The Kerguelen Province revisited: Additional constraints on the early development of the Southeast Indian Ocean

The Kerguelen Province, consisting of two oceanic plateaus (Kerguelen, Broken Ridge) and three basins (Enderby, Labuan and Diamantina), covers a large area of ocean floor in the southeast Indian Ocean. As very few magnetic anomalies have been identified in this area and only a few basement ages from the Kerguelen Plateau are known, reconstruction models of the Kerguelen Province are not well constrained. In an effort to gain more understanding about the evolution of this area, we have used satellite gravity to identify additional fracture zones. As they are likely to be associated with high frequency and low amplitude gravity anomalies, we have computed the vertical derivative map instead of the regular satellite gravity map. Using this approach, we have identified a series of fracture zones in the Enderby Basin, which are aligned with the Mesozoic fracture zones in the Perth Basin and converge to the Kerguelen Fracture Zone. In the conjugate Bay of Bengal, we traced an equivalent pattern of fracture zones which, together, better constrain the early evolution of this part of the Indian Ocean. Synthesis of these images and the other available data from the Kerguelen Province, suggests that the spreading of India from both Australia and Antarctica is closely related. Spreading between the three continents appears to have begun about the same time, in the early Cretaceous and thus, the accretion of some parts of the Kerguelen Province must have occurred before the onset of the quiet magnetic period at 118 Ma. At about 96–99 Ma, when the spreading direction in the Indian Ocean had changed into a N-S direction, it also took place throughout the Kerguelen Province. We find that previously proposed slow spreading in the Diamantina Zone and Labuan Basins, between 96–99 Ma and the initiation of the Southeast Indian Ridge at 43 Ma, could not have taken place. Furthermore, we suggest that there is growing evidence that the same is true for spreading in the eastward continuation of the Diamantina Zone and Labuan Basin, between Australia and Antarctica. Initiation of spreading in this area is likely to be contemporaneous with the spreading in the Kerguelen Province and, thus, older than 96–99 Ma. This revised version was published online in November 2006 with corrections to the Cover Date.     

Frontal structure and Antarctic Bottom Water flow through the Princess Elizabeth Trough,Antarctica

Hydrographic, current meter and ADCP data collected during two recent cruises in the South Indian Ocean (RRS Discovery cruise 200 in February 1993 and RRS Discovery cruise 207 in February 1994) are used to investigate the current structure within the Princess Elizabeth Trough (PET), near the Antarctic continent at 85°E, 63–66°S. This gap in topography between the Kerguelen Plateau and the Antarctic continent, with sill depth 3750 m, provides a route for the exchange of Antarctic Bottom Water between the Australian–Antarctic Basin and the Weddell–Enderby Basin. Shears derived from ADCP and hydrographic data are used to deduce the barotropic component of the velocity field, and thus the volume transports of the water masses. Both the Southern Antarctic Circumpolar Current Front (SACCF) and the Southern Boundary of the Antarctic Circumpolar Current (SB) pass through the northern PET (latitudes 63 to 64.5°S) associated with eastward transports. These are deep-reaching fronts with associated bottom velocities of several cm s^-1. Antarctic Bottom water (AABW) from the Weddell–Enderby Basin is transported eastwards in the jets associated with these fronts. The transport of water with potential temperatures less than 0°C is 3 (±1) Sv. The SB is shown to meander in the PET, caused by the cyclonic gyre immediately west of the PET in Prydz Bay. The AABW therefore also meanders before continuing eastwards. In the southern PET (latitudes 64.5 to 66°S) a bottom intensified flow of AABW is observed flowing west. This AABW has most likely formed not far from the PET, along the Antarctic continental shelf and slope to the east. Current meters show that speeds in this flow have an annual scalar mean of 10 cm s^-1. The transport of water with potential temperatures less than 0°C is 20 (±3) Sv. The southern PET features westward flow throughout the water column, since the shallower depths are dominated by the flow associated with the Antarctic Slope Front. Including the westward flow of bottom water, the total westward transport of the whole water column in the southern PET is 45 (±6) Sv.     

