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1.
Explosions at two large open cut mines (Leigh Creek and Iron Baron) were used as sources of seismic energy to record along two linear profiles, parallel and approximately transverse to the axis of the Adelaide geosyncline in South Australia. Records at approximately 120 sites were obtained out to distances of the order of 350 km with Kinemetrics PS‐1A portable seismographs, using smoked paper and a recording speed of 4 mm/s. Times of blasting were determined from records at some of the permanent stations of the University of Adelaide seismograph network. Station spacing was normally 5 km, but at large distances from the source this increased to the order of 10 km. The simplest model of the crust consistent with the observed travel times comprises two essentially homogeneous layers overlying the mantle. The average P wave velocities in the upper and lower crustal layers are 5.94 km/s and 6.46 km/s, with the boundary between the layers at approximately 18 km and possibly 8 km below Eyre Peninsula. Although such a division has been found in other parts of Australia, none of the earlier studies in S.A. found evidence for such a discontinuity or velocity gradient. The P wave velocity in the upper mantle is 7.97 km/s and the mean thickness of the crust is 39 km. Both the intermediate and Moho “discontinuities” may vary by up to 5 km from their mean depths. Shear waves have velocities of 3.43 and 4.45 km/s in the upper crustal layer and the upper mantle, respectively. 相似文献
2.
The Lachlan Fold Belt has the velocity‐depth structure of continental crust, with a thickness exceeding 50 km under the region of highest topography in Australia, and in the range 41–44 km under the central Fold Belt and Sydney Basin. There is no evidence of high upper crustal velocities normally associated with marginal or back‐arc basin crustal rocks. The velocities in the lower crust are consistent with an overall increase in metamorphic grade and/or mafic mineral content with depth. Continuing tectonic development throughout the region and the negligible seismicity at depths greater than 30 km indicate that the lower crust is undergoing ductile deformation. The upper crustal velocities below the Sydney Basin are in the range 5.75–5.9 km/s to about 8 km, increasing to 6.35–6.5 km/s at about 15–17 km depth, where there is a high‐velocity (7.0 km/s) zone for about 9 km evident in results from one direction. The lower crust is characterised by a velocity gradient from about 6.7 km/s at 25 km, to 7.7 km/s at 40–42 km, and a transition to an upper mantle velocity of 8.03–8.12 km/s at 41.5–43.5 km depth. Across the central Lachlan Fold Belt, velocities generally increase from 5.6 km/s at the surface to 6.0 km/s at 14.5 km depth, with a higher‐velocity zone (5.95 km/s) in the depth range 2.5–7.0 km. In the lower crust, velocities increase from 6.3 km/s at 16 km depth to 7.2 km/s at 40 km depth, then increase to 7.95 km/s at 43 km. A steeper gradient is evident at 26.5–28 km depth, where the velocity is about 6.6—6.8 km/s. Under part of the area an upper mantle low‐velocity zone in the depth range 50–64 km is interpreted from strong events recorded at distances greater than 320 km. There is no substantial difference in the Moho depth across the boundary between the Sydney Basin and the Lachlan Fold Belt, consistent with the Basin overlying part of the Fold Belt. Pre‐Ordovician rocks within the crust suggest fragmented continental‐type crust existed E of the Precambrian craton and that these contribute to the thick crustal section in SE Australia. 相似文献
3.
