Drainage evolution and exhumation history of the eastern Himalaya: Insights from the Nicobar Fan, northeastern Indian Ocean

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Highlights

  • The Nicobar Fan was supplied by the Brahmaputra River from the eastern Himalaya.

  • Presence of Gangdese material denote a Yarlung Tsangpo-Brahmaputra River since ∼19 Ma.

  • The Nicobar Fan registers a two-stage exhumation of eastern syntaxis since ∼9.2 Ma.

Abstract

The eastern Himalayan syntaxis, where the Yarlung Tsangpo sharply bends, is one of the areas experiencing most rapid exhumation on Earth. The rapid exhumation is often regarded as the result of capture of the Yarlung Tsangpo by the Brahmaputra River. However, both the timing of integration of the Yarlung Tsangpo-Brahmaputra River and initiation of the rapid syntaxial exhumation are debated. As the ultimate sedimentary trap of the Yarlung Tsangpo-Brahmaputra River, the Nicobar Fan is a window to look into the drainage evolution and exhumation history of the eastern Himalaya. International Ocean Discovery Program Expedition 362 drilled the Nicobar Fan for the first time, recovering fan sediments dating back to the Early Miocene (∼19 Ma). We apply trace elements and Sr-Nd isotopes to investigate the provenance of the sediments in the Nicobar Fan with the aim of constraining the timing of integration of the Yarlung Tsangpo-Brahmaputra River and initiation of the rapid syntaxial exhumation. The geochemical and Sr-Nd isotope compositions indicate an eastern Himalayan source dominated by the Greater Himalaya, with significant Gangdese arc contribution and primarily carried by the Brahmaputra River. Flux of Gangdese arc material appears to have been continuous from the base of the Nicobar Fan, suggesting that the Yarlung Tsangpo-Brahmaputra River has been established at least since ∼19 Ma. Synchronously with the sharp rise in sedimentation rate, the abrupt change of geochemical and isotope compositions at ∼9.2 Ma indicates an increase in erosion of the Greater Himalaya as the result of initiation of rapid exhumation in the broad syntaxial region. The proportion of Greater Himalayan material increased again at 3.5–1.7 Ma, consistent with a younger pulse of rapid exhumation focused in the core of the syntaxis since ∼3.5 Ma. Our results show that initiation of the rapid syntaxial exhumation postdated integration of the Yarlung Tsangpo-Brahmaputra River by at least ∼10 m.y. Therefore, tectonic uplift rather than river capture could be responsible for the initiation of the rapid syntaxial exhumation.

Introduction

Collisional tectonics induces topographic variation and results in drainage reorganization via mechanisms such as river capture, diversion and reversal (e.g. Brookfield, 1998; Clark et al., 2004). Drainage reorganization markedly affects the distribution and intensity of erosion over an orogen, which in turn influences the style and location of crustal deformation (Cina et al., 2009). Therefore, study of drainage evolution during collisional orogeny is crucial to understanding the complex interplay between tectonic deformation and surface erosion (e.g. Bracciali et al., 2015). The Himalayan orogen, where active collision is ongoing and major South Asian rivers originate, is an ideal place in which to recognize tectonic-erosion interactions. The eastern Himalayan syntaxis is one of the most active tectonic regions on Earth and characterized by very rapid rock uplift and exhumation (Burg et al., 1997; Ding et al., 2001) (Fig. 1). The Yarlung Tsangpo runs eastward along the suture between the Himalaya and the Lhasa block, and then sharply bends southward through the eastern syntaxis, carving one of the world's largest and deepest gorges, the Yarlung Tsangpo Gorge. Downstream of the gorge, the Yarlung Tsangpo is called the Siang River and becomes the southwestward-flowing Brahmaputra River in the Himalayan foreland. It then meets the Ganges River, and finally discharges into the Bay of Bengal and the northeastern Indian Ocean, where it accumulates as the Bengal-Nicobar Fan system (Fig. 1). The sediments are now transferred to the Bengal Fan by turbidite currents via the Swatch-of-No-Ground submarine canyon and the Active Channel (Fig. 1) which has been active since 12.5 ka BP, however, there were probably more than one canyon-channel system at times before then (Curray et al., 2003). The Bengal-Nicobar Fan system extends southward for over 3000 km to ∼7°S and covers an area of ∼4 × 106 km2 with a volume of >8 × 106 km3 since ca. 20 Ma (Curray et al., 2003; Pickering et al., 2020a). As the ultimate sediment trap of the Ganges River and Yarlung Tsangpo-Brahmaputra River, the Bengal-Nicobar Fan preserves records of Himalayan erosion and is therefore vital to deciphering the drainage evolution and exhumation history of the eastern Himalaya.

