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\begin{document}

\title{Zircon U-Pb Age Constraints on the Exhumation of the Lesser Himalayas
from the Laxmi Basin, Arabian Sea}



\author[1]{Peng Zhou}%
\author[2]{Daniel Fritz Stockli}%
\author[3]{Thomas Ireland}%
\author[3]{Richard W. Murray}%
\author[4]{Peter Clift}%
\affil[1]{Unknown}%
\affil[2]{University of Texas at Austin}%
\affil[3]{Boston University}%
\affil[4]{Louisiana State University}%


\vspace{-1em}



  
  \date{May 24, 2024}


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\selectlanguage{english}
\begin{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.%
\end{abstract}%



\sloppy


Peng Zhou\textsuperscript{1,2}, Daniel F. Stockli\textsuperscript{3},
Thomas Ireland\textsuperscript{4}, Richard W. Murray\textsuperscript{5},
Peter D. Clift\textsuperscript{1}

\textsuperscript{1} Department of Geology and Geophysics, Louisiana
State University, Baton Rouge, LA 70803, USA

\textsuperscript{2}Department of Physics, Geology, and Engineering
Technology, Northern Kentucky University, Highland Heights, KY 41099,
USA

\textsuperscript{3} Department of Geological Sciences, Jackson School of
Geosciences, University of Texas, Austin, TX 78712-1722, USA

\textsuperscript{4} Department of Earth and Environment, Boston
University, Boston, MA 02215, USA

\textsuperscript{5} Woods Hole Oceanographic Institution, Woods Hole, MA
02543, USA

Corresponding author: Peter Clift (pclift@lsu.edu)

Key Points:

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

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
{[}\emph{Garzanti et al.} , 1987; \emph{Jaeger et al.} ,
1989;\emph{Klootwijk et al.} , 1992; \emph{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 \selectlanguage{ngerman}± 1 Ma {[}\emph{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 {[}\emph{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 {[}\emph{Bookhagen et al.} , 2005; \emph{Clift et al.} ,
2008b; \emph{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
{[}\emph{Clift et al.} , 2001; \emph{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 \textasciitilde{}45
Ma {[}\emph{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 {[}\emph{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 {[}\emph{Bookhagen et al.} , 2005; \emph{Clift et al.}
, 2008a; \emph{Clift et al.} , 2008b{]}.

Debate continues regarding what controls the erosion of the Himalaya,
with some workers favoring tectonic processes that drive rock uplift
{[}\emph{Burbank et al.} , 2003{]}, as being the critical control, while
others have argued for a dominance by monsoon rainfall and/or glaciation
{[}\emph{Whipple} , 2009; \emph{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 {[}\emph{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 {[}\emph{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 \textasciitilde{}10.8 Ma, with one
sample dated at \textasciitilde{}15.5 Ma {[}\emph{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 {[}\emph{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 {[}\emph{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 {[}\emph{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
{[}\emph{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 {[}\emph{Calvès et al.} , 2015; \emph{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 {[}\emph{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
{[}\emph{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 {[}\emph{Clift and Blusztajn} ,
2005; \emph{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 {[}\emph{Clift et al.} ,
2002a;\emph{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 {[}\emph{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
{[}\emph{Hodges} , 2000; \emph{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 {[}\emph{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 {[}\emph{Khan et al.} , 1997; \emph{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 {[}\emph{Ravikant et al.} , 2009; \emph{Searle et
al.} , 1989{]}. The Karakoram region was uplifted in response to both
compressional tectonics and strike-slip displacement on the Karakoram
Fault {[}\emph{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 {[}\emph{Hildebrand et al.} ,
2001;\emph{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 {[}\emph{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 {[}\emph{Chirouze et al.} ,
2015{]}.

The Greater Himalaya were emplaced along the Main Central Thrust (MCT)
after \textasciitilde{}24 Ma, placing them over the Lesser Himalaya
{[}\emph{Catlos et al.} , 2001; \emph{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
{[}\emph{Bollinger et al.} , 2004; \emph{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 {[}\emph{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 {[}\emph{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 {[}\selectlanguage{english}\emph{Lave 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
{[}\emph{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 {[}\emph{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
{[}\emph{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 {[}\emph{Mukhopadhyay et al.}
, 2010; \emph{Yin et al.} , 2010{]}, characterized by old
(\textgreater{}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 {[}\emph{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 {[}\emph{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
{[}\emph{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
{[}\emph{Zhou et al.} , 2019{]}.

3 Sedimentology and Stratigraphy

Drilling at Sites U1456 and U1457 penetrated \textasciitilde{}1100 m
below the seafloor in both locations, with the basement being reached at
Site U1457 (Fig. 2){[}\emph{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 {[}\emph{Pandey et al.} ,
2016a{]}. Age assignments at both sites are made by combining
biostratigraphy and magnetostratigraphy and we follow the age model of
Pandey \emph{et al.} {[}2016a{]} for Site U1456 and Pandey \emph{et
al.}{[}2016b{]} for Site U1457 (Fig. 2), with updates from
Routledge\emph{et 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
{[}\emph{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 {[}\emph{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 {[}\emph{Deptuck et al.} ,
2008;\emph{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
{[}\emph{Amidon et al.} , 2005{]}, but the reliability of this approach
has recently been questioned {[}\selectlanguage{ngerman}\emph{Malusà et al.} , 2016{]}.
Malusà\emph{et 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 (\textless{}50
cm\textsuperscript{3}) 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
{[}\emph{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 {[}\emph{Alizai et al.} , 2011; \emph{Zhuang et al.}
, 2018{]}. Several studies have also used detrital zircon dating to
investigate the provenance of the Siwalik Group foreland basin
sedimentary rocks {[}\emph{Baral et al.} , 2015; \emph{Bernet et al.} ,
2006; \emph{DeCelles et al.} , 2004; \emph{Zhuang et al.} , 2015{]} and
Quaternary sediments in the delta and offshore {[}\emph{Clift et al.} ,
2008a; \emph{Li et al.} , 2019{]}, allowing evolving erosion patterns to
be reconstructed.

