Deep-towed Sidescan Research Articles

New insights into geology and geochemistry of the Kerch seep area in the Black Sea

High-resolution 3D seismic data in combination with deep-towed sidescan sonar data and porewater analysis give insights into the seafloor expression and the plumbing system of the actively gas emitting Kerch seep area, which is located in the northeastern Black Sea in around 900 m water depth, i.e. well within the gas hydrate stability zone (GHSZ). Our analysis shows that the Kerch seep consists of three closely spaced but individual seeps above a paleo-channel-levee system of the Don Kuban deep-sea fan. We show that mounded seep morphology results from sediment up-doming due to gas overpressure. Each of the seeps hosts its own gas pocket underneath the domes which are fed with methane of predominantly microbial origin along narrow pipes through the GHSZ. Methane transport occurs dominantly in the form of gas bubbles decoupled from fluid advection. Elevated sediment temperatures of up to 0.3 °C above background values are most likely the result of gas hydrate formation within the uppermost 10 m of the sediment column. Compared to other seeps occurring within the GHSZ in the Black Sea overall only scarce gas indications are present in geoacoustic and geophysical data. Transport-reaction modeling suggests that the Kerch seep is a young seep far from steady state and probably not more than 500 years old.

Resource Assessment of Polymetallic Nodules Using Acoustic Backscatter Intensity Data from the Korean Exploration Area, Northeastern Equatorial Pacific

A high level of confidence in resource data is a key prerequisite for conducting a reliable economic feasibility study in deep water seafloor mining. However, the acquisition of accurate resource data is difficult when employing traditional point-sampling methods to assess the resource potential of polymetallic nodules, given the vast size of the survey area and high spatial variability in nodule distribution. In this study, we analyzed high-resolution acoustic backscatter intensity images to estimate nodule abundance and increase confidence levels in nodule abundance data. We operated a 120 kHz deep-towed sidescan sonar (DSL-120) system (1×1 m resolution) across a 75 km2 representative area in the Korean Exploration Area for polymetallic nodules in the Northeastern Equatorial Pacific. A deep-towed camera system was also run along two tracks in the same area to estimate the abundance of polymetallic nodules on the seafloor. Backscatter data were classified into four facies based on intensity. The facies with the weakest and strongest backscatter intensities occurred in areas of high slope gradient and basement outcrops, respectively. The backscatter intensities of the two other facies correlated well with the nodule abundances estimated from still-camera images. A linear fit between backscatter intensity and mean nodule abundance for 10 zones in the study area yielded an excellent correlation (r2 = 0.97). This allowed us to compile a map of polymetallic nodule abundance that shows greater resolution than a map derived from the extrapolation of point-sampling data. Our preliminary analyses indicate that it is possible to greatly increase the confidence level of nodule resource data if the relationship between backscatter intensity and nodule abundance is reliably established. This approach has another key advantage over point sampling and image analyses in that detailed maps of mining obstacles along the seafloor are produced when acquiring data on the abundance of polymetallic nodules. The key limitation of this work is a poor correlation between nodule coverage, as observed from photographs, and nodule abundance. Significant additional ground truth sampling using well located box cores should be completed to determine whether or not there is a real correlation between the backscatter and abundance.

中部マリアナトラフ拡大軸における火山活動位置についての考察 : 深海曳航式サイドスキャンソーナーと「しんかい6500」による高解像度観察の記録から

中部マリアナトラフ拡大軸における火山活動位置についての考察 : 深海曳航式サイドスキャンソーナーと「しんかい6500」による高解像度観察の記録から

Fluid venting and seepage at accretionary ridges: the Four Way Closure Ridge offshore SW Taiwan

Within the accretionary prism offshore SW Taiwan, widespread gas hydrate accumulations are postulated to occur based on the presence of a bottom simulating reflection. Methane seepage, however, is also widespread at accretionary ridges offshore SW Taiwan and may indicate a significant loss of methane bypassing the gas hydrate system. Four Way Closure Ridge, located in 1,500 m water depth, is an anticlinal ridge that would constitute an ideal trap for methane and consequently represents a site with good potential for gas hydrate accumulations. The analysis of high-resolution bathymetry, deep-towed sidescan sonar imagery, high-resolution seismic profiling and towed video observations of the seafloor shows that Four Way Closure Ridge is and has been a site of intensive methane seepage. Continuous seepage is mainly evidenced by large accumulations of authigenic carbonate precipitates, which appear to be controlled by the creation of fluid pathways through faulting. Consequently, Four Way Closure Ridge is not a closed system in terms of fluid migration and seepage. A conceptual model of the evolution of gas hydrates and seepage at accretionary ridges suggests that seepage is common and may be a standard feature during the geological development of ridges in accretionary prisms. The observation of seafloor seepage alone is therefore not a reliable indicator of exploitable gas hydrate accumulations at depth.

