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1.
A seismic refraction/wide-angle reflection experiment was undertaken in the Levant Basin, eastern Mediterranean. Two roughly east–west profiles extend from the continental shelf of Israel toward the Levant Basin. The northern profile crosses the Eratosthenes Seamount and the southern profile crosses several distinct magnetic anomalies. The marine operation used 16 ocean bottom seismometers deployed along the profiles with an air gun array and explosive charges as energy sources. The results of this study strongly suggest the existence of oceanic crust under portions of the Levant Basin and continental crust under the Eratosthenes Seamount. The seismic refraction data also indicate a large sedimentary sequence, 10–14 km thick, in the Levant Basin and below the Levant continental margin. Assuming the crust is of Cretaceous age, this gives a fairly high sedimentation rate. The sequence can be divided into several units. A prominent unit is the 4.2 km/s layer, which is probably composed of the Messinian evaporites. Overlying the evaporitic layer are layers composed of Plio–Pleistocene sediments, whose velocity is 2.0 km/s. The refraction profiles and gravity and magnetic models indicate that a transition from a two layer continental to a single-layer oceanic crust takes place along the Levant margin. The transition in the structure along the southern profile is located beyond the continental margin and it is quite gradual. The northern profile, north of the Carmel structure, presents a different structure. The continental crust is much thinner there and the transition in the crustal structure is more rapid. The crustal thinning begins under western Galilee and terminates at the continental slope. The results of the present study indicate that the Levant Basin is composed of distinct crustal units and that the Levant continental margin is divided into at least two provinces of different crustal structure. 相似文献
2.
During the continental rifting the upper mantle was unroofed, and the mantle rocks were transformed into serpentinite at the ocean-continent transition of the west Galicia margin (Spain). The serpentinite layer, several km thick, extends probably eastwards, beneath the highly thinned continental crust of the margin.
The serpentinite layer was recently imaged by seismic reflection. It is discontinuously and deeply layered. As serpentinized peridotite can have densities and seismic velocities comparable to those of the lower continental crust, we suggest that undercrusting by serpentinite can play a part in building the lower seismic crust in highly stretched continental rifted areas. 相似文献
The serpentinite layer was recently imaged by seismic reflection. It is discontinuously and deeply layered. As serpentinized peridotite can have densities and seismic velocities comparable to those of the lower continental crust, we suggest that undercrusting by serpentinite can play a part in building the lower seismic crust in highly stretched continental rifted areas. 相似文献
3.
By compiling wide-angle seismic velocity profiles along the 400-km-long Lofoten–Vesterålen continental margin off Norway, and integrating them with an extensive seismic reflection data set and crustal-scale two-dimensional gravity modelling, we outline the crustal margin structure. The structure is illustrated by across-margin regional transects and by contour maps of depth to Moho, thickness of the crystalline crust, and thickness of the 7+ km/s lower crustal body. The data reveal a normal thickness oceanic crust seaward of anomaly 23 and an increase in thickness towards the continent–ocean boundary associated with breakup magmatism. The southern boundary of the Lofoten–Vesterålen margin, the Bivrost Fracture Zone and its landward prolongation, appears as a major across-margin magmatic and structural crustal feature that governed the evolution of the margin. In particular, a steeply dipping and relatively narrow, 10–40-km-wide, Moho-gradient zone exists within a continent–ocean transition, which decreases in width northward along the Lofoten–Vesterålen margin. To the south, the zone continues along the Vøring margin, however it is offset 70–80 km to the northwest along the Bivrost Fracture Zone/Lineament. Here, the Moho-gradient zone corresponds to a distinct, 25-km-wide, zone of rapid landward increase in crustal thickness that defines the transition between the Lofoten platform and the Vøring Basin. The continental crust on the Lofoten–Vesterålen margin reaches a thickness of 26 km and appears to have experienced only moderate extension, contrasting with the greatly extended crust in the Vøring Basin farther south. There are also distinct differences between the Lofoten and Vesterålen margin segments as revealed by changes in structural style and crustal thickness as well as in the extent of elongate potential-field anomalies. These changes may be related to transfer zones. Gravity modelling shows that the prominent belt of shelf-edge gravity anomalies results from a shallow basement structural relief, while the elongate Lofoten Islands belt requires increased lower crustal densities along the entire area of crustal thinning beneath the islands. Furthermore, gravity modelling offers a robust diagnostic tool for the existence of the lower crustal body. From modelling results and previous studies on- and off-shore mid-Norway, we postulate that the development of a core complex in the middle to lower crust in the Lofoten Islands region, which has been exhumed along detachments during large-scale extension, brought high-grade, lower crustal rocks, possibly including accreted decompressional melts, to shallower levels. 相似文献
4.