The eastern Himalayan syntaxis presently feeds 45–70% of the bulk sediment flux of the Brahmaputra River, suggesting an exhumation rate up to 10 mm/yr or more (e.g. Singh and France-Lanord, 2002; Bracciali et al., 2016). Such high exhumation rates are also supported by the bedrock cooling age as young as <1 Ma (e.g. Seward and Burg, 2008). To explain the extremely rapid syntaxial exhumation, most researchers emphasize the role of tectonics caused by northward indentation of the northeastern corner of the Indian plate into Eurasia, which leads to growth of the syntaxis (e.g. Burg et al., 1997; Seward and Burg, 2008; Bendick and Ehlers, 2014). However, the “tectonic aneurysm” model highlights the potential coupling between tectonics and erosion and associates the rapid exhumation with the incision of the Yarlung Tsangpo Gorge (Zeitler et al., 2001, 2014). This model suggests that rapid, focused erosion weakens the upper crust, leading to lower crustal flow into the weakened zone, and promoting the doming of the upper crust, thus generating a self-sustaining feedback between tectonic deformation and surface erosion (Zeitler et al., 2001). This model also suggests that initiation of the rapid syntaxial exhumation could be triggered by capture of the Yarlung Tsangpo by the Brahmaputra River (Zeitler et al., 2001). However, both the timing of integration of the Yarlung Tsangpo-Brahmaputra River and initiation of the rapid syntaxial exhumation continue to be debated (e.g. Bracciali et al., 2015, 2016; Najman et al., 2019).

It has been proposed that the Yarlung Tsangpo flowed southeastward into the Irrawaddy River through the Parlung Tsangpo, before it was captured by the Siang-Brahmaputra River at ∼10 Ma (Brookfield, 1998) or 3–4 Ma (Zeitler et al., 2001; Clark et al., 2004). The timing of this capture event was however estimated based on the proposed age of the localized uplift of the eastern syntaxis. Recent provenance analyses of the paleo-Brahmaputra deposits provide new time constraints on integration of the Yarlung Tsangpo-Brahmaputra River, to be either in the Late Miocene or the Early Miocene. The first appearance of Gangdese arc detritus, indicative of a Yarlung Tsangpo contribution, in the eastern Himalayan foreland basin were detected at ∼10 Ma (Cina et al., 2009), at ∼7 Ma (Chirouze et al., 2013) and in the Early Miocene (Lang and Huntington, 2014) in various sections. Meanwhile, Bracciali et al. (2015) observed the first influx of Gangdese arc material in Lower Miocene sediments of the Surma Basin (northeastern Bengal Basin) downstream of the Brahmaputra River. The timing of onset of the rapid syntaxial exhumation remains poorly constrained, varying from the Late Miocene to the Plio-Pleistocene (11–2 Ma). Most of the bedrock thermochronology data from the syntaxial region show young cooling ages and indicate onset of rapid exhumation since ∼3.5 Ma (e.g. Burg et al., 1997; Seward and Burg, 2008). However, bedrock zircon U-Pb geochronology denoted local melting accompanying rapid cooling since ∼11–9.7 Ma (Ding et al., 2001; Booth et al., 2004). A synthesis study of cooling history within and around the syntaxis indicates a significant pulse of rapid exhumation at 10–5 Ma (Zeitler et al., 2014). The paleo-Brahmaputra detrital records could offer a long-term exhumation history of the syntaxis after the integration of the Yarlung Tsangpo-Brahmaputra River, which avoids problems associated with study of the syntaxis bedrock where erosion and metamorphism have removed or obscured the early exhumation history. However, estimates of the onset of rapid exhumation interpreted from detrital thermochronology are also variable. Rapid exhumation was recorded in the eastern Himalayan foreland basin by 7–5 Ma (Lang et al., 2016), in the Surma Basin since ∼3.5–2 Ma (Bracciali et al., 2016) and in the Bengal Fan since ∼3.5 Ma (Najman et al., 2019), as indicated by the first occurrence of detrital minerals with short lag times.

