Lithospheric structure below the eastern Arabian Sea and adjoining West Coast of India based on integrated analysis of gravity and seismic data

Four uniformly spaced regional gravity traverses and the available seismic data across the western continental margin of India, starting from the western Indian shield extending into the deep oceanic areas of the eastern Arabian Sea, have been utilized to delineate the lithospheric structure. The seismically constrained gravity models along these four traverses suggest that the crustal structure below the northern part of the margin within the Deccan Volcanic Province (DVP) is significantly different from the margin outside the DVP. The lithosphere thickness, in general, varies from 110–120 km in the central and southern part of the margin to as much as 85–90 km below the Deccan Plateau and Cambay rift basin in the north. The Eastern basin is characterised by thinned rift stage continental crust which extends as far as Laxmi basin in the north and the Laccadive ridge in the south. At the ocean–continent transition (OCT), crustal density differences between the Laxmi ridge and the Laxmi basin are not sufficient to distinguish continental as against an oceanic crust through gravity modeling. However, 5-6 km thick oceanic crust below the Laxmi basin is a consistent gravity option. Significantly, the models indicate the presence of a high density layer of 3.0 g/cm³ in the lower crust in almost whole of the northern part of the region between the Laxmi ridge and the pericontinental northwest shield region in the DVP, and also below Laccadive ridge in the southern part. The Laxmi ridge is underlain by continental crust upto a depth of 11 km and a thick high density material (3.0 g/cm³) between 11–26 km. The Pratap ridge is indicated as a shallow basement high in the upper part of the crust formed during rifting. The 15 –17 km thick oceanic crust below Laccadive ridge is seen further thickened by high density underplated material down to Moho depths of 24–25 km which indicate formation of the ridge along Reunion hotspot trace.     

Features of the crustal structure of the Arabian Sea

An analysis of the gravity field and geoid heights allowed us to distinguish a third buried basin filled with sediments located in the southwestern part of the sea in the regions adjacent to the Carlsberg Ridge. From the previously known basins, it is separated by saddles. The saddles correspond to a series of faults and are possibly related to the pulse character of the northwestward prograding of the spreading axes of the Carlsberg Ridge. The continental origin of the Laxmi ridge is confirmed. The results of an analysis of the gravity field and its transformants, together with the two-dimensional density modeling, agree with the possibility of the existence of a spreading type of the crust (I) in the region of the Laxmi Basin. An analysis of the geoid height anomalies allows us to suggest that, with respect to the upper layers of the lithosphere, the Laxmi Ridge is not connected with the Chagos-Laccadive Ridge.     

Tectonic core of a sedimentary drift: a potential ridge propagation feature beneath the Blake Outer Ridge

The Blake Outer Ridge is a 480–kilometer long linear sedimentary drift ridge striking perpendicular to the North American coastline. By modeling free-air gravity anomalies we tested for the presence of a crustal feature that may control the location and orientation of the Blake Outer Ridge. Most of our crustal density models that match observed gravity anomalies require an increase in oceanic crustal thickness of 1–3 km on the southwest side of the Blake Outer Ridge relative to the northeast side. Most of these models also require 1–4 km of crustal thinning in zone 20–30 km southwest of the crest of the Blake Outer Ridge. Although these features are consistent with the structure of oceanic fracture zones, the Blake Outer Ridge is not parallel to adjacent known fracture zones. Magnetic anomalies suggest that the ocean crust beneath this feature formed during a period of mid-ocean ridge reorganization, and that the Blake Outer Ridge may be built upon the bathymetric expression of an oblique extensional feature associated with ridge propagation. It is likely that the orientation of this trough acted as a catalyst for sediment deposition with the start of the Western Boundary Undercurrent in the mid-Oligocene.     

