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1.
The distribution of seismic units in deposits of the basins near the Antarctic–Scotia plate boundary is described based on the analysis of multichannel seismic reflection profiles. Five main seismic units are identified. The units are bounded by high-amplitude continuous reflectors, named a to d from top to bottom. The two older units are of different age and seismic facies in each basin and were generally deposited during active rifting and seafloor spreading. The three youngest units (3 to 1) exhibit, in contrast, rather similar seismic facies and can be correlated at a regional scale. The deposits are types of contourite drift that resulted from the interplay between the northeastward flow of Weddell Sea Bottom Water (WSBW) and the complex bathymetry in the northern Weddell Sea, and from the influence of the Antarctic Circumpolar Current and the WSBW in the Scotia Sea. A major paleoceanographic event was recorded by Reflector c, during the Middle Miocene, which represents the connection between the Scotia Sea and the Weddell Sea after the opening of Jane Basin. Unit 3 (tentatively dated ∼Middle to Late Miocene) shows the initial incursions of the WSBW into the Scotia Sea, which influenced a northward progradational pattern, in contrast to the underlying deposits. The age attributed to Reflector b is coincident with the end of spreading at the West Scotia Ridge (∼6.4 Ma). Unit 2 (dated ∼Late Miocene to Early Pliocene) includes abundant high-energy, sheeted deposits in the northern Weddell Sea, which may reflect a higher production of WSBW as a result of the advance of the West Antarctic ice-sheet onto the continental shelf. Reflector a represents the last major regional paleoceanographic change. The timing of this event (∼3.5–3.8 Ma) coincides with the end of spreading at the Phoenix–Antarctic Ridge, but may be also correlated with global events such as initiation of the permanent Northern Hemisphere ice-sheet and a major sea level drop. Unit 1 (dated ∼Late Pliocene to Recent) is characterized by abundant chaotic, high-energy sheeted deposits, in addition to a variety of contourites, which suggest intensified deep-water production. Units 1 and 2 show, in addition, a cyclic pattern, more abundant wavy deposits and the development of internal unconformities, all of which attest to alternating periods of increased bottom current energy.  相似文献   

2.
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   

3.
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   

4.
The Adare Trough, located 100 km NE of Cape Adare, Antarctica, is the extinct third arm of a Tertiary spreading ridge that separated East from West Antarctica. We use seismic reflection data, tied to DSDP Site 274, to link our seismic stratigraphic interpretation to changes in ocean-bottom currents, Ross Sea ice cover, and regional tectonics through time. Two extended unconformities are observed in the seismic profiles. We suggest that the earliest hiatus (early Oligocene to Mid-Miocene) is related to low sediment supply from the adjacent Ross Shelf, comprised of small, isolated basins. The later hiatus (mid-Miocene to late Miocene) is likely caused by strong bottom currents sourced from the open-marine Ross Sea due to increased Antarctic glaciation induced by mid-Miocene cooling (from Mi-3). Further global cooling during the Pliocene, causing changes in global ocean circulation patterns, correlates with Adare Basin sediments and indicate the continuing but weakened influence of bottom currents. The contourite/turbidite pattern present in the Adare Trough seismic data is consistent with the 3-phase contourite growth system proposed for the Weddell Sea and Antarctic Peninsula. Multibeam bathymetry and seismic reflection profiles show ubiquitous volcanic cones and intrusions throughout the Adare Basin that we interpret to have formed from the Oligocene to the present. Seismic reflection profiles reveal trans-tensional/strike-slip faults that indicate oblique extension dominated Adare Trough tectonics at 32–15 Ma. Observed volcanism patterns and anomalously shallow basement depth in the Adare Trough area are most likely caused by mantle upwelling, an explanation supported by mantle density reconstructions, which show anomalously hot mantle beneath the Adare Trough area forming in the Late Tertiary.  相似文献   

