Central role of detachment faults in accretion of slow-spreading oceanic lithosphere

The formation of oceanic detachment faults is well established from inactive, corrugated fault planes exposed on sea floor formed along ridges spreading at less than 80 km Myr(-1) (refs 1-4). These faults can accommodate extension for up to 1-3 Myr (ref. 5), and are associated with one of the two contrasting modes of accretion operating along the northern Mid-Atlantic Ridge. The first mode is asymmetrical accretion involving an active detachment fault along one ridge flank. The second mode is the well-known symmetrical accretion, dominated by magmatic processes with subsidiary high-angle faulting and the formation of abyssal hills on both flanks. Here we present an examination of approximately 2,500 km of the Mid-Atlantic Ridge between 12.5 and 35 degrees N, which reveals asymmetrical accretion along almost half of the ridge. Hydrothermal activity identified so far in the study region is closely associated with asymmetrical accretion, which also shows high levels of near-continuous hydroacoustically and teleseismically recorded seismicity. Increased seismicity is probably generated along detachment faults that accommodate a sizeable proportion of the total plate separation. In contrast, symmetrical segments have lower levels of seismicity, which occurs primarily at segment ends. Basalts erupted along asymmetrical segments have compositions that are consistent with crystallization at higher pressures than basalts from symmetrical segments, and with lower extents of partial melting of the mantle. Both seismic evidence and geochemical evidence indicate that the axial lithosphere is thicker and colder at asymmetrical sections of the ridge, either because associated hydrothermal circulation efficiently penetrates to greater depths or because the rising mantle is cooler. We suggest that much of the variability in sea-floor morphology, seismicity and basalt chemistry found along slow-spreading ridges can be thus attributed to the frequent involvement of detachment faults in oceanic lithospheric accretion.     

Contrasting crustal production and rapid mantle transitions beneath back-arc ridges

The opening of back-arc basins behind subduction zones progresses from initial rifting near the volcanic arc to seafloor spreading. During this process, the spreading ridge and the volcanic arc separate and lavas erupted at the ridge are predicted to evolve away from being heavily subduction influenced (with high volatile contents derived from the subducting plate). Current models predict gradational, rather than abrupt, changes in the crust formed along the ridge as the inferred broad melting region beneath it migrates away from heavily subduction-influenced mantle. In contrast, here we show that across-strike and along-strike changes in crustal properties at the Eastern Lau spreading centre are large and abrupt, implying correspondingly large discontinuities in the nature of the mantle supplying melt to the ridge axes. With incremental separation of the ridge axis from the volcanic front of as little as 5?km, seafloor morphology changes from shallower complex volcanic landforms to deeper flat sea floor dominated by linear abyssal hills, upper crustal seismic velocities abruptly increase by over 20%, and gravity anomalies and isostasy indicate crustal thinning of more than 1.9?km. We infer that the abrupt changes in crustal properties reflect rapid evolution of the mantle entrained by the ridge, such that stable, broad triangular upwelling regions, as inferred for mid-ocean ridges, cannot form near the mantle wedge corner. Instead, the observations imply a dynamic process in which the ridge upwelling zone preferentially captures water-rich low-viscosity mantle when it is near the arc. As the ridge moves away from the arc, a tipping point is reached at which that material is rapidly released from the upwelling zone, resulting in rapid changes in the character of the crust formed at the ridge.     

Discovery of a magma chamber and faults beneath a Mid-Atlantic Ridge hydrothermal field

Crust at slow-spreading ridges is formed by a combination of magmatic and tectonic processes, with magmatic accretion possibly involving short-lived crustal magma chambers. The reflections of seismic waves from crustal magma chambers have been observed beneath intermediate and fast-spreading centres, but it has been difficult to image such magma chambers beneath slow-spreading centres, owing to rough seafloor topography and associated seafloor scattering. In the absence of any images of magma chambers or of subsurface near-axis faults, it has been difficult to characterize the interplay of magmatic and tectonic processes in crustal accretion and hydrothermal circulation at slow-spreading ridges. Here we report the presence of a crustal magma chamber beneath the slow-spreading Lucky Strike segment of the Mid-Atlantic Ridge. The reflection from the top of the magma chamber, centred beneath the Lucky Strike volcano and hydrothermal field, is approximately 3 km beneath the sea floor, 3-4 km wide and extends up to 7 km along-axis. We suggest that this magma chamber provides the heat for the active hydrothermal vent field above it. We also observe axial valley bounding faults that seem to penetrate down to the magma chamber depth as well as a set of inward-dipping faults cutting through the volcanic edifice, suggesting continuous interactions between tectonic and magmatic processes.     

