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1.
The Japan Trench is a plate convergent zone where the Pacific Plate is subducting below the Japanese islands. Many earthquakes occur associated with plate convergence, and the hypocenter distribution is variable along the Japan Trench. In order to investigate the detailed structure in the southern Japan Trench and to understand the variation of seismicity around the Japan Trench, a wide-angle seismic survey was conducted in the southern Japan Trench fore-arc region in 1998. Ocean bottom seismometers (15) were deployed on two seismic lines: one parallel to the trench axis and one perpendicular. Velocity structures along two seismic lines were determined by velocity modeling of travel time ray-tracing method. Results from the experiment show that the island arc Moho is 18–20 km in depth and consists of four layers: Tertiary and Cretaceous sedimentary rocks, island arc upper and lower crust. The uppermost mantle of the island arc (mantle wedge) extends to 110 km landward of the trench axis. The P-wave velocity of the mantle wedge is laterally heterogeneous: 7.4 km/s at the tip of the mantle wedge and 7.9 km/s below the coastline. An interplate layer is constrained in the subducting oceanic crust. The thickness of the interplate layer is about 1 km for a velocity of 4 km/s. Interplate layer at the plate boundary may cause weak interplate coupling and low seismicity near the trench axis. Low P-wave velocity mantle wedge is also consistent with weak interplate coupling. Thick interplate layer and heterogeneous P-wave velocity of mantle wedge may be associated with the variation of seismic activity.  相似文献   

2.
A two-dimensional model of the crust and uppermost mantle for the western Siberian craton and the adjoining areas of the Pur-Gedan basin to the north and Baikal Rift zone to the south is determined from travel time data from recordings of 30 chemical explosions and three nuclear explosions along the RIFT deep seismic sounding profile. This velocity model shows strong lateral variations in the crust and sub-Moho structure both within the craton and between the craton and the surrounding region. The Pur-Gedan basin has a 15-km thick, low-velocity sediment layer overlying a 25-km thick, high-velocity crystalline crustal layer. A paleo-rift zone with a graben-like structure in the basement and a high-velocity crustal intrusion or mantle upward exists beneath the southern part of the Pur-Gedan basin. The sedimentary layer is thin or non-existent and there is a velocity reversal in the upper crust beneath the Yenisey Zone. The Siberian craton has nearly uniform crustal thickness of 40–43 km but the average velocity in the lower crust in the north is higher (6.8–6.9 km/s) than in the south (6.6 km/s). The crust beneath the Baikal Rift zone is 35 km thick and has an average crustal velocity similar to that observed beneath the southern part of craton. The uppermost mantle velocity varies from 8.0 to 8.1 km/s beneath the young West Siberian platform and Baikal Rift zone to 8.1–8.5 km/s beneath the Siberian craton. Anomalous high Pn velocities (8.4–8.5 km/s) are observed beneath the western Tunguss basin in the northern part of the craton and beneath the southern part of the Siberian craton, but lower Pn velocities (8.1 km/s) are observed beneath the Low Angara basin in the central part of the craton. At about 100 km depth beneath the craton, there is a velocity inversion with a strong reflecting interface at its base. Some reflectors are also distinguished within the upper mantle at depth between 230 and 350 km.  相似文献   

