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1.
Throughout the Early Tertiary the area of the Farallon oceanic plate was episodically diminished by detachment of large and small northern regions, which became independently moving plates and microplates. The nature and history of Farallon plate fragmentation has been inferred mainly from structural patterns on the western, Pacific-plate flank of the East Pacific Rise, because the fragmented eastern flank has been subducted. The final episode of plate fragmentation occurred at the beginning of the Miocene, when the Cocos plate was split off, leaving the much reduced Farallon plate to be renamed the Nazca plate, and initiating Cocos–Nazca spreading. Some Oligocene Farallon plate with rifted margins that are a direct record of this plate-splitting event has survived in the eastern tropical Pacific, most extensively off northern Peru and Ecuador. Small remnants of the conjugate northern rifted margin are exposed off Costa Rica, and perhaps south of Panama. Marine geophysical profiles (bathymetric, magnetic and seismic reflection) and multibeam sonar swaths across these rifted oceanic margins, combined with surveys of 30–20 Ma crust on the western rise-flank, indicate that (i) Localized lithospheric rupture to create a new plate boundary was preceded by plate stretching and fracturing in a belt several hundred km wide. Fissural volcanism along some of these fractures built volcanic ridges (e.g., Alvarado and Sarmiento Ridges) that are 1–2 km high and parallel to “absolute” Farallon plate motion; they closely resemble fissural ridges described from the young western flank of the present Pacific–Nazca rise. (ii) For 1–2 m.y. prior to final rupture of the Farallon plate, perhaps coinciding with the period of lithospheric stretching, the entire plate changed direction to a more easterly (“Nazca-like”) course; after the split the northern (Cocos) part reverted to a northeasterly absolute motion. (iii) The plate-splitting fracture that became the site of initial Cocos–Nazca spreading was a linear feature that, at least through the 680 km of ruptured Oligocene lithosphere known to have avoided subduction, did not follow any pre-existing feature on the Farallon plate, e.g., a “fracture zone” trail of a transform fault. (iv) The margins of surviving parts of the plate-splitting fracture have narrow shoulders raised by uplift of unloaded footwalls, and partially buried by fissural volcanism. (v) Cocos–Nazca spreading began at 23 Ma; reports of older Cocos–Nazca crust in the eastern Panama Basin were based on misidentified magnetic anomalies.There is increased evidence that the driving force for the 23 Ma fission of the Farallon plate was the divergence of slab-pull stresses at the Middle America and South America subduction zones. The timing and location of the split may have been influenced by (i) the increasingly divergent northeast slab pull at the Middle America subduction zone, which lengthened and reoriented because of motion between the North America and Caribbean plates; (ii) the slightly earlier detachment of a northern part of the plate that had been entering the California subduction zone, contributing a less divergent plate-driving stress; and (iii) weakening of older parts of the plate by the Galapagos hotspot, which had come to underlie the equatorial region, midway between the risecrest and the two subduction zones, by the Late Oligocene. 相似文献
2.
Recent high-resolution models of past plate motions and their comparison with plate motion models inferred from space geodetic techniques reveal a number of short-term variations in global plate velocities over the past 10 Myrs. Such variations serve as powerful probe into the nature and magnitude of plate boundary forces, because they are unlikely to originate from changes in mantle buoyancy forces, which evolve on longer time scales. Here we explore the constraints of the velocity record using a novel coupled modeling-approach of global neo-tectonic simulations combined with realistic plate driving forces obtained from mantle circulation models (MCMs) to arrive at simple global budgets of mantle, lithosphere and plate boundary forces. We focus on three plate boundary systems along the Nazca/South America plate margin, the Aleutian trench and the India/Australia plate boundary to show that gravitational spreading from high topography in the Andes and Tibet contributes substantially to the global plate tectonic force balance and that this contribution is sufficient to explain some 35% of recent velocity changes over the Earth's surface, including among others the observed 30% convergence reduction between the Nazca/South America plates. Our models make a number of specific predictions such as significant lateral variations in plate coupling forces along a given margin revealed by trench-parallel gravity and bathymetry anomalies and the occurrence of large earthquakes, as well as differences by as much as a factor of five from margin to margin. They also support the notion of a relatively young plate boundary separating the India and Australia plates, which has been previously suggested based on independent observations. Importantly, we find that the modeled Nazca/South America convergence reduction explains recent spreading-rate variations in the South Atlantic and South Pacific, which points to the importance of far field effects on the adjacent continents in explaining the spreading record of oceanic basins. Our numerical results demonstrate (a) that detailed budgets of forces acting upon plates can be obtained and (b) support the notion of strong forcing along weak plate boundaries. 相似文献
3.
