Crustal Thinning of the Northern Continental Margin of the South China Sea

Magnetic data suggest that the distribution of the oceanic crust in the northern South China Sea (SCS) may extend to about 21 °N and 118.5 °E. To examine the crustal features of the corresponding continent–ocean transition zone, we have studied the crustal structures of the northern continental margin of the SCS. We have also performed gravity modeling by using a simple four-layer crustal model to understand the geometry of the Moho surface and the crustal thicknesses beneath this transition zone. In general, we can distinguish the crustal structures of the study area into the continental crust, the thinned continental crust, and the oceanic crust. However, some volcanic intrusions or extrusions exist. Our results indicate the existence of oceanic crust in the northernmost SCS as observed by magnetic data. Accordingly, we have moved the continent–ocean boundary (COB) in the northeastern SCS from about 19 °N and 119.5 °E to 21 °N and 118.5 °E. Morphologically, the new COB is located along the base of the continental slope. The southeastward thinning of the continental crust in the study area is prominent. The average value of crustal thinning factor of the thinned continental crust zone is about 1.3–1.5. In the study region, the Moho depths generally vary from ca. 28 km to ca. 12 km and the crustal thicknesses vary from ca. 24 km to ca. 6 km; a regional maximum exists around the Dongsha Island. Our gravity modeling has shown that the oceanic crust in the northern SCS is slightly thicker than normal oceanic crust. This situation could be ascribed to the post-spreading volcanism or underplating in this region.     

New Bathymetry and Magnetic Lineations Identifications in the Northernmost South China Sea and their Tectonic Implications

The seafloor spreading of the South China Sea (SCS) was previously believed to take place between ca. 32 and 15 Ma (magnetic anomaly C11 to C5c). New magnetic data acquired in the northernmost SCS however suggests the existence of E–W trending magnetic polarity reversal patterns. Magnetic modeling demonstrates that the oldest SCS oceanic crust could be Late Eocene (as old as 37 Ma, magnetic anomaly C17), with a half-spreading rate of 44 mm/yr. The new identified continent–ocean boundary (COB) in the northern SCS generally follows the base of the continental slope. The COB is also marked by the presence of a relatively low magnetization zone, corresponding to the thinned portion of the continental crust. We suggest that the northern extension of the SCS oceanic crust is terminated by an inactive NW–SE trending trench-trench transform fault, called the Luzon–Ryukyu Transform Plate Boundary (LRTPB). The LRTPB is suggested to be a left-lateral transform fault connecting the former southeast-dipping Manila Trench in the south and the northwest-dipping Ryukyu Trench in the north. The existence of the LRTPB is demonstrated by the different patterns of the magnetic anomalies as well as the different seafloor morphology and basement relief on both sides of the LRTPB. Particularly, the northwestern portion of the LRTPB is marked by a steep northeast-dipping escarpment, along which the Formosa Canyon has developed. The LRTPB probably became inactive at ca. 20 Ma while the former Manila Trench prolonged northeastwards and connected to the former Ryukyu Trench by another transform fault. This reorganization of the plate boundaries might cause the southwestern portion of the former Ryukyu Trench to become extinct and a piece of the Philippine Sea Plate was therefore trapped amongst the LRTPB, the Manila Trench and the continental margin.     

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.     

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.     

东海莫霍面起伏与地壳减薄特征初步分析

收集、整理大量由地震剖面提供的沉积层厚度资料,得到东海沉积层等厚图。对完全布格重力异常进行沉积层重力效应改正后,得到剩余重力异常,利用地震资料揭示的莫霍面深度值来约束界面反演得到东海莫霍面埋深。结果表明,东海陆架盆地莫霍面深度在25～28 km之间平缓变化,地壳厚度为14～26 km,西厚东薄;冲绳海槽盆地莫霍面深度为16～26 km,地壳厚度为12～22 km,北厚南薄。东海陆架盆地东部与冲绳海槽盆地南部地壳减薄明显,拉张因子分别达到2.6和3。初步分析认为冲绳海槽地壳以过渡壳为主,并未形成洋壳。     

Crustal velocity structure off SW Taiwan in the northernmost South China Sea imaged from TAIGER OBS and MCS data