\emph{4.1 Major Element Analyses}

In addition to detrital zircon U-Pb dating, bulk sediment samples were
analyzed for their major element contents by Inductively Coupled Plasma
Emission Spectrometry (ICP-ES) at Boston University (BU), USA. Sediment
samples were decarbonated with acetic acid, washed with distilled and
deionized water with a purity of 9--12 megaohms, and hand powdered at
Louisiana State University (LSU) before total fusion preparation at BU.
Glass beads for each sample were made in a muffle furnace under 1050\selectlanguage{english}@C
by fusing 100 \selectlanguage{ngerman}± 0.5 mg of sample mixed with 400 ± 0.5 mg lithium
metaborate (LiBO\textsubscript{2}). The melted mixture was then
dissolved in 5\% HNO\textsubscript{3}, sonicated, manually shaken until
no visible grains were observed, and further diluted for analysis
{[}\emph{Dunlea et al.} , 2015{]}. Precision for all elements was better
than 1\% of the measured value, and accuracy was confirmed by repeated
analyses of International Standard Reference Materials (Basalt, Hawaiian
Volcano Observatory, BHVO-2){[}\emph{Wilson} , 1997{]}. Results of the
geochemical measurements are shown in Table 1.

\emph{4.2 Grain size Analysis}

For quantitative grain size analysis, samples were prepared using
standard procedures as described by Howell \emph{et al.} {[}2014{]}. We
put a small amount of sample into a cleaned 50 ml plastic centrifuge
tube and added 5--7 ml of sodium phosphate solution. The tube was capped
and vortexed to deflocculate clay-sized sediment and separate organic
particles. The sample was poured through an 850 \selectlanguage{greek}µ\selectlanguage{english}m sieve and funneled
into a 15 ml glass test tube. After centrifuging and removing the clear
supernatant, 2--3 ml of sodium phosphate and 5 ml of 30\% H2O2 were
added. Tubes were vortexed again and then put into a hot bath that was
heated to 70@\selectlanguage{english}C. This step requires persistent monitoring to prevent loss
of reactant by spraying it with acetone until the reaction is
stabilized. Reactants then sat overnight to completely oxidize organic
matter. Reacted supernatant was removed, and 5 ml of sodium phosphate
was added. These treated samples were then rinsed with deionized water,
transferred into clean 50 ml plastic centrifuge tubes, and topped with
sodium phosphate into a sample solution of up to 40 ml. Samples were
vortexed again prior to grain size analysis. Grain size analysis was
conducted on a Beckmann Coulter LS13 320 laser diffraction particle size
analyzer at LSU. The obscuration of all running samples in the aqueous
liquid module (ALM) was between 8--12 \%. Result of the analysis are
provided in Table 2.

\emph{4.3 Zircon U-Pb dating}

After standard mineral separation, zircon grains were sprinkle-mounted
onto double-sided tape on 1'' acrylic discs and analyzed at random using
depth-profiling LA-ICP-MS U-Pb geochronology {[}\emph{Marsh and Stockli}
, 2015{]}. Although this method differs from the more common analysis of
cut and polished grains there is no indication that this profiling
approach yields results that differ significantly from earlier work,
including work done in the Himalaya {[}\emph{Colleps et al.} , 2019{]}.
For each sample at least 120 zircons were analyzed to obtain provenance
datasets that resolve all components that comprise \textgreater{}5\% of
the total population {[}\emph{Vermeesch} , 2004{]}. The analyses were
completed using a PhotonMachine Analyte G.2 Excimer laser (30 \selectlanguage{greek}μ\selectlanguage{english}m laser
spot size) with a large-volume Helex sample cell and a Thermo Element2
ICP-MS using procedures described in Hart \emph{et al.} {[}2016{]} at
the UTChron facilities at the Jackson School of Geosciences at the
University of Texas at Austin. GJ1 was used as the primary reference
standard {[}\emph{Jackson et al.} , 2004{]} and a secondary in-house
zircon standard (Pak1 with a
TIMS\textsuperscript{206}Pb/\textsuperscript{239}U age of 43.0 Ma). The
data from the analyses were then reduced using the Iolite data reduction
software VizualAge {[}\emph{Paton et al.} , 2011; \emph{Petrus and
Kamber} , 2012{]}. For analyzed detrital zircons,
the\textsuperscript{206}Pb/\textsuperscript{238}U age was used for
grains younger than 850 Ma and
the\textsuperscript{207}Pb/\textsuperscript{206}Pb age was used for
grains older than 850 Ma {[}\emph{Gehrels et al.} , 2008{]}. All ages
reported use 2\selectlanguage{greek}σ \selectlanguage{english}absolute propagated
uncertainties.\textsuperscript{207}Pb/\textsuperscript{206}Pb ages are
less than 30\% discordant, and
\textsuperscript{206}Pb/\textsuperscript{238}U ages are less than 10\%
discordant {[}\emph{Gehrels et al.} , 2011{]}. The discordance reported
is calculated with the\textsuperscript{206}Pb/\textsuperscript{238}U
and\textsuperscript{207}Pb/\textsuperscript{235}U ages if \textless{}850
Ma and the \textsuperscript{206}Pb/\textsuperscript{238}U
and\textsuperscript{207}Pb/\textsuperscript{206}Pb ages if
\textgreater{}850 Ma. Although some studies have suggested older
crossovers between the \textsuperscript{206}Pb/\textsuperscript{238}U
and \textsuperscript{207}Pb/\textsuperscript{235}U ages and
the\textsuperscript{206}Pb/\textsuperscript{238}U
and\textsuperscript{207}Pb/\textsuperscript{206}Pb ages (e.g., 1.5 Ga
{[}\emph{Spencer et al.} , 2016{]}) blindly picking a crossover at 1.5
Ga leads to culling of
discordant\textsuperscript{206}Pb/\textsuperscript{238}U ages, or
unacceptable smearing and loss of age mode definition between 800--1500
Ma for many samples. Picking a 1500 Ma crossover cutoff and a 20\%
discordance filter would result in the loss of 80\% of the data between
850 and 1500 Ma making the data bad provenance proxies. The 850 Ma
crossover was chosen in accordance with the approach of Spencer et al.
{[}2016{]} and Marsh et al. {[}2019{]}. The data are reported in Table
3.

