TAO, Vol.3, No.3, 419-434, September 1992 Physiographic Characteristics of the Continental Margin, Northeast Taiwan

The continental margin off northeastern Taiwan consists of five major physio­ graphic units which are the East China Sea continental shelf, the East China Sea con· tinental slope, the Southern Okinaw� Trough, the Ilan continental shelf and the Ilan Ridge. This physiographic framework is provisional and may be revised if better phys· iographic names are proposed. The East China Sea continental shelf is relatively wide with a smooth surface marked locally by low-relief features resulting from sea-level fluctuation during the late Pleistocene and by volcanic activities of the back-arc rifting. The East China Sea continental slope varies laterally in slope gradient and to­ pography. The continental slope is marked by a variety of slumping or sliding scars, erosional gullies, and debris flow deposits or turbidites. The submarine canyons are another prominent topographic feature occurring on the slope. The submarine canyons indent the continental shelf and modify the linear shelf-slope transtition region into an irregular shape. The western end of the Southern Okinawa Trough has curvelinear bathymetric contours at the maximum depth of about 2,000m. The trough is mainly confined by the continental slope to the north and by slopes off the Ryukyu volcanic islands. The Ilan continental shelf is the seaward continuation of the Ilan Plain along the northeastern coast of Taiwan. The shelf is fan-shaped, and the width varies from about 10 km off the Lan Yan Hsi River to about 2km at the coastal cities of Touchen and Suao. This shelf protrudes onto the slope seaward with the accumulation of the sediments transported by the Lan Yan Hsi River. The Ilan Ridge is a long and narrow ridge extending seaward from the Ilan shelf. The ridge separates the forearc basin of the Nanao basin to the south from the backarc basin of the Okinawa Trough to the north. The Ilan Ridge is confinec;I by the Ryukyu volcanic islands to the east and is relatively narrow in areal extent as compared to the Ryukyu island chain. · The shelf-slope transition region where the water mass exchange process takes place between the East China Sea and the Kuroshio Current is characterized by an irregular topography. This topographic effect on the Kuroshio edge exchange process is worthwhile to be investigated in greater detail. 1 Institute of Oceanography, National Taiwan University, Taipei, Taiwan, R.O.C. 2 Department of Oceanography, National Taiwan Ocean University, Keelung, Taiwan, R.0.C. 419 420 TAO, Vol.3, No.3, Sep. 1992

The East China Sea continental slope varies laterally in slope gradient and to pography.The continental slope is marked by a variety of slumping or sliding scars, erosional gullies, and debris flow deposits or turbidites.The submarine canyons are another prominent topographic feature occurring on the slope.The submarine canyons indent the continental shelf and modify the linear shelf-slope transtition region into an irregular shape.The western end of the Southern Okinawa Trough has curvelinear bathymetric contours at the maximum depth of about 2,000m.The trough is mainly confined by the continental slope to the north and by slopes off the Ryukyu volcanic islands.
The Ilan continental shelf is the seaward continuation of the Ilan Plain along the northeastern coast of Ta iwan.The shelf is fan-shaped, and the width varies from about 10 km off the Lan Yan Hsi River to about 2km at the coastal cities of Touchen and Suao.This shelf protrudes onto the slope seaward with the accumulation of the sediments transported by the Lan Yan Hsi River.
The Ilan Ridge is a long and narrow ridge extending seaward from the Ilan shelf.The ridge separates the forearc basin of the Nanao basin to the south from the backarc basin of the Okinawa Trough to the north.The Ilan Ridge is confinec;I by the Ryukyu volcanic islands to the east and is relatively narrow in areal extent as compared to the Ryukyu island chain.

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The shelf-slope transition region where the water mass exchange process takes place between the East China Sea and the Kuroshio Current is characterized by an irregular topography.This topographic effect on the Kuroshio edge exchange process is worthwhile to be investigated in greater detail.

