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1.
The data from a recent magnetic compilation by Verhoefet al. (1991) off west Africa were used in combination with data in the western Atlantic to review the Mesozoic plate kinematic evolution of the central North Atlantic. The magnetic profile data were analyzed to identify the M-series sea floor spreading anomalies on the African plate. Oceanic fracture zones were identified from magnetic anomalies and seismic and gravity measurements. The identified sea floor spreading anomalies on the African plate were combined with those on the North American plate to calculate reconstruction poles for this part of the central Atlantic. The total separation poles derived in this paper describe a smooth curve, suggesting that the motion of the pole through time was continuous. Although the new sea floor spreading history differs only slightly from the one presented by Klitgord and Schouten (1986), it predicts smoother flowlines. On the other hand, the sea floor spreading history as depicted by the flowlines for the eastern central Atlantic deviates substantially from that of Sundvik and Larson (1988). A revised spreading history is also presented for the Cretaceous Magnetic Quiet Zone, where large changes in spreading direction occurred, that can not be resolved when fitting magnetic isochrons only, but which are evident from fracture zone traces and directions of sea floor spreading topography.Deceased 11 November 1991  相似文献   

2.
The ocean bottom seismograph described in this paper has been developed primarily for recording earthquakes on the mid-oceanic ridges. The instrument is suitable for dropping onto the most rugged areas of the ocean floor. Acoustic tracking with the ship's precision echo sounder enables it to be located there relative to both the topography of the sea bed and the ship. The outputs of a 3-component seismometer and a hydrophone are recorded in FM form on a low-power magnetic tape recorder designed specifically for the instrument.  相似文献   

3.
The relationship between the magnetic anomalies over Iceland and those over Reykjanes Ridge is investigated using the data of the 1965 Dominion Observatory survey. A method is developed for determining the two-dimensionality of the anomalies from the component data measured in this survey. This method is based on testing the first and the second derivative of the magnetic potential with respect to the direction of two-dimensionality, using the component data along a single flight line. Testing the first derivative also yields the direction of two-dimensionality. The outcomes of the two tests (based on a single line) are compared with the observed two-dimensionality (established by narrowly spaced earlier surveys) of Reykjanes Ridge, showing good agreement. As the outcomes of the two tests provide complementary information they are combined into a single factor: A. This factor of two-dimensionality is very low for the anomalies over the shelf of Iceland indicating that the anomalies over Iceland cannot be continued directly into those over Reykjanes Ridge. Over Iceland A is generally low. Over the neovolcanic zone in eastern Iceland twodimensionality is associated with long wavelengths that are not present in the spectrum of the anomalies over Reykjanes Ridge. Thus, Reykjanes Ridge-type anomalies are absent with the exception of the central anomaly. This may not be used as evidence against crustal spreading since the kinematic model proposed by Pálmason for Iceland has a wide transition zone between rock of opposite polarity. The same model if computed for a mid-ocean ridge has narrow transition zones. The larger width of the transition zone blurs the anomalies related to the reversals of the earth magnetic field.  相似文献   

4.
The sea floor of Fram Strait, the over 2500 m deep passage between the Arctic Ocean and the Norwegian-Greenland Sea, is part of a complex transform zone between the Knipovich mid-oceanic ridge of the Norwegian-Greenland Sea and the Nansen-Gakkel Ridge of the Arctic Ocean. Because linear magnetic anomalies formed by sea-floor spreading have not been found, the precise location of the boundary between the Eurasian and the North American plate is unknown in this region. Systematic surveying of Fram Strait with SEABEAM and high resolution seismic profiling began in 1984 and continued in 1985 and 1987, providing detailed morphology of the Fram Strait sea floor and permitting better definition of its morphotectonics. The 1984 survey presented in this paper provided a complete set of bathymetric data from the southernmost section of the Svalbard Transform, including the Molloy Fracture Zone, connecting the Knipovich Ridge to the Molloy Ridge; and the Molloy Deep, a nodal basin formed at the intersection of the Molloy Transform Fault and the Molloy Ridge. This nodal basin has a revised maximum depth of 5607 m water depth at 79°8.5N and 2°47E.  相似文献   

