Morphological and Seismic Characteristics of the North Mien�Hua Submarine Canyon off Northeastern Taiwan

The North Mien-Hua Canyon on the East China Sea Slope off north­ eastern Taiwan is a multi-headed canyon. Four distinct heads immediately below the shelf edge coalesce to form a single canyon near the 900 m isobath in the lower slope region. Morphologically, the North Mien-Hua Canyon can be divided into two distinct units: the upcanyon segment which is the fan-shaped sloping region extensively cut by these four tributary canyons , and the downcanyon section which is short and straight with no tributar­ ies. These four tributary canyons are characterized by steep-sided V-shaped troughs with high reliefs in excess of 500 m, suggesting intense down-cut· ting of the canyon. The single canyon course on the lower slope still main­ tains a V-shape in cross-section but with a curved and irregular canyon bottom, implying partial sediment infilling. The seismic profiles show ter­ mination of parallel reflectors of slope sediments against the canyon walls and slumping features, providing evidence of downslope submarine exca· vation along the canyon course. Cut-and-fill features are only recognized in the downcanyon section. Morphological and seismic characteristics indi· cate that the upcanyon segment is dominated by erosion and the downcanyon section is represented by multiple episodes of canyon erosion and deposi­ tion. The North Mien-Hua Canyon began as submarine incisions or gullies on the upper p art of the East Chin a Sea Slope. Downslope excavation deep­ ened and widened the gullies into tributary canyons which in turn flowed downslope and joined together to form a single canyon. The seismic data provide no evidence of faults along or across the canyon course, suggesting the origin of the canyon might not be fault-related.

The sea floor off northeastern Taiwan consists of three major physiographic provinces: the East China Sea Shelf, the East China Sea Slope and the Okinawa Trough. Near Taiwan, the East China Sea Slope is a gentle slope with an average angle of 1.3 degrees. The upper bound ary of the slope is placed at the shelfbreak, which is about 120 m deep, and the base of the slope is at a depth which ranges from 1700 to 2000 m near the bottom of the Okinawa Trough (Yu et al., 1997). Near Taiwan the East China Sea Slope is extensively cut by two major submarine canyons and many gullies which incise intercanyon slopes producing irregular to pographic features (Figure 1).
Submarine canyons on the East China Sea Slope off northeastern Taiwan have been dis covered but not fully investigated. Boggs et al. (1979) suggested that these unnamed subma rine canyons were initiated by subaerial erosion during the low stand of sea level in the Late Pleistocene and were later submerged, due to Quaternary transgression which preserved them. The naming of sea valleys and submarine canyons off northeastern Taiwan was discussed by Yu (1992,1993) and Song and Chang (1993). Yu and Hong (1993) described the morphology of the Mien-Hua Canyon which is characterized by a channeVcanyon system. The North Mien Hua Canyon is located northeast of the Mien-Hua Canyon on the East China Sea Slope ( Figure   25N . The purpose of this paper is to describe the morphology of the North Mien-Hua Canyon in detail and the relationship between the morphology and canyon-forming processes. It also discusses a possible origin of the canyon using bathymetric data and multi-channel seismic reflection profiles.

2.DATA
Bathymetric data and multi-channel seismic reflection profiles in the areas covering the North Mien-Hua Canyon were acquired on board RN Ocean Researcher I. Bathymetric data (about 450 km tracklines) were recorded by a Simrad EK 500 Sonar. Newly collected bathy metric data were integrated into the bathymetric data bank compiled by the Institute of Ocean ography, National Taiwan University ( Figure 2). The bathymetric chart showing the North Mien-Hua Canyon is then generated from the bathymetric data bank using the GMT system (Wessel and Smith, 1991). An air-gun array is the seismic source. The DFS-V floating gain digital system on board RN Ocean Researcher I is the recording device. Field seismic data are processed using the SIOSEIS system at the Institute of Oceanography, National Taiwan    3. MORPHOLOGY

Shape
The North Mien-Hua Canyon consists of four discrete tributary canyons on the upper slope of the East China Sea Slope and a single straight canyon on the lower slope ( Figure 3). These four tributaries in the headward region are tentatively named from west to east as can yons W, X, Y and Z, respectively. In plan view, the North Mien-Hua Canyon displays a fan shaped or triangular erosional feature. The apex of this triangle is located at approximately 122°50' E and 25° 30' N where the water depth of the canyon floor is about 1400 m ( Figure 3). Clearly, the areal extent of the East China Sea Slope affected by canyon erosion decreases from the heads to the canyon mouth. The area occupied by the four tributary canyons at the upper slope is about 1,200 square km. In contrast, the single straight main canyon at the lower slope occupies about 120 square km. Note that the cross-sectional span opened up the four tributary canyons is about 60 km and that this narrows to a 7 km-wide cut for the single canyon. Morphologically, the North Mien-Hua Canyon can be divided into two distinct units: an upcanyon section, a fan-shaped sloping region extensively cut by four tributary canyons, and a downcanyon section, a straight and deeply-cut V-shaped trough with steep walls in the lower slope, which merges on the upper slope with four discrete tributary canyons.

