Showing papers in "Marine Geophysical Researches in 1998"

Morpho-tectonic analysis of the Azores Volcanic Plateau from a new bathymetric compilation of the area

Abstract: The existing studies of the Azores triple junction, although based on specific geological or geophysical data, largely rely upon morphological considerations. However, there is no systematic bathymetric coverage of this area, and the quality of the available bathymetric charts does not allow consistent morpho-structural analysis. In this work we present a new bathymetric grid elaborated with all the available data sources in an area comprised between 24° W to 32° W and 36° N to 41° N. The basic data set corresponds to the merge of NGDC data with new swath profiles. This new map, included as an Appendix, combined with other results from seismology and neotectonics, is the basis for the study of the morpho-structural pattern of the Azores area, the present day stress field and its implications on the current view of the Azores geodynamics. As a major result, we conclude that the Azores region is controlled by two sets of conjugated faults with 120° and 150° strikes that establish the framework for the onset of volcanism, expressing as linear volcanic ridges or as point source volcanism. This interaction develops what can be considered as the morphological signature of the Azores Spreading axis segmentation. We argue that the Azores domain, presently in a broad transtensional regime, is acting simultaneously as a ultra slow spreading centre and as a transfer zone between the MAR and the dextral Gloria Fault, as it accommodates the differential shear movement between the Eurasian and African plates.

A New Scenario of the Parece Vela Basin Genesis

Abstract: A new high density geophysical data set in the Parece Vela Basin north of 15°N has been obtained through surveys conducted by the Hydrographic Department of Japan. The combined analyses of the swath bathymetry, magnetic and gravity anomalies from these surveys reveal a new scenario for the genesis of this basin. The evolutionary process is as follows: rifting and crust thinning (29–26 Ma), northward propagation of east-west opening (26-23 Ma) , east-west opening together with the Shikoku Basin (23–21 Ma), and the northeast-southwest opening (20/19–15 Ma). The western part of the basin is complicated, displaying some traces of northward propagation of the spreading center. The change between early east-west opening and the final stage of northeast-southwest spreading is marked by a distinct north-south boundary in both structural and magnetic patterns. Deep and rough topography of the extinct Parece Vela Rift is due to magma starvation in the terminal phase of the spreading.

Namibia volcanic margin

Abstract: Two end members of passive margin types are present on the Namibia margin: volcanic and non-volcanic. The central and southern parts of the Namibia margin feature characteristic volcanic margin elements such as thickened initial oceanic crust, seaward dipping reflectors, and high-velocity/density lower crust that extends beneath the rift zone that was formed during initial seafloor spreading in Early Cretaceous. The margin north of the Walvis FZ is non-volcanic in character and probably formed as a result of a ridge jump that occurred after cessation of the initial magmatic activity. The Walvis Ridge forms the boundary between the two margin types and resulted from the persistent magmatism associated with the Tristan plume. MCS data in conjunction with gravity modeling reveal a Paleozoic rift zone beneath the Namibia margin south of the Walvis FZ. The Paleozoic rift zone partly overlaps the Jurassic/Early Cretaceous rift zone which produced the breakup between Africa and South America. We calculate an average stretching value of β=1.4 for the Paleozoic rift, based on subsidence modeling. The rift is partly bounded by low-angle faults, related to the orogenic collapse of the Pan-African fold belt, which provided a major Paleozoic sediment source. The offshore continuity of onshore ophiolitic complexes is suggested by the coast parallel high-amplitude magnetic anomaly ’G‘, and low-angle detachment faults along the southern part of the margin. The average stretching value for the Jurassic/Early Cretaceous rift is β=1.7, which implies a syn-rift displacement on this margin of ∼70 km. The minimum igneous volume of the South Atlantic LIP was found to be in excess of 3.62×106 km3.

Continental margin off Western India and Deccan Large Igneous Province

Abstract: Massive, transient late syn-rift-to-breakup volcanism during separation between the Seychelles microcontinent and India formed the Deccan continental flood basalts and their equivalents on the Seychelles-Mascarene Plateau and on the conjugate continental margins, i.e. the Deccan Large Igneous Province. We estimate an original extrusive area of at least 1.8×106 km2, and a volume >1.8×106 km3, and suggest a plate tectonic model comprising: (1) development of the Seychelles microplate by fan-shaped spreading in the Mascarene Basin, and continental extension followed by fan-shaped spreading between India and the Seychelles during A29-27 time. (2) Cessation of fan-shaped spreading just after A27 time, followed by spreading along the India-Seychelles plate boundary. (3) Margin subsidence, modified south of Goa by the persistent, time-transgressive effects along the plume trail. The margin is divided into three regional provinces by the prolongation of regional transforms which formed the east and west boundaries of the Seychelles microplate during breakup and early sea floor spreading. In some aspects, the conjugate margins are different from other volcanic margins; e.g. regional wedges of seaward dipping reflectors along the continent-ocean transition have not yet been reported. We ascribe this to the eruption of the most voluminous lavas during chron 29r, i.e on continental lithosphere in a late syn-rift setting. The enigmatic Laxmi Ridge is a complex marginal high comprised of both continental and oceanic crust. It was probably created during breakup, but may have experienced later magmatic and/or tectonic deformation.