5.
The South Yellow Sea basin is filled with Mesozoic–Cenozoic continental sediments overlying pre-Palaeozoic and Mesozoic–Palaeozoic marine sediments. Conventional multi-channel seismic data cannot describe the velocity structure of the marine residual basin in detail, leading to the lack of a deeper understanding of the distribution and lithology owing to strong energy shielding on the top interface of marine sediments. In this study, we present seismic tomography data from ocean bottom seismogra...  相似文献   

6.
M. S. Barash 《Oceanology》2011,51(2):306-314
Among the abiotic factors that determined via the paleoceanographic processes development and evolution of the oceanic biota in the Neogene, noteworthy are the tectonic, volcanic, climatic and extraterrestrial events. The most important tectonic events of such kind include the subsidence of the Faroe-Iceland Threshold 14–13 Ma ago, the closure of the Tethys Ocean in the east 19–12 Ma ago, the orogenesis in the western Mediterranean region and closure of the Mediterranean Sea (Messinian Crisis) 5.59–5.33 Ma ago, the formation of the Central American Isthmus 6.0–3.5 Ma ago, and the opening of the Bering Strait that occurred (according to different data) in the period of 7.4 to 3.1 Ma ago. The most significant climatic consequence resulted from the formation of the Circum-Antarctic Current, the irregular growth of the Antarctic ice shield, the cooling in the Arctic region 3.2–3.1 Ma ago, and the development of continental glaciations in the Northern Hemisphere approximately 2.5 Ma ago. The variations in the atmospheric CO2 content are correlative with the climatic fluctuations. The entire Cenozoic climatic record reflects the influence of the orbital parameters of the Earth. The Neogene was marked by several significant extraterrestrial events: the fireball falling in southwestern Germany in the middle Miocene 14.8–14.5 Ma ago probably accompanied by enhanced volcanic activity particularly in the rift valley of eastern Africa; the drastically increased influx of interplanetary dust due to the disruption caused by a large asteroid in the late Miocene 8.3 ± 0.5 Ma ago, the fall of a large (>1 km in diameter) asteroid in the Eltanin Fault zone of the Southern Ocean in the terminal Pliocene 2.15 Ma ago; and the explosion of a supernova star, which was probably responsible for the partial extinction of marine organisms at the Pliocene-Pleistocene transition approximately 2 Ma ago.  相似文献   

7.
On the basis of new geophysical data acquired by the Federal Institute of Geosciences and Natural Resources (BGR) and the Polar Marine Geological Research Expedition (PMGRE) as well as existing data new geophysical maps were compiled for the Lazarev Sea and the Riiser-Larsen Sea between 10°W and 25°E. The new results are: – The drastic change in the strike direction of the volcanic Explora Wedge between longitudes 10°W and 5°W is accompanied with a gradual change from one major wedge, i.e. the Explora Wedge, into at least two wedge-shaped volcanic constructions, each manifested by a sequence of seaward-dipping reflectors in the seismic records. – The southern Lazarev Sea is best described as a continental margin affected by multiple rifting episodes accompanied with transient volcanism. – A distinct N80°E striking basement depression separates the volcanic-prone continental margin of the southern Lazarev Sea from oceanic crust upon which the Maud Rise rests. The southern scarp of the narrow depression was presumably aligned with the eastern scarp of the Mozambique Ridge during the Early Cretaceous. – The Astrid Ridge proper occupies the transition from the volcanic-prone continental margin of the Lazarev Sea to old oceanic crust of the Riiser -Larsen Sea, and it rests upon a large volcanic apron which covers the basement of the southwestern Riiser-Larsen Sea. – No evidence was found that prolific volcanism has affected the early opening of the Riiser-Larsen Sea. – The Lazarev Sea is a sediment-starved region.  相似文献   

