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1.
 Continent–ocean fracture zones are the fossil transform offsets located along passive rifted continental margins. Kinematic models identify at least two principal stages in their evolution. During the first stage as rifting proceeds, continent–continent shearing dominates a narrow region in which the transform fault will eventually rupture. High-standing continental marginal ridges 50–100 km wide and bounding deep sedimentary basins, are derived in such settings. In stage two as sea-floor spreading proceeds, the younger oceanic block slides along the active transform, heating the older continental block, and possibly induces thermal uplift and accompanying denudation. Magnetic injection into the continental block at depth may also induce an isostatic uplift. After ridge–transform intersection time, mechanical coupling between the continental and oceanic blocks may influence the stratigraphy and structure of these margins. Received: 12 March 1996 / Revision received: 23 April 1996  相似文献   

2.
 The southwest Newfoundland transform margin has been studied by deep seismic reflection and refraction. Lower crustal reflectivity strengthens towards the margin, where there is a shear zone of thinned continental crust overplated with oceanic material. The reflectivity may be due to shear fabrics in the crust. Crustal thinning probably took place by flow in the lower crust. The Ungava transform margin has been less studied but has been explored and drilled. It appears more volcanic in character. The north Baffin region has undergone a complex tectonic history and provides an example of the transition from continent–ocean to continent–continent transform motion. Received: 9 March 1995 / Revision received: 25 July 1995  相似文献   

3.
 Bathymetry, satellite-derived gravity, and interpreted seismic reflection data across the northern Falkland/Malvinas Plateau fossil continent–ocean transform rim may record the degree of mechanical coupling across the boundary after ridge–transform intersection time. The rim comprises a broad microcontinental block in the east and a continental marginal fracture ridge 50–100 km wide elsewhere. Free-air gravity anomalies tentatively suggest that the fracture ridge is locked against oceanic elastic lithosphere both to the north (Argentine Basin) and south (Central Falkland Basin). Received: 18 January 1996 / Revision received: 25 March 1995  相似文献   

4.
 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   

5.
 Debris lobes with characteristic lengths, widths, and thickness of 30–200 km, 2–10 km, and 10–50 m, respectively, represent the main building blocks of deep-sea fans along the Norwegian–Barents Sea continental margin. Their formation is closely related to the input of clay-rich sediments to the upper continental slope by glaciers during periods of maximum ice advance. It is likely that slide release was a consequence of an instability arising from high sedimentation rates on the upper continental slope. The flow behavior of the debris lobes can be described by a Bingham flow model. Received: 17 November 1995 / Revision received: 24 June 1996  相似文献   

6.
 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   

7.
 Numerous small (50- to 300-m-diameter) strong-backscatter objects were imaged on the 1200- to 1350-m deep crest of Vestnesa Ridge (Fram Strait) and along the 900- to 1000-m deep northeast margin of the Storegga slide valley. Ground-truthing identified most of these objects as 2- to 10-m-deep pockmarks, developed within soft, acoustically stratified silty clays (typical wet bulk density: 1400–1600 kg m-3; sound speed: 1480– 1505 m s-1; porosity, 65–75%; shear strength: 5–10 kPa; water content: 80–120%; and thermal conductivity: 0.8–0.9 W m-1 deg C-1 in the top 3 m). Gas wipeouts, enhanced reflectors, and reflector discontinuities indicate recent or ongoing activity, but the absence of local heat flow anomalies suggests that any upward fluid flows are modest and/or local.  相似文献   

8.
 Estimates of uplift at ocean–continent transform margins vary significantly between models incorporating both thermal conduction and viscous coupling across the transform and models taking only one of these processes into account. More heat is predicted to be conducted and advected into the continental lithosphere in combined models than in conductive models. Nevertheless, tectonic uplift predicted by the combined models is only half that predicted conductive models. This is because viscous coupling implies crustal thinning close to the transform. The form and amplitude of uplift in the combined models agree well with erosion estimates along a seismic transect across the Senja shear margin, in the southwestern Barents Sea. Received: 31 January 1995 / Revision received: 20 July 1995  相似文献   