During TAiwan Integrated GEodynamics Research of 2009, we investigated data from thirty-seven ocean-bottom seismometers (OBS) and three multi-channel seismic (MCS) profiles across the deformation front in the northernmost South China Sea (SCS) off SW Taiwan. Initial velocity-interface models were built from horizon velocity analysis and pre-stack depth migration of MCS data. Subsequently, we used refracted, head-wave and reflected arrivals from OBS data to forward model and then invert the velocity-interface structures layer-by-layer. Based on OBS velocity models west of the deformation front, possible Mesozoic sedimentary rocks, revealed by large variation of the lateral velocity (3.1–4.8 km/s) and the thickness (5.0–10.0 km), below the rift-onset unconformity and above the continental crust extended southward to the NW limit of the continent–ocean boundary (COB). The interpreted Mesozoic sedimentary rocks NW of the COB and the oceanic layer 2 SE of the COB imaged from OBS and gravity data were incorporated into the overriding wedge below the deformation front because the transitional crust subducted beneath the overriding wedge of the southern Taiwan. East of the deformation front, the thickness of the overriding wedge (1.7–5.0 km/s) from the sea floor to the décollement decreases toward the WSW direction from 20.0 km off SW Taiwan to 8.0 km at the deformation front. In particular, near a turn in the orientation of the deformation front, the crustal thickness (7.0–12.0 km) is abruptly thinner and the free-air (?20 to 10 mGal) and Bouguer (30–50 mGal) gravity anomalies are relatively low due to plate warping from an ongoing transition from subduction to collision. West of the deformation front, intra-crustal interfaces dipping landward were observed owing to subduction of the extended continent toward the deformation front. However, the intra-crustal interface near the turn in the orientation of the deformation front dipping seaward caused by the transition from subduction to collision. SE of the COB, the oceanic crust, with a crustal thickness of about 10.0–17.0 km, was thickened due to late magmatic underplating or partially serpentinized mantle after SCS seafloor spreading. The thick oceanic crust may have subducted beneath the overriding wedge observed from the low anomalies of the free-air (?50 to ?20 mGal) and Bouguer (40–80 mGal) gravities across the deformation front.     

南极布兰斯菲尔德海峡及邻区地壳结构反演及构造解析

南极布兰斯菲尔德海峡及邻区是南极半岛海域火山、地震等新构造运动最活跃的地区,由于前人对资料处理解释的差异,导致盆地的构造格局仍部分存疑。本文以研究区的卫星重力数据为基础,以多道反射地震和部分岩性资料为约束,采用重震联合反演方法构建了三条横跨研究区的地壳结构剖面,并进一步研究布兰斯菲尔德海峡盆地的地壳结构。研究结果表明布兰斯菲尔德海峡盆地莫霍面深度为33—38km。菲尼克斯板块俯冲消减下沉至南设得兰岛弧之下,导致南设得兰海沟的俯冲带后撤,产生3—4km厚的岩浆混染地壳,密度为2.9g/cm~3。分析认为受板块运动和弧后扩张影响,沿布兰斯菲尔德海峡盆地扩张脊分布的海底火山裂隙式喷发,并进一步导致盆地的持续性扩张。     

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.     

Møre Margin: Crustal structure from analysis of Expanded Spread Profiles

Analysis in both the x—t and —p domains of high-quality Expanded Spread Profiles across the Møre Margin show that many arrivals may be enhanced be selective ray tracing and velocity filtering combined with conventional data reduction techniques. In terms of crustal structure the margin can be divided into four main areas: 1) a thicker than normal oceanic crust in the eastern Norway Basin; 2) expanded crust with a Moho depth of 22 km beneath the huge extrusive complex constructed during early Tertiary breakup; 3) the Møre Basin where up to 13–14 km of sediments overlie a strongly extended outer part with a Moho depth at 20 km west of the Ona High; and 4) a region with a 25–27 km Moho depth between the high and the Norwegian coast. The velocity data restricts the continent-ocean boundary to a 15–30 km wide zone beneath the seaward dipping reflector wedges. The crust west of the landward edge of the inner flow is classified as transitional. This region as well as the adjacent oceanic crust is soled by a 7.2–7.4 km s^–1 lower crustal body which may extend beneath the entire region that experienced early Tertiary crustal extension. At the landward end of the transect a 8.5 km s^–1 layer near the base of the crust is recognized. A possible relationship with large positive gravity anomalies and early Tertiary alkaline intrusions is noted.     

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.     

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.     