Oblique subduction of the Gagua Ridge beneath the Ryukyu accretionary wedge system: Insights from marine observations and sandbox experiments

Abstract: The Gagua Ridge, carried by the Philippine Sea Plate, is subducting obliquely beneath the southernmost Ryukyu Margin. Bathymetric swath-mapping, performed during the ACT survey (Active Collision in Taiwan), indicates that, due to the high obliquity of plate convergence, slip partitioning occurs within the Ryukyu accretionary wedge. A transcurrent fault, trending N95° E, is observed at the rear of the accretionary wedge. Evidence of right lateral motion along this shear zone, called the Yaeyama Fault, suggests that it accommodates part of the lateral component of the oblique convergence. The subduction of the ridge disturbs this tectonic setting and significantly deforms the Ryukyu Margin. The ridge strongly indents the front of the accretionary wedge and uplifts part of the forearc basin. In the frontal part of the margin, directly in the axis of the ridge, localized transpressive and transtensional structures can be observed superimposed on the uplifted accretionary complex. As shown by sandbox experiments, these N330° E to N30° E trending fractures result from the increasing compressional stress induced by the subduction of the ridge. Analog experiments have also shown that the reentrant associated with oblique ridge subduction exhibits a specific shape that can be correlated with the relative plate motion azimuth. These data, together with the study of the margin deformation, the uplift of the forearc basin and geodetic data, show that the subduction of the Gagua Ridge beneath the accretionary wedge occurs along an azimuth which is about 20° less oblique than the convergence between the PSP and the Ryukyu Arc. Taking into account the opening of the Okinawa backarc basin and partitioning at the rear of the accretionary wedge, convergence between the ridge and the overriding accretionary wedge appears to be close to N345° E and thus, occurs at a rate close to 9 cm yr−1. As a result, we estimate that a motion of 3.7 cm yr−1±0.7 cm should be absorbed along the transcurrent fault. Based on these assumptions, the plate tectonic reconstruction reveals that the subducted segment of the Gagua Ridge, associated with the observable margin deformations, could have started subducting less than 1 m.y. ago.

Late Cretaceous-Paleocene extension on the Vøring Volcanic Margin

Abstract: High-quality seismic data document a Maastrichtian-Paleocene rift episode on the Voring margin lasting for ∼20 m.y. prior to continental breakup. The rift structures are well imaged in the Fenris Graben and Gjallar Ridge region in the western Voring Basin, and are characterized by low-angle detachment faults with variable fault geometries from south to north. The structural restoration has facilitated the division of pre- and syn-rift sediments across the extensional terrain, which is subsequently used to evaluate mode and mechanism for the lithospheric deformation. Extension estimates based on the structural restoration, subsidence analysis and crustal thickness evaluations yield stretching factors ranging between 1.5 to 2.3 across the main fault zone just landward of the early Tertiary flood basalts. The structural restoration also shows that a middle crustal dome structure, observed beneath the low-angle faults, can be explained by extensional unroofing. Thus, the dome structure may represent a possible metamorphic core complex. Calculations of the effects on vertical motion, assuming uniform and two-layer differential stretching models combined with the arrival of the Iceland mantle plume during rifting, indicate that the uniform extension model may account for both observed early rift subsidence and subsequent late rift uplift and erosion. Although the differential model can not be excluded, it implies early rift uplift which is not compatible with our seismic interpretation. The direct and indirect effects of the Iceland mantle plume may have caused as much as 1.2 km of late rift uplift. Comparison of the volcanic Voring margin and the non-volcanic West Iberian margin shows similarities in terms of structural style as well as in mode and distribution of extension.

Laxmi Ridge – A continental sliver in the Arabian Sea

Abstract: Identification by Bhattacharya et al. (1994) of seafloor spreading type magnetic anomalies in the basin lying between Laxmi Ridge in the Arabian Sea and the Indian continent necessitates a change in plate tectonic reconstruction. Naini and Talwani (1982) named this basin the Eastern Basin and we will continue to use this term in this paper. Others, in the literature, have called this the Laxmi Basin. Previous reconstructions had assumed that the Eastern Basin is underlain by continental crust. The new reconstruction moves Seychelles' original location closer to India and ameliorates a ‘space’ problem in the Mascarene Basin. A new rotation pole between anomaly 28 and 34 times avoids ‘skipping’ of fracture zones resulting from rotation poles described earlier. The negative gravity anomaly over the Eastern Basin is a necessary consequence of a continental sliver lying between oceanic crust on either side. Seismic velocities that are slightly greater than 7 km s−1 under the Eastern need not be necessarily interpreted as material that underplates continental crust.