Near-bottom current speed maxima in North Atlantic contourite environments inferred from current-induced bedforms and other seabed evidence

Seabed acoustic data (e.g. deep-tow side scan sonar) together with information from seabed photography is presented for estimating near-bottom current speed maxima in North Atlantic contourite environments. These include the southern Iceland–Faroe Ridge flank, the southeast and southwest Greenland Margin, and the Greater Antilles Outer Ridge in the southwestern part of the North Atlantic. Geomorphological response to North Atlantic deep water circulation is expressed in a variety of dynamic bedforms and other current-controlled seabed features. Various types of dynamic bedforms reflect different hydraulic energy levels. Maximum near-bottom flow speed of almost 1.0m/s, or more, is illustrated by ‘infinite’ sand ribbons found on lag sediment, while a much lower maximum of near-bottom water flow speed (<0.5m/s) can be inferred for fine-grained sediment environments where small erosional furrows and small-scale ripple marks occur. These various dynamic bedforms notably are indicators of (past) extreme current events often not captured by current measurements, which is due to decadal-scale variability in deep-ocean circulation. The results from our study demonstrate that at (multi)decadal time scale near-bottom current speed may in some areas reach values well in excess of what is known from current measurements.

Morpho-acoustic variability of cold seeps on the continental slope offshore Nicaragua: Result of fluid flow interaction with sedimentary processes

Based on multibeam bathymetry, high-resolution deep-towed sidescan sonar and Chirp subbottom profiling 32 cold seep sites, already identified in Sahling et al. (2008a), have been studied in an approximately 1000 km 2 large area ranging from 800 to 2600 m water depth along the middle slope of the active continental margin offshore Nicaragua. Ground truthing is available from towed camera surveys and coring on seven of the structures. The seeps occur in different settings on the slope: upslope and along the headwall of large submarine slides, as isolated eroded massifs, and forming linear ridges between deeply incised canyons. The seep sites show a wide range regarding their size and morphology, their backscatter intensity patterns, their structure in subbottom profiles, and their fluid venting activity inferred from seafloor observations. Surface extension of the seep sites ranges from less than 200 to more than 1500 m in diameter, and relief height varies between no relief and 180 m. Indications of extruded materials such as mud flows are not observed in the area of the seep sites. Instead the seeps are characterized by high proportions of authigenic carbonates. The carbonates occur as crusts, detritus, or single layers embedded in the seafloor sediments. They appear as high backscatter intensities on sidescan sonar images. On some seep sites living vent fauna indicative of active seepage is observed, but gas bubbles have not been observed. To explain the high morphological variability of the features, we propose a generic model including the interaction of several processes: (1) episodic fluid venting and associated authigenic carbonate formation; (2) background sedimentation and subsidence; (3) linear erosion along canyons and denudation on the slope surface.

