# Understanding Himalayan erosion and the significance of the Nicobar Fan

Lisa C. Mcneill, Brandon Dugan, Jan Backman, Kevin T. Pickering, Hugo Pouderoux, Timothy J. Henstock, Katerina E. Petronotis, Andrew Carter, Farid Chemale Junior, Kitty Milliken, et al.

► To cite this version:

Lisa C. Mcneill, Brandon Dugan, Jan Backman, Kevin T. Pickering, Hugo Pouderoux, et al. Understanding Himalayan erosion and the significance of the Nicobar Fan. Earth and Planetary Science Letters, 2017, 475, pp.134-142. 10.1016/j.epsl.2017.07.019. insu-01579460

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Understanding Himalayan erosion and the significance
of the Nicobar Fan

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CrossMark

Lisa C. McNeill<sup>a,*</sup>, Brandon Dugan<sup>b</sup>, Jan Backman<sup>c</sup>, Kevin T. Pickering<sup>d</sup>,
Hugo F.A. Pouderoux<sup>e</sup>, Timothy J. Henstock<sup>a</sup>, Katerina E. Petronotis<sup>f</sup>, Andrew Carter<sup>g</sup>,
Farid Chemale Jr.<sup>h</sup>, Kitty L. Milliken<sup>i</sup>, Steffen Kutterolf<sup>j</sup>, Hideki Mukoyoshi<sup>k</sup>,
Wenhuang Chen<sup>l</sup>, Sarah Kachovich<sup>m</sup>, Freya L. Mitchison<sup>n</sup>, Sylvain Bourlange<sup>o</sup>,
Tobias A. Colson<sup>p</sup>, Marina C.G. Frederik<sup>q</sup>, Gilles Guèrin<sup>r</sup>, Mari Hamahashi<sup>s,1</sup>,
Brian M. House<sup>t</sup>, Andre Hüpers<sup>u</sup>, Tamara N. Jeppson<sup>v</sup>, Abby R. Kenigsberg<sup>w</sup>,
Mebae Kuranaga<sup>x</sup>, Nisha Nair<sup>y</sup>, Satoko Owari<sup>z</sup>, Yehua Shan<sup>l</sup>, Insun Song<sup>aa</sup>,
Marta E. Torres<sup>ab</sup>, Paola Vannucchi<sup>ac</sup>, Peter J. Vrolijk<sup>ad</sup>, Tao Yang<sup>ae,2</sup>, Xixi Zhao<sup>af</sup>,
Ellen Thomas<sup>ag</sup>

<footnote>a Ocean and Earth Science, National Oceanography Centre Southampton, University of Southampton, Southampton, S014 3ZH, United Kingdom</footnote>

<footnote>b Department of Geophysics, Colorado School of Mines, Golden, CO 80401, USA</footnote>

<footnote>c Department of Geological Sciences, Stockholm University, SE-106 91 Stockholm, Sweden</footnote>

<footnote>d Earth Sciences, University College London, London, WC1E 6BT, United Kingdom</footnote>

<footnote>e CNRS, UMR6118 - Geosciences Rennes, University de Rennes I, Campus de Beaulieu, 35042 Rennes Cedex, France</footnote>

<footnote>f International Ocean Discovery Program, Texas A&M University, 1000 Discovery Drive, College Station, TX 77845, USA</footnote>

<footnote>g Department of Earth & Planetary Sciences, Birkbeck College, London, WC1E 7HX, UK</footnote>

<footnote>h Programa de Pôs-Graduação em Geologia, Universidade do Vale do Rio dos Sinos, 93.022-000 São Leopoldo, RS, Brazil</footnote>

<footnote>i Bureau of Economic Geology, University Station, Box X, Austin, TX 78713, USA</footnote>

<footnote>j GEOMAR, Helmholtz Center for Ocean Research Kiel, Wischhofstr. 1-3, Kiel 24148, Germany</footnote>

<footnote>k Department of Geoscience, Shimane University, 1060 Nishikawatsu-cho, Matsue, Shimane 690-8504, Japan</footnote>