Tectonic setting of the Azores Plateau deduced from a OBS survey

Abstract: The studies of Azores seismicity generally show shocks with either normal faulting or right-lateral strike-slip along the ESE direction, compatible with a eastward relative motion of the Eurasian (EU) relative to the African (AF) plate. However, the 1 January 1980 earthquake was interpreted as a clear left-lateral strike-slip shock along the N150E direction. This pattern is difficult to explain in terms of the relative motion between the EU, AF and North American (NA) plates: all available models for the present day movement of this triple junction fail to explain the regional variability in the stress conditions of the area. Here we present data from a 34-day long Ocean Bottom Seismograph array deployment. We show that the seismicity is distributed along a band aligned with the island chain itself, and is concentrated along several faults with an approximate N150E strike, cutting the Azores plateau in all the area covered by the OBS network. The combination of these new results with other geophysical data permits us to conclude that the tectonic setting of the Azores plateau is characterised by the existence of two sets of faults, in the N120E and N150E directions, defining several crustal blocks, whose relative motion accommodates the interaction of the three megaplates. The deformation of these tectonic blocks is probably driven by the shear between the EU and AF plates. This model explains well the spatial variability of the stress conditions in the Azores domain, the combination of dextral and sinistral strike slip mechanisms and the observed seismotectonics of the Azores islands.

Constraints on the structure of the Azores spreading center from gravity data

Abstract: The Azores Archipelago is believed to be the site of the third arm of a Triple Junction between the Eurasia/Africa/North America plates. However, to the present no study has been able to identify its segmentation pattern, the spreading mechanism and its relationship with the well-known topographic features of the Mid-Atlantic Ridge (MAR). Here we present a new gravity compilation obtained with the existing National Geophysical Data Center (NGDC) data, merged with new gravity profiles collected during the ESCAPE cruise in 1995. This compilation is used to calculate a Free Air Anomaly (FAA) map, which is used to test two different models, the Mantle Bouguer Anomaly model and the elastic plate model, for the study of the thermal regime of the Terceira Axis. The analysis of the results from both models demonstrates that the elastic plate model successfully models the gravity data from the Azores Plateau and that there is no gravity evidence for the existence of a spreading axis. The elastic plate thickness Te, with a value of 7–8 km, suggests a very young lithosphere (about 10 Ma) at the time of the load of the Azores Plateau.

A detailed study of the Gagua Ridge: A fracture zone uplifted during a plate reorganisation in the Mid-Eocene

Abstract: Recent multibeam bathymetric and geophysical data recorded in the West Philippine Basin, east of Taiwan, reveal new information on the structure and the tectonic origin of the oceanic Gagua Ridge. This linear, 300 km-long, 4 km-high, north-south-trending ridge, is being subducted beneath the Ryukyu Trench along 123 E. This basement high separates two basins of different ages. Its summit is marked by two crests and an axial valley. A map of the basement top shows the region of the ridge to be composed of a set of linear and parallel ridges and troughs. All these elements suggest that the development of the ridge, and its surroundings, has been influenced by strikeslip deformation. Nevertheless, the height of the ridge indicates also an important compressive component in the deformation. Gravity models across the ridge show local compensation with a crustal root, indicating that an overthickening of the crust occurred when it was young and thus more easily deformable. This idea is strengthened with flexural modeling of the lithosphere that bends under the load of the ridge, indeed it indicates that the high probably formed when the underlying lithosphere was young. We interpret the Gagua Ridge as a fracture zone transverse ridge uplifted during a transpressive episode along a north-south -trending fracture zone in the middle Eocene time, if we accept Hilde and Lee’s (1984) model of magnetic lineations. This tectonic event could be contemporaneous with a change of the pole of rotation of the West Philippine Basin which occurred about 43/45 Ma ago.

Optimal processing of marine high-resolution seismic reflection (Chirp) data

Abstract: Chirp frequency-modulated (FM) systems offer deterministic, repeatable source-signatures for high-resolution, normal incidence marine seismic reflection data acquisition. An optimal processing sequence for uncorrelated Chirp data is presented to demonstrate the applicability of some conventional seismic reflection algorithms to high-resolution data sets, and to emphasise the importance of a known source-signature. An improvement of greater than 60dB in the signal- to-noise ratio is realised from correlating the FM reflection data with the transmitted pulse. Interpretability of ringy deconvolved data is enhanced by the calculation of instantaneous amplitudes. The signal-to-noise ratio and lateral reflector continuity are both improved by the application of predictive filters whose effectiveness are aided by the repeatability of the Chirp source.

Reflection coefficient calculation from marine high resolution seismic reflection (Chirp) data and application to an archaeological case study

Abstract: Chirp sub-bottom profilers produce high-resolution images of the near-surface. An attribute of the sea-bed reflection in chirp data are fluctuations in polarity between adjacent traces. Two models are proposed and presented to explain this: the first incorporates changes in an acoustic impedance gradient at the sea bed; the second uses changes in the thickness of the uppermost sediment layer. Mixing of adjacent traces produces a consistent polarity for the sea-bed reflector. Reflection coefficients are calculated, using amplitude information derived from single-traces, and polarity information from trace mixing, with application to a marine archaeological case study. The reflection coefficient calculated for the top of a buried 18th century wooden wreck is -0.26.