A free air gravity anomaly contour map of the Irish continental margin

Compilation of currently-available gravity data permits the construction of a free-air anomaly contour map of the continental margin west of Ireland (51–54 N). Major elements in the structure of the margin, previously delineated on the basis of seismic reflection and magnetic surveys, are clearly seen on the FAA contour map, notably the Porcupine Seabight Trough, and Porcupine Ridge. However, contrary to earlier ideas, the gravity data imply that the Seabight Trough extends northwards onto the Slyne Ridge; and the Slyne Trough, formerly regarded as northeasterly prolongation of the Seabight Trough, appears to be a discrete, fault-bounded, feature separated from the latter by a basement ridge. East-west gravity profiles are modelled in terms of thinned crust with the Moho at a minimum depth of 15 km beneath the axis of the Seabight Trough. The models tend to support hypotheses invoking formation of the Seabight Trough by simple westward translation of Porcupine Ridge with respect to the Irish Mainland.     

Laxmi Ridge – A continental sliver in the Arabian Sea

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.     

The deep structure of the Iceland-Faeroe Ridge

Two long seismic refraction lines along the crest of the Iceland-Faeroe Ridge reveal a layered crust resembling the crust beneath Iceland but differing from normal continental or oceanic crust. The Moho was recognised at the south-eastern end of the lines at an apparent depth of 16–18 km. A refraction line in deeper water west of the ridge and south of Iceland indicates a thin oceanic type crust underlain by a 7.1 km/s layer which may be anomalous upper mantle.An extensive gravity survey of the ridge shows that it is in approximate isostatic equilibrium; the steep gravity gradient between the Norwegian Sea and the ridge indicates that the ridge is supported by a crust thickened to about 20 km rather than by anomalous low density rocks in the underlying upper mantle, in agreement with the seismic results. An increase in Bouguer anomaly of about 140 mgal between the centre of Iceland and the ridge is attributed to lateral variation in upper mantle density from an anomalous low value beneath Iceland to a more normal value beneath the ridge. Local gravity anomalies of medium amplitude which are characteristic of the ridge are caused by sediment troughs and by lateral variations in the upper crust beneath the sediments. A steep drop in Bouguer anomaly of about 80 mgal between the ridge and the Faeroe block is attributed partly to lateral change in crustal density and partly to slight thickening of the crust towards the Faeroe Islands; this crustal boundary may represent an anomalous type of continental margin formed when Greenland started to separate from the Faeroe Islands about 60 million years ago.We conclude that the Iceland-Faeroe Ridge formed during ocean floor spreading by an anomalous hot spot type of differentiation from the upper mantle such as is still active beneath Iceland. This suggests that the ridge may have stood some 2 km higher than at present when it was being formed in the early Tertiary, and that it has subsequently subsided as the spreading centre moved away and the underlying mantle became more normal; this interpretation is supported by recognition of a V-shaped sediment filled trough across the south-eastern end of the ridge, which may be a swamped sub-aerial valley.     

Gravity and multichannel seismic reflection constraints on the lithospheric structure of the Canary Swell

Deep penetrating multichannel seismic reflection and gravity data have been used to study the lithospheric structure of the Canary Swell. The seismic reflection data show the transition from undisturbed Jurassic oceanic crust, away from the Canary Islands, to an area of ocean crust strongly modified by the Canary volcanism (ACV). Outside the ACV the seismic records image a well layered sedimentary cover, underlined by a bright reflection from the top of the igneous basement and also relatively continuous reflections from the base of the crust. In the ACV the definition of the boundary between sedimentary cover and igneous basement and the crust-mantle boundary remains very loose. Two-dimensional gravity modelling in the area outside the influence of the Canary volcanism, where the reflection data constrain the structure of the ocean crust, suggests a thinning of the lithosphere. The base of the lithosphere rises from 100 km, about 400 km west of the ACV, to 80 km at the outer limit of the ACV. In addition, depth conversion of the seismic reflection data and unloading of the sediments indicate the presence of a regional depth anomaly of an extension similar to the lithospheric thinning inferred from gravity modelling. The depth anomaly associated with the swell, after correction for sediment weight, is about 500 m. We interpret the lithospheric thinning as an indication of reheating of old Mesozoic lithosphere beneath the Canary Basin and along with the depth anomaly as indicating a thermal rejuvenation of the lithosphere. We suggest that the most likely origin for the Canary Islands is a hot spot.     

