Morphology and Canyon Forming Processes of Upper Reach of the Penghu Submarine Canyon off Southwestern Taiwan

The main course of the Penghu Submarine Canyon is located along the intersection of bottoms of the Kaoping Slope and the South China Sea Slope in a nearly north-south direction. The Penghu Canyon can be considered a single head canyon with three major tributary canyons joining into the main course to form the fan-shaped upper reach of the canyon. The upper reach begins near the shelf edge of the Taiwan Strait Shelf and extends about 150 km southwards to a water depth about 2200 m at the lower slope where no tributary canyons are present. The upp�r reach of the Penghu Canyon shows high relief, steep walls and V-shaped cross sections, showing typical canyon morphology. Characteristics of seismic profiles suggest that the formation of the upper reach of the Penghu Canyon is mainly attributed to foreland basin sedimentation and accompanying incision of the syn-depositional orogenic sediments of the basin. Orogenic sediments derived from Taiwan progres­ sively onlap westward and bury the Chinese passive margin and deposit in the bottom of the foreland basin. The main course of the Penghu Canyon has resulted mainly from excavating the orogneic sediments along the axis, tilting southward, of the deep foreland basin. Slumping and sliding and downward excavation into the sea floors by downslope sediment flows are the major forming processes in the canyon head and upper canyon part. Diapiric intrusion becomes important in the formation of the lower parts of the upper reach of the Penghu Canyon. (


Tectonic Setting
The island of Taiwan is located at the junction between the Ryukyu and the Luzon Arcs in 1 Institute of Oceanography, National Taiwan University, Taipei, Taiwan, ROG *Corresponding author address: Prof. Ho-Shing Yu, Institute of Oceanography, National Taiwan University, P. 0. Box 23-13, Taipei, Taiwan, 106; E-mail: yuhs@ccms.ntu.edu.tw the.northwestern Pacific (Fig. 1). About five millions years ago, the oblique collision between the Chinese margin and the Luzon Arc formed the Taiwan Island (Suppe 198 1;Ho 1988) and an accompanying foreland basin west of the Taiwan orogen (Covey 1984  southwestern Taiwan, including offshore areas, is an immature foreland basin receiving oro genic sediments from the rising Taiwan orogen (Covey 1984;Yu 1993;Brusset et al. 1999).
The seaward progradation of sediments from the coastal plain of southwest Taiwan has formed the Kaoping Shelf with a regional trend in a northwest to southeast direction. The regional trend of the southwest Taiwan margin is mainly in response to the southward propagation of arc-continent collision in the Taiwan region (Yu and Chiang 1997). Pliocene-Quaternary sedi ments underlying the Kaoping Shelf and Kaoping Slope are deformed into a series west-vergent folds and thrusts (Liu et al. 1997), representing an active margin.
West of the Kaoping Slope, rock sequences underlying the Chinese margin, including the South China Sea Slope (Yu and Song 2000), are mainly Cenozoic elastic sediments with a thickness more than 5,000 meters (Sun 1982(Sun , 1985. A widespread regional unconformity from the Middle Oligocene separates the Tertiary strata into Paleogene and Neogene sequences (Sun 1982). The Neogene sequences have resulted from regional thermal subsidence and depo sition of shallow marine sediments with relatively mild normal faulting, showing a typical passive margin.

Physiography
The sea floor off southwestern Taiwan is occupied by the narrow Kaoping Shelf and the relatively broad Kaoping Slope which extends to a depth of about 3,000 ma t the northern limit of the abyssal plain of the South China Sea (Fig. 2). West of the Kaoping Slope lies the South China Sea Slope which extends from the shelf edge of the South China Sea Shelf to the north ern abyssal plain of the South China Sea, and merges eastward into the Kaoping Slope. The sea floor between Southwest Taiwan and South China is characterized by two broad, deep water (3000 m) submarine slopes that are marked by convex bending of bathymetric contours.
The isobaths on the Kaoping Slope mainly trend northwest to southeast. In contrast, the bathy metric contours on the South China Sea Slope trend in NE-SW and are aligned to the regional structural trend of the southeastern Chinese margin. The physiographic boundary separating these two deep submarine slopes is coincident with the main course of the Penghu Submarine Canyon (Fig. 2).
Submarine canyons and gullies are the most prominent undersea features on the south western Taiwan margin and mainly occur on the upper slope. Some canyons are named after nearby towns, for example, the prominent Kaoping Canyon crossing the Kaoping Shelf and the Kaoping Slope (Fig. 2). Similarly, the upper part of the South Chian Sea Slope is trans versely dissected by unnamed submarine gullies and canyons, producing an irregular sea floor topography (Fig. 2). It is noticed that the courses of these canyons on both submarine slopes are more or less normal to shorelines, respectively, and can be classified as slope canyons (Shepard 1981). They represent short range linear down-hill mass transport to erode, transport and deposit mass on the continental slopes and upper slopes, in particular.

