has been dated between ~22.8 and ~10.7 ka and is mainly composed

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of angular to subangular pebbles and gravels. Unit D has been dated as ~30 ka and covers the

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northern slopes of the outer Siwalik range at the southern part of the valley. The total thickness

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of fan deposits in the Dun using tube-well depths, is estimated to be ~100 m in the proximal part

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and >300 m in the distal part (Thakur et al., 2007). The gravel units in fan deposits are massive

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to thickly bedded, and poorly consolidated to unconsolidated. Clasts are randomly oriented and

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angular to subrounded (Singh et al., 2001; Thakur et al., 2007).

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3. Data and methods

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Geomorphic mapping of the Dehra Dun valley was carried out using satellite images, elevation

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data, and topographic maps. Satellite data consisted of IRS P6 LISS 3, LANDSAT image,

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Shuttle Radar Topography Model (SRTM) version 4 (spatial resolution 90 m), and Advanced

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Spaceborne Thermal Emission and Reflection Radiometer (ASTER) v. 1 (spatial resolution 30

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m). The IRS P6 LISS 3 (spatial resolution 23.5 m) images were provided by NRSC (National

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Remote Sensing Center, India) and the rest of the data were downloaded from the USGS web

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site (http://earthexplorer.usgs.gov). The LANDSAT (resolution 30 m) and SPOT (resolution 5

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m) images were fused together in order to enhance the spectral and spatial properties. Major

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geomorphic features were mapped using the LISS 3 image, fusion image of LANDSAT and

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SPOT, topographic maps of 1:50,000 scale and the ASTER DEM. The ArcGIS 10 software was

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used for mapping and geomorphic analysis. The ERDAS imagine 8.5 software was used for

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image processing and River Tools v. 2.4, and 'System for Automated Geo Scientific Analyses'

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(SAGA) 1.2 were used for drawing the profiles across the valley to distinguish the geomorphic

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units on the basis of their relief. Figure 2 shows the geomorphic map of the study area along with

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major geological structures. Table 2 provides a summary of different geomorphic units mapped

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in this work along with corresponding terminologies used by previous workers.

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Six lithologs were prepared from the exposed sections across different fan surfaces to

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describe the stratigraphic framework of fans. To establish the chronology of the fan units, 11

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samples were collected from the fan surfaces and terraces in the study area. Samples were

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collected in iron pipes and immediately sealed to prevent the exposure of light. These samples

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were processed using optically stimulated luminescence (OSL) dating technique at Wadia

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Institute of Himalayan Geology, India. The available OSL dates from previous literature (Singh

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et al., 2001; Thakur et al., 2007; Dutta et al., 2012) were also used, although limited information

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was available for these samples. Table 3 presents the OSL data generated in this work.

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4. Major geomorphic units and fan stratigraphy

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4.1. Lesser Himalaya hills (LHH)

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Lesser Himalaya hills are situated at the hanging wall of the Main Boundary thrust to the north

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(Fig. 3A). This unit is mainly composed of Lesser Himalayan rocks, which include argillites,

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phyllites, quartzite, carbonate, and slates (Hagen, 1969; Stocklin, 1980; Valdiya, 1980). This

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area can easily be demarcated on the satellite images because of its rugged and high relief pattern

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on the DEM. Slope ranges from 10° to 50° and altitude varies from 500 to 3000 m in this unit.

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The presence of active lineaments makes this area prone to landslides and debris falls (Fig. 3B)

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and sediments are transported to the Dun through dendritic to subdendritic drainage network.

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4.2. Dissected hills (DH)

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The dissected hills unit is composed of middle and upper Siwalik rocks that are dominantly

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sandstone and conglomerate, respectively. This unit has been mapped as ‘inner’ and ‘outer’

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dissected hills based on its position in the valley, i.e., north and south of the Dun, respectively.

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The 'inner dissected hills' unit is situated on the footwall of the MBT and on the hanging wall of

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the Santaurgarh thrust (Fig. 3A). The elevation of ‘inner dissected hills' ranges from 1000 to

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1200 m and slope varies from 10° to 30°. The activity of the Santaurgarh thrust is manifested as

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deeply incised valley in the inner dissected hills and presence of large boulders upstream of the

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Koti (Fig. 3C), a very small stream draining the dun surface. The 'outer dissected hills' constitute

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the SW flank of the Mohand anticline. The elevation of the outer dissected hills ranges from 550

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to 800 m and slope varies from 5° to 30°. This unit is highly dissected by several small streams

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like the Suarna, Koti, Mauti, Tons, and many minor ephemeral streams that flow parallel to each

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other.

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4.3. Isolated hills (IH)

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Isolated hills are low-elevation, rounded residual hills adjacent to the Himalayan front (inner

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Siwalik hills, Fig. 3A). The elevation ranges from 620 to 880 m and the slope varies from 5° to

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20°. On satellite images, this unit can be identified as disjointed hills separated from each other

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and at higher elevations compared to the alluvial fans. This unit is covered with thick vegetation

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(reserved forests) and yields highly positive NDVI values (0.40–0.60; Table 2). Based on their

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composition, this unit has been further divided into two subunits, namely isolated hills of fan

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material (IHF) and isolated hills of Siwaliks (IHS). The IHS subunit is mainly composed of

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sandstones of middle Siwaliks and conglomerate deposits of upper Siwaliks. This unit is mainly

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exposed at the eastern and western margins of the study area (Fig. 2). The conglomerate deposits

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are mainly made up of subrounded gravels of quartzite and mudstone derived from the Lesser

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Himalayan region. The IHS unit is at a higher altitude in comparison to the IHF and shows a

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radial drainage pattern.

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The IHF subunit is mainly composed of massive boulders, gravels, and pebbles of quartzite,

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sandstone, and limestone with a sandy matrix. In general, the IHF sediments represent debris

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flow deposits with intermittent sheet flows as suggested by the dominance of disorganized

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gravels and thin layers of matrix-supported organized gravels. Isolated hills of fan (IHF) unit was

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identified as GD-I depositional unit by Singh et al. (2001) and depositional unit 'A' by Thakur et

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al. (2007) (Table 2). Three lithologs, IHF1, IHF2, and IHF3, were prepared from the exposed

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sections in this geomorphic unit (Fig. 4A). The IHF1 section is only 3 m thick with ~15–20 m of

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overburden. The matrix-supported organized (imbricated) gravels (80 cm thick) are present at the

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base of the section, and the sandy matrix within the basal gravel layer yielded an OSL age of

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41.3 +1.2 ka. A 20-cm-thick layer of disorganized gravel overlies the base layer that is in turn,

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overlain by an ~1-m-thick layer of matrix-supported organized gravels and finally by a 1-m-thick

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surface soil (Fig. 5b). The IHF2 section at Dharer nala, a small tributary of the Tons River, is

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~20 m thick and starts with a 50-cm-thick layer of organized gravels at base suggesting

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channelized flow. This layer is overlain by 2.5 m of a massive sand layer that yielded an OSL

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age of 43.1 ± 6.8 ka (Table 3, Fig. 4B). The IHF3 section has Siwalik rocks at the base (Fig. 5b),

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and the IHF deposits are made up of a 2-m-thick layer of disorganized gravels with some

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boulders (maximum size ~50 cm). The upper parts of the IHF2 and IHF3 sections have been

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interpreted as proximal fan deposits (discussed next).