3.
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   

4.
Ultrasonic laboratory measurements of P-wave velocity (Vp) were carried out up to 1.0 GPa in a temperature range of 25–400 °C for crustal and mantle xenoliths of Ichino-megata, northeast Japan. The rocks used in the present study cover a nearly entire range of lithological variation of the Ichino-megata xenoliths and are considered as representative rock samples of the lower crust and upper mantle of the back arc side of the northeast (NE) Honshu arc. The Vp values measured at 25 °C and 1.0 GPa are 6.7–7.2 km/s for the hornblende gabbros (38.6–46.9 wt.% SiO2), 7.2 km/s for the hornblende-pyroxene gabbro (43.8 wt.% SiO2), 6.9–7.3 km/s for the amphibolites (36.1–44.3 wt.% SiO2), 8.0–8.1 km/s for the spinel lherzolites (46.2–47.2 wt.% SiO2) and 6.30 km/s for the biotite granite (72.1 wt.% SiO2). Combining the present data with the Vp profile of the NE Honshu arc [Iwasaki, T., Kato, W., Moriya, T., Hasemi, A., Umino, N., Okada, T., Miyashita, K., Mizogami, T., Takeda, T., Sekine, S., Matsushima, T., Tashiro, K., Miyamachi, H. 2001. Extensional structure in northern Honshu Arc as inferred from seismic refraction/wide-angle reflection profiling. Geophys. Res. Lett. 28 (12), 2329–2332], we infer that the 15 km thick lower crust of the NE Honshu arc is composed of amphibolite and/or hornblende (±pyroxene) gabbro with ultrabasic composition. The present study suggests that the Vp range of the lower crustal layer (6.6–7.0 km/s) in the NE Honshu arc, which is significantly lower than that obtained from various seismic measurements (e.g. the northern Izu-Bonin-Mariana arc: 7.1–7.3 km/s), is due to the thick hydrous lower crustal layer where hornblende, plagioclase and magnetite are dominant.  相似文献   

5.
Christoffer Nielsen  H. Thybo   《Tectonophysics》2009,470(3-4):298-318
The Cenozoic Baikal Rift Zone (BRZ) is situated in south-central Siberia in the suture between the Precambrian Siberian Platform and the Amurian plate. This more than 2000-km long rift zone is composed of several individual basement depressions and half-grabens with the deep Lake Baikal at its centre. The BEST (Baikal Explosion Seismic Transect) project acquired a 360-km long, deep seismic, refraction/wide-angle reflection profile in 2002 across southern Lake Baikal. The data from this project is used for identification of large-scale crustal structures and modelling of the seismic velocities of the crust and uppermost mantle. Previous interpretation and velocity modelling of P-wave arrivals in the BEST data has revealed a multi layered crust with smooth variation in Moho depth between the Siberian Platform (41 km) and the Sayan-Baikal fold belt (46 km). The lower crust exhibits normal seismic velocities around the rift structure, except for beneath the rift axis where a distinct 50–80-km wide high-velocity anomaly (7.4–7.6 ± 0.2 km/s) is observed. Reverberant or “ringing” reflections with strong amplitude and low frequency originate from this zone, whereas the lower crust is non-reflective outside the rift zone. Synthetic full-waveform reflectivity modelling of the high-velocity anomaly suggests the presence of a layered sequence with a typical layer thickness of 300–500 m coinciding with the velocity anomaly. The P-wave velocity of the individual layers is modelled to range between 7.4 km/s and 7.9 km/s. We interpret this feature as resulting from mafic to ultra-mafic intrusions in the form of sills. Petrological interpretation of the velocity values suggests that the intrusions are sorted by fractional crystallization into plagioclase-rich low-velocity layers and pyroxene- and olivine-rich high-velocity layers. The mafic intrusions were probably intruded into the ductile lower crust during the main rift phase in the Late Pliocene. As such, the intrusive material has thickened the lower crust during rifting, which may explain the lack of Moho uplift across southern BRZ.  相似文献   

