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1.
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.  相似文献   

2.
The 1994 Tasmante swath-mapping and reflection seismic cruise covered 200 000 km2 of sea floor south and west of Tasmania. The survey provided a wealth of morphological, structural and sedimentological information, in an area of critical importance in reconstructing the break-up of East Gondwana.The west Tasmanian margin consists of a non-depositional continental shelf less than 50 km wide and a sedimented continental slope about 100 km wide. The adjacent 20 km of abyssal plain to the west is heavily sedimented, and beyond that is lightly sedimented Eocene oceanic crust formed as Australia and Antarctica separated. The swath data revealed systems of 100 m-deep downslope canyons and large lower-slope fault-blocks, striking 320° and dipping landward. These continental blocks lie adjacent to the continent ocean boundary (COB) and are up to 2500 m high and have 15°–20° scarps.The South Tasman Rise (STR) is bounded to the west by the Tasman Fracture Zone extending south to Antarctica. Adjacent to the STR, the fracture zone is represented by a scarp up to 2000 m high with slopes of 15–20°. The scarp consists of continental faultblocks dipping landward. Beyond the scarp to the west is a string of sheared parallel highs, and beyond that is lightly sedimented Oligocene oceanic crust 4200–4600 m deep with distinct E-W spreading fabric. The eastern margin of the bathymetric STR trends about 320° and is structurally controlled. The depression between it and the continental East Tasman Plateau (ETP) is heavily sedimented; its western part is underlain by thinned continental crust and its central part by oceanic crust of Late Cretaceous to Early Tertiary age. The southern margin of the STR is formed by N-S transform faults and south-dipping normal faults.The STR is cut into two major terrains by a N-S fracture zone at 146°15E. The western terrain is characterised by rotated basement blocks and intervening basins mostly trending 270°–290°. The eastern terrain is characterised by basement blocks and intervening strike-slip basins trending 300°–340°. Recent dredging of basement rocks suggests that the western terrain has Antarctic affinities, whereas the eastern terrain has Tasmanian affinities.Stretching and slow spreading between Australia and Antarctica was in a NW direction from 130–45 Ma, and fast spreading was in a N-S direction thereafter. The western STR terrain was attached to Antarctica during the early movement, and moved down the west coast of Tasmania along a 320° shear zone, forming the landward-dipping continental blocks along the present COB. The eastern terrain either moved with the western terrain, or was welded to it along the 146°15 E fracture zone in the Early Tertiary. At 45 Ma, fast spreading started in a N-S direction, and after some probable movement along the 146°15E fracture zone, the west and east STR terrains were welded together and became part of Australia.  相似文献   

3.
Crustal Thinning of the Northern Continental Margin of the South China Sea   总被引：2，自引：0，他引：2  
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.  相似文献   

4.
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.  相似文献   

5.
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.  相似文献   

6.
The Jan Mayen microcontinent was as a result of two major North Atlantic evolutionary cornerstones—the separation of Greenland from Norway (~54 Ma), accompanied by voluminous volcanic activity, and the jump of spreading from the Aegir to the Kolbeinsey ridge (~33 Ma), which resulted in the separation of the microcontinent itself from Eastern Greenland (~24 Ma). The resulting eastern and western sides of the Jan Mayen microcontinent are respectively volcanic and non-volcanic rifted margins. Until now the northern boundary of the microcontinent was not precisely known. In order to locate this boundary, two combined refraction and reflection seismic profiles were acquired in 2006: one trending S–N and consisting of two separate segments south and north of the island of Jan Mayen respectively, and the second one trending SW–NE east of the island. Crustal P-wave velocity models were derived and constrained using gravity data collected during the same expedition. North of the West Jan Mayen Fracture Zone (WJMFZ) the models show oceanic crust that thickens from west to east. This thickening is explained by an increase in volcanic activity expressed as a bathymetric high and most likely related to the proximity of the Mohn ridge. East of the island and south of the WJMFZ, oceanic Layers 2 and 3 have normal seismic velocities but above normal average crustal thickness (~11 km). The similarity of the crustal thickness and seismic velocities to those observed on the conjugate M?re margin confirm the volcanic origin of the eastern side of the microcontinent. Thick continental crust is observed in the southern parts of both profiles. The northern boundary of the microcontinent is a continuation of the northern lineament of the East Jan Mayen Fracture Zone. It is thus located farther north than previously assumed. The crust in the middle parts of both models, around Jan Mayen island, is more enigmatic as the data suggest two possible interpretations—Icelandic type of oceanic crust or thinned and heavily intruded continental crust. We prefer the first interpretation but the latter cannot be completely ruled out. We infer that the volcanism on Jan Mayen is related to the Icelandic plume.  相似文献   