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4.4. Proximal fan (PF)

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The proximal fan unit occupies the area between the 'isolated hills' unit close to the mountain

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front (Fig. 3D) but extends farther downstream of the Santaurgarh thrust, ~5-10 km. The

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preserved fan surface has a radius of 4-5 km and surface area of ~54 km2. The elevation of this

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unit ranges from 500 to 840 m, and slope varies from 2° to 5° southward (Table 1). The NDVI

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values range between –0.04 and +0.04, suggesting comparatively less vegetation cover with flat

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cultivated areas.

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The proximal fan unit was identified as depositional unit GD–II (Singh et al., 2001) and unit 'B'

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by Thakur et al. (2007) (Table 2). This unit lies unconformably over the steeply-dipping to

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overturned middle Siwalik sandstones (Thakur et al., 2007) at the hanging wall of the

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Santaurgarh thrust but shows no evidence of offset or deformation across the Santaurgarh fault.

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The exposed thickness of this unit is ~50 m. The proximal fan unit is exposed continuously for

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~1.5 km upstream of the Santaurgarh thrust along several major drainages, including the Koti

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and the Suarna rivers. At the footwall of the Santaurgarh thrust, this unit appears to overlie the

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IHF unit (Fig. 3D), and the fan head has been entrenched by later fluvial processes. The proximal

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fan sediments have clasts ranging from pebbles to gravels and rarely boulders. Clasts are

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relatively poorly to fairly sorted but fining upward (normal grading), suggesting these to be sheet

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flow deposits. These sediments were deposited in paleovalleys that were incised by at least 70–

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75 m into the underlying deposits of the IHF. The axes and orientation of these paleo–valleys are

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slightly offset from those of the modern drainage system, which means that their width cannot be

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observed directly (Densmore et al., 2015). The drainages flowing over the fan surface are

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parallel to subparallel. Large boulders were observed in this unit toward the basin margin (Fig.

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5A).

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Two lithologs from PF1 and PF2 sections document the stratigraphy of proximal fan

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sediments. A 6m high PF1 section is located downstream of Dharer nala, a small tributary of the

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Tons River. The proximal fan sediments in this section consist of an organized gravel bed (base

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not exposed) overlain by a 3-m-thick disorganised boulder–gravel layer with randomly oriented

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clasts and loose matrix (Fig. 4B). A thin sand layer above gave an OSL age of 33.8 ±3.2 ka

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(Figure 4B). An 80-cm-thick paleosol layer separates the lower proximal fan sediments in this

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section from the upper organized gravel layer, which was interpreted to be a part of distal fan

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deposits (discussed later). The PF2 section is a reinterpreted section (section D3 of Singh et al.,

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2001). This section has a massive sand layer (1 m thick) at the base for which an OSL age of

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21.2 ±1.2 ka was obtained by Singh et al. (2001). A major part of this section (10.5 m thick)

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consists of alternating layers of clast-supported and matrix-supported gravels. A 2-m-thick

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paleosol above the sand layer represents a major discontinuity and marks the cessation of

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proximal fan deposition.

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The upper part of the IHF2 section (0-8 m) and the middle part of the IHF3 section (6-10

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m) have also been interpreted as proximal deposits onlapping the IHF sediments. In the IHF2

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section, the proximal fan deposits consist of a massive sand layer overlain by a 3-m-thick layer

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of pebbly sand and 6-m-thick layer of matrix-supported organized gravels. The upper part of this

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log consists of another massive sand layer (4 m thick) that was dated as 30.1 ±3.5 ka (Table 3,

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Fig. 4B). The sand layer is overlain by a 2-m-thick layer of clast-supported organized gravels,

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and the section is capped by a 2-m-thick surface soil. In the IHF3 section, the proximal fan

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deposits are made up of a sand layer (2-m-thick) with a date of 21.2 ±1.2 ka (Fig. 4B) and a thin

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layer (1 m) of the imbricated gravels, suggesting a brief period of channelized flow. The

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overlying paleosol layer (80 cm thick) marks a minor discontinuity in the sequence, and the

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overlying sediments were interpreted to be a part of distal fan deposits (discussed next).

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4.5. Distal fan (DF)

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The distal fan has been mapped farther south of the proximal fan. The southern stretch of this

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unit is bounded by the Asan River. The distal fan unit is a near-planar surface with elevation

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varying from 480 to 700 m sloping <2˚ southward. The observed fan radius is ~10–17 km, and

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the total surface area is ~131 km2. This unit consists of poorly sorted, angular to subangular

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pebbles with rare boulders (depositional unit 'C' of Thakur et al., 2007). Singh et al. (2001)

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identified the same unit as depositional unit GD III and GD IV (Table 2) with pedofacies III. The

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exposed thickness of this surface varies from ~15 m in the northern part to ~5 m in the southern

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part. The NDVI values vary from –0.04 to +0.40 for this unit, suggesting barren lands or

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settlements with some areas of thick vegetation cover. The topography of this unit is highly

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undulating owing to dissection by several minor streams.

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A ~10-m-thick exposed section (DF1) on the western bank of the Sitala Rao has a 4.5-m-

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thick layer of disorganised gravels at the base overlain by a 1-m-thick sand layer that was dated

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as 14.4 ±0.6 ka (Fig. 5B). The sand layer is overlain by a 1.5-m-thick clast-supported organised

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gravels, and then by a 1-m-thick matrix-supported organized gravel. In the IHF3 section, the

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uppermost (0-5.5 m) part of the sequence has been interpreted as distal fan deposits consisting of

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two layers of matrix-supported organized gravels, each ~1 m thick, separated by a thin sandy

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layer. The section is capped by sandy soil (1 m thick) that is highly disturbed owing to the

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presence of thick vegetation. Distal fan deposits are also exposed in the upper part of the PF2

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section (0-8 m) consisting of thick alternating layers of clast- and matrix-supported gravels. An

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intervening sand layer at ~3 m depth in PF2 section was dated to be ~9.4 ka by Singh et al.

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(2001).