6.
Anomalous crustal and upper mantle structure of northern Juan de Fuca plate is revealed from wide-angle seismic and gravity modelling. A 2-D velocity model is produced for refraction line II of the 1980 Vancouver Island Seismic Project (VISP80). The refraction data were recorded on three ocean bottom seismometers (OBSs) deployed at the ends and middle of a 110 km line oriented parallel to the North American continental margin. The velocity model is constructed via ray tracing and conforms to first-arrival amplitude observations and travel time picks of direct, converted and reflected phases. Between sub-sediment depths of 3 to 11 km, depths normally associated with the lower crust and upper oceanic mantle, the final model shows that compressional-wave velocities decrease significantly from southeast to northwest along the profile. At sub-sediment depths of 11 km at the northwestern end of the profile, P-wave velocities are as low as 7.2 km/s. A complementary 2-D gravity model using the geometry of the velocity model and velocity–density relationships characteristic of oceanic crust is produced. The high densities required to match the gravity field indicate the presence of peridotites containing 25–30% serpentine by volume, rather than excess gabbroic crust, within the deep low velocity zone. Anomalous travel time delays and unusual reflection characteristics observed from proximal seismic refraction and reflection experiments suggest a broader zone of partially serpentinized peridotites coincident with the trace of a pseudofault. We propose that partial serpentinization of the upper mantle is a consequence of slow spreading at the tip of a propagating rift.  相似文献   

7.
The VRANCEA99 seismic refraction experiment is part of an international and multidisciplinary project to study the intermediate depth earthquakes of the Eastern Carpathians in Romania. As part of the seismic experiment, a 300-km-long refraction profile was recorded between the cities of Bacau and Bucharest, traversing the Vrancea epicentral region in NNE–SSW direction.

The results deduced using forward and inverse ray trace modelling indicate a multi-layered crust. The sedimentary succession comprises two to four seismic layers of variable thickness and with velocities ranging from 2.0 to 5.8 km/s. The seismic basement coincides with a velocity step up to 5.9 km/s. Velocities in the upper crystalline crust are 5.9–6.2 km/s. An intra-crustal discontinuity at 18–31 km divides the crust into an upper and a lower layer. Velocities within the lower crust are 6.7–7.0 km/s. Strong wide-angle PmP reflections indicate the existence of a first-order Moho at a depth of 30 km near the southern end of the line and 41 km near the centre. Constraints on upper mantle seismic velocities (7.9 km/s) are provided by Pn arrival times from two shot points only. Within the upper mantle a low velocity zone is interpreted. Travel times of a PLP reflection define the bottom of this low velocity layer at a depth of 55 km. The velocity beneath this interface must be at least 8.5 km/s.

Geologic interpretation of the seismic data suggests that the Neogene tectonic convergence of the Eastern Carpathians resulted in thin-skinned shortening of the sedimentary cover and in thick-skinned shortening in the crystalline crust. On the autochthonous cover of the Moesian platform several blocks can be recognised which are characterised by different lithological compositions. This could indicate a pre-structuring of the platform at Mesozoic and/or Palaeozoic times with a probable active involvement of the Intramoesian and the Capidava–Ovidiu faults. Especially the Intramoesian fault is clearly recognisable on the refraction line. No clear indications of the important Trotus fault in the north of the profile could be found. In the central part of the seismic line a thinned lower crust and the low velocity zone in the uppermost mantle point to the possibility of crustal delamination and partial melting in the upper mantle.  相似文献   


8.
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   

9.
Numerous ge ological and geophysical investigations within the past decades have shown that the Rhinegraben is the most pronounced segment of an extended continental rift system in Europe. The structure of the upper and lower crust is significantly different from the structure of the adjacent “normal” continental crust.

Two crustal cross-sections across the central and southern part of the Rhinegraben have been constructed based on a new evaluation of seismic refraction and reflection measurements. The most striking features of the structure derived are the existence of a well-developed velocity reversal in the upper crust and of a characteristic cushion-like layer with a compressional velocity of 7.6–7.7 km/sec in the lower crust above a normal mantle with 8.2 km/sec. Immediately below the sialic low-velocity zone in the middle part of the crust, an intermediate layer with lamellar structure and of presumably basic composition could be mapped.