When accurately dating a geological event, high concordance is a
requirement but when assigning grains to broad age populations for
provenance work the emphasis is on high numbers of grains rather than on
high precision in order to improve the statistical reliability. The
appropriate level of discordance filter needs to be determined for each
data set in light of the goals of the study and the complexities
encountered. If a study yields a mix of Phanerozoic and Archean ages,
and the relative proportions of these ages are important, a generous
(e.g., 30\%) discordance cutoff is appropriate so that most Precambrian
ages are retained {[}\emph{Gehrels} , 2012{]}.

5 Results

\emph{5.1 Grain-size of sediments}

The sediments were assessed using the classification scheme of Folk
{[}1974{]}(Fig. 3). Sediments range from silty sand to silt and mud,
both the new samples processed here and those from the earlier study we
integrate with here \{Clift, 2019 \#152\}. The grain size variation in
single samples can be better assessed by plotting the proportion of each
grain size fraction as a spectrum (Fig. 4). We see generally good
sorting (positive kurtosis) and a negative skew, meaning a dominance of
the finer grain sizes and a tail of coarser grains comprising a
diminishing proportion of the sediment. This is especially true for the
coarsest grained sediments. The vast majority of the sediment considered
here is classified as fine sand to silt, with only small amounts of
medium and coarse sand in a minority of samples, three from Site U1456
dated at 1.92--1.32 Ma and one from Site U1457 at 3.02 Ma. Three of the
new samples do contain significant volumes of medium and even coarse
sand (3.39, 7.00 and 7.27 Ma). The spot size of the laser used for the
U-Pb dating means that grains smaller than \textasciitilde{}30 \selectlanguage{greek}µ\selectlanguage{english}m were
not considered in this study. Depending on the sample this represents a
wide range of the total sediment load. Only 9\% of Sample U1456A-51F-3,
100-110 cm was less than 30 \selectlanguage{greek}µ\selectlanguage{english}m, while 89\% of Sample U1456A-70F-2, 10-16
cm is smaller than that threshold. See Table 2 for full results.

\emph{5.2 Bulk Sediment Chemistry}

The general geochemical character of the sediments can be seen on a
CN-A-K ternary diagram {[}\emph{Fedo et al.} , 1995{]}(Fig. 5A). The
IODP samples plot in an array with a Chemical Index of Alteration (CIA)
of \textasciitilde{}65 to 73 {[}\emph{Nesbitt et al.} , 1980{]}. They
form a roughly linear array trending towards the illite end member and
suggestive of its progressive involvement as the primary mineral
breakdown product. The Laxmi Basin samples can be compared with
sediments from the Quaternary Indus delta {[}\emph{Clift et al.} ,
2010{]}, Indus Canyon {[}\emph{Li et al.} , 2018{]}, the Indus Marine
A-1 borehole, as well as modern sediments from the western Indian shelf
and slope between the Saurashtra peninsula and Bhatikal {[}\emph{Kurian
et al.} , 2013{]}(Fig. 1). Rivers south of Bhatikal have a different
composition and are unlikely to be sources to the Laxmi Basin sites.

The Laxmi Basin sands have very similar bulk compositions to the
Quaternary Indus canyon and delta, as well as the Indus Marine A-1
samples (Fig. 5A), but plot below or to the right of the array of the
western Indian shelf sediments. Only the shelf sample taken near
Bhatikal (the southernmost shelf sample that overlies the Precambrian
crystalline basement of India, rather than the Deccan Traps), plots
below the Laxmi Basin sediments, with a lower CIA value. This plot
confirms that the analyzed sands have little in common with material
eroded from peninsular India and appear consistent with an Indus River
origin. Likewise, the sediments plot close to the Quaternary Indus
sediments and those of Indus Marine A-1 on the discrimination diagram of
Herron {[}1988{]}(Fig. 5B). The IODP samples plot with slightly lower
Fe\textsubscript{2}O\textsubscript{3}/K\textsubscript{2}O values
compared to the proximal sediments. The Laxmi Basin sediments form an
array defined as shales and wackes, while the western Indian shelf
sediments fall into the Fe shale, litharenite, sublitharenite and Fe
sand fields.

We further assess whether grain size has any relationship to zircon
concentrations by plotting Zr contents against median grain size (Fig.
6). Although the original source characteristics and hydrodynamic
sorting of the sediment might be expected to concentrate zircons in
certain size fractions this does not appear to be a significant factor
within the range of grain sizes considered here.

\emph{5.3 Detrital Zircon U-Pb}

We examined the range of zircon U-Pb ages using a kernel density
estimate (KDE) diagram (Fig. 7) to assess similarities between different
sampled sediments and potential source regions (Fig. 7). All of the
sediments analyzed in this study show a significant zircon U-Pb
component younger than 200 Ma. In addition, we see significant
components dated at 350--1250 Ma and 1500--2300 Ma. The abundance of
these older age components overall increases with decreasing sample
depositional age. The 350--1250 Ma age component appears to increase in
all sediment samples dated at 5.87 Ma or younger compared to the older
sediments. A particularly prominent age mode at \textasciitilde{}1800 Ma
first occurs in sediments deposited at 3.43 Ma and becomes extremely
prominent in all samples younger than 1.92 Ma. This age mode is also
observed in the modern sediment from the Indus river mouth
{[}\emph{Clift et al.} , 2004{]}.

Examining the \textless{}200 Ma zircon U-Pb ages in detail, we see that
the vast majority of grains are younger than 120 Ma with prominent age
peaks at around 100--120 Ma and 40--70 Ma (Fig. 8). In the youngest
samples, especially those deposited starting at 3.02 Ma, we see another
age mode at \textasciitilde{}20 Ma, although this is also seen in the
sample dated at 5.78 Ma. One sample deposited at 3.17 Ma differs in its
\textless{}200 Ma age spectra from the other samples as it is
characterized by a prominent age peak at 100--120 Ma, with a general
lack of other young zircon grains.

6 Discussion

Major element discrimination diagrams (Fig. 5) suggest that the Laxmi
Basin sediments are most similar to deposits found in the Quaternary and
modern Indus River/delta/canyon, as well as the older sedimentary rocks
from Indus Marine A-1 (Fig. 5). However, they are distinctly different
from sediments sampled from the modern western Indian shelf, and largely
derived from the Deccan Plateau and underlying units {[}\emph{Kurian et
al.} , 2013{]}. These geochemical data suggest that the Laxmi Basin
sediments, most likely originated from the Indus River mouth.