Previous Studies and Purpose
The Penghu Submarine Canyon was named by Lee (1992) after the Penghu Islands which lie about 100 km north of the Penghu Canyon in the Taiwan Strait. This canyon was consid-

Data
A marine survey of the upper slopes south of the Taiwan Strait Shelf was carried out to map the upper reach of the Penghu Canyon and its adjacent tributary canyons. Bathymetric profiles and four-channel reflection seismic sections in the study area were acquired during the cruise aboard RIV Ocean Researcher I during June, 2000 (Fig. 3). The bathymetric profiles were collected by using a Simrad EK 500 Sonar. Newly acquired bathymetric data were inte grated into the bathymetric databank at the Center for Ocean Research, National Taiwan University. Bathymetric data (one data point every 100 m) were then gridded and contoured using the GMT system (Wessel and Smith 1991) to generate a bathymetric chart covering the offshore areas southwest of Taiwan (Fig. 2). Nine seismic reflection profiles trending E-W across the upper reach of the Penghu Canyon were acquired. The total length of these seismic profiles is about 500 km. An air-gun array was deployed as seismic energy. The DFS-V float ing gain digital system was the recording device for the reflection seismic signals. Seismic reflection data were processed using the SIOSEIS system at the Institute of Oceanography, National Taiwan University.

Plan View
Examining the newly generated bathymetric chart (    (Fig. 4 ). This trough has a width of about 1.4 km and the relief between bottom and edges of about 54 meters and steep walls of slope ranging from 3 to 8 degrees. Farther south lies Profile C that crosses over the upper slopes of the Kaoping Slope and the South China Sea Slope. A V-shaped trough, as shown on Profile C, appears at the intersection of these two facing slopes. This trough is about 2.5 km wide and 423 m deep, with a relief of about 200 meters and steep walls of slope angles about 9 degrees. These two V-shaped troughs are aligned along bottoms of the slopes shown on Profiles B and C, respectively, and can be traced southward to the main course of the Penghu Canyon . Therefore, they are considered to be parts of the head of Penghu Canyon. The upper slope immediately below the shelf edge is the common place for inception of canyon incision by sediment failure (Twichell and Roberts 1982;Farre et al. 1983;Pratson and Coakley 1996). Then, the head of Penghu Canyon is suggested to be located at the intersection of the uppermost parts of the Kaoping Slope and the South China Sea Slope, as a small trough with a bottom depth around 246m, as shown on Profile B. In summary, plan view and cross-sectional form of the upper reach of the Penghu Canyon show a submarine valley with a V-shaped cross section, high and steep walls, winding course and numerous tributaries. It is a typical submarine canyon (Shepard and Dill 1966).

CANYON FORMING PROCESSES
Nine seismic reflection profiles trending E-W across the upper reach of the Penghu Can yon are examined to infer the tectonic influences on the canyon formation and to determine the sedimentary processes forming the canyon (Figs. 5 through 9). Locations of these seismic profiles are shown in Fig. 3.

Tectonic Influences
Seismic lines A through I across both the Southwest Taiwan and South China Sea mar gins showjuxtaposition of sedimentary sequences of quite different seismic characteristics, indicating differences in structures and stratigraphy of the South China passive margin to the west and the Southwest Taiwan active margin to the east, respectively. Seismic Line A (Fig.   5) shows that seismic characteristics can be distinguishable between these two margins. Sedi mentary sequences between sea floor and 400 ms time interval of the Southwest Taiwan mar gin are displayed by discontinuous, low-amplitude and sub-parallel reflectors. Below 300 ms, strata of the Southwest Taiwan margin are represented by discontinuous and hummocky cha otic reflectors. In contrast, the South China margin shows characteristic continuous, parallel and divergent reflectors tilting eastwards. In response to the southward propagation of arc continent collision along the strike of the Taiwan orogen contraction occurs between these two margins, some strata of the Southwest Taiwan margin are mildly deformed into anticlines, while those of the South China margin are deformed into thrust faults (Fig. 5). Alternatively,  (Chou 1999). The contact between the frontal strata of the Southwest Taiwan margin and the adjacent buried South China margin is probably a basal decollement, tilting upward and westwards (Fig. 5). Both seismic Lines B and C (Fig. 6) have seismic characteristics similar to that of Line A, except for the presence of V-shaped troughs on the sea floor (Fig. 6). rapidly, as shown on Line E (Fig. 7). We infer that orogenic sediments derived from Taiwan Farther down-canyon, seismic Lines H (Fig. 8) and I (Fig. 9)  southern Taiwan (Sinclair 1997;Chou 1999). It is noted that the frontal sediments are de formed into anticlines and diapiric structures (Fig. 9).