Cross-sectional Forms
As shown in Figure 4, six bathymetric profiles across the North Mien-Hua Canyon reveal steep-sided V-shaped troughs with varying incision depth which are a typical cross-sectional morphology for submarine canyons (Shepard and Dill, 1966;Shepard, 1973;Bouma, 1990).
Profile A near the shelf edge shows that the sea floor is a smooth and flat surface, suggest ing no down-cutting features. Profile B, which cuts across canyon W , shows that the head of canyon Wis a shallow V-shaped notch. Profile C, located further downslope, transverses the upper slope and shows three distinct narrow V-shaped troughs with steep sides. These three troughs are designated as canyons W, X and Y, respectively. Canyons X and Y have widths ranging from 4 to 5 km and reliefs from about 360 to 420 m, reflecting relatively intense erosion. Profile G is a short section trending NW-SE across the head of tributary canyon Z which is characterized by a steep-sided trough with relief in excess of 400 meters. Further downslope, Profile D shows four discrete tributary canyons within a distance of about 35 km. Note that these four tributary canyons are characteristic V-shaped cross-sections with steep walls.
Tributary canyons W, X, Y and Z merge together to form a single steep-sided canyon at a water depth of about 800 ma t the canyon edge (Profile E). This canyon segment is about 7 km wide with a bottom depth of about 1400 m and a relief in excess of 600 m. Note that the bottom of this canyon segment is relatively irregular, implying erosional/depositional features. In con trast, Profile F shows a single U-shaped trough near the 1000 m isobath. It is noted that the canyon floor is relatively flat, probably resulting from sediments filling up the deepest part of the canyon axis. The main course of the North Mien-Hua Canyon continues to extend south eastward to the deep basin of the Okinawa Trough.

Dimensions
Depth, width and relief measurements for the North Mien-Hua Canyon are described here. The width of the head of Canyon Wis about 2.7 km and the depth of the canyon floor is around 616 m. As shown on profiles B, C and D, in a downcanyon direction, Canyon W increases its depth from 616 to 1310 m and its width from 2.7 to 4.2 km. The reliefs of Canyon W alorig its course range from 337 to 518 m. The slope angles of both walls of Canyon W range from about 5.5 to 20.l degrees, showing typical steep canyon walls.  In general, these four tributary canyons W, X, Y and Z increase their width and deepen their canyon floor in a downcanyon direction and the steep canyon walls maintain slope angles of between 10 and 15 degrees. The straight main canyon course is about 6.5 km wide and 17 km long and has steep walls angled at around 10 degrees.

Longitudinal Profile and Relief
As shown in Figure 5,  The main course of the North Mien-Hua Canyon, with an average slope angle of 0.12 degrees, begins at a water depth of about 1463 m and extends southeast for about 17 km where the canyon discharges on the basin floor of the southern Okinawa Trough. In general, the floors of the North Mien-Hua Canyon, including the four tributaries, show gentle continuous slopes which are typical of continental slope canyons (Bouma, 1990).
Reliefs of the North Mien-Hua Canyon increase along its courses, from the heads of its tributaries to the mouth where it ends. For instance, the relief of tributary canyon W increases from 337 min the head portion to 580 mat the canyon mouth. The increasing incision depth along the canyon course suggests that down-cutting into slope sediments predominated in the downcanyon section, although the gradients (about 1.8 degrees) of canyon thalwegs in the upcanyon region are an order greater than that (0.12 degrees) in the downcanyon segment. It seems that the gradient of the canyon axis is not directly related to canyon erosion of the main canyon course. Alternatively, the energy level of downslope-eroding sediment flows is prob ably an important factor in the deepening and widening of the North Mien-Hua Canyon.
Morphological measurements suggest that the North Mien-Hua Canyon is a relatively small canyon in terms of width (2 to 7 km) and length (about 80 km). Bering Sea canyons (100 km wide, 800 km long) are the world's largest canyons Karl, 1984, 1988). Mor phological comparison of canyons of the North Mien-Hua Canyon and the Bering Sea indi cates that the North Mien-Hua Canyon is indeed a very small canyon. The V-shaped troughs, steep canyon walls with slope angles of between 10 and 15 degrees and great reliefs ranging from about 300 to 600 m together indicate intense incision of slope sediments. Therefore, erosion of down-cutting predominated on the East China Sea Slope near northeastern Taiwan, resulting in the formation of the North Mien-Hua Canyon.

