首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 437 毫秒
1.
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   

2.
Eleven seismic reflection profiles across Shirshov Ridge and the adjacent deep-water sedimentary basins (Komandorsky and Aleutian Basins) are presented to illustrate the sediment distribution in the western Bering Sea. A prominent seismic reflecting horizon, Reflector P (Middle—Late Miocene in age), is observed throughout both the Aleutian and Komandorsky Basins at an approximate subbottom depth of 1 km. This reflector is also present, in places, on the flanks and along the crest of Shirshov Ridge. The thickness of sediments beneath Reflector P is significantly different within the two abyssal basins. In the Aleutian Basin, the total subbottom depth to acoustic basement (basalt?) is about 4 km, while in the Komandorsky Basin the depth is about 2 km.Shirshov Ridge, a Cenozoic volcanic feature that separates the Aleutian and Komandorsky Basins, is an asymmetric bathymetric ridge characterized by thick sediments along its eastern flank and steep scarps on its western side. The southern portion of the ridge has more structural relief that includes several deep, sediment-filled basins along its summit.Velocity data from sonobuoy measurements indicate that acoustic basement in the Komandorsky Basin has an average compressional wave velocity of 5.90 km/sec. This value is considerably larger than the velocities measured for acoustic basement in the northwestern Aleutian Basin (about 5.00 km/sec) and in the central Aleutian Basin (5.40–5.57 km/sec). In the northwestern Aleutian Basin, the low-velocity acoustic basement may be volcaniclastic sediments or other indurated sediments that are overlying true basaltic basement. A refracting horizon with similar velocities (4.6–5.0 km/sec) as acoustic basement dips steeply beneath the Siberian continental margin, reaching a maximum subbottom depth of about 8 km. The thick welt of sediment at the base of the Siberian margin may be the result of sediment loading or tectonic depression prior to Late Cenozoic time.  相似文献   

3.
New seismic data off East Greenland were acquired in the summer of 2002, between 77°N and 81°N, north of the Greenland Fracture zone. The data were combined with results from the Greenland Basin and ODP site 909, and indicate a pronounced middle Miocene unconformity within the deep sea basins between 72°N and 81°N. Seismic unit NA-1 consists of sediments older than middle Miocene age and unit NA-2 contains sediments younger than the middle Miocene. Classification of a thinly bedded succession in the Molloy Basin resulted in a subdivision into four units (unit I, unit II, unit IIIA and unit IIIB). A comparison of volume estimations and sediment thickness maps between 72°N and 81°N indicates differences in sediment accumulation in the Greenland, Boreas and Molloy basins. Important controls on the variation of accumulation included different opening times of the basins, as well as tectonic conditions and varying sources of sediment transport.Due to prominent basement structures and the varying reflection character of the sediments along the entire East Greenland margin, we defined an age model of shelf sediments on the basis of similar sediment deposit geometry and known results from other regions. The seismic sequences on the shelf up to an age of middle Miocene are divided into three sub-units along the East Greenland margin: middle Miocene–middle late Miocene (SU-3), middle late Miocene–Pleistocene (SU-2), Pleistocene (SU-1). The differences in the geometry of the sequences show more ice stream related sedimentation between 72°N and 77°N and more ice sheet related sedimentation north of 78°N. The region south of 68°N is dominated by more aggradational sedimentary strata so that a glacio-fluvial drainage seems the main transport mechanism. Due to the Greenland Inland–ice borderlines, we assume the glaciers between the Scoresby Sund and 68°N did not reach the shelf break. A first comparison of the sediment structure of the Northeast Greenland margin with the Southeast Greenland margin made it possible to demonstrate significant differences in sedimentation along this margin.  相似文献   

4.
The LOMROG 2007 expedition targeted the previously unexplored southern part of the Lomonosov Ridge north of Greenland together with a section from the Morris Jesup Rise to Gakkel Ridge. The oceanographic data show that Canadian Basin Deep Water (CBDW) passes the Lomonosov Ridge in the area of the Intra Basin close to the North Pole and then continues along the ridge towards Greenland and further along its northernmost continental slope. The CBDW is clearly evident as a salinity maximum and oxygen minimum at a depth of about 2000 m. The cross-slope sections at the Amundsen Basin side of the Lomonosov Ridge and further south at the Morris Jesup Rise show a sharp frontal structure higher up in the water column between Makarov Basin water and Amundsen Basin water. The frontal structure continues upward into the Atlantic Water up to a depth of about 300 m. The observed water mass division at levels well above the ridge crest indicates a strong topographic steering of the flow and that different water masses tend to pass the ridge guided by ridge-crossing isobaths at local topographic heights and depressions. A rough scaling analysis shows that the extremely steep and sharply turning bathymetry of the Morris Jesup Rise may force the boundary current to separate and generate deep eddies.  相似文献   