The tracer signature of Antarctic Bottom Water and its spread in the Southwest Indian Ocean: Part I—CFC-derived translation rate and topographic control around the Southwest Indian Ridge and the Conrad Rise

In this paper we use a temperature and salinity based mixing model to assess the dilution of Antarctic Bottom Water (AABW) as it moves away from the Weddell Sea and into the Southwest Indian Ocean. By combining these results with CFC tracer measurements we have been able to make direct estimates of the large-scale translation rates of AABW in this region. We confirm that there is a major northward flow of AABW via a gap in the Southwest Indian Ridge at 30°E, and thence across the Agulhas Basin into the Mozambique Basin, with a translation rate from the Greenwich Meridian of 0.8–1.0 cm s⁻¹ and a volume transport between the two basins of 1.5×10⁶ m³ s⁻¹. A second, smaller flow cuts the Del Cano Rise through the Prince Edward Fracture Zone but is indistinguishable from the general bottom waters once on the northern side of the rise. The third flow moves eastward along the southern flank of the Del Cano Rise to pass north of the Conrad Rise. This has bottom velocities of 0.7 cm s⁻¹ and a volume transport of 1.6×10⁶ m³ s⁻¹. This water is probably the source of the AABW-rich Circumpolar Deep Water that flows through the gap to the west of Crozet Island, and which is traceable again at stations on the northern flanks of the ridge. Flow between the Conrad Rise and the Del Cano Rise is complicated by the influence of a fourth flow, the AABW that passes south of the former and thence into the Crozet Basin via the Crozet-Kerguelen Gap. We suggest that a portion of this flow loops into the channel between the Del Cano Rise and the Conrad Rise, modifying the bottom waters at the easternmost stations within this channel. We will go on in Part 2 of this paper to use these results to estimate the dissolution rates of silica in the SWINDEX area.     

Transport of Antarctic Bottom Water through passages in the East Azores Ridge (37° N) in the East Atlantic

The structure of northerly overflow of Antarctic Bottom Water (AABW) through passages in the East Azores Ridge (37° N) in the East Atlantic from the Madeira Basin to the Iberian Basin is studied on the basis of hydrographic measurements carried out by the Institute of Oceanology, Russian Academy of Sciences (RAS) in October 2011, historical World Ocean Data Base 2009, and recent data on the bottom topography. The overflow of the coldest layers of this water occurs through two passages with close depths at 16° W (Discovery Gap) and at 19°30′ W (nameless Western Gap). It is shown that it is likely that the role of the latter passage in water transport was underestimated in earlier publications because the water (2.01°C) found in the region north of the Western Gap was cooler than in the region north of the Discovery Gap (2.03°C). In 2011, we found a decrease of 0.01°C in the AABW temperature near the bottom compared to previous measurements in 1982 (from 2.011°C to 2.002°C). Analysis of the historical database shows that this decrease is most likely caused by the cooling trend in the abyssal waters in the East Atlantic basins.     

Evolution of the Mascarene Basin,western Indian Ocean,and the significance of the Amirante ARC

The northern Mascarene Basin, lying between Madagascar and the Seychelles Plateau in the north-west Indian Ocean, is marked at its north-western end by the Amirante Arc, an enigmatic ridge-trench complex superficially resembling an island arc. Structural trends in the area have been mapped using GLORIA sidescan sonar data, seismic reflection profiles and bathymetric maps. It is concluded that the north-west Mascarene Basin was created during the Late Cretaceous by sea-floor spreading about a north-west trending spreading axis cut by northeast trending transform faults. A major transform fault between the northern tip of Madagascar and the western margin of the Seychelles Plateau is proposed as a boundary between the Late Cretaceous Mascarene basin and the older Somali Basin to the north-west. The northern segment of the Amirante Ridge may mark part of the transform. The southern segment of the Ridge and its associated trench are, however, wholly contained within the Late Cretaceous ocean floor of the Mascarene Basin, and are best explained as compressional features related to a change in sea-floor spreading geometry in the Late Cretaceous or earliest Tertiary. Two models for the evolution of the Mascarene Basin are proposed, the major differences between them being the amount of subduction at the southern Amirante Arc and the timing of the initial separation between India and the Seychelles.     