8.
Using recently gathered onland structural and 2D/3D offshore seismic data in south and central Palawan (Philippines), this paper presents a new perspective in unraveling the Cenozoic tectonic history of the southeastern margin of the South China Sea. South and central Palawan are dominated by Mesozoic ophiolites (Palawan Ophiolite), distinct from the primarily continental composition of the north. These ophiolites are emplaced over syn-rift Eocene turbidites (Panas Formation) along thrust structures best preserved in the ophiolite–turbidite contact as well as within the ophiolites. Thrusting is sealed by Early Miocene (∼20 Ma) sediments of the Pagasa Formation (Isugod Formation onland), constraining the younger limit of ophiolite emplacement at end Late Oligocene (∼23 Ma). The onset of ophiolite emplacement at end Eocene is constrained by thrust-related metamorphism of the Eocene turbidites, and post-emplacement underthrusting of Late Oligocene – Early Miocene Nido Limestone. This carbonate underthrusting at end Early Miocene (∼16 Ma) is marked by the deformation of a seismic unit corresponding to the earliest members of the Early – Middle Miocene Pagasa Formation. Within this formation, a tectonic wedge was built within Middle Miocene (from ∼16 Ma to ∼12 Ma), forming a thrust-fold belt called the Pagasa Wedge. Wedge deformation is truncated by the regionally-observed Middle Miocene Unconformity (MMU ∼12 Ma). A localized, post-kinematic extension affects thrust-fold structures, the MMU, and Late Miocene to Early Pliocene carbonates (e.g. Tabon Limestone). This structural set-up suggests a continuous convergent regime affecting the southeastern margin of the South China Sea between end Eocene to end Middle Miocene. The ensuing structures including juxtaposed carbonates, turbidites and shallow marine clastics within thrust-fold belts have become ideal environments for hydrocarbon generation and accumulation. Best developed in the Northwest Borneo Trough area, the intensity of thrust-fold deformation decreases towards the northeast into offshore southwest Palawan.  相似文献   

9.
Bathymetric, 9.5-kHz long-range sidescan sonar (OKEAN), seismic reflection and sediment-core data are used in the analysis of two tectonic troughs south of Crete, Eastern Mediterranean Sea. Here, up to 1.2 s two-way travel time (TWTT) of strata have accumulated since the Middle Miocene in association with extension in the South Aegean region. The study area comprises >100-km- long by >25-km-wide basins filled by sediments subdivided into two seismic units: (1) an upper Unit 1 deposited in sub-basins which follow the present-day configuration of the southern Cretan margin; (2) a basal Unit 2, more than 500 ms (TWTT) thick, accumulated in deeper half-graben/grabens distinct from the present-day depocentres. Both units overlap a locally stratified Unit 3 comprising the pre-Neogene core complex of Crete and Gavdos. In this work, the interpreted seismic units are correlated with the onshore stratigraphy, demonstrating that denudation processes occurring on Crete and Gavdos in response to major tectonic events have been responsible for high sedimentation rates along the proximal southern Cretan margin. Consequently, topographically confined sedimentary units have been deposited south of Crete in the last 12 Ma, including turbidites and other mass-flow deposits fed by evolving transverse and axial channel systems. Surface processes controlling facies distribution include the direct inflow of sediment from alluvial-fan systems and incising mountain rivers onto the Cretan slope, where significant sediment instability processes occur at present. In this setting, seismic profiles reveal eight different types of stratigraphic contacts on basin-margin highs, and basinal areas show evidence of halokinesis and/or fluid escape. The acquired data also show that significant changes to the margin’s configuration occurred in association with the post-Alpine tectonic and eustatic episodes affecting the Eastern Mediterranean.  相似文献   

