Peng Zhou1,2, Daniel F. Stockli3, Thomas Ireland4, Richard W. Murray5, Peter D. Clift1
1 Department of Geology and Geophysics, Louisiana State University, Baton Rouge, LA 70803, USA
2Department of Physics, Geology, and Engineering Technology, Northern Kentucky University, Highland Heights, KY 41099, USA
3 Department of Geological Sciences, Jackson School of Geosciences, University of Texas, Austin, TX 78712-1722, USA
4 Department of Earth and Environment, Boston University, Boston, MA 02215, USA
5 Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA
Corresponding author: Peter Clift (pclift@lsu.edu)
Key Points:
Abstract
The Indus Fan, located in the Arabian Sea, contains the bulk of the sediment eroded from the Western Himalaya and Karakoram. Scientific drilling in the Laxmi Basin by the International Ocean Discovery Program (IODP) provides an erosional record from the Indus River drainage dating back to 10.8 Ma, and with a single sample from 15.5 Ma. We dated detrital zircon grains by U-Pb geochronology to reconstruct how erosion patterns changed through time. Long-term increases in detrital zircon U-Pb components of 750–1200 Ma and 1500–2300 Ma show increasing preferential erosion of the Himalaya relative to the Karakoram at 7.99–7.78 Ma and more consistently starting by 5.87 Ma. An increase in the contribution of 1500–2300 Ma zircons starting by 1.56 Ma indicates significant unroofing of the Inner Lesser Himalaya (ILH) by that time. The trend in zircon U-Pb age populations is consistent with bulk sediment Nd isotope data implies greater zircon fertility in Himalayan bedrock compared to the Karakoram and Transhimalaya. The initial change in spatial erosion patterns at 7.0–5.87 Ma occurred during a time of drying climate in the Indus foreland. The increase in ILH erosion postdates the onset of dry-wet glacial-interglacial cycles suggesting some role for climate control. However, erosion driven by rising topography in response to formation of the Lesser Himalayan thrust duplex, especially during the Pliocene may also be important. The influence of the Nanga Parbat Massif to the bulk sediment flux is modest, in contrast to the situation in the eastern Himalaya syntaxis.
Keywords: Erosion, zircon, monsoon, Himalaya.
1 Introduction
Collision between India and Eurasia, starting about 50–60 Ma [Garzanti et al. , 1987; Jaeger et al. , 1989;Klootwijk et al. , 1992; Najman et al. , 2010], has resulted in the formation the largest mountain ranges on Earth. The timing of collision remains controversial but is best addressed by consideration of the stratigraphic record that shows the onset of mixed Indian-Eurasian sediments. Sedimentary rocks in the central and eastern Himalaya imply initial collision at 59 ± 1 Ma [Hu et al. , 2016]. Recent work combining Hf isotopes with U-Pb ages in zircon grains from the Tethyan Himalaya now show that sediment eroded from Eurasia, rather than oceanic island arcs, was reaching NW India by 54 Ma, requiring India-Eurasia collision before than time [Najman et al. , 2017]. The Himalaya have continued to evolve both in topography and structure as a result of ongoing tectonic deformation coupled with erosion, largely modulated by the strength of summer monsoon rains [Bookhagen et al. , 2005; Clift et al. , 2008b; Wobus et al. , 2003]. Sediments eroded from the Western Himalaya has been deposited in the Arabian Sea where they form the second largest sediment body on Earth, the Indus submarine fan [Clift et al. , 2001; Kolla and Coumes , 1987].
The sedimentary deposits of the Indus submarine fan represent an archive of the erosion and weathering processes in the Western Himalaya since the onset of continental collision, at least since ~45 Ma [Clift et al. , 2001]. While bedrocks exposed at the surface in the mountains can be used to reconstruct the uplift and exhumation of those particular rock formations, the submarine fan sedimentary record captures spatial and temporal variations of the long-term history of denudation, albeit one buffered by sediment transport processes. Because older portions of bedrocks have been completely removed by erosion and their exhumation history no longer accessible, the sedimentary record becomes the only record of the earlier erosion and exhumation history. Although this record is partially available in the Himalayan foreland basin, these proximal, continental syn-tectonic deposits are more difficult to date at high resolution, and the sequence is truncated by significant unconformities, and deformed by progressive incorporation into the sub-Himalayan fold and thrust belt [Najman , 2006]. Moreover, any given section in the accreted foreland basin can only represent the sediment deposited from paleo-rivers that once flowed in front of the mountains in that region. As such a given section would preserve a history of erosion in a limited catchment of a particular part of the mountains, but does not provide a more integrated orogen-scale overview.