Gas seepage, pockmarks and mud volcanoes in the near shore of SW Taiwan

In order to understand gas hydrate related seafloor features in the near shore area off SW Taiwan, a deep-towed sidescan sonar and sub-bottom profiler survey was conducted in 2007. Three profiles of high-resolution sub-bottom profiler reveal the existence of five gas seeps (G96, GS1, GS2, GS3 and GS4) and one pockmark (PM) in the study area. Gas seeps and pockmark PM are shown in lines A and C, while no gas venting feature is observed along line B. This is the first time that a gas-hydrate related pockmark structure has been imaged off SW Taiwan. The relatively high backscatter intensity in our sidescan sonar images indicates the existence of authigenic carbonates or chemosynthetic communities on the seafloor. More than 2,000 seafloor photos obtained by a deep-towed camera (TowCam) system confirm the relatively high backscatter intensity of sidescan sonar images related to bacteria mats and authigenic carbonates formation at gas seep G96 and pockmark PM areas. Water column gas flares are observed in sidescan sonar images along lines A and C. Likewise, EK500 echo sounder images display the gas plumes above gas seep G96, pockmark PM and gas seep GS1; the gas plumes heights reach about 150, 100 and 20 m from seafloor, respectively. Based on multichannel seismic reflection (MCS) profiles, an anticline structure trending NNE-SSW is found beneath gas seep G96, pockmark PM and gas seep GS2. It implies that the gas venting features are related to the anticline structure. A thermal fluid may migrate from the anticline structure to the ridge crest, then rises up to the seafloor along faults or fissures. The seafloor characteristics indicate that the gas seep G96 area may be in a transitional stage from the first to second stage of a gas seep self-sealing process, while the pockmark PM area is from the second to final stage. In the pockmark PM area, gas venting is observed at eastern flank but not at the bottom while authigenic carbonates are present underneath the pockmark. It implies that the fluid migration pathways could have been clogged by carbonates at the bottom and the current pathway has shifted to the eastern flank of the pockmark during the gas seep self-sealing process.

Fluid seepage and mound formation offshore Costa Rica revealed by deep-towed sidescan sonar and sub-bottom profiler data

Our analysis of geoacoustic data from the Middle American margin provides an insight into the formation and evolution mechanisms of mound structures observed on the continental slope offshore Costa Rica. Based on high-resolution deep-towed sidescan sonar and sub-bottom profiler (SBP) data six different mound and fluid seepage structures at the Hongo mound field are characterized in detail. The Hongo mound field is located on the lower continental slope offshore Nicoya peninsula in the prolongation of the subducting Nicoya fracture zone. The mounds have oval to circular shapes with diameters of 500–1600 m and relief heights of 60–100 m. High backscatter anomalies near the mound summits indicate carbonate precipitation and focused fluid seepage activity within the last 10 ka. The data do not show evidence for mud extrusions and the structures were probably formed by a combination of carbonate precipitation and mud diapirism. Based on seismic stratigraphy analysis, mud diapirism is at least active since 42.5–57 ka and average vertical growth rates vary between 6–24 cm/ka. However, if diapirism represents the dominant mound evolution mechanism, mound heights of 80 m point to much older mound ages of 330–1330 ka.

Hydrothermal activity on the southern Mid-Atlantic Ridge: Tectonically- and volcanically-controlled venting at 4–5°S

We report results from an investigation of the geologic processes controlling hydrothermal activity along the previously-unstudied southern Mid-Atlantic Ridge (3–7°S). Our study employed the NOC (UK) deep-tow sidescan sonar instrument, TOBI, in concert with the WHOI (USA) autonomous underwater vehicle, ABE, to collect information concerning hydrothermal plume distributions in the water column co-registered with geologic investigations of the underlying seafloor. Two areas of high-temperature hydrothermal venting were identified. The first was situated in a non-transform discontinuity (NTD) between two adjacent second-order ridge-segments near 4°02′S, distant from any neovolcanic activity. This geologic setting is very similar to that of the ultramafic-hosted and tectonically-controlled Rainbow vent-site on the northern Mid-Atlantic Ridge. The second site was located at 4°48′S at the axial-summit centre of a second-order ridge-segment. There, high-temperature venting is hosted in an ∼ 18 km 2 area of young lava flows which in some cases are observed to have flowed over and engulfed pre-existing chemosynthetic vent-fauna. In both appearance and extent, these lava flows are directly reminiscent of those emplaced in Winter 2005−06 at the East Pacific Rise, 9°50′N and reference to global seismic catalogues reveals that a swarm of large (M 4.6−5.6) seismic events was centred on the 5°S segment over a ∼ 24 h period in late June 2002, perhaps indicating the precise timing of this volcanic eruptive episode. Temperature measurements at one of the vents found directly adjacent to the fresh lava flows at 5°S MAR (Turtle Pits) have subsequently revealed vent-fluids that are actively phase separating under conditions very close to the Critical Point for seawater, at ∼ 3000 m depth and 407 °C: the hottest vent-fluids yet reported from anywhere along the global ridge crest.