H. Laubscher 《地学学报》1990,2(6):645-652
Gravity surveys of the past century established that mountains have roots, seismic refraction lines shot in the second half of this century confirmed the downbulge of the Moho under the Alps, and recent reflection traverses provided new details on the behaviour of crustal layers in the deep part of the Alps. However, geophysical data are ambiguous geologically. For models of the root in terms of rock distribution to be tectonophysically acceptable, they must be the retrodeformable result of kinematic sequence that fits the geological surface data. For a cross-section through the Swiss Alps based on refraction data and somewhat modified by the recent reflection traverses, a kinematic model compatible with large-scale geological data may be obtained by the superposition of three Neogene phases with alternating vergence. Although Alpine collision is largely dextrally compressive in the central Alps, the N-S component may be discussed in a cross-section. Particularly puzzling geophysical features include a high-velocity body in the middle crust and the disappearance of the layered foreland crust in the root. In order to account for these phenomena, it is proposed that the crustal root is interpreted as the result of complex reshuffling of middle and lower crustal masses as well as large-scale phase transformations. The mid-crustal highvelocity body is interpreted as a delaminated section of the lower crust of the Adria plate that was wedged into the middle crust of the Alps in the middle Miocene. The disappearance of the foreland lower crust is attributed to eclogitization attendant on the subduction of continental crust. Material balance estimates suggest that during Alpine collision large volumes of continental crust have disappeared through subduction. 相似文献
5.
Crustal structure of the southern Dabie ultrahigh-pressure orogen and Yangtze foreland from deep seismic reflection profiling 总被引：8，自引：0，他引：8
Shuwen Dong Rui Gao Bolin Cong † Zhongyan Zhao Xiaochun Liu Sanzhong Li Qiusheng Li Dongding Huang 《地学学报》2004,16(6):319-324
A new 140‐km‐long seismic reflection profile provides a high‐resolution crustal‐scale image of the southern Dabieshan high‐pressure (HP) metamorphic belt and the Yangtze foreland fold‐and‐thrust belt. The seismic image of the stacked section shows that the southern Dabieshan metamorphic terrane and Yangtze foreland belt are separated by a large north‐dipping fault. In the foreland the upper crust is dominated by a series of folds and thrusts formed during the collisional stage in the mid‐Triassic; it was reworked by crustal extension resulting in the formation of a late Jurassic and Cretaceous red‐bed basin. The southern Dabieshan profile shows stacked crustal slabs developed along the margin of the collisional orogenic belt. The Moho reflectors at 10–11 s (～30–33 km) are seismically prominent and segmented by a number of south‐verging thrusts that were probably developed by foreland‐directed thrusting of the deeply subducted continental crust during exhumation. The seismic reflection profile suggests that structures related to the Triassic–Jurassic subduction and exhumation of the Yangtze plate are preserved despite the severe crustal extension superimposed during the late Mesozoic and Cenozoic. 相似文献
6.
7.