Basement-involved faults and deep structures in the West Philippine Basin: constrains from gravity field

To reveal the basement-involved faults and deep structures of the West Philippine Basin (WPB), the gravitational responses caused by these faults are observed and analyzed based on the latest spherical gravity model: WGM2012 Model. By mapping the free-air and Bouguer gravity anomalies, several main faults and some other linear structures are located and observed in the WPB. Then, by conducting a 2D discrete multi-scale wavelet decomposition, the Bouguer anomalies are decomposed into the first- to eighth-order detail and approximation fields (the first- to eighth-order Details and Approximations). The first- to third-order Details reflect detailed and localized geological information of the crust at different depths, and of which the higher-order reflects gravity field of the deeper depth. The first- to fourth-order Approximations represent the regional gravity fields at different depths of the crust, respectively. The fourth-order Approximation represents the regional gravity fluctuation caused by the density inhomogeneity of Moho interface. Therefore, taking the fourth-order Approximation as input, and adopting Parker-Oldenburg interactive inversion, We calculated the depth of Moho interface in the WPB. Results show that the Moho interface depth in the WPB ranges approximately from 8 to 12 km, indicating that there is typical oceanic crust in the basin. In the Urdaneta Plateau and the Benham Rise, the Moho interface depths are about 14 and 16 km, respectively, which provides a piece of evidence to support that the Banham Rise could be a transitional crust caused by a large igneous province. The second-order vertical derivative and the horizontal derivatives in direction 0° and 90° are computed based on the data of the third-order Detail, and most of the basement-involved faults and structures in the WPB, such as the Central Basin Fault Zone, the Gagua Ridge, the Luzon-Okinawa Fault Zone, and the Mindanao Fault Zone are interpreted by the gravity derivatives.     

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.     

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.     

Continental margin off Western India and Deccan Large Igneous Province

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.     

Crustal structure of the ultra-slow spreading Knipovich Ridge,North Atlantic,along a presumed amagmatic portion of oceanic crustal formation

The ultra-slow, asymmetrically-spreading Knipovich Ridge is the northernmost part of the Mid Atlantic ridge system. In the autumn of 2002 a combined ocean-bottom seismometer multichannel seismic (OBS/MCS) and gravity survey along the spreading direction of the Knipovich Ridge was carried out. The main objective of the study was to gain an insight into the crustal structure and composition of what is assumed to be an amagmatic segment of oceanic crust. P-wave velocity and Vp/Vs models were built and complemented by a gravity model. The 190 km long transect reveals a much more complex crustal structure than anticipated. The magmatic crust is thinner than the global average of 7.1 ± 1.0 km. The young fractured portion of Oceanic Layer 2 has low seismic velocities while the older part has normal seismic velocities and is broken into several rotated fault blocks seen as thickness variations of Layer 2. The youngest part of Oceanic Layer 3 is also dominated by low velocities, indicative of fracturing, seawater circulation and thermal expansion. The remaining portion of Layer 3 exhibits inverse variations in thickness and seismic velocity. This is explained by a sequence of periods of faster spreading (estimated to be up to 8 mm/year from interpretation of magnetic anomalies) when more normal gabbroic crust was being generated and periods of slower spreading (5.5 mm/year) when amagmatic stretching and serpentinization of the upper mantle occurred, and crust composed of mixed gabbro and serpentinized mantle was generated. The volumetric changes and upward fluid migration, associated with the process of serpentinization in this part of the crust, caused disruption to the overlying sedimentary layers.     