Seismic stratigraphy and structure of the Northland Plateau and the development of the Vening Meinesz transform margin,SW Pacific Ocean

The Northland Plateau and the Vening Meinesz “Fracture” Zone (VMFZ), separating southwest Pacific backarc basins from New Zealand Mesozoic crust, are investigated with new data. The 12–16 km thick Plateau comprises a volcanic outer plateau and an inner plateau sedimentary basin. The outer plateau has a positive magnetic anomaly like that of the Three Kings Ridge. A rift margin was found between the Three Kings Ridge and the South Fiji Basin. Beneath the inner plateau basin, is a thin body interpreted as allochthon and parautochthon, which probably includes basalt. The basin appears to have been created by Early Miocene mainly transtensive faulting, which closely followed obduction of the allochthon and was coeval with arc volcanism. VMFZ faulting was eventually concentrated along the edge of the continental shelf and upper slope. Consequently arc volcanoes in a chain dividing the inner and outer plateau are undeformed whereas volcanoes, in various stages of burial, within the basin and along the base of the upper slope are generally faulted. Deformed and flat-lying Lower Miocene volcanogenic sedimentary rocks are intimately associated with the volcanoes and the top of the allochthon; Middle Miocene to Recent units are, respectively, mildly deformed to flat-lying, calcareous and turbiditic. Many parts of the inner plateau basin were at or above sea level in the Early Miocene, apparently as isolated highs that later subsided differentially to 500–2,000 m below sea level. A mild, Middle Miocene compressive phase might correlate with events of the Reinga and Wanganella ridges to the west. Our results agree with both arc collision and arc unzipping regional kinematic models. We present a continental margin model that begins at the end of the obduction phase. Eastward rifting of the Norfolk Basin, orthogonal to the strike of the Norfolk and Three Kings ridges, caused the Northland Plateau to tear obliquely from the Reinga Ridge portion of the margin, initiating the inner plateau basin and the Cavalli core complex. Subsequent N115° extension and spreading parallel with the Cook Fracture Zone completed the southeastward translation of the Three Kings Ridge and Northland Plateau and further opened the inner plateau basin, leaving a complex dextral transform volcanic margin.     

The role of the Spitsbergen shear zone in determining morphology,segmentation and evolution of the Knipovich Ridge

In 1989–1990 the SeaMARC II side-looking sonar and swath bathymetric system imaged more than 80 000 km² of the seafloor in the Norwegian-Greenland Sea and southern Arctic Ocean. One of our main goals was to investigate the morphotectonic evolution of the ultra-slow spreading Knipovich Ridge from its oblique (115° ) intersection with the Mohns Ridge in the south to its boundary with the Molloy Transform Fault in the north, and to determine whether or not the ancient Spitsbergen Shear Zone continued to play any involvement in the rise axis evolution and segmentation. Structural evidence for ongoing northward rift propagation of the Mohns Ridge into the ancient Spitsbergen Shear Zone (forming the Knipovich Ridge in the process) includes ancient deactivated and migrated transforms, subtle V-shaped-oriented flank faults which have their apex at the present day Molloy Transform, and rift related faults that extend north of the present Molloy Transform Fault. The Knipovich Ridge is segmented into distinct elongate basins; the bathymetric inverse of the very-slow spreading Reykjanes Ridge to the south. Three major fault directions are detected: the N-S oriented rift walls, the highly oblique en-echelon faults, which reside in the rift valley, and the structures, defining the orientation of many of the axial highs, which are oblique to both the rift walls and the faults in the axial rift valley. The segmentation of this slow spreading center is dominated by quasi stationary, focused magma centers creating (axial highs) located between long oblique rift basins. Present day segment discontinuities on the Knipovich Ridge are aligned along highly oblique, probably strike-slip faults, which could have been created in response to rotating shear couples within zones of transtension across the multiple faults of the Spitsbergen Shear Zone. Fault interaction between major strike slip shears may have lead to the formation of en-echelon pull apart basins. The curved stress trajectories create arcuate faults and subsiding elongate basins while focusing most of the volcanism through the boundary faults. As a result, the Knipovich Ridge is characterized by Underlapping magma centers, with long oblique rifts. This style of basin-dominated segmentation probably evolved in a simple shear detachment fault environment which led to the extreme morphotectonic and geophysical asymmetries across the rise axis. The influence of the Spitsbergen Shear Zone on the evolution of the Knipovich Ridge is the primary reason that the segment discontinuities are predominantly volcanic. Fault orientation data suggest that different extension directions along the Knipovich Ridge and Mohns Ridge (280° vs. 330°, respectively) cause the crust on the western side of the intersection of these two ridges to buckle and uplift via compression as is evidenced by the uplifted western wall province and the large 60 mGal free air gravity anomalies in this area. In addition, the structural data suggest that the northwards propagation of the spreading center is ongoing and that a `normal' pure shear spreading regime has not evolved along this ridge. This revised version was published online in November 2006 with corrections to the Cover Date.     