Rifting and early evolution of ancient ocean basins: the record of the Mesozoic Tethys and of the Galicia-Newfoundland margins

Abstract: Although of different age, the undeformed Cretaceous Iberia/Newfoundland margins and the relics of the Jurassic Brianconnais/Adriatic margins preserved in the Alps document a similar spatial and temporal evolution of rifting suggesting that the evolution of both pairs of margins was controlled by the same processes. Rifting appears to depend strongly on the thermal history of the lithosphere, which controls the rheology and consequently also the structural evolution of the margin. The tectonic evolution of non-volcanic margins appears to be distinctly different from that of volcanic ones.

Seismic study of the Ulleung Basin crust and its implications for the opening of the East Sea (Japan Sea)

Abstract: The Ulleung Basin (Tsushima Basin) in the southwestern East Sea (Japan Sea) is floored by a crust whose affinity is not known whether oceanic or thinned continental This ambiguity resulted in unconstrained mechanisms of basin evolution The present work attempts to define the nature of the crust of the Ulleung Basin and its tectonic evolution using seismic wide-angle reflection and refraction data recorded on ocean bottom seismometers (OBSs) Although the thickness of (10 km) of the crust is greater than typical oceanic crust, tau-p analysis of OBS data and forward modeling by 2-D ray tracing suggest that it is oceanic in character: (1) the crust consists of laterally consistent upper and lower layers that are typical of oceanic layers 2 and 3 in seismic velocity and gradient distribution and (2) layer 2C, the transition between layer 2 and layer 3 in oceanic crust, is manifested by a continuous velocity increase from 57 to 63 km/s over the thickness interval of about 1 km between the upper and lower layers Therefore it is not likely that the Ulleung Basin was formed by the crustal extension of the southwestern Japan Arc where crustal structure is typically continental Instead, the thickness of the crust and its velocity structure suggest that the Ulleung Basin was formed by seafloor spreading in a region of hotter than normal mantle surrounding a distant mantle plume, not directly above the core of the plume It seems that the mantle plume was located in northeast China This suggestion is consistent with geochemical data that indicate the influence of a mantle plume on the production of volcanic rocks in and around the Ulleung Basin Thus we propose that the opening models of the southwestern East Sea should incorporate seafloor spreading and the influence of a mantle plume rather than the extension of the crust of the Japan Arc

A real-time observation network of ocean-bottom-seismometers deployed at the Sagami trough subduction zone, central Japan

Abstract: We installed a real-time operating regional observation network of Ocean-Bottom-Seismometers, connected to an electro-optical fiber communication cable, at the Sagami trough subduction zone, just south of the Tokyo metropolitan area, central Japan The network, called ETMC, has six seismic observation sites at approximately 20 km spacing In addition, there are three tsunami observation sites along the ETMC network to monitor the propagation process of tsunamis around the Sagami trough region The on-line data from the ETMC has been improving the detection capability of smaller-magnitude earthquakes even at areas close to the margin of the trough The ETMC data analyzing system, which has a function of real-time digital filtering for each seismic channel, can read the arrival times of P- and S-waves precisely, constraining well the automatic on-line hypocenter locations The network has been providing useful information regarding the bending and downgoing process of the Philippine sea plate at the Sagami trough subduction zone The pressure sensors of the installed network have a detection capability of tsunami wave trains with an amplitude of less than 1 cm For example, the sensors recorded the full time history of tsunami wave trains, with mm order resolution, originating from a ‘tsunami earthquake’ with 57 MW and the tsunami magnitude of 75 occurred near Tori Shima (Tori Is) of the Izu-Bonin Is arc on September 4, 1996 The maximum amplitude of the tsunami signals on the trough-floor was approximately 1 cm (P-P), in contrast with approximately 20 cm (0-P) at a coastal site on Izu-Oshima, near the trough Also, the pressure sensors observed tsunamis due to a large tsunami earthquake (71 MW) at the northern New Guinea, on July 17, 1998

The Vema Transverse Ridge (Central Atlantic)

Abstract: Transverse ridges are elongate reliefs running parallel and adjacent to transform/fracture zones offsetting mid-ocean ridges. A major transverse ridge runs adjacent to the Vema transform (Central Atlantic), that offsets the Mid-Atlantic Ridge by 320 km. Multibeam morphobathymetric coverage of the entire Vema Transverse ridge shows it is an elongated (∼300 km), narrow (<30 km at the base) relief that constitutes a topographic anomaly rising up to 4 km above the predicted thermal contraction level. Morphology and lithology suggest that the Vema Transverse ridge is an uplifted sliver of oceanic lithosphere. Topographic and lithological asymmetry indicate that the transverse ridge was formed by flexure of a lithospheric sliver, uncoupled on its northern side by the transform fault. The transverse ridge can be subdivided in segments bound by topographic discontinuities that are probably fault-controlled, suggesting some differential uplift and/or tilting of the different segments. Two of the segments are capped by shallow water carbonate platforms, that formed about 3–4 m.y. ago, at which time the crust of the transverse ridge was close to sea level. Sampling by submersible and dredging indicates that a relatively undisturbed section of oceanic lithosphere is exposed on the northern slope of the transverse ridge. Preliminary studies of mantle-derived ultramafic rocks from this section suggest temporal variations in mantle composition. An inactive fracture zone scarp (Lema fracture zone) was mapped south of the Vema Transverse ridge. Based on morphology, a fossil RTI was identified about 80 km west of the presently active RTI, suggesting that a ridge jump might have occurred about 2.2 m.a. Most probable causes for the formation of the Vema Transverse ridge are vertical motions of lithospheric slivers due to small changes in the direction of spreading of the plates bordering the Vema Fracture Zone.

