Attenuation of seismic waves in the Jan Mayen island area

An analysis of the attenuation of seismic waves as measured by the quality factorQc (for coda waves) has been performed for the volcanic Jan Mayen island in the Norwegian Sea, using earthquakes near the Jan Mayen Fracture Zone and local seismic stations on the Jan Mayen island.Qc values of the order of 100 at a frequency of 1 Hz are found, increasing to about 300 at 10 Hz. These values are typical of what usually is observed in tectonically influenced areas near oceanic/continental plate boundaries. It is considered likely that these results are influenced by the fact that the Jan Mayen island, in spite of its proximity to a fracture zone, is located in the northern end of the Jan Mayen Ridge, which now is accepted as being a micro-continent. The presence of the active Beerenberg volcano on the Jan Mayen island does give rise to a somewhat stronger attenuation for waves traversing that area, but this effect is weak and quite limited in spatial extent. There is also a slight increase in attenuation as a function of depth, but less than what is observed in terms of lateral variations. This is reasonable in view of the very strong lateral variations in lithospheric structure exhibited in this area.     

Geological history and magmatism of the Jan Mayen microcontinent (Polar Atlantic)

Two main events determined the formation, geological history, magmatism, and geodynamics of the Jan Mayen microcontinent: (1) drift of this segment of the Laurasian plate over the Iceland plume in the Early Paleogene; (2) propagation of the rift zone of the mid-Atlantic Ridge into this region and separation of the Jan Mayen lithospheric block from northeastern Greenland. The lithosphere was reduced at the block boundary when it was separated. This process was accompanied by the formation of depressions intruded by magma of the Iceland plume, which resulted in the appearance of a new volcanic center with active volcanoes of the central type. They supplied pyroclastic material to the sedimentary cover of the expanding Norwegian?Greenland Basin in the Eocene and Oligocene. The wedging of the Jan Mayen plate (microcontinent) into the triple junction of the plates (Greenland, Eurasian, Jan Mayen) promoted intense volcanism and the formation of two large volcanic complexes: (1) the Greenland?Faroes and the (2) Trail?Vøring. Recent volcanoes of the Jan Mayen hot spot are fed by magma from the Iceland plume as well as from relict and newly formed cambers in a zone of deep-seated Jan Mayen transform faults.     

First results from the North Iceland experiment

Between June 2004 and September 2004 a temporary seismic network was installed on the northern insular shelf of Iceland and onshore in north Iceland. The seismic setup aimed at resolving the subsurface structure and, thus, the geodynamical transition from Icelandic crust to typical oceanic crust along Kolbeinsey Ridge. The experiment recorded about 1,000 earthquakes. The region encloses the Tjörnes Fracture Zone containing the Husavik–Flatey strike-slip fault and the extensional seismic Grimsey Lineament. Most of the seismicity occurs in swarms offshore. Preliminary results reveal typical mid-ocean crust north of Grimsey and a heterogeneous structure with major velocity anomalies along the seismic lineaments and north–south trending subsurface features. Complementary bathymetric mapping highlight numerous extrusion features along the Grimsey Lineament and Kolbeinsey Ridge. The seismic dataset promises to deliver new insights into the tectonic framework for earthquakes in an extensional transform zone along the global mid-ocean ridge system.     

Early Eocene sea floor spreading and continent-ocean boundary between Jan Mayen and Senja fracture zones in the Norwegian-Greenland Sea

Sea floor spreading anomalies in the Lofoten-Greenland basins reveal an unstable plate boundary characterized by several small-offset transforms for a period of 4 m.y. after opening. North of the Jan Mayen Fracture Zone, integrated analysis of magnetic and seismic data also document a distinct, persistent magnetic anomaly associated with the continent-ocean boundary and a locally, robust anomaly along the inner boundary of the break-up lavas. These results provide improved constraints on early opening plate reconstructions, which include a new anomaly 23-to-opening pole of rotation yielding more northerly relative motion vectors than previously recognized; and a solution of the enigmatic, azimuthal difference between the conjugate Eocene parts of the Greenland-Senja Fracture Zone if the Greenland Ridge is considered a continental sliver. The results confirm high, 2.36–2.40 cm yr^–1, early opening spreading rates, and are consistent with the start of sea floor spreading during Chron 24r. The potential field data along the landward prolongations of the Bivrost Fracture Zone suggest that its location is determined by a Mesozoic transfer system which has acted as a first-order, across-margin tectono-magmatic boundary between the regional Jan Mayen and Greenland-Senja Fracture Zone systems, greatly influencing the pre-, syn- and post-breakup margin development.     