INTRODUCTION
The East China Sea is one of the marginal seas along the rim of the western Pacific.
It is bordered by the Yellow Sea to the north.The boundary line between the East China Sea and the Yellow Sea extends from the location of 33°17 1 lat.at the coast of China to the western end of Cheju Island, Korea.The East China Sea is separated from the Ta iwan Strait by a line connecting from the Fukueichiao at northernmost Taiwan to Haitan island off Fujian Province, China (Ma, 1986).The Ryukyu islands are the eastern boundary of the East China Sea (Figure 1 ).It covers an area of about 770,000 square km ar..d has an elongated shape trending northeast-southwest.It has a length of about l ,300km from northeast to southwest and the average width from east to west is around 540krn (Zhou et al., 1989).
The continental margin off northeastern Ta iwan is characterized by the shallow broad East China Sea shelf, the relatively narrow and steep continental slope and the deep Okinawa Trough.In general, the shelfbreak around Taiwan occurs at a depth of about 200m (Boggs et al., 1979).It has been noted that off the eastern and northeastern coast of Ta iwan, the Kuroshio Current flows northeasterly along the shelf edge towards Japan (Figure 2).The Kuroshio Current is a major western boundary current with warm (20-27°C), saline(33%) water and originates east of Luzon Island around 15°N, 125°E.The importance of shelf edge exchange processes of the Kuroshio Current flowing along the segment off northeast Taiwan has been recognized in recent years, especially the significance of water-mass exchange between the Kuroshio Current and the East China Sea (Chu, 1976).This study is a geological part of the joint research project of Kuroshio edge exchange processes (KEEP) which is an integrated research of marine sciences including physical oceanography, marine chemistry, biology, and geology sponsored by the National Science Council of Taiwan (Chu, 1989).
The movement of the Kuroshio Current is closely related to the topography of the shelf slope region off eastern Taiwan (Chung and Yu, 1988 ), and the topography of the sea floor around Taiwan has resulted from the hydrologic conditions on the shelf and slope and partly from Late Pleistocene lowstands of sea level (Boggs, et al., 1979).The need for further information about the topography for the KEEP project is apparent and leads to investigation of the bathymetry and physiography of the region which affects the shelf-edge exchange processes of the Kuroshio Current in the areas off northeast Taiwan.The main purpose of this paper are twofold: (1) to emphasize prominent topographic features which may be related to the hydrodynamics along the shelf-slope region and (2) to present a physiographic framework for the KEEP study in the region off northeastern Taiwan.The advantage of having a physiographic framework is to set the references for specific locations with distinct topographic and bathymetric characters in the KEEP study areas.Hopefully, these proposed physiographic names could be followed by the marine scientists participating in the KEEP project and the topography infonnation can be useful for other disciplines.This research emphasizes the prominent topographic features for each major physiographic unit.There is no intention of discussing the geomorphic characters of each individual feature in detail.
The study area is located at the shelf-slope region off northeastern Taiwan (Figure 3).The site for investigations is bounded to the north at 26°N, to the east at 123°E and to the south at 24°N.

Data
Surface-ship echo-sounding tracks and high frequency (3.5KHz) subbottom profiles and, where available, multichannel seismic reflection profiles in the areas off northeast Taiwan were acquired during three cruises aboard RIV Ocean Resea rcher I during 1990-1991.Bathymetric data were determined with a Simard EK 500 Sonar.High frequency (3.5KHz) transects were collected by using Ocean Researcher Equipment transducers.In total, about 3,000km lines were collected, of which three multichannel seismic profi les are also included (Figure 3).Ten selected 3.5KHz profiles were used to illustrate the characteristic sea floor topography in the shelf-slope transition region off northeastern Taiwan (Figure 4).The 3.5KHz profiling is an effective tool to reveal the topography and surface sediments of the sea floor (Kennett, 1982).

BATHYMETRY
The bathymetric maps around Taiwan commonly used include the maps of Boggs et al. (1979), Suppe (1984)  The sea floor of the continental shelf, shallower than 150m, is generally smooth and fiat.However, many small islands, ridges, channels or canyons interrupt the smoothness of the sea floor of the continental shelf.About 15,000 years before the present time, in the glacial period of Late Pleistocene, the sea level was about 140m below present sea level and caused the conti.nentalshelf to be subjected to subaerial erosion (Emery et al., 1971).When the shelf was submerged again, partly due to the melting of Quaternary glacier, some relict features such as river valleys or rocky banks were preserved.However, these relict geomorphologic features do not seriously change the mean relief of the shelf.For instance, profile 1 shows the typical flatness of the shelf morphology (Figure 5A), the relict features such as chann els and ridges are shown on the profile 2 (Figure 5B).We realize that high resolution seismic profiles can better demonstrate these relict features than the 3.5KHz profi les do.The average gradient of the East China Sea shelf is about 0.07.