D. M. Finlayson R. J. Korsch R. A. Glen J. H. Leven D. W. Johnstone 《Australian Journal of Earth Sciences》2013,60(2):311-321
A 2‐D crustal velocity model has been derived from a 1997 364 km north‐south wide‐angle seismic profile that passed from Ordovician volcanic and volcaniclastic rocks (Molong Volcanic Belt of the Macquarie Arc) in the north, across the Lachlan Transverse Zone into Ordovician turbidites and Early Devonian intrusive granitoids in the south. The Lachlan Transverse Zone is a proposed west‐northwest to east‐southeast structural feature in the Eastern Lachlan Orogen and is considered to be a possible early lithospheric feature controlling structural evolution in eastern Australia; its true nature, however, is still contentious. The velocity model highlights significant north to south lateral variations in subsurface crustal architecture in the upper and middle crust. In particular, a higher P‐wave velocity (6.24–6.32 km/s) layer identified as metamorphosed arc rocks (sensu lato) in the upper crust under the arc at 5–15 km depth is juxtaposed against Ordovician craton‐derived turbidites by an inferred south‐dipping fault that marks the southern boundary of the Lachlan Transverse Zone. Near‐surface P‐wave velocities in the Lachlan Transverse Zone are markedly less than those along other parts of the profile and some of these may be attributed to mid‐Miocene volcanic centres. In the middle and lower crust there are poorly defined velocity features that we infer to be related to the Lachlan Transverse Zone. The Moho depth increases from 37 km in the north to 47 km in the south, above an underlying upper mantle with a P‐wave velocity of 8.19 km/s. Comparison with velocity layers in the Proterozoic Broken Hill Block supports the inferred presence of Cambrian oceanic mafic volcanics (or an accreted mafic volcanic terrane) as substrate to this part of the Eastern Lachlan Orogen. Overall, the seismic data indicate significant differences in crustal architecture between the northern and southern parts of the profile. The crustal‐scale P‐wave velocity differences are attributed to the different early crustal evolution processes north and south of the Lachlan Transverse Zone. 相似文献
4.
近年来,有关长白山火山是否存在潜在喷发危险的讨论引起了国内外地学研究者的广泛关注,但人们目前对其壳幔岩浆系统的了解却十分不足,已有的研究对长白山火山壳内岩浆房存在的深度位置、形态规模及其部分熔融程度的认识仍存在较大分歧。本研究通过汇集长白山火山及邻区(包括朝鲜境内)多个密集流动地震台阵和区域固定地震台网的观测资料,采用背景噪声成像方法获得了长白山火山区高精度的Rayleigh面波相速度模型。成像结果表明:长白山火山中-下地壳深度位置存在显著低波速异常,可能指示了岩浆房的存在。上地壳局部低速区可能反映了壳内深部岩浆向上运移的通道或者是区域小尺度的岩浆聚集体。长白山火山上地幔顶部的低速可能揭示了软流圈热物质上涌,其减压熔融为壳内岩浆房提供了幔源岩浆补给。 相似文献
5.
There is an ongoing debate about the tectonic evolution of southeast Australia, particularly about the causes and nature of its accretion to a much older Precambrian core to the west. Seismic imaging of the crust can provide useful clues to address this issue. Seismic tomography imaging is a powerful tool often employed to map elastic properties of the Earth's lithosphere, but in most cases does not constrain well the depth of discontinuities such as the Mohorovi?i? (Moho). In this study, an alternative imaging technique known as receiver function (RF) has been employed for seismic stations near Canberra in the Lachlan Orogen to investigate: (i) the shear-wave-velocity profile in the crust and uppermost mantle, (ii) variations in the Moho depth beneath the Lachlan Orogen, and (iii) the nature of the transition between the crust and mantle. A number of styles of RF analyses were conducted: H-K stacking to obtain the best compressional–shear velocity (V P /V S) ratio and crustal thickness; nonlinear inversion for the shear-wave-velocity structure and inversion of the observed variations in RFs with back-azimuth to investigate potential dipping of the crustal layers and anisotropy. The thick crust (up to 48 km) and the mostly intermediate nature of the crust?mantle transition in the Lachlan Orogen could be due to the presence of underplating at the base of the crust, and possibly to the existing thick piles of Ordovician mafic rocks present in the mid and lower crust. Results from numerical modelling of RFs at three seismic stations (CAN, CNB and YNG) suggest that the observed variations with back-azimuth could be related to a complex structure beneath these stations with the likelihood of both a dipping Moho and crustal anisotropy. Our analysis reveals crustal thickening to the west beneath CAN station which could be due to slab convergence. The crustal thickening may also be related to the broad Macquarie volcanic arc, which is rooted to the Moho. The crustal anisotropy may arise from a strong N–S structural trend in the eastern Lachlan Orogen and to the preferred crystallographic orientation of seismically anisotropic minerals in the lower and middle crust related to the paleo-Pacific plate convergence. 相似文献
6.
