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 [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 [Najman et al. ,
2009]. Our data support the findings of Clift 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 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 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 [Huyghe et al. , 2001; Webb ,
2013]. Integrated metamorphic and geochronologic data indicate rapid
cooling of the Inner Lesser Himalaya before 6 Ma, following peak
metamorphism around 10 Ma [Caddick et al. , 2007; 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 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 ~8 Ma [Dettman et
al. , 2001], or after 7.7 Ma based on new environmental data from Site
U1456 [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 [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 [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
(<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 [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 [Garzanti et al. , 2004; Stewart
et al. , 2008]. This is consistent with the U-Pb zircon ages in the
modern Indus River downstream of Nanga Parbat [Alizai et al. ,
2011] that show neither many <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 [Whittington et al. , 1999].
We compare our detrital zircon budget with that of the Nd budget
published by Clift 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 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 εNd values
for these different units then it is possible to predict the average
composition of the bulk sediment through time. We use an
εNd 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 [Clift et al. ,
2002b]. Transhimalaya Nd data are from Rolland et al.[2002], Singh et al. [2002], and Khan et al.[1997]. Greater and Lesser Himalayan data are from Ahmad et
al. [2000], Deniel et al. [1987], Inger et al.[1993] and Parrish and Hodges [1996]. Karakoram data are from
Crawford and Searle [1992] and Schärer et al. [1990].
The results of this estimate are shown next to the smoothed long-term Nd
isotope evolution from bulk sediment analysis [Clift et al. ,
2019b] (Fig. 13). We account for the ±1 εNduncertainty value estimated from the Indus Quaternary [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
εNd 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 [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.