Ho-Shing Yu

Records of submarine natural hazards off SW Taiwan

Abstract In the past few years, large earthquakes and torrential rain hit southern Taiwan and induced severe submarine hazards off the SW coast. Marine sediments (turbidites) provide valuable records with which to study and understand the formation of these submarine geo-hazards. The Pingtung Earthquake (two major events (ML=7.0) plus many aftershocks), on 26 December 2006, triggered turbidity currents that severed submarine cables in the Fangliao and Gaoping submarine canyons. This caused significant economic loss. In addition to earthquake activity, typhoons and torrential rains that induced flooding are also important mechanisms responsible for the formation of turbidites. On 8–9 August 2009 Typhoon Morakot brought heavy rains to southern Taiwan, causing serious landslides and flooding on land. The typhoon also caused submarine cable breaks in the Gaoping Canyon. All such events are likely to be recorded in the marine sediments of the canyon system, and by analysing these records we may be able to reconstruct the history of past earthquakes and floods in the region. Chirp sonar profiles, in conjunction with core analysis, including X-ray radiographs, grain size and 210Pb analysis, are used to identify the sources, transport and deposition of the turbidites (or hyperpycnite) and to reconstruct the history of earthquakes and flooding in the study area. Results indicate that these submarine hazards are not only related to earthquake and floods but that the unique geological and hydrological setting also plays an important role in the initiation of these submarine geo-hazards.

Modeling of thermo-rheological structure of lithosphere under the foreland basin and mountain belt of Taiwan

Abstract The system of Taiwan mountain belt and the West Taiwan foreland basin is a manifestation of the collision between eastern Asian continental margin and the Philippine volcanic arc. It provides an ideal place for the study of the flexure behavior of lithosphere under the mountain–basin system. The paper presents results of thermal and rheological modeling of the system along Profile B, which extends 200 km in a NW–SE direction from the Taiwan Strait across the foreland basin and mountain range to the Longitudinal Valley. Along the profile, the crustal structure is constrained by wide-angle seismic and gravity data as well as P-wave tomography, while the structure of the foreland basin is constrained by multichannel seismic and drilling. Assuming that a steady-state deformation has proceeded in Taiwan since the collision of the Luzon Arc with the SE Eurasian margin at 6.5 Ma, the present-day thermal and rheological structures of the lithosphere are modeled by finite element analysis. The base of lithosphere is assumed to be isothermal, with a temperature of 1300 °C. The interior heat source consists of radiogenic heating, frictional heating on thrust faults and basal decollement, and body heating by internal friction within thrust sheets. In addition to conduction, the heat convection is carried out by the subduction of the Eurasian margin, the exhumation and erosion in the mountain range, and the sedimentation in the foreland basin. Thermal parameters are carefully selected and updated based on available data on surface heat flow, downhole thermal gradient, and thermal conductivity from Taiwan and nearby regions. The mechanical parameters are determined based on regional geology. The resulting thermal model predicts well the elevated surface heat flow in the mountain range and the depressed surface heat flow in the thrust front. A high-temperature core of >500 °C appears in the bottom of thrust sheets and the uppermost upper crust. Sensitivity analysis indicates that this elevated temperature is mainly due to the frictional heating along thrust faults and basal decollement, and to the heat carried up by the exhumation of the deeper rocks and subsequent erosion. The depth-dependent strength envelopes were computed based on the thermal model and commonly used rheological parameters. Rheological stratifiction of the lithosphere along the profile is depicted. Although the estimation of the thickness of the rheological strata is affected by the uncertainty in lithosphere thickness, the modeling results indicate clearly that the rheological structure of the lithosphere under the mountain range is remarkably different from that in the foreland basin. While the foreland basin has a typical thermal and rheological structure of the rifted continental margin with three brittle layers above three ductile layers, the raised temperature under the mountain range has weakened the lithosphere dramatically. Under compressional stress, the lithosphere beneath the mountain range becomes ductile almost entirely, except a thin (about 6 km in the central part) brittle layer near the surface and perhaps a thin brittle layer in the uppermost mantle. Such a significant weakening of the lithosphere in the mountain range should not be overlooked while discussing the flexure behavior of the lithosphere in the area.

Development of the shale diapir-controlled Fangliao Canyon on the continental slope off southwestern Taiwan

Abstract Fangliao Canyon is one of several major canyons on the continental slope off southwestern Taiwan. This paper evaluates the canyon morphology and its formative processes and origin using multichannel seismic reflection profiles and bathymetric data. Fangliao Canyon is a small canyon around 10 km wide and 60 km long, an order of magnitude smaller than the large canyons of the world. This canyon can be divided into two morphologically contrasting parts: the upper canyon, a relatively straight part beginning at the shelf edge and ending approximately at the 600 m isobath, and the lower canyon, consisting of two segments separated by a rising linear ridge (shale diapir) and extending downslope to about the 1000 m isobath where its mouths lack submarine fans. Seismic profiles and bathymetric data provide evidence of submarine erosion forming the upper canyon and the uplift of a shale diapir controlling the formation of the lower canyon. In the upper canyon, truncation of parallel flat-lying strata and sliding/slumping features on the canyon walls are indicative of downcutting and lateral widening of the canyon. In the lower canyon, the shale diapir uplifted the slope strata and protruded through the overlying slope sediments, producing a ridge rising from the sea floor. Here the steep flanks of the shale diapir become the walls of the steep-sided canyons. The interaction of these sedimentary and tectonic processes on the continental slope off southwestern Taiwan forms the present Fangliao Canyon.