Investigations of the East Greenland continental margin between 70° and 72° N by deep seismic sounding and gravity studies

Results are presented from a deep seismic sounding experiment with the research vessel POLARSTERN in the Scoresby Sund area, East Greenland. For this continental margin study 9 seismic recording landstations were placed in Scoresby Sund and at the southeast end of Kong Oscars Fjord, and ocean bottom seismographs (OBS) were deployed at 26 positions in and out of Scoresby Sund offshore East Greenland between 70° and 72° N and on the west flank of the Kolbeinsey Ridge. The landstations were established using helicopters from RV POLARSTERN. Explosives, a 321 airgun and 81 airguns were used as seismic sources in the open sea. Gravity data were recorded in addition to the seismic measurements. A free-air gravity map is presented. The sea operations — shooting and OBS recording — were strongly influenced by varying ice conditions. Crustal structure 2-D models have been calculated from the deep seismic sounding results. Free-air gravity anomalies have been calculated from these models and compared to the observed gravity. In the inner Scoresby Sund — the Caledonian fold belt region — the crustal thickness is about 35 km, and thins seaward to 10 km. Sediments more than 10 km thick on Jameson Land are of mainly Mesozoic age. In the outer shelf region and deep sea a ‘Moho’ cannot clearly be identified by our data. There are only weak indications for the existence of a ‘Moho’ west of the Kolbeinsey Ridge. Inside and offshore Scoresby Sund there is clear evidence for a lower crust refractor characterised byp-velocities of 6.8–7.3 km s^?1 at depths between 6 and 10 km. We believe these velocities are related to magmatic processes of rifting and first drifting controlled by different scale mantle updoming during Paleocene to Eocene and Late Oligocene to Miocene times: the separation of Greenland/Norway and the separation of the Jan Mayen Ridge/Greenland, respectively. A thin igneous upper crust, interpreted to be of oceanic origin, begins about 50 km seaward of the Liverpool Land Escarpment and thickens oceanward. In the escarpment zone the crustal composition is not clear. Probably it is stretched and attenuated continental crust interspersed with basaltic intrusions. The great depth of the basement (about 5000 m) points to a high subsidence rate of about 0.25 mm yr^?1 due to sediment loading and cooling of the crust and upper mantle, mainly since Miocene time. The igneous upper crust thickens eastward under the Kolbeinsey Ridge to about 2.5 km; the thickening is likely caused by higher production of extrusives. The basementp-velocity of 5.8–6.0 km s^?1 is rather high. Such velocities are associated with young basalts and may also be caused by a higher percentage of dykes. Tertiary to recent sediments, about 5000 m thick, form most of the shelf east of Scoresby Sund, Liverpool Land and Kong Oscars Fjord. This points to a high sedimentation rate mainly since the Miocene. The deeper sediments have a rather high meanp-velocity of 4.5 km s^?1, perhaps due to pre-Cambrian to Caledonian deposits of continental origin. The upper sediments offshore Scoresby Sund are thick and have a rather low velocity. They are interpreted as eroded material transported from inside the Sund into the shelf region. Offshore Kong Oscars Fjord the upper sediments, likely Jurassic to Devonian deposits, are thin in the shelf region but thicken to more than 3000 m in the slope area. The crust and upper mantle structure in the ocean-continent transition zone is interpreted to be the result of the superposition of the activities of three rifting phases related to mantle plumes of different dimensions:

1. the ‘Greenland/Norway separation phase’ of high volcanic activity,
2. the ‘Jan Mayen Ridge/Greenland separation phase’ and
3. the ‘Kolbeinsey Ridge phase’ of ‘normal’ volcanic activity related to a more or less normal mantle temperature.

During period 2 and 3 only a few masses of extrusives were produced, but large volumes of intrusives were emplaced. So the margin between Scoresby Sund and Jan Mayen Fracture Zone is interpreted to be a stretched margin with low volcanic activity.     