5.
Analysis of the multi-channel seismic reflection, magnetic and bathymetric data collected along a transect, 1110 km long parallel to 13° N latitude across the Bay of Bengal was made. The transect is from the continental shelf off Madras to the continental slope off Andaman Island in water depths of 525 m to 3350 m and across the Western Basin (bounded by foot of the continental slope of Madras and 85° E Ridge), the 85° E Ridge, the Central Basin (between the 85° E Ridge and the Ninetyeast Ridge), the Ninetyeast Ridge and the Sunda Arc. The study revealed eight seismic sequences, H1 to H8 of parallel continuous to discontinuous reflectors. Considering especially depth to the horizons, nature of reflection and on comparison with the published seismic reflection results of Currayet al. (1982), the early Eocene (P) and Miocene (M) unconformities and the base of the Quaternary sediments (Q) are identified on the seismic section. Marked changes in velocities also occur at their boundaries.In the Western Basin the acoustic basement deepening landward is inferred as a crystalline basement overlain by about 6.7 km of sediment. In the Central Basin possibly thicker sediments than in the Western Basin are estimated. The sediments in the Sunda Arc area are relatively thick and appears to have no distinct horizons. But the entire sedimentary section appears to be consisting of folded and possibly faulted layers.The comparatively broader wavelength magnetic anomalies of the Central Basin also indicate deeper depth of their origin. Very prominent double humped feature of the 85° E Ridge and broad basement swell of the Ninetyeast Ridge are buried under about 2.8 km thick sediments except over the prominent basement high near 92° E longitude. The positive structural relief of the buried 85° E Ridge in the area is reflected in magnetic signature of about 450 nT amplitude. Flexural bulge of the 85° E Ridge and subsidence of the Ninetyeast Ridge about 24 cm my–1 rate since early Eocene period have been inferred from the seismic sequence analysis.  相似文献   

6.
Approximately 147000 km of low-level (450 m) aeromagnetic tracks were flown over the Arctic Ocean and adjacent Greenland and Norwegian Seas, for the greater part with a digitally recording nuclear precession magnetometer designed and built by Wold (1964). The digital recording feature of the system facilitated numerous data processing and analytical techniques which are described herein. These include: noise filtering coordinate conversion, removal of the regional field, second derivatives, downward continuations, polynomial fits of varying degrees to profiles and surfaces, numerical approximations, and depth to source calculations. Using these data and interpretative techniques some inferences could be made about the geologic structure and evolution of the Arctic Ocean Basin. Salient amongst these are: both gravity and magnetic data suggest that there is a 2 1/2 km basement uplift in the eastern Chukchi Shelf associated with the Tigara structure which truncates the western end of Lisburne Peninsula. A 30–40 km wide basement root encircles the Chukchi Rise and extends over 30 km into the mantle. Within the Canda Basin there is a thickening of sediments from the Asian continental margin toward the Canadian Arctic Archipelago. Sediment thickness in the Makarov Basin is 1–1 1/2 km. There appears to be only about a 1/2 km sediment cover in the Fram and Nautilus Basins. The absence of large amplitude magnetic anomalies over these basins is attributed to a 10 km elevation of the Curie isotherm. The Alpha and Nansen ridges produce magnetic profiles that show axial symmetry and correlate with profiles in the North Atlantic. A quantitative attempt has been made to verify these correlations, which infer that the Alpha Cordillera became inactive 40 mybp when the locus of rifting shifted to the Nansen Cordillera. The absence of significant magnetic anomalies over the Lomonosov Ridge reinforces the hypothesis that it is a section of the former Eurasian continental margin that was translated into the Arctic Basin by sea-floor spreading along the Nansen Cordillera axis.  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.
The western Svalbard continental margin contains thick sediment sequences with areas known to contain gas hydrates. Together with a dynamic tectonic environment, this makes the region prone to submarine slides. This paper presents results from geophysical mapping of the deepest part of the high Arctic environment, the Molloy Hole. The mapping includes multibeam bathymetry, acoustic backscatter and sub-bottom profiling. The geophysical data reveal seabed features indicative of sediment transport and larger-scale mass wasting. The large slide scar is here referred to as the Molloy Slide. It is located adjacent to the prominent Molloy Hole and Ridge system. The slide is estimated to have transported >65 km3 of sediments over the deep axial valley of the Molloy Ridge, and further into the Molloy Hole. A unique feature of this slide is that, although its run-out distance is relatively short (<5 km), it extends over an enormous vertical depth (>2,000 m) as a result of its position in a complex bathymetric setting. The slide was most likely triggered by seismic activity caused by seafloor spreading processes along the adjacent Molloy Ridge. However, gas-hydrate destabilization may also have played a role in the ensuing slide event.  相似文献   