D.M. Finlayson 《Tectonophysics》1982,84(2-4)
Cannikin atomic bomb recordings indicate that there are differences in travel-times from the Aleutian Islands test site to Phanerozoic and Precambrian provinces in Australia of up to 1.1 s. Explosion seismic studies in central and southeastern Australia enable travel-time corrections for crustal and upper mantle structure to be made to recordings of such teleseismic events. Structure in the upper 60 km can account for, at most, about 0.2 s of the residual difference, but attempts to constrain the remaining residual time to the region above the Lehmann discontinuity at about 200 km depth are difficult to reconcile with explosion seismic models. Regional differences in seismic velocity structure between Phanerozoic and Precambrian Australia therefore appear to exist at depths greater than 200 km.Electrical conductivities within the mantle have been investigated using two methods. Long-period electromagnetic depth sounding using magnetometer arrays demonstrates that conductivities increase at about 200 km under Phanerozoic Australia but not until about 500 km depth under Precambrian Australia. Shorter period magnetotelluric measurements can only resolve shallower structures; these too indicate a similar trend but with sub-crustal conductivities increasing at less than 100 km under Phanerozoic Australia. Magma at these depths and shallower may be the source for Cainozoic volcanism in eastern Australia. Under Precambrian central and northern Australia magnetotelluric investigations indicate that pronounced conductivity increases do not occur until depths of 150–200 km are reached.Oceanic magnetic observations indicate that the Australian lithospheric plate as a whole is separating from Antarctica at a rate of about 7 cm/yr. The seismic and conductivity structures under the continental region of this plate indicate that lateral inhomogeneities possibly extend to depths as great as 500 km and are probably caused by the passage of eastern Australia over a hot spot. Hawaiian studies indicate that hot spots are not local features but result from large scale disturbances in the mantle. Conductivity increases commencing in the depth range 100–250 km may give an indication of uppermost zones within which the Palaeozoic lithospherc has been substantially modified resulting in elevated surface heat flow, volcanism and seismic travel-time anomalies. 相似文献
7.
The crustal structure of the western Arabian platform is derived using the spectral analysis of long-period P-wave amplitude ratios. The ratio of the vertical to the horizontal component is used to obtain the crustal transfer function based on thickness variations, crustal velocities, densities and the angle of emergence at the lower crust and upper mantle interface. Eleven well-defined earthquakes recorded at the long-period RYD station during the period from 1985 to 1994 were selected for analysis based on the following criteria: focal depths with a range between 7 and 89 km, body-wave magnitudes greater than 4.7, epicentral distances with a range from 8.8° to 26.5°, and back azimuthal coverage from 196° to 340°. Spectral analysis calculations were based on the comparison of the observed spectral ratios with those computed from theoretical P-wave motion obtained using the Thomson–Haskell matrix formulation for horizontally layered crustal models. The selection of the most suitable model was based on the identification of the theoretical model which exhibits the highest cross-correlation coefficient with the observed transfer function ratio. By comparing the spectral peak positions of the observed and theoretical values, the thickness and velocity can be resolved within 3 km and 1 km/s, respectively, of the observed values. The spectral analysis of long-period P-waves can detect a thin layer near the surface of about 1.6 km thick and a velocity contrast of about 10% with that of the underlying layer. A strong velocity gradient of about 0.05 km/s per km was found in the upper crust and 0.02 km/s per km in the lower crust. The derived crustal model is not unique due to the theoretical assumptions (horizontal layering, constant densities and velocities in each layer), quality of the data and complexities of the crustal structure. The crustal model suggests that the crust consists of five distinct layers. The upper crustal layer has a P-wave velocity of about 5.6 km/s and is about 1.6 km thick. The second layer has a velocity of about 6.2 km/s and is 10.2 km thick. The third layer shows a velocity of 6.6 km/s and is 6.8 km thick. The fourth layer has a velocity of about 6.8 km/s and is 12.3 km thick. The lower crustal layer has a velocity of about 7.5 km/s and is 9.3 km thick. The Mohorovicic discontinuity beneath the western Arabian platform indicates a velocity of 8.2 km/s of the upper mantle and 42 km depth. 相似文献
8.
