Continent-Ocean Interactions
within the East Asian Marginal Seas
 
 

blueline

Regional Dynamics
Rift Tectonics
The Sedimentary Fill
Paleoceanographic Record
Summary
References
 

Regional Dynamics

The origin of the East Asian Marginal Seas is a controversial topic that revolves around the relative importance of continental versus oceanic influences. Resolving this problem is important for understanding the modern geology of the region, but is also of significance to the general problem of why marginal seas open, and indeed how and when they close. This cycle is of interest to orogenic geologists, notably those studying the Appalachian-Caledonian and Tethyan systems, who use the region as a template for understanding the complex series of rifts and collisions they see in the ancient record. The significance of understanding the processes that formed the basins of eastern Asian thus extends beyond the immediate vicinity and the regional community.

 In one type of model for basin development lateral extrusion of southeast Asia, followed by southern and central China, towards the east may cause basins to open as a result of lateral shear between major continental blocks (e.g., Tapponnier et al., 1986). Alternatively, some have argued that it the regional extensional stresses are induced by subduction that caused continental arc crust to rift and form marginal seas (e.g., Hall, 1996). Although radiometric dating of rocks within the major strike-slip fault zones is broadly coincident with the age of spreading, most notably in the South China Sea (Harrison et al., 1996), these ages postdate the start of extensional deformation at least in the Pearl River Mouth basin, making a convincing correlation difficult. Although the age of spreading is well known from marine magnetic anomaly patterns, the age of onset of extension is not tightly constrained because the syn-rift continental facies of many of the basins have limited biostratigraphic data. Where marine units are found, extension can be seen to have dated from the Maastrichtian, somewhat before the start of India-Asia collision. Some workers (e.g., Wang et al., 1998) have even argued that radiometric ages do not support extrusion-driven rifting. Arguments continue as to whether the timing and degree of offset on the major faults is consistent with a purely continental explanation for basin opening. On the other hand regional basin analysis work (e.g., Wheeler and White, 2000) indicates that the amount of modern dynamic topography induced by subduction is small. If induced stresses from subduction zones bordering the eastern side of the basins are low then these may not be effective in driving the extension during the Cenozoic.
 
 

Rift Tectonics

The East Asian Marginal Seas are ideal places to examine the nature of continental rifting, break-up, and the onset of seafloor spreading. Although they are mostly inactive today, the young age of the basins means that the basement and syn-rift sediments are not deeply buried by post-rift strata, as seen in Atlantic margins. The oldest oceanic crust has not yet had time to subside to great water depths. The thin cover is important for both seismic imaging and for drilling and sampling of the sediments and basement. Accessibility to the syn-rift sediments and structures makes these basins ideal for studying extension and the rift-to-drift transition. This latter process is generally poorly documented in major ocean basins, and those that have been well studied typically fall into two very different groups, the non-volcanic (Iberia-type) and volcanic (Greenland-type) margins. Existing studies of the East Asian Marginal Seas suggest that these may not fit neatly into either of these categories. Impressive submarine volcanic centers are seen along southern Chinese margin, as well as from offshore Vietnam and the Dangerous Grounds area (Figure 2). Seismic images show volcanic edifices whose form is suggestive of magmas of intermediate compositions.

 The Miocene is a period of rapid magnetic reversals and consequently good marine magnetic anomalies in the oceanic crust, allowing detailed correlation from oceanic spreading history to rifting in the continental crust (Figure 3). The biostratigraphic framework for this period allows a high resolution record to be derived from the sedimentary fill of the rift basins. An important objective of the meeting will be to apply modern Cenozoic time scales to the Asian Marginal Seas and their associated continental geology. The need to make regional correlations between independent basins if we are to recognize regional trends in the tectonic, sedimentary or climatic development requires the use of a single accurate time scale. Because the basins typically stopped extending when their width was modest matching conjugate margins is not difficult, even when flow lines are hard to trace because of poorly defined fracture zones. In the South China Sea the propagating spreading center is frozen at the SW end, which allows the two conjugate margins to be pinned there, and aids the correlation of conjugate margins farther east.

