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 [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 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
(>100 µm) 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 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
<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
[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 [Gehrels et al. ,
2011], and these are in any case not always mapped consistently by
different groups [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 [DeCelles et al. , 2000; 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
[Clift et al. , 2004; 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
[Alizai et al. , 2011; 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 [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 (<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
[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 [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 [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 [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
[Clift et al. , 2008a] compared to the modern river or during
the Holocene when the summer monsoon was strong [Caley et al. ,
2014; Fleitmann et al. , 2003; 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 [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 [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.