Evolution and depositional structure of earthquake-induced mass movements and gravity flows: Southwest Orphan Basin, Labrador Sea

A series of submarine canyons on the southwest slope of Orphan Basin experienced complex failure at 7–8 cal ka that resulted in the formation of a large variety of mass-transport deposits (MTDs) and sediment gravity flows. Ultra-high-resolution seismic-reflection profiles and multiple sediment cores indicate that evacuation zones and sediment slides characterize the canyon walls, whereas the canyon floors and inner-banks are occupied by cohesive debris-flow deposits, which at the mouths of the canyons on the continental rise form large, coalescing lobes (up to 20 m thick and 50 km long). Erosional channels, extending throughout the length of the study area (<250 km), are observed on the top of the lobes. Piston cores show that the channels are partially filled by poorly sorted muddy sand and gravel, capped by inversely to normally graded gravel and sand. Such deposits are interpreted to originate from multi-phase gravity flows, consisting of a lower part behaving as a cohesionless debris flow and an upper part that was fully turbulent.The Holocene age and the widespread synchronous occurrence of these failures indicate a large magnitude earthquake as their possible triggering mechanism. The large debris-flow deposits on the continental rise originated from large failures on the upper continental slope, involving proglacial sediments. Retrogression of these failures led to the eventual failure of marginal sandy till deposits on the upper slope and outer shelf, which due to their low cohesion disintegrated into multi-phase gravity flows. The evacuation zones and slide deposits on the canyon walls were triggered either by the earthquake, or from erosion of the canyon walls by the debris flows. The slides, debris-flows, and multi-phase gravity flows observed in this study are petrographically different, indicating different sediment sources. This indicates that not all failures lead through flow transformation to the production of a multi-phase gravity flow, but only when the sediment source contains ample coarse-grained material. The spatial segregation of the slide, debris-flow, and multi-phase gravity-flow deposits is attributed to the different mobility of each transport process.     

Modes of development of slope canyons and their relation to channel and levee features on the Ebro sediment apron, off-shore northeastern Spain

Six submarine slope canyons in an area of the northwestern Mediterranean, offshore from the Ebro River and Delta, were surveyed with bathymetric swathmapping (SeaBeam) and mid-range side-looking sonar (SeaMARC I). All of the canyons have slightly winding paths with concave-upwards gradients that are relatively steep shallower than 1,200 m. Two major types of canyons are identified on the basis of their morphologic character at the base of the slope; Type-I canyons lead to an unchannelled base-of-slope deposit and Type-II canyons are continuous with channel-levee systems that cross the rise.Four Type-I canyons were surveyed in the area. Two of these are broad, U-shaped, steep (average gradients of 1:14), do not indent the shelf, and terminate downslope at debris-flow deposits. These two canyons, the most northern in the area, have rounded heads with extensive gullies separated by knife-edge ridges. Relief of the canyon walls is about equal on both sides of the canyons, although the right-hand walls (looking downslope) are generally steeper. The other two Type-I canyons in the area are similar in that they do not indent the shelf, but they are much smaller and shallower and coalesce before terminating in the base-of-slope region. The two Type-II canyons that feed leveed-channels are U-shaped with flatter floors, longer profiles and gentler gradients than Type-I canyons. They are closer to the Valencia Valley and have relatively small cross-sectional areas.We propose a four-stage evolutionary sequence to explain the development of the canyons observed in this section on the prograding Ebro margin. During the initial stage, slumping and erosion on the slope creates a network of small gullies. During the next stage, headward growth of one (or more) gully leads to a major indentation of the shelf. This is the critical factor for developing a channel that will incise the slope and provide a major conduit for moving sediment to the basin. Stage 3 is characterized by the development of a continuous channel accompanied by levee growth across the lobe. In the final stage, the channel-levee system becomes inactive either through destruction by mass wasting, infilling of the channel, or loss of the major sediment source.     