洋-陆过渡带是理解大陆岩石圈破裂和海底初始扩张的关键位置,但是在南海北部地区仍然存在关于相关地质过程的诸多疑问.通过近年开展的国际大洋发现计划航次以及深部地质地球物理探测,取得以下4个方面的认识.（1）南海北部的洋-陆边界一般与自由空间重力异常的正-负值过渡位置对应,而更加准确地限定需要结合反射、折射地震资料.稳定大洋岩石圈生成与大陆岩石圈最终破裂之间的洋-陆过渡边界的位置比以往认为的还应往深海盆方向移动.（2）洋-陆过渡带代表了远端带构造作用减弱和岩浆作用逐渐增强的区域.陆坡地壳发育扩张后岩浆底侵、洋-陆过渡带发育同破裂期岩浆喷出结构和侵入反射体.（3）在中生代的古俯冲带弧前区域,新生代的断裂沿着早期的构造开始活动,岩石圈多处发生强烈的共轭韧性剪切作用.随着大陆岩石圈的进一步拉伸减薄,部分靠陆一侧的裂谷中心停止张裂,成为夭折裂谷,以台西南盆地南部凹陷、白云凹陷、西沙海槽为代表,而南海陆缘异常伸展和最终破裂的地方集中在南侧裂谷中心.夭折裂谷下亦发现地幔蛇纹石化,进一步反映了较弱的同破裂岩浆活动.（4）南海初始洋壳的增生沿着大陆边缘走向具有显著的变化,南海东北部洋-陆过渡带下伏地幔明显抬升和部分蛇纹石化,地震纵、横波速度以及折射波衰减特征都支持此观点,反映南海东北部是一个贫岩浆型大陆边缘.未来,南海北部洋-陆过渡带有望成为南海“莫霍钻”的理想备选钻探区. 相似文献
8.
We present results from a 484 km wide-angle seismic profile acquired in the northwest part of the South China Sea (SCS) during OBS2006 cruise. The line that runs along a previously acquired multi-channel seismic line (SO49-18) crosses the continental slope of the northern margin, the Northwest Subbasin (NWSB) of the South China Sea, the Zhongsha Massif and partly the oceanic basin of the South China Sea. Seismic sections recorded on 13 ocean-bottom seismometers were used to identify refracted phases from the crustal layer and also reflected phases from the crust-mantle boundary (Moho). Inversion of the traveltimes using a simple start model reveals crustal images in the study area. The velocity model shows that crustal thickness below the continental slope is between 14 and 23 km. The continental part of the line is characterized by gentle landward mantle uplift and an abrupt oceanward one. The velocities in the lower crust do not exceed 6.9 km/s. With the new data we can exclude a high-velocity lower crustal body (velocities above 7.0 km/s) at the location of the line. We conclude that this part of the South China Sea margin developed by a magma-poor rifting. Both, the NWSB and the Southwest Sub-basin (SWSB) reveal velocities typical for oceanic crust with crustal thickness between 5 and 7 km. The Zhongsha Massif in between is extremely stretched with only 6–10 km continental crust left. Crustal velocity is below 6.5 km/s; possibly indicating the absence of the lower crust. Multi-channel seismic profile shows that the Yitongansha Uplift in the slope area and the Zhongsha Massif are only mildly deformed. We considered them as rigid continent blocks which acted as rift shoulders of the main rift subsequently resulting in the formation of the Northwest Sub-basin. The extension was mainly accommodated by a ductile lower crustal flows, which might have been extremely attenuated and flow into the oceanic basin during the spreading stage. We compared the crustal structures along the northern margin and found an east-west thicken trend of the crust below the continent slope. This might be contributed by the east-west sea-floor spreading along the continental margin. 相似文献
9.
南海及邻区处于欧亚大陆与冈瓦纳古陆拼合带的东南端,是特提斯构造域和濒太平洋构造域交汇的重要地区.特提斯缝合带沿金沙江-哀牢山构造带进入南海,人们从而认为南海可能存在特提斯洋遗迹,并认为缝合带存在于磁静区中.本文通过对南海北部陆坡地球物理资料的解释结果,包括重力、磁力、海底地震和深反射地震数据,以及区域地质特征分析,研究了南海北部陆缘高磁异常带和磁静区的成因.结果表明高磁异常带是中白垩世时期古太平洋板块转向俯冲形成的陆缘火山弧,当时存在古俯冲带.磁静区经历了后期大陆边缘张裂和古南海和南海的打开,并经历了高温热物质的底辟作用,使得地壳拉张减薄,居里面抬升形成磁静区.经历了南海的扩张后,原始的俯冲带可能已经向南迁移到南海南部或者已经俯冲消失,其中也不存在缝合带. 相似文献
10.