Sediments from the western Himalaya are delivered to the Arabian Sea by the Indus River and its eastern tributaries in the Punjab (Fig. 1A). The Indus is particularly sensitive to variations in the strength of the Asian monsoon because it lies on the western edge of the zone affected by this climatic phenomenon. As a result, variations in monsoon strength can have a major impact on both patterns and rates of erosion in the various ranges that comprise the western end of the Himalayan mountain chain (Fig. 1B). A number of studies have suggested that changes in monsoon intensity have significantly impacted the erosion history of the western Himalaya [Bookhagen et al. , 2005; Clift et al. , 2008a; Clift et al. , 2008b].
Debate continues regarding what controls the erosion of the Himalaya, with some workers favoring tectonic processes that drive rock uplift [Burbank et al. , 2003], as being the critical control, while others have argued for a dominance by monsoon rainfall and/or glaciation [Whipple , 2009; Wobus et al. , 2003]. These focus the sediment producing regions across a relatively narrow band of the range front and in turn drive exhumation of deep buried rocks [Thiede et al. , 2004]. It is however known that the erosion of the Himalaya is sensitive to climate change because sediment supply during and shortly after the Last Glacial Maximum (LGM) was preferentially focused in the Karakoram, while the strongest erosion has shifted into the Lesser Himalaya since the onset of the Holocene [Clift et al. , 2008a].
In this study we focus on the Late Miocene-Recent history and examine evidence for coupling between the tectonic evolution and the changing strength of summer monsoon rains. We take advantage of recently recovered sediments collected by the International Ocean Discovery Program (IODP) in 2015 from the Eastern Arabian Sea, which provide a record of erosion extending back to ~10.8 Ma, with one sample dated at ~15.5 Ma [Pandey et al. , 2016d]. An earlier lower-resolution study using detrital zircon grains and numerous bulk sediment Nd and Sr isotopes argued that changes in erosion across the Indus Basin were unconnected to climate change and largely manifest as increasing erosion of the Lesser Himalaya, especially starting at 1.9 Ma [Clift et al. , 2019b]. We test this model using an expanded set of new U-Pb ages from detrital zircon sand grains (1882 new ages from 15 additional samples, compared to 1335 ages from ten samples in the earlier work) coupled with a more sophisticated statistical treatment of the total data set in order to reconstruct the evolving patterns of erosion.
2 Geologic Setting
The sediments analyzed in this study were retrieved from the Laxmi Basin in the Eastern Arabian Sea (Fig. 1A and 1B). This basin is separated from the rest of the Arabian Sea by a continental block known as Laxmi Ridge [Pandey et al. , 1995]. Rifting in Laxmi Basin preceded the breakup of the main Arabian basin, west of Laxmi Ridge, and likely occurred in the latest Cretaceous [Bhattacharya et al. , 1994]. Since that time 2–3 km of sediment have accumulated in the Laxmi Basin. Initial provenance investigation of these sediments using Nd isotopes and limited zircon U-Pb dating indicates that while some fine-grained material might be derived in from peninsular India, immediately to the east of the Laxmi Basin, most of the sediment was sourced from the Indus Delta, located around 800 km towards the north [Clift et al. , 2019b]. Continuous sedimentation in Laxmi Basin was interrupted by the emplacement of a large mass transport complex (MTC) just before 10.8 Ma, which eroded most of the Middle Miocene at Site U1456 [Calvès et al. , 2015; Dailey et al. , 2019]. At Site U1457 the MTC removed almost the entire sediment fill from the edge of Laxmi Ridge leaving only a thin deposit of red Paleocene mudstones [Pandey et al. , 2016b].
We also compare our sediments with those recovered as drill cuttings from the industrial borehole Indus Marine A-1 located on the Indus shelf (Fig. 1A). Because Indus Marine A-1 is located close to the Indus River mouth the source of sediment is more straight forward, and cannot have involved influence from the Indian Peninsula, as might be the case in the Laxmi Basin. This site penetrated into the Middle Miocene [Shuaib , 1982] and drill cuttings have been used to look at the evolving provenance using Nd isotope methods, going back further in time than possible at the IODP sites [Clift and Blusztajn , 2005; Clift et al. , 2019b]. The Indus Marine A-1 drill site is located on the relatively flat continental shelf and is only affected by growth faulting, but has otherwise escaped major tectonic deformation since the breakup of the Arabian Sea, except along its western edge adjacent to the Murray Ridge [Clift et al. , 2002a;Gaedicke et al. , 2002]. Unfortunately, the recovered sediments from Indus Marine A-1 are fine-grained and are not conducive to detrital zircon U-Pb dating in this proximal area. We examined the major element chemistry of the sediments at Indus Marine A-1 for comparison with the more distal drill sites sampled by IODP in order to demonstrate their Indus provenance. Neodymium isotope data indicate that Indus Marine A-1 sediments were derived from the Indus River, consistent with their proximal location, providing a useful comparison with the deep-water materials [Clift and Blusztajn , 2005].