Depositional characteristics and spatial distribution of deep-water sedimentary systems on the northwestern middle-lower slope of the Northwest Sub-Basin, South China Sea

Based upon 2D seismic data, this study confirms the presence of a complex deep-water sedimentary system within the Pliocene-Quaternary strata on the northwestern lower slope of the Northwest Sub-Basin, South China Sea. It consists of submarine canyons, mass-wasting deposits, contourite channels and sheeted drifts. Alongslope aligned erosive features are observed on the eastern upper gentle slopes (<1.2° above 1,500 m), where a V-shaped downslope canyon presents an apparent ENE migration, indicating a related bottom current within the eastward South China Sea Intermediate Water Circulation. Contourite sheeted drifts are also generated on the eastern gentle slopes (~1.5° in average), below 2,100 m water depth though, referring to a wide unfocused bottom current, which might be related to the South China Sea Deep Water Circulation. Mass wasting deposits (predominantly slides and slumps) and submarine canyons developed on steeper slopes (>2°), where weaker alongslope currents are probably dominated by downslope depositional processes on these unstable slopes. The NNW–SSE oriented slope morphology changes from a three-stepped terraced outline (I–II–III) east of the investigated area, into a two-stepped terraced (I–II) outline in the middle, and into a unitary steep slope (II) in the west, which is consistent with the slope steepening towards the west. Such morphological changes may have possibly led to a westward simplification of composite deep-water sedimentary systems, from a depositional complex of contourite depositional systems, mass-wasting deposits and canyons, on the one hand, to only sliding and canyon deposits on the other hand.     

Slope-confined submarine canyons in the Baiyun deep-water area,northern South China Sea: variation in their modern morphology

On the basis of newly collected multibeam bathymetric data, chirp profiles and existing seismic data, we presented a detailed morphological interpretation of a series of slope-confined canyons in water depths of 300–2000 m in the Baiyun deep-water area, northern margin of the South China Sea. Although these canyons are commonly characterized by regular spacing and a straight-line shape, they vary in their lengths, starting and ending water depths, canyon relief, slope gradients, wall slope gradients and depth profiles along the axis. The eastern canyons (C1–C8) have complex surface features, low values in their slope gradient, canyon relief and wall slope gradient and high values in their length and starting and ending depth contrasting to the western ones (C9–C17). From the bathymetric data and chirp profiles, we interpret two main processes that have controlled the morphology and evolution of the canyons: axial incision and landsliding. The western part of the shelf margin where there were at least four stages of submerged reefs differs from the eastern part of the shelf margin where sedimentary undulations occurred at a water depth of ~650 m. We consider that the variation in morphology of submarine canyons in the study area is the result of multiple causes, with the leading cause being the difference in stability of the upper slope which is related to the submerged reefs and sedimentary undulations.     

Geomorphological evidence for upslope canyon-forming processes on the northern KwaZulu-Natal shelf,SW Indian Ocean,South Africa

A geomorphological and statistical analysis of slope canyons from the northern KwaZulu-Natal continental margin is documented and compared with submarine canyons from the Atlantic margin of the USA. The northern KwaZulu-Natal margin is characterized by increasing upslope relief, concave slope-gradient profiles and features related to upslope growth of the canyon forms. Discounting slope-gradient profile, this morphology is strikingly similar to canyon systems of the New Jersey slope. Several phases of canyon incision indicate that downslope erosion is also an important factor in the evolution of the northern KwaZulu-Natal canyon systems. Despite the strong similarities between the northern KwaZulu-Natal and New Jersey slope-canyon systems, key differences are evident: (1) the concavity of the northern KwaZulu-Natal slope, contrasting with the ∼linear New Jersey slope; (2) the relative isolation of the northern KwaZulu-Natal canyons, rather than the dense clustering of the New Jersey canyons; and (3) the absence of strongly shelf-breaching canyons along the northern KwaZulu-Natal margin. In comparison with the New Jersey margin, we surmise a more youthful stage of canyon evolution, a result of either the canyons themselves being younger or the formative processes being less active. Less complicated patterns of erosion resulting from reduced sediment availability have developed in northern KwaZulu-Natal. The reduction in slope concavity on the New Jersey margin may be the result of grading of the upper slope by intensive headward erosion, a process more subdued—or less evident—on the KwaZulu-Natal margin.     