M. Sachpazi A. Hirn A. Nercessian F. Avedik J. Mc Bride M. Loucoyannakis R. Nicolich the STREAMERS-PROFILES group 《Tectonophysics》1997,270(3-4):301-312
The Aegean Sea is a broad area of submerged continental crust undergoing active extension to varying degrees. A combined near-normal incidence and wide-angle seismic recording programme was conducted in the western Aegean Sea in 1993, with the principal objective of testing the popular hypothesis that lower crustal deformation (particularily extension) is expressed as a seismically “layered lower crust” (LLC). Across the southern margin of the Cretan trough (i.e. North Cretan offshore margin), a LLC was indicated by wide-angle arrivals that was not apparent on either the coincident near-normal-in-cidence profile or on older low-frequency refraction records. North of the northern margin of the Cretan Trough, beneath the Cyclades, a domain of strong reflectivity is recorded from the middle to lower crust. Here, the near-normal incidence sections also show this typical LLC reflectivity. On the wide-angle sections, a distinct interface is suggested in addition, at a larger depth than that previously assumed for the Moho discontinuity. The structural images and interpretations derived from the new seismic data so far do not clearly support either a pure-shear crustal stretching or an asymmetric simple-shear extension model for the Aegean Sea. Our results appear to be consistent with a tectonic model, where middle crust mobilised by flow coincides spatially with upper crust that has been thinned by active extension of an orogenically thickened crust and expressed near the surface as an exhumed metamorphic core complex. 相似文献
11.
Subduction erosion, which occurs at all convergent plate boundaries associated with magmatic arcs formed on crystalline forearc basement, is an important process for chemical recycling, responsible globally for the transport of ~1.7 Armstrong Units (1 AU = 1 km3/yr) of continental crust back into the mantle. Along the central Andean convergent plate margin, where there is very little terrigenous sediment being supplied to the trench as a result of the arid conditions, the occurrence of mantle-derived olivine basalts with distinctive crustal isotopic characteristics (87Sr/86Sr ≥ 0.7050; εNd ≤ −2; εHf ≤ +2) correlates spatially and/or temporally with regions and/or episodes of high rates of subduction erosion, and a strong case can be made for the formation of these basalts to be due to incorporation into the subarc mantle wedge of tectonically eroded and subducted forearc continental crust. In other convergent plate boundary magmatic arcs, such as the South Sandwich and Aleutian Islands intra-oceanic arcs and the Central American and Trans-Mexican continental margin volcanic arcs, similar correlations have been demonstrated between regions and/or episodes of relatively rapid subduction erosion and the genesis of mafic arc magmas containing enhanced proportions of tectonically eroded and subducted crustal components that are chemically distinct from pelagic and/or terrigenous trench sediments. It has also been suggested that larger amounts of melts derived from tectonically eroded and subducted continental crust, rising as diapirs of buoyant low density subduction mélanges, react with mantle peridotite to form pyroxenite metasomatites that than melt to form andesites. The process of subduction erosion and mantle source region contamination with crustal components, which is supported by both isotopic and U-Pb zircon age data implying a fast and efficient connectivity between subduction inputs and magmatic outputs, is a powerful alternative to intra-crustal assimilation in the generation of andesites, and it negates the need for large amounts of mafic cumulates to form within and then be delaminated from the lower crust, as required by the basalt-input model of continental crustal growth. However, overall, some significant amount of subducted crust and sediment is neither underplated below the forearc wedge nor incorporated into convergent plate boundary arc magmas, but instead transported deeper into the mantle where it plays a role in the formation of isotopically enriched mantle reservoirs. To ignore or underestimate the significance of the recycling of tectonically eroded and subducted continental crust in the genesis of convergent plate boundary arc magmas, including andesites, and for the evolution of both the continental crust and mantle, is to be on the wrong side of history in the understanding of these topics. 相似文献
12.