International Ocean Discovery Program (IODP) Expedition 362 successfully cored Nicobar Fan sediments dating back to the Early Miocene (∼19 Ma) (Fig. 1). These sediments represent a key sedimentary archive of eastern Himalayan evolution (McNeill et al., 2017a, 2017b). The Nicobar Fan sediments was transported for a long distance over 1700 km from the outlet. Therefore, compared to the proximal records of the fluvial-deltaic Himalayan foreland basin and Surma Basin, the Nicobar Fan sediments might be expected to minimize local effects that would obscure the upstream signal, and also provide a continuous marine sedimentary succession with better depositional age constraints. Evidence from the Bengal Basin and the Bay of Bengal indicates that the Ganges River and the Brahmaputra River entered the Bay of Bengal separately at some time in the past (e.g. Curray et al., 2003; Govin et al., 2018), rather than merging together before entering the bay as they do today. Therefore, compared to the Bengal Fan, the Nicobar Fan in the east should be less influenced by the signal of the Ganges River that drains the central Himalaya and peninsular India. Any significant contribution from the Irrawaddy River to the Nicobar Fan is unlikely as it requires transfer of the sediments across the forearc to the west, which was restricted by the then exposed Yadana-M8 Highs and the uplifted Sewell-Alcock Rises west of the Andaman Sea (Racey and Ridd, 2015) (Fig. 1). Sediment isopaches of the Cenozoic Martaban basin in the Andaman Sea related to the development of the Salween-Irrawaddy delta also show no significant sediment transfer to the west (Racey and Ridd, 2015) (Fig. 1). Because the Himalayan units and the Gangdese arc have contrasting lithologies and geochemistry (e.g. Singh et al., 2008; Wu et al., 2010) (Table 1), we conduct a provenance analysis of the Nicobar Fan sediments using trace elements and Sr-Nd isotope composition. Specifically, we aim to constrain the timing of integration of the Yarlung Tsangpo-Brahmaputra River by detecting the first appearance of Gangdese arc material, and attempt to estimate the timing of initiation of rapid exhumation of the eastern Himalayan syntaxis from the perspective of a provenance shift. Lastly, we evaluate the interaction between the rapid syntaxial exhumation and the Yarlung Tsangpo-Brahmaputra River evolution.

Section snippets

Tectonic setting of the eastern Himalaya

The collision between the Indian and Eurasian plates, beginning at 65–43 Ma, deformed and uplifted the northern margin of the Indian continent to form the Himalayan orogen (Yin, 2006). The Himalaya is separated from the Lhasa block of the Eurasian plate by the Indus-Yarlung Suture Zone (Fig. 1). The southern Lhasa block is intruded by Cretaceous–Paleogene magmatic and volcanic rocks of the Andean-type Gangdese arc, resulting from the northward subduction of the Tethyan Ocean (e.g. Copeland et

Methods

We conducted trace element and Sr-Nd isotopic analyses on the bulk silicate fraction of the Nicobar Fan muds/mudstones, in an attempt to trace provenance variations. A total of sixty-eight core samples were collected (Fig. 2), primarily from the mud/mudstone of the upper part of turbidite beds. The samples were leached with 2N acetic acid to remove carbonates prior to element and isotopic analyses at the State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese

Results

Results of the trace element and Sr-Nd isotopic analyses of the silicate fraction of the Nicobar Fan samples are listed in Supplementary Tables A.1 and A.2, respectively.

Eastern Himalayan provenance for the Nicobar Fan sediments

The trace element and REE distribution patterns of the Nicobar Fan sediments denote a Himalayan provenance dominated by sedimentary-metasedimentary rocks. However, enrichment of the transitional elements relative to UCC indicates additional input of more mafic rocks. The Niocbar Fan sediments also reflect mixing between felsic and intermediate sources on a plot of Cr/Th versus Th/Sc (Fig. A.1). It is also noteworthy that the Nicobar Fan sediments show higher εNd values and lower 87Sr/86Sr

Conclusions

We conducted a provenance study on well-dated Nicobar Fan sediments (19 Ma–Recent) using trace element and Sr-Nd isotopic methods. We provide new time constraints for integration of the Yarlung Tsangpo-Brahmaputra River and initiation of rapid exhumation of the eastern Himalayan syntaxis.

The trace element and REE distribution patterns of the Nicobar Fan sediments indicate a Himalayan provenance. The Sr-Nd isotope compositions of the Nicobar Fan sediments further show a close affinity with

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This research used samples and data provided by the International Ocean Discovery Program (IODP). We appreciate Science Party of IODP Expedition 362, in particular the Co-Chief Scientists (McNeill L.C. and Dugan B.E.), the Staff Scientist (Petronotis K.E.) and the leader of the Paleontology team (Backman J.), for their efforts. We also thank the IODP staff and JOIDES Resolution crew for their contributions during the expedition. This research was financially supported by the National Natural

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