西北次海盆的深部地壳结构蕴含着南海北部陆缘拉张过程的重要信息.广角反射/折射测线(OBS2006-2)长386 km,是目前唯一的一条沿NEE向穿过西沙地块、并平行于西北次海盆扩张脊的深地震测线.通过射线追踪与走时模拟方法(RAYINVR),获得了OBS2006-2测线下方的速度结构.结果表明:西沙地块的沉积层厚度约为1~2 km,而西北次海盆的沉积层厚度大约为2~3 km;Moho界面从西沙地块的27 km逐步抬升到西北次海盆的12 km,Moho界面下方的速度为7.8~8.0 km/s;未发现壳内高速层和低速层.在西沙地块和西北次海盆的过渡区,有着较大量的岩浆活动信息,推测与西北次海盆的初始扩张有关.OBS2006-2测线中114.5°E以西的地区为减薄的陆壳,而114.5°E以东的地区为洋壳,莫霍面在陆壳与洋壳的结合处剧烈抬升,地壳厚度明显减薄.西北次海盆的扩张脊下方可能有残余岩浆的存在. 相似文献
9.
MODELING v_P AND Q ON EXPLOSION SEISMOLOGY DATA IN NE TIBET 相似文献
10.
The crustal and upper mantle compressional-wave velocity structure across the southwestern Arabian Shield has been investigated by a 1000-km-long seismic refraction profile. The profile begins in Mesozoic cover rocks near Riyadh on the Arabian Platform, trends southwesterly across three major Precambrian tectonic provinces, traverses Cenozoic rocks of the coastal plain near Jizan, and terminates at the outer edge of the Farasan Bank in the southern Red Sea. More than 500 surveyed recording sites were occupied, and six shot points were used, including one in the Red Sea.Two-dimensional ray-tracing techniques, used to analyze amplitude-normalized record sections indicate that the Arabian Shield is composed, to first order, of two layers, each about 20 km thick, with average velocities of about 6.3 km/s and 7.0 km/s, respectively. West of the Shield-Red Sea margin, the crust thins to a total thickness of less than 20 km, beyond which the Red Sea shelf and coastal plain are interpreted to be underlain by oceanic crust.A major crustal inhomogeneity at the northeast end of the profile probably represents the suture zone between two crustal blocks of different composition. Elsewhere along the profile, several high-velocity anomalies in the upper crust correlate with mapped gneiss domes, the most prominent of which is the Khamis Mushayt gneiss. Based on their velocities, these domes may constitute areas where lower crustal rocks have been raised some 20 km. Two intracrustal reflectors in the center of the Shield at 13 km depth probably represent the tops of mafic intrusives.The Mohorovičić discontinuity beneath the Shield varies from a depth of 43 km and mantle velocity of 8.2 km/s in the northeast to a depth of 38 km and mantle velocity of 8.0 km/s depth in the southwest near the Shield-Red Sea transition. Two velocity discontinuities occur in the upper mantle, at 59 and 70 km depth.The crustal and upper mantle velocity structure of the Arabian Shield is interpreted as revealing a complex crust derived from the suturing of island arcs in the Precarnbrian. The Shield is currently flanked by the active spreading boundary in the Red Sea. 相似文献
11.
中国大陆现今构造运动的GPS速度场与活动地块 总被引：141，自引：11，他引：130
GPS观测结果给出了在欧亚参考框架下周边板块的运动状态 ,印度板块的运动方向约NE2 0° ,速度是 40～ 42mm/a ;北美板块的运动方向约NW 2 80°～ 2 90° ,速度是 2 1～ 2 3mm/a ;菲律宾板块的运动方向是NW 2 90°～ 310° ,速度是 37～ 45mm/a ;哈萨克—西伯利亚地盾的运动方向约NE130° ,速度是 3～ 5mm/a。GPS所揭示的中国大陆现今运动场清晰地表现出了以活动地块为单元的分块运动特征。文中给出了各主要活动地块的运动方向和速度。大部分活动地块内部结构完整 ,以整体性的运动为主 ;个别活动地块内部发生构造变形 ,地块的整体性不好。中国大陆以活动地块为单元的现今构造变形可能与大陆岩石圈的结构和性质有关 ,上地壳以脆性变形为主 ,下地壳和上地幔以粘塑性的流变为特征 ,从底部驱动着上覆脆性地块的整体运动。 相似文献
12.