An off-axis hydrothermal vent field near the Mid-Atlantic Ridge at 30 degrees N.

Evidence is growing that hydrothermal venting occurs not only along mid-ocean ridges but also on old regions of the oceanic crust away from spreading centres. Here we report the discovery of an extensive hydrothermal field at 30 degrees N near the eastern intersection of the Mid-Atlantic Ridge and the Atlantis fracture zone. The vent field--named 'Lost City'--is distinctly different from all other known sea-floor hydrothermal fields in that it is located on 1.5-Myr-old crust, nearly 15 km from the spreading axis, and may be driven by the heat of exothermic serpentinization reactions between sea water and mantle rocks. It is located on a dome-like massif and is dominated by steep-sided carbonate chimneys, rather than the sulphide structures typical of 'black smoker' hydrothermal fields. We found that vent fluids are relatively cool (40-75 degrees C) and alkaline (pH 9.0-9.8), supporting dense microbial communities that include anaerobic thermophiles. Because the geological characteristics of the Atlantis massif are similar to numerous areas of old crust along the Mid-Atlantic, Indian and Arctic ridges, these results indicate that a much larger portion of the oceanic crust may support hydrothermal activity and microbial life than previously thought.     

Evidence from gabbro of the Troodos ophiolite for lateral magma transport along a slow-spreading mid-ocean ridge

The lateral flow of magma and ductile deformation of the lower crust along oceanic spreading axes has been thought to play a significant role in suppressing both mid-ocean ridge segmentation and variations in crustal thickness. Direct investigation of such flow patterns is hampered by the kilometres of water that cover the oceanic crust, but such studies can be made on ophiolites (fragments of oceanic crust accreted to a continent). In the Oman ophiolite, small-scale radial patterns of flow have been mapped along what is thought to be the relict of a fast-spreading mid-ocean ridge. Here we present evidence for broad-scale along-axis flow that has been frozen into the gabbro of the Troodos ophiolite in Cyprus (thought to be representative of a slow-spreading ridge axis). The gabbro suite of Troodos spans nearly 20 km of a segment of a fossil spreading axis, near a ridge-transform intersection. We mapped the pattern of magma flow by analysing the rocks' magnetic fabric at 20 sites widely distributed in the gabbro suite, and by examining the petrographic fabric at 9 sites. We infer an along-axis magma flow for much of the gabbro suite, which indicates that redistribution of melt occurred towards the segment edge in a large depth range of the oceanic crust. Our results support the magma plumbing structure that has been inferred indirectly from a seismic tomography experiment on the slow-spreading Mid-Atlantic Ridge.     

Mineralogical and geochemical features of sulfide chimneys from the 49°39′E hydrothermal field on the Southwest Indian Ridge and their geological inferences

During January–May in 2007,the Chinese research cruise DY115-19 discovered an active hydrothermal field at 49°39′E/37°47′S on the ultraslow spreading Southwest Indian Ridge (SWIR).This was also the first active hydrothermal field found along an ultraslow-spreading ridge.We analyzed mineralogical,textural and geochemical compositions of the sulfide chimneys obtained from the 49°39′E field.Chimney samples show a concentric mineral zone around the fluid channel.The mineral assemblages of the interiors consist ...     

Mantle thermal pulses below the Mid-Atlantic Ridge and temporal variations in the formation of oceanic lithosphere

A 20-Myr record of creation of oceanic lithosphere is exposed along a segment of the central Mid-Atlantic Ridge on an uplifted sliver of lithosphere. The degree of melting of the mantle that is upwelling below the ridge, estimated from the chemistry of the exposed mantle rocks, as well as crustal thickness inferred from gravity measurements, show oscillations of approximately 3-4 Myr superimposed on a longer-term steady increase with time. The time lag between oscillations of mantle melting and crustal thickness indicates that the mantle is upwelling at an average rate of approximately 25 mm x yr(-1), but this appears to vary through time. Slow-spreading lithosphere seems to form through dynamic pulses of mantle upwelling and melting, leading not only to along-axis segmentation but also to across-axis structural variability. Also, the central Mid-Atlantic Ridge appears to have become steadily hotter over the past 20 Myr, possibly owing to north-south mantle flow.     

