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.
板块三节点     
三条板块边界常相交于一点，这一个交点就叫做板块三联接合点(简称三联点，或称三节点)。三联点相接的3个板块之间的边界可以是拉张型、挤压型或剪切型的边界，它们的组合类型比较复杂，而且稳定性不同，这对精细研究海底构造格局与演化具有重要的意义。三节点的稳定性取决于相邻板块运动的方向，一个空间位置稳定的三节点不一定在演化期间其构造活动性是稳定的或空间位置永远不变，一个不稳定的三节点必然发展到稳定的三节点。文中系统分析了海沟、洋脊以及转换断层的16种组合的几何学和相关三节点的稳定性。  相似文献   

2.
The analysis of multibeam bathymetric data of the Southwest Indian Ridge(SWIR) domain between the triple junction traces from 68° E to theRodrigues Triple Junction (RTJ; 70° E) reveals the evolution of thisridge since magnetic anomaly 4 (8 Ma). Image processing has been used toshow that the horizontal component of strain due to a network of normal stepfaults increases dramatically between 69°30 E and the RTJ. Thisarea close to the RTJ is characterized by a deep graben at the foot of thetriple junction trace on the African plate and by a narrow fault-boundedridge that joins an offset of the trace on the Antarctic plate. In thatarea, spreading is primarily amagmatic and dominated by tectonic extensionprocesses. To the west of 69°30 E, some lobate bathymetricfeatures atop of a large topographic high suggest volcanic constructions.Between 68°10 E and 69°25 E the southern flank of theSWIR domain is wider than the northern one and is characterized by a series of 7 en echelon bathymetric highs similar in size,shape and orientation to the one centred at 69°30E near the present-day triple junction. Their en echelon organization along the triple junction trace on the Antarctic plate and the typical lack of conjugated parts on the northern flank show that these bathymetric highs have been shifted to the south by successive northward relocalisations of the SWIR rifting zone. This evolution results in the asymmetric spreading of the SWIR in the survey area. The off-axis bathymetric highs connect to the offsets of the triple junction trace on the Antarctic plate when the Southeast Indian Ridges lightly lengthenstoward the northwest and the triple junction is relocated to the north. We propose that the SWIR lengthens toward the northeast with two propagation modes: 1) a continuous and progressive propagation with distributed deformation in preexisting crust of the Central Indian Ridge, 2) a discontinuous propagation with focusing of the deformation in a rift zone when the triple junction migrates rapidly to the north. The modes of propagation of the SWIR are related to different localisation and distribution of strain which are in turn controlled by changes of the triple junction configurations due to propagation, recession or a symmetric spreading on the Central and Southeast Indian Ridges.  相似文献   

3.
A 2°×2° map of spreading centres and fracture zones surrounding the Indian Ocean RRR triple junction, at 25.5°S, 70°E, is described from a data set of GLORIA side-scan sonar images, bathymetry, magnetic and gravity anomalies. The GLORIA images show a pervasive fabric due to linear abyssal hills oriented parallel to the two medium-spreading ridges (the Central Indian Ridge (CIR) and Southeast Indian Ridge (SEIR)). A cuvature of the fabric occurs along fracture zones, which are also located by lows in the bathymetry and gravity data and by offsets between magnetic anomalies. The magnetic anomalies also record periods of asymmetric spreading marking the development of the fracture zones, including the birth, at anomaly 2A, of a short fracture zone 50 km north of the triple junction on the CIR, and its death near the time of the Jaramillo anomaly. In some localities, a fine-scale fabric corresponds to a coarser fabric on the opposite flank of the CIR, possibly indicating a persistent asymmetry in the faulting at the median valley walls if the fabric has a tectonic and not a volcanic origin. A plate velocity analysis of the triple junction shows that both the CIR and Southwest Indian Ridge (SWIR) are propagating obliquely; the CIR appears to form an oblique trend by segmenting into a series of almost normally-oriented segments separated by short-offset fracture zones. For the last 4 m.y., the abyssal hill lineations indicate that the CIR segment immediately north of the triple junction has been spreading with an average 10° obliquity. The present small 5 km offset of the centres of the CIR and SEIR median valleys (Munschy and Schlich, 1989) is shown to be the result of this obliquity and a 30% spreading asymmetry between anomaly 2 and the Jaramillo on the CIR segment immediately north of the triple junction.  相似文献   