A multiyear solution of the SIRGAS-CON network was used to estimate the strain rates of the earth surface from the changing directions of the velocity vectors of 140 geodetic points located in the South American plate. The strain rate was determined by the finite element method using Delaunay triangulation points that formed sub-networks; each sub-network was considered a solid and homogeneous body. The results showed that strain rates vary along the South American plate and are more significant on the western portion of the plate, as expected, since this region is close to the subduction zone of the Nazca plate beneath the South American plate. After using Euler vectors to infer Nazca plate movement and to orient the velocity vectors of the South American plate, it was possible to estimate the convergence and accommodation rates of the Nazca and South American plates, respectively. Strain rate estimates permitted determination of predominant contraction and/or extension regions and to establish that contraction regions coincide with locations with most of the high magnitude seismic events. Some areas with extension and contraction strains were found to the east within the stable South American plate, which may result from different stresses associated with different geological characteristics. These results suggest that major movements detected on the surface near the Nazca plate occur in regions with more heterogeneous geological structures and multiple rupture events. Most seismic events in the South American plate are concentrated in areas with predominant contraction strain rates oriented northeast-southwest; significant amounts of elastic strain can be accumulated on geological structures away from the plate boundary faults; and, behavior of contractions and extensions is similar to what has been found in seismological studies. 相似文献
4.
The Andaman arc in the northeastern Indian Ocean defines nearly 1100 km long active plate margin between the India and Burma plates where an oblique Benioff zone develops down to 200 km depth. Several east-trending seismologic sections taken across the Andaman Benioff Zone (ABZ) are presented here to detail the subduction zone geometry in a 3-D perspective. The slab gravity anomaly, computed from the 3-D ABZ configuration, is a smooth, long-wavelength and symmetric gravity high of 85 mGal amplitude centering to the immediate east of the Nicobar Island, where, a prominent gravity “high” follows the Nicobar Deep. The Slab-Residual Gravity Anomaly (SRGA) and Mantle Bouguer Anomaly (MBA) maps prepared for the Andaman plate margin bring out a double-peaked SRGA “low” in the range of − 150 to − 240 mGal and a wider-cum-larger MBA “low” having the amplitude of − 280 to − 315 mGal demarcating the Andaman arc–trench system. The gravity models provide evidences for structural control in propagating the rupture within the lithosphere. The plate margin configuration below the Andaman arc is sliced by the West Andaman Fault (WAF) as well as by a set of sympathetic faults of various proportions, often cutting across the fore-arc sediment package. Some of these fore-arc thrust faults clearly give rise to considerably high post-seismic activity, but the seismic incidence along the WAF further east is comparatively much less particularly in the north, although, the lack of depth resolution for many of the events prohibits tracing the downward continuity of these faults. Tectonic correlation of the gravity-derived models presented here tends to favour the presence of oceanic crust below the Andaman–Nicobar Outer Arc Ridge. 相似文献
5.