Determining the provenance of the sediment delivered to the Arabian Sea is facilitated by the leverage of the significant spatial diversity of bedrock ages and lithologies within the Indus drainage basin [Hodges , 2000; Searle , 1996]. Geochemical and isotopic differences between bedrock sources are transferred to the eroded sediment and although grains may be altered during the transport process, many of these differences are preserved in the final deposited sediment, allowing us to deconvolve the sources and variations using appropriate proxies. Figure 1C shows the various mountain ranges that comprise the main distinct source regions to the modern Indus River, including the Greater and Lesser Himalaya, the Tethyan Himalaya that lie further north, and that represent the telescoped, passive continental margin of Greater India [Garzanti et al. , 1987]. This unit is separated by the Indus Suture Zone from magmatic arc rocks of the Transhimalaya and Kohistan (Fig. 1C) that were largely emplaced in the Cretaceous and Paleogene [Khan et al. , 1997; Rolland et al. , 2002]. Further north, across the Shyok Suture Zone, lie the Karakoram, the old active margin of continental Eurasia, which also comprises Mesozoic arc rocks, and experienced magmatism after India-Euasia collision, most notably in the form of the Early Miocene Karakoram Batholith [Ravikant et al. , 2009; Searle et al. , 1989]. The Karakoram region was uplifted in response to both compressional tectonics and strike-slip displacement on the Karakoram Fault [Searle and Phillips , 2007]. Farther to the west the Hindu Kush mountains are characterized by a similar pre-collisional history as the Karakoram, but subsequently did not experience such dramatic or rapid unroofing [Hildebrand et al. , 2001;Zhuang et al. , 2018]. In addition, the Western Syntaxis of the mountain chain is marked by the Nanga Parbat Massif (Fig. 1C), characterized by high-grade metamorphic and igneous intrusive rocks that experienced recent, very rapid exhumation [Zeitler et al. , 1989]. However, it is unclear exactly when this process began because the rocks now at the surface are very young. Nonetheless, this does not preclude an earlier onset to erosion [Chirouze et al. , 2015].
The Greater Himalaya were emplaced along the Main Central Thrust (MCT) after ~24 Ma, placing them over the Lesser Himalaya [Catlos et al. , 2001; Stephenson et al. , 2001]. These in turn were unroofed and brought to the surface due to motion along the Main Boundary Thrust (MBT) and associated thrust duplexing [Bollinger et al. , 2004; Huyghe et al. , 2001]. Evidence from the Siwalik Group foreland basin sedimentary strata indicates that the Lesser Himalaya were exposed locally only after 9 Ma and more widely after 6 Ma in NW India [Najman et al. , 2009], although the Nd isotopes at Sites U1456 and U1457 imply that widespread unroofing of the Inner (Crystalline) Lesser Himalaya only began at 1.9 Ma [Clift et al. , 2019b]. The Siwalik Group rocks themselves have been up-thrusted and are presently eroding, recycling older sediments back into the river system. However, estimates derived from the incision of terraces in the Nepalese frontal Himalaya suggest that the Siwaliks contribute no more than about 15% of the total flux [Lavé and Avouac , 2000]. The western edge of the Indus drainage basin is characterized by fold and thrust belts (Sulaiman and Kirthar ranges, Fig. 1A), similar to the Siwalik Group in character [Roddaz et al. , 2011], but experiencing a more arid climate. Nonetheless, this environment need not limit erosion rates because of the strong erosion associated with occasional flash flooding events in vegetation-poor settings [Molnar , 2001], although study of heavy minerals in rivers draining these ranges and the lower Indus indicate that their contribution to the net sediment load is minor [Garzanti et al. , 2020].
Other potential sources of sediment delivered into the Laxmi Basin include the Precambrian cratonic rocks of peninsular India and associated Gondwanan sedimentary sequences [Mukhopadhyay et al. , 2010; Yin et al. , 2010], characterized by old (>500 Ma) bedrock zircon U-Pb ages, similar to those observed in the Himalaya, especially the Lesser Himalaya. Zircon U-Pb dating, given its high closure temperature [Hodges , 2003], only records the initial crystallization or high-temperature metamorphism, and thus, does not allow us to exclude such old grains as having been derived from peninsular India rather than the Himalaya. Sediments eroded from the Deccan Plateau, the latest Cretaceous flood basalt province that dominates the Western Ghats, immediately onshore from the drilling area, were erupted around 65 Ma [Courtillot et al. , 2000] over a relatively short period of time. While these would be very distinctive, basalt is characterized by a very low zircon fertility and might not provide significant zircon grains of that age into the adjoining basin. Nd and Sr isotopic evidence suggests enhanced flux in muddy sediments to the Laxmi Basin during interglacial times [Khim et al. , 2019]. Low-resolution apatite fission track and zircon U-Pb studies have so far identified just a single sand at the IODP drill sites that was derived from the Indian peninsula [Zhou et al. , 2019].