广东恩开断裂带新构造活动特征

恩开断裂带是华南沿海地区重要的断裂带之一,长约160km,由3条大致平行展布的北东向断裂组成,其中苍城—海陵断裂(F₁)、均安断裂(F₂)倾向北西,鹤城—金鸡断裂(F₃)倾向南东。这3条断裂在第三纪主要作张性正断层运动。约在上新世末或第四纪初,断裂带受到北西西—南东东方向的强烈挤压,北东向断裂转为压性活动。然而,约在中更新世开始,挤压力逐渐减弱,加之,华南沿海地区水平挤压力有从南往北减弱的趋势,断裂带南段,挤压力仍足以使断裂上盘逆冲上升,形成逆断层地貌;继续往北,水平挤压力与重力大小相近,两者平衡使断层两盘处于动态稳定之中,差异升降弱;再往北,水平挤压力更弱,重力均衡使断裂再次发生正断层运动。约在晚更新世开始,区内的一系列北西向断裂形成或复活,由于与主压应力方向夹角小,断裂面摩擦力小而比北东向断裂更易发生错动,使北东向断裂多处被切。北西向断裂的活动一方面限制了北东向断裂的平移,另一方面释放了部分应变能,最终使北东向断裂的活动性降低。     

Seismic identification of along-axis hydrothermal flow on the East Pacific Rise

Hydrothermal circulation at the axis of mid-ocean ridges affects the chemistry of the lithosphere and overlying ocean, supports chemosynthetic biological communities and is responsible for significant heat transfer from the lithosphere to the ocean. It is commonly thought that flow in these systems is oriented across the ridge axis, with recharge occurring along off-axis faults, but the structure and scale of hydrothermal systems are usually inferred from thermal and geochemical models constrained by the geophysical setting, rather than direct observations. The presence of microearthquakes may shed light on hydrothermal pathways by revealing zones of thermal cracking where cold sea water extracts heat from hot crustal rocks, as well as regions where magmatic and tectonic stresses create fractures that increase porosity and permeability. Here we show that hypocentres beneath a well-studied hydrothermal vent field on the East Pacific Rise cluster in a vertical pipe-like zone near a small axial discontinuity, and in a band that lies directly above the axial magma chamber. The location of the shallow pipe-like cluster relative to the distribution and temperature of hydrothermal vents along this section of the ridge suggests that hydrothermal recharge may be concentrated there as a consequence of the permeability generated by tectonic fracturing. Furthermore, we interpret the band of seismicity above the magma chamber as a zone of hydrothermal cracking, which suggests that hydrothermal circulation may be strongly aligned along the ridge axis. We conclude that models that suggest that hydrothermal cells are oriented across-axis, with diffuse off-axis recharge zones, may not apply to the fast-spreading East Pacific Rise.     

Reykjanes "V"-shaped ridges originating from a pulsing and dehydrating mantle plume

Prominent crustal lineations straddle the Reykjanes ridge, south of Iceland (Fig. 1). These giant V-shaped features are thought to record temporal variations in magma production at the Reykjanes ridge axis, associated with along-axis flow of Icelandic plume material. It has been proposed that this flow is channelled preferentially along the ridge axis, and that temporal variability is induced by fluctuations of the Iceland plume itself or, alternatively, by relocations of the ridge axis on Iceland. Here I present a geodynamic model that predicts the formation of crustal V-shaped ridges from a pulsing and radially flowing mantle plume. In this model, plume pulses produce mantle temperature perturbations that expand away from the plume in all directions beneath the zone of partial melting. The melting zone has a high viscosity owing to mantle dehydration at the onset of partial melting. This high-viscosity region allows for reasonable variations in crustal thickness, produces crustal Vs that extend hundreds of kilometres along the axis, and prevents the plume material from being preferentially channelled along the ridge axis. The angle of the crustal V-shaped features relative to the ridge axis reflects the rate of lateral plume flow, which remains several times greater than the ridge half-spreading rate over the length of a crustal V. Consequently, this radially expanding plume produces lineations in crustal thickness and free-air gravity anomalies that appear to be nearly straight.     