东海陆坡不同类型海底峡谷的分布构造响应

利用1997-2001年在东海海域获得的多波束全覆盖测深数据和收集的部分高分辨率单道地震剖面,对东海陆坡海底峡谷的地形要素,如长度、弯曲度、平均坡度、剖面特征、头部和末端的水深等进行了详细的分析和统计.根据平面形态将海底峡谷划分为3种类型:直线型、蛇曲型和树枝型.海底峡谷主要分布于中段和南段的东海陆坡之上,平面形态特征总体上自北向南趋于复杂(直线型→蛇曲型→树枝型),规模逐渐增大,不同平面类型代表了峡谷不同的演化阶段.峡谷的分布格局和变化趋势主要受控于冲绳海槽扩张背景下的构造活动.     

Characteristics of migrating submarine canyons from the middle Miocene to present: Implications for paleoceanographic circulation,northern South China Sea

Previously undocumented, migrating submarine canyons have developed in the Pearl River Mouth Basin along the northern continental margin of the South China Sea from the middle Miocene to present. A grid of high-resolution, 2-D multi-channel seismic profiles calibrated by borehole information permits documentation of these northeastward migrating submarine canyons, as the result of the interplay of gravity flows and bottom currents. The modern canyons have lengths of 30–60 km, widths of 1–5.7 km, and relief of 50–300 m in water depths of 450–1500 m. Buried ancient submarine canyon successions were originally eroded by basal erosional discontinuities and partially filled by canyon thalweg deposits. These are overlain by lateral inclined packages and hemipelagic drape deposits. Basal erosional discontinuities and thalweg deposits are probably created principally by turbidity currents and filled with turbidites. Lateral inclined packages likely were formed by along-slope bottom currents. The evolution of these migrating submarine canyons reveals that northeastward bottom currents have consistently occurred at least from the middle Miocene to present in the study area. It might further imply that thermohaline intermediate water circulation of the South China Sea has been anti-cyclonic from the middle Miocene to present. The initiation of migrating submarine canyons possibly signals commencement of strong bottom currents after the middle Miocene in the South China Sea. The intensification of bottom currents also possibly may reflect shoaling of the major ocean seaways and increased vigor in oceanic circulation forced by global cooling after the middle Miocene.     

Qualitative assessment of gas hydrate and gas concentrations from the AVO characteristics of the BSR in the Ulleung Basin,East Sea (Japan Sea)

The presence of gas hydrate in the Ulleung Basin, East Sea (Japan Sea), inferred by various seismic indicators, including the widespread bottom-simulating reflector (BSR), has been confirmed by coring and drilling. We applied the standard AVO technique to the BSRs in turbidite/hemipelagic sediments crosscutting the dipping beds and those in debris-flow deposits to qualitatively assess the gas hydrate and gas concentrations. These BSRs are not likely to be affected by thin-bed tuning which can significantly alter the AVO response of the BSR. The BSRs crosscutting the dipping beds in turbidite/hemipelagic sediments are of low-seismic amplitude and characterized by a small positive gradient, indicating a decrease in Poisson’s ratio in the gas-hydrate stability zone (GHSZ), which, in turn, suggests the presence of gas hydrate. The BSRs in debris-flow deposits are characterized by a negative gradient, indicating decreased Poisson’s ratio below the GHSZ, which is likely due to a few percent or greater gas saturations. The increase in the steepness of the AVO gradient and the magnitude of the intercept of the BSRs in debris-flow deposits with increasing seismic amplitude of the BSRs is probably due to an increase in gas saturations, as predicted by AVO model studies based on rock physics. The reflection strength of the BSRs in debris-flow deposits, therefore, can be a qualitative measure of gas saturations below the GHSZ.     