Deep crustal structure and rheology of the Gascoyne volcanic margin, western Australia

Abstract: Interpretation of deep seismic reflection data across the Gascoyne Margin reveals six distinct seismic facies units related to the tectono-magmatic breakup history. On the outer Exmouth Plateau four large scale units are identified: (1) an extensively block-faulted upper crust; (2) a middle-crustal unit of discontinuous, undulatory reflectors; (3) a reflection-free deep crustal unit; and (4) a lower-crustal band of low-frequency, high-amplitude reflectors. Two additional units are found near the continent-ocean boundary (COB); (5) seaward-dipping reflectors (SDR); and (6) landward-dipping reflectors in the lower crust below the SDR. The lower-crustal high-reflectivity band, located near the top of a high-velocity unit (Vp > 7 kms−1), is interpreted as magmatic underplating. There is a spatial correlation between the underplated area and the presence of extensive upper-crustal block-faulting and intrusive rocks in the shallow crust. The undulatory middle-crustal reflector unit is also only identified in the outer plateau area, and is interpreted as a zone in which the upper-crustal faults terminate. The inner parts of the margin consist of a deep basin showing little upper-crustal faulting and no evidence of middle crustal deformation or underplating. Theoretical modeling of the effect of rifting and magmatic underplating on crustal strength profiles suggests that the brittle-ductile transition may migrate at least 5 km upwards during several million years after the underplating event. Based on the seismic interpretation and crustal strength modeling we propose that the seismic structure of the outer Exmouth Plateau is severely modified by a transient change in the crustal rheological structure associated with magmatic underplating.

New Observations on the Distribution of Past and Present Hydrothermal Activity in the TAG Area of the Mid-Atlantic Ridge (26°08′ N)

Abstract: Seafloor acoustic and photographic imagery combined with high- resolution bathymetry are used to investigate the geologic and tectonic relations between active and relict zones of hydrothermal venting in the TAG (Trans-Atlantic Geotraverse) hydrothermal field at 26°08′N on the Mid-Atlantic Ridge (MAR) The TAG field consists of a large, currently active, high-temperature mound, two relict zones (the Alvin and Mir zones), and an active low-temperature zone The active mound and the Alvin relict zone lie along a series of closely-spaced, axis-parallel (NNE-trending) faults in an area of active extension east of the neovolcanic zone The Alvin zone extends for 25 km along these faults from the valley floor onto the eastern wall, and consists of at least five mounds identified using DSL-120 sidescan sonar and bathymetric data The existence of sulfide structures on most of these mounds is verified with near-bottom electronic still camera (ESC) images from the Argo-II deep-towed vehicle, and is confirmed in at least one case with collected samples Two of these mounds were previously unidentified The existence of these mounds extends the length of the Alvin zone by ~05 km to the south Much of the Alvin relict zone appears to be buried by debris from a large mass wasting event on the eastern wall of the median valley The Mir zone, located on normal fault blocks of the eastern valley wall, cannot be clearly identified in the sidescan data and no structural connections from it to the active mound or Alvin zone can be discerned The active mound is located at the intersection of an older oblique fault set with the younger axis- parallel faults which extend into the Alvin relict zone, and no fresh volcanics are observed in the vicinity of the mound The fact that both the active mound and the Alvin relict zone lie along the same set of active, axis-parallel faults suggests that the faults may be a major control on the location of hydrothermal activity by providing pathways for fluid flow from a heat source at the ridge axis

Acoustic imagery evidence for methane hydrates in the Ulleung Basin

Abstract: Approximately 12,000 km2 of acoustic backscatter imagery (sidescan) data and swath bathymetry data were collected jointly by Republic of Korea (ROK) Navy, the Naval Oceanographic Office (NAVOCEANO), Hawaii Mapping Research Group (HMRG) and the Naval Research Laboratory (NRL) in the East Sea (Sea of Japan) in 1995. Preliminary analysis of these data have revealed a large network of canyons with well-developed fan deposits and slumps which were not previously mapped. Also identified is a 1400 km2 area occupied by more than 300 circular, low-backscatter features ca. 50–1000 m in diameter which are interpreted to be pockmarks or mounds created by escaping methane gas, methane-rich porewater and mud.

Arctic margin gravity highs: Deeper meaning for sediment depocenters?

