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1.
Magnetic profiles obtained during the Hesant 92/93 cruise with the R/V Hesperides show large amplitude anomalies (up to 1000 nT) along a 100 km wide band in the northern margin of the Powell Basin. The anomalies, which are also locally identified in the eastern and western margins, are attributed to the continuation of the two branches of the Antarctic Peninsula Pacific Margin Anomaly (PMA). Interactive modelling of two-dimensional bodies in four profiles oriented NNW-SSE allows us to determine the main features of the magnetic source bodies within the continental crust. These are elongated in a N60/degE trend, and their base is located at a depth exceeding 15 km. Equivalent magnetic susceptibilities mostly between 0.07 and 0.1 (SI) are obtained. These values are consistent with the hypothesis that remanent magnetisation of the magnetic source bodies is sub-parallel to the present geomagnetic field (norÍmally magnetised). The general trends of the bathymetry a nd the geometry of the acoustic basement on multichannel seismic profiles are consistent with the upper surface of magnetic bodies. In order to match the observed anomalies it is also necessary to consider a second tabular shaped body with induced magnetisation in almost all the profiles, which could represent layers 2 and 3 of the oceanic crust of the Powell Basin. Three different geometries of connection between the anomalies in the Powell Basin margins and the PMA branches are discussed. The most plausible one is the occurrence of two branches, although they are closer together than in the Bransfield Strait. The northern branch would continue along the fragments of continental crust of the South Scotia Ridge located at the northern boundary of the Powell Basin, whereas the southern branch would be located only in the eastern and western passive margins of the Powell Basin. The apparent splitting of the southern branch of the anomalous body indicates that it was emplaced before Oligo cene times, when the opening of this basin occurred, and that it was subsequently fragmented during the Cenozoic. A possible time of formation of the PMA body would be during the long Cretaceous normal polarity interval, which also coincides with a peak in magmatic activity along the Antarctic Peninsula.  相似文献   

2.
Distribution of 230Th and 231Pa in the water column was measured in the Eurasian basins of the Arctic Ocean, i.e. the Makarov, Amundsen and Nansen basins. The profiles obtained were compared to one previously published from the Alpha Ridge (Bacon et al., Earth and Planetary Science Letters, 95 (1989) 15–22. The Amundsen and Nansen basins are similar in their 23OTh and 231Pa distribution, whereas in the Makarov Basin and at the Alpha Ridge these radionuclides are comparatively enriched in the deep basin. This may be due to low particle flux in conjunction with low water mass renewal rates. The 230Th concentration in water depth < 1000 m in the Makarov Basin is relatively low compared with the Alpha Ridge, which may be caused by an exchange of water masses between the Amundsen and Makarov basins.In the Eurasian Basin Deep Water, 230Th profiles can be explained both by a scavenging model and by a mixing-scavenging model. Enhanced particle load caused by resuspension may explain the increased scavenging in the Eurasian Basin Bottom Water. The differences in scavenging between the Norwegian-Greenland Sea and the Eurasian basins at present are not large enough to cause significant fractionation between 230Th and 231Pa.  相似文献   

3.
Magnetic total intensity values and bathymetric data collected on the continental margin off Saurashtra were, used to prepare magnetic anomalies and bathymetric contour maps. The magnetic anomalies are considered to have been caused by the Deccan Trap flood basalts which underlie the Tertiary sediments. Interpretation of the magnetic data using two-dimensional modelling method suggests that the magnetic basement is block faulted and deepens in steps from less than 1.0 km in the north to about 8.0 km towards the southern portion of the study area. The WNW-ESE trending faults identified in the present study extend across the Saurashtra continental margin between Porbandar and Veraval and appear to represent a major linear tectonic feature. The relationship of these fault lineaments with the regional tectonic framework have been discussed to indicate that they conform better as the northern boundary faults of the Narmada rift graben on the continental margin off Saurashtra.  相似文献   