9.
Analyses of about 6000 km of processed magnetic data in the central Bay of Bengal using Analytical Signal Processing and Werner Deconvolution techniques revealed that the depth to top of the magnetic basement varies between 5 and 12 km from the sea surface, where the water column thickness is about 3.4 km. These inferred depths are comparable to the reported acoustic basement depths. The basement map derived from magnetic interpretation defines the general configuration of the central Bay of Bengal. The N10–12° W trending subsurface 85° E Ridge buried under 2 to 3 km thick sediments is a prominent tectonic feature. Offshore basins characterised by deeper magnetic basement (9 km) and 100–200 km wide are present on either sides of the ridge. These basins were filled with 6–8 km thick lower Cretaceous to recent sediments. Integrated geophysical study depicts that the magnetic basement is characterised by NW-SE, NE-SW, NNE-SSW, N10-12° W and E-W trending structural features that are associated with the lower Cretaceous ocean floor. The Analytical Signal Processing and Werner Deconvolution techniques proved to be effective in determining the depth to the basement in areas covered by thick sediment overburden and characterized by a complex geologic/tectonic framework.  相似文献   

10.
As the Mesozoic sediments contribute most of the oil and gas reserves of the world, we present an integrated interpretation approach using magnetotellurics (MT) and surface geochemical prospecting studies to demarcate hydrocarbon prospective Gondwana (Mesozoic) formations underneath the Deccan flood basalts of Late Cretaceous age across Narmada-Tapti rift (between Bhusawal and Barwah) in Central India. The MT interpretation shows deep (∼5 km) basement structure between southern and central part of the MT profile however, it gradually becomes shallower to either ends of the profile with a predominant basement depth reduction in the northern end compared to the southern end. The geophysical results suggest thick (2-3.5 km) Mesozoic sediments in the area characterized by deep basement structure. The geochemical analysis of the near surface soil samples indicate higher concentrations of light gaseous hydrocarbons constituents over the area marked with thick sub-basalt Mesozoic formations. Analyses of the geochemical data imply that these hydrocarbons are genetically related, generated from a thermogenic source and these samples fall in the oil-producing zone. The temperature-depth estimations in the region supports favorable temperature conditions (80-120 °C) for oil generation at basement depths.  相似文献   

11.
Investigations of lithogenic and biogenic particle fluxes using long-term sediment traps are still very rare in the northern high latitudes and are restricted to the arctic marginal seas and sub-arctic regions. Here data on the variability of fluxes of lithogenic matter, CaCO3, opal, and organic carbon and biomarker composition from the central Arctic Ocean are presented for a 1-year period. The study was carried out on material obtained from a long-term mooring system equipped with two multi-sampling traps, at 150 and 1550 m depth, and deployed on the southern Lomonosov Ridge close to the Laptev Sea continental margin from September 1995 to August 1996. In addition, data from surface sediments were included in the study. Annual fluxes of lithogenic matter, CaCO3, opal, and particulate organic carbon were 3.9, 0.8, 2.6, and 1.5 g m−2 y−1, respectively, in the shallow trap and 11.3, 0.5, 2.9, and 1.05 g m−2 y−1, respectively, in the deep trap.Both the shallow and the deep trap showed significant variations in vertical flux over the year. Higher values were found from mid-July to the end of October (total mass flux of 75–130 mg m−2 d−1 in the shallow trap and 40–225 mg m−2 d−1 in the deep trap). During all other months, fluxes were fairly low in both traps (most total mass flux values <10 mg m−2 d−1). The interval of increased fluxes can be separated into (1) a mid-July/August maximum caused by increased primary production as documented in high abundances of marine biomarkers and diatoms and (2) a September/October maximum caused by increased influence of Lena River discharge indicated by maximum lithogenic flux and large amounts of terrigenous/fluvial biomarkers in both traps. During September/October, total mass fluxes in the deep trap were significantly higher than in the shallow trap, suggesting a lateral sediment flux at greater depth. The lithogenic flux data also support the importance of sediment input from the Laptev Sea for the sediment accumulation on the Lomonosov Ridge on geological time scales, as indicated in sedimentary records from this region.  相似文献   