<footnote>l Key Laboratory of Marginal Sea Geology, Chinese Academy of Sciences, #511 Kehua Street, Tianhe District, Guangzhou 510640, PR China</footnote>

<footnote>m Department of Geography Planning and Environmental Management, Level 4, Building 35, The University of Queensland, Brisbane, QLD, Australia</footnote>

<footnote>n School of Earth and Ocean Sciences, Cardiff University, Park Place, Cardiff, CF10 3XQ, United Kingdom</footnote>

<footnote>o Laboratoire GeoResources, CNRS-Université de Lorraine-CREGU, Ecole Nationale Supérieure de Géologie, Rue du Doyen Marcel Roubault, TSA 70605-54518, Vandoeuvre-lès-Nancy, France</footnote>

<footnote>p School of Earth Sciences, University of Western Australia, 35 Stirling Highway, Crawley 6009, Australia</footnote>

<footnote>q Center for Regional Resources Development Technology (PTPSW-TPSA), Agency for the Assessment and Application of Technology (BPPT), Bldg. 820, Earth System Technology (Geostech), Kawasan Puspitek Serpong, South Tangerang, Banten, 15314, Indonesia</footnote>

<footnote>r Lamont-Doherty Earth Observatory, Columbia University, Borehole Research Group, 61 Route 9W, Palisades, NY 10964, USA</footnote>

<footnote>s Geophysics Research Group, Institute of Geology and Geoinformation, Geological Survey of Japan (AIST), AIST Tsukuba Central 7, 1-1-1 Higashi, Tsukuba Ibaraki 305-8567, Japan</footnote>

<footnote>t Scripps Institution of Oceanography, University of California, San Diego, Vaughan Hall 434, 8675 Discovery Way, La Jolla, CA 92037, USA</footnote>

<footnote>u MARUM-Center for Marine Environmental Sciences, University of Bremen, PO Box 330 440, D-28334 Bremen, Germany</footnote>

<footnote>v Department of Geology and Geophysics, University of Wisconsin-Madison, 1215 W. Dayton Street, Madison, WI 53706, USA</footnote>

<footnote>w Department of Geosciences, Pennsylvania State University, 503 Deike Building, University Park, PA 16802, USA</footnote>

<footnote>x Graduate School of Science and Engineering, Yamaguchi University, 1677-1 Yoshida, Yamaguchi City 753-8512, Japan</footnote>

<footnote>y CLCS/Marine Geophysical Division, National Centre for Antarctic and Ocean Research, Earth System Science Organization, Ministry of Earth Sciences, Government of India, Headland Sada, Vasco-da-Gama, Goa 403804, India</footnote>

<footnote>z Department of Earth Sciences, Chiba University, 1-33 Yayoi-cho, Image-ku, Chiba City 263-8522, Japan</footnote>

<footnote>aa Geologic Environmental Division, Korea Institute of Geoscience & Mineral (KIGAM), 124 Gwahak-ro, Yuseong-gu, Daejeon 34132, Republic of Korea</footnote>

<footnote>ab College of Earth, Ocean and Atmospheric Sciences, Oregon State University, 104 CEOAS Admin Building, Corvallis, OR 97331-5503, USA</footnote>

<footnote>ac Royal Holloway and Bedford New College, Royal Holloway University of London, Queens Building, Egham, TW20 0EX, United Kingdom</footnote>

<footnote>ad New Mexico Tech, 801 Leroy Place, Socorro, NM 87801, USA</footnote>

<footnote>ae Institute of Geophysics, China Earthquake Administration, #5 Minzu Daxue Nanlu, Hiadian District, Beijing 100081, PR China</footnote>

<footnote>af Department of Earth and Planetary Sciences, University of California, Santa Cruz, 1156 High Street, Santa Cruz, CA 95064, USA</footnote>

<footnote>ag Department of Geology and Geophysics, Yale University, New Haven, CT 06520-8109, USA</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: lcmn@noc.soton.ac.uk (L.C. McNeill).</footnote>

<footnote n="1">1 Now at: Earth Observatory of Singapore (EOS), Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798.</footnote>

<footnote n="2">2 Now at: Institute of Geophysics and Geomatics, China University of Geosciences, 388 Lumo Road, Wuhan 430074, PR China.</footnote>
ARTICLE INFO