Abstract: Morphologic, gravity, and seismic reflection/refraction data from ca. 10,000 km of Arctic passive continental margins suggest that the numerous oval free-air gravity anomalies, their +50–150 mGal extrema typically located just landward of shelf breaks, are caused by combinations of rapidly deposited Plio-Pleistocene glacial marine sediment loads, older post-breakup sediments, and perhaps causally related density anomalies (mascons) in the underlying oceanic crust. Dispersed seismicity associated with some gravity highs may reflect ongoing brittle, flexural adjustment to the loads. Multi-channel-seismic-controlled depocenter models for several prominent highs (including the Hornsund gravity high re-examined here) suggest that sediments alone do not suffice to explain the gravity highs, unless depocenter seismic velocities have been significantly underestimated. A flexural backstripping model for the Hornsund anomaly only roughly replicates observed gravity. Subjacent 'mascons', if present below some depocenters, may be caused by (1) anomalous subsidence of initially formed dense/thin crust; (2) depocenter blanketing of early-formed crust, mitigating hydrothermal fracturing and related density reduction; or (3) metastable phase transitions, converting basalt/gabbro to denser phases (Neugebauer–Spohn hypothesis), while cracks close or fill under the increased pressures and temperatures.

Contrasting volcanic-tectonic processes during the past 2 Ma on the Mid-Atlantic Ridge: submersible mapping, petrological and magnetic results at lat. 34°52′ N and 33°55′ N

Abstract: In August–September of 1995, 20 Nautile dives and detailed magnetic surveys (spaced every 1.8 km) were undertaken on two segments of the Mid-Atlantic Ridge between the Oceanographer and Hayes fractures zones. These two segments are only 65 km apart and show strong morphology and gravity contrasts. OH1 is shallower and has a large mantle Bouguer anomaly (MBA) bull's eye, whereas OH3 is deeper and has a smaller MBA bull's eye. Thirteen dives were devoted to segment OH1. The Median Ridge (MR) located on the central high (1700 m deep) is topped by 100 to 300 m high circular volcanoes. The volcanics consists mainly of porphyritic and/or vesicular pillows and volcaniclastics. The NVZ (2200 m deep), located in the valley floor east of the MR, consists of near aphyric fluid lava flows. A chain of off-axis volcanoes, displaying a magnetic continuity with surroundings, extends on both sides of the axis. Three volcanoes on the east side and one on the west side of the axis were explored and sampled by submersible. The off-axis increase of weathering, Fe-Mn coating and magnetic signature suggest that the volcanoes were built at or near the ridge axis. The spacing of NS elongated hills bearing circular volcanoes and separated large magnetic signature (2 to 4 km) depressions suggests that several similar volcanic events occured during the past 2 Ma. The last 1 Ma episode involves (1) the construction of an axial ridge (MR) by fissure eruptions and the formation of circular summit volcanoes by focused volcanism, and (2) the extrusion of fluid magma in the depressions formed by further fissuring and faulting of the MR.

Late Quaternary Seismic Stratigraphy of the Eastern Bengal Shelf

Abstract: An ultra-high-resolution seismic study of the eastern Bengal Shelf with the parametric narrow-beam echosounder Parasound allows the interpretation of late Quaternary depositional patterns in terms of seismic stratigraphy. Accommodation space was still present on the outer shelf during the last lowstand, where a prograding delta developed in the western survey area. Oolitic beach ridges were later formed on top of this lowstand delta. Farther east, large parts of the shelf were exposed to subaerial erosion and a river system extended seaward across the area. A subaqueous highstand delta prograded southwards following the maximum transgression about 7,000 years ago. Its foreset beds exhibit acoustic voids very likely generated by sediment liquefaction, possibly caused by episodic energetic events such as major cyclones and/or earthquakes. Bottomset sediments extend seaward close to the shelf break in the west, whereas no Holocene sediments cover the outer shelf in the east.

Variability of temperature and currents measured near Pipe Organ hydrothermal vent site

Abstract: Temperature and currents were measured ∼100 m south of the Pipe Organ vent site on the Juan de Fuca Ridge during a 5-day interval in September 1997 to examine the temporal variability of hydrothermal heat signals close to a source. Temperatures were sampled at three depths every 15 s while current speed and direction were sampled at a single depth hourly. Results show that rapid potential temperature (θ) changes of as much as 0.02 °C occurred at all three depths, partly caused by changes in current speed and direction. Spectra of θ showed no prominent spectral peaks, however. An analytic model of heat transport under variable currents was used to help put observed θ temporal variability into context and to point out problems with estimating hydrothermal heat fluxes from mooring data. Model θ distributions, when contoured on a horizontal plane and animated, demonstrate the pooling of effluent over vents at various times, the streaming of effluent as a very narrow plume away from vents at times of large current speed, and the spawning of boluses of heated fluid and their transport away from venting regions when a pooling period is followed by a streaming period. Model results also show that estimated heat flux based on mooring data is strongly dependent on analysis assumptions and suggest that vents are capable of causing measurable θ anomalies as far as several kilometers downstream.