7.
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.  相似文献   

8.
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.  相似文献   

9.
To facilitate geological analyses of the Ulleung Basin in the East Sea (Japan Sea) between Korea and Japan, shipborne and satellite altimetry-derived gravity data are combined to derive a regionally coherent anomaly field. The 2-min gridded satellite altimetry-based gravity predicted by Sandwell and Smith [Sandwell DT, Smith WHF (1997) J Geophys Res 102(B5):10,039–10,054] are used for making cross-over adjustments that reduce the errors between track segments and at the cross-over points of shipborne gravity profiles. Relative to the regionally more homogeneous satellite gravity anomalies, the longer wavelength components of the shipborne anomalies are significantly improved with minimal distortion of their shorter wavelength components. The resulting free-air gravity anomaly map yields a more coherent integration of short and long wavelength anomalies compared to that obtained from either the shipborne or satellite data sets separately. The derived free-air anomalies range over about 140 mGals or more in amplitude and regionally correspond with bathymetric undulations in the Ulleung Basin. The gravity lows and highs along the basin’s margin indicate the transition from continental to oceanic crust. However, in the northeastern and central Ulleung Basin, the negative regional correlation between the central gravity high and bathymetric low suggests the presence of shallow denser mantle beneath thinned oceanic crust. A series of gravity highs mark seamounts or volcanic terranes from the Korean Plateau to Oki Island. Gravity modeling suggests underplating by mafic igneous rocks of the northwestern margin of the Ulleung Basin and the transition between continental and oceanic crust. The crust of the central Ulleung Basin is about a 14–15 km thick with a 4–5 km thick sediment cover. It may also include a relatively weakly developed buried fossil spreading ridge with approximately 2 km of relief.  相似文献   

10.
The Pelotas Basin is the classical example of a volcanic passive margin displaying large wedges of seaward-dipping reflectors (SDR). The SDR fill entirely its rifts throughout the basin, characterizing the abundant syn-rift magmatism (133–113 Ma). The Paraná–Etendeka Large Igneous Province (LIP), adjacent to west, constituted the pre-rift magmatism (134–132 Ma). The interpretation of ultra-deep seismic lines showed a very different geology from the adjacent Santos, Campos and Espírito Santo Basins, which constitute examples of magma-poor passive margins. Besides displaying rifts totally filled by volcanic rocks, diverse continental crustal domains were defined in the Pelotas Basin, such as an outer domain, probably constituted by highly stretched and permeated continental igneous crust, and a highly reflective lower crust probably reflecting underplating.The analysis of rifting in this portion of the South Atlantic is based on seismic interpretation and on the distribution of regional linear magnetic anomalies. The lateral accretion of SDR to the east towards the future site of the breakup and the temporal relationship between their rift and sag geometries allows the reconstitution of the evolution of rifting in the basin. Breakup propagated from south to north in three stages (130–127.5; 127.5–125; 125–113 Ma) physically separated by oceanic fracture zones (FZ). The width of the stretched, thinned and heavily intruded continental crust also showed a three-stage increase in the same direction and at the same FZ. Consequently, the Continental-Oceanic Boundary (COB) shows three marked shifts, from west to east, from south to north, resulting into rift to margin segmentation. Rifting also propagated from west to east, in the direction of the final breakup, in each of the three segments defined. The importance of the Paraná–Etendeka LIP upon the overall history of rupturing and breakup of Western Gondwanaland seems to have been restricted in time and in space only to the Pelotas Basin.  相似文献   