It is interesting to note that the asymmetry of the sedimentary fill in the central Rhinegraben seems to extend down deeper into the upper crust as indicated by the focal depths of earthquakes. The top of the rift “cushion” shows a marked relief which has no obvious relation to the crustal structure above it or the visible rift at the surface.  相似文献   


10.
We present results from a seismic refraction experiment on the northern margin of the Guayana Shield performed during June 1998, along nine profiles of up to 320 km length, using the daily blasts of the Cerro Bolívar mines as energy source, as well as from gravimetric measurements. Clear Moho arrivals can be observed on the main E–W profile on the shield, whereas the profiles entering the Oriental Basin to the north are more noisy. The crustal thickness of the shield is unusually high with up to 46 km on the Archean segment in the west and 43 km on the Proterozoic segment in the east. A 20 km thick upper crust with P-wave velocities between 6.0 and 6.3 km/s can be separated from a lower crust with velocities ranging from 6.5 to 7.2 km/s. A lower crustal low velocity zone with a velocity reduction to 6.3 km/s is observed between 25 and 25 km depth. The average crustal velocity is 6.5 km/s. The changes in the Bouguer Anomaly, positive (30 mGal) in the west and negative (−20 mGal) in the east, cannot be explained by the observed seismic crustal features alone. Lateral variations in the crust or in the upper mantle must be responsible for these observations.  相似文献   

11.
Claus Prodehl 《Tectonophysics》1981,80(1-4):255-269
The crustal structure of the central European rift system has been investigated by seismic methods with varying success. Only a few investigations deal with the upper-mantle structure. Beneath the Rhinegraben the Moho is elevated, with a minimum depth of 25 km. Below the flanks it is a first-order discontinuity, while within the graben it is replaced by a transition zone with the strongest velocity gradient at 20–22 km depth. An anomalously high velocity of up to 8.6 km/s seems to exist within the underlying upper mantle at 40–50 km depth. A similar structure is also found beneath the Limagnegraben and the young volcanic zones within the Massif Central of France, but the velocity within the upper mantle at 40–50 km depth seems to be slightly lower. Here, the total crustal thickness reaches only 25 km. The crystalline crust becomes extremely thin beneath the southern Rhônegraben, where the sediments reach a thickness of about 10 km while the Moho is found at 24 km depth. The pronounced crustal thinning does not continue along the entire graben system. North of the Rhinegraben in particular the typical graben structure is interrupted by the Rhenohercynian zone with a “normal” West-European crust of 30 km thickness evident beneath the north-trending Hessische Senke. A single-ended profile again indicates a graben-like crustal structure west of the Leinegraben north of the Rhenohercynian zone. No details are available for the North German Plain where the central European rift system disappears beneath a sedimentary sequence of more than 10 km thickness.  相似文献   

12.
Possibilities for the fate of oceanic plateaus at subduction zones range from complete subduction of the plateau beneath the arc to complete plateau–arc accretion and resulting collisional orogenesis. Deep penetration, multi-channel seismic reflection (MCS) data from the northern flank of the Solomon Islands reveal the sequence stratigraphy, structural style, and age of deformation of an accretionary prism formed during late Neogene (5–0 Ma) convergence between the 33-km-thick crust of the Ontong Java oceanic plateau and the 15-km-thick Solomon island arc. Correlation of MCS data with the satellite-derived, free-air gravity field defines the tectonic boundaries and internal structure of the 800-km-long, 140-km-wide accretionary prism. We name this prism the “Malaita accretionary prism” or “MAP” after Malaita, the largest and best-studied island exposure of the accretionary prism in the Solomon Islands. MCS data, gravity data, and stratigraphic correlations to islands and ODP sites on the Ontong Java Plateau (OJP) reveal that the offshore MAP is composed of folded and thrust faulted sedimentary rocks and upper crystalline crust offscraped from the Solomon the subducting Ontong Java Plateau (Pacific plate) and transferred to the Solomon arc. With the exception of an upper, sequence of Quaternary? island-derived terrigenous sediments, the deformed stratigraphy of the MAP is identical to that of the incoming Ontong Java Plateau in the North Solomon trench.We divide the MAP into four distinct, folded and thrust fault-bounded structural domains interpreted to have formed by diachronous, southeast-to-northwest, and highly oblique entry of the Ontong Java Plateau into a former trench now marked by the Kia–Kaipito–Korigole (KKK) left-lateral strike-slip fault zone along the suture between the Solomon arc and the MAP. The structural style within each of the four structural domains consists of a parallel series of three to four fault propagation folds formed by the seaward propagation of thrust faults roughly parallel to sub-horizontal layering in the upper crystalline part of the OJP. Thrust fault offsets, spacing between thrusts, and the amplitude of related fault propagation folds progressively decrease to the west in the youngest zone of active MAP accretion (Choiseul structural domain). Surficial faulting and folding in the most recently deformed, northwestern domain show active accretion of greater than 1 km of sedimentary rock and 6 km, or about 20%, of the upper crystalline part of the OJP. The eastern MAP (Malaita and Ulawa domains) underwent an earlier, similar style of partial plateau accretion. A pre-late Pliocene age of accretion (3.4 Ma) is constrained by an onshore and offshore major angular unconformity separating Pliocene reefal limestone and conglomerate from folded and faulted pelagic limestone of Cretaceous to Miocene age. The lower 80% of the Ontong Java Plateau crust beneath the MAP thrust decollement appears unfaulted and unfolded and is continuous with a southwestward-dipping subducted slab of presumably denser plateau material beneath most of the MAP, and is traceable to depths >200 km in the mantle beneath the Solomon Islands.  相似文献   