A submersible study of the western intersection of the Mid-Atlantic ridge and Kane fracture zone (WMARK)

Abstract: In 1994, a joint Japanese-American dive program utilizing the worlds deepest diving active research submersible (SHINKAI 6500) was carried out at the western ridge-transform intersection (RTI) of the Mid-Atlantic Ridge and Kane transform in the central North Atlantic Ocean. A total of 15 dives were completed along with surface-ship geophysical mapping of bathymetry, magnetic and gravity fields. Dives at the RTI traced the neovolcanic zone up to, and for a short distance (2.5 km) along, the Kane transform. At the RTI, the active trace of the transform is marked by a narrow valley (<50 m wide) that separates the recent lavas of the neovolcanic zone from the south wall of the transform. The south wall of the transform at the western RTI consists of a diabase section near its base between 5000 and 4600 m depth overlain by basaltic lavas, with no evidence of gabbro or deeper crustal rocks. The south wall is undergoing normal faulting with considerable strike-slip component. The lavas of the neovolcanic zone at the RTI are highly magnetized (17 A m−1) compared to the lavas of the south wall (4 A m−1), consistent with their age difference. The trace of the active transform changes eastwards into a prominent median ridge, which is composed of heavily sedimented and highly serpentinized peridotites. Submersible observations made from SHINKAI find that the western RTI of the Kane transform has a very different seafloor morphology and lithology compared to the eastern RTI. Large rounded massifs exposing lower crustal rocks are found on the inside corner of the eastern RTI whereas volcanic ridge and valley terrain with hooked ridges are found on the outside corner of the eastern RTI. The western RTI is much less asymmetric with both inside and outside corner crust showing a preponderance of volcanic terrain. The dominance of low-angle detachment faulting at the eastern RTI has resulted in a seafloor morphology and architecture that is diagnostic of the process whereas crust formed at the WMARK RTI must clearly be operating under a different set of conditions that suppresses the initiation of such faulting.

Fault scarp statistics at the Galapagos spreading centre from Deep Tow data

Abstract: Much of the relief of the abyssal hills covering the ocean basins is believed to originate from faulting of oceanic crust at mid-ocean ridges. The timescale over which faults grow is controversial, however, with some authors arguing that faults continue to grow in places for 0.5 m.y. or more based on increasing relief of fault scarps with distance from ridge axes. We examine Deep Tow profiler records of the Galapagos Spreading Centre, in which basement reflections allow scarp relief to be measured beneath the sediment cover, and find that relief does not increase but decreases systematically to 40 km off-axis (1.5 Ma seafloor). Since reversal of fault offsets is unlikely in this tectonic setting, we interpret this result as indicating that variations in fault statistics could reflect temporal variations in the tectonic or volcanic state of the ridge crest, not necessarily progressive fault growth with age as previously assumed. Resolving the issue of fault longevity will therefore require independent data on the timing of fault growth and distribution of present growth activity. We suggest some possible alternative indicators of fault longevity and discuss more generally the implications of volcanic flows to studies of faulting at ridges.

Highly magnetised and differentiated basalts at the 18–19° S propagating spreading centre in the North Fiji Basin

Abstract: Geochemical and magnetic properties of basalts dredged along a propagating spreading centre (central part of the North Fiji Basin) have been analysed. Two phenomena appear to be important in controlling variations of natural remanent magnetisation (NRM): 1) an extensive magma differentiation to Fe-Ti basalts increases with the intensity of NRM; and both increase towards the propagator tip, 2) low temperature oxidation (maghemitisation) seems to have already occurred along the ridge axis for the zero age basalts. This paper shows that despite the apparent lack of correlation between NRM and the maghemitisation process (masked by the effect of extensive magma differentiation), the latter involves a change of the domain state of magnetic carriers, from pseudo single-domain to single-domain. We separated the NRM into 4 partial NRMs (pNRM) depending on the coercivity of grains. This analysis showed that the contribution to the remanence of grains with high coercivity increases with maghemitisation, whereas the contribution of grains with weak coercivity decreases.

Cross-sectional areas of mid-ocean ridge axes bounding the Easter and Juan Fernandez microplates

Abstract: We have calculated cross-sectional areas for the ridges bounding the Easter and Juan Fernandez microplates, 22°–28°S and 31°–35°S, obtaining accurate results where complete bathymetric data exist and estimates in other regions with partial bathymetric coverage and predicted bathymetry We consider the reliability and usefulness of global predicted bathymetry in these calculations and the possible application of this dataset in other localities The spreading rates on ridges bounding these microplates span the range from slow to superfast, allowing an investigation of ridge axis inflation over most of the rates active on Earth today The across-axis areas of the Easter microplate ridge axes range from −29 km2 to 7 km2, while the Juan Fernandez ridge axis areas range from −27 km2 to 8 km2 Positive values correlate with regions usually interpreted as magmatically robust Negative values arise from calculations in areas of propagating rift tips and deep grabens, such as Pito and Endeavor Deeps Geochemical trends of Easter microplate axial basalts show decreasing MgO toward propagating rift tips and slight positive correlations between variables such as MgO vs cross-sectional area, Na80 vs axial depth, and Na80 vs cross-sectional area We document the decrease in the axial area approaching segment ends and propagating rift tips along both the West and East ridges of the microplates On the Easter microplate both East and West ridge systems undergo large variations in spreading rate from >130 km Myr−1 to <50 km Myr−1 Inflation on these ridge segments is highly variable and only weakly correlated with spreading rate On the Juan Fernandez microplate, West ridge spreading rates vary only between ∼115–140 km Myr−1 and are systematically faster than on the East ridge, where rates vary between ∼10–35 km Myr−1 Cross axis areas are systematically greater and significantly less variable on the faster spreading West ridge Overall, compared to oceanic spreading centers bounding major plates with similar spreading rates, the axial areas are smaller on the microplate ridge systems, possibly because their rapidly changing configurations create a lag in the mantle response to the rigid plate boundary