《International Geology Review》2012,54(5):437-447
Subduction-zone magmatism became extensive along the west coast of South America during the Ordovician, soon after Gondwana was assembled. During the remainder of the Paleozoic and the early Mesozoic, eastward subduction of the Farallon plate led to emplacement of a succession of granitic and volcanic rocks. During the Cretaceous, when South America broke away from Africa and began moving independently toward the Pacific Basin, the resulting opposite motions of the South American and Farallon plates toward the subduction zone caused vigorous tectonic mountain building. But by the Oligocene, South America had advanced more than 2000 km beyond the position of the Cretaceous subduction zone's root in the lower mantle. The South American plate, moving westward over the subducting plate, pushed down and flattened the curved top of the subducting slab, as indicated by today's flattened earthquake zone under South America. I hypothesize that this flattening increased the subducting slab's resistance with the underlying lower mantle. Crustal deformation slowed, and the mountains built during the Cretaceous and later were eroded to a peneplane. During the Oligocene, about 25 Ma, the Farallon plate broke into the Cocos and Nazca plates, and I suggest that along the west coast of South America a shear at a slope of about 30° cut through the subducting slab. The oceanic (Nazca) part of the slab then entered the lower mantle below the Andes with a steeper dip than before. As the newly sheared obtuse upper corner of the Nazca plate pushed eastward and downward, it buckled the rigid edge of the continent and began the folding and thrusting of the Andean (Quechua) orogeny. The orogeny continues, but earthquake foci indicate that as South America continues to move westward, the subduction zone once again is flattening; in the future we can expect the Nazca slab to shear once more and its new wedge-shaped end to enter the lower mantle again. 相似文献
6.
Seismic coupling and uncoupling at subduction zones 总被引：1，自引：0，他引：1
Seismic coupling has been used as a qualitative measure of the “interaction” between the two plates at subduction zones. Kanamori (1971) introduced seismic coupling after noting that the characteristic size of earthquakes varies systematically for the northern Pacific subduction zones. A quantitative global comparison of many subduction zones reveals a strong correlation of earthquake size with two other variables: age of the subducting lithosphere and convergence rate. The largest earthquakes occur in zones with young lithosphere and fast convergence rates, while zones with old lithosphere and slow rates are relatively aseismic for large earthquakes. Results from a study of the rupture process of three great earthquakes indicate that maximum earthquake size is directly related to the asperity distribution on the fault plane (asperities are strong regions that resist the motion between the two plates). The zones with the largest earthquakes have very large asperities, while the zones with smaller earthquakes have small scattered asperities. This observation can be translated into a simple model of seismic coupling, where the horizontal compressive stress between the two plates is proportional to the ratio of the summed asperity area to the total area of the contact surface. While the variation in asperity size is used to establish a connection between earthquake size and tectonic stress, it also implies that plate age and rate affect the asperity distribution. Plate age and rate can control asperity distribution directly by use of the horizontal compressive stress associated with the “preferred trajectory” (i.e. the vertical and horizontal velocities of subducting slabs are determined by the plate age and convergence velocity). Indirect influences are many, including oceanic plate topography and the amount of subducted sediments.All subduction zones are apparently uncoupled below a depth of about 40 km, and we propose that the basalt to eclogite phase change in the down-going oceanic crust may be largely responsible. This phase change should start at a depth of 30–35 km, and could at least partially uncouple the plates by superplastic deformation throughout the oceanic crust during the phase change. 相似文献
7.
Antonio Introcaso Myriam P. Martínez Mario E. Gimnez Francisco Ruiz 《Gondwana Research》2004,7(4):1117-1132
A gravimetric and magnetometric study was carried out in the north-eastern portion of the Cuyania terrane and adjacent Pampia terrane. Gravimetric models permitted to interpret the occurrence of dense materials at the suture zone between the latter terranes. Magnetometric models led to propose the existence of different susceptibilities on either side of the suture. The Curie temperature point depth, representing the lower boundary of the magnetised crust, was found to be located at 25 km, consistent with the lower limit of the brittle crust delineated by seismic data; this unusually thick portion of the crust is thought to release stress producing significant seismicity.
Moho depths determined from seismic studies near western Sierras Pampeanas are significantly greater than those obtained from gravimetric crustal models.
Considering mass and gravity changes originated by the flat-slab Nazca plate along Cuyania and western Pampia terranes, it is possible to reconcile Moho thickness obtained either by seismic or by gravity data. Thus, topography and crustal thickness are controlled not only by erosion and shortening but by upper mantle heterogeneities produced by: (a) the oceanic subducted Nazca plate with “normal slope” also including asthenospheric materials between both continental and oceanic lithospheres; (b) flat-slab subducted Nazca plate (as shown in this work) without significant asthenospheric materials between both lithospheres. These changes influence the relationship between topographic altitudes and crustal thickness in different ways, differing from the simple Airy system relationship and modifying the crustal scale shortening calculation. These changes are significantly enlarged in the study area. Future changes in Nazca Plate slope will produce changes in the isostatic balance. 相似文献
8.