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Three additional OSL dates were obtained for distal fan deposits (Table 3). The sample SU-1

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came from an exposed section on the eastern flank of the Suarna River at a depth of 0.9 m from a

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location close to the IHF2 section (Fig. 5A) and gave an OSL date of 19.9 ±2.1 ka (Table 3). The

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samples LIS-TOP and FS 2.1 were collected from distal fan deposits overlying the IHF/PF

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deposits (Fig. 5A) and yielded OSL ages of 16.2 ±1.4 and 13.8 ±0.9 ka, respectively. Distal fan

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sediments are onlapping those of the proximal fan as observed in several lithologs (IHF3, PF1,

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PF2) but not all the way up to the hanging wall of the Santaurgarh thrust; this suggests

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backfilling of this unit over the older fan units.

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4.6. Dip slope (DS)

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The dip slope unit has been mapped on the northern flank of the Mohand anticline and is mainly

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controlled by the dip of the Siwalik Ranges (mapped as dip-controlled slope by Thakur et al.,

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2007). Lithologically, it is mainly composed of unsorted subrounded boulders and pebbles,

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predominantly of quartzite and sandstone derived from the Siwalik Ranges of the Mohand

289

anticline. The total surface area of this unit is ~115 km2, mostly covered by thick vegetation

290

cover characterised by highly positive (0.4–0.6) NDVI values (Table 1). The regional slope of

291

this unit varies from 5° to 10°, and the elevation ranges between 500 and 680 m. Based on

292

TRMM data, Bookhagen and Burbank (2006) interpreted very high annual rainfall (~100–300

293

cm) along the Mohand stretch because of a significant topographic high. As a result, hillslope

294

failures and landslides are common in this unit (Fig. 5B) and they constitute the dominant

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sediment transport processes in the area (Barnes et al., 2011). The OSL age of ~30 ka (Thakur et

296

al., 2007) suggests a major unconformity between the underlying Siwalik strata (age ~0.5 Ma;

297

Sangode et al., 1996) and the dip slope unit.

298

4.7. Channel belt

299

Channel belts of the major rivers such as the Koti, Mauti, Sitala Rao, and Suarna occupy the

300

lowest elevations in the Dehra Dun. These seasonal rivers dissect the fan surfaces and flow for a

301

length varying from ~20 to 25 km from their point of origin in the Lesser Himalaya to their

302

confluence with the axial river, the Asan (Tons). The Asan (Tons) originates from southern

303

slopes of the Mussoorie hills and drains for ~50 km before meeting the Yamuna. Channel belts

304

of most of the rivers are quite narrow (<0.5 km) in the upstream reaches, but they become wider

305

(1 to 1.5 km) in downstream reaches. Small streams such as the Nimi, Dharer, Ghulata, Udmadi

306

Rao, and Chor khala originate within the valley, mainly from the detached residual hills (IH

307

unit). These are ephemeral streams and most of them are <15 km long. Large boulders were

308

noted in the channel belt of the Kosi and Mauti rivers in their upstream reaches. In the

309

downstream reaches, gravels and pebbles constitute a major part of channel sediments.

310

4.8. Floodplain (FP)

311

The major rivers draining the study area, the Yamuna, Suarna, Sitala Rao, and Asan, have

312

prominent floodplains. The general slope of the floodplains ranges from 0° to 1° and the

313

elevation ranges from 420 to 660 m. The floodplain unit was demarcated on the satellite images

314

on the basis of image characteristics such as high moisture content and NDVI values (negative to

315

zero; see Table 1). The floodplain along the Asan River is 0.5–2.5 km wide but is much narrower

316

(600–900 m) along the Suarna River (Fig. 5C). The floodplain unit is composed of sand and silt

317

and little mud with some pedogenic modification and intercalated with pebbles. At the western

318

end of the study area, the Yamuna has a well-developed floodplain, and the width varies from

319

0.5 km in the upstream to 3 km in downstream reaches.

320

4.9. Terraces (T)

321

One set of paired fill terraces were mapped along the Sitala Rao and Suarna rivers. Terraces are

322

cultivated areas with elevation ranging from 480 to 720 m. Terraces are mainly composed of

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gravels and pebbles in sandy matrix. The NDVI values of these surfaces are nearly zero or

324

slightly positive because of the presence of barren land and cultivated areas. Terraces along the

325

Sitala Rao are ~5 km long and ~0.5–0.8 km wide. We obtained an OSL age of 0.9 ±0.2 ka from a

326

sample taken from this terrace (Table 3). Terraces along the Suarna River are ~4 to 5 km long

327

and ~0.5 km wide, located ~2 to 3 m above the river bed and are flat with sparse vegetation

328

cover (Fig. 5C). An approximately ~30-km-long and 0.7 to 2.5 km wide asymmetric terrace was

329

identified along the southern bank of the Asan river. This terrace is bordered by the dip slope

330

(DS) unit at the south and is cut by several ephemeral streams flowing from the northern flank of

331

the Mohand anticline that join the Asan River to the south. The formation of this asymmetric

332

terrace along the Asan may be related to the uplift of the Mohand anticline. Another small

333

symmetric depositional terrace surface, ~0.5 km long, ~0.2 km wide, and ~1 m high, was

334

identified along the Chor Khala River (Fig. 5D), a small tributary of the Asan River. An OSL

335

date of 3.5 ±0.2 ka was obtained for a sample from this terrace (Table 3).

336

4.10. Undifferentiated gravels (UGF)

337

This unit, composed mainly of gravels, is flat and occupies a total area of ~200 km2 north of the

338

Asan river and east of the Tons river. In the surface profile, there is no major break between

339

distal fan unit and undifferentiated gravels unit. A large part of this unit is dotted with cultivated

340

land and settlements including the city of Dehradun. Although it has been mapped as a fairly

341

continuous unit, it is quite scattered and disjointed owing to human interventions.

342

5. Discussion

343

5.1. Tectonogeomorphic evolution and depositional history of the Dehra Dun

344

The IHF unit is a steeply dipping fan surface and is mainly composed of clast- and matrix-

345

supported boulders to pebble conglomerate. The clasts are poorly sorted and randomly oriented

346

suggesting dominant debris flow deposits (Blair and McPherson, 1994; Singh et al., 2001;

347

Nichols, 2009). The deposition of isolated hills of fan IHF unit occurred during ~43–33 ka

348

coinciding with the precipitation minima around 40 ka (Prell and Kutzbach, 1987). We argue that

349

the availability of sediments in the system was related to reactivation of MBT during 70–55 ka

350

(Banerjee et al., 1999) and 50–35 ka (Singhvi et al., 1994; Singh et al., 2001) that led to the

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deposition of thick strata of IHF (≥41 ka) in the Dun (Fig. 6). Low transport capacity caused by

352

the precipitation minimum during this period resulted in deposition close to the mountain front.