A mechanism to thin the continental lithosphere at magma-poor margins

Where continental plates break apart, slip along multiple normal faults provides the required space for the Earth's crust to thin and subside. After initial rifting, however, the displacement on normal faults observed at the sea floor seems not to match the inferred extension. Here we show that crustal thinning can be accomplished in such extensional environments by a system of conjugate concave downward faults instead of multiple normal faults. Our model predicts that these concave faults accumulate large amounts of extension and form a very thin crust (< 10 km) by exhumation of mid-crustal and mantle material. This transitional crust is capped by sub-horizontal detachment surfaces over distances exceeding 100 km with little visible deformation. Our rift model is based on numerical experiments constrained by geological and geophysical observations from the Alpine Tethys and Iberia/Newfoundland margins. Furthermore, we suggest that the observed transition from broadly distributed and symmetric extension to localized and asymmetric rifting is directly controlled by the existence of a strong gabbroic lower crust. The presence of such lower crustal gabbros is well constrained for the Alpine Tethys system. Initial decoupling of upper crustal deformation from lower crustal and mantle deformation by progressive weakening of the middle crust is an essential requirement to reproduce the observed rift evolution. This is achieved in our models by the formation of weak ductile shear zones.     

Spreading rate dependence of gravity anomalies along oceanic transform faults

Mid-ocean ridge morphology and crustal accretion are known to depend on the spreading rate of the ridge. Slow-spreading mid-ocean-ridge segments exhibit significant crustal thinning towards transform and non-transform offsets, which is thought to arise from a three-dimensional process of buoyant mantle upwelling and melt migration focused beneath the centres of ridge segments. In contrast, fast-spreading mid-ocean ridges are characterized by smaller, segment-scale variations in crustal thickness, which reflect more uniform mantle upwelling beneath the ridge axis. Here we present a systematic study of the residual mantle Bouguer gravity anomaly of 19 oceanic transform faults that reveals a strong correlation between gravity signature and spreading rate. Previous studies have shown that slow-slipping transform faults are marked by more positive gravity anomalies than their adjacent ridge segments, but our analysis reveals that intermediate and fast-slipping transform faults exhibit more negative gravity anomalies than their adjacent ridge segments. This finding indicates that there is a mass deficit at intermediate- and fast-slipping transform faults, which could reflect increased rock porosity, serpentinization of mantle peridotite, and/or crustal thickening. The most negative anomalies correspond to topographic highs flanking the transform faults, rather than to transform troughs (where deformation is probably focused and porosity and alteration are expected to be greatest), indicating that crustal thickening could be an important contributor to the negative gravity anomalies observed. This finding in turn suggests that three-dimensional magma accretion may occur near intermediate- and fast-slipping transform faults.     

Intense mixing of lower thermocline water on the crest of the Mid-Atlantic Ridge

Buoyancy exchange between the deep and the upper ocean, which is essential for maintaining global ocean circulation, mainly occurs through turbulent mixing. This mixing is thought to result primarily from instability of the oceanic internal wave field, but internal waves tend to radiate energy away from the regions in which they are generated rather than dissipate it locally as turbulence and the resulting distribution of turbulent mixing remains unknown. Another, more direct, mixing mechanism involves the generation of turbulence as strong flows pass through narrow passages in topography, but the amount of turbulence generated at such locations remains poorly quantified owing to a lack of direct measurements. Here we present observations from the crest of the Mid-Atlantic Ridge in the subtropical North Atlantic Ocean that suggest that passages in rift valleys and ridge-flank canyons provide the most energetic sites for oceanic turbulence. Our measurements show that diffusivities as large as 0.03 m2 s(-1) characterize the mixing downstream of a sill in a well-stratified boundary layer, with mixing levels remaining of the order of 10(-4) m2 s(-1) at the base of the main thermocline. These mixing rates are significantly higher than the diffusivities of the order of 10(-5) m2 s(-1) that characterize much of the global thermocline and the abyssal ocean. Our estimates suggest that overflows associated with narrow passages on the Mid-Atlantic Ridge in the North Atlantic Ocean produce as much buoyancy flux as has previously been estimated for the entire Romanche fracture zone, a large strait in the Mid-Atlantic Ridge that connects the North and South Atlantic basins. This flux is equivalent to the interior mixing that occurs in the entire North Atlantic basin at the depth of the passages, suggesting that turbulence generated in narrow passages on mid-ocean ridges may be important for buoyancy flux at the global scale.     