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Maximum relief is found on the continental slope.The water depth to the top of the slope •ranges from 150 to 300m, and the depth to the toe of the slope ranges from 1,800 to 2,200m.The average angle of the slope is between 2 and 5 with local slopes of greater than 30 as indicated by the 3.5KHz echo records.
The bathymetric contours between 200 and 1,200m are closely spaced which illustrate that the upper slope is steeper than the lower slope (Figure 4).The bathymetric chart also indicates indentation of bathymetric contours in the upslope areas.These convex contours suggest the presence of submarine canyons at the upper slopes as reported by Wang and Hilde (1973) and Boggs et al. (1979).
The bathymetric chart indicates that the floor of the Southern Okinawa Trough is con fined to below the 2,000m isobath.The bathymetric contours shallower than 2000m converge landward at the Ilan Plain.Bathymetric contours indicate that the shelf areas off Ilan Plain are relatively wide as compared to the rest of the shelves off the eastern coast of Taiwan to the south which are relatively narrow (2km) and linear.

PHYSIOGRAPHY
Three major physiographic units are recognized in the offshore region of northeast Taiwan: East China Sea shelf, East China Sea slope and basin floor of the Southern Okinawa Trough.In addition, two less prominent topographic features are distinguished: the Ilan continental shelf and the Ilan Ridge (Figure 4).

East China Sea Continental Shelf
Structurally, the East China Sea continental shelf is the seaward continuation of the extensive coastal plain along the coastal provinces of southeast China.Geomorphologically, the East China Sea shelf (200km) is relatively wide as compared to the continental shelves around the world with an average width of around 70-80km (Boillot, 1981).At the shelf edge (shelfbreak), there is a marked increase in gradient (Vanney and Stanley, 1983;Bouma, 1990).The depth of the East China Sea shelf edge around northeastern Taiwan is delineated by the 200m isobath (Boggs et al., 1979).This is an approximation for the sake of convenient separation between the shelf and slope.Our study indicates that the depth of the shelf edge off northeast Taiwan has great lateral variations and ranges from 140 to 220m deep.For in stance, profile 7 shows that there is a marked increase in gradient at the depth of about 140m along the outer margin of the East China Sea shelf (Figure 6).At profi le 10 the shelf is  terraced (Figure 7) with the inner shelf of the East China Sea shelf shallower than 150m and the outer shelf about 220m in depth.However, the depth of shelf edge mainly is around 150m which may refl ect the paleo-shoreline during the Late Pleistocene about 15,000 years ago.Note that the shelf-slope transition region around 200m is crooked which apparently reflects the indentation of the submarine canyons (Figure 4).The crooked topography of the shelf-slope zone may affect the water mass exchange between the Kuroshio Current and the East China Sea.It has been recognized that a cold water mass of upwelling is located adjacent to the Menhuahsu island and the head of one of the canyons (KEEP & WOCE Conference, Shilo, Taiwan, 1992).The inner shelf is confined to depths shallower than 150m, but the outer shelf shows a marked change of slope angle at the depth about 220m at the outer margin of the shelf.Notice that the continental slope is very steep, and the slope angle is greater than 30.Location of the profiles is shown in Figure 4.
Several small islands, Pengchiahsu, Menhuahsu, Huapinghsu and Keelung Tao, occur on the broad and flat East China Sea shelf (Figure 4).These islets are about 30 to 60 km northeast of Ta iwan and each have areas of fewer than several square kilometers.They are approximately 50 to 180 meters above sea level, and are mainly composed of andesites with tuff and agglomerates (Lin and Chou, 1974).They are Pleistocene volcanic islands and belong to the Tatung andesitic volcanic group of northern Taiwan (Yen, 1971).