Current states of stress in the northern Andes as indicated by focal mechanisms of earthquakes 总被引：1，自引：3，他引：1
Systematic inversion of double couple focal mechanisms of shallow earthquakes in the northern Andes reveals relatively homogeneous patterns of crustal stress in three main regions. The first region, presently under the influence of the Caribbean plate, includes the northern segment of the Eastern Cordillera of Colombia and the western flank of the Central Cordillera (north of 4°N). It is characterized by WNW–ESE compression of dominantly reverse type that deflects to NW–SE in the Merida Andes of Venezuela, where it becomes mainly strike–slip in type. A major bend of the Eastern thrust front of the Eastern Cordillera, near its junction with the Merida Andes, coincides with a local deflection of the stress regime (SW–NE compression), suggesting local accommodation of the thrust belt to a rigid indenter in this area. The second region includes the SW Pacific coast of Colombia and Ecuador, currently under the influence of the Nazca plate. In this area, approximately E–W compression is mainly reverse in type. It deflects to WSW–ENE in the northern Andes south of 4°N, where it is accommodated by right-lateral displacement of the Romeral fault complex and the Eastern front of the northern Andes. The third, and most complex, region is the area of the triple junction between the South American, Nazca and Caribbean plates. It reveals two major stress regimes, both mainly strike–slip in type. The first regime involves SW–NE compression related to the interaction between the Nazca and Caribbean plates and the Panama micro-plate, typically accommodated in an E–W left-lateral shear zone. The second regime involves NW–SE compression, mainly related to the interaction between the Caribbean plate and the North Andes block which induces left-lateral displacement on the Uramita and Romeral faults north of 4°N.Deep seismicity (about 150–170 km) concentrates in the Bucaramanga nest and Cauca Valley areas. The inversion reveals a rather homogeneous attitude of the minimum stress axis, which dips towards the E. This extension is consistent with the present plunge of the Nazca and Caribbean slabs, suggesting that a broken slab may be torn under gravitational stresses in the Bucaramanga nest. This model is compatible with current blocking of the subduction in the western northern Andes, inhibiting the eastward displacement of slabs, which are forced to break and sink in to the asthenosphere under their own weight. 相似文献
9.
全球火山活动分布特征 总被引：14，自引：0，他引：14
根据全球活动火山目录 ,分析研究了全球火山分布的特征 ,描述了各区的火山活动分布 ,总结了火山活动强度的时、空分布特征。全球火山活动可分为三大区 ,西太平洋火山活动区 ,主要与太平洋板块向北西西方向的俯冲活动有关 ;东太平洋火山活动区 ,主要与太平洋东面的小板块 (胡安德富卡板块、科科斯、纳斯卡板块 )向美洲板块的俯冲有关 ;大西洋火山活动区 ,与大西洋和非洲的裂开 ,以及地中海带的活动有关。不同火山区带具有各自的最大喷发等级与相应的复发周期。一条火山弧上活动强度的分布往往是不对称的 ,意味着火山弧在整体上有其动力学的控制机理。火山活动显示了随纬度成带状分布。在 - 10～ 0° ,10 2 0° ,30 4 0°,5 0 6 0°分布有高值带。火山喷发活动还与当地的重力势有关 ,重力势正异常可能与高的正压力有关 ,有利于产生特大喷发。火山活动与大角度的正面俯冲带的弧后火山活动最强 ,当板块运动方向与板块边缘走向成小角度相交时 ,缺少正面俯冲的动力 ,火山活动相对平静。 相似文献
10.