We assessed the overall geochemical characteristics of the sediments by
plotting the major element composition of each sample normalized to the
upper continental crust (UCC; Fig. 9){[}\emph{Taylor and McLennan} ,
1995{]}. Most of the samples display a relatively uniform topology in
these diagrams and are broadly similar to both post-LGM sediments from
the Indus Delta (KB-40-4), the Holocene delta (TH-10-1) and, the modern
Indus river (Thatta TH-1). Most of the samples show a similar major
element composition compared to the UCC, with a consistent enrichment in
TiO\textsubscript{2}, suggestive of a higher content of Ti-bearing heavy
minerals (e.g., rutile, anatase, brookite, ilmenite, titanite). This
enrichment is particularly strong in the 0.93 Ma sample which apatite
fission track data indicate to have a unique provenance {[}\emph{Zhou et
al.} , 2019{]}. There are also relative depletions in CaO and
Na\textsubscript{2}O, as well as P\textsubscript{2}O\textsubscript{5},
implying both a lower plagioclase and apatite content relative to the
UCC. This relative depletion in CaO is strongest in the modern river
mouth sediment and weakest in the post-glacial delta sediments, with the
fan sediments plotting between these extremes. The systematically lower
abundance of plagioclase and apatite likely reflects chemical weathering
in the floodplains prior to deposition in the ocean, because these
phases are less stable under conditions dominated by chemical weathering
{[}\emph{Guidry and Mackenzie} , 2000; \emph{White and Brantley} ,
1995{]}. However, all samples show this effect and there is a general
consistency in the overall composition, we conclude that we are
comparing sediments of a similar bulk character. All fan sediments show
Zr abundances relatively close to the UCC average

\emph{6.1 Changing Provenance}

Before using the changing zircon U-Pb age spectra to infer changing
sediment provenance we examine the possible role of grain size in
controlling the results. Sediment grains are fractionated during
transport because different densities and shapes affect their settling
characteristics {[}\emph{Garzanti et al.} , 2009{]}. Zircons all have
the same density but the size and shape of the grains from a given
source may vary and influence the final conclusions. If one source is
associated with smaller or larger grains compared to other sources then
this may prejudice the analysis, especially if the grains are too small
to be analyzed. Garzanti \emph{et al.} {[}2009{]} concluded that this
effect was moderate in the Ganges-Brahmaputra catchment, which has
strong similarities to the Indus. We plot major, provenance-related age
populations (0--25, 40--70, 70--120, 300--750, 750--1250 and 1500--2300
Ma) against median grain size for all samples considered here to see if
grain size plays a strong role in controlling the age spectra. Figure 10
shows that there is not a strong correlation between sediment median
grain size and the proportion of various provenance-sensitive age
groups. However, we note that the four coarsest sediments
(\textgreater{}100 \selectlanguage{greek}µ\selectlanguage{english}m) do contain more 750--1250 and 1500--2300 Ma
grains compared to the 40--70 and 70--120 Ma groups. The effect is
especially strong with the 1500--2300 Ma group. In contrast to work on
the Amazon River by Lawrence \emph{et al.} {[}2011{]} who showed that
older grains were significantly smaller than younger ones, the reverse
may be true in the Indus. It is however noteworthy that the coarser
sediments are all young 3.02 Ma and younger and as demonstrated below
the provenance inferred from similar aged finer sediment is not greatly
different and also consistent with neighboring bulk sediment Nd isotope
constraints. We conclude that there may be a grain size issue with the
coarsest sediment, but that this is not dominant in controlling the U-Pb
age spectra.

The zircon U-Pb age spectra are used to track the source evolution of
sediment reaching the Arabian Sea and compared to bedrock zircon U-Pb
age signatures of possible source areas (Fig. 7). The abundance of
grains younger than 200 Ma correlates well with young bedrock from the
Indus Suture Zone, particularly in Kohistan, the Transhimalaya and
Karakoram, as well as Nanga Parbat (Fig. 8). The abundance of these
young zircon grains clearly points to sediment being supplied by the
Indus River and not by peninsular India, where no magmatism
\textless{}200 Ma is known outside the Deccan Plateau. Detrital zircon
grains older than 350 Ma also largely correlate with various bedrock
sources known in the Himalaya. Detrital zircon age modes between 350 and
750 Ma have been correlated with bedrock sources in the Tethyan Himalaya
{[}\emph{Alizai et al.} , 2011{]}, although it is generally agreed that
there is little real difference in terms of U-Pb ages, between Tethyan
and Greater Himalaya zircon signatures {[}\emph{Gehrels et al.} ,
2011{]}, and these are in any case not always mapped consistently by
different groups {[}\emph{Webb} , 2013{]}. Consequently, zircons with
ages between 350 and 1250 Ma could be derived from either source. The
older samples show relatively low abundance of grains in this age range,
but these increased significantly starting at 5.87 Ma and become very
abundant in the last few million years. Older grains, dating between
1500 and 2300 Ma, are particularly common in Lesser Himalayan sources,
although they are also present in smaller amounts in the Tethyan and
Greater Himalaya {[}\emph{DeCelles et al.} , 2000; \emph{Gehrels et al.}
, 2011{]}. These mainly Paleoproterozoic zircon grains are almost
entirely absent from the Laxmi Basin Miocene samples, but show a marked
increase beginning at 5.72 Ma, and becoming very abundant beginning at
1.56 Ma (Fig. 7). We therefore interpret these patterns to indicate a
progressive increase in erosion from the Himalaya starting after 7.0 Ma,
and especially starting at 5.72 Ma, with strong erosion from the Tethyan
and Greater Himalaya. After 3.02 Ma there is a dramatic increase in
erosional flux from the Lesser Himalaya, which have had a strong
influence on the river system since the onset of the Holocene
{[}\emph{Clift et al.} , 2004; \emph{Clift et al.} , 2008a{]}.