Glacially influenced basin plain sedimentation in the southern Faeroe-Shetland Channel, northwest United Kingdom continental margin

Quaternary sediments at the southwest end of the Faeroe-Shetland Channel are preserved as a basin plain succession of variable fill geometry and lithology. In high-resolution seismic profiles the basin plain succession is characterised by laterally discontinuous and transparent, mounded lensoid bodies interbedded with acoustically well-layered sediments which display drape and onlapping reflection configurations. The lensoid bodies comprise an up to 50 m thick amalgamated package of mass-flow deposits consisting primarily of debris-flow diamictons. They represent resedimented glacigenic deposits derived from the West Shetland Shelf. Accumulation of these packages was episodic and related to specific rapid phases of downslope resedimentation, most probably concomitant with ice-marginal deposition on the West Shetland Slope. The acoustically well-layered sediments include glaciomarine hemipelagites and contourites. These indicate phases of reduced sediment supply from the adjacent shelf and slope areas, and probably represent the more pervasive “background” sedimentation in the basin. Although weak bottom-current activity may have prevailed throughout the glacial episodes, the onset of vigorous bottom-current circulation occurred at the changeover from a glacial to an interglacial regime. The debris flow packages form about 50% of the basin-plain sediments in this part of the Faeroe-Shetland Channel, thereby forming a significant component of the deep-water succession.     

Mass-transport complex evolution in a tectonically active margin (Gioia Basin,Southeastern Tyrrhenian Sea)

Along the southeastern Tyrrhenian Sea margin, the Gioia Basin formed as a result of extensional tectonics at the rear of the Maghrebian thrust belt. In the central part of the basin, mass-transport deposits represent up to 80% of its recent infill. The basin-wide Nicotera slump is the deepest mass-transport deposit present in the basin and was followed by sheet turbidite deposition. Above the turbidite package, a mass-transport complex (MTC) formed through the stacking of different mass-transport deposits due to repeated failures of the continental slope and of a base of slope channel levee wedge, which is still preserved in the western side of the basin. The Villafranca frontally-confined slide, a body mainly consisting of coherent blocks, represents the bulk of the MTC. The failure of the Villafranca slide was due to asymmetric loading of a permeable condensed horizon in the thinnest, distal lateral part of the channel levee wedge. The relatively large thickness of the Villafranca slide caused it to remain confined at its toe region. Smaller scale mass-transport deposits, a debris-flow sheet and a debris-flow lobe, followed the Villafranca slide and were sourced from the same headwall area. Their different run out and internal character are possibly a function of the lithology of the material involved in the collapse. A slab slide, characterized by little internal deformation and frontal contractional ridges, originated when seafloor instability propagated towards the north, causing clockwise rotation of a sediment wedge. Along the linear headwall of the slab slide, a localized upslope failure propagation is shown by a small scale re-entrant. The Sicilian margin, along which the Gioia Basin develops, is characterized by strong differential vertical movements due to ongoing extensional tectonics. The effects of both local and regional strong earthquakes are frequently felt in the area. Thus, slope oversteepening and earthquakes are suggested as the more likely causes for the observed repeated events of seafloor failure. In addition, an evolution of the MTC through larger slides controlled by the migration of uplift of the basin bounding submarine ridge, followed by smaller scale failures due to the consequent slope profile modification, is here advanced.     

Submarine Landslides on the Western Slope of the Kuril Basin,Sea of Okhotsk

Landslide processes on the western slope of the Kuril Basin were studied using bathymetry and seismic data obtained under the international KOMEX and SSGH projects. Slope areas containing landslides, landslide blocks and mass-transport deposits were distinguished. Large-scale landslides occupying an area of more than 100 km² are located in such areas of open continental margins as the slopes of the North Hokkaido Marginal Plateau and Terpeniya Ridge. Landslide blocks up to 2 km in size and mass-transport deposits are located in submarine canyons and fans in Terpeniya Bay. The age of landslides has been estimated as Middle Pleistocene–Holocene. Landslides are most likely triggered by seismic activity and gas saturation of sediments. Subsequent slope failure seems quite probable within the study area, and landslides capable of generating tsunamis may occur.     