Pockmarks in the Northern Congo Fan area, SW Africa: Complex seafloor features shaped by fluid flow

Three pockmarks named “Hydrate Hole”, “Black Hole”, and “Worm Hole” were studied in the northern Congo Fan area at water depths around 3100 m. The cross-disciplinary investigations include seafloor observations by TV-sled, sampling by TV-guided grab and multicorer as well as gravity coring, in addition to hydroacoustic mapping by a swath system, a parametric sediment echosounder and a deep-towed sidescan sonar. The pockmarks are morphologically complex features consisting of one or more up to 1000 m wide and 10–15 m deep depressions revealed by swath-mapping. High reflection amplitudes in the sediment echosounder records indicate the presence of a 25–30 m thick shallow sediment section with gas hydrates, which have been recovered by gravity corer. Hydrates, chemosynthetic communities, and authigenic carbonates clearly indicate fluid flow from depths, which we propose to be mainly in the form of ascending gas bubbles rather than advection of methane-rich porewater. Evidence for seepage at the seafloor is confined to small areas within the seafloor depressions and was revealed by characteristic backscatter facies. Small meter-scale sized depressions signified as “pits” exist in or close to the pockmarks but seafloor observations did not reveal evidence for the presence of typical seep organisms or authigenic carbonates. Areas of intermediate backscatter were inhabited by vesicomyid clams in soft sediments. High backscatter was associated with vestimentiferan tubeworms (Siboglinidae) and authigenic carbonates. We discuss the three different environments “pits”, “vesicomyid clams”, “vestimentifera/carbonate” in the light of differences in the geochemical setting. Pits are probably formed by escaping gas bubbles but seepage is too transient to sustain chemosynthetic life. Vesicomyid clams are present in sediments with gas hydrate deposits. However, the hydrates occur several meters below the surface indicating a lower flux compared to the vestimentifera/carbonate environment. In the latter environment, accumulated carbonates and clam shells indicate that fine grained particles have been eroded away. Gas hydrates were found in this environment at depths below about 50 cm suggesting the highest supply with methane compared to the other environments.

Shelf to slope sedimentation processes and the impact of Plio–Pleistocene glaciations in the northeast Atlantic, west of Ireland

The eastern margin of the Rockall Trough, between 54°30′ and 56°30′N, straddles the southern limit of the glaciated North Atlantic margin. A complex system of canyons, channels and slope failure features on the steep margins of the trough is resolved on the TRIM (TOBI Rockall Irish Margins Project) deep-towed sidescan survey. The canyons consist of a complex headwall gullied region, produced by multiple slope failure events, and interconnect to form a cauliform pattern in plan view. Individual gullies are U-shaped in cross-section in the upper parts of the margin and are connected downslope to single, deeply-incised, V-shaped channels. These channels feed extensive aprons of sediment on the lower slope supplied by failure of the headwall regions and sediment transport along the shelf into the canyon headwalls. The change in the canyon morphology systems reflects a more abundant supply of glaciomarine sedimentary debris from Pleistocene ice sheets, whose southerly limit lay along this part of the eastern Rockall margin. A good correlation between acoustic sidescan backscatter and more deeply penetrating 3.5 kHz profiler data suggests that sediment transport pathways were stable through time. Slope failure escarpments occur in association with the canyon systems along the upper part of the slope. Canyons are absent in the north of the area where the slope gradient is lowest. This is interpreted as the result of progradation of sheet-like flows of glaciomarine debris across the southern part of the Donegal Fan. A poor correlation between acoustic backscatter and sub-bottom profile data indicates that sediment transport pathways varied through time in the region. Evidence for strong northeast-flowing bottom currents is present along the upper slope and shelf. These currents transport and sort sediments along the entire eastern margin of the Rockall Trough from the south of the Porcupine Bank to the Hebrides Terrace Seamount.

Carbonate mound development at the SW Rockall Trough margin based on high resolution TOBI and seismic recording