5.
Sea floor spreading anomalies in the Lofoten-Greenland basins reveal an unstable plate boundary characterized by several small-offset transforms for a period of 4 m.y. after opening. North of the Jan Mayen Fracture Zone, integrated analysis of magnetic and seismic data also document a distinct, persistent magnetic anomaly associated with the continent-ocean boundary and a locally, robust anomaly along the inner boundary of the break-up lavas. These results provide improved constraints on early opening plate reconstructions, which include a new anomaly 23-to-opening pole of rotation yielding more northerly relative motion vectors than previously recognized; and a solution of the enigmatic, azimuthal difference between the conjugate Eocene parts of the Greenland-Senja Fracture Zone if the Greenland Ridge is considered a continental sliver. The results confirm high, 2.36–2.40 cm yr–1, early opening spreading rates, and are consistent with the start of sea floor spreading during Chron 24r. The potential field data along the landward prolongations of the Bivrost Fracture Zone suggest that its location is determined by a Mesozoic transfer system which has acted as a first-order, across-margin tectono-magmatic boundary between the regional Jan Mayen and Greenland-Senja Fracture Zone systems, greatly influencing the pre-, syn- and post-breakup margin development.  相似文献   

6.
A regional survey of the southern Reykjanes Ridge (52°N to 57°N) shows an irregular topography: a rift valley which is only partly recognizable as such, with varying azimuth and some fracturezone-like interruptions. The survey also comprised gravity and magnetic measurements.The course of the axis as well as the perpendicular fractures show up well in the free air anomalies as relative troughs within an area of positive free air gravity (Figure 5). There is no indication of density variations within the topographic masses.The anomaly pattern of total magnetic intensity indicates the exact position of the rift axis and a bifurcation at about 55°N. From the parallel magnetic anomalies south of 55°N (Figure 2) a spreading rate can be deduced of 1.10 cm/yr perpendicular to the rift axis (Figure 3). This spreading rate is at the same time the plate movement involved.A survey of the Iceland-Faeroe Ridge with a 3–5 miles grid shows large gravity and magnetic anomalies over a smooth topography, indicating large pockets of light material, probably of volcanic origin. These areas have normal magnetization. Positive gravity anomalies forming a ring structure along the 200 m isobath are characterized by reversed magnetization.The dissimilarity in morphology, seismicity and inner structure between the two ridges that intersect in Iceland suggest that there is no relation between the two phenomena.Paper presented at the meeting of the International Gravity Commission, Paris, on September 8, 1970.  相似文献   

7.
From reflection and refraction data a crustal model of the Concepcion Bank NE of Lanzarote Island has been calculated. It is interpreted as a tectonic unit of volcanic origin. Its evolution history of mainly Tertiary age does not depend on sea floor spreading. The Bank is built of intruded basaltic layers over material with a reduced mantle p-velocity.  相似文献   

8.
The Rodriguez Triple Junction (RTJ) corresponds to the junction of the three Indian Ocean spreading ridges. A detailed survey of an area of 90 km by 85 km, centered at 25°30 S and 70° E, allows detailed mapping (at a scale of 1/100 000) of the bathymetry (Seabeam) and the magnetic anomalies. The Southeast Indian Ridge, close to the triple junction, is a typical intermediate spreading rate ridge (2.99 cm a-1 half rate), trending N140°. The Central Indian Ridge rift valley prolongs the Southeast Indian Ridge rift valley with a slight change of orientation (12°). The half spreading rate and trend of this ridge are 2.73 cm a-1 and N152° respectively. In contrast, the Southwest Indian Ridge close to the triple junction is expressed by two deep-valleys (4300 and 5000 m deep) which abut the southwestcrn flanks of the two other ridges, and appears to be a stretched area without axial neovolcanic zone. The evolution of the RTJ is analysed for the past one million years. The instantaneous velocity triangle formed by the three ridges cannot be closed indicating that the RTJ is unstable. A model is proposed to explain the evolution of the unstable RRF Rodriguez Triple Junction. The model shows that the axis of the Central Indian Ridge is propressively offset from the axis of the Southeast Indian Ridge at a velocity of 0.14 cm a-1, the RTJ being restored by small jumps. This unstable RRF model explains the directions and offsets which are observed in the vicinity of the triple junction. The structure and evolution of the RTJ is similar to that of the Galapagos Triple Junction located in the East Pacific Ocean and the Azores Triple Junction located in the Central Atlantic Ocean.  相似文献   