If we only consider the zircon grains younger than 200 Ma then we can
see that there is evidence of erosion, from both Kohistan and from the
Karakoram, in most of the samples analyzed (Fig. 8). Kohistan is
particularly noteworthy for having zircon dated between 40 and 70 Ma
{[}\emph{Alizai et al.} , 2011; \emph{Zhuang et al.} , 2018{]}, although
there are similar aged units in the Karakoram as well. However, zircon
grains older than 70 Ma but younger than 120 Ma are almost exclusively
known only from Karakoram bedrock sources {[}\emph{Searle} , 1996{]}.
The 3.17 Ma sample does not show the younger 40--70 Ma population,
suggesting that it did not receive any significant material from
Kohistan/Ladakh.

The youngest (\textless{}25 Ma) zircon grains are more enigmatic in
terms of their provenance. While very young zircons are known from the
present-day Nanga Parbat massif, these are generally younger even than
the 25 Ma zircon U-Pb age component observed in many of the samples
{[}\emph{Zeitler et al.} , 1993{]}. Our new data also show an increased
influx from bedrock sources with very young zircon starting at 3.02 Ma,
as well as a brief appearance at around 5.78 Ma. It is possible that
this increase starting at 3.02 Ma reflects the emergence of Nanga
Parbat, although we cannot exclude the influence of other young sources
in the southern Karakoram metamorphic belt, which also contains rocks of
this age and have experienced very rapid exhumation in the last few
million years {[}\emph{Wallis et al.} , 2016{]}. Because the Deccan
Plateau volcanic rocks were erupted rather quickly around 65 Ma, it is
hard to completely exclude their influence because grains of a similar
age are also known in Kohistan, and in the Karakoram. However, the
erosion from the Deccan Plateau would not account for the other young
grains and an influx from that area should result in a clear peak age at
65 Ma, which is not observed.

We also assessed the evolving provenance patterns of sediments in Laxmi
Basin using a multidimensional scalar (MDS) analysis of the detrital
zircon U-Pb dates {[}\emph{Vermeesch et al.} , 2016{]}. In this plot,
which is a type of principle component analysis, samples with similar
age spectra plot close to one another, while distinct samples are far
separated. Figure 11A shows all the detrital samples data, along with a
modern river mouth and a delta sample (KB-40) dating from shortly after
the LGM {[}\emph{Clift et al.} , 2008a{]}. The MDS analysis shows clear
and coherent patterns. Samples deposited at and after 1.56 Ma, are
relatively similar to the modern river. In contrast, the oldest samples
plot in a cluster suggesting a similar Miocene provenance and a
subsequent progressive shift from right to left with decreasing
depositional age, although with some reversals, most notably at 3.17 and
3.57 Ma. This reflects an overall shift in the zircon age spectra
through time. Nonetheless, the LGM sample has stronger similarities with
sediments deposited on the fan during the Late Miocene. Earlier work
implied that erosion during the LGM was focused in the Karakoram
{[}\emph{Clift et al.} , 2008a{]} compared to the modern river or during
the Holocene when the summer monsoon was strong {[}\emph{Caley et al.} ,
2014; \emph{Fleitmann et al.} , 2003; \emph{Gupta et al.} , 2003{]}. The
new data indicate that older Miocene samples were also deriving their
material from Karakoram sources, and this was followed by a shift to
more Himalayan sources, especially in the last few million years. The
plot implies that the change might be step wise, with a change starting
between 7.0 and 5.87 Ma and again at 1.56 Ma.

The fact that the youngest turbidite sands are most similar to the
modern interglacial river, and not the compositions of the Indus shortly
after the LGM, also implies that most of the sediment deposited in the
Indus Fan has been eroded during interglacial times when the monsoon was
strong, even if final deposition did not occur until the sea level fell
during the onset of the subsequent glaciation. We envisage fast
interglacial erosion generating great volumes of sediment, which is then
mobilized, transported, and delivered to the delta as the rains
strengthened {[}\emph{Jonell et al.} , 2017{]}. The sediment would then
be stored on the shelf or in the upper canyon during sea level high
stands before being eroded and redeposited as sea level fell {[}\emph{Li
et al.} , 2018{]}. This emphasizes the importance of monsoon intensity
in controlling erosion and sediment delivery in the Western Himalaya.

We also compared the Arabian Sea sediments with known zircon ages from
bedrock sources themselves. Figure 11B shows the progressive changes
from the Miocene to the present and emphasizes the fact that the
stratigraphically oldest detrital zircon samples plot closest to sources
in the Karakoram and have similarities with analyses from the trunk
stream (upper reaches) of the main Indus River, before it mixes with the
Himalaya-draining Eastern tributaries, such as the Jhellum, Chenab,
Ravi, Sutlej and Beas (Fig. 1). Conversely, the stratigraphically
youngest sediments plot on this diagram closest to Himalayan sources and
have greater similarity not only to the modern river mouth, but also
Himalayan tributaries such as the Ravi, Chenab and Jhellum rivers.

These data also imply that Nanga Parbat has not been a very important
contributor to the bulk sediment flux. Whether this is actually true or
not is not entirely apparent because the bedrock analyses from Nanga
Parbat were focused on igneous rocks in the center of that metamorphic
massif, and might not be representative of the net erosional flux from
this particular source. However, the relationships displayed in Figure
11B can be readily explained as a simple mixing between Karakoram and
Himalayan sources, with a progressive shift towards the Himalaya through
time.

\emph{6.2 Unmixing Sources}

In order to further characterize the evolving source of sediments to the
Indus Fan we employ the unmixing software of Sundell and Saylor
{[}2017{]}, which analyzes the U-Pb age spectra from each of the samples
and compares them with the defined end-member compositions of the
different source ranges compiled from the published literature. This
approach works particularly well in the western Himalaya where the
sources are well defined and often unique. Data from the Tethyan,
Greater and Lesser Himalaya were compiled from DeCelles \emph{et
al.}{[}2004{]}. Data from the Karakoram are from Le Fort \emph{et
al.}{[}1983{]}, Parrish and Tirrul {[}1989{]}, Sch\selectlanguage{ngerman}ärer \emph{et
al.}{[}1990{]}, Fraser \emph{et al.} {[}2001{]} and Ravikant \emph{et
al.}{[}2009{]}. Data from Nanga Parbat are from Zeitler and Chamberlain
{[}1991{]} and Zeitler \emph{et al.} {[}1993{]}. Data from the
Transhimalayan are from Honegger \emph{et al.} {[}1982{]},
Schärer\emph{et al.} {[}1984{]}, Krol \emph{et al.} {[}1996{]}, Weinberg
and Dunlap {[}2000{]}, Zeilinger \emph{et al.} {[}2001{]}, Dunlap and
Wysoczanski {[}2002{]}, Singh \emph{et al.} {[}2007{]}, and
Ravikant\emph{et al.} {[}2009{]}.