East China Sea Continental Slope
The East China Sea continental slope begins at the shelf edge and descends downslope towards the floor of the Southern Okinawa Trough at the 2000m isobath.Structurally, the East China Sea slope is the northern flank of a young back-arc basin (Okinawa Trough).Tue continental slope, therefore, terminates at the axis of basin center without the presence of continental rise, abyssal plain or trench.Bathymetric profiles across the East China Sea slope vary greatly in terms of slope gradient and topography.Some sections of the continental slopes have gradients exceeding 30 (Figure 7).This very steep feature could be related to faulting at the shelibreak.Other sections of the continental slopes are gently inclined and have average gradients less than 4 degrees (Figure 8).Wang and Hilde (1973) reported that  Location of the profiles is shown in Figure 4.
the East China Sea continental slope defining the northern side of the Southern Okinawa Trough has an average gradient of 1/100 to 1/50.Our observations of the slopes in the study areas seem to be compatible with their finding.In addition, Chou (1987) also reported that the East China Sea slope is characterized by an average angle of greater than 10 degrees.
The profiles studied indicate that the continental slope has both smooth sloping surface and relatively irregular topography.For instance, profiles 4 and 5 both show relatively smooth surfaces and underneath the smooth sea floor may be the turbidites or debris flows with stratifications (Figure 8).On the other hands, profiles at different localities are characterized by slumping scars and channels (Figure 9).The relationships between the bedforms, sediment properties, near-bottom sedimentary processes and microtopography of the sea floor and certain specific 3.SKHz echoes reflected from the sea floor are well documented (Damuth, 1980;Klaus and Ledbetter, 1988;Laine et al., 1986;McClennen, 1989 andPratson andLaine, 1989).Based on the comparison between the 3.5KHz echograms on the East China Sea slope and that of the published studies , the sedimentary/topographic features such as the turbidites (Figure 8) and sliding scars (Figure 9) were determined.The slope topography has  The submarine canyon is another prominent topographic feature occurring on the East China Sea slope (Figure 10).Profile 8, trending approximately in a EW direction across the submarine caynon shows that the uppercanyon part is characterized by a V shape with relatively steep slope extending from walls of two sides.The maximum width of the canyon is around 6km.The canyon floor is about 500 meters deep (Figure lOA).In a short distance downcanyon, profile 9 across the canyon shows the similar cross-sectional morphology to that of profile 8 but the canyon becomes wider (1 Okm) and deeper ( 600m) as shown in Figure lOB.The cross-sectional morphology and dimensions of the canyon surveyed in the study area are similar to those found in the slope areas off southwestern Taiwan ( Yu and Wen, 1991).

Southern Okinawa Trough
The Okinawa Trough is a back-arc basin located behind the Ryukyu Trench and the Ryukyu Island Arc (Lee et al., 1980)  The western part of the Southern Okinawa Trough is surrounded by the East China Sea slope to the north, the Han Plain continental shelf to the west, the Ilan plain continental slope to the southwest and the slope off the Ryukyu volcanic island of YonagWlisima to the south (Figure 4).

Ilan continental shelf
The continental shelf off the northeastern corner of Taiwan is a relatively narr ow plat form which extends from the protrusion of about 122°E and 25°N southward and ends up at the coastal city, Suao (Figure 4).This shelf is named here as the llan continental shelf after the llan Plain.The shelf is fed by sediments from Taiwan via the Lan-Yan Hsi River.Apparently, the Ilan Plain continental shelf is the natural continuation of the Ilan Plain under the sea as revealed by the geologic map of Taiwan (1986).The width of the llan shelf varies from about 2km off the northeastern protrusion to more than lOkm east of the lan-Yan Hsi and becomes narrow again towards the city of Suao.The fan-shaped shelf areas off the Lan-Yan Hsi is called Lan-Yan Fan by Chen (1991).Sediments derived from the Lan-Yan Hsi may prograde outward and accumulate on the shelf in a lobate form (see Hong, et al., this volume).Kueishantao is a prominent volcanic island that appears on the Ilan Plain shelf.
The island is located at 121 °55 1 E, 24 °53 1 N and has an area of about 2.7 square kilometers.Radiometric dating indicates that the volcanic island formed about 200,000 years ago as re ported by the 1990 yearbook of the Central Geologic Survey of Ta iwan.Kueishantao island consists mainly of andesite and is considered the western extension of the Ryukyu Arc.

