Geoscience Reference
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inferred two main uplift phases, one at 10.9-7.5 Ma and the other after 0.9 Ma. They
also concluded that rivers in the upper Indus captured the headwaters of the Ganga
between 7.5 and 6.5 Ma, probably as a result of tectonic movements. Burbank (
1992
)
investigated Neogene sediments deposited by rivers coming from the Himalayas onto
the Indo-Gangetic foreland at the foot of the Himalayas. He found a marked change
in sediment accumulation in Pliocene-Pleistocene times, which was consistent with
an increase in erosional unloading in the Himalayas during the past 4 Ma. Here
we have a nice example of uplift triggered by denudation rather than by tectonic
processes, showing that it is sometimes possible to distinguish between uplift arising
from tectonic forces and uplift caused by erosion and, indirectly, by climate.
Li et al. (
2011
) analysed the neodymium (Nd) and
87
Sr/
86
Sr isotope ratios in dust
that was derived from Asia over the past 20 Ma and deposited downwind in the
Pacific. They found from the change in isotope ratios that dust derived from the north
Tibetan Plateau (NTP) showed an increase relative to that derived from the Central
Asian Orogen after 15 Ma, and they concluded that the elevation of the NTP had
increased gradually from about 2,700 to 4,500 m in the last 15 million years. The
central and south Tibetan Plateau were already high by 15 Ma. Rohrmann et al. (
2012
)
used a battery of thermochronologic techniques to establish that in central Tibet, the
plateau had begun to form during the Late Cretaceous, expanding to cover much of
central Tibet by 45 Ma. Methods used included apatite fission track dating and apatite
[(U-Th)/He] dating of rates of exhumation and uplift.
Hetzel et al. (
2011
) used a combination of cosmogenic
10
Be exposure dating (see
Chapter 6
) and thermal modelling based on the (U-Th)/He ages of apatite and zircon
to date a well-preserved peneplain situated at an elevation of about 5,300 m in the
northern Lhasa block. They concluded that the peneplain had formed at low elevations
until India's collision with Asia around 50 Ma ago resulted in crustal thickening,
surface uplift and the subsequent preservation of the peneplain. Since Tapponnier et al.
(
2001
) had already demonstrated that south Tibet had attained an elevation of at least
4,000 m by 35 Ma, Hetzel et al. (
2011
) inferred that there must have been rapid uplift
of the Tibetan Plateau between the onset of the 50 Ma collision and 35 Ma.
Heller and Liu (
1982
) reported an age of about 2.4 Ma for the base of the wind-
blown dust in the Loess Plateau of central China. However, this does not date the onset
of aridity in this region. Sun et al. (
2009
) obtained an age of about 7 Ma for eolian
dune sands in the central Taklamakan Desert, which is older than the age of 5.3 Ma
for eolian siltstone on the windward edge of the Kunlun Shan flanking the southern
Taklamakan (Sun and Liu,
2006
). Further to the north-west in central Asia, the first
evidence of eolian dust is far older, with strong evidence of aridity in that region by
around 24 Ma (Sun et al.,
2010
).
Dupont-Nivet et al. (
2007
) obtained a fine-resolution magnetostratigraphic chro-
nology for the Eocene-Oligocene transition (34-33 Ma) in the Xining Basin at the
north-east edge of the Tibetan Plateau. Widespread sedimentation in playa lakes