Paleo-bathymetry of the northern North Atlantic and consequences for the opening of the Fram Strait

In the past decade, the geophysical database in the northern North Atlantic and central Arctic Ocean constantly grew. Though far from being complete, the information from new aeromagnetic and seismic data north of the Jan Mayen Fracture Zone and in the Arctic Ocean, in combination with existing compiled geological and geophysical data, is used to produce paleo-bathymetric maps for several Cenozoic time intervals. This paleo-bathymetric model provides evidence for an initial deep-water exchange through the Fram Strait starting around 17 Ma. Furthermore, the model suggests that crustal rifting prior to initial seafloor spreading might have facilitated an earlier deep-water connection. This confirms that the paleo-topography of the Yermak Plateau played an important role in allowing at least the exchange of shallow water between the northern North Atlantic and the Arctic Ocean before the opening of the deep-water Fram Strait gateway. In the south of the research area the paleo-bathymetric model indicates that the first possibility for a deep-water overflow from the Norwegian-Greenland Sea to the North Atlantic could have been between 15 and 20 Ma.     

The crustal nature of the northern Mozambique Ridge,Southwest Indian Ocean

The Mozambique Ridge (MOZR) is one of the basement high structures located in the Southwest Indian Ocean, parallel to the Southeast African continental margin. It was formed as a result of the tectono-magmatic evolution of the Gondwana breakup. The origin of the MOZR has been highly debated, with models suggesting either continental or oceanic origin. With new free-air gravity anomaly and multichannel seismic (MCS) reflection data, we present results of 2D density modeling along two seismic profiles acquired by R/V Xiangyanghong 10 at the northern Mozambique Ridge (N-MOZR) between 26°S and 28°S. We observed high free-air gravity anomaly and strong positive magnetic anomaly related to the emplaced seaward dipping reflectors (SDR) and high density lower crustal body (HDLCB), and high Bouguer gravity anomaly associated with the thinning of the continental crust underneath the N-MOZR over a distance of ~82 km. This suggests a thinned and intruded continental crust bound by the Mozambique Fracture Zone (MFZ) that is characterized by gravity low and negative magnetic anomaly. This fracture zone marks the continent-ocean boundary (COB) while the N-MOZR is the transform margin high, i.e., marks the continent-ocean transition (COT) of the Southern Mozambique margin, following the definition of transform margins. We suggest that the N-MOZR was formed by continental extension and subsequent breakup of the MFZ, accompanied by massive volcanism during the southward movement of the Antarctica block. The presence of SDR, HDLCB, and relatively thick oceanic crust indicates the volcanic nature of this transform margin.     

Controls on the tectono-magmatic evolution of a volcanic transform margin: the Vøring Transform Margin,NE Atlantic

Ocean bottom seismograph (OBS), multichannel seismic and potential field data reveal the structure of the Vøring Transform Margin (VTM). This transform margin is located at the landward extension of the Jan Mayen Fracture Zone along the southern edge of the Vøring Plateau. The margin consists of two distinctive segments. The northwestern segment is characterized by large amounts of volcanic material. The new OBS data reveal a 30–40 km wide and 17 km thick high-velocity body between underplated continental crust to the northeast and normal oceanic crust in the southwest. The southeastern segment of the mar is similar to transform margins elsewhere. It is characterized by a 20–30 km wide transform margin high and a narrow continent-ocean transition. The volcanic sequences along this margin segment are less than 1 km thick. We conclude from the spatial correspondence of decreased volcanism and the location of the fracture zone, that the amount of volcanism was influenced by the tectonic setting. We propose that (1) lateral heat transport from the oceanic lithosphere to the adjacent continental lithosphere decreased the ambient mantle temperature and melt production along the entire transform margin and (2) that right-stepping of the left-lateral shear zone at the northwestern margin segment caused lithospheric thinning and increased volcanism. The investigated data show no evidence that the breakup volcanism influenced the tectonic development of the southeastern VTM.     