This unmixing method uses a Monte Carlo approach to estimate the
contributions from the different sources that would be required to
generate the modes and modal abundances of U-Pb ages seen in the
sediment samples. Because this is relatively objective the method is
considered robust for analyzing potential source contributions, assuming
that the sources themselves have been well characterized. The bedrock
sources of the Indus catchment have significant differences between many
of them and are some of the best characterized worldwide. Results of the
Monte Carlo simulation are provided in Table 4, showing the output using
all three statistical comparison methods, cross-correlation, the best V
value in the Kuiper test. as well as the best D value in the K-S test.
The method involves creating 10,000 model mixed sediments using the
defined bedrock source end members. The DZMix software then compares the
model with the measured spectra and retains the best 1\% of these models
in order to estimate which sources were contributing the sampled
material. We favor the unmixing models derived from the
cross-correlation approach as being geological reasonable and favored by
Sundell and Saylor {[}2017{]}.

The results of our unmixing calculations show a progressive provenance
evolution that is consistent with that seen in the MDS diagram (Figs. 11
and 12). The very oldest sample deposited at 15.5 Ma shows a dominance
of sediment eroded from the Karakoram and from the Tethyan and Greater
Himalaya. Most of the Miocene samples dated between 8.2 and 7.0 Ma are
more dominated by material from the Karakoram but also usually show
significant Tethyan and Greater Himalayan contributions. This Himalayan
component is particularly noteworthy at 7.99, 7.84, 7.78, 7.66, and 7.0
Ma during this interval. The proportion of Karakoram zircons shows a
significant decrease starting no later than 5.72 Ma and again at 3.02
Ma. The sediment deposited at 3.17 Ma shows the greatest amount of
modeled erosion from Karakoram sources of any sample.

From 3.02 Ma onwards the Himalaya dominate as sources to the submarine
fan, with significant amounts of material from the Lesser Himalaya first
appearing at 1.56 Ma. The sample dated as being deposited at 0.93 Ma is
anomalous for being very similar in source signature to Tethyan and
Greater Himalayan bed rocks sources. However, we note that fission track
data indicate that this sample was derived from peninsular India
{[}\emph{Zhou et al.} , 2019{]}. The unmixing analysis largely mirrors
the pattern shown by the MDS diagram, in showing a progressive long-term
increase in erosion from the Himalaya relative to the Karakoram,
although with significant steps making the evolution nonlinear. All of
the samples contain a small amount of very young \textless{}25 Ma
zircons. None of the samples analyzed show a close similarity with
post-LGM river compositions. Delta samples deposited at 6.6 and 15 ka
are strongly enriched in Karakoram-derived grains compared to fan
sediments deposited at and after 3.02 Ma. This short term variability is
interpreted to reflect the short-term changes in erosion patterns linked
to monsoon strength, modulated by glacial cycles since the onset of the
NHG.

\emph{6.3 Relationships to Climate Change and Tectonics}

The progressive increase in the relative flux from the Himalaya since
the Middle Miocene represents the progressive unroofing of these units.
Structural reconstructions of the Western Himalaya predict that prior to
5.4 Ma the Greater and Lesser Himalaya were not exposed {[}\emph{Webb} ,
2013{]} implying that the Himalayan contribution was derived entirely
from the Tethyan Himalaya during the Miocene. As we are not able to
distinguish between Tethyan and Greater Himalaya derived sediment we
focused on the first appearance of significant amounts of 1500--2300 Ma,
Inner Lesser Himalayan detritus starting at 1.56 Ma. Previous studies
considered these ranges to have been exposed somewhat before 1.6 Ma.
Study of the Siwalik Group in the area of the Beas River Valley
indicated an initial exposure of these units around 9 Ma and significant
exposure by 6 Ma based on Nd isotope data {[}\emph{Najman et al.} ,
2009{]}. Our data support the findings of Clift \emph{et al} .
{[}2019b{]} that this exposure may only reflect the local situation in
the paleo-Beas River area, but that widespread regional exposure of the
Inner Lesser Himalayan units comes somewhat later. While Clift \emph{et
al} . {[}2019b{]}favored increased Inner Lesser Himalaya erosion
starting at 1.9 Ma our new zircon data imply that 1.56 Ma is a more
accurate age for this this transition.

Our result also contrasts with the suggestion by Myrow \emph{et
al.}{[}2015{]} that the Inner Lesser Himalaya were widely exposed and
eroding by 16 Ma. Although we cannot exclude this from happening further
east in the Ganges Basin our data do not support this over a wide area
of the western Himalaya until much later.

The timing of Lesser Himalayan unroofing may reflect the development of
the thrust duplex, which characterizes the structure of the Lesser
Himalaya in this area {[}\emph{Huyghe et al.} , 2001; \emph{Webb} ,
2013{]}. Integrated metamorphic and geochronologic data indicate rapid
cooling of the Inner Lesser Himalaya before 6 Ma, following peak
metamorphism around 10 Ma {[}\emph{Caddick et al.} , 2007; \emph{Thiede
et al.} , 2009{]}\}. We note that rapid cooling does not however require
synchronous unroofing. The first major flux of Himalayan zircons to the
submarine fan is dated at 7.99 to 7.78 Ma, although widespread Himalayan
unroofing may not have started until 5.72 Ma, followed by Inner Lesser
Himalayan unroofing starting around 1.56 Ma. This timing is younger than
reconstructed by Colleps \emph{et al.} {[}2018{]} who favor exposure of
the Outer Lesser Himalaya after 16 Ma and of the Inner Lesser Himalaya
after 11 Ma, although that study was again located in an area father
east, within the wetter Ganges catchment, and need not apply to the
drier Indus basin. A more erosive climate further east might favor
earlier unroofing in that area. The erosion data support the concept of
significant along strike diachroneity of unroofing.