4.
The junction between oceanic crust generated, within the Antarctic plate, at the Southeast Indian Ridge and the Southwest Indian Ridge has been studied using a SEABEAM swathe bathymetry mapping system and other geophysical techniques between the Indian Ocean Triple Junction (approximately 25°S, 70° E), and a point some 500 km to the southwest (at 28°25 S, 66°35 E). The morphotectonic boundary which marks this trace of the ridge-ridge-ridge triple junction is complex and varies with age. Recent theories proposing a cyclicity of volcanic and tectonic processes at this mode of triple junctions appear to be supported by a series of regularly spaced, en echelon escarpments facing the slowly spreading (0.6 to 0.8 cm a-1, half rate) Southwest Indian Ridge axis. The en echelon escarpments intersect at approximately right angles with the regularly spaced oceanic spreading fabric formed on the Antarctic plate at the Southeast Indian Ridge and together locally flank uplifted northward-pointing corner sections of ocean floor. The origins for the localised elevations are unclear, but may relate to intermittent and/or alternating rifting and volcanic episodes. Variations of degree of asymmetry and/or obliquity in spreading on the Central Indian Ridge and the Southwest Indian Ridge are suggested to explain detailed structural changes along the triple junction trace. It is suggested that discontinuities of the trace may be related to an intermittent development of new spreading centres beneath the most easterly part of the Southwest Indian Ridge, coupled with a more continuous process beneath the faster spreading Central Indian Ridge (2 to 2.5 cm a-1) and the Southeast Indian Ridge (2.5 to 3 cm a-1). A detailed history of triple junction evolution may be thus inferred from basic morphological and structural mapping along the three triple junction traces.  相似文献   

5.
The geography of the East Pacific Rise (EPR) between 10°N and 6°S, redetermined by new surface ship surveys, is characterized by long spreading axes orthogonal to infrequent transform faults. Near 2°10N the EPR is intersected by the Cocos-Nazca spreading center at the Galapagos triple junction. The present pattern was established 27-5.5 m.y.b.p. by a complex sequence of rise-crest jumps and reorientations from a section of the Pacific-Farallon plate boundary. Transverse profiles of the rise flanks can be matched by thermal contraction curves for aging lithosphere, except between the triple junction and 4°S, where the east flank is anomalously shallow and almost horizontal. Most sections of spreading axis have the 10–30 km wide, 100–400 m high, axial ridge that is characteristic of fast spreading centers. However, within 60 km of the triple junction the rise crest structure is atypical, with an axial rift valley and elevated rift mountains, despite a spreading rate of 140 mm/yr. With the exception of this atypical section, the bathymetric profile along the spreading axis is remarkably even, with continuous, gentle slopes for hundreds of kilometers between major transform faults, where step-like offsets in axial depths occur. Most of the observations can be accommodated by a model in which the long spreading axes are underlain by continuous crustal magma chambers that allow easy longitudinal flow of magma, and whose size controls the style and dimensions of EPR crestal topography.Contribution of the Scripps Institution of Oceanography, new series.  相似文献   