10.
We studied the active deformation zone of the middle strand of the North Anatolian Fault Zone through the southern part of the Sea of Marmara by means of high-resolution as well as deep seismic reflection data. Our main objective was to investigate the active deformation within the uppermost sedimentary layers at high resolution as well as deeper sedimentary layers, focusing on the tectonic and stratigraphic setting between Gemlik and Bandırma. The middle strand of the North Anatolian Fault reaching the Gulf of Gemlik is a main fault which has a lazy-S shape in the Gulf of Gemlik, and extends westwards to Bandırma as a main fault which is an E–W-trending single right-lateral fault controlling the zone along the Gemlik and Bandırma sub-basins. Small-scale faults, consistent with a dextral shear regime, are present in the vicinity of the main fault. Several oblique fault groups parallel to the main fault were detected. The deformation in the Gulf of Gemlik is characterized by a series of synthetic and antithetic faults emanating from the main fault. The boundary faults in the Gulf of Gemlik have a compressive component, which indicates the sill areas of the gulfs of Gemlik and Bandırma to be push-up structures. Four seismic stratigraphic units were identified in the sediments of the gulfs of Gemlik and Bandırma, providing evidence of tectonic influence. The present tectonic structure between Gemlik and Bandırma is not a pull-apart structure. The microseismic study in this area has shown that fault planes are either strike-slip or compressional, and that the stress tensor is compatible with pure strike-slip in the E–W fault system.  相似文献   

11.
The structural evolution of the Messinian evaporites in the Levantine Basin   总被引：2，自引：0，他引：2  
The Levantine Basin in the South-eastern Mediterranean Sea is a world class site for studying the initial stages of salt tectonics driven by differential sediment load, because the Messinian evaporites are comparatively young, the sediment load varies along the basin margin, they are hardly tectonically overprinted, and the geometry of the basin and the overburden is well-defined. In this study we analyse depositional phases of the evaporites and their structural evolution by means of high-resolution multi-channel seismic data. The basinal evaporites have a maximum thickness of about 2 km, precipitated during the Messinian Salinity Crisis, 5.3–5.9 Ma ago. The evaporite body is characterized by 5 transparent layers sequenced by four internal reflections. We suggest that each of the internal reflection bands indicate a change of evaporite facies, possibly interbedded clastic sediments, which were deposited during temporal sea level rises. All of these internal reflections are differently folded and distorted, proving that the deformation was syn-depositional. Thrust angles up to 14° are observed. Backstripping of the Pliocene–Quaternary reveals that salt tectonic is mainly driven by the sediment load of the Nile Cone. The direction of lateral salt displacement is mainly SSW–NNE and parallel to the bathymetric trend. Apparent rollback anticlines off Israel result rather from differential subsidence than from lateral salt displacement. In the south-eastern basin margin the deposition of the Isreali Slump Complex (ISC) is coeval with the onset of salt tectonic faulting, suggesting a causal link between slumping processes and salt tectonics.

The superposition of ‘thin-skinned’ tectonics and ‘thick-skinned’ tectonics becomes apparent in several locations: The fold belt off the Israeli Mediterranean slope mainly results from active strike-slip tectonics, which becomes evident in faults which reach from the seafloor well below the base of the evaporites. Owing to the wrenching of the crustal segments which are bounded by deep-rooted fault lines like the Damietta–Latakia, Pelusium and Shelf Edge Hinge line the setting is transpressional south of 32°N, where the fault lines bend further towards the west. This adds a component of ‘thick-skinned’ transpression to the generally ‘thin-skinned’ compressional regime in the basin. Above 1.5 km of evaporites, a mud volcano is observed with the mud source seemingly within the evaporite layer. At the eastern Cyprus Arc, the convergence zone of the African and the Anatolian plates, deep-rooted compression heavily deformed the base of the evaporites, whereas at the Eratosthenes Seamount mainly superficial compression affecting the Post-Messinian sediments and the top of the evaporites is observed.  相似文献   