Sediment velocity estimation using iterative 3-D migrations of short offset seismic reflection data in deep water

Abstract: In deep ocean settings where water depth greatly exceeds the source-to-receiver length, the geometry is insufficient for accurate determinations of velocity from reflection-moveout. However, velocities are crucial for estimates of physical properties and image processing. ‘Focusing analyses’ with conventional post-stack two-dimensional migration improves images, but does not produce geologically meaningful velocities except in the special case of a two-dimensional earth. For the more general case of the three-dimensional earth there is no a priori method to determine the degree of geometrical complexity. We present a technique using a short-offset three-dimensional (3-D) data set over the 5 km deep trench west of the Lesser Antilles. These data illustrate highly sensitive post-stack 3-D focusing analyses (± 20 m s−1 interval velocities), and the relationship of these seismically derived velocities to rock velocities. In our Barbados example we were able to establish the presence of a widespread 80-160 m thick low-velocity zone at and above the main low-angle fault. This observation suggests the water-rich decollement ‘leaks’ water into the overlying sections. Also evident is a low-velocity section associated with turbidite sands. These results are confirmed with sparse logging data and well samples. Deep-water short offset 3-D experiments provide a potentially effective approach for velocity estimation, replacing the operational complexity of long-offsets with simpler short-offset techniques. In areas of structural complications and abundant diffracted energy, it is a surprisingly accurate method, utilizing the high fidelity 3-D wavefield and the information carried in ‘zero-offset’ diffraction ellipsoids. The velocity used to properly collapse a diffraction ellipsoid is explicitly the velocity of propagation in the media since the travel path is known exactly. Thus, the derived velocities should closely represent rock velocities, unlike the 2-D case where the propagation geometry is not known.

An implosive seismoacoustic source for seismic experiments on the ocean floor

Abstract: Bottom shots have been used for a number of years in seismic studies on the ocean floor. Most experiments utilized explosives as the energy source, though researchers have recognized the usefulness of collapsing water voids to produce seismoacoustic signals. Implosive sources, however, suffered generally from a lack of control of source depth. We present a new experimental tool, called SEEBOSEIS, to carry out seismic experiments on the seafloor utilizing hollow glass spheres as controlled implosive sources. The source is a 10-inch BENTHOS float with penetrator. Inside the sphere we place a small explosive charge (two detonators) to destabilize the glass wall. The time of detonation is controlled by an external shooting device. Test measurements on the Ninetyeast Ridge, Indian Ocean, show that the implosive sources can be used in seismic refraction experiments to image the subbottom P-wave velocity structure in detail beyond that possible with traditional marine seismic techniques. Additionally, the implosions permit the efficient generation of dispersed Scholte waves, revealing upper crustal S-wave velocities. The frequency band of seismic energy ranges from less than 1 Hz for Scholte modes up to 1000 Hz for diving P-waves. Therefore, broadband recording units with sampling rates >2000 Hz are recommended to sample the entire wave field radiated by implosive sources.

Mapping the sedimentary basins of the Barents and Kara Seas using ERS-1 altimetry-geodetic mission

Abstract: The continental shelf in the Arctic north of Russia consists of a series of epicontinental seas, which are the offshore continuation of potentially oil and gas basins on land. The geology of all these epicontinental seas is poorly known, due to the remoteness, the extreme climatic conditions and the extensive costs associated with seismic exploration. Radar altimeter sensors thus provide an invaluable tool for studying the geological structures off the coast. The unique ERS-1 contribution comes from its high latitude coverage (81.5 deg south to north), and the space and time density of its measurements (168-day repeat-orbit). The gravity anomaly field is derived from the geoid height measurements by computing the deflections of the vertical in the north-south and east-west directions and transforming these deflections into gravity anomalies. The gravimetry reveals interesting features of the basement of the Barents and Kara Seas which have not been chartered in recent, previous compilation maps of sedimentary thickness in the Arctic Ocean (Jackson and Oakey, 1988; Gramberg and Puscharovski, 1989). We obtain no indication of the SE-NW offshore Baikalian trend described by Fichler et al (1997) using ERS-1 gravimetry. Instead, the data indicate the presence of a north-south trending gravity high associated with the maximum sediment thickness within the South Barents Sea and the North Barents Sea Basins. Further geological studies are needed to interpret the gravimetric data, which directly addresses the problem of understanding the gravity signature of deep, old, sedimentary basins.