Rises of the Amerasia Basin,Arctic Ocean,and Possible Equivalents in the Atlantic Ocean

Oceanology - Abstract—The main positive morphostructures of the Amerasia Basin — the Lomonosov Ridge, Alpha Ridge, Mendeleev Rise, Chukchi Plateau, and Northwind Ridge — are...     

Accumulation rates of hydrothermal metalliferous sediments in the Lau Basin,S. W. Pacific

Five cores were recovered on a traverse in the Lau Basin at 18°30′S crossing a supposed active spreading center. The sediments were subjected to selective chemical leaching for Mn, Fe, Cu, Zn, and As. Accumulation rates were determined using¹⁴C. These rates increase from west to east, reflecting the influence of volcaniclastic inputs from the Tonga- Kermadec Ridge. All elements display highest non-detrital accumulation rates closest to the supposed spreading center, suggesting a hydrothermal input to the sediments there. Variable hydrothermal inputs also influence the other cores.     

On the circulation of bottom water in the region of the Vema Channel

The circulation and transport of Antarctic Bottom Water (σ₄<45.87) in the region of the Vema Channel are studied along three WOCE hydrographic lines, the geostrophic velocities referenced to previously published direct current measurements. The primary supply of water to the deep Vema Channel is from the Argentine Basin's deep western boundary current, with no indication of an inflow from the southeast. In the northern Argentine Basin, detachment of lower North Atlantic Deep Water from the continental slope is associated with a deep thermohaline front near 34°S. To the north of this front, the upper part of the AABW bound for the Vema Channel (σ₄<46.01) exhibits a significant NADW influence. Further modification of the throughflow water occurs near 30°30′S, where the channel orientation changes by ∼50°. Southward flow of bottom water on the eastern flank of the Vema Channel, amounting to ∼1.5 Sv, represents a significant countercurrent to the deep channel transport. Inclusion of this countercurrent reduces the net flow of AABW through the Vema Channel from 3.2±0.7 to 1.7±1.1 Sv. Water properties imply that the near-zero net flow over the Santos Plateau results from a near-closed cyclonic circulation fed by the deep Vema Channel throughflow. A disruption of the northward boundary current in the upper AABW (lower circumpolar water) is required by this flow pattern. The extension of the cyclonic circulation on the Santos Plateau enters the Brazil Basin as a ∼1 Sv flow distinct from the outflow in the Vema Channel Extension (6.2 Sv). The high magnitude of the latter suggests a southward recirculation of bottom water near the western boundary to the north of the region of study.     