Qualitative assessment of gas hydrate and gas concentrations from the AVO characteristics of the BSR in the Ulleung Basin, East Sea (Japan Sea)

The presence of gas hydrate in the Ulleung Basin, East Sea (Japan Sea), inferred by various seismic indicators, including the widespread bottom-simulating reflector (BSR), has been confirmed by coring and drilling. We applied the standard AVO technique to the BSRs in turbidite/hemipelagic sediments crosscutting the dipping beds and those in debris-flow deposits to qualitatively assess the gas hydrate and gas concentrations. These BSRs are not likely to be affected by thin-bed tuning which can significantly alter the AVO response of the BSR. The BSRs crosscutting the dipping beds in turbidite/hemipelagic sediments are of low-seismic amplitude and characterized by a small positive gradient, indicating a decrease in Poisson’s ratio in the gas-hydrate stability zone (GHSZ), which, in turn, suggests the presence of gas hydrate. The BSRs in debris-flow deposits are characterized by a negative gradient, indicating decreased Poisson’s ratio below the GHSZ, which is likely due to a few percent or greater gas saturations. The increase in the steepness of the AVO gradient and the magnitude of the intercept of the BSRs in debris-flow deposits with increasing seismic amplitude of the BSRs is probably due to an increase in gas saturations, as predicted by AVO model studies based on rock physics. The reflection strength of the BSRs in debris-flow deposits, therefore, can be a qualitative measure of gas saturations below the GHSZ.     

南海北部白云深水区东北部小型峡谷内的块体搬运

The process of mass movements and their consequent turbidity currents in large submarine canyons has been widely reported, however, little attention was paid to that in small canyons. In this paper, we document mass movements in small submarine canyons in the northeast of Baiyun deepwater area, north of the South China Sea(SCS), and their strong effects on the evolution of the canyons based on geophysical data. Submarine canyons in the study area arrange closely below the shelf break zone which was at the depth of –500 m. Within submarine canyons, seabed surface was covered with amounts of failure scars resulted from past small-sized landslides. A complex process of mass transportation in the canyons is indicated by three directions of mass movements.Recent mass movement deposits in the canyons exhibit translucent reflections or parallel reflections which represent the brittle deformation and the plastic deformation, respectively. The area of most landslides in the canyons is less than 3 km2. The trigger mechanisms for mass movements in the study area are gravitational overloading, slope angle and weak properties of soil. Geophysical data indicate that the genesis of submarine canyons is the erosion of mass movements and consequent turbidity currents. The significant effects of mass movements on canyon are incision and sediment transportation at the erosion phases and fillings supply at the fill phases. This research will be helpful for the geological risk assessments and understanding the sediment transportation in the northern margin of the SCS.     

Characteristics and occurrence of submarine canyon-associated landslides in the middle of the northern continental slope,South China Sea

High-resolution and high-density 2-D multichannel seismic data, combined with high-precision multibeam bathymetric map, are utilized to investigate the characteristics and distribution of submarine landslides in the middle of the northern continental slope, South China Sea. In the region, a series of 19 downslope-extending submarine canyons are developed. The canyons are kilometers apart, and separated by inter-canyon sedimentary ridges. Numerous submarine landslides, bounded by headscarps and basal glide surfaces, are identified on the seismic profiles by their distorted to chaotic reflections. Listric faults and rotational blocks in head areas and compressional folds and inverse faults at the toes of the landslides are possibly developed. Three types of submarine landslides, i.e., creeps, slumps, and landslide complexes, are recognized. These landslides are mostly distributed in the head areas and on the flanks of the canyons. As the most widespread landslides in the region, creeps are usually composed of multiple laterally-coalesced creep bodies, in which the boundaries of singular component creep bodies are difficult to delineate. In addition, a total of 77 landslides are defined, including 61 singular slumps and 16 landslide complexes that consist of two or more component landslides. Statistics show that most landslides are of a small dimension (0.53–18.09 km² in area) and a short runout distance (less than 3.5 km). Regional and local slope gradients and rheological behavior of the displaced materials might play important roles in the generation and distribution of the submarine landslides. A conceptual model for the co-evolution of the canyons and the associated landslides in the study area is presented. In the model it is assumed that the canyons are initiated from gullies created by landslides on steeper sites of the continental slope. The nascent canyons would then experience successive retrogressive landsliding events to extend upslope; at the same time canyon downcutting or incision would steepen the canyon walls to induce more landslides.     