12.
The northeastern high-latitude North Atlantic is characterised by the Bellsund and Isfjorden fans on the continental slope off west Svalbard, the asymmetrical ultraslow Knipovich spreading ridge and a 1,000 m deep rift valley. Recently collected multichannel seismic profiles and bathymetric records now provide a more complete picture of sedimentary processes and depositional environments within this region. Both downslope and alongslope sedimentary processes are identified in the study area. Turbidity currents and deposition of glacigenic debris flows are the dominating downslope processes, whereas mass failures, which are a common process on glaciated margins, appear to have been less significant. The slide debrite observed on the Bellsund Fan is most likely related to a 2.5–1.7 Ma old failure on the northwestern Barents Sea margin. The seismic records further reveal that alongslope current processes played a major role in shaping the sediment packages in the study area. Within the Knipovich rift valley and at the western rift flank accumulations as thick as 950–1,000 m are deposited. We note that oceanic basement is locally exposed within the rift valley, and that seismostratigraphic relationships indicate that fault activity along the eastern rift flank lasted until at least as recently as 1.5 Ma. A purely hemipelagic origin of the sediments in the rift valley and on the western rift flank is unlikely. We suggest that these sediments, partly, have been sourced from the western Svalbard—northwestern Barents Sea margin and into the Knipovich Ridge rift valley before continuous spreading and tectonic activity caused the sediments to be transported out of the valley and westward.  相似文献   

13.
During late Pliocene to Pleistocene times, prominent prograding wedges were deposited along the continental margin of NW Europe, resulting in seaward shelf break migration of up to 150 km. Much of the sediment accumulation occurred marginal to the former mid- to high-latitude ice sheets. The geographical distribution, and stratigraphical and chronological data may suggest that the instigation of the wedges was variously related to tectonic uplift as well as a response to the late Pliocene to Pleistocene climate deterioration and onset of major northern hemisphere glaciations. The onset of wedge growth on the NW UK and Irish margins was initiated at about 4 Ma in response to tectonic tilting of the margin in that region. However, glacially derived sediments here comprise a significant proportion of the wedges, especially since 0.44 Ma. For the Faroe margin, no detailed chronology is available; however, it may be inferred that onset of glacigenic wedge growth here did not post-date that observed on the NW UK and Irish margins. Offshore Norway, wedge growth has largely occurred since ca. 2.7 Ma in response to northern hemisphere glaciations, also recording a major change in sediments transport routes at 0.8–1.1 Ma (reflecting larger Fennoscandian Ice Sheets). Presently, it is uncertain whether the glacigenic wedge growth was preceded by a fluvial phase (in response to uplift) in this area. In the western Barents Sea, an early phase of wedge growth was (glacio) fluvial in character. Off western Spitsbergen, the development was similar to that of the Barents Sea although the glacigenic wedge-growth phase may have started somewhat earlier.The wedges commonly display gently inclined seaward prograding clinoforms, and transparent to chaotic internal acoustic facies. Sampling of their sediments reveals that they are mainly composed of glacigenic diamicton interbedded with marine and glaciomarine sediments that, to various extents, have been affected by bottom-current action. The clinoforms of these wedges vary in geometry from oblique to sigmoidal, and they also show varying degrees of aggradation throughout their development. The resulting stratal stacking pattern can be attributed to a combination of variations in sediment supply, sedimentary processes, and accommodation space, the latter being a function of tectonic movements and/or loading induced subsidence as well as eustatic sea-level fluctuations.  相似文献   

14.
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   

15.
In the austral summer of 2007, 20.5 km of high-resolution over-sea-ice seismic reflection data were collected in the Granite Harbor region of southern McMurdo Sound over the Mackay Sea Valley. The goal of the survey was to image thin pelagic sediment deposited in the Mackay Sea Valley after the Last Glacial Maximum. A generator–injector air gun was lowered beneath the sea ice through holes drilled by an auger drill system. The recording system was a 60 channel snow streamer with vertically oriented gimbaled geophones spaced 25 m apart. Unique problems in the over-sea-ice seismic reflection survey—noise from the ice column flexing and timing delays caused by trapped air at previous shot points—were overcome to improve the quality of the seismic data. The Mackay Sea Valley survey produced seismic data with a vertical resolution of 6.3 m. The processed seismic data show pelagic sediment thickness of up to 50 m within the Mackay Sea Valley with some locations showing possible older sediments beneath the pelagic sediment layer.  相似文献   