Ilan Ridge
The Ilan Ridge is a long and narrow ridge extending seaward from the llan continental shelf.The Ilan ridge extends seaward and terminates at the Yonagunisima island of the Ryukyu island chain.The Ilan Ridge is relatively small in areal extent as compared to the Ryukyu island chain.
The word RIDGE has several meanings, and one of them is defined as a long, narrow elevation often separating ocean basins (Bouma, 1990).Morphologically, the Ilan Ridge is an elevation that rises steeply from the Okinawa Trough to the north and the Nanao basin to the south.Tectonically, the ridge is a part of the Ryukyu volcanic islands and separates the backarc ba sin of the Okinawa Trough from the forearc basin of the Nanao basin to the south.Therefore, we assign the term of ridge to the slope areas between the Ilan shelf and Ryukyu islands because it fits the definition as suggested by Bouma (1990).

DISCUSSION
The naming of these physiographic units is provisional and may be revised if better names are proposed.We are fully aware of the guidelines for naming undersea features recommended by such organizations as the Intergovernmental Oceanographic Commission (IOC), the International Hydrographic Organization (IHO) and the joint IOC/IHO Guiding Committee for the General Bathymetric Chart of the Ocean (GEBCO), as cautioned by Bouma (1990).

CONCLUSIONS
The continental margin off northeastern Taiwan can be divided into five physiographic units mainly by bathymetry and topography and secondarily by structure and tectonic setting.
These physiographic features from north to south are the East China Sea continental shelf, the East China Sea continental slope, the Southern Okinawa Trough, the Ilan continental shelf and the Ilan Ridge.This physiographic framework is provisional and may be revised if better names are proposed.
The indentation of the submarine canyons onto the shelf and the protrusion of the Ilan shelf onto the slope have modifi ed the linear shelf-slope transition region into an irregular shape.The shelf-slope zone is also characterized by great lateral variations in gradient and topographic characters.The relationships between the topography and hydrodynamics in the shelf-slope region off northeastern Taiwan are worthy of investigation.

Fig. 1 .
Fig. 1.Map showing the areal extent of the East China Sea which is characterized by relatively shallow (100m) and wide (400 to 500m) continental shelf.Note that the Ryukyu volcanic islands serve as the boundary to the east.Dashed line is the shelf break at approximately 200 meters depth.

Fig. 3 .
Fig. 3. Location of echo-sounding tracks and high frequency (3.5KHz) subbottom profi les.The study area is restricted at the north of 26°N, at east of 123 °E and at the south of 24°N.

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Chinese Navy and the Institute of Oceanography, National Taiwan University (IONTU).There are discrepancies between these maps and charts.In order to provide an update and accurate bathymetric map covering the KEEP study areas off northeastern Taiwan, G.S. Song from IONTU has carried out a three-year plan to survey the KEEP study areas sponsored by the National Science Council of Taiwan.Song (1 992) has presented a preliminary result of the topographic survey off northeastern Taiwan.He expects a standard bathymetric map for the KEEP study to be ready by 1994.We would not duplicate Song's work , and hence used the bathymetric map of Marssett et al.(1987)  in this paper (Figure4).The bathymetric map in the study area mainly covers the East China Sea shelf and slope and the western part of the Southern Okinawa Trough; the water depths range from approximately 100 to 2,200m below sea level.The bathymetry shows a regional trend in an NEE direction.Apparently, the water depth increases seaward in a SSE direction..3,No.3, Sep.1992

Fig. 5 .
Fig. 5. Profile 1 showing the typically smooth and fl at nature of the East China Sea shelf (A).However, the relict features such as channels and ridges resulting from the late Pleistocene glaciation on the shelf are shown by profile 2 (B).Location of the profiles is shown in Figure 4.
Fig. 6.A dip line (profile 7) across the shelf onto the slope showing that there is a marked increase of slope angle at the depth of about 140m at the outer margin of the East China Sea shelf.Location of the profiles is shown in .Figure 4.

Fig. 8 .
Fig. 8. Profiles 4 and 5 showing that the gently inclined slopes which are charac terized by slope angles smaller than 4 degrees.Notice that the slopes may result from the accumulation of the turbidites and/or debris fl ow deposits.Location of the profiles is shown in Figure 4.

Fig. 9 .
Fig. 9. Profile 6 across the slope showing the typical topographic expressions such as slide scars and channels.Location of the profiles is shown inFigure 4.

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resulted from the combining effect of the progradational and/or erosional processes, sediment input, slope stability and faulting(Pickering et al., 1989).
Fig. 10.Cross-sectional morphology of the segments of uppercanyon which is characterized by V-shaped configuration and high and steep walls.Loca tion of the profile is shown in Figure 4.