华南大陆是新元古代以来全球地质演化历史最复杂的地区之一,也是欧亚板块东南缘地壳生长和大陆增生最活跃,大规模构造变形、岩浆活动和多金属矿产资源最丰富的地区。揭示该区浅表构造与岩浆活动和成矿作用机制离不开对深部壳幔结构的研究。宽频带地震学是开展深部壳幔结构探测的重要手段,基于宽频带地震学数据可以刻画地壳-岩石圈-上地幔-地幔过渡带不同深度和尺度的深部结构,为深入理解研究区的深部构造、动力学过程、岩浆活动与成矿作用提供有效约束。本文较全面地总结了近二十年来在华南大陆东部地区开展的宽频带流动地震探测工作,并对研究区的地壳厚度、Vp/Vs比值、岩石圈底界(LAB)深度、上地幔速度结构与各向异性等进行了分析与讨论。本文旨在为相关研究人员和团队提供未来在该区新布设地震探测台站时的参考,也可为后续深入研究该区的深部结构与成矿过程提供一些深部要素约束。 相似文献
13.
D. M. Finlayson I. Lukaszyk C. D. N. Collins E. C. Chudyk 《Australian Journal of Earth Sciences》2013,60(5):717-732
The Otway Basin in southeastern Australia formed on a triangular‐shaped area of extended continental lithosphere during two extensional episodes in Cretaceous to Miocene times. The extent of the offshore continental margin is highlighted by Seasat/Geosat satellite altimeter data. The crustal architecture and structural features across this southeast Australian margin have been interpreted from offshore‐onshore wide‐angle seismic profiling data along the Otway Continental Margin Transect extending from the onshore Lake Condah High, through the town of Portland, to the deep Southern Ocean. Along the Otway Continental Margin Transect, the onshore half‐graben geometry of Early Cretaceous deposition gives way offshore to a 5 km‐thick slope basin (P‐wave velocity 2.2–4.6 km/s) to at least 60 km from the shoreline. At 120 km from the nearest shore in a water depth of 4220 m, sonobuoy data indicate a 4–5 km sedimentary sequence overlying a 7 km thick basement above the Moho at 15 km depth. Major fault zones affect the thickness of basin sequences in the onshore area (Tartwaup Fault Zone and its southeast continuation) and at the seaward edge of the Mussel Platform (Mussel Fault). Upper crustal basement is interpreted to be attenuated and thinned Palaeozoic rocks of the Delamerian and Lachlan Orogens (intruded with Jurassic volcanics) that thin from 16 km onshore to about 3.5 km at 120 km from the nearest shore. Basement rocks comprise a 3 km section with velocity 5.5–5.7 km/s overlying a deeper basement unit with velocity 6.15–6.35 km/s. The Moho shallows from a depth of 30 km onshore to 15 km depth at 120 km from the nearest shore, and then to about 12 km in the deep ocean at the limits of the transect (water depth 5200 m). The continent‐ocean boundary is interpreted to be at a prominent topographic inflection point 170 km from shore at the bottom of the continental slope in 4800 m of water. P‐wave velocities in the lower crust are 6.4–6.8 km/s, overlying a thin transition zone to an upper mantle velocity of 8.05 km/s beneath the Moho. Outstandingly clear Moho reflections seen in deep‐marine profiling data at about 10.3 s two‐way time under the slope basin and continent‐ocean boundary place further strong controls on crustal thickness. There is no evidence of massive high velocity (>7 km/s) intrusives/underplate material in the lower crust nor any synrift or early post‐rift subaerial volcanics, indicating that the Otway continental margin can be considered a non‐volcanic margin, similar in many respects to some parts of the Atlantic Ocean margins e.g. the Nova Scotia ‐ Newfoundland margin off Canada and the Galicia Bank off the Iberian Peninsula. Using this analogue, the prominent gravity feature trending northwest‐southeast at the continent‐ocean boundary may indicate the presence of highly serpentinised mantle material beneath a thin crust, but this has yet to be tested by detailed work. 相似文献
14.