The western Svalbard margin (74°–80°N)

A regional study of the continental margin between the Senja and Molloy-Spitsbergen fracture zones reveals that the transition from continental to oceanic crust occurs in a narrow zone beneath the outer shelf and uppermost slope. The postulated continent-ocean boundary appears to be fault-related consisting of sheared and rifted segments. The marginal structures are compatible with a plate tectonic model in which the southern Greenland Sea opened along a northeasterly propagating plate boundary in the Eocene, whereas the northern Greenland Sea started opening in the early Oligocene. The main structure at the margin is the Hornsund Fault Zone which probably reflects an old zone of weakness rejuvenated in the Tertiary, first by shear and later by extensional movements. In the early Tertiary local transpressional and transtensional components along the plate boundary are associated with the Spitsbergen Orogeny, emplacement of belts of high-density oceanic crust and tectonism in the western Barents Sea. A complex volcanic rifted margin characterized by the Bjørnøya Marginal High links the predominantly sheared margin segments on either side. The main ridge-like segment of the Hovgaard Fracture Zone was originally part of the Spitsbergen margin. In a regional sense, the Hornsund Fault Zone demarcates the eastern boundary of the Tertiary sedimentary wedge which reaches a total thickness of more than 7 km. There appears to have been a considerable increase in deposition of sediments the last 5–6 my. Depocentres located seaward of the east-west fjord systems and submarine depressions indicate a relationship between late Cenozoic glaciations and high sedimentation rates.     

Deep Crustal Structure of the Continental Margin off the Explora Escarpment and in the Lazarev Sea, East Antarctica

This study presents the results of a seismic refraction experiment that was carried out off Dronning Maud Land (East Antarctica) along the Explora Escarpment (14° W–12° W) and close to Astrid Ridge (6°E). Oceanic crust of about 10 km thickness is observed northwest of the Explora Escarpment. Stretched continental crust, observed southeast of the escarpment, is most likely intruded by volcanic material at all crustal levels. Seismic velocities of 7.0–7.4 km/s are modelled for the lower crust. The northern boundary of this high velocity body coincides approximately with the Explora Escarpment. The upper crystalline crust is overlain by a 4-km thick and 70-km wide wedge of volcanic material: the Explora Wedge. Seismic velocities for the oceanic crust north of the Explora Escarpment are in good agreement with global studies. The oceanic crust in the region of the Lazarev Sea is also up to 10-km thick. The lower crystalline crust shows seismic velocities of up to 7.4 km/s. This, together with the larger crustal thickness might point to higher mantle temperatures during the formation of the oceanic crust. The more southerly rifted continental crust is up to 25-km thick, and also has seismic velocities of 7.4 km/s in the lower crystalline crust. This section is interpreted to consist of stretched continental crust, which is heavily intruded by volcanic material up to approximately 8-km depth. Multichannel seismic data indicate that, in this region, two volcanic wedges are present. The wedges are interpreted to have evolved during different time/rift periods. The wedges have a total width of at least 180 km in the Lazarev Sea. Our results support previous findings that the continental margin off Dronning Maud Land between ≈2°E and ≈13°E had a complex and long-lived rift history. Both continental margins can be classified as rifted volcanic continental margins that were formed during break-up of Gondwana.     

Gravity and S-wave modelling across the Jan Mayen Ridge,North Atlantic; implications for crustal lithology

The horizontal components from fourteen Ocean Bottom Seismometers deployed along four profiles focused along the western margin of the Jan Mayen microcontinent, North Atlantic, have been modelled with regard to S-waves, based on P-wave models obtained earlier. The seismic models have furthermore been constrained by 2D gravity modelling. High V _p/V _s-ratios (2.3–7.9) within the Cenozoic sedimentary section are attributed to significant porosities, whereas V _p/V _s-ratios in the order of 1.9–2.2 for the Mesozoic and Paleozoic sedimentary rocks indicate shale-dominated lithology throughout the area. The eastern side of the Jan Mayen Ridge is interpreted as a passive, volcanic margin, based on relatively high crustal V _p/V _s-ratios (1.9), whereas lower V _p/V _s-ratios (1.75–1.8) suggest the presence of intermediate composition crust and non-volcanic margin on the western side of the ridge. In the westernmost part of the Jan Mayen Basin, slightly increased upper mantle V _p/V _s-ratios may indicate some degree of serpentization of upper mantle peridotites.     

