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 31 毫秒
1.
Five expeditions (1965–1970) across parts of the Aleutian Abyssal Plain and adjacent areas in the Gulf of Alaska, and results of the Deep Sea Drilling Project, provide new information for the geologic history of the region which forms restrictive limits on models of plate tectonics. In general: (1) the Eocene-Oligocene, turbidite Aleutian Abyssal Plain was deposited from channelized turbidity currents from the north or northeast; (2) the plain is bounded on the south by the northern ridges of the Surveyor Fracture Zone, and is isolated from the Tufts Abyssal Plain; (3) turbidites were deposited from many buried channels and smaller surficial channels, but mainly from four great channels: Seamap, Sagittarius, Aquarius, and Taurus.The channels are depositional features; accumulation of sediments causes the channels to lie, topographically, along low ridges, with channels above distal portions of their levees. Western levees are higher and broader than eastern levees. Levee heights decrease from 30–100 m in the north to 15–25 m in the south.Rates of deposition and thicknesses of pelagic sediments in the northwest are 3 to 4 times greater than in the southeast. The data indicate the pelagics were deposited near the margin of the Pacific, at or near present locations. Thus, little or no northward plate motion is indicated.Turbidite thicknesses decrease from about 400–800 m in the north to about 200 m in the south. Turbidite thicknesses in the east-central plain are greater than in the Alaskan Abyssal Plain (formed since the Miocene), the northern Tufts Abyssal Plain, or the Sohm Abyssal Plain in the North Atlantic.Faulting and flexure of the oceanic crust seaward of the Aleutian Trench have strongly affected the channels. Seamap Channel has its high point midway along its course. The other three major channels are uplifted and faulted in the north.Required volumes of off-scraped sediments, undisturbed turbidites in the Aleutian Trench floor, and paleoclimatology also argue for little northward plate movement.The total evidence indicates that the turbidite Aleutian Abyssal Plain was formed in the Eocene-Oliogocene at, or near, its present position, and that the sediment source was probably Alaska. Cretaceous flysch of the Alaska Peninsula continental terrace was a possible source.The evidence does not require, but does not exclude, plate tectonics hypotheses. The evidence apparently excludes those continuous spreading models which cannot explain deposition of an Eocene-Oligocene turbidite plain over the magnetic bight, or which require an active, subducting, paleogene Aleutian Trench. Plate movements to the north over small distances cannot be excluded. The evidence is consistent with concepts of discontinuous sea-floor spreading with episodic subduction, or discontinuous, relative plate motion in this area. Two models are outlined which are consistent with the regional evidence: (1) a model with discontinuous relative plate motion and episodic subduction (a variation of one published by Hayes and Pitman, 1970); or (2) a no-plate-motion, or very-little-motion, model with long periods of inter-plate inactivity without subduction.  相似文献   

2.
Heck and Heckle are seamount chains trending approximately northwest on the western flank of Juan de Fuca Ridge near its northern end. Evidence from magnetic anomalies and from chemistry and relative ages of dredged basalt suggests that the seamounts in these chains are produced near the spreading centre on Juan de Fuca Ridge and do not continue to grow as they are carried away by sea-floor spreading. Their development is possibly related to transverse fractures on Juan de Fuca Ridge resulting from reorientation of the ridge from north to north-northeast which began about 8 m.y. ago, combined with tension in the Pacific Plate. In contrast the Eickelberg Chain to the south may have been produced by a fixed-mantle plume now located near Juan de Fuca Ridge, as suggested by limited basalt geochemistry and by the long and productive life of that chain. The Pratt-Welker Chain may also have been produced by a mantle plume, but most other seamounts on the western flanks of Juan de Fuca and Explorer ridges are thought to have formed at crustal fractures near the spreading centres in the same way as the seamounts of the Heck and Heckle chains.  相似文献   

3.
Magnetic and bathymetric studies on the Konkan basin of the southwestern continental margin of India reveal prominent NNW-SSE, NW-SE, ENE-WSW, and WNW-ESE structural trends. The crystalline basement occurs at about 5–6 km below the mean sea level. A mid-shelf basement ridge, a shelf margin basin, and the northern extension of the Prathap Ridge complex are also inferred. The forces created by the sea-floor spreading at Carlsberg Ridge since late Cretaceous appears to shape the present-day southwestern continental margin of India and caused the offsets in the structural features along the preexisting faults.  相似文献   

