Figure Captions
Figure 1. A) Shaded bathymetric and topographic map of the Arabian Sea and surrounding area showing the location of the drilling sites considered by this study. Map also shows the major tributary systems of the Indus River, as well as smaller peninsular India rivers and their source mountains. B) Inset map shows detail of the Laxmi Basin and location of the drill sites considered in this study. Numbered red circles indicate existing scientific boreholes from Deep Sea Drilling Project (DSDP) and Ocean Drilling Program (ODP). KK = Karakoram; NP = Nanga Parbat. C) Geological map of the western Himalaya showing the major tectonic units that are eroded by the Indus River and its tributaries. Map is modified after Garzanti et al. [2005]. Rivers as shown in thick black lines. ISZ = Indus Suture Zone, MCT = Main Central Thrust, MBT = Main Boundary Thrust and MFT = Main Frontal Thrust. Thick black line shows the boundary of the Indus drainage, while thinner lines demark the limits of the major Himalayan tributaries. Figure is modified from Clift et al. [2019b].
Figure 2. Simplified lithologic logs of the two drill sites considered in this study. Black arrows show the location of the samples analyzed. Modified from Pandey et al. [2016c]. Pale shaded intervals show inferred lithologies based on small amounts of recovered core. Because induration is progressive and there is no sharp division, we make no attempt to distinguish between sediments and indurated rocks. Numerical ages are from Pandey et al. [2016a] for Site U1456 and from Pandey et al. [2016b] for Site U1457, with updates from Routledge et al. [2019].
Figure 3. Grainsize range of all samples analyzed for U-Pb zircon dating from the Laxmi Basin shown on the scheme of Folk [1974]. Samples are marked to show those published by Clift et al. [2019b], rather than presented new here (Table 2). Note the dominance of silty sand and sandy silt in the analyzed samples.
Figure 4. Detailed grain size spectra showing the range of sizes of the different samples considered within this study. Most of the sediment is fine sand to coarse silt in size and typically shows a coarse-skewed. A) Samples younger than 7 Ma, b) samples older than 7 Ma. Samples are marked to show those published by Clift et al.[2019b](gray text labels and white ringed symbol), rather than presented new here (Table 2) (black text labels and black ringed symbol).
Figure 5. (A) Geochemical signature of the analyzed samples illustrated by a CN-A-K ternary diagram [Fedo et al. , 1995]. CN denotes the mole weight of Na2O and CaO* (CaO* represent the CaO associated with silicate, excluding all the carbonate). A and K indicate the content of Al2O3 and K2O respectively. Samples closer to A are rich in kaolinite, chlorite and/or gibbsite (representing by kao, chl and gib). CIA values are also calculated and shown on the left side, with its values are correlated with the CN-A-K. Samples from the delta have the lowest values of CIA and indicates high contents of CaO and Na2O and plagioclase. Abbreviations: sm (smectite), pl (plagioclase), ksp (K-feldspar), il (illite), m (muscovite). B) Geochemical classification of sediments from this study as well as those from the Indus delta [Clift et al. , 2010], Indus Canyon [Li et al. , 2018] and western Indian shelf [Kurian et al. , 2013] following the scheme of Herron [1988].
Figure 6. Cross plot of Zr concentration against median sample grain size. No strong correlation is observed.
Figure 7. Kernal density estimate (KDE) diagram showing the range of the zircon U-Pb ages for individual sand grains back to 3000 Ma. Colored strips show the range of populations with diagnostic links to critical source terrains in the headwaters of the Indus. Data from the Siwaliks, as well as the Tethyan, Greater and Lesser Himalaya are compiled from DeCelles et al. [2004]. Karakoram data is from is from Le Fort et al. [1983], Parrish and Tirrul [1989], Schärer et al. [1990], Fraser et al. [2001] and Ravikant et al. [2009]. Nanga Parbat data is from Zeitler and Chamberlain [1991] and Zeitler et al. [1993]. Transhimalayan data is from Honegger et al. [1982], Schäreret al. [1984], Krol et al. [1996], Weinberg and Dunlap [2000], Zeilinger et al. [2001], Dunlap and Wysoczanski [2002], Singh et al. [2007], and Ravikantet al. [2009]. Samples are marked to show those published by Clift et al. [2019b], and those presented new here.
Figure 8. Kernal density estimate (KDE) diagram showing the range of the zircon U-Pb ages for individual sand grains back to 200 Ma. Colored strips show the range of populations with diagnostic links to critical source terrains in the headwaters. See Figure 6 caption for data sources.
Figure 9. Upper continental crust normalized compositions of the sediments whose zircons are the focus of the study. Bulk settlement compositions are normalized according to the average of the continental crust from Taylor and McLennan [1995].
Figure 10. Plots of relative abundance of provenance sensitive zircon age populations in individual samples compared with sample median grain size. The coarsest samples show preference for the oldest U-Pb ages and a relative lack of the younger populations.
Figure 11. Multidimensional scalar (MDS) diagrams made from zircon U-Pb age data showing (A) how the different sediment samples from IODP Expedition 355 compare with one another and post-glacial sediments from the Indus delta (TH-10-8 and KB-40-4) and (B) with the major source terranes in the Indus catchment, as well as the modern rivers of the Indus catchment, i.e., the main or trunk stream of the Indus, upstream of Attock, and its major eastern tributaries. Solid lines join sediments to their most similar neighbor, while dashed lines join the next most similar. Sources of bedrock age data come from the literature, as described in Figure 6. River data is from Alizai et al.[2011]. Note that sediments older than 5 Ma plot towards the right in Figure 10B, in the direction of Karakoram bedrock sources, whereas there is a progressive migration towards the left, towards Himalayan sources after that time. Diagram was constructed using the statistical package of Vermeesch et al. [2016].
Figure 12. Pie diagrams showing the predicted source compositions of the zircon populations in sands from the Laxmi Basin as unmixed using the software of Sundell and Saylor [2017]. Note the significant reduction in flux from the Karakoram starting ~5.72 and again at 3.02 Ma. Samples are marked to show those published by Clift et al. [2019b], and those presented new here.
Figure 13. Comparison of climate, erosion and exhumation proxies in the Himalaya. (a) Smoothed Nd isotope history for the Indus River with grey background showing effective uncertainties from Cliftet al. [2018]. (b) Breakdown of the sources of detrital zircons based on the unmixing procedure of Sundell and Saylor [2017]. (c) Carbon isotope character of pedogenic carbonate in Pakistan as an indicator of dominant vegetation in the Potwar Plateau of Pakistan [Quade et al. , 1989], and NW India [Singh et al. , 2011]. (d) Relative exhumation rates of the Greater Himalaya tracked by bedrock Ar-Ar dating [Clift et al. , 2008b] and zircon fission track from foreland basin sediment [Chirouze et al. , 2015]. (e) Rates of sediment supply to the Arabian Sea calculated from regional seismic [Clift , 2006].