6.
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   

7.
The study of very low-spreading ridges has become essential to ourunderstanding of the mid-oceanic ridge processes. The Southwest Indian Ridge(SWIR) , a major plate boundary of the world oceans, separating Africa fromAntarctica for more than 100 Ma, has such an ultra slow-spreadingrate. Its other characteristic is the fast lengthening of its axis at bothBouvet and Rodrigues triple junctions. A survey was carried out in thespring of 1993 to complete a multibeam bathymetric coverage of the axisbetween Atlantis II Fracture Zone (57° E) and the Rodrigues triplejunction (70° E). After a review of what is known about the geometry,structure and evolution of the SWIR, we present an analysis of the newalong-axis bathymetric data together with previously acquiredacross-axis profiles. Only three transform faults, represented byAtlantis II FZ, Novara FZ, and Melville FZ, offset this more than 1000 kmlong section of the SWIR, showing that the offsets are more generallyaccommodated by ridge obliquity and non-transform discontinuities. From comparison of the axial geometry, bathymetry, mantle Bouguer anomaly and central magnetic anomaly, three large sections (east of Melville FZ, between Melville FZ and about 65°30 E, and from there to the Rodrigues triple junction) can be distinguished. The central member, east of Melville FZ, does not resemble any other known mid-oceanic ridge section: the classical signs of the accretion (mantle Bouguer anomaly, central magnetic anomaly) are only observed over three very narrow and shallow axis sections. We also apply image processing techniques to the satellite gravity anomaly map of Smith and Sandwell (1995) to determine the off-axis characteristics of the Southwest Indian Ridge domain, more especially the location of the triple junction and discontinuities traces. We conclude that the large-scale segmentation of the axis has been inherited from the evolution of the Rodrigues triple junction.  相似文献   

8.
During the last three years, the North Fiji Basin (SW Pacific) has been intensively studied on three oceanographic cruises carried out by French, American and Japanese ships. One of the main goals of these cruises was to study by means of precise SeaBeam, SEAMARC II, seismic and magnetic surveys, the active spreading system and its associated hydrothermal processes. The North Fiji basin, bounded by the major Pacific and Indo-Australian plates, shows a complex polyphased tectonic evolution. One of the last phases of this evolution is the functioning since 3 Ma of a NS spreading center in the axial part of the basin. The tectonic instability of the area resulted in a permanent rearrangement of the ridge axis. Among others, the 16°40′ S triple junction is one of the major manifestations of such an instability. Sinistral strike-slip motion 1 Ma ago, along the North Fiji Fracture Zone induced the change in direction of two segments of the axis from NS to N15 and N160. The first segment is characterized by a typical spreading ridge similar to various parts of the EPR, while the second shows an atypical ‘en echelon’ fan-shape opening. The N15 and N160 ridges converging with the North Fiji Fracture Zone constitute the 16°40′ S Ridge-Ridge-Fracture Zone triple junction. The detailed morphologic and kinematic study of this junction allows us to understand one of the mechanisms of the deformation in the North Fiji basin.  相似文献   

9.
The Woodlark triple junction region, a topographically and structurally complex triangular area of Quaternary age, lies east of Simbo Ridge and southwest of the New Georgia island group, Solomon Islands, at the junction of the Pacific, Australian and Solomon Sea plates. SeaMARC II side-scan imagery and bathymetry in conjunction with seismic reflection profiles, 3.5 kHz records, and petrologic, magnetic and gravity data show that the active Woodlark spreading centre does not extend into this region.South of the triple junction region, the Woodlark spreading centre reoriented at about 2 Ma into a series of short ESE-trending segments. These segments continued to spread until about 0.5 Ma, when the lithosphere on their northern sides was transferred from the Solomon Sea plate to the Australian plate. Simultaneously the Simbo transform propagated northwards along the western side of the transferred lithosphere, forming a trench-trench-transform triple junction located NNW of Simbo island and a new leaky plate boundary segment that built Simbo Ridge.As the Pacific plate approached, the area east of northern Simbo Ridge was tilted northwards, sheared by dominantly right-lateral faults, elevated, and intruded by arc-related magmas to form Ghizo Ridge. Calc-alkalic magmas sourced beneath the Pacific plate built three large strato-volcanic edifices on the subducting Australian plate: Simbo at the northern end of Simbo Ridge, and Kana Keoki and Coleman seamounts on an extensional fracture adjoining the SE end of Ghizo Ridge.A sediment drape, supplied in part from Simbo and Kana Keoki volcanoes, mantles the east-facing slopes of northern Simbo and Ghizo Ridges and passes distally into sediment ponded in the trench adjoining the Pacific plate. As a consequence of plate convergence, parts of the sediment drape and pond are presently being deformed, and faults are dismembering Kana Keoki and Coleman seamounts.The Woodlark system differs from other modern or Tertiary ridge subduction systems, which show wide variation in character and behaviour. Existing models describing the consequences of ridge subduction are likely to be predictive in only a general way, and deduced rules for the behaviour of oceanic lithosphere in ridge subduction systems may not be generally applicable.  相似文献   