12.
The Agulhas Bank, which forms the continental margin on the southern tip of Africa, consists of a thick Meso-Cainozoic sedimentary sequence (up to 6.2 km) resting on and behind pre-Mesozoic continental acoustic basement. The stratigraphy of this sequence is outlined and its history and facies variations mentioned where they are known. Refraction seismic velocity and bottom sample data indicate a basic three-fold subdivision of the Mesozoic sequence, which can be correlated with the onshore succession in the Algoa Basin. It is separated by a major hiatus from the Cainozoic sediments, which consist of a Palaeogene and Neogene sequence subdivided by another well-defined level of erosion. Various formations within the Cainozoic are defined and named. An outline of the bathymetry of the eastern Agulhas Bank is also given.  相似文献   

13.
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   

14.
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.  相似文献   

15.
Ground Penetrating Radar(GPR) measurements of sea ice thickness including undeformed ice and ridged ice were carried out in the central north Canadian Archipelago in spring 2010. Results have shown a significant spatial heterogeneity of sea ice thickness across the shelf. The undeformed multi-year fast ice of(2.05±0.09) m thick was investigated southern inshore zone of Borden island located at middle of the observational section,which was the observed maximum thickness in the field work. The less thick sea ice was sampled across a flaw lead with the thicknesses of(1.05±0.11) m for the pack ice and(1.24±0.13) m for the fast ice. At the northernmost spot of the section, the undeformed multi-year pack ice was(1.54±0.22) m thick with a ridged ice of 2.5 to 3 m,comparing to the multi-year fast ice with the thickness of(1.67±0.16) m at the southernmost station in the Prince Gustaf Adolf Sea.  相似文献   

16.
With the recently recovered organic-rich sediments of early Tertiary age from the Lomonosov Ridge by the Integrated Ocean Drilling Program (IODP) Expedition 302, the first data collection directly from source rocks of the central basins of the Arctic Ocean is now available. Using the results of seismic interpretations and published sedimentological and organic geochemical data from Expedition 302, the framework for the first quantitative assessment of source-rock quality and distribution of the Palaeogene sediments was modelled in the central Arctic Ocean. The modelling results suggest that an approximately 100-m-thick Early to Middle Eocene sedimentary sequence of good to very good source rocks exists along a 75 km long transect across the Lomonosov Ridge. In-situ generation of hydrocarbons is unlikely because the overburden (200–250 m) and consequently the thermal maturity are too low. Burial history and thermal modelling reveal that an additional overburden of at least 1000 m is necessary to start hydrocarbon generation along the ridge. However, source-rock modelling results show that good source-rock potential may exist in correlative units in the adjacent Amundsen Basin. Simulated organic carbon contents of 1.5–5%, coupled with an overburden of 1000–1200 m, and heat flow anomalies (117 and 100 mW m−2) due to the vicinity to the Gakkel Ridge spreading centre indicate that necessary conditions for hydrocarbon expulsion are already reached, and point to viability of a potential petroleum system. Our results support the hypothesis that deposition of a potentially good hydrocarbon source rock occurred across the entire Arctic Basin and adjacent margins during the early Tertiary.  相似文献   

17.
The Mediterranean Ridge is an arcuate ridge of deformed sediment caught up in the convergent plate margin between the African plate and the Aegean. An intensive campaign of SeaMARC I and SeaBeam surveys followed by piston coring has been conducted along the contact between undeformed turbidites of the Sirte Abyssal Plain and folded and faulted sediments of the Mediterranean Ridge. Along the outer edge of the Ridge, surficial sediments have been deformed into sinusoidal ridges and troughs (wavelengths 0.5–2 km, amplitude 20–150 m), which we interpret as folds. In plan view, the ridge and the trough fabric parallels the NW-SE trending regional contours, suggesting that the folds formed in response to compression orthogonal to the Mediterranean Ridge. The outermost ridge is shedding a debris apron out onto the abyssal plain, implying that uplift and deformation are ongoing. We show that the geometry of the outermost folds can be produced by elastic bending of a packet of 5–10 relatively strong layers, each 10–20 m thick, interbedded between weaker layers; we equate the strong layers with gypsum beds in the Messinian upper evaporites. Folding the seafloor from a flat layer into the observed ridge and trough topography would shorten the layer by less than 2%. Two percent shortening (equals two percent thickening) is insufficient to create the observed relief of the Mediterranean Ridge even if the entire sediment column down to basement were involved; we infer that additional shortening/thickening is accommodated by thrust faulting above a decollement at the top of the Messinian salt layer. At distances > 15 km from the deformation front and more than 500 m from the abyssal plain, sharp-edged, fine-grained side-scan lineations with very little vertical relief cut across the kilometer-scale ridge and trough topography. These fine-grained lineations fall in two groups trending N/S to NNE/SSW and ~ENE. We interpret these lineaments as traces of conjugate strike-slip faults formed in the same compressional regime which formed the NW/SE trending folds. The onset of strike-slip faulting may coincide with the cessation of imbricate thrust fan development above the initial salt-controlled decollement surface. The following characteristics of the Mediterranean Ridge are attributed to the presence of evaporites in the incoming sedimentary section: (1) initial deformation by folding rather than thrust faulting; (2) narrow taper; (3) rapid rate of outward growth; (4) karstification.  相似文献   