A seismic refraction study of cretaceous oceanic lithosphere in the Northwest Pacific Basin

A seismic refraction study on old (110 Myr) lithosphere in the northwest Pacific Basin has placed constraints on crustal and uppermantle seismic structure of old oceanic lithosphere, and lithospheric aging processes. No significant lateral variation in structure other than azimuthally anisotropic mantle velocities was found, allowing the application of powerful amplitude modeling techniques. The anisotropy observed is in an opposite sense to that expected, suggesting the tectonic setting of the area may be more complex than originally thought. Upper crustal velocities are generally larger than for younger crust, supporting current theories of decreased porosity with crustal aging. However, there is no evidence for significant thickening of the oceanic crust with age, nor is there any evidence of a lower crustal layer of high or low velocity relative to the velocity of the rest of Layer 3. The compressional and shear wave velocities rule out a large component of serpentinization of mantle materials. The only evidence for a basal crustal layer of olivine gabbro cumulates is a 1.5 km thick Moho transition zone. In the slow direction of anisotropy, upper mantle velocities increase from 8.0 km s^-1 to 8.35 km s^-1 in the upper 15 km below the Moho. This increase is inconsistent with an homogeneous upper mantle and suggests that compositinal or phase changes occur near the Moho.     

Geology of the Continental Margin of Enderby and Mac. Robertson Lands, East Antarctica: Insights from a Regional Data Set

In 2001 and 2002, Australia acquired an integrated geophysical data set over the deep-water continental margin of East Antarctica from west of Enderby Land to offshore from Prydz Bay. The data include approximately 7700 km of high-quality, deep-seismic data with coincident gravity, magnetic and bathymetry data, and 37 non-reversed refraction stations using expendable sonobuoys. Integration of these data with similar quality data recorded by Japan in 1999 allows a new regional interpretation of this sector of the Antarctic margin. This part of the Antarctic continental margin formed during the breakup of the eastern margin of India and East Antarctica, which culminated with the onset of seafloor spreading in the Valanginian. The geology of the Antarctic margin and the adjacent oceanic crust can be divided into distinct east and west sectors by an interpreted crustal boundary at approximately 58° E. Across this boundary, the continent–ocean boundary (COB), defined as the inboard edge of unequivocal oceanic crust, steps outboard from west to east by about 100 km. Structure in the sector west of 58° E is largely controlled by the mixed rift-transform setting. The edge of the onshore Archaean–Proterozoic Napier Complex is downfaulted oceanwards near the shelf edge by at least 6 km and these rocks are interpreted to underlie a rift basin beneath the continental slope. The thickness of rift and pre-rift rocks cannot be accurately determined with the available data, but they appear to be relatively thin. The margin is overlain by a blanket of post-rift sedimentary rocks that are up to 6 km thick beneath the lower continental slope. The COB in this sector is interpreted from the seismic reflection data and potential field modelling to coincide with the base of a basement depression at 8.0–8.5 s two-way time, approximately 170 km oceanwards of the shelf-edge bounding fault system. Oceanic crust in this sector is highly variable in character, from rugged with a relief of more than 1 km over distances of 10–20 km, to rugose with low-amplitude relief set on a long-wavelength undulating basement. The crustal velocity profile appears unusual, with velocities of 7.6–7.95 km s⁻¹ being recorded at several stations at a depth that gives a thickness of crust of only 4 km. If these velocities are from mantle, then the thin crust may be due to the presence of fracture zones. Alternatively, the velocities may be coming from a lower crust that has been heavily altered by the intrusion of mantle rocks. The sector east of 58° E has formed in a normal rifted margin setting, with complexities in the east from the underlying structure of the N–S trending Palaeozoic Lambert Graben. The Napier Complex is downfaulted to depths of 8–10 km beneath the upper continental slope, and the margin rift basin is more than 300 km wide. As in the western sector, the rift-stage rocks are probably relatively thin. This part of the margin is blanketed by post-rift sediments that are up to about 8 km thick. The interpreted COB in the eastern sector is the most prominent boundary in deep water, and typically coincides with a prominent oceanwards step-up in the basement level of up to 1 km. As in the west, the interpretation of this boundary is supported by potential field modelling. The oceanic crust adjacent to the COB in this sector has a highly distinctive character, commonly with (1) a smooth upper surface underlain by short, seaward-dipping flows; (2) a transparent upper crustal layer; (3) a lower crust dominated by dipping high-amplitude reflections that probably reflect intruded or altered shears; (4) a strong reflection Moho, confirmed by seismic refraction modelling; and (5) prominent landward-dipping upper mantle reflections on several adjacent lines. A similar style of oceanic crust is also found in contemporaneous ocean basins that developed between Greater India and Australia–Antarctica west of Bruce Rise on the Antarctic margin, and along the Cuvier margin of northwest Australia.     

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.     

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.     

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.     