Oceanic crust west of North America at the beginning of the Jurassic belonged to the Kula plate. The development of the western margin of North America since the Jurassic reflects interaction with the Kula plate, the Kula-Farallon spreading center and the Farallon plate. The Kula plate ceased to exist in the Paleocene and later developments were caused by interaction of the Farallon plate and, subsequently, collision with the East Pacific Rise.At the beginning of the Jurassic, when spreading between North and South America began, the Kula-Farallon-Pacific triple junction moved to the north relative to North America, and the eastern end of the Kula-Farallon spreading center swept northwards along the continental margin.During the Paleocene, Kula-Pacific spreading ceased and the Kula plate fused to the Pacific plate. Throughout the Mesozoic, subduction of the Kula plate took place along the Alaskan continental margin. When the Kula plate joined the Pacific plate a new subduction zone formed along the line of the present Aleutian chain.Wrangellia and Stikinia, anomalous terrains in Alaska and northwestern Canada respectively, were emplaced by transport on the Kula plate from lower latitudes. Hypotheses which require transport of these plates in the Mesozoic from the “far reaches of the Pacific” ignore the problem of transport across either the Kula-Pacific or Kula-Farallon spreading centers. The interaction of the Kula plate and western North America throughout the Jurassic and the Cretaceous should result in emplacement of these terrains by motion oblique to the continental margin. Tethyan faunas in Stikinia must come from the western end of Tethys between North and South America, not the Indonesian region at the eastern end of Tethys.As the northeastern end of the Kula-Farallon ridge moved northward, the sense of motion changed from right lateral shear between the Kula and North American plates to collision or left lateral shear between the Farallon and North American plates. Left lateral shear along zones analogous to the Mojave-Sonora megashear may have been the means by which anomalous terrains were transported to the southeast into the gap between North and South America forming present day Central America. Such a model overcomes the overlap difficulties suffered in previous attempts to reconstruct the Mesozoic paleogeography of Central America. 相似文献
11.
Ewald Lüschen 《Tectonophysics》1986,130(1-4):141-146
Crustal studies in western Colombia, by deep seismic, gravity and geomagnetic surveys, during the last two decades have revealed an extremely anomalous crustal structure as compared to the South American Andes further south. Strong gravity gradients and differences in seismic velocities showed a transition from oceanic to continental character between the Western and Central Andes.
Measured gravity and height variations of opposite sign and lengths of 50 to 100 km on three east-west running profiles correlate surprisingly well with the typical positive Bouguer anomaly of the Western Andes which represents an isostatic instability. A gravity decrease of 0.5–1.0 mGal on two profiles and an increase on an intermediate one and corresponding ratios of gravity to apparent height variations of nearly −20 mGal/m are interpreted as consequences of deep-seated density variations. They may be related to collision tectonics and recent obduction processes between aseismic ridges riding on the Pacific Nazca plate and the continent. 相似文献
12.
We present a new three-dimensional SV-wave velocity model for the upper mantle beneath South America and the surrounding oceans, built from the waveform inversion of 5850 Rayleigh wave seismograms. The dense path coverage and the use of higher modes to supplement the fundamental mode of surface waves allow us to constrain seismic heterogeneities with horizontal wavelengths of a few hundred kilometres in the uppermost 400 km of the mantle.The large scale features of our tomographic model confirm previous results from global and regional tomographic studies (e.g. the depth extent of the high velocity cratonic roots down to about 200–250 km).Several new features are highlighted in our model. Down to 100 km depth, the high velocity lid beneath the Amazonian craton is separated in two parts associated with the Guyana and Guapore shields, suggesting that the rifting episode responsible for the formation of the Amazon basin has involved a significant part of the lithosphere. Along the Andean subduction belt, the structure of the high velocity anomaly associated with the sudbduction of the Nazca plate beneath the South American plate reflects the along-strike variation in dip of the subducting plate. Slow velocities are observed down to about 100 km and 150 km at the intersection of the Carnegie and Chile ridges with the continent and are likely to represent the thermal anomalies associated with the subducted ridges. These lowered velocities might correspond to zones of weakness in the subducted plate and may have led to the formation of “slab windows” developed through unzipping of the subducted ridges; these windows might accommodate a transfer of asthenospheric mantle from the Pacific to the Atlantic ocean. From 150 to 250 km depth, the subducting Nazca plate is associated with high seismic velocities between 5°S and 37°S. We find high seismic velocities beneath the Paraná basin down to about 200 km depth, underlain by a low velocity anomaly in the depth range 200–400 km located beneath the Ponta Grossa arc at the southern tip of the basin. This high velocity anomaly is located southward of a narrow S-wave low velocity structure observed between 200 and 500–600 km depth in body wave studies, but irresolvable with our long period datasets. Both anomalies point to a model in which several, possibly diachronous, plumes have risen to the surface to generate the Paraná large igneous province (LIP). 相似文献
13.