Article history:

Received 3 February 2017
Received in revised form 7 July 2017
Accepted 10 July 2017
Available online 10 August 2017
Editor: M. Bickle

Keywords:

Bengal–Nicobar Fan
submarine fan
Himalayan tectonics
Asian monsoon
Indian Ocean

A B S T R A C T

A holistic view of the Bengal–Nicobar Fan system requires sampling the full sedimentary section of the Nicobar Fan, which was achieved for the first time by International Ocean Discovery Program (IODP) Expedition 362 west of North Sumatra. We identified a distinct rise in sediment accumulation rate (SAR) beginning ~9.5 Ma and reaching 250–350 m/Myr in the 9.5–2 Ma interval, which equal or far exceed rates on the Bengal Fan at similar latitudes. This marked rise in SAR and a constant Himalayan-derived provenance necessitates a major restructuring of sediment routing in the Bengal–Nicobar submarine fan. This coincides with the inversion of the Eastern Himalayan Shillong Plateau and encroachment of the west-propagating Indo–Burmese wedge, which reduced continental accommodation space and increased sediment supply directly to the fan. Our results challenge a commonly held view that changes in sediment flux seen in the Bengal–Nicobar submarine fan were caused by discrete tectonic or climatic events acting on the Himalayan–Tibetan Plateau. Instead, an interplay of tectonic and climatic processes caused the fan system to develop by punctuated changes rather than gradual progradation.

© 2017 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license

(http://creativecommons.org/licenses/by/4.0/).

1. Introduction

The Bengal–Nicobar Fan, Indian Ocean (Fig. 1), has the greatest area and length of any submarine fan, and has long been studied to investigate possible links between Himalayan tectonics and the Asian monsoon (e.g., An et al., 2001; Bowles et al., 1978; Clift et al., 2008; Curray, 2014; Curray and Moore, 1974; France-Lanord et al., 2016; Schwenk and Spiess, 2009; Moore et al., 1974). To date, a holistic synthesis of the Indian Ocean fan system history and related processes of tectonics, climate and erosion has been hampered by a lack of data from the Nicobar Fan. The importance of sampling widely across a sedimentary system to avoid biases due to major temporal changes in channel and lobe activity was noted (Stow et al., 1990), and highlighted that the under-sampled Nicobar Fan may hold a key component of the eastern Indian Ocean sedimentation record.

International Ocean Discovery Program (IODP) Expedition 362 sampled and logged the Nicobar Fan offshore North Sumatra in 2016 (Fig. 1). The stratigraphic results from this expedition (Dugan et al., 2017) are integrated here with results from previous sites on the Bengal–Nicobar Fan and Ninetyeast Ridge (NER) of the Deep Sea Drilling Program (DSDP Leg 22, von der Borch et al., 1974), Ocean Drilling Program (ODP Leg 116, Cochran et al., 1989; Leg 121, Peirce et al., 1989) and IODP (Expedition 353, Clemens et al., 2016; Expedition 354, France-Lanord et al., 2016). We present the first stratigraphic data from the Nicobar Fan and reappraise published chronostratigraphic data from Bengal Fan and NER drillsites into a unified modern timescale to facilitate accurate comparison of depositional records across the whole system. Comparing sediment accumulation rates (SARs) between these sites gives a new and integrative understanding of the timing of fan growth and distribution of fan deposits.

2. Nicobar Fan stratigraphy and sediment source

Expedition 362 drilled two sites on the northern Nicobar Fan east of the NER (Fig. 1), sampling the complete sedimentary section at Site U1480 to a basement depth of 1415 m below seafloor (mbsf), and from 1150 mbsf to within 10s m of basement at Site U1481 at 1500 mbsf. At both sites Units I and II represent the Nicobar Fan, with Units III–V representing intervals dominated by pelagic sedimentation with significantly reduced SARs (Fig. 1).