11.
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.  相似文献   

12.
The sedimentary structure in the Gulf of Cadiz has been extensively studied by oil exploration companies. However, up to now little is known about its deep crustal structure. Moreover, the total thickness of the sedimentary layers remains unknown in large areas. The purpose of this paper is the crustal-scale interpretation of deep seismic near-vertical reflection and refraction/wide-angle reflection data obtained during the IAM (Iberian Atlantic Margins) project, carried out in 1993. Our results indicate that a continental type crust is underlying the entire Gulf of Cadiz, with progressive thinning from east to west. The sedimentary cover shows a great thickness, reaching 8 km in the center of the Gulf. Three main sedimentary units can be recognized: Jurassic-Cretaceous calcareous rocks, continuation of Algarve outcrops; the Allochthonous Units of Guadalquivir/Gulf of Cadiz, the offshore continuation of the inland Carmona nappe; and sub-horizontal post-Miocene marine sediments. The crystalline crust is divided into three main layers: the upper crust is characterized by P-wave velocity values of 5.7–6.1 km/s; the middle crust shows values of 6.3–6.4 km/s; the lower crust has a mean vertical velocity gradient of 0.02 km/s/km, with velocities between 6.9 to 7.1 km/s. The total crustal thickness varies from 27 km for the eastern part of the studied area, to 20 km for the westernmost part. The crustal thinning is more pronounced in a N-S direction than in an E-W direction. No major structures related with a defined Iberia-Africa plate boundary could be found. This revised version was published online in November 2006 with corrections to the Cover Date.  相似文献   

13.
 Six sandstone blocks sampled during dives along the southern slope of the Ivory Coast–Ghana continental margin have been studied using fission tracks in apatite and zircon. Measurements demonstrate that the rocks were heated above 120°C but below 390°C and cooled quickly. The ages of cooling recorded by the apatite crystals are 90 Ma in the western part of the margin, and 80–70 Ma in the central and eastern part. Heating is interpreted by the heat liberation due to the friction along the active transform fault and by the vicinity of an oceanic spreading center, which slipped along the margin. Cooling is interpreted by two stages of denudation due to minor faults and landslides produced by the increasing of the bathymetric step between the continental margin and the oceanic crust. Received: 12 April 1995 / Revision received: 20 December 1995  相似文献   

14.
In 1982, a geophysical survey of the Antarctic margin, including multichannel seismic, gravity, magnetic and bathymetric surveying, was carried out off Adélie Coast-Wilkes Land and in the eastern Ross Sea. Of the 5000 km of lines recorded, 3000 km were in the Adélie Coast area. Lines ATC 101–102, approximately following meridian 138°E, show the first complete transect of the Adélie Coast margin from the Southeast Indian abyssal plain at DSDP Site 269 to the continental shelf. These lines reveal a thick sedimentary series divided into three main acoustic units by two major unconformities considered to be Upper Eocene (42 Ma) and late Oligocene (25 Ma). Oceanic or continental basement can be traced under the whole area, and the ocean-continent boundary clearly lies beneath the lower continental slope. A deep, high-amplitude, low-frequency horizon, extending under the oceanic basement for over 300 km of line, is considered to be the Moho. Our interpretation supports a recent revision be Cande and Mutter proposing an early Upper Cretaceous opening between Australia and East Antarctica.  相似文献   

15.
Information concerning two seismic lines, the first located northwest of the Lefkada Island and the second from the deep Ionian basin to the gulf of Patras, is used to trace the Kefalonia Transform Zone (KTZ) and to explore its relation with the sedimentary sequences and the deeper geologic structures in the study area. In addition, sea bottom topography and fault plane solutions are combined in order to explore the prolongation of the KTZ into the Ionian Abyssal Plain (IoAP) and to describe its properties. The boundary between the subduction of the eastern Mediterranean oceanic crust under the overriding continental crust and the KTZ is well constrained by the seismic data in association with seismicity and regional stress field. The southern prolongation of the KTZ is located in the IoAP towards the direction between Kefalonia and Zakynthos Islands at depth greater than 15 km. The southern part of the KTZ exhibits a strike–slip motion with a thrust component according to fault plane solutions of moderate and strong earthquakes. The seismic section mostly confirms the existence of the thrust component and gives information about the tectonic status east and west of the KTZ.  相似文献   