13.
The CELEBRATION 2000 together with the earlier POLONAISE'97 deep seismic sounding experiments was aimed at the recognition of crustal structure in the border zone between the Precambrian East European Craton (Baltica) and Palaeozoic Europe. The CEL02 profile of the CELEBRATION family is a 400-km long SW–NE transect, running in Poland from the Upper Silesia Block (USB), across the Małopolska Block (MB) and the Trans-European Suture Zone (TESZ) to the East European Craton (EEC). The structure along CEL02 was interpreted using both 2D tomography and forward ray-tracing techniques as well as 2D gravity modelling.The crustal thickness along CEL02 varies from 32–35 km in the USB to 45–47 km beneath the TESZ and the EEC. The USB is a clearly distinctive crustal block with the characteristic high velocity lower crust (7.1–7.2 km/s), interpreted as a fragment of Gondwana. The Kraków–Lubliniec Fault is a terrane boundary produced by soft docking of the USB with the MB. The Małopolska crust fundamentally differs from the USB and has a strong connection with Baltica. It is a transitional, 150- to 200-km wide unit composed of the extended Baltican lower crust and the overlying low velocity (5.15–5.9 km/s) Neoproterozoic metasediments in the up to 18-km thick upper crust. The Łysogóry Unit has its crustal structure identical with that of Małopolska, thus it is connected with Baltica and cannot be interpreted as a Gondwana-derived terrane. Higher velocity and density bodies found below the Mazovia–Lublin Graben at a depth of 12 km and at the base of the lower crust, might be a result of mantle-derived mafic intrusions accompanying the extension of Baltica. By the preliminary 2D gravity modelling, we have reconfirmed the need for considering the increased TESZ mantle density in comparison to the EEC and USB mantle.  相似文献   