Asbjrn Johan Breivik Rolf Mjelde Paul Grogan Hideki Shimamura Yoshio Murai Yuichi Nishimura 《Tectonophysics》2003,369(1-2):37-70
The Barents Sea is located in the northwestern corner of the Eurasian continent, where the crustal terrain was assembled in the Caledonian orogeny during Late Ordovician and Silurian times. The western Barents Sea margin developed primarily as a transform margin during the early Tertiary. In the northwestern part south of Svalbard, multichannel reflection seismic lines have poor resolution below the Permian sequence, and the early post-orogenic development is not well known here. In 1998, an ocean bottom seismometer (OBS) survey was collected southwest to southeast of the Svalbard archipelago. One profile was shot across the continental transform margin south of Svalbard, which is presented here. P-wave modeling of the OBS profile indicates a Caledonian suture in the continental basement south of Svalbard, also proposed previously based on a deep seismic reflection line coincident with the OBS profile. The suture zone is associated with a small crustal root and westward dipping mantle reflectivity, and it marks a boundary between two different crystalline basement terrains. The western terrain has low (6.2–6.45 km s−1) P-wave velocities, while the eastern has higher (6.3–6.9 km s−1) velocities. Gravity modeling agrees with this, as an increased density is needed in the eastern block. The S-wave data predict a quartz-rich lithology compatible with felsic gneiss to granite within and west of the suture zone, and an intermediate lithological composition to the east. A geological model assuming westward dipping Caledonian subduction and collision can explain the missing lower crust in the western block by subduction erosion of the lower crust, as well as the observed structuring. Due to the transform margin setting, the tectonic thinning of the continental block during opening of the Norwegian-Greenland Sea is restricted to the outer 35 km of the continental block, and the continent–ocean boundary (COB) can be located to within 5 km in our data. Distinct from the outer high commonly observed on transform margins, the upper part of the continental crust at the margin is dominated by two large, rotated down-faulted blocks with throws of 2–3 km on each fault, apparently formed during the transform margin development. Analysis of the gravity field shows that these faults probably merge to one single fault to the south of our profile, and that the downfaulting dominates the whole margin segment from Spitsbergen to Bjørnøya. South of Bjørnøya, the faulting leaves the continental margin to terminate as a graben 75 km south of the island. Adjacent to the continental margin, there is no clear oceanic layer 2 seismic signature. However, the top basement velocity of 6.55 km s−1 is significantly lower than the high (7 km s−1) velocity reported earlier from expanding spread profiles (ESPs), and we interpret the velocity structure of the oceanic crust to be a result of a development induced by the 7–8-km-thick sedimentary overburden. 相似文献
13.
Recent geophysical measurements, including multi-channel seismic reflection, on the Svalbard passive margin have revealed that it has undergone a complex geological history which largely reflects the plate tectonic evolution of the Greenland Sea and the Arctic Ocean. The western margin (75–80°N) is of a sheared-rifted type, along which the rifted margin developed subsequent to a change in the pole of plate rotation about 36 m.y. B.P. The north-trending Hornsund Fault on the central shelf and the eastern escarpment of the Knipovich Ridge naturally divide the margin into three structural units. These main marginal structures strike north, paralleling the regional onshore fault trends. This trend also parallels the direction of Early Tertiary plate motion between Svalbard and Greenland. Thus, the western Svalbard margin was initially a zone of shear, and the shear movements have affected the adjacent continental crust. Although, the nature and location of the continent—ocean crustal transition is somewhat uncertain, it is unlikely to lie east of the Hornsund Fault. The northern margin, including the Yermak marginal plateau, is terminated to the west by the Spitsbergen Fracture Zone system. This margin is of a rifted type and the preliminary analysis indicates that the main part of the investigated area is underlain by continental crust. 相似文献
14.