18.
Seismic-reflection profile investigations of the California continental terrace and Deep Plain, between 35°N and 39°N, support the hypothesis that the continental shelf and slope consist of alternating blocks of Franciscan and granitic-metamorphic basement overlain by varying thicknesses of younger sediments. North of 37°N, the seismic profiles confirm the distribution of turbidites shown by other workers. A significant proportion of the sediments on the middle and lower continental rise, south of 37°N, appears to be unrelated to the present Monterey deep-sea canyon system.Near 39°N the ridge which forms the topographic axis of the Delgada deep-sea fan consists of a thin cover of acoustically-transparent sediment unconformably overlying a thick sequence of turbidites; the southern part of this ridge is composed of well-defined short reflectors of highly variable dip. The ridge is incised by a steep-walled, flat-floored valley which follows a nearly straight course across its eastern flank. Among possible explanations for this pattern is uplift of the sea floor beneath the ridge.Our data and investigations of others indicate that acoustic basement north of 38°40N is at least 0.5 sec (two-way travel time) shoaler than it is south of Pioneer Ridge; when present, the ridge may represent as much as 0.5 sec additional basement relief. This structural pattern probably does not extend east of 127°40W, although the magnetic expression of the ridge persists to 127°W.Disappearance of the distinctive abyssal hills topography from west to east within the area of investigation usually can be attributed to burial by turbidites. Normal pelagic sediments form a veneer, rarely more than 0.15 sec thick, which conforms with the basement topography; some localities are devoid of discernible sediment.  相似文献   

19.
Some seismic refraction observations undertaken during the IGY are reported here together with a summary of other refraction studies carried out within the Transkei Basin, the Mozambique Ridge and the South African continental shelf area.A 2.5 km section of Cretaceous and younger rocks is associated with profiles observed on the continental shelf; directly below this group are rocks with velocities in the range 4.0–5.5 km s-1, probably representatives of the Karroo and Cape supergroups. The basement material velocity variations were from 5.3 to 6.5 with an average of 5.9 km s-1, and is correlated with granite or Malmesbury Formation plus granite. This crustal structure is similar to that found on the eastern continental shelf of southern South America.The profiles in the Transkei Basin show a thick layer of sediment with velocity range 1.50 to 3.50 km s-1, underlain by a refracting layer in which the average velocity is 4.5 km s-1. The velocity of 6.6 km s-1 obtained for the oceanic layer is similar to the velocities of the crustal layer measured in the Argentine Basin. The mantle velocity (8.1 km s-1) is consistent with the average mantle velocity for the Indian Ocean but significantly lower than the Pacific Ocean average of 8.20 km s-1. The depth to Moho is about 12.0 km and the crustal section is typical oceanic. A plate tectonic model of the early opening of the South Atlantic is used to describe the evolution of the Transkei Basin.On the Mozambique Ridge the thin sediments (0.7 km) are underlain by rocks with velocities averaging 5.6 km s-1. This is more than 1.0 km s-1 faster than the velocity for layer 2 from the Transkei Basin and the Agulhas Plateau, indicating rocks of a younger age or of a different type. Moreover the crustal section of the Ridge has a thickness in excess of 22 km and is in isostatic equilibrium when compared with the adjacent Transkei Basin and Agulhas Plateau. DSDP site 249, situated on the Ridge, penetrated basalt at a depth of 0.4 km. Whether this is continental or oceanic basalt is not known; when this site 249 basalt was compared to the cored basalts of the adjacent Mozambique Basin, inconclusive results were obtained. The essential constitution of the Mozambique Ridge remains an enigma, but solution of this problem is vital for the proper understanding of the Mesozoic history of this oceanic region.  相似文献   

20.
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.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号