In 2002, high-resolution sidescan sonar images of a mound area at the SW Rockall Trough margin were recorded with the TOBI deep towed sidescan sonar. Processed TOBI images with a pixel resolution of 6 m provide a unique overview of the carbonate mound distribution and related sedimentary features around the mounds. Three morphologically distinct areas can be recognised on the TOBI mosaic. In area I (between 500 and 600 m water depth), upslope of the mounds, giant sediment waves are found with wavelengths up to 500 m, with wave crests of more than 10 km long and with an amplitude of up to 30 m, reflected with a high backscatter on the TOBI image. Area II (between 600 and 1000 m water depth), along the upper margin flank, is characterised by clustered and isolated carbonate mounds, forming elongated ridges with an orientation perpendicular to the slope. Sedimentary structures such as flow ridges, sediment waves, local scouring at the foot of some mounds and draping of sediment around mounds indicate the influence of strong near-bed currents, oriented in two main current directions parallel to the margin, as well as in a similar direction as the mound clusters. The mound clusters are several kilometres long, can be over 380 m high and are dissected by downslope directed channels. On the TOBI image, the mounds appear as regions of high backscatter caused by the presence of cold water coral colonies on the sediment and by distinct shadows. All mounds have their tops at a specific depth level (500–600 m water depth). Area III (between 900 and 1200 m water depth) below the mounds is characterised by disturbed sediments with distinct slump scars and flow ridges. 2D high resolution seismic profiling perpendicular and parallel to the slope across the mounds does in general not reveal strong internal reflectors within the mounds. In contrast, three strong reflectors can be observed in the sedimentary sequence underneath the mounds. The upper reflector C10 (of early Pliocene age) is an erosional unconformity, above which most of the mounds and sediment waves seem to have developed. The second erosional unconformity C20 (of middle Miocene age) is formed by an apparent irregular surface that locally is dissected by some small faults. Reflectors C10 and C20 have been found in two different mound clusters in the mound area, indicating that mound development possibly started after formation of reflector C20. The third reflector forms the acoustic basement, which is locally dissected by some small faults that are located below the mounds. At least two stages of mound formation are recognised on the seismic profiles.

The death mask of the antarctic ice sheet: Comparison of glacial geomorphic features across the continental shelf

Multibeam swath bathymetry data, seismic lines, and shallow cores were collected seaward of all of the major drainage outlets of the Antarctic Ice Sheet from the Pennell Coast of North Victoria Land to the northwestern Weddell Sea. The results show that during the last glacial maximum, the ice sheet extended onto the outer shelf in all areas, to the shelf break in many. Swath bathymetry and deep-tow sidescan sonar data from the continental shelf also record the geomorphology left by the retreating ice sheet. Glacial troughs occur offshore of all major glacial outlets. In each drainage area except for parts of the Ross Sea, the inner shelf is characterized by acoustic basement interpreted as crystalline substrate. The geomorphology of these inner shelf areas consists of erosional features such as grooves that show that ice flow tended to follow the structural grain of the bedrock. Outer shelf areas are floored by sedimentary substrate where the direction of ice flow was more directly offshore and depositional features characterize the seafloor. In these areas the signature of the grounded ice consists of till deposits and large-scale geomorphic features, mega-scale glacial lineations. Drumlins occur within the region of contact between crystalline and sedimentary substrates. Meltwater channels also have been imaged in both inner and outer shelf settings. While the presence of meltwater across the shelf has long been suspected, we present here the first multibeam image of a meltwater channel on the outer continental shelf of Antarctica. At the shelf break gullies are in all areas where we know ice reached the shelf break. We note the remarkable similarity in form of mega-scale glacial lineations observed on the outer shelf over sedimentary substrate across all areas surveyed, typically measuring 200–600 m crest to crest. This suggests that the shape of these landforms is governed by the same process in each drainage area and that process was occurring under a large area of the ice sheet. The erosional and transitional features show much variability in form and distribution.

Slope failure, mass flow and bottom current processes in the Rockall Trough, offshore Ireland, revealed by deep-tow sidescan sonar

The margins of the Rockall Trough, currently the focus of oil and gas exploration interest, incorporate an environmentallyimportant and lightly studied shelf region. To address the paucity of information within this region an extensive high resolution sidescan sonar survey was carried out in the Irish sector of the Rockall Trough in June/July 1998. This valuable natural laboratory contains information on slope stability, sediment transport, nutrient upwelling, bottom current, and biological activity at a shelf/slope/basin-floor transition. Detailed information on the nature of many of these processes can be provided from shallow geophysical techniques. The TRIM (TOBI Rockall Irish Margins) project (Shannon et al., 2001), funded by the Irish Petroleum Infrastructure Programme (Rockall Studies Group), acquired 3100 line-km of TOBI sidescan sonar and 3700 km of 3.5 kHz profiler data along the margins of the Rockall Trough (Figure 1). This project followed on from the earlier reconnaissance AIRS (Atlantic Irish Regional Survey) survey conducted over the entire southern Rockall Trough region, using the lower resolution surface towed sonar side-scan system known as GLORIA (Unnithan et al., 2001). The TOBI sidescan system, developed at the Southampton Oceanography Centre (Flewellen et al., 1993), is deep-towed at 300-400 m above the seafloor. It transmits at a frequency of 32 kHz and samples the acoustic response of the seabed and shallow sedimentary structure to a centimetre to metre depth scale. This acoustic response therefore reflects not only the backscattering properties of the seabed but also the shallow detailed structure of sediments formed by recent sedimentological processes.

