The Marine Geological Record of Neogene Erosional in Asia


Interpreting the sedimentary record to understand tectonic
and climatic evolution in the wake of India-Asia Collision



Indus fan channels

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.



Introduction

    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.

Yellow Sea


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 Sea.





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).

drainage network

Shaded topographic map of East Asia showing the different drainage basins feeding
sediment to the marginal seas.  Each sea contains a sedimentary record of the evolving
erosion and weathering regimes onshore. Capture of headwaters of one river to another
can result in changes in sediment provenance in the marine sediment record.
Map produced by M.K. Clark (MIT).

Continued on Page 2

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