353

The proximal fan unit primarily consists of matrix- and clast-supported pebbles to gravels with

354

some boulders having loose contact with matrix that suggests mud flow or sheet flow deposits

355

(Blair and McPherson, 1994; Singh et al., 2001) with minor debris flows. The proximal fan unit

356

is the second phase (34–21 ka) of major aggradation in the Dun that coincides with a period of

357

high sediment production from the Himalaya (Sharma and Owen, 1996; Taylor and Mitchell,

358

2000; Pant et al., 2006). This is also the period of high precipitation that led to high intensity

359

floods in the foreland induced by high solar insolation at ~31 ka (Goodbred, 2003). High

360

sediment supply (Lecce, 1990) and an increase in water-sediment ratio (Ballance, 1984; Wells

361

and Harvey, 1987; Harvey and Wells, 2003; Densmore et al., 2007; Nichols, 2009) during this

362

period were the primary reasons for transition from debris flow deposits (IHF unit) to sheet flow

363

deposits (PF unit). The elevation difference of ~100 m between the IHF and PF units (Table 2)

364

suggests an incision of the IHF unit prior to the deposition of proximal fan unit.

365

The onlapping of the proximal fan deposits over the IHF deposits (lithologs IHF2 and IHF3) at

366

the hanging wall of the Santaurgarh thrust suggests widespread deposition of the proximal fan

367

unit during the LGM and eventually backfilling toward and across the Santaurgarh thrust. This

368

backfilling of the proximal fan deposits could have been triggered because of tectonic influences

369

(e.g., increase in slip rate; Bull, 1961, 1964; DeCelles et al., 1991; Gordon and Heller, 1993;

370

Mellere, 1993; Allen and Densmore, 2000). The increase in slip rate provides accommodation

371

space and the younger fan unit can onlap the older units (Densmore et al., 2007). Backfilling can

372

also be influenced by hydraulic conditions such as less water availability or low water-sediment

373

ratio that can lead to the onlapping of the younger unit over the older one close to the mountain

374

front (Goudie, 2004; Densmore et al., 2007; Hamilton et al., 2013). However, as no deformation

375

was observed on the fan surfaces in the Dun and the widespread filling occurred around the

376

LGM time, we argue that climatic factors have had a dominant role in back filling of the

377

proximal fan unit.

378

The presence of boulders in the distal part of the fan unit (Fig. 4A) has been debated and

379

alternative hypotheses have been proposed. Kumar et al. (2007) and Giano (2011) interpreted the

380

presence of boulders in fan deposits toward the basin margin because of high precipitation in the

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catchment. On the contrary, Singh et al. (2001) suggested that the development of the pediments

382

1 and 2 (IHF in the present work) caused by the activity of the Bhauwala thrust and headward

383

erosion of the streams. They also suggested that the pedimented Siwalik (IHS in the present

384

work) and fan area were uplifted caused by the activity of the Bansiwala thrust (Asan fault

385

according to Thakur et al., 2007). In the hanging wall of the Santaurgarh thrust, fan deposits

386

were observed lying unconformably over the steeply dipping to overturned middle Siwalik

387

sandstones (Thakur et al., 2007), but no evidence was recorded for any deformation in the fan

388

units across the Santaurgarh thrust. We therefore favour the climatic interpretation (high

389

precipitation) for such deposits.

390

The distal fan unit consisting of poorly sorted, angular to subangular pebbles have been

391

interpreted as braided stream deposits and overbank deposits. The distal fan unit (20–11 ka)

392

represents the third phase of deposition in the Dun. Sheet-like fan deposits indicate high

393

precipitation in the catchment area (Densmore et al., 2007). The timing of distal fan deposition

394

coincides with the post-LGM period that corresponds to high sediment supply from the

395

hinterland because of monsoonal intensification (Overpeck et al., 1996) and to extensive slope

396

failures and hill slope sediment transport (Pratt et al., 2002). The lithologs PF1 and PF2 clearly

397

show that the distal fan sediments onlapped the proximal fan deposits. This again suggests

398

backfilling of distal fan deposits (Fig. 6) but not up to the hanging wall of the Santaurgarh thrust.

399

We argue that such backfilling is related to water-sediment ratio influencing the transport

400

capacity of the river systems (Goudie, 2004; Densmore et al., 2007; Hamilton et al., 2013).

401

During the early to mid-Holocene period, minor terrace deposits dating 11 ka (Singh et al., 2001)

402

and 3 ka (present work) have been documented along the rivers in the Dun. This period was the

403

beginning of the incision phase in the Dun because of intense monsoonal activity (Singh et al.,

404

2001). An incision phase started in the Duns at ~12/10 ka and has continued until present (Singh

405

et al., 2001; Suresh and Kumar, 2009). No deposition was recorded after 3 ka except a minor

406

recent aggradational terrace (OSL age 0.9 ka, sample ST-1) along the Sitala Rao (Fig. 4A).

407

5.2. Conceptual model

408

Figure 7 shows the sequential development of the different fan units in the Dehra Dun. The

409

deposition of the IHF unit (Stage I) began at ~43 ka and probably ended around ~33 ka. The

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reactivation of MBT during 75–65 and 50–35 ka (Singhvi et al., 1994) provided high sediment

411

availability and low precipitation minima (Prell and Kutzbach, 1987), resulting in low transport

412

capacity and deposition of sediments close to the mountain front. The geomorphic expression of

413

this fan is not well preserved because of several cycles of incision. Therefore, the precise timing

414

of deposition and abandonment of this unit could not be ascertained. The proximal fan formed

415

because of continuous deposition from 34 to 21 ka and developed between isolated hills. Field

416

evidences suggest incision of isolated hills and refilling of the valley. The activity of the

417

Santaurgarh thrust before 23 ka (Singh et al., 2001) provided high sediment availability during

418

this period and led to continuous deposition of PF until 21 ka. The distal fan (stage III) formed

419

owing to further southward shift of depositional locus after the abandonment of the proximal fan

420

and fanhead entrenchment. Deposition of the distal fan sediments (~20–11 ka) may have

421

progressed as backfilling over the proximal fan deposits (34–21 ka), but no distal fan deposits are

422

observed over the hanging wall of the Santaurgarh thrust. A minor depositional phase is also

423

documented in the intermontane basin during 3–1 ka (stage IV) that led to thin depositional

424

terraces along the modern rivers.

425

All four depositional units, namely isolated hills of fan, proximal fan, distal fan, and younger

426

depositional terraces, are separated by strong incision phases (Fig. 7). The timing and duration of

427

the incision phases are not precisely constrained, but these must have occurred after the

428

deposition of fan units, at ~33 ka for IHF, ~20 ka for PF, and ~10 ka for DF. Earlier work

429

(Densmore et al., 2015) estimated that these incision episodes contributed a significant amount

430

of sediment discharge into the plains that were ca. 1-2% of the present-day suspended load of the

431

Ganga and Yamuna rivers that traverse the margin of the Dun.