The structural elements on the shallow (Sunda Shelf) and deep seas of east and south—east Asia are interpreted as the result of past interaction between lithospheric plates. During the Mesozoic the western Pacific Ocean and the eastern Indian Ocean were parts of the Tethys Sea and were moving to the north relative to Antarctica. A Mesozoic ridge system trending east—west produced east—west trending magnetic anomalies throughout the entire area. The ridge system was bisected by large north—south transform faults which divided the eastern Indian Ocean—western Pacific Ocean into sub-plates traveling at different speeds. The Mesozoic evolution of the Sunda Shelf and the deep seas resulted from such horizontal differential movement in a north—south direction. During Late Cretaceous—Eocene the various segments of the spreading ridge gradually submerged beneath the deep sea trenches to the north, causing a gradual change in the direction of motion of the Pacific plate. The change in motion of the Pacific plate resulted in the separation between the Pacific and the eastern Indian Ocean plates, the formation of large northeast—southwest tectonic elements on the Sunda Shelf and elsewhere in south—east Asia, the formation of the western Philippine Basin and the rapid northward motion of Australia. The only remnant of the Mesozoic ridge system exists today at the western Philippine Basin. 相似文献
14.
Subduction zones with deep seismicity are believed to be associated with the descending branches of convective flows in the mantle and are subordinated to them. Therefore, the position of subduction zones can be considered as relatively fixed with respect to the steady-state system of convective flows. The lithospheric plate overhanging a subduction zone (as a rule of continental type) may:
- 1. (1) either move away from the subduction zone; or
- 2. (2) move onto it. In the first case extensional conditions originate behind the subduction zone and the new oceanic crust of back-arc basins forms. In the second case active Andean-type continental margins with thickening of the crust and lithosphere are observed.
15.
《International Geology Review》2012,54(6):493-565
An abbreviated variant (condensed and updated by an American specialist) of a research monograph focuses on the gravity field, geoid height, seismicity, rheology, and Phanerozoic tectonic history of the Pacific Ocean and the surrounding Pacific Mobile Belt from a computational geodynamic perspective. The state of stress calculated for the Pacific Mobile Belt incidates that it is a geologically persistent tectonic boundary separating the Pacific hemisphere from the Indo-Afro-Atlantic hemisphere, which contains the bulk of the earth's continental crust. The gravity field of the Pacific Basin has a concentric structure and the region has a counter-clockwise sense of rotation relative to the Pacific Mobile Belt and surrounding hemisphere. The relative displacement is believed to have been approximately periodic through Phanerozoic time. 相似文献
16.
The continental margin orogenic systems of the western Americas are enormous features that formed along the Pacific margins of the North and South American plates during late Mesozoic through Cenozoic time. There has been considerable debate concerning their origin, and they are often compared with intra-oceanic fringing arc-trench systems more typical of the Australasian margins of the Pacific Ocean, in that both involve the subduction of oceanic lithosphere, often with similar convergent relative motion vectors. The onset of orogenesis in the two Cordilleras, as shown in reversal of sedimentary polarity from sources generally on the continent to sources along the Pacific margin, seems to date from shortly after emplacement of the oldest oceanic crust in that part of the Atlantic Ocaen east of each continent — i.e., about 170 Ma, or Middle Jurassic, in the case of the Central Atlantic, and about 135 to 100 Ma, or Early to mid-Cretaceous, in the case of the South Atlantic. These ages also seem to mark the onset of westward motion of the two continents over the Pacific Ocean basin and subsequent crustal thickening and uplift, with development of thrust belts, foreland basins, and foredeeps. Prior to this prolonged westward drift, both margins had been convergent for at least several hundred million years, but no massive mountain building had taken place. Instead, the margins were tectonically “neutral”, with typically submarine fringing arc-trench systems or shallow marine to continental margin arcs which stood “outboard” of shallow marine platformal shelves or basins whose main sedimentary polarity was from the continent. Although accretion of “suspect” terranes, high rates of convergence, and age of subducting lithosphere all may have influenced particularly local tectonic response and/or phases of orogenic activity in the two chains, the absolute motion of the two continental margins over the Pacific Ocean basin is considered to have been the dominant factor in Cordilleran tectonic evolution. 相似文献
17.