16.
The Southwest Subbasin (SWSB) is an abyssal subbasin in the South China Sea (SCS), with many debates on its neotectonic process and crustal structure. Using two-dimensional seismic tomography in the SWSB, we derived a detailed P-wave velocity model of the basin area and the northern margin. The entire profile is approximately 311-km-long and consists of twelve oceanic bottom seismometers (OBSs). The average thickness of the crust beneath the basin is 5.3 km, and the Moho interface is relatively flat (10–12 km). No high velocity bodies are observed, and only two thin high-velocity structures (~7.3 km/s) in the layer 3 are identified beneath the northern continent-ocean transition (COT) and the extinct spreading center. By analyzing the P-wave velocity model, we believe that the crust of the basin is a typical oceanic crust. Combined with the high resolution multi-channel seismic profile (MCS), we conclude that the profile shows asymmetric structural characteristics in the basin area. The continental margin also shows asymmetric crust between the north and south sides, which may be related to the large scale detachment fault that has developed in the southern margin. The magma supply decreased as the expansion of the SWSB from the east to the west.  相似文献   

17.
Gravity studies over the continental margin of the central west coast of India show a sediment thickness of 2–3 km on the shelf associated with deeper horst and graben structures, of 6 km in the shelf margin basin, and about 1 km in the deep sea. The upward trend in free-air gravity anomaly toward the deep sea region is interpreted as crustal thinning. Model studies indicate a 25-km-thick crust in the shelf region and a minimum of 18 km in the more offshore region. An abrupt magnetic signature change suggests differential basement depths in the shelf region. Major faulting in the region is confirmed in water depths of approximately 100–200 m.  相似文献   

18.
This paper presents actuality of investigation and study of the crustal structure characters of East China Sea at home and abroad. Based on lots of investigation and study achievements and the difference of the crustal velocity structure from west to east, the East China Sea is divided into three parts - East China Sea shelf zone, Okinawa Trough zone and Ryukyu arc-trench zone. The East China Sea shelf zone mostly has three velocity layers, i.e., the sediment blanket layer （the velocity is 5.8-5.9 km/s）, the basement layer （the velocity is 6.0-6.3 km/s）, and the lower crustal layer （the velocity is 6.8-7.6 km/s）. So the East China Sea shelf zone belongs to the typical continental crust. The Okinawa Trough zone is located at the transitional belt between the continental crust and the oceanic crust. It still has the structural characters of the continental crust, and no formation of the oceanic crust, but the crust of the central trough has become to thinning down. The Ryukyu arc-trench zone belongs to the transitional type crust as a whole, but the ocean side of the trench already belongs to the oceanic crust. And the northwest Philippine Basin to the east of the Ryukyu Trench absolutely belongs to the typical oceanic crust.  相似文献   

19.
 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  相似文献   

20.
 Magnetic data over the eastern continental margin of India and adjacent Bengal fan demarcate two major lineaments. A high amplitude N–S-trending lineation of the Cauvery offshore Basin corresponds to the offshore fragment of the 80°E lineament recorded onland. A N–S lineation of very high amplitude anomaly off Chilka lake considered as the possible northward extension of the 85°E ridge delineated, hitherto in Bengal Fan. A subdued magnetic anomaly zone is demarcated seaward of the continent–ocean boundary (COB) in the Bengal Fan. Over the northern Bengal Fan this zone is delineated east of 85°E lineation. This quiet zone might have evolved during the Early Cretaceous period of normal magnetic polarity between M0 and 34 (120–84 Ma) anomalies. Received: 6 April 1995 / Revision received: 3 September 1996  相似文献   

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