Modification and pathways of Southern Ocean Deep Waters in the Scotia Sea

An unprecedented high-quality, quasi-synoptic hydrographic data set collected during the ALBATROSS cruise along the rim of the Scotia Sea is examined to describe the pathways of the deep water masses flowing through the region, and to quantify changes in their properties as they cross the sea. Owing to sparse sampling of the northern and southern boundaries of the basin, the modification and pathways of deep water masses in the Scotia Sea had remained poorly documented despite their global significance.Weddell Sea Deep Water (WSDW) of two distinct types is observed spilling over the South Scotia Ridge to the west and east of the western edge of the Orkney Passage. The colder and fresher type in the west, recently ventilated in the northern Antarctic Peninsula, flows westward to Drake Passage along the southern margin of the Scotia Sea while mixing intensely with eastward-flowing Circumpolar Deep Water (CDW) of the antarctic circumpolar current (ACC). Although a small fraction of the other WSDW type also spreads westward to Drake Passage, the greater part escapes the Scotia Sea eastward through the Georgia Passage and flows into the Malvinas Chasm via a deep gap northeast of South Georgia. A more saline WSDW variety from the South Sandwich Trench may leak into the eastern Scotia Sea through Georgia Passage, but mainly flows around the Northeast Georgia Rise to the northern Georgia Basin.In Drake Passage, the inflowing CDW displays a previously unreported bimodal property distribution, with CDW at the Subantarctic Front receiving a contribution of deep water from the subtropical Pacific. This bimodality is eroded away in the Scotia Sea by vigorous mixing with WSDW and CDW from the Weddell Gyre. The extent of ventilation follows a zonation that can be related to the CDW pathways and the frontal anatomy of the ACC. Between the Southern Boundary of the ACC and the Southern ACC Front, CDW cools by 0.15°C and freshens by 0.015 along isopycnals. The body of CDW in the region of the Polar Front splits after overflowing the North Scotia Ridge, with a fraction following the front south of the Falkland Plateau and another spilling over the plateau near 49.5°W. Its cooling (by 0.07°C) and freshening (by 0.008) in crossing the Scotia Sea is counteracted locally by NADW entraining southward near the Maurice Ewing Bank. CDW also overflows the North Scotia Ridge by following the Subantarctic Front through a passage just east of Burdwood Bank, and spills over the Falkland Plateau near 53°W with decreased potential temperature (by 0.03°C) and salinity (by 0.004). As a result of ventilation by Weddell Sea waters, the signature of the Southeast Pacific Deep Water (SPDW) fraction of CDW is largely erased in the Scotia Sea. A modified form of SPDW is detected escaping the sea via two distinct routes only: following the Southern ACC Front through Georgia Passage; and skirting the eastern end of the Falkland Plateau after flowing through Shag Rocks Passage.     

Isopycnal displacements within the Cape Basin thermocline as revealed by the Hydrographic Data Archive

The transfer of upper kilometer water from the Indian Ocean into the South Atlantic, the Agulhas leakage, is believed to be accomplished primarily through meso-scale eddy processes. There have been various studies investigating eddies of the “Cape Basin Cauldron” from specific data sets. The hydrographic data archive acquired during the last century within the Cape Basin region of the South Atlantic provides additional insight into the distribution and water mass properties of the Cape Basin eddies. Eddies are identified by mid-thermocline isopycnal depth anomalies relative to the long-term mean. Positive depth anomalies (the reference isopycnal is deeper than the long-term mean isopycnal depth) mark the presence of anticyclonic eddies; negative anomalies mark cyclonic eddies. Numerous eddies are identified in the whole region; the larger isopycnal displacements are attributed to the energetic eddies characteristic of the Cape Basin and indicate that there is a 2:1 anticyclone/cyclone ratio. Smaller displacements of the less energetic features are almost equally split between anticyclones and cyclones (1.4:1 ratio). Potential temperature, salinity and oxygen relationships at thermocline and intermediate levels within each eddy reveal their likely origin. The eddy core water is not solely drawn from Indian Ocean: tropical and subtropical South Atlantic water are also present. Anticyclones and cyclones carrying Agulhas Water properties are identified throughout the Cape Basin. Anticyclones with Agulhas Water characteristics show a predominant northwest dispersal, whereas the cyclones are identified mainly along the western margin of the African continent, possibly related to their origin as shear eddies at the boundary between the Agulhas axis and Africa. Cyclones and anticyclones carrying pure South Atlantic origin water are identified south of 30°S and west of the Walvis Ridge. Tropical Atlantic water at depth is found for cyclones north of the Walvis Ridge, west of 10°E and for stations deeper than 4000 m, and a few anticyclones with the same characteristics are found south of the ridge.     