4.
冲绳海槽北段地磁场特征及其地质解释   总被引：4，自引：0，他引：4  
根据磁异常的分布特征,冲绳海槽北段可以分成三个异常区：东海陆架边缘异常区、东海陆坡异常区和冲绳海槽北段异常区。在东海陆架边缘区,磁异常以正为主,最大可达＋ ３５０ｎＴ,该区所对应的磁性基底埋深较浅,一般在２～３ｋｍ 。在东海陆坡异常区,除了有ＮＮＥ向展布的负异常外,在此背景上还发育了一些沿ＮＷＷ 向展布的次级异常,推测本异常区的磁异常与ＮＷ 向的基底断裂有关。冲绳海槽北段异常区所在宽缓负异常的大背景下又有几处正异常条带分布,则是在吐噶喇断裂带控制作用下所产生的磁性较强而又不均匀的火成岩体的反应  相似文献   

5.
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   

6.
The Uruguayan continental margin comprises three sedimentary basins: the Punta del Este, Pelotas and Oriental del Plata basins, the genesis of which is related to the break-up of Gondwana and the opening of the Atlantic Ocean. Herein the continental margin of Uruguay is studied on the basis of 2D multichannel reflection seismic data, as well as gravity and magnetic surveys. As is typical of South Atlantic margins, the Uruguayan continental margin is of the volcanic rifted type. Large wedges of seaward-dipping reflectors (SDRs) are clearly recognizable in seismic sections. SDRs, flat-lying basalt flows, and a high-velocity lower crust (HVLC) form part of the transitional crust. The SDR sequence (subdivided into two wedges) has a maximum width of 85 km and is not continuous parallel to the margin, but is interrupted at the central portion of the Uruguayan margin. The oceanic crust is highly dissected by faults, which affect post-rift sediments. A depocenter over oceanic crust is reported (deepwater Pelotas Basin), and volcanic cones are observed in a few sections. The structure of continental crust-SDRs-flat flows-oceanic crust is reflected in the magnetic anomaly map. The positive free-air gravity anomaly is related to the shelf-break, while the most prominent positive magnetic anomaly is undoubtedly correlated to the landward edge of the SDR sequence. Given the attenuation, interruption and/or sinistral displacement of several features (most notably SDR sequence, magnetic anomalies and depocenters), we recognize a system of NW-SE trending transfer faults, here named Río de la Plata Transfer System (RPTS). Two tectono-structural segments separated by the RPTS can therefore be recognized in the Uruguayan continental margin: Segment I to the south and Segment II to the north.  相似文献   

7.
《Marine Geology》2006,225(1-4):265-278
The first seismic reflection data from the shallowest part of the submarine Lomonosov Ridge north of Arctic Canada and North Greenland comprise two parallel single channel lines (62 and 25 km long, offset 580 m) acquired from a 10 day camp on drifting sea ice. The top of southern Lomonosov Ridge is bevelled (550 m water depth) and only thin sediments (< 50 ms) cover acoustic basement. We suggest erosion of a former sediment drape over the ridge crest was either by a grounded marine ice sheet extending north from Ellesmere Island and/or deep draft icebergs. More than 1 km of sediments are present at the western entrance to the deep passage between southern Lomonosov Ridge and the Lincoln Sea continental margin. Here, the uppermost part (+ 0.3 s thick) of the section reflects increased sediment input during the Plio–Pleistocene. The underlying 0.7 s thick succession onlaps the slope of a subsiding Lomonosov Ridge. An unconformity at the base of the sedimentary section caps a series of NW–SE grabens and mark the end of tectonic extension and block faulting of an acoustic basement represented by older margin sediments possibly followed by minor block movements in a compressional regime. The unconformity may relate to termination of Late Cretaceous deformation between Lomonosov Ridge and Alpha Ridge or be equivalent to the Hauterivian break-up unconformity associated with the opening of the Amerasia Basin. A flexure in the stratigraphic succession above the unconformity is most likely related to differential compaction, although intraplate earthquakes do occur in the area.  相似文献   

8.
通过中国第1至第3次北极科学考察在北冰洋西部所采集的99个表层沉积物中生源与陆源粗组分的分析,研究了该海域表层生产力的变化,有机质来源以及陆源粗颗粒物质的输入方式和影响因素.研究区域生源组分所反映的表层生产力变化与通过白令海峡进入楚科奇海的3股太平洋洋流密切相关.楚科奇海西侧高盐高营养盐的阿纳德尔流流经区域,表层生产力...  相似文献   