Takashi Iidaka Tetsuya Takeda Eiji Kurashimo Tomonori Kawamura Yoshiyuki Kaneda Takaya Iwasaki 《Tectonophysics》2004,388(1-4):7
A seismic experiment with six explosive sources and 391 seismic stations was conducted in August 2001 in the central Japan region. The crustal velocity structure for the central part of Japan and configuration of the subducting Philippine Sea plate were revealed. A large lateral variation of the thickness of the sedimentary layer was observed, and the P-wave velocity values below the sedimentary layer obtained were 5.3–5.8 km/s. P-wave velocity values for the lower part of upper crust and lower crust were estimated to be 6.0–6.4 and 6.6–6.8 km/s, respectively. The reflected wave from the upper boundary of the subducting Philippine Sea plate was observed on the record sections of several shots. The configuration of the subducting Philippine Sea slab was revealed for depths of 20–35 km. The dip angle of the Philippine Sea plate was estimated to be 26° for a depth range of about 20–26 km. Below this depth, the upper boundary of the subducting Philippine Sea plate is distorted over a depth range of 26–33 km. A large variation of the reflected-wave amplitude with depth along the subducting plate was observed. At a depth of about 20–26 km, the amplitude of the reflected wave is not large, and is explained by the reflected wave at the upper boundary of the subducting oceanic crust. However, the reflected wave from reflection points deeper than 26 km showed a large amplitude that cannot be explained by several reliable velocity models. Some unique seismic structures have to be considered to explain the observed data. Such unique structures will provide important information to know the mechanism of inter-plate earthquakes. 相似文献
15.
Two NE-SW trending wide-angle seismic profiles were surveyed across the Chinese side Two NE-SW trending wide-angle seismic profiles were surveyed across the Chinese side of the Yinggehai (莺歌海) basin (YGHB) with ocean bottom hydrophones (OBHs) and piggyback recorded by onshore stations located on the Hainan (海南)Island.Detailed velocity-depth models were obtained through traveltime modeling and partially constrained by amplitude calculations.More than 15 km Tertiary sedimentary infill within the YGHB can be divided in to three layers with distinct velocity-depth distribution.Overall,the upper layer has a high velocity gradient with 3.8-4.1 km/s at its bottom,consistent with progressive compaction and diagenesls.Its thickness increases gradually towards the basin center,reaching 4.5 km along the southern profile.The middle layer is characterized in its most part as a pronounced low velocity zone (LVZ) with average velocity as low as 3.0 km/s.Its thickness increases from 3.0 to over 4.5 km from NW towards SE.The primary causes of the velocity inversion are high accumulation rate and subsequent under-compaction of sediments.The velocity at the top of the lower layer is estimated at about 4.5 km/s.Despite strong energy source used (4 x 12L airgun array),no reflections can be observed from deeper levels within the basin.Towards NE the basin is bounded sharply by a clear and deep basement fault (Fault No.1),which seems to cut through the entire crust.A typical continental crust with low-velocity middle crust is found beneath the coast of the western Hainan Island.Its thickness is determined to be 28 km and shows no sign of crustal thinning towards the basin.The sharp change in crustal structure across Fault No.1 indicates that the fault is a strike-slip fault.The crustal structure obtained in this study clearly favors the hypothesis that the YGHB is a narrow pull-apart basin formed by strike-slip faulting of the Red River fault zone. 相似文献
16.
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
17.