4.
Macquarie Island is composed of rocks petrologically similar to those formed at accreting ocean ridges. For this reason a geophysical study of the island and the surrounding ocean was undertaken to examine the possible presence of sea-floor spreading magnetic anomalies. The marine magnetic phase of the study allowed sea-floor spreading anomalies from the Australian plate to be fentatively correlated across Macquarie Island. Macquaric Island occurring about anomaly 7 time (27 m.y.b.p.). The palaeomagnetic phase of the study produced an apparent palaeomagnetic pole for the island consistent with the above. Modelling of the on-land magnetics indicates that the on-land magnetic data reflect dominantly the remanent magnetic properties of the rocks. Exact identification of the time-reversal sequence was not possible, however, because of distruption caused by major faulting on the island.  相似文献   

5.
南海北部陆缘位于特提斯与古太平洋两大构造域的叠合部位,构造特征十分复杂,其构造属性一直是国内外学者争论的焦点,从主动陆缘到被动陆缘,火山型被动陆缘到非火山型被动陆缘等均有表述。南海复杂的形成机制以及东、西部构造差异性所引起的地球物理、岩浆活动等认识的异同,是造成南海北部陆缘构造属性认识差异的主要原因。通过与全球典型地区的比较研究,进一步加强对南海形成演化过程分析,开展大洋钻探与多学科综合分析,揭示南海海盆的多期扩张与多盆张裂特征,是认识南海北部陆缘构造属性的关键。探讨了南海三叉裂谷张裂模式,初步认为南海第1次扩张具有非火山型被动陆缘性质,第2次扩张具有火山型陆缘性质。  相似文献   

6.
Abyssal hills were delineated in a 185 × 185-km area by an 18.5 × 18.5-km grid of narrow-beam bathymetric and geophysical profiles in oceanic crust of Cretaceous age near 23°N latitude, 31°W longitude. The abyssal hills are similar to features located along flow lines of sea-floor spreading near the crest of the Mid-Atlantic Ridge. This similarity indicates a primary origin for these abyssal hills related to axial processes at a mid-oceanic ridge involving construction (igneous) and tectonics (faulting), and secondary modification by volcanic activity.  相似文献   

7.
Morphology and tectonics of the Galapagos Triple Junction   总被引：1，自引：0，他引：1  
We describe the results of GLORIA and SEABEAM surveys, supplemented by other marine geophysical data, of the Galapagos Triple Junction where the Pacific, Cocos and Nazca plates meet. The data allowed detailed topographic and tectonic maps of the area to be produced. We located each spreading axis with a precision of about 1 km. All three plate boundaries change character as the triple junction is approached to take on morphologies typical of slower spreading axes: the fast-spreading East Pacific Rise develops the morphology of a medium-spreading rise, and the medium-spreading Cocos-Nazca Rise takes on the appearance of a slow-spreading ridge. The axis of the East Pacific Rise was found to be completely continuous throughout the survey area, where it runs along the 102°05 W meridian. The Cocos-Nazca axis, however, fails to meet it, leaving a 20-km-wide band of apparently normal East Pacific Rise crust between its tip and the East Pacific Rise axis. As a consequence there must be considerable intra-plate deformation within the Cocos and Nazca plates. A further 40 km of the Cocos-Nazca axis is characterised by oblique faulting that we interpret to be a sign of rifting of pre-existing East Pacific Rise crust. We infer that true sea-floor spreading on the Cocos-Nazca axis does not begin until 60 km east of the East Pacific Rise axis. Other areas of similar oblique faulting occur on the Pacific plate west of the triple junction and along the rough-smooth boundaries of the Galapagos Gore. We present a model involving intermittent rifting, rift propagation, and sea-floor spreading, to explain these observations.  相似文献   