Crustal structure in the North Arabian Sea from magnetic surveys

The spectral study of the aero-magnetic map of the North Arabian Sea (above 20°N) has delineated three horizons at average depths of 45 km, 21 km, and 8 km. Spectral estimates from smaller blocks of data drawn from the original map suggest that the 21 km horizon varies in depth from 14 km on the abyssal plain (oceanic crust) to 24 km towards the north and 28 km towards the east onto the continental shelf. This appears to correspond to the crust-mantle interface (Moho). The 8 km horizon corresponds to the top of the igneous basement. The significance of the deepest layer (45 km) is discussed as the maximum depth of the Curie point geotherm in this region. The spectral estimate of the block of data on the continental shelf off the west coast of India (above 20°N) has brought out some magnetic inhomogeneity at a shallower depth of 4 km. This appears to be connected with the sea-floor spreading phenomenon from the Carlsberg ridge. The presence of such a magnetic inhomogeneity at a depth of 4 km is further confirmed by the spectral estimate of a marine magnetic map off the west coast of India around Bombay. The depth of the basement inferred from this study is in close agreement with that obtained from other studies in this region, such as seismics.     

Crustal structure of the deepwater west Niger Delta passive margin from the interpretation of seismic reflection data

The interpretation of 2D and 3D seismic reflection data complemented with gravity data allows the crustal architecture of the deepwater west Niger Delta passive margin to be defined. The data show that the area is underlain by oceanic crust that is characterised by a thickness of 5–7 km and by internal reflectivity consisting of both dipping and sub-horizontal reflectors. Some of the dipping reflections can be traced up to the top of the basement where they offset it across a series of minor to major thrust faults. Other internal reflections are attributed to extensional shear zones and possibly due to intrusions in the lower crust. The Moho can be correlated as a discrete reflection over >70% of the study area. It is generally smooth, but localised relief of up to 1 km is observed. The southeastern part of the study area is dominated by a zone of SW–NE striking basement thrusts.     

南海西北部与红河地区地球物理场及其地壳深部结构特征

分析了南海西北部与红河地区地球物理场特征，计算了研究区重、磁资料的一阶小波细节变换、四阶小波逼近变换。根据重力场资料以及南海北部盆地钻井取样的测试结果，同时参考在研究区进行的地震勘探结果，对研究区的地壳结构进行了反演计算。结果表明，研究区域地壳结构较为复杂，地壳厚度在17—38km之间，总的趋势由陆向洋地壳厚度逐渐减薄，反映出该区域地壳具有陆壳、过渡壳的性质，同时存在上地幔隆起区及凹陷区。用地震层折成像结果与重力资料计算出的地壳分布趋势进行了对比验证。根据地幔对流结果探讨了研究区深部地球动力学特征及其与深部地壳结构的关系。     

Geophysical investigations of crust-scale structural model of the Qiongdongnan Basin, Northern South China Sea

Rifting of the Qiongdongnan Basin was initiated in the Cenozoic above a pre-Cenozoic basement, which was overprinted by extensional tectonics and soon after the basin became part of the rifted passive continental margin of the South China Sea. We have integrated available grids of sedimentary horizons, wells, seismic reflection data, and the observed gravity field into the first crust-scale structural model of the Qiongdongnan Basin. Many characteristics of this model reflect the tectonostratigraphic history of the basin. The structure and isopach maps of the basin allow us to reconstruct the history of the basin comprising: (a) The sediments of central depression are about 10 km thicker than on the northern and southern sides; (b) The sediments in the western part of the basin are about 6 km thicker than that in the eastern part; (c) a dominant structural trend of gradually shifting depocentres from the Paleogene sequence (45–23.3 Ma) to the Neogene to Quaternary sequence (23.3 Ma–present) towards the west or southwest. The present-day configuration of the basin reveals that the Cenozoic sediments are thinner towards the east. By integrating several reflection seismic profiles, interval velocity and performing gravity modeling, we model the sub-sedimentary basement of the Qiongdongnan Basin. There are about 2–4 km thick high-velocity bodies horizontal extended for a about 40–70 km in the lower crust (v > 7.0 km/s) and most probably these are underplated to the lower stretched continental crust during the final rifting and early spreading phase. The crystalline continental crust spans from the weakly stretched domains (about 25 km thick) near the continental shelf to the extremely thinned domains (<2.8 km) in the central depression, representing the continental margin rifting process in the Qiongdongnan Basin. Our crust-scale structural model shows that the thinnest crystalline crust (<3 km) is found in the Changchang Sag located in the east of the basin, and the relatively thinner crystalline crust (<3.5 km) is in the Ledong Lingshui Sag in the west of the basin. The distribution of crustal extension factor β show that β in central depression is higher (>7.0), while that on northern and southern sides is lower (<3.0). This model can illuminate future numerical simulations, including the reconstruction of the evolutionary processes from the rifted basin to the passive margin and the evolution of the thermal field of the basin.