Sequential faulting explains the asymmetry and extension discrepancy of conjugate margins

During early extension, cold continental lithosphere thins and subsides, creating rift basins. If extension continues to final break-up, the split and greatly thinned plates subside deep below sea level to form a conjugate pair of rifted margins. Although basins and margins are ubiquitous structures, the deformation processes leading from moderately extended basins to highly stretched margins are unclear, as studies consistently report that crustal thinning is greater than extension caused by brittle faulting. This extension discrepancy might arise from differential stretching of brittle and ductile crustal layers, but that does not readily explain the typical asymmetric structure of conjugate margins-in cross-section, one margin displays gradual thinning accompanied by large faults, and the conjugate margin displays abrupt thinning but smaller-scale faulting. Whole-crust detachments, active from early in the rifting, could in theory create both thinning and asymmetry, but are mechanically problematical. Furthermore, the extension discrepancy occurs at both conjugate margins, leading to the apparent contradiction that both seem to be upper plates to a detachment fault. Alternative models propose that much brittle extension is undetected because of seismic imaging limitations caused either by subseismic-resolution faulting, invisible deformation along top-basement 100-km-scale detachments or the structural complexity of cross-cutting arrays of faults. Here we use depth-migrated seismic images to accurately measure fault extension and compare it with crustal thinning. The observations are used to create a balanced kinematic model of rifting that resolves the extension discrepancy by producing both fault-controlled crustal thinning which progresses from a rift basin to the asymmetric structure, and extreme thinning of conjugate rifted margins. Contrary to current wisdom, the observations support the idea that thinning is to a first degree explained by simple Andersonian faulting that is unambiguously visible in seismic data.     

The role of fluids in lower-crustal earthquakes near continental rifts

The occurrence of earthquakes in the lower crust near continental rifts has long been puzzling, as the lower crust is generally thought to be too hot for brittle failure to occur. Such anomalous events have usually been explained in terms of the lower crust being cooler than normal. But if the lower crust is indeed cold enough to produce earthquakes, then the uppermost mantle beneath it should also be cold enough, and yet uppermost mantle earthquakes are not observed. Numerous lower-crustal earthquakes occur near the southwestern termination of the Taupo Volcanic Zone (TVZ), an active continental rift in New Zealand. Here we present three-dimensional tomographic imaging of seismic velocities and seismic attenuation in this region using data from a dense seismograph deployment. We find that crustal earthquakes accurately relocated with our three-dimensional seismic velocity model form a continuous band along the rift, deepening from mostly less than 10 km in the central TVZ to depths of 30-40 km in the lower crust, 30 km southwest of the termination of the volcanic zone. These earthquakes often occur in swarms, suggesting fluid movement in critically loaded fault zones. Seismic velocities within the band are also consistent with the presence of fluids, and the deepening seismicity parallels the boundary between high seismic attenuation (interpreted as partial melt) within the central TVZ and low seismic attenuation in the crust to the southwest. This linking of upper and lower-crustal seismicity and crustal structure allows us to propose a common explanation for all the seismicity, involving the weakening of faults on the periphery of an otherwise dry, mafic crust by hot fluids, including those exsolved from underlying melt. Such fluids may generally be an important driver of lower-crustal seismicity near continental rifts.     

Damage to the shallow Landers fault from the nearby Hector Mine earthquake

Crustal faults have long been identified as sites where localized sliding motion occurs during earthquakes, which allows for the relative motion between adjacent crustal blocks. Although there is a growing awareness that we must understand the evolution of fault systems on many timescales to relate present-day crustal stresses and fault motions to geological structures formed in the past, fault-zone damage and healing have been documented quantitatively in only a few cases. We have been monitoring the healing of damage on the shallow Johnson Valley fault after its rupture in the 1992 magnitude-7.3 Landers earthquake, and here we report that this healing was interrupted in 1999 by the magnitude-7.1 Hector Mine earthquake rupture, which occurred 20-30 km away. The Hector Mine earthquake both strongly shook and permanently strained the Johnson Valley fault, adding damage discernible as a temporary reversal of the healing process. The fault has since resumed the trend of strength recovery that it showed after the Landers earthquake. These observations lead us to speculate that fault damage caused by strong seismic waves may help to explain earthquake clustering and seismicity triggering by shaking, and may be involved in friction reduction during faulting.     

The influence of ridge migration on the magmatic segmentation of mid-ocean ridges

The Earth's mid-ocean ridges display systematic changes in depth and shape, which subdivide the ridges into discrete spreading segments bounded by transform faults and smaller non-transform offsets of the axis. These morphological changes have been attributed to spatial variations in the supply of magma from the mantle, although the origin of the variations is poorly understood. Here we show that magmatic segmentation of ridges with fast and intermediate spreading rates is directly related to the migration velocity of the spreading axis over the mantle. For over 9,500 km of mid-ocean ridge examined, leading ridge segments in the 'hotspot' reference frame coincide with the shallow magmatically robust segments across 86 per cent of all transform faults and 73 per cent of all second-order discontinuities. We attribute this relationship to asymmetric mantle upwelling and melt production due to ridge migration, with focusing of melt towards ridge segments across discontinuities. The model is consistent with variations in crustal structure across discontinuities of the East Pacific Rise, and may explain variations in depth of melting and the distribution of enriched lavas.     