3 Sedimentology and Stratigraphy
Drilling at Sites U1456 and U1457 penetrated ~1100 m below the seafloor in both locations, with the basement being reached at Site U1457 (Fig. 2)[Pandey et al. , 2016b]. Drilling at Site U1456 only just penetrated through the MTC, allowing a very short core of Middle Miocene sandstone to be recovered [Pandey et al. , 2016a]. Age assignments at both sites are made by combining biostratigraphy and magnetostratigraphy and we follow the age model of Pandey et al. [2016a] for Site U1456 and Pandey et al.[2016b] for Site U1457 (Fig. 2), with updates from Routledgeet al. [2019]. Ages of individual samples are calculated assuming linear sedimentation between the dated points. At Site U1456 the sediments are relatively mud-rich, but with a number of silt and fine sand turbidite interbeds at 460–730 mbsf (meters below seafloor; Fig. 2A), which are overlain by a sequence of mud and carbonate-rich sediments. A thick, sand-rich package was recovered between 360 and 140 mbsf and interpreted as a submarine fan lobe [Pandey et al. , 2016a]. Above this sand-rich package, the section is dominated by mud and carbonate, interpreted as the product of hemipelagic sedimentation. Site U1457 is characterized by much lower proportions of sand, reflecting the drilling location on the flanks of the Laxmi Ridge. However, a sand-rich interval between 670 and 810 mbsf is overlain by a carbonate and mud-rich interval between 600 and 670 mbsf. More sand-rich beds were encountered between 470 and 600 mbsf. As at Site U1456, sediments shallower than 200 mbsf at Site U1457, are mud and carbonate-rich (Fig. 2). The coarse-grained intervals are again interpreted as lobe deposits [Pandey et al. , 2016b]. The sandy sediments are interpreted to be deposited by turbidity currents, with the muddy sediments representing hemipelagic intervals between depositional events. Changes in grain size might be driven by changes in the erosional power in the source regions, the discharge stream power of the river, or by changes in sea level, but could also reflect avulsion in the main depositional lobes in and out of Laxmi Basin and the main part of the Arabian Sea towards the West. Such auto cyclic behavior is commonly observed in submarine fans [Deptuck et al. , 2008;Shanmugam and Moiola , 1991].
4 Methods
U-Pb geochronology of detrital zircon grains has become a powerful and widely employed tool for discerning provenance in siliciclastic sedimentary systems. The methodology is based on the concept that different bedrock source rocks are characterized by distinct and/or different age populations of zircons. A zircon budget is not the same as an eroded rock budget because of differences in the relative fertility of bedrock sources with regard to zircon. Zr concentrations have been used as a proxy for the relative abundance of zircons in sediment [Amidon et al. , 2005], but the reliability of this approach has recently been questioned [Malusà et al. , 2016]. Malusàet al. [2016] developed a method using mineralogy and density data from the sediment to infer the fertility of the source bedrock. Unfortunately, this approach is not practical for this work because the sample sizes available from IODP were small (<50 cm3) so that all the material had to be processed for zircon extraction and even required amalgamating neighboring samples in order to generate enough data to make a statistically meaningful result in some cases. We use previously published geochemical data from modern rivers as a guide to zircon fertility as this data already exists and we cross check this prediction against other provenance methods to assess its credibility. Our erosion budgets are however largely zircon based, not bulk sediment.
Zircon is a robust mineral and its grains do not generally experience significant physical abrasion during transport, unless they had previously accumulated major radiation damage. Hence, zircon can undergo multiple episodes of recycling and redeposition. Although the concentration of zircon in any given sediment can be affected by hydrodynamic sorting, this process may not be a strong influence on the resulting detrital age spectra unless there is a relationship between grain size and crystallization age, which we investigate below. Work on Yangtze River sediments indicates that the typical grain size range analyzed using LA-ICP-MS technology is representative of the overall population in the sediment without a bias related to grains size [Yang et al. , 2012]. Detrital zircon U-Pb dating has been widely applied in provenance studies in the Western Himalaya due to the large range of zircon U-Pb age differences between the various source terrains described above. Furthermore, studies of the modern Indus River documented a close correlation between the modern zircon U-Pb age spectra and the bedrock sources, albeit one implying focused erosion in several sub-basins [Alizai et al. , 2011; Zhuang et al. , 2018]. Several studies have also used detrital zircon dating to investigate the provenance of the Siwalik Group foreland basin sedimentary rocks [Baral et al. , 2015; Bernet et al. , 2006; DeCelles et al. , 2004; Zhuang et al. , 2015] and Quaternary sediments in the delta and offshore [Clift et al. , 2008a; Li et al. , 2019], allowing evolving erosion patterns to be reconstructed.