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Earth Surf. Dynam., 9, 463–485, 2021 https://doi.org/10.5194/esurf-9-463-2021 © Author(s) 2021. This work is distributed under the Creative Commons Attribution 4.0 License. Implications of the ongoing rock uplift in NW Himalayan interiors Saptarshi Dey1, Rasmus C. Thiede2, Arindam Biswas3, Naveen Chauhan4, Pritha Chakravarti1, and Vikrant Jain1 1Earth Science Discipline, IIT Gandhinagar, Gandhinagar-382355, India 2Institute of Geosciences, Christian Albrechts University of Kiel, 24118 Kiel, Germany 3Department of Applied Geology, IIT-ISM Dhanbad, Jharkhand-826004, India 4Atomic Molecular and Optical Physics Division, Physical Research Laboratory, Ahmedabad, India Correspondence: Saptarshi Dey ([email protected]) Received: 28 April 2020 – Discussion started: 18 June 2020 Revised: 1 April 2021 – Accepted: 19 April 2021 – Published: 2 June 2021 Abstract. The Lesser Himalaya exposed in the Kishtwar Window (KW) of the Kashmir Himalaya exhibits rapid rock uplift and exhumation (∼ 3 mm yr−1) at least since the late Miocene. However, it has remained unclear if it is still actively deforming. Here, we combine new field, morphometric and structural analyses with dating of geomorphic markers to discuss the spatial pattern of deformation across the window. We found two steep stream segments, one at the core and the other along the western margin of the KW, which strongly suggest ongoing differential uplift and may possibly be linked to either crustal ramps on the Main Himalayan Thrust (MHT) or active surface-breaking faults. High bedrock incision rates (>3 mm yr−1) on Holocene–Pleistocene timescales are deduced from dated strath terraces along the deeply incised Chenab River valley. In contrast, farther down- stream on the hanging wall of the MCT, fluvial bedrock incision rates are lower (<0:8 mm yr−1) and are in the range of long-term exhumation rates. Bedrock incision rates largely correlate with previously published ther- mochronologic data. In summary, our study highlights a structural and tectonic control on landscape evolution over millennial timescales in the Himalaya. 1 Introduction Miocene (Vannay et al., 2004). Existence of the MHT has been further elaborated in Himalayan cross sections (e.g., Protracted convergence between the Indian and the Eurasian Powers et al., 1998; DeCelles et al., 2001; Webb et al., 2011; Plate resulted in the growth and evolution of the Himalayan Gavillot et al., 2018). orogen and temporal in-sequence formation of the South- Lavé and Avouac (2000) studied the late Pleistocene– ern Tibetan Detachment System (STDS), the Main Central Holocene shortening history of the central Nepal Himalaya Thrust (MCT), the Main Boundary Thrust (MBT) and the where they showed that Holocene shortening is accommo- Himalayan Frontal Thrust (HFT) towards the south (e.g., Yin dated only across the HFT. However, a large body of litera- and Harrison, 2000; DiPietro and Pogue, 2004) (Fig. S1 in ture in the eastern, central and western Himalaya favors the the Supplement Sect. S1). The HFT defines the southern ter- idea that the majority of the late Pleistocene–Holocene short- mination of the Himalayan orogenic wedge and separates the ening is rather partitioned throughout the sub-Himalayan do- orogen from the undeformed foreland basin known as the main (morphotectonic segment between the MBT and the Indo-Gangetic Plain. Seismic reflection profiles reveal that MFT) and not solely accommodated by the HFT (e.g., Wes- all these fault zones emerge from a low-angle basal decolle- nousky et al., 1999; Burgess et al., 2012; Thakur et al., 2014; ment, the Main Himalayan Thrust (MHT), forming the base Mukherjee, 2015; Vassallo et al., 2015; Dey et al., 2016a, of the Himalayan orogenic wedge (e.g., Ni and Barazangi, 2018). The statement above implies that the northerly thrusts, 1984; Nábelekˇ et al., 2009) that was established in the late Published by Copernicus Publications on behalf of the European Geosciences Union. 464 S. Dey et al.: Implications of the ongoing rock uplift in NW Himalayan interiors i.e., the MBT and the brittle faults exposed in the vicinity of tectonic evolution, we combined geological field evidence, the southern margin of the Higher Himalaya, are considered chronologically constrained geomorphic markers and mor- inactive over millennial timescales. However, in recent years, phometric analysis of the KW. The detailed structural infor- several studies focused on low-temperature thermochrono- mation of the window and its surroundings, previously pub- logic data and thermal modeling of the interiors of the NW lished thermochron data, accessibility, well-preserved sedi- Himalaya have raised questions about the statement above. ment archives, and recognizable geomorphic markers across Recent studies have suggested that 1–3 mm yr−1 of the total the Kishtwar Window make it an attractive location for our Quaternary shortening has been accommodated in the north study. of the MBT as out-of-sequence deformation (Thiede et al., In this study, we use the KW to focus on