16.
Subsidence analysis (backstripping) was carried out on a series of wells from the Gulf of Suez and northern Red Sea region of Egypt in order to examine the interplay between tectonic events, basin subsidence, sedimentation and sea level changes in a young, developing ocean basin and continental margin. Using constraints on chronostratigraphy and paleodepth from various sources combined with stratigraphic and structural information from industry wells and other geophysical sources it has been possible to compile the data necessary to perform geohistory analyses throughout the region.Major subsidence due to crustal thinning began ∼25 Ma with sedimentation initially occurring in isolated sub-basins. These earliest sediments record the transition from continental to marine depositional environments. Subsequently during early and middle Miocene times subsidence was rapid and uniform along and across the entire rift basin. Open marine sedimentation occurred across all structural regimes. The mid-Clysmic tectonic event (16.5 Ma) resulted in structural rearrangement of the rift basin and uplift of the rift shoulders. Rapid subsidence continued as global sea level fell, producing a series of prograding, siliciclastic fan-deltas at the rift margins. At ∼15.5 Ma, opening of the Suez rift was terminated, tectonic subsidence decreased dramatically in the southern rift and ceased entirely in the northern rift. Tensional plate motion probably was transferred from the Gulf of Suez to sinistral strike-slip movement on the Dead Sea transform at this time. The quiescence in subsidence combined with a lowered global sea level resulted in the deposition of a thick (up to 4 km) series of evaporites within the central trough of the rift from the middle to latest Miocene. The accumulation of such a thick sequence of sediments during a phase of decreased tectonic subsidence is interpreted as a ‘filling-in’ of the rift topography which developed during the earlier period of rapid subsidence and rift-shoulder uplift and continued compaction.A rapid global sea level rise concomitant with a subsequent pulse of increased tectonic activity in the latest Miocene—earliest Pliocene returned the rift to dominantly marine conditions.  相似文献   

17.
The first high resolution multichannel seismic data from the Mendeleev and Alpha Ridges in the Arctic Ocean have been used to investigate the depositional history, and compare acoustic stratigraphies of the three main sub-marine ridges (Mendeleev, Alpha and Lomonosov) in the polar ocean. Acoustic basement on the Mendeleev Ridge is covered by a ~0.6–0.8 s thick sediment drape over highs and up to 1.8 s within grabens. A pronounced angular discordance at 0.18–0.23 s below the seafloor along the middle to upper slopes divides the succession into an upper, undisturbed, uniformly thick, hemipelagic drape (Unit M1) and a partially truncated lower unit (Unit M2) characterized by strong reflection bands. Unit M2 is thicker in intra-ridge grabens and includes three sub-units with abundant debris flows in the uppermost subunit (M2a). The discordance between Units M1 and M2 most likely relates to instability along the middle to upper slopes and mass wasting, triggered by tectonic activity. The scars were further smoothed by bottom current erosion. We observe comparable acoustic stratigraphy and discordant relationships on the investigated northwestern part of Alpha Ridge. Similarly, on the central Lomonosov Ridge, Paleocene and younger sediments sampled by scientific drilling include an uppermost ~0.2 s thick drape overlying, highly reflective deposits with an angular unconformity confined to the upper slope on both sides of the ridge. Sediment instability on the three main ridges was most likely generated by a brief phase of tectonic activity (~14.5–22 Ma), coinciding with enhanced bottom circulation. These events are coeval with the initial opening of the Fram Strait. The age of the oldest sediments above acoustic basement on the Mendeleev- and west-central Alpha Ridges is estimated to be 70–75 Ma.  相似文献   