Han-Joon Kim Hyeong-Tae Jou Hyun-Moo Cho Harmen Bijwaard Takeshi Sato Jong-Kuk Hong Hai-Soo Yoo Chang-Eob Baag 《Tectonophysics》2003,364(1-2):25-42
Despite the various opening models of the southwestern part of the East Sea (Japan Sea) between the Korean Peninsula and the Japan Arc, the continental margin of the Korean Peninsula remains unknown in crustal structure. As a result, continental rifting and subsequent seafloor spreading processes to explain the opening of the East Sea have not been adequately addressed. We investigated crustal and sedimentary velocity structures across the Korean margin into the adjacent Ulleung Basin from multichannel seismic (MCS) reflection and ocean bottom seismometer (OBS) data. The Ulleung Basin shows crustal velocity structure typical of oceanic although its crustal thickness of about 10 km is greater than normal. The continental margin documents rapid transition from continental to oceanic crust, exhibiting a remarkable decrease in crustal thickness accompanied by shallowing of Moho over a distance of about 50 km. The crustal model of the margin is characterized by a high-velocity (up to 7.4 km/s) lower crustal (HVLC) layer that is thicker than 10 km under the slope base and pinches out seawards. The HVLC layer is interpreted as magmatic underplating emplaced during continental rifting in response to high upper mantle temperature. The acoustic basement of the slope base shows an igneous stratigraphy developed by massive volcanic eruption. These features suggest that the evolution of the Korean margin can be explained by the processes occurring at volcanic rifted margins. Global earthquake tomography supports our interpretation by defining the abnormally hot upper mantle across the Korean margin and in the Ulleung Basin. 相似文献
15.
Maryline Moulin Daniel Aslanian Mikael Evain Anglique Leprêtre Philippe Schnurle Fanny Verrier Joseph Thompson Philippe De Clarens Sylvie Leroy Nuno Dias 《地学学报》2020,32(3):205-214
The Natal Valley, offshore Mozambique, is a key area for understanding the evolution of East Gondwana. Within the scope of the integrated multidisciplinary PAMELA project, we present new wide‐angle seismic data and interpretations, which considerably alter Geoscience paradigms. These data reveal the presence of a 30‐km‐thick crust that we argue to be of continental nature. This falsifies all the most recent palaeo‐reconstructions of the Gondwana. This 30‐km‐thick continental crust 1,000 m below sea level implies a complex history with probable intrusions of mantle‐derived melts in the lower crust, connected to several occurrences of magmatism, which seems to evidence the crucial role of the lower continental crust in passive margin genesis. 相似文献
16.
造山带地壳结构和成分的基本特征对于认识大陆地壳成分演化和区域成矿背景具有重要意义.综合青藏高原拉萨地体东南部地球物理、高温高压岩石物性和岩浆岩地球化学资料,分析该地区地壳整体成分特征,并探讨其可能成因.该地区平均地壳波速显著低于全球大陆和造山带地壳的平均值,表明地壳整体具有中酸性成分,下地壳特征也可由中性岩石(残余体性质的中性含石榴石麻粒岩)解释.拉萨地体东南部整体地壳成分特征应与多阶段长英质化有关,包括碰撞前大陆弧演化阶段(以堆晶或残余体下地壳拆沉为主)和碰撞后高原垮塌阶段(以加厚下地壳拆沉为主,伴随印度古老长英质陆壳物质的俯冲回返/构造底侵).拉萨地体是研究大陆地壳成分演化的绝佳区域,亟待进一步开展多学科综合研究. 相似文献
17.
Analogies are drawn between continental and continental margin structures on the basis of seismic data on the crustal structure of Eurasia and its Atlantic margins. Crustal thinning from the inner parts of the continent to its margins is observed to be a general feature common to the formation of deep midland depressions and sedimentary basins of shelf zones. The latter are characterized by crustal thinning and its assimilation. These phenomena cannot be explained solely be sea-floor spreading effects in the process of active rifting and formation of oceanic crust. It appears that the main role in the formation of the margins in played by processes of mantle erosion in connection with heating at continental margins and with the migration of mantle material to the lower part of the crust. 相似文献
18.