9.
根据11段洋脊上已经公布的热液活动区数目和扩张速率,建立了洋脊扩张速率和热液活动区数目之间的关系,发现洋脊的扩张速率越高,其上发育的热液活动区数目就越多。两者之间可用一个线性关系式来描述:Ph=0.000 199 294 89Us-0.000 625 479 19,其中,Us为洋脊的扩张速率(mm/a),Ph为单位洋脊上热液活动区的数目(个/km)。依据上述关系,在已知洋脊扩张速率的情况下可以预测洋脊上热液活动区的数目。冲绳海槽前期的扩张(拉张)速率很低,一般为15 mm/a左右。根据地震剖面计算了冲绳海槽现代扩张速率约为42 mm/a。总体上,冲绳海槽直到目前仍为一个低速扩张(拉张)的孤后盆地,但扩张(拉张)速率有明显加快的趋势,所得到的关系式能够较好地符合冲绳海槽的情况,这一方面验证了给出的热液活动区数目和扩张速率之间关系的合理性,另一方面也说明我们对冲绳海槽近期扩张(拉张)速率讨论的合理性。根据在冲绳海槽的应用结果,我们预测冲绳海槽仍然存在尚未查明的热液活动区。  相似文献   

10.
南海及其周缘中新生代火山活动时空特征与南海的形成模式   总被引：13，自引：2，他引：11  
根据南海海区、华南和中南半岛的地面露头、钻井、拖网及地球物理资料,分析了南海地区火山活动的时空分布特点。在南海陆缘和周边陆区中生代末期花岗岩分布非常广泛。新生代火山岩活动规模较小,主要是海底扩张之后在洋盆扩张脊、北部陆缘的陆洋边界附近、雷琼地区和中南半岛南部的玄武岩。在南海北部陆缘的深部地震调查中发现,在地壳下部存在小规模的高速异常体,结合浅部的晚第三纪一第四纪火山活动,认为该高速体形成于南海扩张之后。这些特征表明,在南海的拉张过程中岩浆供应不丰富,在陆缘未形成大规模的侵入和喷出岩。南海陆缘属于岩浆匮乏型被动大陆边缘。南海海区残留多个刚性断裂陆块,反映了裂谷拉张过程中脆性破裂。根据这些特征,南海形成难以用印藏碰撞引起的软流圈物质上涌导致岩石圈破裂这样的模式来解释。  相似文献   

11.
Spectral analysis techniques have been applied to data sets of gravity and topography selected across fracture zones with large offset in the Equatorial and Central Atlantic Ocean and processed independently for each fracture zone. Three simple compensation models, two in local isostatic equilibrium (Airy I and II) and one in regional equilibrium (Plate model) have been tested. It is found that the free-air anomalies are primarily controlled by the topography and its isostatic response. For short wavelengths, admittance can be explained either by the effect of uncompensated sea floor topography with high density basement or by the effect of uncompensated sea floor topography with normal oceanic basement density but accompanied by a crust of constant thickness. For intermediate wavelengths, admittance for the Romanche fracture zone agrees best with a local isostatic model in which compensation is achieved by a less dense material in the upper mantle. No such evidence exists for the Vema and Chain fracture zones for which topography could just as well be regionally supported by an elastic plate 3 to 10 km thick. For longer wavelengths, the admittances computed for the three fracture zones are compatible with a thermal compensation of the topography.  相似文献   