Crustal transect across the North Atlantic

Two dimensional crustal models derived from four different ocean bottom seismographic (OBS) surveys have been compiled into a 1,580 km long transect across the North Atlantic, from the Norwegian Møre coast, across the extinct Aegir Ridge, the continental Jan Mayen Ridge, the presently active Kolbeinsey Ridge north of Iceland, into Scoresby Sund in East Greenland. Backstripping of the transect suggests that the continental break-up at ca. 55 Ma occurred along a west-dipping detachment localized near the western end of a ca. 300 km wide basin thinned to less than 20 km crustal thickness. It is likely that an east-dipping detachment near the present day Liverpool Land Escarpment was active during the late stages of continental rifting. A lower crustal high-velocity layer (7.2–7.4 km/s) interpreted as mafic intrusions/underplating, was present beneath the entire basin. The observations are consistent with the plume hypothesis, involving the Early Tertiary arrival of a mantle plume beneath central Greenland and focused decompression melting beneath the thinnest portions of the lithosphere. The mid-Eocene to Oligocene continental extension in East Greenland is interpreted as fairly symmetric and strongly concentrated in the lower crustal layer. Continental break-up which rifted off the Jan Mayen Ridge, occurred at ca. 25 Ma, when the Aegir Ridge became extinct. The first ca. 2 m.y. of oceanic accretion along the Kolbeinsey Ridge was characterized by thin magmatic crust (ca. 5.5 km), whereas the oceanic crustal formation since ca. 23 Ma documents ca. 8 km thick crust and high magma budget.     

Structure of the Malpelo Ridge (Colombia) from seismic and gravity modelling

Wide-angle and multichannel seismic data collected on the Malpelo Ridge provide an image of the deep structure of the ridge and new insights on its emplacement and tectonic history. The crustal structure of the Malpelo Ridge shows a 14 km thick asymmetric crustal root with a smooth transition to the oceanic basin southeastward, whereas the transition is abrupt beneath its northwestern flank. Crustal thickening is mainly related to the thickening of the lower crust, which exhibits velocities from 6.5 to 7.4 km/s. The deep structure is consistent with emplacement at an active spreading axis under a hotspot like the present-day Galapagos Hotspot on the Cocos-Nazca Spreading Centre. Our results favour the hypothesis that the Malpelo Ridge was formerly a continuation of the Cocos Ridge, emplaced simultaneously with the Carnegie Ridge at the Cocos-Nazca Spreading Centre, from which it was separated and subsequently drifted southward relative to the Cocos Ridge due to differential motion along the dextral strike-slip Panama Fracture Zone. The steep faulted northern flank of the Malpelo Ridge and the counterpart steep and faulted southern flank of Regina Ridge are possibly related to a rifting phase that resulted in the Coiba Microplate’s separation from the Nazca Plate along the Sandra Rift.     

Contribution of global potential field data to the tectonic reconstruction of the Rio Grande Rise in the South Atlantic

The application of advanced enhancement techniques for geophysical anomalies to global gravity (WGM2012) and magnetic (EMAG2) models sheds light on the complex tectonic evolution of the Rio Grande Rise (RGR) in the southern South Atlantic. Long wavelength Bouguer gravity lows indicate a thicker crust beneath of the ridge, whose nature can be related to a microcontinent or an excess of volcanism within the oceanic realm. Recently dredged continental rocks reinforce the hypothesis of a microcontinent or, at least, slivers of continental crust. However, the reserval magnetic pattern of the processed magnetic anomalies provide no evidence of aborted spreading center similar to the well-studied Jan Mayen microcontinent and the surrounding (inactive) Aegir and (active) Kolbeinsey ridges in the North Atlantic Ocean. The reversal magnetic anomalies show a series N-S trending parallel stripes roughly follow the current South American coastline and segmented by E-W oriented oceanic fracture zones (FZs). The magnetic stripes are bended eastwards at the RGR, showing a more complex magnetic pattern similar to that in the Iceland. The aborted Cruzeiro do Sul Rift (CSR) and the Jean Charcot Chain (JCC) are structures that cross the RGR and contribute to the understanding of the tectonic evolution of the South Atlantic Ocean. NW-SE oriented extensive gravity lows and bathymetric valleys, which mark the CSR, are segmented by E-W trending magnetic lineaments related to FZs. This structural configuration suggests that the extensional event, which formed the rift and the seamounts chain, was replaced by strike-slip movements along the FZs. In addition, we constructed a plate kinematic model for the evolution of the RGR based on bathymetric, free-air and Bouguer gravity and magnetic data. Our model comprises five main stages of the RGR formation and evolution between late Cretaceous and Paleocene (ca. 95 - 60 Ma), separated by published seafloor isochrones. The proposed model suggests that the RGR was built at the mid-Atlantic ridge by increased magmatism probably related to the Tristan da Cunha hotspot.     