 All of these factors make East Asian Marginal Seas good places to examine rifting in arc crust, if not in cratonic crust, such as seen in the Baikal Rift and the Red Sea. This style of extension represents a large fraction of global rifted margins. A good deal of drilling and seismic data has already been collected by both industrial and academic groups in many of these basins, allowing their evolution to be reconstructed in some detail. As well as shallow crustal reflection seismic images(Figure 4), some basins also have deep penetrating seismic refraction data (e.g., South China, Nissen et al., 1995; Sea of Japan, Kurashimo et al., 1996). The additional control on the degree of crustal extension provides confidence in the rifting models derived by traditional subsidence modeling techniques, or from shallow structural data. Seismic refraction data also provides the possibility of identifying underplated magmatic bodies that accreted during break-up, similar to those seen in the North Atlantic (Nissen et al., 1995). Their recognition and quantification is essential to understanding melting processes during the rift-to-drift transition.

 The East Asian Marginal Seas may be used to address issues such as the nature of strain partitioning along rifted continental margins. Since the recognition of the importance of low-angle detachment faults in governing continental extension in the Basin and Range Province, marine geologists have attempted to apply the same model to passive margins, albeit with greater amounts of extension. Unfortunately, as noted by Driscoll and Karner (1998), workers have tended to interpret all margins as upper plates within simple shear systems, even when the conjugate has already been interpreted as an upper plate. The most common reason for this apparent problem is that total observed subsidence normally exceeds that expected from the degree of normal faulting imaged seismically. Clift et al. (2001) have suggested from data in the South China Sea that this apparent anomaly reflects not simple shear, but instead the preferential loss of the ductile lower continental crust adjacent to the continent-ocean boundary for a distance of 50-100 km. The same loss and subsidence mis-match is not seen in basins within the same arc crust but far from the continent-ocean boundary (e.g., Beibu Gulf Basin). The style of rifting may derive from the thermally juvenile nature of many East Asian Marginal Seas, with their recent association to active subduction systems. However, the recognition of low crustal viscosity under Tibet and much of southern China (Clark and Royden, 2000) may instead place the East Asian Marginal Seas in a regional context of lower crustal flow.

 The East Asian Marginal Seas allow models of strain accommodation to be assessed because both sides of a margin can be analyzed and directly compared. Allowing a full mass balance of both margins prevents the generation of inconsistent models for conjugate margins.
 
 

The Sedimentary Fill

The sedimentary fill of the basins of east and southeast Asia represents an important repository of information on the tectonic and climatic evolution of the region. Many of the basins are fed by the large rivers that drain east Asia and consequently the sediments may record the uplift and erosion of the orogenic belts and plateaus that largely formed as a result of the India-Asia collision. Some sediment, especially in the East and South China Seas is derived from the island arcs of the region, in those examples from the collision of the Luzon Arc with the passive margin of southern China in Taiwan. The sedimentary fill of the marginal seas thus provides information on the nature of arc-continent collision the probable method by which continental crust has been constructed, at least since the late Precambrian. Southern Asia accounts for ~75% of the fluvial sediment to the global ocean; the six high-standing islands of Indonesia alone accounting for 20-25%, despite only representing about 2% of the land area draining into the ocean. In addition, records of the evolving arc magmatism are found in the form of tephra. Serious attempts to mass balance the flux of material through active margins on geologic time-scales requires the use of these deposits to assess arc output beyond that exposed in modern arc volcanic islands.

Uplift patterns onshore reflect the nature of strain accommodation during the India-Asia collision, an issue that is still controversial. Continental geologists continue to debate the competing roles of lateral extrusion (e.g., Molnar and Tapponnier, 1975), continental crustal subduction and horizontal compression in the India-Asia collision. Understanding which of these processes was dominant and during specific stages in the collision history is important to general and regional orogenic models and also has significance to the ongoing debate concerning the interaction between climate and orogenic uplift. The timing of Tibetan uplift in particular has proven to be a difficult issue because of lack of a Tibetan erosional signal in the Indian foreland basin and because of the general lack of a well dated complete sedimentary record on the plateau itself. Uplift may be traced in part through the marine erosional record, because although the central plateau is flat, the eastern flank is heavily incised by large rivers that carry material into the Asian marginal seas.