12.
A detailed survey of a 1°×1°-square of seafloor 100 miles south-east of the Azores shows a strong correlation between directions of regional topographic and magnetic lineations. The area is dissected by the East Azores Fracture Zone at 36°55N, identified as the active Eurasian-African plate boundary, and by another large, non-active fracture zone at 36°10N. Both fracture zones strike 265° and are accompanied by large amplitude magnetic anomalies. The general strike in the area in between is 000°–015°. The skewing effect at this magnetic latitude is very sensitive to variations in strike of the magnetic contrasts. This effect was eliminated by a non-linear transformation which also gives the positions of magnetic contrasts. Some N-S contrasts were identified as sea floor spreading polarity contrasts (anomalies 31 and 32). Weak contrasts could be identified as topographic effects and gave a magnetization intensity of 5 A m-1. The identified sea floor spreading anomalies to both sides of the fracture zone at 36°10N agree very well, also quantatively, with a three-dimensional model for the fracture zone anomalies. This model describes the non-linear anomalies as end effects of the magnetic layer which is divided in blocks of alternating polarity.  相似文献   

13.
The volumes of formed volcanoes are an important indicator of the formation intensity of the ocean floor volcanogenic morphostructure under different geodynamic conditions. The distribution of the volumes of volcanoes that formed within the oceanic lithosphere of different geological ages is considered to analyze the spatial and time dynamics of the scopes of the seamount formation. The greatest volumes of effused rocks concentrate in volcanoes of different heights on the different-age segments of the lithosphere. The total volumes of volcanogenic substance growing with increasing age and thickness of the lithosphere is caused by the increasing range of the heights of the formed volcanoes and the growing number of big seamounts. Despite that the volcanic seamounts formed within the mid-oceanic ridges, on the divergent boundaries of the lithospheric plates and in the transform faults most often, half of the volcanogenic material was emitted to the earth’s surface during the spreading of the intraplate volcanism. A leading part in the formation of the volcanogenic morphostructure of the ocean floor belongs to the formation of big seamounts with heights of 4–6 km and volumes from 1500 to 5100 km3, respectively, in the provinces with ages from 60 to 90 mln years.  相似文献   

14.
New swath bathymetric, multichannel seismic and magnetic data reveal the complexity of the intersection between the extinct West Scotia Ridge (WSR) and the Shackleton Fracture Zone (SFZ), a first-order NW-SE trending high-relief ridge cutting across the Drake Passage. The SFZ is composed of shallow, ridge segments and depressions, largely parallel to the fracture zone with an `en echelon' pattern in plan view. These features are bounded by tectonic lineaments, interpreted as faults. The axial valley of the spreading center intersects the fracture zone in a complex area of deformation, where N120° E lineaments and E–W faults anastomose on both sides of the intersection. The fracture zone developed within an extensional regime, which facilitated the formation of oceanic transverse ridges parallel to the fracture zone and depressions attributed to pull-apart basins, bounded by normal and strike-slip faults.On the multichannel seismic (MCS) profiles, the igneous crust is well stratified, with numerous discontinuous high-amplitude reflectors and many irregular diffractions at the top, and a thicker layer below. The latter has sparse and weak reflectors, although it locally contains strong, dipping reflections. A bright, slightly undulating reflector observed below the spreading center axial valley at about 0.75 s (twt) depth in the igneous crust is interpreted as an indication of the relict axial magma chamber. Deep, high-amplitude subhorizontal and slightly dipping reflections are observed between 1.8 and 3.2 s (twt) below sea floor, but are preferentially located at about 2.8–3.0 s (twt) depth. Where these reflections are more continuous they may represent the Mohorovicic seismic discontinuity. More locally, short (2–3 km long), very high-amplitude reflections observed at 3.6 and 4.3 s (twt) depth below sea floor are attributed to an interlayered upper mantle transition zone. The MCS profiles also show a pattern of regularly spaced, steep-inclined reflectors, which cut across layers 2 and 3 of the oceanic crust. These reflectors are attributed to deformation under a transpressional regime that developed along the SFZ, shortly after spreading ceased at the WSR. Magnetic anomalies 5 to 5 E may be confidently identified on the flanks of the WSR. Our spreading model assumes slow rates (ca. 10–20 mm/yr), with slight asymmetries favoring the southeastern flank between 5C and 5, and the northwestern flank between 5 and extinction. The spreading rate asymmetry means that accretion was slower during formation of the steeper, shallower, southeastern flank than of the northwestern flank.  相似文献   