432

5.3. Comparison with other Duns (intermontane valleys) and regional correlation

433

The OSL ages suggest that the fan development in western Dehradun started at ~43 ka and

434

ceased around 10 ka. Aggradational phase I (43-33 ka) that corresponds to formation of the IHF

435

unit is comparable to the first aggradation phase in Yamuna valley (~37–24 ka) between the

436

MBT and the HFT (Dutta et al., 2012). An aggradational phase was also documented during

437

~49–25 ka at various upstream sites in the Ganga basin (Srivastava et al., 2008; Ray and

438

Srivastava, 2010). Fan aggradation in the Pinjaur Dun was also documented from 72 to 24 ka

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(Kumar et al., 2007; Suresh et al., 2007) but without any incision phase as recorded in Dehra

440

Dun (Fig. 7).

441

Phase II (~34–21 ka) of proximal fan deposition correlates with depositional phases in the Soan

442

Dun in NW Himalaya during ~36–29 ka (Suresh and Kumar 2009) and falls within the fan

443

aggradation phase (~72–24 ka ) in the Pinjaur Dun in NW Himalaya (Kumar et al., 2007; Suresh

444

et al., 2007). Fan aggradation phases in the Kiarda and the Parduni basins in NW Himalaya –

445

recorded from ~34 to 19 ka and from ~27 to 20 ka, respectively (Philip et al., 2009) – are also

446

correlatable to this phase (Fig. 7). In Chitwan Dun in the Narayani basin, southern Nepal, no

447

absolute ages of the fill deposits are available, but Kimura (1995) suggested ages of the

448

depositional units as ~26–16 and <10 ka separated by an incision event.

449

Aggradational phase III (20–11 ka) represented by distal fan deposition in Dehra Dun, correlates

450

with the second aggradation phase (15–12 ka) in the Yamuna valley between the MBT and the

451

HFT (Dutta et al., 2012). Aggradation in the Ganga valley between MBT and HFT was also

452

documented at ~11 ka followed by incision during 11–9.7 ka (Sinha et al., 2010). A phase of

453

aggradation was documented at several upstream sites in the Ganga basin during 18–11 ka

454

(Srivastava et al., 2008; Ray and Srivastava, 2010). A minor depositional phase was also

455

observed in the Pinjaur Dun during 16 ka (Suresh et al., 2007). Minor terrace deposits during 3–1

456

ka in the Dehra Dun (aggradational phase IV) correlates with a brief depositional event in the

457

Yamuna valley during (~5–1 ka) (Dutta et al., 2012) and the Pinjaur Dun during 4.5 ka (Fig. 7)

458

(Suresh et al., 2007). Fan aggradation ceased ~11 ka ago in the Dehra Dun (Singh et al., 2001)

459

and so did the second phase of fan deposition in the Kangra region around 10 ka (Suresh and

460

Kumar, 2009).

461

It has been observed that major fan aggradational and incision phases in the Duns along the

462

Himalayan arc (Kumar et al., 2007; Suresh et al., 2007; Philip et al., 2009; Suresh and Kumar,

463

2009; Sinha et al., 2010; Dutta et al., 2012) correlate reasonably well, but spatial variation in

464

timing of individual events is noted. We therefore suggest that these Duns have gone through

465

similar cycles of sedimentation/evacuation under the influence of climatic variation with

466

interruptions from tectonic activity and sediment supply from the mountainous area. Major

467

aggradational phases are related to the Quaternary glacial period, whereas the degradational

468

phases are related to interglacial periods (Harvey, 1990). We suggest that the role of tectonic

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activities has been limited to creation of accommodation space in the Dehra Dun region and has

470

not influenced the fan development process in a major way.

471

Our observations are in line with landscape development in different parts of the Himalaya

472

documented on the basis of sedimentological, geomorphic and chronostratigraphic analysis using

473

OSL and terrestrial cosmogenic nuclide (TCN). These studies have suggested that erosional and

474

depositional processes throughout the Himalaya and Tibet have been mainly controlled by

475

climatic changes, specifically the transition between glacial to deglacial periods (Barnard et al,

476

2004, 2006a, 2006b; Dortch et al., 2009; Seong et al., 2009; Ray and Srivastava, 2010). In the

477

Central Karakoram region, glacier geochronology and outburst flood deposits have indicated that

478

landform development was dominantly influenced by glaciation to paraglaciation during the late

479

Quaternary (Seong et al., 2009). Paleoclimatic records from the Alaknanda–Ganga system in the

480

Lesser Himalaya indicate that aggradation and incision phases between 63 and 11 ka resulted

481

primarily because of glaciation–deglaciation processes (Ray and Srivastava, 2010). Mass wasting

482

(landslides) and sheet flow processes during the transition between glacial and nonglacial periods

483

(Barnard et al., 2006a, 2006b; Dortch et al., 2009; Seong et al., 2009) also contributed to the

484

sediment input in the Duns (Chandel and Singh, 2000; Singh et al., 2001). However, it has been

485

suggested that tectonic activities along longitudinal as well as transverse faults have continuously

486

modified the landforms in the Duns (Kimura, 1994, 1999; Chandel and Singh, 2000; Singh et al.,

487

2001; Virdi et al., 2006; Philip and Virdi, 2007; Singh and Tandon, 2008; Philip et al., 2009).

488

6. Conclusions

489

Geomorphic and stratigraphic investigations in the Dehra Dun in NW Himalaya have suggested

490

that at least three phases of fan sedimentation occurred during the late Quaternary period (I) 43-

491

33 ka, (II) 34-21 ka, and (III) 20-11 ka followed by minor terrace formation during 3-1 ka. Each

492

depositional phase was followed by fanhead incision, abandonment of the active depositional

493

locus and shifting of the depocenter toward the basin margin. Climate change (high precipitation)

494

was the primary reason for the transition from debris flow deposits to sheet flow and stream flow

495

deposits in the Dun. While the evolutionary history of the alluvial fans in the Dun was influenced

496

by tectonic activities, particularly in terms of providing accommodation space, the aggradation

497

and entrenchment of fans were controlled by climatic perturbations.

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Acknowledgements

499

This work was supported by a research grant from the Department of Science and Technology,

500

Government of India including the studentship to SS and we thankfully acknowledge this

501

support. A part of the field investigations and OSL dating were supported from a grant from

502

UKIERI, British Council that was crucial for this research. Discussions with Alexander

503

Densmore, Jason Barnes, Sampat Tandon, Malay Mukul, Vikrant Jain, and Vimal Singh during

504

field work and project review meetings were most useful to develop the ideas presented in this

505

paper. We thank all the reviewers for their detailed comments that helped to improve the

506

manuscript significantly. We are grateful to Richard Marston for his painstaking editorial

507

comments.