Gravity anomalies over inactive fracture zones in the central North Atlantic

The basement topography and the free-air gravity along two profiles in the central North Atlantic between 16° and 25° N, crossing a number of fracture zones, were divided in three wavelength intervals. Two-dimensional modelling shows that the short wavelength (>50 km) gravity is well explained by uncompensated topography (mainly spreading topography). For the long wavelengths (>200 km) there is no correlation of topography and gravity. In principle this topography is compensated. Residual anomalies comprise the Ridge effect as well as regional anomalies related to depth anomalies. The 50 to 200km band-pass filtered topography and gravity contain relevant information on fracture zones. Models require a base of the crust that parallels the topography rather than a form of regional compensation. For an explanation of this crustal model that has the appearance of frozen in normal faults we have to consider the typical morphology as created in the transform domain. The geophysical processes that cause this morphology are still an object of study.     

Morphology and crustal structure of a small transform fault along the Mid-Atlantic Ridge: The Atlantis Fracture Zone

The Atlantis Fracture Zone (30° N) is one of the smallest transform faults along the Mid-Atlantic Ridge with a spatial offset of 70 km and an age offset of ~ 6 Ma. The morphology of the Atlantis Fracture Zone is typical of that of slow-slipping transforms. The transform valley is 15–20 km wide and 2–4 km deep. The locus of strike-slip deformation is confined to a narrow band a few kilometers wide. Terrain created at the outside corners of the transform is characterized by ridges which curve toward the ridge-transform intersections and depressions which resemble nodal basins. Hooked ridges are not observed on the transform side of the ridge-transform intersections. Results of the three-dimensional inversion of the surface magnetic field over our survey area suggest that accretionary processes are sufficiently organized within 3–4 km of the transform fault to produce lineated magnetic anomalies. The magnetization solution further documents a 15-km, westward relocation of the axis of accretion immediately south of the transform about 0.25 Ma ago. The Atlantis Transform is associated with a band of high mantle Bouguer anomalies, suggesting the presence of high densities in the crust and/or mantle along the transform, or anomalously thin crust beneath the transform. Assuming that all the mantle Bouguer anomalies are due to crustal thickness variations, we calculate that the crust may be 2–3 km thinner than a reference 6-km thickness beneath the transform valley, and 2–3 km thicker beneath the mid-points of the spreading segments which bound the transform. Our results indicate that crustal thinning is not uniform along the strike of the fracture zone. Based on studies of the state of compensation of the transform, we conclude that the depth anomaly associated with the fracture zone valley is not compensated everywhere by thin crust. Instead, the regional relationship between bathymetry and gravity is best explained by compensation with an elastic plate with an effective thickness of ~ 4 km or greater. However, the remaining isostatic anomalies indicate that there are large variations away from this simple model which are likely due to variations in crustal thickness and density near the transform.     

THE BOUGUER ANOMALIES AND DEPTHS OF MOHOROVICIC DISCONTINUITY IN THE SOUTH CHINA SEA REGION

The South China Sea is situated at the continsntal margin of South China. In this region, there are both continental and oceanic crusts. The values of Bouguer gravity anomalies on the continental shelf are low positive or low negative. Because the depth of the Mohorovicic discontinuity in this region is about 26-32 km below sea level, the crust belongs to the continental type. The values of Bouguer gravity anomalies in the deep-sea region are more than 250 mgal and the depth of the Moho-surface is about 10-15 km below sea level, so the crust is of oceanic type. The values of gravity anomalies and depths of the Moho-surface, obtained over the continental (and island) slope, range between those regions mentioned above, so the crust belongs to the transitional type. The continental crust is inferred to be directly in contact with the oceanic crust as a result of a lithospheric fault.     

Gravity crustal models and heat flow measurements for the Eurasia Basin,Arctic Ocean

The Gakkel Ridge in the Arctic Ocean with its adjacent Nansen and Amundsen Basins is a key region for the study of mantle melting and crustal generation at ultraslow spreading rates. We use free-air gravity anomalies in combination with seismic reflection and wide-angle data to compute 2-D crustal models for the Nansen and Amundsen Basins in the Arctic Ocean. Despite the permanent pack-ice cover two geophysical transects cross both entire basins. This means that the complete basin geometry of the world’s slowest spreading system can be analysed in detail for the first time. Applying standard densities for the sediments and oceanic crystalline crust, the gravity models reveal an unexpected heterogeneous mantle with densities of 3.30 × 10³, 3.20 × 10³ and 3.10 × 10³ kg/m³ near the Gakkel Ridge. We interpret that the upper mantle heterogeneity mainly results from serpentinisation and thermal effects. The thickness of the oceanic crust is highly variable throughout both transects. Crustal thickness of less than 1 km dominates in the oldest parts of both basins, increasing to a maximum value of 6 km near the Gakkel Ridge. Along-axis heat flow is highly variable and heat flow amplitudes resemble those observed at fast or intermediate spreading ridges. Unexpectedly, high heat flow along the Amundsen transect exceeds predicted values from global cooling curves by more than 100%.     