Uplift of the Lesser Himalayan Duplex would have created a topographic
barrier, susceptible to erosion as monsoon rains were focused along this
topographic front. The increasing Himalayan character of the total
zircon input comes at a time when the summer monsoon rains were
generally weakening after \textasciitilde{}8 Ma {[}\emph{Dettman et al.}
, 2001{]}, or after 7.7 Ma based on new environmental data from Site
U1456 {[}\emph{Clift et al.} , 2019a{]}(Fig. 13). Moisture delivery to
this area from the winter westerlies has also been shown to have reduced
around 7 Ma {[}\emph{Vögeli et al.} , 2017{]}. In the recent geologic
past, since the LGM, strong Himalayan rather than Karakoram erosion has
occurred when the summer monsoon was strong, during interglacial times
and not when it was weak during glacial times {[}\emph{Clift et al.} ,
2008a{]}. The increase in Himalayan erosion over longer periods of time,
correlating with the weakening monsoon, is the opposite of this
shorter-term trend. It is possible that solid Earth tectonic forces,
rather than climate, have dominated the long-term evolution of erosion,
although the temporal correlation of provenance and aridity is
suggestive of a climatic control.

All of the samples show the presence of very young zircons
(\textless{}25 Ma) that possibly correlate with bedrock dates from Nanga
Parbat, although these are never very numerous. It is also possible that
some of these young ages may in fact be derived from erosion of fast
exhuming rocks in parts of the southern Karakoram {[}\emph{Wallis et
al.} , 2014{]}. However, even if that this material was derived from
Nanga Parbat, the low abundance of such zircon grains in the Laxmi Basin
sediments would suggest that this massif was not generating very high
proportions of sediment in the trunk Indus river, unlike the situation
in the eastern syntaxis {[}\emph{Garzanti et al.} , 2004; \emph{Stewart
et al.} , 2008{]}. This is consistent with the U-Pb zircon ages in the
modern Indus River downstream of Nanga Parbat {[}\emph{Alizai et al.} ,
2011{]} that show neither many \textless{}25 Ma zircons or older
1500--2300 Ma grains that would be associated with less deeply buried
rocks but with the Lesser Himalayan affiliation typically made with
Nanga Parbat {[}\emph{Whittington et al.} , 1999{]}.

We compare our detrital zircon budget with that of the Nd budget
published by Clift \emph{et al.} {[}2019b{]}. Translating zircon budgets
into rock erosion budgets is not easy due to bedrock zircon fertility
variations. However, whole-rock geochemical analysis of Alizai \emph{et
al.} {[}2012{]} suggested that on average the eastern, Himalaya-draining
tributaries are around 2.2 times more fertile in zircon than the trunk
Indus. If we simply use the source percentages from the zircon unmixing
calculation described above and the average \selectlanguage{greek}ε\selectlanguage{english}\textsubscript{Nd} values
for these different units then it is possible to predict the average
composition of the bulk sediment through time. We use an
\selectlanguage{greek}ε\selectlanguage{english}\textsubscript{Nd} value of -14.6 for the Greater and Tethyan Himalaya,
-21.7 for the Lesser Himalaya, -9.3 for the Karakoram, -20 for Nanga
Parbat and +5.1 for Kohistan and the Transhimalaya based on synthesis of
the bedrock data, but especially the composition of river sediments that
are derived from wide areas of these ranges {[}\emph{Clift et al.} ,
2002b{]}. Transhimalaya Nd data are from Rolland \emph{et
al.}{[}2002{]}, Singh \emph{et al.} {[}2002{]}, and Khan \emph{et
al.}{[}1997{]}. Greater and Lesser Himalayan data are from Ahmad
\emph{et al.} {[}2000{]}, Deniel \emph{et al.} {[}1987{]}, Inger
\emph{et al.}{[}1993{]} and Parrish and Hodges {[}1996{]}. Karakoram
data are from Crawford and Searle {[}1992{]} and Sch\selectlanguage{ngerman}ärer \emph{et al.}
{[}1990{]}.

The results of this estimate are shown next to the smoothed long-term Nd
isotope evolution from bulk sediment analysis {[}\emph{Clift et al.} ,
2019b{]} (Fig. 13). We account for the ±1 \selectlanguage{greek}ε\selectlanguage{english}\textsubscript{Nd}uncertainty
value estimated from the Indus Quaternary {[}\emph{Jonell et al.} ,
2018{]}. We note that before 6 Ma the estimates overlap with the bulk
sediment record that was derived from muddy lithologies, suggesting
similar sources. After this time both the estimated and measured
\selectlanguage{greek}ε\selectlanguage{english}\textsubscript{Nd} values became more negative. However, the predicted
Nd isotope compositions are always more negative than those measured
from the bulk sediment and this implies an over estimation in the flux
from isotopically negative sources, i.e. the Himalaya, using the zircon
method. This is consistent with the geochemical data indicating that the
Himalaya are more abundant in zircon than the Karakoram, but have
similar concentrations in Nd {[}\emph{Alizai et al.} , 2011{]}. As a
result, our zircon budget (Fig. 13) represents an overestimate of the
influence of the Himalaya compared the Karakoram through time in terms
of total rock eroded. Nonetheless, the overall trends in the two data
sets are consistent and the reconstruction of increasing Himalayan
erosion since the 5.72 Ma may be considered robust.

7. Conclusions

Sandy and silty sediments recovered from the Laxmi Basin in the Eastern
Arabian Sea provide a relatively continuous erosional record derived
from the Indus River and spanning the last 15.5 m.y. In this study
samples were taken from IODP Sites U1456 and U1457 for geochemical and
geochronological analyses. Detrital zircon grains were dated by U-Pb
methods to determine their provenance. The sediments themselves are
defined as wackes and are relatively immature in composition, with bulk
sediment characters similar to those found in the Quaternary delta of
the Indus and in its submarine Canyon. They are readily distinguishable
from sediments on the Western Indian Shelf, confirming their derivation
from the Indus River and not the peninsula with one exception. The
sediments are mostly of silty sand to silt size, with only a few being
classified as fine sand. Although the sediments are relatively depleted
in Ca, Na and P relative to the upper continental crust this reflects
chemical weathering during transport and does not affect the provenance
analysis conducted here.