Kaoping shelf: morphology and tectonic significance

Abstract Thirty bathymetric profiles across the southwestern Taiwan margin reveal two distinct physiographic features: a shelf and a slope separated at Fangliao. The Kaoping Shelf northwest of Fangliao is the offshore extension of the Pingtung Valley, and the unnamed slope west of the Hengchun Peninsula is a part of the submerged southern Central Range. The Kaoping Shelf is a short, narrow and shallow shelf (100 km long. 20 km wide and 80 m deep). This shelf can be divided into two subshelves. The one to the north is terraced with an average width of 28 km and the other to the south is a very narrow (9 km) and shallow (40 m) platform. The average gradient (5 m/km) of the shelf is greater than that (2.5 m km) of the average shelf worldwide. The width of the shelf. ranging from 7 to 40 km, increases progressively from southeast to northwest and is a factor of 2 4 narrower than that of others in the world. The gradient and width reflect the youthful stage of development of the Kaoping Shelf. The seaward progradation of the sediments from the coastal plain of the Pingtung Valley resulted in the prograding Kaoping Shelf, as suggested by cored sediment samples and seismic profiles. The morphology of the Kaoping Shelf depends mainly on the tectonic setting of the uplifted Taiwan orogen and the accompanying foreland-basin sedimentation. This young (less than 400,000 years) shelf is still growing and prograding southward in a parallel direction with the southward propagating are continent collision in the Taiwan region.

An Under‐Filled Foreland Basin in the Northern South China Sea off Southwest Taiwan: Incipient Collision and Foreland Sedimentation

Folding-and-thrusting in southern Taiwan is accompanied by formation of a marine under-filled foreland basin flanking the orogen. The South China Sea north of 21°N is characterized by initial collision between the Luzon Arc and the Chinese margin, and by sediment flux both from Taiwan and China. The resultant foreland basin is bounded by the passive Chinese margin to the west and on the eastern flank by a westward-migrating orogenic wedge, shedding relatively large amounts of sediment into the basin. The basin seafloor is >3000 m in water depth. Sediments eroded from Taiwan are transported and overlap the toe of the South China Sea Slope. Longitudinal transport is the major process of sediment dispersal from orogenic highlands in southern Taiwan to the adjacent foreland basin. Sediments derived from Taiwan are syn-depositionally deformed into folds and thrust faults. The deformation fronts overlap onto the Chinese craton. Seismic characteristics and basin geometry infer a general sequential sediment filling of initiation, growth and near completion of the under-filled basin. The uplifted Taiwan orogen is the major sediment source.

Characteristics of the outer rise seaward of the Manila Trench and implications in Taiwan–Luzon convergent belt, South China Sea

The outer rise on the distal periphery of a subduction system is caused by emplacement of an accreted load onto the flexed oceanic lithosphere. By examining the bathymetry and free-air gravity anomaly data collected by satellite observations and marine reflection seismic data collected during the TAIGER project, we demonstrate the characteristics of the flexural outer rise seaward of the Manila Trench. The region of the outer rise on the westernmost periphery of the Manila subduction system is characterized by the positive free-air gravity anomaly seaward parallel to the Manila Trench and the morphological rise at the south of the Manila subduction system. A flexure simulation is performed based on the flexural profiles along the southern Manila Trench-outer system and the resulting effective elastic thickness values may provide an alternative aspect for the spreading rates of the South China Sea basin. Since both the western periphery of the Taiwan collision belt and Manila subduction belt are dominated by the strain regime of extension of flexural origin, it appears that the strain regime of flexural extension associated with the flexural forebulge of the Western Taiwan Foreland Basin to the north, and the strain regime of flexural extension associated with the outer rise seaward of the Manila Trench to the south are meridionally interconnected. This revised understanding of the strain regime of flexural extension origin west of the Taiwan–Luzon convergent belt provides an alternative point of view on the strain regime offshore SW Taiwan.

Seismic characteristics, morphology and formation of the ponded Fangliao Fan off southwestern Taiwan, northern South China Sea