Bengal Fan sediments are predominantly micaceous quartzo-
feldspathic sands of the Ganges and Brahmaputra that drain the
Himalaya and southern Tibet plus contributions from the Meghna

river that drains northeastern India and Bangladesh (France-Lanord et al., 2016). Despite proximity to the Sunda forearc, the Nicobar Fan sediments at Sites U1480 and U1481 (Fig. S1) contain a similar range of siliciclastic sediment gravity-flow (SGF) deposits (mostly turbidites) as Bengal Fan sites. The sand- and silt-size grain assemblage in the Nicobar Fan is relatively uniform downhole as quartzo-feldspathic (arkosic) sands, with pelitic metamorphic lithic grains, mica, minor detrital carbonate (<5%), minor woody debris, and an abundant and diverse assemblage of mostly high-grade metamorphic heavy minerals (including kyanite and sillimanite). Candidate sources for the Nicobar Fan include the Himalayan-derived Ganges-Brahmaputra, Indo-Burman Ranges/West Burma, Sunda forearc and arc, and NER. Detrital zircon age spectra of samples from the Nicobar Fan sand-silt SGF deposits are dominantly sourced from the Greater and Tethyan Himalaya mixed with sediment from the Burmese arc-derived Paleogene Indo-Burman Ranges, similar to the provenance of Neogene sands deposited in the eastern Bengal and Surma basins (Najman et al., 2008, 2012) (Figs. 1–3). The limited arc-derived ash content in sediments at Sites U1480–1481 suggests that the Sunda forearc makes only a minor contribution. Significant input from the Irrawaddy drainage is unlikely as it would require transfer of material across the forearc and possibly the trench. Cenozoic sediment isopachs of the Martaban back arc basin, the main north–south-oriented depocentre in the Andaman Sea related to the development of the Thanlwin–Irrawaddy delta system, show no obvious evidence for major routing to the west where carbonate-capped volcanic highs (e.g. Yadana High) served as a barrier for most of the Neogene (Racey and Ridd, 2015). Nevertheless we consider Irrawaddy sources in our provenance interpretations.

**3. Bengal–Nicobar Fan sediment accumulation patterns**

Age-depth distributions of calcareous nannofossils, planktonic
foraminifers, diatoms, silicoflagellates, and radiolarians were used
to create tie points for SARs for Expedition 362 sites (Dugan et
al., 2017) (Fig. 4A; Supplementary Material). Also, published lat-
est Eocene-Recent biomagnetostratigraphic data from six represen-
tative DSDP, ODP and IODP Bengal Fan and NER sites were re-
assessed and placed on a common time scale (Hilgen et al., 2012;
Pälike et al., 2006) (Fig. 4; Supplementary Material). The reassess-
ment of previous biostratigraphic data and the choice of age model
tie points considered recent developments in understanding the
consistency and reliability of biohorizons in lower latitude environ-
ments. At all sites, the age-depth relationship is non-linear. At Sites
U1480 and U1481 on the Nicobar Fan, the SAR increases dramati-
<figure><img src="image_4.png" /><figcaption>**Fig. 1.** Regional map of study area. Map includes Bengal–Nicobar Fan system, river systems, eastern Himalayan provinces, and relevant DSDP/ODP/IODP sites. BB = Bengal Basin; SP = Shillong Plateau; SB = Surma Basin; IBR = Indo-Burman Ranges. Inset summarizes Site U1480 lithostratigraphy.</figcaption></figure>

cally at 9.5–9 Ma, from <15 m/Myr to >200 m/Myr, which corresponds to the onset of significant fan deposition at the Unit III–II boundary. Specifically, at Site U1480 rates increase from 2–15 to 223 m/Myr, and at Site U1481 from 11–27 to 207 m/Myr (Figs. 4, S2). At Site U1480, high rates persist and in the earliest Pleistocene (~2–2.5 Ma), they increase further to 360 m/Myr (Fig. S2). In the Bengal Fan, such high rates, of the order of 250–300 m/Myr or greater, are only found within the more proximal fan (e.g., Weber et al., 1997, although greater spatial and temporal variability in rates might be expected here due to high impact of sea level fluctuation coupled with shifting channel/levée systems) or in the latest Pleistocene (e.g., Expedition 354 results of France-Lanord et al., 2016). These results emphasize the significance of the Nicobar Fan within the wider Bengal–Nicobar Fan system from the late Miocene to early Pleistocene (~9–2 Ma).