西南印度洋脊“断桥”热液区深部地质构造研究

Polymetalic sulfide is the main product of sea-floor hydrothermal venting, and has become an important sea-floor mineral resources for its rich in many kinds of precious metal elements. Since 2007, a number of investigations have been carried out by the China Ocean Mineral Resources Research and Development Association(COMRA)cruises(CCCs) along the Southwest Indian Ridge(SWIR). In 2011, the COMRA signed an exploration contract of sea-floor polymetallic sulfides of 10 000 km2 on the SWIR with the International Seabed Authority. Based on the multibeam data and shipborne gravity data obtained in 2010 by the R/V Dayang Yihao during the leg 6 of CCCs21, together with the global satellite surveys, the characteristics of gravity anomalies are analyzed in the Duanqiao hydrothermal field(37°39′S, 50°24′E). The "subarea calibration" terrain-correcting method is employed to calculate the Bouguer gravity anomaly, and the ocean bottom seismometer(OBS) profile is used to constrain the two-dimensional gravity anomaly simulation. The absent Moho in a previous seismic model is also calculated.The results show that the crustal thickness varies between 3 and 10 km along the profile, and the maximum crustal thickness reaches up to 10 km in the Duanqiao hydrothermal field with an average of 7.5 km. It is by far the most thicker crust discovered along the SWIR. The calculated crust thickness at the Longqi hydrothermal field is approximately 3 km, 1 km less than that indicated by seismic models, possibly due to the outcome of an oceanic core complex(OCC).     

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.     

Crustal seismic structure beneath the West Philippine Sea, 17°–18° North

Crustal seismic structures beneath the West Philippine Sea are determined by using explosive sources (0.5–108.6 kg) and ocean bottom seismometers to measure refracted compressional waves. Total crustal thicknesses are shown to be thinner in the eastern part of the ocean basin, approaching only 3.5 km. Crustal thinning toward the east is consistent with the Palau Kyushu Ridge being a remnant transform fault connecting the Central Basin Ridge and the Kula Pacific Ridge in the past. A velocity-depth inversion from the westernmost refraction profile indicates the upper transitional crust layer to have strong velocity gradients which gradually decrease with depth; the lower crust is characterized by a nearly constant velocity gradient. The western part of the ocean basin is also shown to have more typical oceanic thicknesses, as is found in deep ocean basins of the Pacific. Spectral energy models using WKBJ synthetic seismograms suggest that there is a sharp seismic discontinuity between the crust and moho in the western part of the basin. Predicted water depths for the West Philippine Basin using an age-depth relation and corrected for an isostatic response to the measured crustal thicknesses, are still 300 meters shallower than observed depths. The depth anomaly can not be fully reconciled by thinner crust in the eastern part of the basin. This observation implies that a deeper seated anomaly is present beneath the West Philippine Basin.     

Evaporite deposition in the Aptian South Atlantic Ocean

The Aptian salt deposits of the South Atlantic Ocean extend for 250 km offshore and underlie the continental rise off Brazil and Angola in 4000 m water depth. These salt deposits do not occur south of the Walvis Ridge and Rio Grande Rise, and apparently lie on oceanic and continental crust. Geological evidence and an Aptian-Middle Barremian reconstruction of the South Atlantic suggests the pre-Aptian South Atlantic was closed at both ends, thus favouring widespread salt deposition on oceanic and continental crust. Three models of halite deposition are discussed briefly.