Using bathymetry and reflection seismic profiles this study reveals the nature of the modern ponded Fangliao Fan within a framework of sediment infilling of an intra-slope basin on a tectonically active margin off southwestern Taiwan. The Fangliao Fan begins at the mouth of Fangliao Canyon at a water depth of 900 m and terminates down-slope at the escarpment of a linear ridge north of the Kaoping Slope Valley at a water depth of about 1,100 m, sediment gravity flows being prevented from farther down-slope transport due to ponding against this bathymetric high. The fan appears as a distinct basinward-opening triangular depocenter confined by ridges on both sides and the NW–SE trending ridge aligned normal to the elongation of the fan. These topographic ridges were formed by mud-diapiric intrusions. The external form of the ponded Fangliao Fan is characterized by a fan-valley fill pattern that has a concave cross-sectional morphology, in contrast to typical mounded fans deposited on slope-basin plains having a smooth topography. Sediment episodically funneled through the Fangliao Canyon from upslope areas and derived from the flanks of the mud-diapiric ridges are mainly transported by mass movement before being re-dispersed by unconfined channels to infill the intra-slope basin, thereby building up channelized fan complexes with poorly developed levees. The sediment flows from the mouth of Fangliao Canyon flow down-slope along the west flank of the Fangliao Ridge. In the process, a feeder channel has been eroded into the seafloor along which sediment is transported to the distal parts of the fan. Sediment west of the feeder channel is mainly redistributed by mass movement and/or fan channels to fill up the irregular topographic low in the slope. Due to a very low sediment supply, Fangliao Fan represents a starved ponded slope fan. As such it provides insights into the processes by which ponded fans develop and can therefore serve as an analog for similar fans developed on topographically complex slopes elsewhere. The morpho-structural features of the Fangliao Fan resulted from the interplay between sediment supply, uplift of the mud-diapiric ridge, mass movements, and alternating incision and deposition.

Geological Characteristics and Distribution of Submarine Physiographic Features in the Taiwan Region

The sea floor topography around Taiwan is characterized by the asymmetry of its shallow and flat shelves to the west and markedly deep troughs and basins to the south and east. Tectonics and sedimentation are major controls in forming the submarine physiographic features around Taiwan. Three Pliocene-Quaternary shelves are distributed north and west of Taiwan: East China Sea Shelf (passive margin shelf), the Taiwan Strait Shelf (foreland shelf), and Kaoping Shelf (island shelf) from north to south parallel to the strike of Taiwan orogen. Off northeastern Taiwan major morpho/tectonic features associated with plate subduction include E-W trending Ryukyu Trench, Yaeyama accretionary wedge, forearc basins, the Ryukyu Arcs, and the backarc basin of southern Okinawa Trough. Off eastern Taiwan lies the deep Huatung Basin on the Philippine Sea plate with a relatively flat floor, although several large submarine canyons are eroding and crossing the basin floor. Off southeastern Taiwan, the forearc region of the Luzon Arc has been deformed into five alternating N-S trending ridges and troughs during initial arc-continent collision. Among them, the submarine Hengchun Ridge is the seaward continuation of the Hengchun peninsula in southern Taiwan. Off southwestern Taiwan, the broad Kaoping Slope is the major submarine topographic feature with several noticeable submarine canyons. The Penghu Canyon separates this slope from the South China Sea Slope to the west and merges southwards into the Manila Trench in the northern South China Sea. Although most of sea floors of the Taiwan Strait are shallower than 60 m in water depth, there are three noticeable bathymetric lows and two highs in the Taiwan Strait. There exists a close relationship between hydrography and topography in the Taiwan Strait. The circulation of currents in the Taiwan Strait is strongly influenced by seasonal monsoon and semidiurnal tides. The Penghu Channel-Yunchang Ridge can be considered a modern tidal depositional system. The Taiwan Strait shelf has two phases of development. The early phase of the rift margin has developed during Paleoocene-Miocene and it has evolved to the foreland basin in Pliocene-Quaternary time. The present shelf morphology results mainly from combined effects of foreland subsidence and modern sedimentation overprinting that of the Late Pleistocene glaciation about 15,000 years ago.

Sedimentation in remnant ocean basin off SW Taiwan with implication for closing northeastern South China Sea

Taiwan is one of the most cited examples of modern mountain building in an oblique arc–continent collision. However, the recognition and significance of the Taiwan remnant ocean basin in the final stages of the collision of Luzon with the Chinese margin have long been ignored. This paper emphasizes the Taiwan remnant ocean basin as the main sink for sediments from the uplifted Taiwan suture zone and as a record of clues to the suturing history of the Taiwan collision zone. A part of the palaeo South China Sea has been closed by oblique collision of the Luzon Arc with the Chinese margin in the Taiwan region since about 4 Ma. As collision continued southward along the Taiwan–Luzon convergent zone the remaining South China Sea off SW Taiwan became the Taiwan remnant ocean basin. Beginning about 1.6 Ma much sediment from the longitudinal drainage of the Taiwan orogen was fed into this remaining basin and trapped in the intra-slope basins of the Kaoping slope, with some sediment bypassing via the Kaoping Canyon to the lower reach of the Penghu Canyon. This basin is characteristic of an axial canyon–deep-sea channel–oceanic trench system without significant sediment accumulation in submarine fans. Excess sediments from the Penghu Canyon mainly spill over out of the closing basin to the South China Sea and Manila Trench via the deep-sea Penghu Channel. The Taiwan–Luzon convergent zone continues along the NE South China Sea where the Taiwan remnant ocean basin exists. The implication of the convergence is that the Taiwan remnant ocean basin will keep shrinking in the next few million years southward to the northern end of the Manila Trench at around 20°N, where collision is replaced by subduction. The remnant ocean basin would be closed and deformed, and sediments would be accreted to uplifted collisional terranes north of the Manila Trench.