Crustal features of the northeastern South China Sea: insights from seismic and magnetic interpretations

We interpret seven two-dimensional deep-penetration and long-offset multi-channel seismic profiles in the northernmost South China Sea area, which were collected by R/V Marcus G. Langseth during the TAIwan GEodynamics Research (TAIGER) project in 2009. To constrain the crustal characteristics, magnetic inversion and forward magnetic modeling were also performed. The seismic results clearly show tilted faulting blocks in the upper crust and most of the fault plane connects downward to a quasi-horizontal detachment as its bottom in the south of the Luzon-Ryukyu transform plate boundary. North of the plate boundary, a small-scale failed rifted basin (minimum 5 km in crustal thickness) with negative magnetization probably indicates an extended continental origin. Significant lower crustal material (LCM) was imaged under a crustal fracture area which indicated a continent and ocean transition origin. The thickest LCM (up to 6.5 km) is located at magnetic isochron C15 that is probably caused by the magma supply composite of a Miocene syn-rift volcanic event and Pliocene Dongsha volcanic activity for submarine volcanoes and sills in the surrounding area. The LCM also caused Miocene crustal blocks to be uplifted reversely as 17 km crustal thickness especially in the area of magnetic isochron C15 and C16. In addition, the wide fault blocks and LCM co-existed on the magnetic striped area (i.e. C15–C17) in the south of the Luzon-Ryukyu transform plate boundary. Magnetic forward modeling suggests that the whole thick crustal thickness (>12 km thick) needs to be magnetized in striped way as oceanic crust. However, the result also shows that the misfit between observed and synthetic magnetic anomaly is about 40 nT, north of isochron C16. The interval velocity derived from pre-stack time migration suggests that the crust is composed of basaltic intrusive upper crust and lower crustal material. The crustal nature should refer to a transition between continent and ocean. Thus, the magnetic reversals may be produced in two possible ways: basaltic magma injected along the crustal weak zone across magnetic reversal epoch and because some undiscovered ancient piece of oceanic crust existed. The crustal structure discrimination still needs to be confirmed by future studies.     

Pre-23 magnetic anomalies between Jan Mayen and Greenland-Senja fracture zones in the Norwegian sea

Analysis of magnetic data between the Jan Mayen and Senja fracture zones indicates that the anomaly 24A-B sequence extends from the Lofoten Basin onto the outer Vøring Plateau. Anomaly patterns, including those on the conjugate margin, suggest that the pre-23 sea floor spreading was characterized by an unstable plate boundary between fracture zones. The pre-23 spreading rate was at least 2.5 cm yr^-1 which is remarkably high compared with the post-23 rates. An evolutionary model which assumes Cenozoic oceanic crust as far landward as the Vöring Plateau and Greenland escarpments is suggested.     

Crustal structure and ocean–continent transition at marginal ridge: the Côte d’Ivoire–Ghana marginal ridge

 Crustal structure of the Co^te d’Ivoire–Ghana marginal ridge and its transition with oceanic lithosphere are deduced from multichannel seismic reflection, wide-angle seismic, and gravity data. The CIGMR is cut into rotated blocks and displays a crustal structure quite similar to that of the nearby northern Ivorian extensional basin. These results strongly support that the CIGMR represents an uplifted fragment of continental crust. Transition with the oceanic crust appears sharp; continental crustal thinning occurs over less than 5 km. We did not find evidence for underplating and/or contamination as anticipated from such a sharp contact between continental and oceanic crust. Received: 12 March 1995/Revision received: 2 July 1996     

Structure of the Basin and Ridge System West of New Caledonia (Southwest Pacific): A Synthesis