9.
全球碳增汇需求高涨,海冰消退后的北冰洋被期待是一个主要的潜在碳增汇区。北冰洋太平洋扇区因受控于楚科奇海及其邻近海域较高的海洋固碳效率和碳深海封存量,在整个北冰洋碳循环中起着举足轻重的作用。开展该海域碳循环过程对环境快速变化的响应机制研究是实现北冰洋碳汇精准预测的基础。本文重点阐述了楚科奇海及其邻近海域碳循环过程（即海洋对大气二氧化碳的吸收、生物固碳、太平洋入流携带碳经陆架生物地化过程后向深海输出封存的陆架泵）对北冰洋环境快速变化的响应,并提出未来研究需要聚焦的关键科学问题。  相似文献   

10.
Total magnetic intensity and bathymetric surveys were carried out in the offshore area of Ratnagiri on the western continental shelf of India and an isomagnetic anomalies map at a contour interval of 50 nT was prepared which reveals N-S trends of magnetic anomalies. Two-dimensional model and spectral studies of these anomalies were carried out, and subsurface models of the geology in the area have been derived from anomalies at a number of places. The results suggest that the anomalies occur over a magnetic crystalline basement at a depth of 1–1.2 km which is similar (in magnetization) to onshore basalts of northwest India. These anomalies are believed to be an expression of a considerable thickness (around 1.7 km) of basalt, underlain by sediments. Identification of these basalts in offshore areas along the northwestern continental shelf of India would support (1) the idea that the onshore Deccan basalts of western India and the rhyolitic tuffs at the Laccadive ridge system (DSDP Site 210) are related to the same volcanic events, and (2) subsequent downfaulting of onshore Deccan basalts into the Arabian Sea and submergence below the Tertiary sediments.  相似文献   

11.
To facilitate geological analyses of the Ulleung Basin in the East Sea (Japan Sea) between Korea and Japan, shipborne and satellite altimetry-derived gravity data are combined to derive a regionally coherent anomaly field. The 2-min gridded satellite altimetry-based gravity predicted by Sandwell and Smith [Sandwell DT, Smith WHF (1997) J Geophys Res 102(B5):10,039–10,054] are used for making cross-over adjustments that reduce the errors between track segments and at the cross-over points of shipborne gravity profiles. Relative to the regionally more homogeneous satellite gravity anomalies, the longer wavelength components of the shipborne anomalies are significantly improved with minimal distortion of their shorter wavelength components. The resulting free-air gravity anomaly map yields a more coherent integration of short and long wavelength anomalies compared to that obtained from either the shipborne or satellite data sets separately. The derived free-air anomalies range over about 140 mGals or more in amplitude and regionally correspond with bathymetric undulations in the Ulleung Basin. The gravity lows and highs along the basin’s margin indicate the transition from continental to oceanic crust. However, in the northeastern and central Ulleung Basin, the negative regional correlation between the central gravity high and bathymetric low suggests the presence of shallow denser mantle beneath thinned oceanic crust. A series of gravity highs mark seamounts or volcanic terranes from the Korean Plateau to Oki Island. Gravity modeling suggests underplating by mafic igneous rocks of the northwestern margin of the Ulleung Basin and the transition between continental and oceanic crust. The crust of the central Ulleung Basin is about a 14–15 km thick with a 4–5 km thick sediment cover. It may also include a relatively weakly developed buried fossil spreading ridge with approximately 2 km of relief.  相似文献   