With the aim of investigating the P-wave velocity structure below the Tertiary volcano Vogelsberg, a network of 10 mobile short period seismograph stations was installed in May 1987 for a period of 20 months. P-Wave travel time residuals relative to the station Kleiner Feldberg/Taunus (TNS) were determined for 168 seismic events using the Jeffreys - Bullen travel time tables. At all stations the relative residuals showed a positive sign, indicating a low velocity zone beneath the Vogelsberg. Maxima were found in the northern part of the Vogelsberg (station VAD +0.5 s) and in the region of the Amöneburger Basin (station RAU +0.28 s).The travel time residuals were inverted using the tomographic inversion method of Aki et al. (1977). The slowness perturbations of the single blocks were calculated relative to a crustal and upper mantle model of the Rhenish Massif. The results show an intracrustal low velocity body (about –9%) striking in a Variscan direction and underlying the north-eastern part of the Vogelsberg, and another velocity minimum (about – 6%) in the region of the Am6neburger Basin. In the lower crust and the upper mantle the velocities are reduced by about 4% relative to the starting model.The Variscan alignment of the low velocity zone under the Vogelsberg correlates with results of other geological studies. It can be assumed that during the rifting phase of the Upper Rhinegraben Variscan lineations have been reactivated, favouring uprising of magma along these old structures. The position and extension of the low velocity zone correlate with the assumed sediment distributions in the area of investigation. This may account for about one-half of the observed anomaly. The reason for the velocity reduction of about 4% in the entire underground region of the Vogelsberg down to a depth of about 70 km can be explained by the intensive fracturing of the lithosphere, caused by thermal and pressure gradients during the magma eruption process. 相似文献
18.
Crustal and upper mantle seismic structure of the Australian Plate, South Island, New Zealand 总被引：7，自引：0，他引：7
Anne Melhuish W. Steven Holbrook Fred Davey David A. Okaya Tim Stern 《Tectonophysics》2005,395(1-2):113-135
Seismic reflection and refraction data were collected west of New Zealand's South Island parallel to the Pacific–Australian Plate boundary. The obliquely convergent plate boundary is marked at the surface by the Alpine Fault, which juxtaposes continental crust of each plate. The data are used to study the crustal and uppermost mantle structure and provide a link between other seismic transects which cross the plate boundary. Arrival times of wide-angle reflected and refracted events from 13 recording stations are used to construct a 380-km long crustal velocity model. The model shows that, beneath a 2–4-km thick sedimentary veneer, the crust consists of two layers. The upper layer velocities increase from 5.4–5.9 km/s at the top of the layer to 6.3 km/s at the base of the layer. The base of the layer is mainly about 20 km deep but deepens to 25 km at its southern end. The lower layer velocities range from 6.3 to 7.1 km/s, and are commonly around 6.5 km/s at the top of the layer and 6.7 km/s at the base. Beneath the lower layer, the model has velocities of 8.2–8.5 km/s, typical of mantle material. The Mohorovicic discontinuity (Moho) therefore lies at the base of the second layer. It is at a depth of around 30 km but shallows over the south–central third of the profile to about 26 km, possibly associated with a southwest dipping detachment fault. The high, variable sub-Moho velocities of 8.2 km/s to 8.5 km/s are inferred to result from strong upper mantle anisotropy. Multichannel seismic reflection data cover about 220 km of the southern part of the modelled section. Beneath the well-layered Oligocene to recent sedimentary section, the crustal section is broadly divided into two zones, which correspond to the two layers of the velocity model. The upper layer (down to about 7–9 s two-way travel time) has few reflections. The lower layer (down to about 11 s two-way time) contains many strong, subparallel reflections. The base of this reflective zone is the Moho. Bi-vergent dipping reflective zones within this lower crustal layer are interpreted as interwedging structures common in areas of crustal shortening. These structures and the strong northeast dipping reflections beneath the Moho towards the north end of the (MCS) line are interpreted to be caused by Paleozoic north-dipping subduction and terrane collision at the margin of Gondwana. Deeper mantle reflections with variable dip are observed on the wide-angle gathers. Travel-time modelling of these events by ray-tracing through the established velocity model indicates depths of 50–110 km for these events. They show little coherence in dip and may be caused side-swipe from the adjacent crustal root under the Southern Alps or from the upper mantle density anomalies inferred from teleseismic data under the crustal root. 相似文献
19.