Complementary to previous work mainly based on seismic interpretation, our compilation of geophysical data (multibeam bathymetry, gravity, magnetic and seismic) acquired within the framework of the ZoNéCo (ongoing since 1993) and FAUST (1998–2001) programs enables us to improve the knowledge of the New Caledonia Basin, Fairway Basin and Fairway Ridge, located within the Southwest Pacific region. The structural synthesis map obtained from geophysical data interpretation allows definition of the deep structure, nature and formation of the Fairway and New Caledonia Basins. Development of the Fairway Basin took place during the Late Cretaceous (95–65 Ma) by continental stretching. This perched basin forms the western margin of the New Caledonia Basin. A newly identified major SW–NE boundary fault zone separates northern NW–SE trending segments of the two basins from southern N–S trending segments. This crustal-scale fault lineament, that we interpret to be related to Cretaceous-early Cainozoic Tasman Sea spreading, separates the NW–SE thinned-continental and N–S oceanic segments of the New Caledonia Basin. We can thus propose the following pattern for the formation of the study area. The end of continental stretching within the Fairway and West Caledonia Basins ( 65–62 Ma) is interpreted as contemporaneous with the onset of emplacement of oceanic crust within the New Caledonia Basin’s central segment. Spreading occurred during the Paleocene (62–56 Ma), and isolated the Gondwanaland block to the west from the Norfolk block to the east. Finally, our geophysical synthesis enables us to extend the structural Fairway Basin down to the structural Taranaki Basin, with the structural New Caledonia Basin lying east of the Fairway Basin and ending further north than previously thought, within the Reinga Basin northwest of New Zealand.     

An integrated geophysical study of Vestbakken Volcanic Province, western Barents Sea continental margin, and adjacent oceanic crust

This paper describes results from a geophysical study in the Vestbakken Volcanic Province, located on the central parts of the western Barents Sea continental margin, and adjacent oceanic crust in the Norwegian-Greenland Sea. The results are derived mainly from interpretation and modeling of multichannel seismic, ocean bottom seismometer and land station data along a regional seismic profile. The resulting model shows oceanic crust in the western parts of the profile. This crust is buried by a thick Cenozoic sedimentary package. Low velocities in the bottom of this package indicate overpressure. The igneous oceanic crust shows an average thickness of 7.2 km with the thinnest crust (5–6 km) in the southwest and the thickest crust (8–9 km) close to the continent-ocean boundary (COB). The thick oceanic crust is probably related to high mantle temperatures formed by brittle weakening and shear heating along a shear system prior to continental breakup. The COB is interpreted in the central parts of the profile where the velocity structure and Bouguer anomalies change significantly. East of the COB Moho depths increase while the vertical velocity gradient decreases. Below the assumed center for Early Eocene volcanic activity the model shows increased velocities in the crust. These increased crustal velocities are interpreted to represent Early Eocene mafic feeder dykes. East of the zone of volcanoes velocities in the crust decrease and sedimentary velocities are observed at depths of more than 10 km. The amount of crustal intrusions is much lower in this area than farther west. East of the Kn?legga Fault crystalline basement velocities are brought close to the seabed. This fault marks the eastern limit of thick Cenozoic and Mesozoic packages on central parts of the western Barents Sea continental margin.     

Crustal structure and tectonic provinces of the Riiser-Larsen Sea area (East Antarctica): results of geophysical studies

About 16,000 km of multichannel seismic (MCS), gravity and magnetic data and 28 sonobuoys were acquired in the Riiser-Larsen Sea Basin and across the Gunnerus and Astrid Ridges, to study their crustal structure. The study area has contrasting basement morphologies and crustal thicknesses. The crust ranges in thickness from about 35 km under the Riiser-Larsen Sea shelf, 26–28 km under the Gunnerus Ridge, 12–17 km under the Astrid Ridge, and 9.5–10 km under the deep-water basin. A 50-km-wide block with increased density and magnetization is modeled from potential field data in the upper crust of the inshore zone and is interpreted as associated with emplacement of mafic intrusions into the continental margin of the southern Riiser-Larsen Sea. In addition to previously mapped seafloor spreading magnetic anomalies in the western Riiser-Larsen Sea, a linear succession from M2 to M16 is identified in the eastern Riiser-Larsen Sea. In the southwestern Riiser-Larsen Sea, a symmetric succession from M24B to 24n with the central anomaly M23 is recognized. This succession is obliquely truncated by younger lineation M22–M22n. It is proposed that seafloor spreading stopped at about M23 time and reoriented to the M22 opening direction. The seismic stratigraphy model of the Riiser-Larsen Sea includes five reflecting horizons that bound six seismic units. Ages of seismic units are determined from onlap geometry to magnetically dated oceanic basement and from tracing horizons to other parts of the southern Indian Ocean. The seaward edge of stretched and attenuated continental crust in the southern Riiser-Larsen Sea and the landward edge of unequivocal oceanic crust are mapped based on structural and geophysical characteristics. In the eastern Riiser-Larsen Sea the boundary between oceanic and stretched continental crust is better defined and is interpreted as a strike-slip fault lying along a sheared margin.