The NER (separating the Bengal and Nicobar fans) is thought to capture an elevated expression of fan deposition despite its predominantly pelagic composition (Peirce et al., 1989), due to in-

creased flow lofting (cf., Stow et al., 1990) and nepheloid layer flux. Remarkably, at all northern NER sites (216, 217 and 758/1443; Fig. 2B) SARs increase by a factor of 2–3 at ~10–8 Ma, coeval with the Nicobar Fan site increases. Decreasing carbonate content values at Sites 217, 758, 1443, support that the increase in SAR resulted from the effect of increased input of clay.

When SARs increase on the Nicobar Fan and NER (at 2–9°N), rates on the Bengal Fan at related mid-fan positions (Fig. 4A) show either a marked decrease (e.g., Site 718, at 1°S) or minimal change (e.g., Site 1451, at 8°N), and all rates are lower than on the Nicobar Fan immediately after 9–9.5 Ma. At Site 718, rates decrease from 275 to 12–13 m/Myr at 9.5 Ma and only exceed 100 m/Myr in the late Pleistocene (Fig. S2). At Site 1451, rates from 9.5 Ma do not exceed 150 m/Myr (range: 70–150 m/Myr; Fig. S4). Local variations in deposition between Leg 116 sites (717, 718, 719) can be explained by late Miocene folding on the Indian plate controlling accommodation and potential submarine-channel routing (Stow et al., 1990).
<figure><img src="image_5.png" /><figcaption>**Fig. 2.** Detrital zircon age plots for samples and equivalent plots of regional rivers and formations. TH = Tethyan Himalaya; GHS = Greater Himalaya Series; LHS = Lesser Himalaya Series. Details of Expedition 362 samples are given in Table S1.</figcaption></figure>

Depositional history at a single site is inevitably affected by individual channel and lobe positions and there will be variability across the fan in terms of sediment accumulation due to depositional environment, e.g., channel-axis, channel-margin, levee-overbank, lobe and lobe fringe etc. (e.g., Stow et al., 1990; Schwenk and Spiess, 2009). However, on the Nicobar Fan seismic horizons and packages can be traced over large distances with confidence in unit correlation and with minimal evidence of unit thickness variation across and along the oceanic plate. No onlap in the vicinity of the drillsites is observed, and channel-levée complexes that might correlate with enhanced deposition are evenly distributed spatially and temporally (Dugan et al., 2017). In addition, our analysis of data from other Expedition 354 Bengal Fan transect sites where SARs may be greater than at Site U1451 (e.g., U1450, France-Lanord et al., 2016) continues to support that during the late Miocene and Pliocene, Nicobar Fan rates either exceeded or were broadly comparable with those on the Bengal Fan. This interpretation is further supported because the Expedition 354 sites on the Bengal Fan are located 5°N of the Expedition 362 sites on the Nicobar Fan (and therefore probably in a more proximal position).

Preliminary benthic foraminiferal data from the Site U1480 pre-fan and lowermost Nicobar Fan deposits indicate that this part of the Indian plate was at upper abyssal depths (2500–3000 m; Supplementary Material), not isolated at a higher elevation which

could delay arrival of fan sediments. Combining these facts, we have confidence that the drillsite stratigraphic record is representative of the wider Nicobar Fan.