18.
In north-eastern Siberia the active mid-ocean Gakkel Ridge interacts with the continental shelf of the Laptev Sea. Extension has affected the shelf since at least the Early Tertiary and has resulted in the formation of a complex horst and graben system. We present new seismic data from the Laptev Sea including deep seismic soundings.The most prominent rift basin is the Ust' Lena Rift with a minimum E–W width of 300 km at latitude 75°N and a Cenozoic infill up to 13 km in thickness. The asymmetric shape of the basin and conclusive evidence for a detachment imply a simple-shear geometry. The suggested rift model combines a ramp and flat geometry for the detachment with ductile stretching beneath the detachment. A major west-dipping, hingeline, listric fault separates the Ust' Lena Rift from the Laptev Horst.The 100–150 km wide Laptev Horst is subdivided into three units by narrow rift grabens. Another prominent rift graben is the Anisin Basin, which is located in the northern shelf area.Though the Laptev Sea Rift formed in interaction with an active mid-oceanic ridge, there are indications that the Laptev Sea rift is of the ‘passive rift’ type. The rift was developed east of a SW–NE trending transfer zone which links the Gakkel Ridge to the Laptev Sea Rift.  相似文献   

19.
The Jane Arc and Basin system is located at the eastern offshore prolongation of the Antarctic Peninsula, along the southern margin of the South Orkney Microcontinent. Three magnetic anomaly profiles orthogonal to the main tectonic and bathymetric trends were recorded during the SCAN97 cruise by the Spanish R/V Hespérides. In our profiles, chron C6n (19.5 Ma) was identified as the youngest oceanic crust of the Northern Weddell Sea, whose northern spreading branch was totally subducted. The profiles from the Jane Basin allow us to date, for the first time, the age of the oceanic crust using linear sea floor magnetic anomalies. The spreading in the Jane Basin began around the age of the oldest magnetic anomaly at 17.6 Ma (chron C5Dn), and ended about 14.4 Ma (chron C5ADn). The distribution of the magnetic anomalies indicate that the mechanism responsible for the development of Jane Basin was the subduction of the Weddell Sea spreading centre below the SE margin of the South Orkney Microcontinent, suggesting a novel mechanism for an extreme case of backarc development.  相似文献   

20.
Sedimentary processes and structures across the continental rise in the western Weddell Sea have been investigated using sediment acoustic and multichannel seismic data, integrated with multibeam depth sounding and core investigations. The results show that a network of channels with associated along-channel ridges covers the upper continental slope. The seismic profiles reveal that the channels initially developed as erosive turbidite channels with associated levees on their northern side due to Coriolis force. Later they were partly or fully infilled, probably as a result of decreasing turbidite activity. Now the larger ones exist as erosive turbidite channels of reduced size, whereas the smaller ones are non-erosive channels, their shape being maintained by contour current activity. Drift bodies only developed where slumps caused a distinctive break in slope inclination on the upper continental rise, which served to initiate the growth of a drift body fed by contour currents or by the combined action of turbidites and contourites. The history of sedimentation can be reconstructed tentatively by correlation of seismo-stratigraphic units with the stages of evolution of the drifts on the western side of the Antarctic Peninsula. Three stages can be distinguished in the western Weddell Sea after a pre-drift stage, which is delimited by an erosional unconformity at the top: (1) a growth stage, dominated by turbidites, with occasional occurrence of slumps during its initial phase; (2) during a maintenance stage turbiditiy-current intensity (and presumably sedimentation rate also) decreased, probably as a result of the ice masses retreating from the shelf edge, and sedimentation became increasingly dominated by contour current activity; and (3) a phase of sheeted-sequence formation. A southward decrease in sediment thickness shows that the Larsen Ice Shelf plays an important role in sediment delivery to the western Weddell Sea. This study shows that the western Weddell Sea has some characteristics in common with the southern as well as the northwestern Weddell Sea: contour currents off the Larsen Ice Shelf have been present for a long time, probably since the late Miocene, but during times of high sediment input from the shelves as a result of advancing ice masses a channel-levee system developed and dominated over the contour-current transport of sediment. At times of relatively low sediment input the contour-current transport dominated, leading to the formation of drift deposits on the upper continental rise. Seaward of areas without shelf ice masses the continental rise mainly shows a rough topography with small channels and underdeveloped levees. The results demonstrate that sediment supply is an important, maybe the controlling factor of drift development on the Antarctic continental rise.  相似文献   

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