The basins of Sundaland (SE Asia): Evolution and boundary conditions

Most of the basins developed in the continental core of SE Asia (Sundaland) evolved since the Late Cretaceous in a manner that may be correlated to the conditions of the subduction in the Sunda Trench. By the end of Mesozoic times Sundaland was an elevated area composed of granite and metamorphic basement on the rims; which suffered collapse and incipient extension, whereas the central part was stable. This promontory was surrounded by a large subduction zone, except in the north and was a free boundary in the Early Cenozoic. Starting from the Palaeogene and following fractures initiated during the India Eurasia collision, rifting began along large faults (mostly N–S and NNW–SSE strike-slip), which crosscut the whole region. The basins remained in a continental fluvio-lacustrine or shallow marine environment for a long time and some are marked by extremely stretched crust (Phu Khanh, Natuna, N. Makassar) or even reached the ocean floor spreading stage (Celebes, Flores). Western Sundaland was a combination of basin opening and strike-slip transpressional deformation. The configuration suggests a free boundary particularly to the east (trench pull associated with the Proto-South China Sea subduction; Java–Sulawesi trench subduction rollback). In the Early Miocene, Australian blocks reached the Sunda subduction zone and imposed local shortening in the south and southeast, whereas the western part was free from compression after the Indian continent had moved away to the north. This suggests an important coupling of the Sunda Plate with the Indo-Australian Plate both to SE and NW, possibly further west rollback had ceased in the Java–Sumatra subduction zone, and compressional stress was being transferred northwards across the plate boundary. The internal compression is expressed to the south by shortening which is transmitted as far as the Malay basin. In the Late Miocene, most of the Sunda Plate was under compression, except the tectonically isolated Andaman Sea and the Damar basins. In the Pliocene, collision north of Australia propagated toward the north and west causing subduction reversal and compression in the short-lived Damar Basin. Docking of the Philippine Plate confined the eastern side of Sundaland and created local compression and uplift such as in NW Borneo, Palawan and Taiwan. Transpressional deformation created extensive folding, strike-slip faulting and uplift of the Central Basin and Arakan Yoma in Myanmar. Minor inversion affected many Thailand rift basins. All the other basins record subsidence. The uplift is responsible for gravity tectonics where thick sediments were accumulated (Sarawak, NE Luconia, Bangladesh wedge).     

塔斯曼海构造特征及其演化过程研究

塔斯曼海位于西南太平洋地区，处于印度-澳大利亚板块和西兰板块之间，大地构造背景复杂。该地区是全球油气资源勘探的重点海域之一，但是国内对该地区的研究相当匮乏。本文根据塔斯曼海海域的自由空气重力异常对塔斯曼海海域的构造单元进行了划分，前人关于塔斯曼海的研究主要集中在Resolution海岭北部，我们认为塔斯曼海的范围应包括Resolution海岭以南，麦夸里海岭以西，塔斯曼断裂带以东的区域（即南部次盆）。结果显示，塔斯曼海域及邻区包括3个一级构造单元:东澳大利亚陆缘、西兰板块和塔斯曼海盆，且塔斯曼海盆可进一步划分为西部次盆、东部次盆和南部次盆。本文基于塔斯曼海域90 Ma以来的洋壳年龄数据编制了构造演化图，将塔斯曼海的形成演化过程分为4个阶段:（1）中生代陆内裂谷期（90~83 Ma BP）；（2）塔斯曼海扩张阶段（83~61 Ma BP）；（3）塔斯曼海北部扩张停止阶段（61~52 Ma BP）；（4）塔斯曼海南部改造阶段（52 Ma BP至今）。     

Seismic reflection-based evidence of a transfer zone between the Wagner and Consag basins: implications for defining the structural geometry of the northern Gulf of California

This study examines the structural characteristics of the northern Gulf of California by processing and interpreting ca. 415 km of two-dimensional multi-channel seismic reflection lines (data property of Petróleos Mexicanos PEMEX) collected in the vicinity of the border between the Wagner and Consag basins. The two basins appear to be a link between the Delfín Superior Basin to the south, and the Cerro Prieto Basin to the north in the Mexicali-Imperial Valley along the Pacific–North America plate boundary. The seismic data are consistent with existing knowledge of four main structures (master faults) in the region, i.e., the Percebo, Santa María, Consag Sur, and Wagner Sur faults. The Wagner and Consag basins are delimited to the east by the Wagner Sur Fault, and to the west by the Consag Sur Fault. The Percebo Fault borders the western margin of the modern Wagner Basin depocenter, and is oriented N10°W, dipping (on average) ～40° to the northeast. The trace of the Santa María Fault located in the Wagner Basin strikes N19°W, dipping ～40° to the west. The Consag Sur Fault is oriented N14°W, and dips ～42° to the east over a distance of 21 km. To the east of the study area, the Wagner Sur Fault almost parallels the Consag Sur Fault over a distance of ～86 km, and is oriented N10°W with an average dip of 59° to the east. Moreover, the data provide new evidence that the Wagner Fault is discontinuous between the two basins, and that its structure is more complex than previously reported. A structural high separates the northern Consag Basin from the southern Wagner Basin, comprising several secondary faults oriented NE oblique to the main faults of N–S direction. These could represent a zone of accommodation, or transfer zone, where extension could be transferred from the Wagner to the Consag Basin, or vice versa. This area shows no acoustic basement and/or intrusive body, which is consistent with existing gravimetric and magnetic data for the region.     