Compilation of age-depth plots and SARs from across the fan system enables us to examine earlier sedimentation patterns. These indicate that by at least 15 Ma, SARs on the Bengal Fan were high with sediment directed west of the NER (Fig. 5C). The oldest recovered sediments in the central part of the Bengal Fan are Oligocene (25–28 Ma) thin-bedded silts, and the first significant sands are late Miocene, although this may be related to changes in coring technique and recovery (9–10 Ma: Site U1451, France-Lanord et al., 2016). At other Expedition 354 sites, the apparent earliest onset of substantial sand was <8 Ma (Sites U1450 and U1455), although we note that low recovery in parts of the deeper section at these sites may allow for the presence of additional earlier thick sand layers. In the distal Bengal Fan (Leg 116 Sites 717–719), early Miocene (back to 17 Ma) silts came from the Himalaya and minor components from the Indian subcontinent (Cochran et al., 1989; Bouqillon et al., 1990; Copeland et al., 1990). At Nicobar Fan Site U1481, a 20-m interval within the period 19–9 Ma (sample 362–9 in Figs. 2 and 3) includes minor very fine-grained sandstones and siltstones, with the same zircon assemblage as other Expedition 362 sand/silt samples (Fig. 2), supporting an eastern Himalayan source. These predate the dramatic increase in sediment flux to the Nicobar Fan sites.
<figure><img src="image_6.png" /><figcaption>**Fig. 3.** Detrital zircon age comparison plots of samples from this study and regional rivers and formations. Multidimensional scaling maps (Vermeesch, 2013) based on calculated K-S distances between U-Pb age spectra, comparing Nicobar Fan sand samples from this study with possible source areas compiled from the literature (Allen et al., 2008; Bracciali et al., 2015, 2016; Campbell et al., 2005; Gehrels et al., 2011; Limonta et al., 2017; Najman et al., 2008). The maps show 362 samples share the same sources as the SGF deposits exposed on the Andaman–Nicobar Islands and Neogene sediments deposited in northeast Bengal that were originally sourced from erosion of the Indo–Burma Ranges. The IODP sands are not directly comparable to sands from the modern Brahmaputra or Irrawaddy. See Fig. 2 for acronyms.</figcaption></figure>

In summary, although the Bengal–Nicobar Fan was clearly developing prior to the late Miocene, the SARs at a range of sites support a marked increase in sediment flux at around 9.5 Ma, in particular to the eastern part of the system, the Nicobar Fan (Fig. 5B; an idea postulated by Bowels et al. (1978), confirmed here with detailed and integrated drilling and seismic data). A major conclusion from our appraisal of SARs is that when high SARs are recorded on the Bengal Fan, they are significantly lower on the Nicobar Fan, and that between ~9.5–2 Ma this switches abruptly (Fig. 3), with highest sediment flux deflected to the east.

## 4. Nicobar Fan volumetrics and late Miocene–recent growth of the Sunda forearc

Using sediment thickness from seismic profiles and ocean drilling boreholes, we have made a new estimate of the late Miocene–Recent Nicobar Fan volume, incorporating the component of fan now accreted into the Sunda subduction margin (see Supplementary Material). Estimates of the present day Nicobar Fan volume are ~0.5 × 10⁶ km³ (Figs. 1, S4; Supplementary Material), without decompaction. An additional 0.4 × 10⁶ km³ is estimated to have been added to the accretionary prism between the Nicobar Islands and Southern Sumatra, using present day sediment thicknesses and plate convergence rates back to 9 Ma (Fig. S5). This generates a minimum late Miocene–Recent (i.e. from ~9.5 Ma to present) Nicobar Fan volume of ~1 × 10⁶ km³. This volume is significant compared with the estimated Bengal Fan volume of 7.2 × 10⁶ km³ for the entire Neogene, a period of ~20 Myr (Clift, 2002).

The increase in thickness of the Indian plate sediment section at ~9.5 Ma would have corresponded to a marked and abrupt change in sediment volume input to the north Sunda margin. Offshore North Sumatra and the Nicobar Islands, the forearc prism is markedly wide (150–180 km) and thick relative to the rest of the margin and to other accretionary prisms (McNeill and Henstock, 2014), and the northern Sumatran prism forms an unusual plateau inferred to be a consequence of internal and basal material properties and/or prism growth history (e.g., Fisher et al., 2007). The large additional sediment input volume from 9.5 Ma to present in the easternmost Indian Ocean is a significant proportion of the prism volume and can explain the anomalously large northern Sunda prism (supporting hypotheses by Hamilton, 1973; Karig et al., 1979).

