The Marine Geological Record of Neogene Erosional in Asia
the sedimentary record to understand tectonic
and climatic evolution in the wake of India-Asia Collision
BGR multichannel seismic profile SO122-26
showing stacked channel-levee complexes
on the upper Indus Fan, the product of erosion
in the Karakoram and Himalaya onshore.
Collision of India with the Asian mainland during the earliest Eocene (~50
Ma) has resulted in the growth of the world's largest orogenic belt and
associated plateau. However, the timing of surface uplift is presently not
well understood, especially in Tibet, because there is no good sediment
record of uplift or convincing paleo-altitude proxy. Understanding when the
uplift occurred is important for models of strain accomodation in Asia and
to models linking solid earth and climatic evolution. Proposed links between
the evolution of the solid earth and the circulation systems of the oceans
and atmosphere represent an exciting frontier to the earth and ocean sciences.
However, most of the proposed links, most notably between Tibetan uplift
and strengthening of the monsoon, remain poorly understood because even when
climatic data are available there are usually no matching records of mountain
uplift or erosion. Of all
possible coupled tectonic-climate systems the relationship between growth
of the Himalaya and Tibetan Plateau, and both regional and global climate
change is recognized as the most dramatic and contentious. The modern Tibetan
Plateau is known to significantly disrupt upper atmospheric circulation.
Consequently, the suggestion that the Tibetan Plateau may have been rapidly
uplifted at ~8 Ma, a time when the South Asian Monsoon was known to strengthen
in the Arabian Sea (e.g., Kroon et al. 1991), resulted in several workers
linking the tectonic evolution of South Asia and the intensity of the monsoon
(e.g., Molnar et al. 1993). Such models can now be tested using the
detrital sediment record in the Asian marginal seas.
Satellite image showing the Bohai, Yellow and East China Seas.
Note the major sediment plumes extending from the Yangtze Mouth
SE into the East China Sea and from the Yellow Sea into the Bohai
Evolving Drainage Basins
Each Asian marginal sea is fed by a river system, many of which have their
head-waters in the Tibetan Plateau. These are some of the largest
rivers in the world, including the Yellow, Yangtze, Pearl, Red, Mekong and
Irrawaddy. These rivers contrast with those in south Asia, the Brahmaputra,
Ganges and Indus which also erode the Himalayas, as well as the Karakoram
and Hindu Kush in the case of the latter. Thus, the east Asian rivers
potentially have a simpler provenance than their southern equivalents. During
periods of plateau uplift these rivers must have excavated the gorges in
which they now flow and delivered this material to the ocean. Thus
Tibetan uplift may be record as a period of accelerating sedimentation rates
offshore. However, as these river courses run close to one another in eastern
Tibet, it have been suggested that they have captured each others head-waters,
complicating the offshore record. Conversely, the marine record may be
used to date the capture events and in turn date the plateau uplift. Métivier
et al. (1999) noted that the present day discharge sites of the Yangtze
and Yellow Rivers are not representative of the drainage pattern prior to
5 or 11 Ma, based on regional study of sedimentation rates. These workers
considered that oscillation and variation in deposition rates in the East
China Sea and other SE Asian basins imply variations in flow and the sediment
load of the river, or alternatively, major changes in the configuration of
drainage in Asia. Isotopic provenance work down in WHOI now shows the sediment
now delivered to the Gulf of Tonkin by the Red River differs from earlier,
Eocene sediments, which also appear to have drained the Yangtze Block. This
requires not only drainage capture from the Red to the Yangtze, but also
flow reversal as the continent deformed. The marine record can be employed
to provide important constraints on the deformation patterns of the continental
interior. At the same time, clay mineralogy in these same sediments can
be employed to assess changing weathering regimes onshore (e.g. South China
Sea: Clift et al., 2002).
Shaded topographic map of East Asia
showing the different drainage basins feeding
erosion and weathering
regimes onshore. Capture of headwaters of one river to another
sediment to the
marginal seas. Each sea contains a sedimentary record of the evolving
can result in changes in sediment provenance
in the marine sediment record.
by M.K. Clark (MIT).
Continued on Page 2
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