The apparent lateral growth of elevated topography due to low viscosity of the crust under Tibet (Clark and Royden, 2000) might be expected to have an influence in diverting river courses, while compression of the large east Asian rivers around the Namche Barwe syntaxis of the Himalaya has allowed the progressive capture of drainage basins from one river to another. The end result is documented in the offshore region, where large sedimentary basins are now fed by rivers with very modest drainage basins, most notably the Irrawaddy and the Red Rivers. In these areas offshore sediment thicknesses reach ~9 km (e.g., Yingehai Basin), but drainage areas are small and marked by low elevations compared to the modern Brahmaputra. The Brahmaputra appears to have captured the drainage basins of the Red and Irrawaddy as it migrates NE with the Namche Barwe orogenic syntaxis. The marine record thus provides a way in which the evolving continental tectonics can be dated and quantified. Changes in the rates of sediment accumulation and the provenance of the sediments will be governed by continental evolution.
 
 

Paleoceanographic Record

The sedimentary fill of the East Asian Marginal Seas is also a valuable record of the climatic evolution of the region. Changes in the paleoceanography, including circulation patterns, upwelling, and biological productivity may be reconstructed from the foraminiferal assemblages within the sediments. The western boundary currents of East Asia, the Kuroshio and Oyashio, flow through several of the East Asian Marginal Seas and exert and important influence on the climatic evolution of the neighboring Asian continent, as well as productivity in the basins themselves. Because the passageways between the seas are often narrow and shallow the boundary currents are extremely sensitive to tectonic uplift and subsidence as well as rises and falls in eustatic sea-level. Circulation is aided by the opening of marginal basins and inhibited by the closure of deep-water gateways. While the rifting of the Sea of Japan allowed flow through that region, the collision of the Luzon Arc with the Chinese passive margin has closed a major deep-water passage, restricting flow to the deepest sill along the arc ridge. This sensitivity by the East Asian currents has the effect of amplifying the response of the Asian Marginal Seas to glacial cycles. Global climatic changes, most notably the northern hemispheric glaciation must have had a profound effect in isolating basins from the open oceans. This is especially true where those basins are rimmed by relatively shallow ridges and continental platforms, as in the Sea of Japan and the South China Sea. In these cases, the basins would have become stagnant or enclosed during glaciations. In some case, e.g., the Yellow Sea the entire basin would have drained or desiccated during major sealevel fall.

 Beyond global climate changes, the Asian monsoon is the greatest influence on the paleoceanography of the East Asian Marginal Seas (Figure 5). The monsoon is one of the major components of the global climate system and its evolution plays a significant role in our understanding of global climates (Webster et al., 1998). The Asian summer and winter monsoons dominate the seasonal winds, precipitation and run-off patterns, and the character of land vegetation over southern and eastern Asia. The monsoon controls the volume and mineralogy of the continental run-off into the seas, as well as the flux of wind-blown dust. In practice the sedimentary fill of the marginal seas provides a record of the continental Asian climate, as well as the paleoceanography, both of which are influenced by the monsoon. The winter monsoon is characterized by high pressure over northern Asia, northeast winds across the South China Sea, and enhanced precipitation in the Austral-Asian equatorial zone. The summer monsoon circulation is characterized by low pressure over Tibet, strong southwesterly winds, upwelling in the Arabian Sea, and high precipitation over southern and eastern Asia. The Asian Marginal Seas are ideally located to record the paleoceanographic responses to both winter and summer monsoons. Recent drilling by the Ocean Drilling Program of the South China Sea (Leg 184; Wang et al., 2000) now provides the opportunity to study the evolution in this region to compare with the existing Arabian Sea record of Leg 117 (Prell et al., 1989). Examining the coupling between these two climatic regions and assessing any long term linkage to the tectonic evolution are major goals of the proposed meeting.