## 5. Discussion

Our new integrated Nicobar–Bengal Fan sediment records show a net increase in flux to the eastern Indian Ocean at 9.5–9 Ma, representing the onset of a new sedimentary regime in the east Indian Ocean. Detrital zircon ages (Fig. 2) and petrology from the Nicobar Fan sediments show the sand provenance remained unchanged throughout the middle Miocene to present. To constrain sources we compared these results with detrital zircon ages from potential source regions in the Himalayas as well as the Burmese arc and Irrawady drainage due to the presence of Cenozoic age zircons (mainly between 60–20 Ma). Comparison of the zircon age distributions (Fig. 3) show SGF deposits exposed on the Andaman–Nicobar Islands are closely similar to Nicobar Fan sediments and that both have affinities with Himalayan-derived units, the Trans-Himalaya and arc-derived input from erosion of the Indo–Burman Ranges that were expanding westwards during the Pliocene. These are the same sources as Neogene sands deposited in the north-east Bengal and Surma basins via the paleo-Brahmaputra River (Najman et al., 2012) (Fig. 1). Between 15–9 Ma the northeast Bengal Basin underwent inversion related to tectonic shortening and exhumation of the Shillong Basin Plateau which accommodates up to 1/3 of the present-day convergence across the Eastern Himalaya (Bilham and England, 2001; Biswas et al., 2007; Clark and Bilham, 2008) (Fig. 1). Exhumation and erosion of the Himalayan-derived Surma Group atop the Shillong Plateau began at this time followed by the main phase of surface uplift <3.5 Ma (Najman et al., 2016) – these timings provide an excellent fit with the high SARs on the Nicobar Fan (from 9.5, and 2.5–2 Ma). We propose that this inversion of the Shillong region and westward migration of the Indo–Burmese wedge reduced accommodation and diverted sediments south to the shelf and Nicobar Fan. The most significant sediment pulse to the Nicobar Fan, at 2.5–2 Ma,
<figure><img src="image_7.png" /><figcaption>**Fig. 4.** Age-depth relationships at ocean drilling sites. Panel (A) shows tie points of biomagnetostratigraphic age-depth relationships for Bengal Fan sites (718C, U1451) and Nicobar Fan sites (U1480, U1481). Panel (B) shows biomagnetostratigraphic tie points of age-depth relationships for sites from the Ninetyeast Ridge crest (216, 217, 758A, 1443A). Inset shows sediment accumulation rate (SAR) increase between 9 and 10 Ma. Data are presented in detail in Figs. S2 and S3, and Tables S2 and S3.</figcaption></figure>

may also record erosion of the exhumed eastern Himalayan syn-
taxis and resulting erosion, with at least 12 km of material since
3 Ma – a signal recorded in the Surma Basin from the latest
Pliocene (Bracciali et al., 2016) but not previously identified in
the Indian Ocean. From ~2 Ma, a SAR reduction on the Nicobar
Fan supports the hypothesis that impingement of the NER on the
Sunda Trench diverted the primary flux west of the ridge with con-
comitant high mid-late Pleistocene SARs on the Bengal Fan (e.g.,
France-Lanord et al., 2016) (Fig. 5A), although westward re-routing
of the Brahmaputra River may also have played a role (Najman et
al., 2016).

The Nicobar Fan is volumetrically significant within the Bengal-
Nicobar Fan system, and at certain times during the late Miocene-
early Pleistocene, such as the near 400 m/Myr SARs of the earliest
Pleistocene, it may have been a dominant sediment sink. Since
10 Ma, sea level has generally fallen (Miller et al., 2005), decreas-
ing accommodation on the shelf, thus amplifying the processes
driving sediment southward to the fan. Erosion was probably aided
by South Asian monsoon strengthening in the mid to late Miocene
(e.g., An et al., 2001; Betzler et al., 2016; Kroon et al., 1991;
Peterson and Backman, 1990; Prell and Kutzbach, 1992) combined
with the orographic forcing that caused the locus of monsoon pre-
cipitation to shift south onto the newly uplifted Shillong Plateau
(Biswas et al., 2007). Enhanced erosion rates (Duvall et al., 2012)
and/or uplift in the eastern Tibetan plateau (e.g., An et al., 2001) in
the late Miocene would also contribute to the volume of sediment
available.