15.
The central part of the northern Labrador Sea is a magnetic quiet zone, and is flanked by regions exhibiting well developed linear magnetic anomalies older than anomaly 24. The quiet zone dies out progressively to the south, where it becomes possible to correlate anomalies between adjacent profiles. A 45 degree change in spreading direction at anomaly 25 time was accompanied by a major jump in ridge position and orientation. As a consequence of this reorganisation, spreading in the northern Labrador Sea next occurred within a rift that was oriented at 45 degrees to the spreading direction, while to the south spreading occurred within in a rift that was orientated at 90 degrees to the spreading direction. Obliquity of spreading changed, between these limits, progressively along the ridge. The quiet zone may be present to the north because the oblique northern geometry resulted in a fragmented ridge composed of many small-offset transform faults joining many short spreading ridge segments. Each magnetic source block produced by magnetisation of sea floor at these small ridge segments will be surrounded by similar small blocks that have opposite polarity, so that none can be resolved at the sea surface. Supporting evidence comes from multi-channel seismic profiles across the Labrador Sea, which show that the basement is more textured within the quiet zone than outside, suggesting the presence of numerous small fracture zones in the quiet zone.A magnetic quiet zone is present in the northern Greenland Sea between margins that are oblique to the spreading direction. In contrast, there are clear lineated magnetic patterns in adjacent areas to north and south where the margins are orthogonal to the spreading direction. This quiet zone may also be due to the geometry of spreading.  相似文献   

16.
每一位海洋地质工作者都非常重视外业调查及第一手资料的获取,而这些往往得益于海洋地质调查仪器的帮助.以前我们常用的表层沉积物取样仪器,有普通蚌式抓斗、小箱式取样器、多管取样器等.这里介绍一种深海新型取样仪器--电视抓斗.目前,它在深海及大洋洋中脊资源调查,特别是在深海底块状硫化物、多金属结核、锰结壳调查等勘测中扮演着重要...  相似文献   

17.
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18.
The East Vietnam Boundary Fault Zone (EVBFZ) forms the seaward extension of the Red River Shear Zone and interacted with the extensional rift systems in basins along the Central Vietnamese continental margin. The structural outline of the central Vietnamese margin and the timing of deformation are therefore fundamental to understanding the development of the South China Sea and its relation to Indochinese escape tectonism and the India-Eurasia collision. This study investigates the structural and stratigraphic evolution of the Central Vietnamese margin in a regional tectonic perspective based on new 2-D seismic and well data. The basin fill is divided into five major Oligocene to Recent sequences separated by unconformities. Deposition and the formation of unconformities were closely linked with transtension, rifting, the opening of the South China Sea and Late Neogene uplift and denudation of the eastern flank of Indochina. The structural outline of the Central Vietnamese margin favors a hybrid tectonic model involving both escape and slab-pull tectonics. Paleogene left-lateral transtension over the NNW-striking EVBFZ, occurred within the Song Hong Basin and the Quang Ngai Graben and over the Da Nang Shelf/western Phu Khanh Basin, related to the escape of Indochina. East of the EVBFZ, Paleogene NE-striking rifting prevailed in the outer Phu Khanh Basin and the Hoang Sa Graben fitting best with a prevailing stress derived from a coeval slab-pull from a subducting proto-South China Sea beneath the southwest Borneo – Palawan region. Major rifting terminated near the end of the Oligocene. However, late stage rifting lasted to the Early Miocene when continental break-up and seafloor spreading commenced along the edge of the outer Phu Khanh Basin. The resulting transgression promoted Lower and Middle Miocene carbonate platform growth on the Da Nang Shelf and the Tri Ton High whereas deeper marine conditions prevailed in the central part of the basins. Partial drowning and platform retreat occurred after the Middle Miocene due to increased siliciclastic input from the Vietnamese mainland. As a result, siliciclastic, marine deposition prevailed offshore Central Vietnam during the Pliocene and Pleistocene.  相似文献   