A deep cyclonic gyre in the Australian–Antarctic Basin

The traditional image of ocean circulation between Australia and Antarctica is of a dominant belt of eastward flow, the Antarctic Circumpolar Current, with comparatively weak adjacent westward flows that provide anticyclonic circulation north and cyclonic circulation south of the Antarctic Circumpolar Current. This image mostly follows from geostrophic estimates from hydrography using a bottom level of no motion for the eastward flow regime which typically yield transports near 170 Sv. Net eastward transport of about 145 Sv for this region results from subtracting those westward flows. This estimate is compatible with the canonical 134 Sv through Drake Passage with augmentation from Indonesian Throughflow (around 10 Sv).A new image is developed from World Ocean Circulation Hydrographic Program sections I8S and I9S. These provide two quasi-meridional crossings of the South Australian Basin and the Australian–Antarctic Basin, with full hydrography and two independent direct-velocity measurements (shipboard and lowered acoustic Doppler current profilers). These velocity measurements indicate that the belt of eastward flow is much stronger, 271 ± 49 Sv, than previously estimated because of the presence of eastward barotropic flow. Substantial recirculations exist adjacent to the Antarctic Circumpolar Current: to the north a 38 ± 30 Sv anticyclonic gyre and to the south a 76 ± 26 Sv cyclonic gyre. The net flow between Australia and Antarctica is estimated as 157 ± 58 Sv, which falls within the expected net transport of 145 Sv.The 38 Sv anticyclonic gyre in the South Australian Basin involves the westward Flinders Current along southern Australia and a substantial 33 Sv Subantarctic Zone recirculation to its south. The cyclonic gyre in the Australian–Antarctic Basin has a substantial 76 Sv westward flow over the continental slope of Antarctica, and 48 ± 6 Sv northward-flowing western boundary current along the Kerguelen Plateau near 57°S. The cyclonic gyre only partially closes within the Australian–Antarctic Basin. It is estimated that 45 Sv bridges westward to the Weddell Gyre through the southern Princess Elizabeth Trough and returns through the northern Princess Elizabeth Trough and the Fawn Trough – where a substantial eastward 38 Sv current is hypothesized. There is evidence that the cyclonic gyre also projects eastward past the Balleny Islands to the Ross Gyre in the South Pacific.The western boundary current along Kerguelen Plateau collides with the Antarctic Circumpolar Current that enters the Australian–Antarctic Basin through the Kerguelen–St. Paul Island Passage, forming an energetic Crozet–Kerguelen Confluence. Strongest filaments in the meandering Crozet-Kerguelen Confluence reach 100 Sv. Dense water in the western boundary current intrudes beneath the densest water of the Antarctic Circumpolar Current; they intensely mix diapycnally to produce a high potential vorticity signal that extends eastward along the southern flank of the Southeast Indian Ridge. Dense water penetrates through the Ridge into the South Australian Basin. Two escape pathways are indicated, the Australian–Antarctic Discordance Zone near 125°E and the Geelvinck Fracture Zone near 85°E. Ultimately, the bottom water delivered to the South Australian Basin passes north to the Perth Basin west of Australia and east to the Tasman Basin.     

Radionuclides as tracers of water fronts in the South Indian Ocean—ANTARES IV Results

Anthropogenic ⁹⁰Sr, ^239,240Pu and ²⁴¹Am were used as tracers of water mass circulation in the Crozet Basin of the South Indian Ocean, represented by three main water fronts—Agulhas (AF), Subtropical (STF) and Subantarctic (SAF). Higher ⁹⁰Sr concentrations observed north of 43°S were due to the influence of AF and STF, which are associated with the south branch of the Subtropical gyre, which acts as a reservoir of radionuclides transported from the North to the South Indian Ocean. On the other hand, the region south of 43°S has been influenced by SAF, bringing to the Crozet Basin Antarctic waters with lower radionuclide concentrations. The ²³⁸Pu/^239,240Pu activity ratios observed in water and zooplankton samples indicated that, even 35 years after the injection of ²³⁸Pu to the Indian Ocean from the burn-up of the SNAP-9A satellite, the increased levels of ²³⁸Pu in surface water and zooplankton are still well visible. The radionuclide concentrations in seawater and their availability to zooplankton are responsible for the observed ²¹⁰Po, ^239,240Pu and ²⁴¹Am levels in zooplankton.     