 Evolution of the Asian monsoon system is thought to reflect at least four types of large-scale climate forcing or boundary conditions: (1) tectonic uplift of the Himalayan-Tibetan orogen, (2) changes in the atmospheric CO2 concentration, (3) changes in the Earth's orbital parameters and the resulting variations in seasonal solar radiation, and (4) changes in the extent of glacial climates. These factors act to amplify or dampen the seasonal development of land-sea heating and pressure gradients, latent heat transport, and moisture convergence over the Asian continent.
 
 

Summary

Despite the success of plate tectonics in providing a global framework to understand the rifting and destruction of ocean basins several major topics remain poorly understood, most notably:


The East Asian Marginal Seas represent some of the best places to examine these questions within the broad framework on continent-ocean interactions. In the proposed meeting we will bring together an international group to present the most recent results from the region and attempt to address the multidisciplinary issues that regular meetings do not discuss. We believe that this is an appropriate time for the meeting because of recent marine sampling and surveying (ODP and SEAS program), coupled with advances in our understanding of the evolution of east Asia.
 


 

References


Clark, M. K., and L. H. Royden, Topographic ooze; building the eastern margin of Tibet by lower crustal flow. Geology, 28, 703-706, 2000.

Clift, P. D., J. Lin, and ODP Leg 184 Scientific Party. Patterns of extension and magmatism along the continent-ocean boundary, South China margin. In Non-volcanic rifting of continental margins: a comparison of evidence from land and sea, eds. R. C. L. Wilson, R. B. Whitmarsh, B. Taylor, and N. Froitzheim, Geol. Soc. Spec. Publ., in press.

Driscoll, N. W., and G. D. Karner, Lower crustal extension across the Northern Carnarvon basin, Australia: Evidence for an eastward dipping detachment. J. Geophys. Res., 103, 4975-4991, 1998.

Hall, R., Reconstructing Cenozoic SE Asia, In Tectonic evolution of Southeast Asia, eds., R. Hall and D. Blundell, Geol. Soc. Spec. Publ, 106, 153-184, 1996.

Harrison, T. M., P. H. Leloup, F. J. Ryerson, P. Tapponnier, R. Lacassin and C. Wenji, Diachronous initiation of transtension along the Ailao Shan-Red River shear zone, Yunnan and Vietnam. In The Tectonic Evolution of Asia, eds., A. Yin and T. M. Harrison, Cambridge University Press, 110-137, 1996.

Kurashimo, E., Shinohara, M., Suyehiro, K., Kasahara, J., Hirata, N., Seismic evidence for stretched continental crust in the Japan Sea, Geophysical Research Letters, 23, p. 3067-3070, 1996.

Molnar, P., and P. Tapponnier, Cenozoic tectonics of Asia; effects of a continental collision. Science, 189, 419-426, 1975.

Prell, W. L., N. Niitsuma, K. C. Emeis, et al., Proc. Ocean Drill. Prog., Part A: Init. Rpts., 117, p. 1236, 1989.

Nissen, S. S., D. E. Hayes, P. Buhl, J. Diebold, Y. Bochu, Z. Weijun, and C. Yongqin, Deep penetration seismic soundings across the northern margin of the South China sea. J. Geophys. Res., 100, 22,407-22,433, 1995.

Tapponnier, P., G. Peltzer, and R. Armijo, On the mechanics of the collision between India and Asia. In Collision Tectonics, eds., M. P. Coward and A. C. Ries, Geol. Soc. Spec. Publ., 19, 115-157, 1986.

Wang, P. L., C. H. Lo, T. Y. Lee, S. L. Chung, N. T. and Yem, Thermochronological evidence for the movement of the Ailao Shan-Red River shear zone: a perspective from Vietnam. Geology, 26, 887-890, 1998.

Wang P., W. L. Prell, P. Blum, Proc. Ocean Drill. Prog., Part A: Init. Rpts, 184, p. 77, 2000.

Webster, P. J., V. O. Magana, T. N. Palmer, J. Shukla, R. A. Tomas, M. Yanai, , and T. Yasunari, Monsoons: Processes, predictability, and the prospects for prediction, in the TOGA decade. J. Geophys. Res., 103, 14,451-14,510, 1998.

Wheeler, P., and N. White, Quest for dynamic topography: Observations from Southeast Asia. Geology, 28, 963-966, 2000.


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