9.
 Recent sediment accumulation rates are 18–230 mg cm-2 yr-1 (0.02–0.2 cm yr-1) based on excess 210Pb activity profiles in the southwestern part of the East Sea (Sea of Japan). Assuming no mixing beneath surface mixed layers, 210Pb-derived sediment accumulation rates are 18–32 mg cm-2 yr-1 in the northern part of the Yamato Ridge and the Ulleung Basin, 29–136 mg cm-2 yr-1 in the Korea Plateau, and 230 mg cm-2 yr-1 in the southern shelf. These values generally agree with long-term sedimentation rates estimated from dated ash layers. Received: 6 October 1995 / Revision received: 31 May 1996  相似文献   

10.
 The stratal architecture of the Gulf of Cádiz continental margin (SW Spain) has been analyzed by using single-channel, very high-resolution seismic reflection profiles. An evolutionary scheme of asymmetrical depositional sequences is proposed that was governed by the Late Pleistocene–Holocene sea-level fluctuations. Stratigraphic analysis defined 14 seismic units, that are configured into two major type-1 depositional sequences related to 4th-order eustatic sea level changes (100–110 ka). Within these sequences, minor asymmetrical depositional sequences have been recognized related to 5th-order eustatic cycles (22–23 ka) superimposed and modulated by the regressive trends of 4th-order cycles. In 5th-order depositional sequences, the forced regressive and lowstand deposits are volumetrically dominant. They cause the main progradation of the margin in such a way that they form the margin structure almost entirely. Received: 6 April 1995 / Revision received: 8 March 1996  相似文献   

11.
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   

12.
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.  相似文献   

13.
 Results of a detailed geophysical transect across the transform continental margin off Ghana, at the eastern end of the Romanche Fracture Zone in the Equatorial Atlantic, are presented. Seismic refraction, single-channel seismic reflection, gravity, and magnetic data were collected, and seismic, gravity, and magnetic models along the transect are shown. The 6- to 11-km-wide ocean–continent transition (OCT) is characterized by a high-velocity, high-density, high-magnetization crustal zone. The models show no evidence for any underplating of the continental crust adjacent to the margin but minor melting and intrusion of the continental crust may have occurred in the vicinity of the OCT. Received: 6 February 1995/Revision received: 24 July 1995  相似文献   

14.
 The Queen Charlotte fault zone along the western margin of Canada, a right-lateral transform, is part of the boundary between the Pacific and North America plates. Combining reflection and refraction surveys and flexural modeling, we place limits on the amount of underthrusting of the Pacific plate beneath the North America plate. Results from our two-dimensional elastic modeling suggest 10–15 km of underthrusting along the northern Queen Charlotte Islands, in agreement with the amount of underthrusting inferred from plate motion models. Received: 12 June 1995/Revision received: 9 December 1995  相似文献   

15.
Down-core variations in illite, chlorite, smectite and kaolinite (the major clays) in two 14C-dated cores collected along the SW continental margin of India show that illite and chlorite have enhanced abundance during 20–17, 12.5, 11–9.5, and 5–4.8 ka b.p., whereas smectite accumulation is higher between 17 and 12.5, and after 9 ka b.p. The climate may have been predominantly arid at 17 (20–17), 12.5, 10.5 (11–9.5), and 4.8 ka b.p. The first three dates correspond to the last glacial maximum, Bolling-Allerod, and Younger Dryas events, respectively. The SW monsoon was variable between 17 and 15 ka b.p., and it was more stable and intense after the Younger Dryas until about 6 ka b.p. Received: 2 December 1999 / Revision accepted: 11 April 2000  相似文献   