Detrital zircon U-Pb ages fall into a number of categories which can be
correlated with bedrock sources in the Himalaya. The ubiquitous presence
of zircon grains younger than 200 Ma requires the sediments to be the
erosional products of the Himalaya/Karakoram and not peninsular India.
The progressive increase in zircon grains dating at 350--1250 Ma, as
well as 1500--2300 Ma, indicates that the erosional flux from the
Himalaya increased through the studied time interval. Almost all the
samples contain grains that could be derived from the Karakoram or from
Kohistan, and there is an appearance of very young zircon grains,
younger than 25 Ma, that is especially marked since 3.17 Ma. Such young
zircon grains may be from Nanga Parbat or parts of the eastern
Karakoram. Statistical analysis shows that there are a number of
groupings and an increase in Himalayan erosion through time. High flux
from the Himalaya was noted at 7.99--7.78 Ma and starting between 7.0
and 5.87 Ma. Since 1.32 Ma the sediments are similar to the modern Indus
River, but not like the glacial-era river, which has more similarities
with the Miocene Laxmi Basin samples and with enhanced erosion in the
Karakoram. Detrital zircon population unmixing techniques allow us to
objectively confirm the progressive increase of Himalayan erosion
relative to the Karakoram, and the sharp jump in erosion from the Inner
Lesser Himalayas starting at 1.56 Ma. This is somewhat younger than the
anticipated unroofing of these ranges derived from earlier foreland
studies, although much of the earlier data comes from further east in
the Ganges catchment. The shift to more Himalayan erosion through time
occurs as the monsoon climate weakened, as well as when the Lesser
Himalayan Duplex formed. This suggests that the changing patterns of
erosion could be largely a function of solid Earth tectonic forces
building topography, although the correlation of unroofing to the Late
Miocene drying trend does raise the possible role for climate too,
albeit in the opposite fashion to that seen since the LGM, when more
Himalayan erosion correlates with strong summer monsoon rains.

\textbf{Acknowledgments}

This study was made possible by samples provided by International Ocean
Discovery Program (IODP). The work was partially funded by a grant from
USSSP, as well as additional funding from the Charles T. McCord Chair in
petroleum geology at LSU, and the Chevron (Gulf) Centennial
professorship and the UTChron Laboratory at the University of Texas. We
also would like to acknowledge Lisa Stockli for assistance with zircon
U-Pb data acquisition and reduction. Data related to this study are
available as tables within the article and from Mendeley
(data.mendeley.com) at DOI: 10.17632/b57z79m4kj.1.



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\textbf{Figure Captions}

\textbf{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 \emph{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 \emph{et al.} {[}2019b{]}.

\textbf{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 \emph{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 \emph{et al.} {[}2016a{]} for Site U1456
and from Pandey \emph{et al.} {[}2016b{]} for Site U1457, with updates
from Routledge \emph{et al.} {[}2019{]}.

\textbf{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 \emph{et
al.} {[}2019b{]}, rather than presented new here (Table 2). Note the
dominance of silty sand and sandy silt in the analyzed samples.

\textbf{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 \emph{et
al.}{[}2019b{]}(gray text labels and white ringed symbol), rather than
presented new here (Table 2) (black text labels and black ringed
symbol).

\textbf{Figure 5.} (A) Geochemical signature of the analyzed samples
illustrated by a CN-A-K ternary diagram {[}Fedo \emph{et al.} , 1995{]}.
CN denotes the mole weight of Na\textsubscript{2}O and CaO* (CaO*
represent the CaO associated with silicate, excluding all the
carbonate). A and K indicate the content of
Al\textsubscript{2}O\textsubscript{3} and K\textsubscript{2}O
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 Na\textsubscript{2}O 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
{[}\emph{Clift et al.} , 2010{]}, Indus Canyon {[}Li \emph{et al.} ,
2018{]} and western Indian shelf {[}Kurian \emph{et al.} , 2013{]}
following the scheme of Herron {[}1988{]}.

\textbf{Figure 6.} Cross plot of Zr concentration against median sample
grain size. No strong correlation is observed.

\textbf{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 \emph{et al.} {[}2004{]}. Karakoram data is from
is from Le Fort \emph{et al.} {[}1983{]}, Parrish and Tirrul {[}1989{]},
Scharer \emph{et al.} {[}1990{]}, Fraser \emph{et al.} {[}2001{]} and
Ravikant \emph{et al.} {[}2009{]}. Nanga Parbat data is from Zeitler and
Chamberlain {[}1991{]} and Zeitler \emph{et al.} {[}1993{]}.
Transhimalayan data is from Honegger \emph{et al.} {[}1982{]},
Scharer\emph{et al.} {[}1984{]}, Krol \emph{et al.} {[}1996{]}, Weinberg
and Dunlap {[}2000{]}, Zeilinger \emph{et al.} {[}2001{]}, Dunlap and
Wysoczanski {[}2002{]}, Singh \emph{et al.} {[}2007{]}, and
Ravikant\emph{et al.} {[}2009{]}. Samples are marked to show those
published by Clift \emph{et al.} {[}2019b{]}, and those presented new
here.

\textbf{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.

\textbf{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{]}.

\textbf{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.

\textbf{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 \emph{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 \emph{et al.} {[}2016{]}.

\textbf{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
\textasciitilde{}5.72 and again at 3.02 Ma. Samples are marked to show
those published by Clift \emph{et al.} {[}2019b{]}, and those presented
new here.

\textbf{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
Clift\emph{et 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 \emph{et al.} , 1989{]}, and NW India {[}\emph{Singh
et al.} , 2011{]}. (d) Relative exhumation rates of the Greater Himalaya
tracked by bedrock Ar-Ar dating {[}\emph{Clift et al.} , 2008b{]} and
zircon fission track from foreland basin sediment {[}Chirouze \emph{et
al.} , 2015{]}. (e) Rates of sediment supply to the Arabian Sea
calculated from regional seismic {[}\emph{Clift} , 2006{]}.

\textbf{Table Captions}

\textbf{Table 1.} Major elements major elements geochemical analysis of
the samples considered in this study.

\textbf{Table 2.} Analytical results from the laser particle size
analyzer for all bulk samples considered in this study.

\textbf{Table 3.} Analytical data for the zircon U-Pb dating of grains
presented in this work.

\textbf{Table 4.} Results of the mixing modelling performed by the DZMix
software of Sundell and Saylor {[}2017{]} on the detrital samples from
the Laxmi Basin.

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