A model for the development of a carbonate mound population in the Rockall Trough based on deep-towed sidescan sonar data

A large carbonate mound population is identified on deep-towed TOBI (towed ocean bottom instrument) sidescan data along the eastern margin of the Rockall Trough, west of Ireland. Individual mounds are circular to elliptical in plan view, varying from 50 to 850 m in width and up to about 200 m in height. Strong NE-flowing contour currents at 800 m water depth are inferred from large-scale sedimentary bedforms. Smaller mound arrays are spatially associated with slope-parallel escarpments produced by mass wastage, while around the largest mounds the sharp escarpments may have been smoothed by the vigorous contour currents. Current streamlining effects control the shape of the mounds, which become more elliptical as their size increases, thereby minimising the hydraulic drag force. The frequency distribution of mound size follows a general power law, which is determined by the growth rate of the framework-building coral species and the rate at which they colonised the substrate. Initially, bottom currents support mound growth until the mounds become so large that hydraulic drag forces retard their growth. A model for the evolution of the population predicts that increased hydraulic drag forces on the larger mounds cause a sharp decrease in their number, in agreement with the observations. The model also allows an age structure for the population to be determined and correlations between the growth of the mound population and palaeoclimatic variations in the NE Atlantic to be attempted.

Magmato‐tectonic cyclicity at the ultra‐slow spreading Southwest Indian Ridge: Evidence from variations of axial volcanic ridge morphology and abyssal hills pattern

On‐axis deep tow side scan sonar data are used together with off‐axis bathymetric data to investigate the temporal variations of the accretion processes at the ultra‐slow spreading Southwest Indian Ridge. Differences in the length and height of the axial volcanic ridges and various degrees of deformation of these volcanic constructions are observed in side scan sonar images of the ridge segments. We interpret these differences as stages in an evolutionary life cycle of axial volcanic ridge development, including periods of volcanic construction and periods of tectonic dismemberment. Using off‐axis bathymetric data, we identify numerous abyssal hills with a homogeneous size for each segment. These abyssal hills all display an asymmetric shape, with a steep faulted scarp facing toward the axis and a gentle dipping volcanic slope facing away. We suggest that these hills are remnants of old split axial volcanic ridges that have been transported onto the flanks and that they result from successive periods of magmatic construction and tectonic dismemberment, i.e., a magmato‐tectonic cycle. We observe that large abyssal hills are in ridge sections of thicker crust, whereas smaller abyssal hills are in ridge sections of thinner crust. This suggests that the magma supply controls the size of abyssal hills. The abyssal hills in ridge sections of thinner crust are regularly spaced, indicating that the magmato‐tectonic cycle is a pseudoperiodic process that lasts ∼0.4 m.y., about 4 to 6 times shorter than in ridge sections of thicker crust. We suggest that the regularity of the abyssal hills pattern is related to the persistence of a nearly constant magma supply beneath long‐lived segments. By contrast, when magma supply strongly decreases and becomes highly discontinuous, regular abyssal hills patterns are no longer observed.

TOBI sidescan sonar imagery of the very slow-spreading Southwest Indian Ridge: evidence for along-axis magma distribution