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Prell, W.L., Kutzbach, J.E., 1987. Monsoon variability over the past 150,000 years. Journal of Geophysical Research 92 (D7), 8411-8425. Raiverman, V., Kunte, S. V. and Mukherjee, A., 1983. Basin geometry, Cenozoic sedimentation and hydrocarbon prospects in north-western Himalaya and Indo-Gangetic plains. Petroleum Asia Journal 6, 67-92. Raiverman, V., Srivastava, A.K., Prasad, D.N., 1994. Structural style in northwestern Himalayan foothills. Himalayan Geology 15, 263–280. Rao, D. P., 1978. Geomorphology and neotectonics: An example from Sub Himalaya, Proc. Sym. on Morphology and Evolution of Landforms, Geology Department, University of Delhi, pp.88-98. Ray, Y., and Srivastava, P., 2010. Widespread aggradation in the mountainous catchment of the Alaknanda-Ganga River System: timescales and implications to hinterland-foreland relationships. Quaternary Science Reviews 29, 2238-2260. Ritter J.B., Miller J.R., Enzel Y. and Wells S.G., 1995: Reconciling the roles of tectonism and climate in Quaternary alluvial fan evolution. Geology 23, 245-248. Robustelli G., Luca L., Corbi F., Pelle T., Dramis F., Fubelli G., Scarciglia F., Muto F. and Cugliari D., 2009. Alluvial terraces on the Ionian coast of northern Calabria, southern Italy: Implications for tectonic and sea level controls. Geomorphology 106, 165-179. Sangode, S. J., Kumar, R. and Ghosh, S. K., 1996. Magnetic polarity of the Siwalik sequence of Haripur area (HP), NW Himalaya. Journal of Geological Society of India 47, 683-704. Seeber, L.and Gornitz, V., 1983. River profiles along the Himalayan arc as indicators of active tectonics. Tectonophysics 92 (4), 335-367. Seong Y.B., Bishop, M.P., Bush, A., Clendon, P., Copland, P., Finkel., R., Kamp, U., Owen, L.A., Shroder, J.F., 2009. Landforms and landscape evolution in the Skardu, Shigar and Braldu Valleys, Central Karakoram Mountains. Geomorphology 103, 251–267. Sharma, M.C., and Owen, L.A., 1996. Quaternary glacial history of the Garhwal Himalaya, India. Quaternary Science Reviews 15, 335-365. Silva, P.G., Harvey, A.M., Zazo C., and Goy, J.L., 1992. Geomorphology, depositional style and morphometric relationships of Quaternary alluvial fans in the Guadalentin Depression (Murcia, Southeast Spain). Z. Geomorphologie, N.F. 36, 325-341. Singh, A.K., 1998. Geomorphology, Sedimentology and Pedology of the intermontane Dehradun valley, Northwest Himalaya. Unpublished PhD. thesis, University of Roorkee, 247p. Singh, A.K., Prakash,B., Mohindra,R. and Thomas,J.V., Singhavi A.K., 2001. Quaternary alluvial fan sedimentation in the Dehradun Valley Piggyback Basin, NW Himalaya: tectonic and palaeoclimatic implications. Basin Research 13, 449-471. Singh, V. and Tandon, S.K., 2007. Evidence and consequences of tilting of two alluvial fans in the Pinjaur Dun, Northwestern Himalayan Foothills. Quaternary International 159, 21-31. Singh, V. and Tandon, S.K., 2008. The Pinjaur Dun (intermontane longitudinal valley) and associated active mountain fronts, NW Himalaya: Tectonic geomorphology and morphotectonic evolution. Geomorphology 102, 376-394. Singh, V. and Tandon, S.K., 2010. Integrated analysis of structures and landforms of an intermontane longitudinal valley (Pinjaur Dun) and its associated mountain fronts in the NW Himalaya. Geomorphology 114, 573-589. Singhvi, A.K., Banerjee, D., Pande, K., Gogte, V. and Valdiya, K.S. (1994). Luminescence studies on neotectonic events in south-central Kumaun Himalaya - A feasibility study. Quaternary Science Reviews 13(5-7), 595-600. Sinha, S., Suresh, N., Kumar, R., Dutta, S. and Arora, B.R., 2010. Sedimentologic and geomorphic studies on the Quaternary alluvial fan and terrace deposits along the Ganga exit. Quaternary International 227, 87-113. Srivastava, P., Tripathi, J.K., Islam, R., and Jaiswal, M.K., 2008. Fashion and phases of late Pleistocene aggradation and incision in the Alaknanda River Valley, western Himalaya, India. Quaternary Research 70, 68-80.

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Suresh, N. and Kumar, R., 2009. Variable period of aggradation and termination history of two late Quaternary aluvial fan in the Soan Dun, NW Sub Himalaya: Impact of tectonic and climate. Himalayan Geology 30,155-165. Suresh, N., Bagati, T. N., Thakur, V . C., Kumar, R. and Sangode, S.J., 2002. Optically stimulated luminescence dating of alluvial fan deposits of Pinjaur Dun, NW Sub Himalaya. Current Science, 82(10), 1267-1272. Suresh, N., Bagati, T.N. Kumar, R. and Thakur, V.C., 2007. Evolution of Quaternary alluvial fans and terraces in the intermontane Pinjaur Dun, Sub-Himalaya, NW India: interaction between tectonics and climate change. Sedimentology 54, 809-833. Stöcklin, J., 1980. Geology of Nepal and its regional frame. Journal of Geological Society of London 137, 1-34. Taylor, P.J. and Mitchell, W.A., 2000. The Quaternary glacial history of the Zanskar range, north-west Indian Himalaya. Quaternary International 65-66, 81-99. Thakur, V.C., 1995. Geology of Dun Valley, Garhwal Himalaya: Neotectonics and coeval deposition with fault-propagation folds. Himalayan Geology, 6 (2), 1-8. Thakur, V.C. and Pandey, A.K., 2004. Late Quaternary tectonic evolution of Dun in fault bend propagated fold system, Garhwal Sub-Himalaya. Current Science 87(11), 1567-1576. Thakur, V.C., Pandey, A.K. and Suresh, N., 2007. Late Quaternary–Holocene evolution of Dun structure and the Himalayan Frontal Fault zone of the Garhwal Sub-Himalaya, NW India. Journal of Asian Earth Sciences 29, 305–319. Valdiya, K. S., 1980. Geology of Kumaun Lesser Himalaya. Dehra Dun, India. Wadia Institute of Himalayan Geology. p. 291. Virdi, N. S., Philip, G. and Bhattacharya, S,. 2006. Neotectonic activity in the Markanda and Bata River basins, Himachal Pradesh, NW Himalaya: A morphotectonic approach. International Journal of Remote Sensing 27(10), 2093–2099. Viseras, C., Calvache, M.L., Soria, J.M. and Fernandez, J., 2003. Differential features of alluvial fans controlled by tectonic or eustatic accommodation space. Examples from the Betic Cordillera, Spain. Geomorphology 50, 181-202. Wells, S.G. and Harvey, A.M., 1987. Sedimentologic and geomorphic variations in storm generated alluvial fans, Howgill Fells, northwest England. Bulletin of the Geological Society of America 98, 182-198.