8.
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   

9.
The physiography of the Amundsen and Bellingshausen Basins and adjacent continental margin of Antarctica is described based on all existing geologic/geophysical data. The sea-floor morphology is the result of a complex spreading history in this region which commenced in the pre-Cretaceous. Abyssal constructional forms including the location of the abyssal plains reflect this spreading history. Another decisive event was the initiation of the circum-Antarctic current and its resultant redistribution of the sedimentary blanket. The third major controlling factor was the commencement of polar conditions on Antarctica which effected a change in continental erosional patterns.  相似文献   

10.
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   

11.
Interpretation of reflection profiles across the Washington continental margin suggests deformation of Cascadia basin strata against the continental slope. Individual reflecting horizons can be traced across the slope-basin boundary. The sense of offset along faults on the continental slope is predominantly, but not entirely, west side up. Two faults of small displacement are seen to be west-dipping reverse faults. Magnetic anomalies on the Juan de Fuca plate can be traced 40–100 km eastward under the slope, and structural interpretation combined with calculated rates of subduction suggests that approximately 50 km of the outer continental slope may have been formed in Pleistocene time. Rocks of Pleistocene age dredge from a ridge exposing acoustic “basement” on the slope, plus the results of deep-sea drilling off northern Oregon, are consistent with this interpretation. The question of whether or not subduction is occurring at present is unresolved because significant strain has not affected the upper 200 m of section in the Cascadia basin. However, deformation of the outer part of the slope has been episodic and may reflect episodic yield, deposition rate, subduction rate, or some combination of these factors.  相似文献   

12.
In previous publications, the relationship between the Sirte Abyssal Plain as foreland and the Mediterranean Ridge as accretionary complex was considered to be simple: the foreland is undeformed, the accretionary complex consumes the foreland, the Messinian evaporites control the internal structure of the growing complex. The compilation of our own and published data results in a more complex tectonic pattern and a new geodynamic interpretation. The Sirte Abyssal Plain is imprinted by extensional tectonics which originated independently from and prior to the approaching process of accretion. The structural setting of the pre-Messinian and Messinian Sirte Abyssal Plain is responsible for the highly variable thickness of Messinian evaporites. The foreland setting in the Sirte Abyssal Plain also controls the internal structure of the Mediterranean Ridge, at least between the deformation front and Bannock Basin, following sediment deformation within the accretionary wedge with a dominating inherited SW-NE orientation. The taper angle of the post-Messinian Mediterranean Ridge is unusually small compared with other accretionary wedges. In the studied area, within a distance of about 45 km from the deformation front, there is no appreciable dip in the décollement. Therefore, the slope of the outer 45 km of the Mediterranean Ridge is considered to be caused only by gravitational spreading of Messinian evaporites deposited on the slope of pre-Messinian accretionary wedge. As a consequence, the Mediterranean Ridge underlying such slope is interpreted to belong to the foreland. The allochthonous evaporites overlie autochthonous evaporites of the Sirte Abyssal Plain. The NE-dipping décollement (and thus of the true tectonically driven deformation front) is expected to initiate at about the present position of Bannock Basin. The Sirte Abyssal Plain, the adjacent Cyrene Seamount and neighbouring seafloor relief on the African continental margin are considered to be the product of tectonic segmentation of the continental crust.  相似文献   

13.
南海北部陆缘张裂--岩石圈拆沉的地壳响应   总被引：4，自引：0，他引：4  
南海北部陆缘在中生代晚期曾形成宏伟的华夏陆缘造山带。火成岩岩石学、岩相古地理学和地球物理学证据显示，该造山带不仅具有巨厚(50～60 km)的陆壳，而且还有巨厚(160～180 km)的岩石圈根，在地势上曾出现过高3 500～4 000 m 的华夏山系。陆缘裂陷盆地的形成发育历史、地壳-岩石圈深部结构、火成岩地球化学特征及理论计算均表明，南海北部陆缘从晚白垩世以来发生的张裂作用起始于华夏陆缘造山带的拉伸塌陷，岩石圈拆沉是南海北部陆缘张裂的重要的引发机制。因此，南海北部陆缘张裂既不同于弧后扩张，也不受控于大西洋式的海底扩张，而是该区大陆构造演化和深部壳幔相互作用的结果。  相似文献   