SEISMIC CHARACTERISTICS AND SEDIMENTARY PROCESSES
Six seismic reflection profiles trending NE-SW across the course of the North Mien-Hua Canyon are shown to display seismic characteristics and to infer sedimentary prncesses form ing the canyon (Figures 6-13). The locations of these seismic sections are in Figure 2.
Profile A, located immediately north of the canyon heads, shows that the sea floor is represented by a strong and relatively flat reflector, suggesting no down-cutting features (Fig  ure 6). However, some slightly inclined reflectors are truncated against the reflector of the sea floor to form stratigraphic pinchouts, suggesting a denudation effect in the Late Pleistocene period of glacially lowered sea level (Boggs et al., 1979). The southwest part of Profile B below the 200 m isobath (Figure 4) shows a small shallow trough characterized by a V-shaped cross-section and terminated parallel reflectors at the walls of the trough (Figure 7). This trough is considered to be the head of tributary canyon W. The northeast portion of Profile B on the shelf shows truncation of inclined reflectors without down-cutting troughs.
The upper slope immediately below a shelf edge is the place where slope failure com monly initiating canyon erosion and development (Twichell and Roberts, 1982;Farre et al., 1983). Therefore, it is reasonably to expect the head of tributary canyon W to begin at such a place just below shelfbreak. Profile C, farther downslope, shows three distinct V-shaped troughs which are designated as tributary canyons W, X and Y, respectively (Figure 8). Thicker se- Later, these sediments could be carried downcanyon by strong sediment flows or turbidity currents along the canyon axis, and repeated deposition and erosion could result in the irregu lar bedform on the canyon floor.
Tributary canyons W, X, Y and Z merge into a single canyon as shown on profile E (Figure 11). Truncation of reflectors of slope sediments on the canyon walls is indicative of down-cutting deep into the underlying strata. Profile E reveals evidence for probable multiple episodes of canyon erosion and deposition. The close-up view of the canyon axis illustrates that multiple cut and fill features probably occur on the bottom of the North Mien-Hua Canyon ( Figure 12). The cut and fill feature is a common erosional/depositonal event throughout the evolution of a submarine canyon (e.g., McGregor, 1981, for Miocene Wilmington Canyon;Liu et al., 1993, for modem Kaoping Canyon). Further downcanyon, profile Falso shows truncation of sediment layers against the canyon walls ( Figure 13) and probably a cut and fill feature on the bottom of the canyon (Figure 14).
Seismic reflection profiles provide no evidence of fault zones along or across the courses of the North Mien-Hua Canyon. Therefore, the origin of the North Mien-Hua Canyon is prob ably not related to strike-slip motion, as suggested by Hsu et al. (1996). Seismic data provide  evidence that submarine erosion of down-cutting is the major factor in the formation of the North Mien-Hua Canyon with multiple episodes of cut and fill. On the basis of morphological characteristics and seismic interpretations, the North Mien-Hua Canyon began as submarine incisions or gullies on the upper part of the East China Sea Slope. Upslope instability, which was probably caused by over-steepness of the slope (Twichell andRoberts, 1982, Farre et al., 1983) and inferred tectonic activities of back-arc rifting (Lee et al., 1980, Sibuet et al., 1987, initiated the incisions or gullies immediately below the shelf edge. Later, downslope excava tion deepened and widened the gullies into tributary canyons which in tum flowed downslope and joined together to form a single canyon.

CONCLUSIONS
Bathymetric data show that the North Mien-Hua Canyon is a multi-headed canyon which developed on the East China Sea Slope near Taiwan. This canyon consists of four tributary canyons which join together to form a single canyon near the 900 m isobath in lower slope region. The North Mien-Hua Ca n yon is characterized by a fan-shaped erosional feature. In cross-section, the canyon shows V-shaped troughs and steep-sided walls with varying incision depths.
Seismic profiles show submarine excavation along the canyon walls that is considered to be the major factor in the formation of North Mien-Hua Canyon. Seismic data provide no evidence of the presence of faults associated with the North Mien-Hua Canyon, suggesting the origin of the canyon might not be fault-related.