Three dimensional seismic studies of deep-water hazard-related features on the northern slope of South China Sea

Mass transport deposits and geological features related to fluid flow such as gas chimneys, mud diapirs and volcanos, pockmarks and gas hydrates are pervasive on the canyon dominated northern slope of the Pearl River Mouth basin of the South China Sea. These deposits and structures are linked to serious geohazards and are considered risk factors for seabed installations. Based on high resolution three dimensional seismic surveys, seismic characteristics, distributions and origins of these features are analyzed. A distribution map is presented and geometrical parameters and spatial distribution patterns are summarized. Results show that various groups of the mapped features are closely tied to local or regional tectonism and sedimentary processes. Mass transport complexes are classified as slides near the shelf break, initially deformed slumps on the flanks of canyons and highly deformed slumps on the lower slope downslope of the mouth of canyons. We propose them to be preconditioned by pore pressure changes related to sea level fluctuations, steep topography, and fluid and fault activities. Gas chimneys are mainly located in the vicinity of gas reservoirs, while bottom-simulating reflectors are observed within the gas chimney regions, suggesting gas chimneys serve as conduits for thermogenic gas. Mud diapirs/volcanos and pockmarks are observed in small numbers and the formation of pockmarks is related to underlying gas chimneys and faults. This study aims at reducing risks for deep-water engineering on the northern slope of South China Sea.     

尼日利亚西部大陆边缘海底峡谷下部结构研究

Multi-beam,sub-bottom and multichannel seismic data acquired from the western Nigerian continental margin are analysed and interpreted to examine the architectural characteristics of the lower parts of the submarine canyons on the margin.The presence of four canyons: Avon,Mahin,Benin,and Escravos,are confirmed from the multi-beam data map and identified as cutting across the shelf and slope areas,with morphological features ranging from axial channels,moderate to high sinuosity indices,scarps,terraces and nickpoints which are interpreted as resulting from erosional and depositional activities within and around the canyons.The Avon Canyon,in particular,is characterised by various branches and sub-branches with complex morphologies.The canyons are mostly U-shaped in these lower parts with occasional V-shapes down their courses.Their typical orientation is NE–SW.Sedimentary processes are proposed as being a major controlling factor in these canyons.Sediments appear to have been discharged directly into the canyons by rivers during the late Quaternary low sea level which allows river mouths to extend as far as the shelf edge.The current sediment supply is still primarily sourced from these rivers in the case of the Benin and Escravos Canyons,but indirectly in the case of the Avon and Mahin Canyons where the rivers discharge sediments into the lagoons and the lagoons bring the sediments on to the continental shelf before they are dispersed into the canyon heads.Ancient canyons that have long been buried underneath the Avon Canyon are identified in the multichannel seismic profile across the head of the Avon Canyon,while a number of normal faults around the walls of the Avon and Mahin Canyons are observed in the selected sub-bottom profiles.The occurrence of these faults,especially in the irregular portions of the canyon walls,suggests that they also have some effect on the canyon architecture.The formation of the canyons is attributed to the exposure of the upper marginal area to incisions from erosion during the sea level lowstand of the glacial period.The incisions are widened and lengthened by contouric currents,turbidity currents and slope failures resulting in the canyons.     

Slope and basin-floor reservoirs from the Miocene and Pliocene of the Veracruz Basin,southeastern Mexico

A regional study of the Veracruz Basin provided an excellent view of long-term deepwater sedimentation patterns from an evolving foreland-type basin. The regional seismic and well-log data set allows for an accurate reconstruction of slope and basin-floor depositional patterns, lithologic compositions, and paleogradients from a continuous succession of bathyal strata that span the Miocene to the lower Pliocene. Variations in Miocene and Pliocene deepwater reservoirs can be linked to prevailing slope characteristics. The Miocene basin had a high-gradient, tectonically generated slope, and the Pliocene basin had a low-gradient constructional slope. The Miocene basin owes its steep margin to the tectonic stacking of early Tertiary, Laramide-age thrust sheets. The Miocene margin shed a mixture of coarse elastic sediments (sands, gravels, and cobbles) and fines (silts and clays) that were transported into the deep basin via turbidity currents and debris flows. Channelized deposits dominate the Miocene slope, and reservoirs occur in long-lasting basement-confined canyons and shorter-lived shallower erosional gulleys. Thick and areally-extensive basin-floor fans exist outboard of the strongly channelized Miocene slope. Fan distribution is strongly controlled by synsedimentary contractional anticlines and synclines. In contrast, the latest Miocene to early Pliocene basin development was dominated by a strongly prograding wedge of shelf and slope deposits that was induced by volcanogenic uplift and increased sediment supply. During this phase, turbidite reservoirs are limited to narrow and sinuous deepwater channels that reside at the toe of the constructional clinoforms and areally limited, thinner basinal fans.     