10.
This paper reports a comparative study of digital enhancement techniques using spatial filtering to improve the geologic interpretation of side-scan sonar GLORIA images. Seven algorithms for speckle reduction with window sizes of 3×3-7×7 pixel and various numbers of iterations were tested for cosmetic purposes, and also to improve subsequent image processing. The filtered images were evaluated using both quantitative and qualitative techniques. It was determined that a normalized inverse gradient weighted smoothing scheme, with a 3×3 pixel filter and five iterations, allows a significant speckle reduction without blurring the edges in the GLORIA image which correspond to geological structures. Three local contrast enhancement techniques were also tested and evaluated to increase the perception of these geologic structures. Subtracting the gradient magnitude twice, calculated with spatial filters of a 5×5 pixel on smoothed images, was found to enhance most GLORIA images. Texture analysis methods developed for GLORIA images of mid-oceanic ridges and based on edge detection and orientation determination by spatial filtering are also presented. It enables the GLORIA mosaic of the Rodriguez triple junction (Indian Ocean) to be partitioned into regions of preferred orientation corresponding to the different seafloor fabrics generated at each arm of the triple junction  相似文献   

11.
The southern end of the Shackleton Fracture Zone is subducted below the South Shetland forearc, while the basal detachment of the forearc continues eastward of the Shackleton ridge as a thrust fault. The western boundary between the Antarctic/Scotia plates is located at the eastern margin of the Shackleton ridge, where an elongated depositional basin and a morphological trough delineates the contact. The boundary of the Antarctic/Scotia plates and the South Shetland forearc form a triple junction at the intersection of the Shackleton Fracture Zone with the trench, between these two plates, and an independent South Shetland block.  相似文献   

12.
It is suggested that Bouvet Island is the surface manifestation of a mantle plume which has resulted in the creation of a chain of seamounts in the South Atlantic and a general shoaling of the region.The strike of two newly defined large fracture zones borth and south of the Bouvet Island pedestal have been utilized to determine a pole of rotation at 12.5°S, 12.5°W for the Africa-Antarctica motion. A pole at 75°S, 13°E has been calculated for the South America-Antarctica motion. At the triple junction the South America-Africa relative motion is 3.3 cm yr-1 (whole rate) at 075°. The Africa-Antarctica motion is 1.7 cm yr-1 at 065° and the South America-Antarctica motion is 1.6 cm yr-1 at 085.  相似文献   

13.
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   

14.
GLORIA imagery of the Lau Basin north of 17°S shows several morphotectonic terrains: a basement ridge and sedimented inter-ridge area in the SE; a nascent triple junction in the NE; a deeply sedimented basinal terrain in the central area; a linear neovolcanic zone striking NNESSW in the NW; and the northern flank of a leaky transform, the Peggy Ridge. Extension is now being accommodated at two main areas of spreading, but as no site of persistent long-term backarc crustal accretion is evident in this 250-km-wide portion of the basin, we conclude that past extension was largely by formation of pull-apart basins and local magmatism.  相似文献   

15.
The geologic history of the eastern Indian Ocean between northwest Australia and the Java Trench is known to involve two separate events of rifting and sea-floor spreading. Late Jurassic spreading in the Argo Abyssal Plain off northwest Australia was followed by Early Cretaceous spreading in the Cuvier and Perth Abyssal Plains off west Australia. However, the evolution and interaction of these events has not been clear. Mesozoic sea-floor spreading anomalies have been identified throughout the Argo Abyssal Plain that define a rifting event and subsequent northward spreading on the northwestern Australian margin at 155 m.y.b.p. Magnetic anomalies northwest of the Argo Abyssal Plain indicate a ridge jump to the south at about 130 m.y.b.p. that is approximately synchronous with east-west rifting along the southwestern Australian margin. The Joey Rise in the Argo Plain was probably formed by volcanism at the intersection of this new rift and the spreading ridge to the north. The southern and northern spreading systems were connected through the Exmouth Plateau which was stretched and faulted as spreading progressed. The RRR triple junction was formed at the intersection of the two spreading systems and appears to have migrated west along the northern edge of the Gascoyne Abyssal Plain. Spreading off northwest Australia cannot be easily related to simultaneous spreading in the west central Pacific via any simple tectonic scheme.  相似文献   