14.
At 11°N latitude, the Mid-Atlantic ridge is offset 300 km by the Vema fracture zone. Between the ridge offset, the fracture consists of an elongate, parallelogram-shaped trough bordered on the north and south by narrow, high walls. The W-E trending valley floor is segmented by basement ridges and troughs which trend W10°N and are deeply buried by sediment. Uniform high heat flow characterizes the valley area. Seismically inactive valleys south of the Vema fracture, also trending W10°N, are interpreted as relict fracture zones. We explain the high heat flow and the shape of the Vema fracture as the results of secondary sea-floor spreading produced by a reorientation of the direction of sea-floor spreading from W10°N to west-east. This reorientation probably began approximately 10 million years ago. Rapid filling of the fracture valley by turbidites from the Demerara Abyssal plain took place during the last million years.The large amount of differential uplift in the Vema fracture is not explained by the reorientation model. Since the spreading rate across the valley is small compared to that across the ridge crest, we suggest that it takes place by intrusion of very thin dikes that cool rapidly and hence have high viscosity. Upwelling in the fracture valley will thus result in cosiderable loss of hydraulic head, according to models by Sleep and Biehler (1970), and recovery of the lost head could produce valley walls higher than the adjacent ridge crest. We further postulate that the spreading takes place along the edges of the fracture zone rather than in the center. This would account for the uniform distribution of heat flow along the fracture valley and for the lack of disturbance of the valley fill. As a consequence, a median ridge should form in the center, where head loss is compensated in the older crust; such a median ridge may be present. The width of the valley should be a function of the angle and time of reorientation, and of the spreading rate; the width so obtained for the Vema fracture is in accordance with the observed width. If this model is correct, the narrowness of the valley walls implies a thin lithosphere of very limited horizontal strength.  相似文献   

15.
The northern Norwegian-Greenland Sea opened up as the Knipovich Ridge propagated from the south into the ancient continental Spitsbergen Shear Zone. Heat flow data suggest that magma was first intruded at a latitude of 75° N around 60 m.y.b.p. By 40–50 m.y.b.p. oceanic crust was forming at a latitude of 78° N. At 12 m.y.b.p. the Hovgård Transform Fault was deactivated during a northwards propagation of the Knipovich Ridge. Spreading is now in its nascent stages along the Molloy Ridge within the trough of the Spitsbergen Fracture Zone. Spreading rates are slower in the north than the south. For the Knipovich Ridge at 78° N they range from 1.5–2.3 mm yr-1 on the eastern flank to 1.9–3.1 mm yr-1 on the western flank. At a latitude of 75° N spreading rates increase to 4.3–4.9 mm yr-1.Thermal profiles reveal regions of off-axial high heat flow. They are located at ages of 14 m.y. west and 13 m.y. east of the northern Knipovich Ridge, and at 36 m.y. on the eastern flank of the southern Knipovich Ridge. These may correspond to episodes of increased magmatic activity; which may be related to times of rapid north-wards rise axis propagation.The fact that the Norwegian-Greenland Sea is almost void of magnetic anomalies may be caused by the chaotic extrusion of basalts from a spreading center trapped within the confines of an ancient continental shear zone. The oblique impact of the propagating rift with the ancient shear zone may have created an unstable state of stress in the region. If so, extension took place preferentially to the northwest, while compression occurred to the southeast between the opening, leaking shear zone and the Svalbard margin. This caused faster spreading rates to the northwest than to the southeast.  相似文献   

16.
The continental breakup which gave way to the formation of the oceanic South China Sea (SCS) basin began in the latest Cretaceous in the northeastern SCS and propagated in southern and western direction over a long period of time, possibly more than 40 m.y. The seafloor spreading history of the South China Sea has been interpreted in different ways in the past and the debate over the correct timing of the major tectonic events continues. We review the different models that have been published and present a revised interpretation of seafloor spreading anomalies based on three datasets with documented high quality which cover all of the SCS but the northernmost and southernmost parts. We can precisely date the onset of seafloor spreading in the central part of the SCS at 32 Ma. After a ridge jump at 25 Ma spreading also began in the southwestern sub-basin and spreading ended at 20.5 Ma in the entire basin, followed by a phase of magmatic seamount formation mainly along the abandoned spreading ridge. Spreading rates vary from 56 mm/yr in the early stages to 72 mm/yr after the ridge jump to 80 mm/yr in the southwestern sub-basin. We find indications for a stepwise propagation of the seafloor spreading from northeast to southwest in segments bounded by major fracture zones. Seafloor spreading ended abruptly probably because the subduction zone along the eastern and southern boundary of the SCS (of which today the Manila Trench remains) was blocked by collision with a continental fragment, possibly the northern part of Palawan or a part of the Dangerous Grounds.  相似文献   

