In trying to interpret the marine sedimentary record it is often important
to also consider the evolution of drainage systems on shore if the full sediment
transport system is to be understood. In Ladakh, northern India for
example, the Indus Molasse Group has been identified as the deposits of a
Paleo-Indus River that was eroding western Tibet shortly after India-Asia
collision in the Early Eocene. It is this system that is considered to be
providing grains from the suture zone to the Arabian Sea by the Middle Eocene
(~45 Ma; Clift et al., 2001). Although the age control in continental sediments
is often poor compared to their marine equivalents, they form an important
part of the system. Similarly, study of the modern river sediments and comparison
with marine sediment allows the volume and composition of inputs from
different sources to be quantified. If the process of marine sedimentation
can be related to the modern mountains and drainage system then there is hope
that ancient sediments can be used to reconstruct what the mountains looked
like in the geologic past.
Aerial view of the Eocene-Oligocene Indus Molasse Group
Indus Suture Zone, Ladakh, India. This sequence is interpreted as the
deposits of a paleo-Indus River and shows that the river started to flow
soon after initial collision and uplift of southern Tibet (Clift et al.,
If sediment budgets derived from offshore
accumulation rates are to be used to reconstruct changing continental erosion
rates through time, and eventually date Tibetan Plateau uplift, then the
source of that sediment needs to be constrained. Basic petrography and mineralogy
are fundamental to this process, yet modern single grain probe techniques
can also play an important role. At WHOI we have been exploring the possibilities
of the Pb isotope approach, which is based on the distinct isotopic character
of the principle source regions. Analysis is made using detrital K-feldspars,
a techniques pioneered by conventional mass spectrometry (e.g., McDaniel
et al., 1994). It has already been shown that the modern rivers of SE Asia
and the tributaries within the Indus system carry K-feldspar grains with
resolvably different Pb isotope characters (Bodet & Schärer, 2001;
Clift et al., 2002). We employ the newly developed technique of measuring
Pb from single grains in situ (Layne & Shimizu, 1998) using the
high resolution Cameca 1270 ion microprobe at WHOI. Used in company with
bulk sediment techniques, such as Nd isotopes, we are able to mass balance
erosion in the mountains with deposition along the continetal margin.
Using the Cameca ims 1270 ion probe at WHOI it has
to constrain the source of sediments in the Asian marginal seas. This
example from the Gulf of Tonkin shows that sediment in the Eocene
gulf had a different source than that in the modern Red River, more
similar to the Yangtze Block, implying stream capture since that time.
The marine sedimentary record can
be used to reconstruct the erosion of the Himalayas and Tibet since the start
of India-Asia collision. This record is essential to any attempts to quantify
the relationships between tectonics, climate change and erosion. In particular,
understanding the relationship between Tibetan uplift and monsoon strengthening
and its erosional response is still unresolved because of uncertainty about
the timing of each process. Early work at WHOI, in collaboration with colleagues
at MIT, now suggests that the plateau uplift may be much older than
previously believed 16-20 Ma, rather than 8 Ma, and that this caused a strengthening
of the monsoon across east and south Asia, increasing erosion and marine clastic
sedimentation in the marginal seas. Future work is now aimed at dating key
drainage capture events and in constructing robust sediment budgets in the
marginal seas using regional seismic profiles and drilling data from industrial
and future IODP sources.
Clift, P. D., 2002. A brief history of the Indus River.
In, Clift, P.D., Kroon, D., Craig, J., and Gaedicke, C. (Editors), The
Tectonic and Climatic Evolution of the Arabian Sea Region, Geological
Society of London special publication, 195, 237–258.
Clift, P.D., Lee, J. I., Blusztajn, J. and Clark, M.K.,
2002. Erosional response of South China to arc rifting and monsoonal strengthening
recorded in the South China Sea. Marine Geology, 184, 207–226.
Clift, P. D., Lee, J.I., Hildebrand, P., Shimizu, N.,
Layne, G. D., Blusztajn, J., Blum, J. D., Garzanti, E., and Khan, A. A.,
2002. Nd and Pb isotope variability in the Indus River system: Implications
for sediment provenance and crustal heterogeneity in the Western Himalaya.
Earth and Planetary Science Letters, 200, 91–106.
Clift, P.D. and Gaedicke, C., 2002. Accelerated mass flux
to the Arabian Sea during the Middle-Late Miocene. Geology, 30, 207–210.
Clift, P.D., 2001. The Indus Fan: Climate tectonic interactions
in the western Himalaya. Geoscientist, 11 (12), 4-9.
Clift, P.D., Shimizu, N., Layne, G., Gaedicke, C., Schlüter,
H.U., Clark, M. and Amjad, S., 2001. Development of the Indus Fan and its
significance for the erosional history of the western Himalaya and Karakoram.
Geological Society of America Bulletin, 113, 1039–1051.
Clift, P.D., Shimizu, N., Layne, G., and Blusztajn, J.,
2001. Tracing patterns of unroofing in the Early Himalaya through microprobe
Pb isotope analysis of detrital K-feldspars in the Indus Molasse, India,
Earth and Planetary Science Letters, 188, 475–491.
Clift, P.D., Shimizu, N., Layne, G., Gaedicke, C., Schülter,
H.U., Clark, M., and Amjad, S., 2000. 55 million years of Tibetan and Karakoram
evolution recorded in the Indus Fan. EOS, 81 (25), p. 277–282.
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