The active faults known and inferred in the area where the major Pacific, North American and Eurasian plates come together group into two belts. One of them comprises the faults striking roughly parallel to the Pacific ocean margin. The extreme members of the belt are the longitudinal faults of islands arcs, in its oceanic flank, and the faults along the continental margins of marginal seas, in its continental flank. The available data show that all these faults move with some strike-slip component, which is always right-lateral. We suggest that characteristic right-lateral, either partially or dominantly, kinematics of the fault movements has its source in oblique convergence of the Pacific plate with continental Eurasian and North American plates. The second belt of active faults transverses the extreme northeast Asia as a continental extension of the active mid-Arctic spreading ridge. The two active fault belts do not cross but come close to each other at the northern margin of the Sea of Okhotsk marking thus the point where the Pacific, North American and Eurasian plates meet. 相似文献
18.
Sediment subduction versus accretion around the pacific 总被引：2，自引：0，他引：2
Thomas W.C. Hilde 《Tectonophysics》1983,99(2-4)
Subducting oceanic plates are typically broken by normal faults as they bend downward into subduction zones, usually forming regular patterns of grabens. The faults strike parallel or subparallel to the trench axes and are most commonly 5–10 km in spacing and width. Rupture occurs initially near the outer topographic high and vertical displacement or graben depth increases as the plate descends, the 400 m or more at many trench axes. It is suggested that the grabens provide void spaces within the surface of the subducting plate, below the plane of subduction, into which the trench sediments are tectonically displaced and thus subducted. Around the Pacific, the only regions of apparent fore-arc sediment accretion are where the graben structures are missing or masked by thick sediment deposits. Even in these cases sediment subduction, by inclusion in subducting plate grabens or by other mechanisms, must be invoked to explain the relatively small fore-arc sediment volumes compared to calculated accretion volumes based on historical convergence. Where trench sediment volumes are small compared to the graben volumes the grabens may abrade the leading edge and underside of the overriding plate and subduct the eroded material. It is concluded that sediment subduction is dominant around the Circum-Pacific and that the bending-induced graben structures of the subducting plates are a major factor for sediment subduction and tectonic erosion. 相似文献
19.
Buckle-controlled seismogenic faulting in peninsular India 总被引：1，自引：0，他引：1
As intraplate earthquakes are often not associated with major known faults their location as well as their timing is unpredictable. In peninsular India the larger (M5.0) events occur mainly on reverse faults in a series of belts 400–800 km apart which are aligned roughly normal to the azimuth of convergence between the Indian and Eurasian plates. The location of the belts is controlled largely by the buckling wavelength of the lithosphere, and the seismogenic faults do not generate folding and sometimes result from it. There is consequently no need to postulate the creation of regularly spaced normal faults in an antecedent extensional phase, and the deformation is consistent with a plate-driving force such as gravity glide which is unlikely to reverse its polarity and which creates structures that are influenced by plate geometry at the leading edge. The thesis is potentially of value to seismic hazard mitigation as it identifies the zones that are most at risk. 相似文献
20.
The `plate tectonic mirror image' to the region of the Cocos and Nazca plates, which are currently being subducted beneath Central America, is preserved in the Central Pacific around 120°W just south of the equator. Cruise SO‐180 investigated this remote area during project CENTRAL and acquired new magnetic and bathymetric data. A plate tectonic model for the ‘mirror image’ is presented based on the newly acquired as well as reprocessed existing data. Discordant magnetic anomaly patterns and bathymetric structures indicate at least two major reorganization events (19.5 and 14.7 Myr), which can be detected both in the Cocos‐Nazca spreading system and in the East Pacific Rise. Irregularities in the anomaly pattern and curvilinear structures on the sea floor of the survey area are interpreted in terms of a fossil overlapping spreading centre at the location where the Farallon break‐up originated. 相似文献