19.
Immediately southwest of Iceland, the Reykjanes Ridge consists of a series ofen échelon, elongate ridges superposed on an elevated, smooth plateau. We have interpreted a detailed magnetic study of the portion of the Reykjanes Ridge between 63°00N and 63°40N on the Icelandic insular shelf. Because the seafloor is very shallow in our survey area (100–500 m), the surface magnetic survey is equivalent to a high-sensitivity, nearbottom experiment using a deep-towed magnetometer. We have performed two-dimensional inversions of the magnetic data along profiles perpendicular to the volcanic ridges. The inversions, which yield the magnetization distribution responsible for the observed magnetic field, allow us to locate the zones of most recent volcanism and to measure spreading rates accurately. We estimate the average half spreading rate over the last 0.72 m.y. to have been 10 mm/yr within the survey area. The two-dimensional inversions allow us also to measure polarity transition widths, which provide an indirect measure of the width of the zone of crustal accretion. We find a mean transition width on the order of 4.5±1.6 km. The observed range of transition widths (2 to 8.4 km) and their mean value are characteristic of slow-spreading centers, where the locus of crustal accretion may be prone to lateral shifts depending on the availability of magmatic sources. These results suggest that, despite the unique volcanotectonic setting of the Reykjanes Ridge, the scale at which crustal accretion occurs along it may be similar to that at which it occurs along other slow-spreading centers. The polarity transition width measurements suggest a zone of crustal accretion 4–9 km wide. This value is consistent with the observed width of volcanic systems of the Reykjanes Peninsula. The magnetization amplitudes inferred from our inversions are in general agreement with NRM intensity values of dredge samples measured by De Boer (1975) and ourselves. Our thermomagnetic measurements do not support the hypothesis that the low amplitude of magnetic anomalies near Iceland is the result of a high oxidation state of the basalts. We suggest that the observed reduction in magnetic anomaly amplitude toward Iceland may be the result of an increase in the size of pillows and other igneous units.  相似文献   

20.
In this paper, residual and regional gravity and magnetic anomalies of Saros Bay are separated using wavelet method. Wavelet is one of the modern, stochastic image approaches processing technique in electronics. The vertical, horizontal and diagonal components of wavelet output are evaluated simultaneously and an underground model is obtained by suitable cross-sections. Thus the geological and tectonic properties of Saros Bay are extracted. Our proposed model is confirmed by deep and shallow seismic researches of Turkish Petroleum Cooperation (TPAO). The South-East region of Saros Graben is formed by strike slip Ganos Fault and Anafartalar reverse Fault that lies on the east of Ganos Fault. On the Northwest region, there is strike slip component fault resulting in the Enez Graben. Here we detected two new oval type geological structure, both of them starting from the Eastern direction of Canakkale Dardanelles and but one of them continuing through Gelibolu Peninsuls, while the other tending to the Enez Graben. We think that these structures are the ruins of a very old sea in this region. This revised version was published online in November 2006 with corrections to the Cover Date.  相似文献   

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