12.
The Mozambique Ridge (MOZR) is one of the basement high structures located in the Southwest Indian Ocean, parallel to the Southeast African continental margin. It was formed as a result of the tectono-magmatic evolution of the Gondwana breakup. The origin of the MOZR has been highly debated, with models suggesting either continental or oceanic origin. With new free-air gravity anomaly and multichannel seismic (MCS) reflection data, we present results of 2D density modeling along two seismic profiles acquired by R/V Xiangyanghong 10 at the northern Mozambique Ridge (N-MOZR) between 26°S and 28°S. We observed high free-air gravity anomaly and strong positive magnetic anomaly related to the emplaced seaward dipping reflectors (SDR) and high density lower crustal body (HDLCB), and high Bouguer gravity anomaly associated with the thinning of the continental crust underneath the N-MOZR over a distance of ~82 km. This suggests a thinned and intruded continental crust bound by the Mozambique Fracture Zone (MFZ) that is characterized by gravity low and negative magnetic anomaly. This fracture zone marks the continent-ocean boundary (COB) while the N-MOZR is the transform margin high, i.e., marks the continent-ocean transition (COT) of the Southern Mozambique margin, following the definition of transform margins. We suggest that the N-MOZR was formed by continental extension and subsequent breakup of the MFZ, accompanied by massive volcanism during the southward movement of the Antarctica block. The presence of SDR, HDLCB, and relatively thick oceanic crust indicates the volcanic nature of this transform margin.  相似文献   

13.
A corridor 315 km wide centered along the southeast projection of the Atlantis fracture zone between 21°W and 29°W was investigated with seismic reflection, bathymetric, gravity, and magnetic profiles. Six sub-parallel, sediment-filled troughs in acoustic basement trend about 106° across the abyssal hills and lower continental rise off northwest Africa. Where the southernmost structural lineations cross the abyssal plain, they are interrupted by a ridge trending 080° surmounted by volcanic peaks.The structural lineations become less distinct landward of the western margin of the abyssal plain coincident with a decrease in topographic relief on acoustic basement and increasing sediment thickness. This transition is coincident with a reduction in the amplitude of gravity and magnetic anomalies.  相似文献   

14.
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   

15.
为了准确解释环境磁学参数记录的极地古气候环境变化信息,本研究对白令海和西北冰洋61个站位的表层沉积物进行了高、低频质量磁化率(χ)、非磁滞磁化率(χARM)和磁化率-温度(k-T)分析,以探明该区沉积物中磁性矿物的种类、来源与搬运路径。结果显示,样品的χ具有明显的地域分布特征。白令海的χ值整体高于楚科奇海,并在育空河口外侧和圣劳伦斯岛南侧较高,向北和向西南方向逐渐减小。楚科奇海中东部陆架上表层沉积χ值高于阿拉斯加沿岸,而西北冰洋深海平原和洋脊区的χ值最低。χARM的变化趋势与质量磁化率相似,但频率磁化率的变化趋势与质量磁化率正好相反。k-T分析结果显示阿留申海盆沉积物中的铁磁性矿物以磁赤铁矿占主导,白令海陆架育空河口外侧和圣劳伦斯岛南北两侧为磁铁矿,白令海陆架西部和楚科奇海陆架中东部为磁赤铁矿和磁铁矿,楚科奇海阿拉斯加沿岸为黄铁矿,而西北冰洋陆坡、深海平原和洋脊区为胶黄铁矿和黄铁矿,但高纬度区沉积物中的胶黄铁矿含量更高。沉积物中磁性矿物的区域性分布受沉积物来源、洋流和底质环境等因素的控制。白令海和楚科奇海陆架磁赤铁矿来源于亚洲大陆,白令海陆架东部的磁铁矿来自育空河流域,阿拉斯加沿岸沉积物中的黄铁矿,应为阿拉斯加西北部陆源侵蚀来源的或早期成岩作用形成的,西北冰洋深海盆区的胶黄铁矿,为自生成因的。  相似文献   