R. A. Cayley R. J. Korsch D. H. Moore R. D. Costelloe A. Nakamura C. E. Willman 《Australian Journal of Earth Sciences》2013,60(2):113-156
A ～400 km long deep crustal reflection seismic survey was acquired in central Victoria, Australia, in 2006. It has provided information on crustal architecture across the western Lachlan Orogen and has greatly added to the understanding of the tectonic evolution. The east-dipping Moyston Fault is confirmed as the suture between the Delamerian and western Lachlan Orogens, and is shown to extend down to the Moho. The Avoca Fault, the boundary between the Stawell and Bendigo Zones, is a west-dipping listric reverse fault that intersects the Moyston Fault at a depth of about 22 km, forming a V-shaped geometry. Both the Stawell and Bendigo Zones can be divided broadly into a lower crustal region of interlayered and imbricated metavolcanic and metasedimentary rocks and an upper crustal region of tightly folded metasedimentary rocks. The Stawell Zone was probably part of a Cambrian accretionary system along the eastern Gondwanaland margin, and mafic rocks may have been partly consumed by Cambrian subduction. Much of the Early Cambrian oceanic crust beneath the Bendigo Zone was not subducted, and is preserved as a crustal-scale imbricate thrust stack. The seismic data have shown that a thin-skinned structural model appears to be valid for much of the Melbourne Zone, whereas the Stawell and Bendigo Zones have a thick-skinned structural style. Internal faults in the Stawell and Bendigo Zones are mostly west-dipping listric faults, which extend from the surface to near the base of the crust. The Heathcote Fault Zone, the boundary between the Bendigo and Melbourne Zones, extends to at least 20 km, and possibly to the Moho. A striking feature in the seismic data is the markedly different seismic character of the mid to lower crust of the Melbourne Zone. The deep seismic reflection data for the Melbourne Zone have revealed a multilayered crustal structure that supports the Selwyn Block model. 相似文献
20.
The crustal structure of the central Eromanga Basin in the northern part of the Australian Tasman Geosyncline, revealed by coincident seismic reflection and refraction shooting, contrasts with some neighbouring regions of the continent. The depth to the crust-mantle boundary (Moho) of 36–41 km is much less than that under the North Australian Craton to the northwest (50–55 km) and the Lachlan Fold Belt to the southeast (43–51 km) but is similar to that under the Drummond and Bowen Basins to the east.The seismic velocity boundaries within the crust are sharp compared with the transitional nature of the boundaries under the North Australian and Lachlan provinces. In particular, there is a sharp velocity increase at mid-crustal depths (21–24 km) which has not been observed with such clarity elsewhere in Australia (the Conrad discontinuity?).In the lower crust, the many discontinuous sub-horizontal reflections are in marked contrast to lack of reflecting horizons in the upper crust, further emphasising the differences between the upper and lower crust. The crust-mantle boundary (Moho) is characterised by an increase in velocity from 7.1–7.7 km/s to a value of 8.15 + 0.04 km/s. The depth to the Moho under the Canaway Ridge, a prominent basement high, is shallower by about 5 km than the regional Moho depth; there is also no mid-crustal horizon under the Canaway Ridge but there is a very sharp velocity increase at the Moho depth of 34 km. The Ridge could be interpreted as a horst structure extending to at least Moho depths but it could also have a different intra-crustal structure from the surrounding area.The sub-crustal lithosphere has features which have been interpreted, from limited data, as being caused by a velocity gradient at 56–57 km depth with a low velocity zone above it.Because of the contrasting crustal thicknesses and velocity gradients, the lithosphere of the central Eromanga Basin cannot be considered as an extension of the exposed Lachlan Fold Belt or the North Australian Craton. The lack of seismic reflections from the upper crust indicates no coherent accoustic impedance pattern at wavelengths greater than 100 m, consistent with an upper crustal basement of tightly folded meta-sedimentary and meta-volcanic rocks. The crustal structure is consistent with a pericratonic or arc/back-arc basin being cratonised in an episode of convergent tectonics in the Early Palaeozoic. The seismic reflections from the lower crust indicate that it could have developed in a different tectonic environment. 相似文献