Submarine canyons on the north of Chiwei Island:influenced by recent extension of the southern Okinawa Trough

Based on new multibeam bathymetric data and about 300 km long single seismic profiles, three topographic units were identified： the canyons, fractural valley and submarine terrace on the north of Chiwei Island where is a structural transition zone between the southern trough and the middle trough. The Chiwei Canyon and the North Chiwei Canyon are two of the largest canyons in the East China Sea （ECS） slope. Topographic features and architectures of them are described. The study shows that both of them are originated along faults. The evolution and spatial distribution of topographic units in the study area are controlled mainly by three groups of faults which were formed and reactive in the recent extensional phase of Okinawa Trough. The Chiwei Canyon was initia- ted during the middle Pleistocene and guided by F4 that is a N--S trending fault on the slope and F1, a large NW--SE trending fault on the trough. The pathway migration from the remnant channel to the present one of Chiwei Canyon is the result of uplift of tilted fault block that is coupled to the recent extension movements of the southern trough. The submarine terrace is detached from the ECS slope by the NEE -trending fault. The North Chiwei Canyon, developing during the late Pleistocene, is guided by FS, a N-S trending fault, diverted and blocked by the submarine terrace.     

Morphology and architecture of the typical erosion-genesis submarine canyons on the East China Sea slope

The Yithi submarine canyons,composed of four canyons less than 60 km in length,are located on the narrowest part of the East China Sea(ECS) slope.They extend from the shelf break at 160 m down to water depth of 1 500 m with an average gradient(along the canyon axis) of 3°(<1 000 m) and 0.7°(>1000 m).The sinuosity of the canyons ranges form 1.02 to 1.14 and their pathways extend radially from the shelf break to the axis of the Okinawa Trough.Structural and evolution pattern of the Yithi canyons are mainly controlled by sediment mass-movements and turbidity current and similar with that of the canyons in Ebro continental slope.The whole canyon system consists of three parts:the canyon,the channel and the fan.Slumps and slides often develop in the upper part of canyon where the water depth is less than 1000 m,and the turbidities usually developed on the fan.The scale of turbidites becomes smaller and their inner structures become more regular towards the ends of the canyons.Canyon-fans are often associated with small angle progradational reflection.Most canyon-fans and levees were transversely cut by active normal faults with NEE-SWW trending that are coupled to the modern extension of the Okinawa Trough.According to the age of formation of canyon-fans and sediments incised by canyons,we can infer that the Yithi canyons were formed since the middle the Medio-Pleistocene.     

Submarine canyons on the north of Chiwei Island: influenced by recent extension of the southern Okinawa Trough

Based on new multibeam bathymetric data and about 300 km long single seismic profiles, three topographic units were identified:the canyons, fractural valley and submarine terrace on the north of Chiwei Island where is a structural transition zone between the southern trough and the middle trough. The Chiwei Canyon and the North Chiwei Canyon are two of the largest canyons in the East China Sea (ECS) slope. Topographic features and architectures of them are described. The study shows that both of them are originated along faults. The evolution and spatial distribution of topographic units in the study area are controlled mainly by three groups of faults which were formed and reactive in the recent extensional phase of Okinawa Trough. The Chiwei Canyon was initiated during the middle Pleistocene and guided by F4 that is a N-S trending fault on the slope and F1, a large NW-SE trending fault on the trough. The pathway migration from the remnant channel to the present one of Chiwei Canyon is the result of uplift of tilted fault block that is coupled to the recent extension movements of the southern trough. The submarine terrace is detached from the ECS slope by the NEE-trending fault. The North Chiwei Canyon, developing during the late Pleistocene, is guided by F5, a N-S trending fault, diverted and blocked by the submarine terrace.