16.
GLORIA side-scan imagery from the northern North Fiji Basin reveals modern and relict sea-floor fabric. The South Pandora Ridge is marked by steep escarpments and small rift basins, but no recent volcanism. The northern and eastern limbs of the 16°58S, 173°55E triple junction are marked by rift grabens flanked by steep escarpments, but little recent volcanism is apparent there. At present, there is no well-organized spreading system in the northern North Fiji Basin; extension and shearing are occurring within narrowly confined areas. It is uncertain how these areas relate to one another and fit into the regional tectonic framework.  相似文献   

17.
利用漂移扩散理论研究了磁性pn结中自旋的输运特性.探讨了外加电压、平衡自旋极化率、外加自旋注入和自旋寿命对磁性pn结电流密度和电阻的影响,讨论了磁性pn结自旋伏特效应与pn结宽度的关系.发现平衡自旋极化率使得不同自旋方向电子具有不同的势垒高度从而能有效调制电流;而外加自旋注入则为磁性pn结提供了非平衡自旋极化电子从而达到对电流的调制作用,同时发现自旋伏特电流随准中性p区宽度减小而增大.  相似文献   

18.
Spreading cycles in the Pacific Ocean   总被引：1，自引：0，他引：1  
Changes in the spreading rates in the Pharallon-Pacific-Izanagi (Kula) triple junction during the Cretaceous and Cenozoic are revised using new data of the dynamics of the Pacific plate. The cyclic character of the spreading is recognized, and the stages of its acceleration and deceleration are distinguished. Approximately 130, 87, and 42 My B.P., at the culminations of the cycles, when maximal spreading rates were reached, the principal rearrangements in the tectonic evolution of the ocean occurred. The spreading rates were minimal about 140, 120, 65, and 15 My B.P. The latter periods are marked by pulses of basalt magmatism in the west, east, and northeast of the Pacific Ocean. The study recognized no signs of the intrusion of the Middle Cretaceous superplume, which was suggested by Larson. Both the cycling revealed and the pulsations of the intraplate volcanism are most probably related to the regularities of the deformations of the oceanic lithosphere and reflect the periodic alternations of regimes of compression and extension of the Pacific plate during the last 180 My.  相似文献   

19.
This paper reports that the 4H-SiC Schottky barrier diode, PiN diode and junction barrier Schottky diode terminated by field guard rings are designed, fabricated and characterised. The measurements for forward and reverse characteristics have been done, and by comparison with each other, it shows that junction barrier Schottky diode has a lower reverse current density than that of the Schottky barrier diode and a higher forward drop than that of the PiN diode. High-temperature annealing is presented in this paper as well to figure out an optimised processing. The barrier height of 0.79 eV is formed with Ti in this work, the forward drop for the Schottky diode is 2.1 V, with an ideality factor of 3.2, and junction barrier Schottky diode with blocking voltage higher than 400 V was achieved by using field guard ring termination.  相似文献   

20.
Abstract

The South China Sea is located within the domain of a plate triple‐junction and can be divided into five major geotectonic blocks that control the formation and distribution of the mineral resources of the region: (1) the southern China faulted block, (2) the eastern Indochina faulted block, (3) the Nansha‐Borneo faulted block, (4) the Taiwan‐Luzon faulted block, and (5) the central ocean basin faulted block. Apart from oil and gas, the most intensively exploited mineral deposits in the South China Sea are near‐shore placer minerals of titaniferous magnetite, zircon, monazite, tin, gold, and chromite. Based on analyses of submarine morphology and sea level change during the past 15,000 years, the South China Sea continental shelves are considered to be highly prospective for additional placer occurrence associated with such submarine features as: submerged platforms and terraces, drowned rivers and sand bars, ancient beaches, and seafloors covered by relict sediments. Additionally, based on available data, polymetallic sulfides and manganese nodules and crusts are considered as speculative resources of the future in the South China Sea.  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