17.
  相似文献   

18.
The continental margin of Western Australia is a rifted or “Atlantic”-type margin, with a complex physiography. The margin comprises a shelf, an upper and lower continental slope, marginal plateaus, a continental rise, and rise or lower slope foothills. Notches or terraces on the shelf reflect pre-Holocene deposition of prograded sediment, whose seaward limit was determined by variations in relative sea level, wave energy, and sediment size and volume. The upper continental slope has four physiographic forms: convex, due to sediment outbuilding (progradation) over a subsiding marginal plateau; scarped, due to erosion of convex slopes; stepped, due to deposition at the base of a scarped slope; and smooth, due to progradation of an upper slope in the absence of a marginal plateau. Lying at the same level as the upper/lower slope boundary are two extensive marginal plateaus: Exmouth and Scott. They represent continental crust which subsided after continental rupture by sea-floor spreading. Differential subsidence, probably along faults, gave rise to the various physiographic features of the plateaus. The deep lower continental slope is broken into straight northeasterly-trending segments, that parallel the Upper Jurassic/Lower Cretaceous rift axis, and northwesterly-trending segments that parallel the transform direction. The trends of the slope foothills are subparallel to the rift direction. The four abyssal plains of the region (Perth, Cuvier, Gascoyne and Argo) indicate a long history of subsidence and sedimentation on Upper Jurassic/Lower Cretaceous oceanic crust.  相似文献   

19.
ALVIN investigations have defined the fine-scale structural and volcanic patterns produced by active rift and spreading center propagation and failure near 95.5° W on the Galapagos spreading center. Behind the initial lithospheric rifting, which is propagating nearly due west at about 50 km m.y.–1, a triangular block of preexisting lithosphere is being stretched and fractured, with some recent volcanism along curving fissures. A well-organized seafloor spreading center, an extensively faulted and fissured volcanic ridge, develops ~ 10 km (~ 200,000 years) behind the tectonic rift tip. Regional variations in the chemical compositions of the youngest lavas collected during this program contrast with those encompassing the entire 3 m.y. of propagation history for this region. A maximum in degree of magmatic differentiation occurs about 9 km behind the propagating rift tip, in a region of diffuse rifting. The propagating spreading center shows a gentle gradient in magmatic differentiation culminating at the SW-curving spreading center tip. Except for the doomed rift, which is in a constructional phase, tectonic activity also dominates over volcanic activity along the failing spreading system. In contrast to the propagating rift, failing rift lavas show a highly restricted range of compositions consistent with derivation from a declining upwelling zone accompanying rift failure. The lithosphere transferred from the Cocos to the Nazca plate by this propagator is extensively faulted and characterized by ubiquitous talus in one of the most tectonically disrupted areas of seafloor known. The pseudofault scarps, where the preexisting lithosphere was rifted apart, appear to include both normal and propagator lavas and are thus more lithologically complex than previously thought. Biological communities, probably vestimentiferan tubeworms, occur near the top of the outer pseudofault scarp, although no hydrothermal venting was observed.  相似文献   

20.
台西南盆地的构造演化与油气藏组合分析   总被引：14，自引：2，他引：14  
本文根据台西南盆地的地质、地球物理资料，对台西南盆地的地壳结构、基底特征、沉积厚度、断裂构造等基本地质构造特征＾［１］作了研究，探讨了台西南盆地的构造发展演化及及油气藏组合。认为该盆地的构造演化为幕式拉张。幕式拉张可分为三大张裂幕，相应的热沉降作用使盆地在不同的张裂幕时期发展为断陷，裂陷，裂拗－拗陷。它们分别与板块作用下的区域构造运动阶段相对应，说明区域构造运动不但控制了盆地的发展演化，同时也制约  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