the follow- 2004; Deeken et al., 2011; Thiede et al., 2017) or in the ing long-standing questions on Himalayan neotectonic evo- form of growth of the Lesser Himalayan Duplex (Gavillot lution, which are the following. et al., 2018) (Fig. S2). For faults within the hinterland of the central Himalaya, the out-of-sequence deformation has been 1. Is there any ongoing neotectonic deformation in the in- explained by two end-member models. One of them favors teriors of the Kashmir Himalaya? the reactivation of the MCT (Wobus et al., 2003), while the 2. Can we determine subsurface structural variations and other tries to explain all changes along the southern margin existence of surface-breaking faults by analyzing terrain of the Higher Himalaya as driven by enhanced rock uplift morphology? over a major ramp on the MHT (Bollinger et al., 2006; Her- man et al., 2010; Robert et al., 2009). Landscape evolution 3. Can we obtain new constraints on deformation over ge- models, structural analysis and thermochronologic data from omorphic timescales? Do millennial-scale fluvial inci- the interior of the Himalaya favor the idea that the Lesser sion rates support long-term exhumation rates? Himalaya has formed a duplex at the base of the south- ern Himalayan front through sustained internal deformation To address these questions, we adopted a combination since the late Miocene (Decelles et al., 2001; Mitra et al., of methods including morphometric analysis using high- 2010; Robinson and Martin, 2014; Gavillot et al., 2016). The resolution digital elevation models, field observations on growth of the duplex resulted in the uplift of the Higher Hi- rock type, structural variations and rock strength data, and malaya, forming the major orographic barrier of the orogen. analysis of satellite images to assess the spatial distribution The Kishtwar Window (KW) in the NW Himalaya represents of the late Quaternary deformation of the KW and surround- the northwestern termination of the Lesser Himalayan Du- ings (Fig. 1). We aimed to evaluate the role of active tec- plex (LHD). While most of the published cross sections of tonics and geometric variations in the basal decollement in the Himalayan orogen today recognize the duplex structures shaping the topography (Fig. 1). We used basin-wide steep- within the Lesser Himalaya (Webb et al., 2011; Mitra et al., ness indices and specific stream power as a proxy for fluvial 2010; DeCelles et al., 2001; Gavillot et al., 2018), little to incision. Lastly, but most importantly, we calculated the flu- no data are available on how the deformation is spatially and vial bedrock incision rates by using depositional ages of ag- temporally distributed or, most importantly, whether a duplex graded sediments along the Chenab River. In this study, we is active over millennial timescales. show that the regional distribution of topographic growth is The low-temperature thermochronology study by Kumar concentrated in the core of the window and along the western et al. (1995) portrayed the first orogen-perpendicular sam- margin of the window. Our new estimates of the bedrock in- pling traverse extending from the Kishtwar tectonic window cision rate agree with Quaternary exhumation rates from the over the Zanskar Range. More recent studies link the evo- KW, which could mean consistent active growth of the Kisht- lution of the KW to the growth of the Lesser Himalayan war Window over million-year to millennial timescales. Al- Duplex structure (Gavillot et al., 2018), surrounded by the though the observed topographic and morphometric patterns Miocene MCT shear zone along the base of the High Hi- indicate a structural and/or tectonic control on topographic malayan Crystalline Sequence, locally called the Kishtwar evolution, with the available data we are not able to resolve Thrust (KT) (Ul Haq et al., 2019). Thermochronological whether it is caused by passive translation on the MHT or by constraints suggest higher rates of exhumation within the active surface-breaking faulting within the duplex. window (3.2–3.6 mm yr−1) with respect to the surroundings − (∼ 0:2 mm yr 1) (Gavillot et al., 2018), corroborating simi- 2 Geological background lar thermochronology-based findings from the of the Kullu– Rampur window along the Beas (Stübner et al., 2018) and Regionally balanced cross sections (DiPietro and Pogue, Sutlej valley (Jain et al., 2000; Vannay et al., 2004; Thiede 2004; Searle et al., 2007; Gavillot et al., 2018) suggest that et al., 2004) over Quaternary timescales. No evidence ex- the Himalayan wedge is bounded at the base by a decolle- ists to distinguish whether the hinterland of the Kashmir Hi- ment called the MHT. All regionally extensive surface- malaya is tectonically active over intermediate timescales. breaking thrust systems are thought to be rooted to the MHT. Therefore, to understand the 103–104-year timescale of neo- The orogenic growth of the Himalaya resulted in an overall Earth Surf. Dynam., 9, 463–485, 2021 https://doi.org/10.5194/esurf-9-463-2021 S.

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