Sedimentary Processes
Seismic Line A (Fig. 5), across the Taiwan Strait Shelf and immediately north of the canyon head, shows no prominent V-shaped troughs and indicates no significant downward incisions. The presence of V-shaped troughs, as shown on Lines B and C (Fig. 6), suggests that 103 results in sediment failure which is a common process in the initiation and development of submarine canyons (Twichell and Roberts 1982;Farre et al. 1983;May et al. 1983). The upper slope segment between Lines B and C has a slope gradient of 2.26 degrees and is steeper than that at the down-canyon segment The head of Penghu Canyon, occurring at the steeper upper slope immediately below the shelf edge of the Taiwan Strait Shelf, is reasonably expected. The west wall of the Penghu Canyon, shown on Line C, shows stepped and curved surfaces that can have resulted from slumping or sliding. Slumping and sliding are volumetrically most important as erosive agents for canyon formation (May et al. 1983). The canyon head shown on Line C also shows slumping on canyon walls and sediment spillover that can widen the canyon. This part of canyon is deepened by excavation of canyon floor by down-canyon sedi ment flows. Farther down-canyon the main course of the Penghu Canyon becomes wider ( 4.3 km) and deeper (1420 m). We infer from previous work of Shepard (1981) that continued failures of over-steeping walls due to slumping or sliding combined with axial down-cutting would widen and deepen the course of the Penghu Canyon. It is noted that the seismic facies west and east of the Penghu Canyon is characterized by chaotic reflection pattern as shown on profile E (Fig.  7). As previously mentioned, orogenic sediments from Taiwan have characteristic chaotic seismic facies. This means that sedimentary materials removed by erosion along the canyon axis are mainly derived from Taiwan. Extensive downward incisions appear on the sea floors The bottom of the canyon shows a continuous gentle inclination and the gradients of the upcanyon segment (average 1.84 degrees) greater than that (average 1 degree) of the downcanyon course. This means that downslope sediment flows are the major eroding process cutting canyon floor along the regional gradient. Lithology or structural deformation of the slope sediments seem to have little effect on the axis gradient of the Penghu Canyon. Changes of relief between canyon bottom and canyon edges along the canyon course are expected to be in accordance with the canyon axial slope. As a matter of fact, relief along the canyon course shows considerable variations, although it increases progressively from the head to lower parts of the canyon (Fig. 10). The variations of canyon relief and the longitudinal profile suggest that canyon forming processes other than downslope sediment flows can be significant. The canyon head, with an incision depth of 54 m, increases to a maximum relief of 909 m at the lower parts of the course, and then abruptly decreases to 315 m farther down (Fig.10). Seismic evidence together with canyon relief along the course indicates that down�cutting dominates at the canyon head, lateral widening by slumping of canyon walls is prevalent at the upcanyon segment, and diapiric intrusion becomes an important agent for shaping the canyon fo rms at the lower slope section of the canyon. The maximum relief of 909 m indicates the greatest extrusion of the mud diapir from the adjacent sea floor (Fig. 10).

SUMMARY AND CONCLUSIONS
Penghu Canyon can be considered a single head canyon with tributary canyons from the Kaoping Slope to the east and the South China Sea Slope from the west joining into the main course to form the upper reach of the canyon. Penghu Canyon and its tributary canyons show high relief, steep walls and a V-shaped cross section, with typical canyon morphology.
Slumping and sliding and downward excavation into the sea floors by downslope sedi ment flows are the major forming processes in the canyon head and upper canyon part. Dia piric intrusion plays an important role in shaping the canyon form in the deeper and lower parts of the Penghu Canyon. Bathymetric ridges arise from sea floor du e to diapiric intrusion and the resulting steep flanks of the ridges become the canyon walls.
Orogenic sediments derived from Taiwan orogen progressively onlap westward and bury the Chinese passive margin and deposit in the bottom of the foreland basin. The main course of the Penghu Canyon has resulted mainly from down-slope sediment flows excavating the orogenic sediments along the axis of de ep marine basin in a nearly north-south direction.
In summary, sedi ment failure of the upper slope initiates the canyon head and determines the canyon course along the bottom of two opposing slopes and focus downslope sediment flows excavating canyon floor during the development of the Penghu Canyon. Downslope erosion continued to deepen and widen the gullies on the upper slopes into tributary canyons that in turn fl owed downslope and coalesced to form the upper reach of the Penghu Canyon characterized by a fan-shaped network.