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ACCEPTED MANUSCRIPT 23 Figure captions

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Fig. 1. (A) Location map of the Dehra Dun. (B) Study area including the western part of the Dehra Dun valley bounded by the Tons River to the east and the Yamuna River to the west. Red coloured arrows indicate flow directions of the rivers. The Main Boundary thrust (MBT) and the Santaurgarh thrust (ST) are shown with thick black lines as their activity has been observed in the field while Bhauwala thrust (BT), Majahaun fault (MJF), Asan, and Tons fault are shown with a dotted black line, as their activity has not been observed in the field.

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Fig. 2. Geomorphic map of the Dehra Dun. The geological structures marked by the previous workers are represented as F1 and F2. F1 represents MBT–Main Boundary thrust and ST– Santaurgarh thrust which are confirmed through field evidence. F2 represents MJF– Majahaun fault, BT– Bhauwala thrust, AF– Asan fault, TF–Tons fault; their presence is not supported by any field evidence. Different geomorphic units and features are described as CB– Channel with bars, DN- Drainage network, NTH– Northern thrust sheet, DH– Dissected hills, IHF– Isolated hills of fan material, IHS– Isolated hills of Siwalik rocks, PF– Proximal fan, DF– Distal fan, UGF– Undifferentiated gravel-fill, FP– Floodplain, DS– Dip slope, T– Fill terraces.

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Fig. 3. Field photographs of different geomorphic units. (A) Overview of geomorphic units from north to south. Hanging wall of the MBT is identified as 'northern thrust sheet'. Dissected hills unit covers the area of the footwall of the MBT, and isolated hills unit are mapped as detached rounded hills south of the dissected hills. (B) ‘Northern thrust sheet' mapped in the hanging wall of the MBT is mainly composed of the Lesser Himalayan rocks, which include argillites, phyllites, quartzite, carbonate, and slates. Slope failure is common in this unit and is the most dominant mechanism for sediment transport. (C) Field photos of ‘dissected hills' mainly consists of middle Siwalik rocks. Presence of big boulders at the Koti River outlet in the dissected hills unit indicate tectonic activity of the Santaurgarh thrust. (D) Spatial distribution of geomorphic units of IHF and proximal fan. Deposits of the IHF unit lie unconformably on the middle Siwalik rocks at the hanging wall of the Santaurgarh thrust, and both units are unconformably overlain by the proximal fan unit.

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Fig. 4. (A) Locations of various lithologs and OSL samples on the satellite image. (B) Lithologs and chronology of all sections exposing different fan units. IHF1, IHF2, and IHF3 logs document IHF deposits that are interpreted as debris flow deposits with maximum OSL ages of ~ 43/41 ka. The upper parts of the IHF2 and IHF3 sections have been interpreted as proximal and distal fan deposits respectively, yielding younger ages. Lithologs PF1 and PF2 document proximal fan deposits with ages of 33-21 ka. The PF1 and PF2 sections record distal fan deposits in the upper parts. Litholog DF1 is exposed at the west bank of Sitala Rao and has 4.5-m-thick unit of disorganized gravels. Thin (~1 m) sandy layer yields OSL age of 14.4 ±0.6 ka for distal fan deposits. Fig. 5. Field photos of different geomorphic units. (A) Location of the sample collected from ‘proximal fan unit'. This ~6-m-high section near Dharer nala has an organised gravel bed at the base that is overlain by disorganised boulder-gravel units. Big boulders were observed in this unit toward the basin margin suggesting high precipitation in catchment. (B) ‘Dip slope’ geomorphic unit, mapped on the NW flank of the Mohand anticline, comprised of unconsolidated gravels, unsorted subrounded pebbles, and boulders of quartzite and sandstone. High rainfall makes this unit vulnerable to slope failures. (C) River terraces along the Suarna River. The terraces are ~4-5 km long and ~0.5 km wide and located ~2–3 m above the river bed. Floodplain width along the river varies from 600 to 900 m. (D) One set of depositional terraces along Chor Khala River. The sample (CKT-1) collected from the right bank of the river, ~1 m below the surface, yielded an OSL age of 3.53 ±0.21 ka.

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Fig. 6. Thematic section across the Mohand anticline and the Lesser Himalaya. Major geomorphic units and structures in the Dun are marked on the section. Isolated hills of fan (IHF), proximal fan (PF), and distal fan (DF) are the three major fan surfaces with the ages >41, 34-21 and 20-11 ka, respectively. Dissected hills unit is mapped at the footwall of the MBT and isolated hills unit is mapped as detached rounded hills south of the dissected hills. Dip slope geomorphic unit was mapped on the NW flank of the Mohand anticline.

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Fig. 7. Evolutionary model of the Dehra Dun fills. (I) IHF deposits formed during 43-33 ka, and the reactivation of MBT during 75-65 and 50-35 ka provided high sediment availability and low precipitation minima, resulting in low transportation capacity that resulted in sediments deposited close to the mountain front. (II) Proximal fan formed because of continuous deposition from 34 to 21 ka and developed between isolated hills; field evidences suggest incision of IH and refilling of the valley. The activity of the Santaurgarh thrust before 23 ka (Singh et al., 2001) provided high sediment availability during this period and led to continuous deposition of PF until 21 ka. (III) Distal fan (2011 ka) formed when the depositional locus shifted southward owing to fanhead entrenchment of the proximal fan. (IV) A dominant incision phase (<5 ka) in the Dun with deposition of minor younger terraces during 3–1 ka. (DD- Dehra Dun; PD – Pinjaur Dun, KD – Kiarda Dun, PaD – Parduni basin, CD- Chitwan Dun)

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770 771 772 773 774 775 776 777 778 779 780 781 782 783 784 785 786 787 788 789

ACCEPTED MANUSCRIPT 25

D

MA

NU

SC

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PT

790

AC CE P

792

TE

791

ACCEPTED MANUSCRIPT

794

AC CE P

793

TE

D

MA

NU

SC

RI

PT

26

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796

AC CE P

795

TE

D

MA

NU

SC

RI

PT

27

ACCEPTED MANUSCRIPT

MA

NU

SC

RI

PT

28

797

AC CE P

TE

D

798

ACCEPTED MANUSCRIPT

800

AC CE P

799

TE

D

MA

NU

SC

RI

PT

29

ACCEPTED MANUSCRIPT

802

AC CE P

801

TE

D

MA

NU

SC

RI

PT

30

ACCEPTED MANUSCRIPT

804

AC CE P

803

TE

D

MA

NU

SC

RI

PT

31

ACCEPTED MANUSCRIPT

806

AC CE P

805

TE

D

MA

NU

SC

RI

PT

32

ACCEPTED MANUSCRIPT 33 807

Table 1

808

Schemes of geomorphic mapping by different authors in the Dehra Dun area

Chandel and Singh (2000)