Avulsion of large-scale channel-levée complexes is common to
many submarine fans (e.g., Amazon Fan; Flood et al., 1991). We
propose that the eastward deflection of sediment towards the
Nicobar Fan at 9.5 Ma was the result of channel avulsion in re-
sponse to increased sediment flux. Consequently, the proportion
of sediment routed from the northeastern Bengal Basin to the
Bengal Fan was significantly reduced. Any differential seafloor to-
pography on the shelf and the subsiding NER could have assisted
this eastward diversion process. Similar processes have been ob-

served in physical flume experiments, for example, lobe switching
observed with change in sedimentation supply/rate, accommoda-
tion, and depositional slope (Fernandez et al., 2014; Parsons et al.,
2002).

An early, lower-volume phase of fan deposition is recorded
in the accreted sediments of the Sunda forearc (to ~40–50 Ma;
Curray and Moore, 1974; Murray et al., 1979; Karig et al., 1980).
A model of trench-axial supply, with sediment now almost entirely
accreted, can explain this earlier phase (nascent Nicobar Fan), with
trench overspill delivering minor sands/silts recorded at Site U1481
by the middle Miocene.

An accurate history of siliciclastic deposition on the Bengal-
Nicobar Fan system necessitates knowledge of both fans. Using
estimates of fan volume, we demonstrate that the Nicobar Fan
is significant within the overall sediment budget of the Bengal-
Nicobar Fan, particularly from 9.5–2 Ma. This previously unquanti-
fied sediment sink likely also contributes to organic carbon burial
budgets (cf. Galy et al., 2007). An interplay of tectonic, climatic
and sedimentological processes, rather than a discrete tectonic or
climatic event or mechanism such as monsoon onset, as often in-
voked (e.g., Betzler et al., 2016; Clift et al., 2008), moved sediment
through a series of staging areas and controlled SARs in the various
sediment sinks of the Indo-Asian system. Our reappraisal of inte-
grated drilled fan data is inconsistent with the long-held notion of
gradual fan progradation (Curray et al., 2003) but rather suggests a
more dynamic system of punctuated and abrupt changes (Fig. 5D).
Our work highlights the importance of sediment routing from the
uplifting Eastern Himalaya along the eastern Indian Ocean to the
Nicobar Fan during the late Neogene, a region whose role has been
significantly underappreciated.

Acknowledgements

This research used samples and data provided by the Interna-
tional Ocean Discovery Program (IODP) (www.iodp.org/access-data-
and-samples). The JOIDES Resolution crew and IODP technical team
<figure><img src="image_8.png" /></figure>

<figure><img src="image_9.png" /></figure>

<figure><img src="image_10.png" /></figure>

<figure><img src="image_11.png" /><figcaption>**Fig. 5.** Conceptual model of Bengal–Nicobar Fan system history (tectonics from Hall (2012); fan morphology from Bowles et al. (1978) and Curray (2014); fan data from DSDP/ODP/IODP sites (white dots; red dots = Expedition 362 sites). A) Late Pleistocene to Recent, sedimentation primarily on Bengal Fan, B) late Miocene–Pliocene, Nicobar Fan dominates, C) pre-late Miocene, Bengal Fan dominates, minor trench-axial supply to Sunda margin/Nicobar Fan. D) Pattern of sedimentation along the Bengal Fan system from proximal (left) to distal (right) indicating absence of simple fan progradational pattern (updated from Curray et al., 2003). Data sources: Indo–Burman Ranges and Andaman–Nicobar forearc data (Bandopadhyay and Ghosh, 2015; Curray et al., 2003 and references therein); ocean drilling site data (Cochran et al., 1989, 1990; France-Lanord et al., 2016; von der Borch et al., 1974; this study). All sites converted to modern timescale (Supplementary Material). Note that 9.5–18 Ma silts for Nicobar Fan sites are only present at Site U1481.</figcaption></figure>

are thanked for their contributions during Expedition 362. We
thank the editor and two reviewers for their valuable comments
which helped to improve the paper. McNeill acknowledges support
during and following the Expedition from Natural Environment Re-
search Council grant NE/P012817/1.

**Appendix A. Supplementary material**

Supplementary material related to this article can be found on-line at http://dx.doi.org/10.1016/j.epsl.2017.07.019.
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