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)
- First basin-wide study of how regional erosion patterns have changed
through time since 15.5 Ma in the Western Himalaya and Karakoram.
- Geochemical and geochronological analyses show increased relative
erosion from the Himalaya compared to the Karakoram at 7.99–7.78 Ma.
- Changing patterns of erosion correlate with climatic drying at
~7.7–6.3 Ma, and relate to solid Earth tectonic
forces building topography.
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.