16.
Extremely low summer sea-ice coverage in the Arctic Ocean in 2007 allowed extensive sampling and a wide quasi-synoptic hydrographic and δ18O dataset could be collected in the Eurasian Basin and the Makarov Basin up to the Alpha Ridge and the East Siberian continental margin. With the aim of determining the origin of freshwater in the halocline, fractions of river water and sea-ice meltwater in the upper 150 m were quantified by a combination of salinity and δ18O in the Eurasian Basin. Two methods, applying the preformed phosphate concentration (PO*) and the nitrate-to-phosphate ratio (N/P), were compared to further differentiate the marine fraction into Atlantic and Pacific-derived contributions. While PO*-based assessments systematically underestimate the contribution of Pacific-derived waters, N/P-based calculations overestimate Pacific-derived waters within the Transpolar Drift due to denitrification in bottom sediments at the Laptev Sea continental margin.Within the Eurasian Basin a west to east oriented front between net melting and production of sea-ice is observed. Outside the Atlantic regime dominated by net sea-ice melting, a pronounced layer influenced by brines released during sea-ice formation is present at about 30–50 m water depth with a maximum over the Lomonosov Ridge. The geographically distinct definition of this maximum demonstrates the rapid release and transport of signals from the shelf regions in discrete pulses within the Transpolar Drift.The ratio of sea-ice derived brine influence and river water is roughly constant within each layer of the Arctic Ocean halocline. The correlation between brine influence and river water reveals two clusters that can be assigned to the two main mechanisms of sea-ice formation within the Arctic Ocean. Over the open ocean or in polynyas at the continental slope where relatively small amounts of river water are found, sea-ice formation results in a linear correlation between brine influence and river water at salinities of about 32–34. In coastal polynyas in the shallow regions of the Laptev Sea and southern Kara Sea, sea-ice formation transports river water into the shelf’s bottom layer due to the close proximity to the river mouths. This process therefore results in waters that form a second linear correlation between brine influence and river water at salinities of about 30–32. Our study indicates which layers of the Arctic Ocean halocline are primarily influenced by sea-ice formation in coastal polynyas and which layers are primarily influenced by sea-ice formation over the open ocean. Accordingly we use the ratio of sea-ice derived brine influence and river water to link the maximum in brine influence within the Transpolar Drift with a pulse of shelf waters from the Laptev Sea that was likely released in summer 2005.  相似文献   

17.
白令海、西北冰洋等高生产力海域在北冰洋“生物泵”中起到重要作用；海水升温、海冰消退等北极快速变化，将强烈影响该海域“生物泵”的结构与规模，并在沉积物中有机质的来源与新鲜程度上有所体现，可用脂肪酸加以指征。对第五次、第六次中国北极科学考察在以上海域采集的表层沉积物进行脂肪酸含量（以沉积物干重计）及组成分析，结果显示楚科奇海陆架总脂肪酸含量非常高((97.15± 55.31) μg/g)，白令海盆最低((15.00±1.30) μg/g)，加拿大海盆、楚科奇海陆坡、白令海陆架居中(分别为(88.65 ± 3.52) μg/g，(70.35±11.32) μg/g与(38.28±14.89) μg/g)。海源脂肪酸占总脂肪酸比例最高(86.82%±7.08%)，陆源次之(8.45%±6.62%)，细菌最低(4.63%±2.24%)；硅藻指数(16:1ω9/16:0)在楚科奇海陆架(> 0.82)、白令海陆架边缘(> 0.65)较高，其他区域均较低。脂肪酸结果表明:(1) 该海域沉积有机质主要来自海源，陆源贡献小；在北部、南部楚科奇海陆架、白令海陆架边缘，硅藻生物量占主要优势；细菌脂肪酸比例显著低于温暖海域，指示低温抑制细菌活动。(2) 楚科奇海陆架区硅藻生产力高、细菌活动弱，新鲜有机质沉降效率高，但对未来海水升温、浮游植物群落变化也较为敏感。(3) 加拿大海盆、楚科奇海陆坡的浮游植物群落由绿藻与金藻主导。以上结论说明脂肪酸可指示表层沉积物中有机质的来源与新鲜程度；未来，脂肪酸有望进一步揭示北冰洋“生物泵”对北极快速变化的响应。  相似文献   

18.
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   

19.
Two profiles have been gravimetrically modelled in the central and eastern parts of the Alboran Sea and Betic Cordillera. The crust in the central part thins from 38 km below the Internal Zones to 18–22 km beneath the Alboran Basin in an area about 30 km wide. The eastern part shows greater thinning, from about 38 to 15 km, distributed over a broader area some 100 km wide. The mode of crustal thinning in the eastern part can be compared with that of a rifted passive margin whereas the central part exhibits a transform style.  相似文献   

20.
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   

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