16.
 Near-surface sediment geoacoustic and physical properties were measured from a variety of unconsolidated carbonate sediments in the Lower Florida Keys. Surficial values of compressional and shear speed correlate with sediment physical properties and near-surface acoustic reflectivity. Highest speeds (shear 125–150 m s-1; compressional 1670–1725 m s-1) are from sandy sediments near Rebecca Shoal and lowest speeds (shear 40–65 m s-1; compressional 1520–1570 m s-1) are found in soft, silty sediments which collect in sediment ponds in the Southeast Channel of the Dry Tortugas. High compressional wave attenuation is attributed to scattering of acoustic waves from heterogeneity caused by accumulation of abundant shell material and other impedance discontinuities rather than high intrinsic attenuation. Compared to siliciclastic sediments, carbonate sediment shear wave speed is high for comparable values of sediment physical properties. Sediment fabric, rather than changes due to the effects of biogeochemical processes, is responsible for these differences.  相似文献   

17.
 Much of the modern upper (proximal) Monterey fan is a channel–levee complex, the Upper Turbidite Sequence (UTS), that was deeply eroded after the channel breached a volcanic ridge to reach a deeper base level. Ages of sediment samples collected with the ALVIN submersible from the deepest outcrop within the channel–levee system, 390 m below the adjacent western levee crest, indicate that the UTS deposits accumulated at ≥1 m ka-1 during the last 500 ka. Neogene and Early Pleistocene sediment accumulation on the fan prior to the UTS was much slower (<0.03 m ka-1), and underlying turbidite systems(?) had substantially different morphologic expression(s). Received: 10 February 1998 / Revision received: 6 July 1998  相似文献   

18.
 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   

19.
Composition and accumulation rates of organic carbon in Holocene sediments provided data to calculate an organic carbon budget for the Laptev Sea continental margin. Mean Holocene accumulation rates in the inner Laptev Sea vary between 0.14 and 2.7 g C cm−2 ky−1; maximum values occur close to the Lena River delta. Seawards, the mean accumulation rates decrease from 0.43 to 0.02 g C cm−2 ky−1. The organic matter is predominantly of terrigenous origin. About 0.9 × 106 t year−1 of organic carbon are buried in the Laptev Sea, and 0.25 × 106 t year−1 on the continental slope. Between about 8.5 and 9 ka, major changes in supply of terrigenous and marine organic carbon occur, related to changes in coastal erosion, Siberian river discharge, and/or Atlantic water inflow along the Eurasian continental margin. Received: 26 October 1998 / Revision accepted: 15 June 1999  相似文献   

20.
Magnetic zoning and seismic structure of the South China Sea ocean basin   总被引：2，自引：0，他引：2  
We made a systematic investigation on major structures and tectonic units in the South China Sea basin based on a large magnetic and seismic data set. For enhanced magnetic data interpretation, we carried out various data reduction procedures, including upward continuation, reduction to the pole, 3D analytic signal and power spectrum analyses, and magnetic depth estimation. Magnetic data suggest that the South China Sea basin can be divided into five magnetic zones, each with a unique magnetic pattern. Zone A corresponds roughly to the area between Taiwan Island and a relict transform fault, zone B is roughly a circular feature between the relict transform fault and the northwest sub-basin, and zones C, D, and E are the northwest sub-basin, the east sub-basin, and the southwest sub-basin, respectively. This complexity in basement magnetization suggests that the South China Sea evolved from multiple stages of opening under different tectonic settings. Magnetic reduction also fosters improved interpretation on continental margin structures, such as Mesozoic and Cenozoic sedimentary basins and the offshore south China magnetic anomaly. We also present, for the first time, interpretations of three new 2D reflection seismic traverses, which are of ~2,000 km in total length and across all five magnetic zones. Integration of magnetic and seismic data enables us to gain a better 3D mapping on the basin structures. It is shown that the transition from the southwest sub-basin to the east sub-basin is characterized by a major ridge formed probably along a pre-existing fracture zone, and by a group of primarily west-dipping faults forming an exact magnetic boundary between zones D and E. The northwest sub-basin has the deepest basement among the three main sub-basins (i.e., the northwest sub-basin, the southwest sub-basin, and the east sub-basin). Our seismic data also reveal a strongly faulted continent–ocean transition zone of about 100 km wide, which may become wider and dominated with magmatism or transit to an oceanic crust further to the northeast.  相似文献   

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