Contrasting origins of the upper mantle revealed by hafnium and lead isotopes from the Southeast Indian Ridge

The origin of the isotopic signature of Indian mid-ocean ridge basalts has remained enigmatic, because the geochemical composition of these basalts is consistent either with pollution from recycled, ancient altered oceanic crust and sediments, or with ancient continental crust or lithosphere. The radiogenic isotopic signature may therefore be the result of contamination of the upper mantle by plumes containing recycled altered ancient oceanic crust and sediments, detachment and dispersal of continental material into the shallow mantle during rifting and breakup of Gondwana, or contamination of the upper mantle by ancient subduction processes. The identification of a process operating on a scale large enough to affect major portions of the Indian mid-ocean ridge basalt source region has been a long-standing problem. Here we present hafnium and lead isotope data from across the Indian-Pacific mantle boundary at the Australian-Antarctic discordance region of the Southeast Indian Ridge, which demonstrate that the Pacific and Indian upper mantle basalt source domains were each affected by different mechanisms. We infer that the Indian upper-mantle isotope signature in this region is affected mainly by lower continental crust entrained during Gondwana rifting, whereas the isotope signature of the Pacific upper mantle is influenced predominantly by ocean floor subduction-related processes.     

Geophysical evidence for reduced melt production on the Arctic ultraslow Gakkel mid-ocean ridge

Most models of melt generation beneath mid-ocean ridges predict significant reduction of melt production at ultraslow spreading rates (full spreading rates &<20 mm x yr(-1)) and consequently they predict thinned oceanic crust. The 1,800-km-long Arctic Gakkel mid-ocean ridge is an ideal location to test such models, as it is by far the slowest portion of the global mid-ocean-ridge spreading system, with a full spreading rate ranging from 6 to 13 mm x yr(-1) (refs 4, 5). Furthermore, in contrast to some other ridge systems, the spreading direction on the Gakkel ridge is not oblique and the rift valley is not offset by major transform faults. Here we present seismic evidence for the presence of exceptionally thin crust along the Gakkel ridge rift valley with crustal thicknesses varying between 1.9 and 3.3 km (compared to the more usual value of 7 km found on medium- to fast-spreading mid-ocean ridges). Almost 8,300 km of closely spaced aeromagnetic profiles across the rift valley show the presence of discrete volcanic centres along the ridge, which we interpret as evidence for strongly focused, three-dimensional magma supply. The traces of these eruptive centres can be followed to crustal ages of approximately 25 Myr off-axis, implying that these magma production and transport systems have been stable over this timescale.     

Tectonic geomorphology of the Ryukyu Trench-Arc-Backarc System： geological-geophysical exploration and mapping

Based on an analysis of full-cover multi-beam bathymetric data, seismic and sub-bottom profiling data, and other geological-geophysical data sets, the geomorphologic features of the Ryukyu trench-arc-backarc (T-A-BA) system are delineated, and a geomorphologic map of the system is compiled. The results show that the evolution and spatial distribution patterns of the geomorphologic types of the Ryukyu T-A-BA system are controlled mainly by tectonic movements. The tectonic geomorphologic characteristics of the Ryukyu Arc (RA) differ distinctly from those of the East China Sea (ECS) continental shelf and slope. In term of geological structures, RA consists of the Tokara volcanic ridge, the Ryukyu folded ridge, the fore-arc accretion-wedge ridge and the Amami Depression and the fore-arc depressions between the ridges, which is composed of a complex of alternating island-slope ridges and fault basins. The slope of the ECS is a passive continental margin with stepwise faults. The Okinawa Trough (OT) is a backarc rift in which tectonicmovements are intensive, with active volcanic and hydro-thermal eruptions and sea floor spreading. The development of geomorphic features of the OT is controlled by the central en echelon spreading axes, the faults along the ECS slope and the marginal faults to the west of the Tokara volcanic ridge. The geomorphic complex of the OT is arranged in the following pattern: the en echelon grabens and volcanic chains formed by rifting and spreading lie in the central part of the trough, the turbidite plains incliningwards from the slope foot of the ECS lie in the westeru-northwesteru parts of the OT, and the volcaniclastic deposit plains inclining westward-northwestwards from the western slope foot of the RA lie in the easteru-southeasteru parts of the OT. In term of tectonic geomorphology, the OT forms a natural division between the sheff of the ECS and the RA.