PT

RI

Nakata (1972, 1975)

Major geomorphic units 1) Doon fans: high level–upper and lower group; 2) Doon fans: principal level– dissected and undissected surface; 3) Doon fans: lower realm of coalescence; 4) Doon fans: terraces below principal level 1) Upper Dun surface, 2) Middle Dun surface, 3) Lower Dun surface and higher river terraces, and 4) Middle river terraces 1) Lesser Himalaya (~2500 m), 2) dissected Siwalik hills (~900 m), 3) pedimented Siwaliks with thick gravel cover (~750–450 m), 4) isolated N–S trending Siwalik hills (~850 m), 4) piedmont (upper and lower piedmonts)

SC

Authors Nossin (1971)

NU

809

AC CE P

TE

D

MA

810

ACCEPTED MANUSCRIPT 34 811

Table 2

812

Summary of geomorphic units and their description in Dehra Dun

PT

813 Description

Northern thrust sheet (NTH)

Area north of MBT with high drainage density; high relief pattern on DEM, elevation varying from 3000-500m, mainly composed of Lesser Himalayan rocks; NDVI values are nearly '0' that indicates barren land.

Mapped as Lesser Himalayaa, b

Dissected hills (DH)

Highly dissected Siwalik rocks, mainly Middle and upper Siwalik rocks (Sandstone, conglomerate); sparse vegetation at inner dissected hills and thick vegetation cover at outer dissected hills; elevation varies from 1200-1000 m, NDVI values (-0.04 to 0.60.

Same as suggested by previous workers

Isolated hills of Siwaliks (IHS)

Low–elevation, rounded residual hills of Upper Siwaliks; elevation ranges from 880m to 620m; positive NDVI values (0.040 to 0.60); distributary streams, thick vegetation cover.

Isolated hills of fan (IHF)

Low–elevation, rounded residual hills of fan material mainly composed of massive boulders & gravels; elevation varies from 840-620m; positive NDVI values (0.040 to 0.60); parallel drainages

AC CE P

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Unit

Comparison with earlier work

Pedimented Siwaliks (hills)a, b

Pediments GD-Ia Piedmont Unit Ab

Proximal Fan (PF)

Composed dominantly of gravels and pebble with some boulders; flat surface with less vegetation; altitude varies from 840-500m,; NDVI values (-0.04 to +0.04)

Pediments GD-Iaa Piedmont Unit B2

Distal fan (DF)

Youngest gravel unit with thick vegetation cover; elevation ranges from 700 to 480 m; NDVI values (-0.04 and +0.04); slope 0-2 degree, parallel drainages, dissected surface, Dun gravels,

Pediments GD-III, IVa Piedmont Unit Cb

Undifferentiated gravel

Undifferentiated gravels, identified as fill surface; elevation ranges from 560 to 460 m; few drainages, less vegetation; NDVI values (-0.04 to +0.04)

-

Flood plain

Active flood plain, fluvial sediments; elevation varies from 660420 m; Gravels, pebbles and coarse sand; NDVI values negative to zero

Floodplainb

Inactive flood plain

Abandoned floodplain of Asan river, cultivated areas; elevation varies from 460-440m; thick soil cover; NDVI values nearly zero.

Floodplainb

Dip slope

Dip slope gravels mainly derived from Mohand anticline; elevation varies from 680-500m; slope 5° to 10°; NDVI values highly positive (0.40-0.60); thickly vegetated, nearly parallel

Piedmont Unit Da

ACCEPTED MANUSCRIPT 35 drainages

814

a

815

b

Fluvial terraces composed of gravels, pebbles, sand and mud; Flat surfaces, younger sediments; elevation varies from 720-480m; NDVI values are nearly zero or slightly positive

PT

Terraces

Singh et al., 2001.

RI

Thakur et al., 2007.

SC

816

AC CE P

TE

D

MA

NU

817

-

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Table 3

819

Age data of fan units and terrace deposits in Dehra Dun based on OSL dating

U

Th

(ppm)

(ppm)

K (%)

Moist. Equivalent Dose Cont (De) Gy (%)

Dose rate

Age

(Gy/ka)

(ka)

2.77±0.48

3.05±0.04

0.9±0.2

ST-1

1

2.17±0.02

10.7±0.11

2.02±0.02

CKT-1

1

2.34±0.02

15.1±0.15

2.45±0.02

21.48

11.70±0.65

3.32±0.07

3.5±0.2

Distal fan

FS 2.1

4

2.23±0.02

15.3±0.15

2.91±0.03

3.44

59.68±3.96

4.33±0.05

13.8±0.9

Distal fan

FS2.2

5

2.43±0.02

16±0.16

2.46±0.02

13.53

51.46±1.87

3.58±0.06

14.4±0.6

2.8

3.3±0.03

3.02±0.03

1.27

80.72±6.82

4.99±0.06

16.2±1.4

12.6

51.25+3.98

2.57+0.18

19.9+2.1

NU

Sitla Rao Terrace

MA

Chor Khala

D

(DF1)

LIS-TOP

TE

Distal

Proximal fan

AC CE P

fan Distal fan

9.44

SC

Terrace

Sample Depth no./Log (m)a

RI

Lithostratigraphic unit

PT

820

SU-1

FS3.1

18.1±0.18

0.9

NA

NA

NA

10

1.89±0.02

16.4±0.16

2.21±0.02

18.16

65.71±3.62

3.10±0.06

21.2±1.2

3

4.56±0.05

21.4±0.21

2.65±0.03

14.38

150.87±14.17

4.47±0.08

33.8±3.2

7.6

111.23+9.84

3.69+0.27

30.1+3.5

1.36

207.20±5.47

5.02±0.06

41.3±1.2

4.2

163.73+23.27

3.8+0.28

43.1+6.8

(IHF3) Proximal fan

FS1.1 (PF1)

Proximal fan

DHR-2

5

NA

NA

NA

(IHF2) Isolated hills

IHF1

2.8

Isolated hills

DHR-1

9

3.1±0.03 NA

(IHF2)

821 822 823

a

Sample depth from surface in m.

18.4±0.18 NA

3.08±0.03 NA

ACCEPTED MANUSCRIPT 37 824

Highlights

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Geomorphic complexities and landform evolution in an intermontane basin in NW Himalaya Multiple phases of fan aggradation related to climate forcings Tectonics provided the accommodation space for valley aggradation and sediment supply Regional correlation of intermontane basin fills along the Himalayan front