The morphotectonics and its evolutionary dynamics of the central Southwest Indian Ridge (49° to 51°E)

The morphotectonic features and their evolution of the central Southwest Indian Ridge （SWIR） are dis- cussed on the base of the high-resolution flfll-coverage bathyraetric data on the ridge between 49°-51°E. A comparative analysis of the topographic features of the axial and flank area indicates that the axial topogra- phy is alternated by the ridge and trough with en echelon pattern and evolved under a spatial-temporal mi- gration especially in 49°-50.17°E. It is probably due to the undulation at the top of the mantle asthenosphere, which is propagating with the mantle flow. From 50.17° to 50.7°E, is a topographical high terrain with a crust much thicker than the global average of the oceanic crust thickness. Its origin should be independent of the spreading mechanism of ultra-slow spreading ridges. The large numbers of volcanoes in this area indicate robust magmatic activity and may be related to the Crozet hot spot according to RMBA （residual mantle Bouguer anomaly）. The different geomorphological feature between the north and south flanks of the ridge indicates an asymmetric spreading, and leading to the development of the OCC （oceanic core complex）. The tectonic activity of the south frank is stronger than the north and is favorable to develop the OCC. The first found active hydrothermal vent in the SWIR at 37°47＇S, 49°39＇E is thought to be associated with the detach- ment fault related to the OCC.     

Regional circulation around Heard and McDonald Islands and through the Fawn Trough,central Kerguelen Plateau

The fine-scale circulation around the Heard and McDonald Islands and through the Fawn Trough, Kerguelen Plateau, is described using data from three high-resolution CTD sections, Argo floats and satellite maps of chlorophyll a, sea surface temperature (SST) and absolute sea surface height (SSH). We confirm that the Polar Front (PF) is split into two branches over the Kerguelen Plateau, with the NPF crossing the north-eastern limits of our survey carrying 25 Sv to the southeast. The SPF was associated with a strong eastward-flowing jet carrying 12 Sv of baroclinic transport through the deepest part of Fawn Trough (relative to the bottom). As the section was terminated midway through the trough this estimate is very likely to be a lower bound for the total transport. We demonstrate that the SPF contributes to the Fawn Trough Current identified by previous studies. After exiting the Fawn Trough, the SPF crossed Chun Spur and continued as a strong north-westward flowing jet along the eastern flank of the Kerguelen Plateau before turning offshore between 50°S and 51.5°S. Measured bottom water temperatures suggest a deep water connection between the northern and southern parts of the eastern Kerguelen Plateau indicating that the deep western boundary current continues at least as far north as 50.5°S. Analysis of satellite altimetry derived SSH streamlines demonstrates a southward shift of both the northern and southern branches of the Polar Front from 1994 to 2004. In the direct vicinity of the Heard and McDonald islands, cool waters of southern origin flow along the Heard Island slope and through the Eastern Trough bringing cold Winter Water (WW) onto the plateau. Complex topography funnels flow through canyons, deepens the mixed layer and increases productivity, resulting in this area being the preferred foraging region for a number of satellite-tracked land-based predators.     

The Trobriand Subduction System in the Western Solomon Sea

A south-dipping Subduction system which underlies the Trobriand Trough and 149° Embayment, on the southern margin of the Solomon Sea, is active or was recently active. Oceanic basement is overlain by 2.5 s, two-way travel time (TWTT), of sediment that shows at least two stages of deformation: early thrusts (inner wall) and normal faults (outer wall), and later normal faults that have elevated the outer trench margin. Thrust anticlines and slope basins are developed on the inner wall. The floor of the Solomon Sea Basin arches upward between the Trobriand Trough and the New Britain Trench to form isolated peaks and ridges in the east (152° Peaks) and an east-west Central Ridge in the west. Structures in the subduction system, and in the Solomon Sea Basin, plunge westward towards the point of collision with the New Britain Trench.