New deep tow sidescan sonar data from the Southwest Indian Ridge reveal complex volcanic/tectonic interrelationships in the axial zone of this ultra-slow spreading ridge. While some constructional volcanic features resemble examples documented at the slow-spreading Mid-Atlantic Ridge, such as axial volcanic ridges, hummocky and smooth lava flows, their distribution and dimensions differ markedly. The largest axial volcanic ridges occur at segment centres, but fresh-looking volcanic constructions also occur at segment ends and in the deep basins marking the non-transform discontinuities. The orientations of the dominant fault population and main volcanic ridges are controlled by tectonic processes such as orthogonal extension in the sections of the ridge perpendicular to the spreading direction and transtensional extension in the obliquely spreading sections of the ridge. Minor faults and small volcanic ridges striking parallel to the axis in the oblique part of the ridge are not controlled by these extensional regimes. This observation suggests that the ridge axis acts as a zone of weakness and that magmatic processes, with associated fractures opening in response to magma pressure, may control local emplacements of axial volcanic ridges at obliquely spreading ridges. This non-systematic pattern of ridge characteristics suggests an along-axis variation between focused and distributed magmatic supply, a model which is supported by our interpretation of low-amplitude mantle Bouguer anomalies calculated for the area. We propose that a change of the axial segmentation pattern, from two segments to the present-day three segments, may have introduced additional instability into the crustal accretion process.

Multibeam bathymetry and Deep Tow sidescan sonar surveys SW of San Clemente Island, CA

In March 1999, about 170 square nm of bathymetry were mapped from the 40- to 700-m contour lines using a 50-kHz Elac BottomChart multibeam echo-sounder temporarily installed aboard Scripps’ R/V NEW HORIZON. This survey provided the base map for a near-bottom 110-kHz sidescan sonar and 4-kHz subbottom profiler survey with the Marine Physical Lab. Deep Tow package. Notable seafloor geological features include a broad terrace, left over from the last glacial low sea level stand and sloping gently from the island’s shore to the 120-m depth contour, canyons and sediment aprons below, steep rocky extensions of China Point (island’s southern-most point), and the sheer wall of the San Clemente Escarpment to the east. The sidescan sonar map has poor contrast over the sandy bottom along the island’s western shore, whereas the rock spurs off China Point and a number of erosional channels carving the slopes between 200- and 800-m depth correspond to areas of high acoustic backscatter. Deep Tow subbottom profiles along the southern slope provide evidence of erosional channels cutting through sediment bedforms that are presumably compressional side effects of the strike-slip faulting along the San Clemente Escarpment.

Second-order segmentation; the relationship between volcanism and tectonism at the MAR, 38°N–35°40′N

Deep-tow sidescan sonar data acquired along 240 km of the Mid-Atlantic Ridge (MAR) between 35°40′N and 38°N have been combined with new bathymetric compilations and used to establish the recent tectonic and volcanic history of six second-order segments and their bounding non-transform offsets (NTOs). The segments show a range of volcanic and tectonic types, but in general the northernmost segments (i.e. those with greater influence from the Azorean hotspot) are shallower and more volcanically robust than those to the south, with hydrothermal activity in segment centres (for example, Menez Gwen and Lucky Strike). Nonetheless, this generalisation requires some modification due to temporal variations in the balance between magmatic supply and tectonic dismemberment. The NTOs are broad right-stepping discontinuities, locally up to 25 km wide, and accommodate offsets between 10 and 50 km. The discontinuities are mostly sediment-floored, and link the spreading segment tips with a range of structures. These include locally dense arrays of en echelon extensional normal faults, short lengths of linear strike–slip fault strands occasionally cutting basement blocks apparently stranded within the offsets. Basement blocks within the offsets are cross-cut by complexes of intersecting faults, suggesting that deformation is distributed across the zone. The pervasive faulting taking place at the NTOs favours fluid circulation and associated hydrothermal activity, as at the Rainbow Site at 36°17′N.

Ground-Truthing 11- to 12-kHz side-scan sonar imagery in the Norwegian-Greenland Sea: Part II: Probable diapirs on the Bear Island fan slide valley margins and the Vøring Plateau

SeaMARC II (11- to 12-kHz) side-scan sonar revealed hundreds of small strong-backscatter spots, tens to 500 m in diameter, along the lips of the Bear Island fan slide valley. New bathymetry, deep-tow side-scan, deep-tow profiles, heatflow, and gravity cores were collected for ground-truth. These mounds are probably mud diapirs (or mud-built mounds) typically 10–75 m high, formed by glacial sediment mobilized by Late Pleistocene slide events. The mounds are arranged along NNE trending lines, suggesting control by intrasedimentary faults ca. 0.5–1 km apart. Diapirs examined on the Voring Plateau exhibit WNW structural control. No heatflow anomaly was found in four stations on or next to diapirs in either area.