Multichannel seismic reflection data acquired by Marine Arctic Geological Expedition (MAGE) of Murmansk, Russia in 1990 provide the first view of the geological structure of the Arctic region between 77–80°N and 115–133°E, where the Eurasia Basin of the Arctic Ocean adjoins the passive-transform continental margin of the Laptev Sea. South of 80°N, the oceanic basement of the Eurasia Basin and continental basement of the Laptev Sea outer margin are covered by 1.5 to 8 km of sediments. Two structural sequences are distinguished in the sedimentary cover within the Laptev Sea outer margin and at the continent/ocean crust transition: the lower rift sequence, including mostly Upper Cretaceous to Lower Paleocene deposits, and the upper post-rift sequence, consisting of Cenozoic sediments. In the adjoining Eurasia Basin of the Arctic Ocean, the Cenozoic post-rift sequence consists of a few sedimentary successions deposited by several submarine fans. Based on the multichannel seismic reflection data, the structural pattern was determined and an isopach map of the sedimentary cover and tectonic zoning map were constructed. A location of the continent/ocean crust transition is tentatively defined. A buried continuation of the mid-ocean Gakkel Ridge is also detected. This study suggests that south of 78.5°N there was the cessation in the tectonic activity of the Gakkel Ridge Rift from 33–30 until 3–1 Ma and there was no sea-floor spreading in the southernmost part of the Eurasia Basin during the last 30–33 m.y. South of 78.5°N all oceanic crust of the Eurasia Basin near the continental margin of the Laptev Sea was formed from 56 to 33–30 Ma. 相似文献
19.
P-wave velocity structure of the margin of the southeastern Tsushima Basin in the Japan Sea using ocean bottom seismometers and airguns 总被引：2，自引：0，他引：2
Takeshi Sato Toshinori Sato Masanao Shinohara Ryota Hino Minoru Nishino Toshihiko Kanazawa 《Tectonophysics》2006,412(3-4):159-171
The Tsushima Basin is located in the southwestern Japan Sea, which is a back-arc basin in the northwestern Pacific. Although some geophysical surveys had been conducted to investigate the formation process of the Tsushima Basin, it remains unclear. In 2000, to clarify the formation process of the Tsushima Basin, the seismic velocity structure survey with ocean bottom seismometers and airguns was carried out at the southeastern Tsushima Basin and its margin, which are presumed to be the transition zone of the crustal structure of the southwestern Japan Island Arc. The crustal thickness under the southeastern Tsushima Basin is about 17 km including a 5 km thick sedimentary layer, and 20 km including a 1.5 km thick sedimentary layer under its margin. The whole crustal thickness and thickness of the upper part of the crust increase towards the southwestern Japan Island Arc. On the other hand, thickness of the lower part of the crust seems more uniform than that of the upper part. The crust in the southeastern Tsushima Basin has about 6 km/s layer with the large velocity gradient. Shallow structures of the continental bank show that the accumulation of the sediments started from lower Miocene in the southeastern Tsushima Basin. The crustal structure in southeastern Tsushima Basin is not the oceanic crust, which is formed ocean floor spreading or affected by mantle plume, but the rifted/extended island arc crust because magnitudes of the whole crustal and the upper part of the crustal thickening are larger than that of the lower part of the crustal thickening towards the southwestern Japan Island Arc. In the margin of the southeastern Tsushima Basin, high velocity material does not exist in the lowermost crust. For that reason, the margin is inferred to be a non-volcanic rifted margin. The asymmetric structure in the both margins of the southeastern and Korean Peninsula of the Tsushima Basin indicates that the formation process of the Tsushima Basin may be simple shear style rather than pure shear style. 相似文献
20.
Along the Rio Muni transform margin, the transition from continental to oceanic crust occurs across a region of approximately 75-km width. The crust in this transition region, termed proto-oceanic crust (POC), is neither purely oceanic nor continental in composition and structure. Improved seismic reflection images from the PROBE deep-imaging dataset, combined with gravity modelling, have shed new light on the structural architecture of the margin and the composition of the POC. On these newly migrated seismic reflection sections, four fracture zones associated with large steps in the Moho are identified, splitting the POC into three segments. Models in which these segments are composed of oceanic or stretched continental crust do not provide satisfactory predictions of the gravity anomaly. A model of serpentinized peridotite for two segments of POC, with the third segment composed of oceanic crust in between, does satisfy the observed gravity anomaly. Three alternative geological scenarios are proposed to explain the segmentation and composition of the POC: (a) serpentinized upper mantle becoming unroofed and emplaced at basement surface level along detachment surfaces confined to discrete segments by the fracture zones, (b) oblique-slip on transform faults that allow the circulation of water into the mantle and emplacement of serpentinized upper mantle material; or (c) intense faulting of anomalous oceanic crust as a result of magma depletion allowing hydrothermal circulation and the emplacement of serpentinized peridotites. 相似文献