14.
One in-line wide-angle seismic profile was conducted in 1990 in the course of the Southeastern China Continental Dynamics project aimed at the study of the contact between the Cathaysia block and the Yangtze block. This 380-km-long profile extended in NW–SE direction from Tunxi, Anhui Province, to Wenzhou, Zhejiang Province. Five in-line shots were fired and recorded at seismic stations with spacing of about 3 km along the recording line. We have used two-dimensional ray tracing to model P- and S-wave arrivals and provide constraints on the velocity structure of the upper crust, middle crust, lower crust, Moho discontinuity, and the top part of the lithospheric mantle. P-wave velocity, S-wave velocity and VP/VS ratio are mapped. The crust is 36-km thick on average, albeit it gradually thins from the northwest end to the southeast end (offshore) of the profile. The average crustal velocity is 6.26 km/s for P-waves but 3.6 km/s for S-waves. A relatively narrow low-velocity layer of about 4 km of thickness, with P- and S-wave velocities of 6.2 km/s and 3.5 km/s, respectively, marks the bottom of the middle crust at a depth of 23-km northwest and 17-km southeast. At the crust–mantle transition, the P- and S-wave velocity change quickly from 7.4 to 7.8 km/s (northwest) and 8.0 to 8.2 km/s (southeast) and from 3.9 to 4.2 km/s (northwest) and 3.9 to 4.5 km/s (southeast), respectively. This result implies a lateral contrast in the upper mantle velocity along the 140 km sampled by the profile approximately. The average VP/VS ratio ranges from 1.68–1.8 for the upper crust to 1.75 for the middle and 1.75–1.85 for lower crust. With the interpretation of the wide-angle seismic data, Jiangshan–Shaoxin fault is considered as the boundary between the Yangtze and the Cathaysia block.  相似文献   

15.
We present the first detailed seismic velocity models of the crust and uppermost mantle around the Mirnyi kimberlite field in Yakutia, Siberia. We have digitized vintage seismograms that were acquired in 1981 and 1983 by use of Taiga analogue seismographs along two perpendicular seismic profiles. The 370-km long, northwest striking profile I across the kimberlite pipe was covered by 41 seismographs, which recorded seismic signals from 21 chemical shots along the line, including one off-end shot. The perpendicular, 340-km long profile II across profile I ca. 30 km to the south of the Mirnyi kimberlite field was covered by 45 seismographs, which recorded seismic signals from 22 chemical shots, including four off-end shots. Each shot involved detonation of between 1.5 and 6.0 tons of TNT, distributed in individual charges of 100–200 kg in shallow water (< 2 m deep). The data is of high quality with high signal/noise ratio to the farthest offsets. We present the results from two-dimensional ray tracing, forward modelling.Both velocity models show normal cratonic structure of the ca. 45-km-thick crust with only slight undulation of the Moho. However, relatively small seismic velocity is detected to 25-km depth in a ca. 60-km wide zone around the kimberlite pipe, surrounded by elevated velocity (> 6.3 km/s) in the upper crust. The lower crust has a relatively constant velocity of 6.8–6.9 km/s. It appears relatively unaffected by the presence of the kimberlite field. Extremely large P-wave velocity (> 8.7 km/s) of the sub-Moho mantle is interpreted along profile I, except for a 70-km wide zone with a “normal” Pn velocity of 8.1 km/s below the kimberlite. Profile II mainly shows Pn velocities of 8.0–8.2 km/s, with unusually large velocity (> 8.5 km/s) in two, ca. 100-km wide zones, at its southwestern end, one zone being close to the kimberlite field. The nature of these exceptionally large, sub-Moho mantle velocities is not yet understood. The difference in velocity in the two profile directions indicates anisotropy, but the effect of unusual rock composition, e.g. from a high concentration of garnet, cannot be excluded.  相似文献   

16.
The large-scale POLONAISE'97 seismic experiment investigated the velocity structure of the lithosphere in the Trans-European Suture Zone (TESZ) region between the Precambrian East European Craton (EEC) and Palaeozoic Platform (PP). In the area of the Polish Basin, the P-wave velocity is very low (Vp <6.1 km/s) down to depths of 15–20 km, and the consolidated basement (Vp5.7–5.8 km/s) is 5–12 km deep. The thickness of the crust is 30 km beneath the Palaeozoic Platform, 40–45 km beneath the TESZ, and 40–50 km beneath the EEC. The compressional wave velocity of the sub-Moho mantle is >8.25 km/s in the Palaeozoic Platform and 8.1 km/s in the Precambrian Platform. Good quality record sections were obtained to the longest offsets of about 600 km from the shot points, with clear first arrivals and later phases of waves reflected/refracted in the lower lithosphere. Two-dimensional interpretation of the reversed system of travel times constrains a series of reflectors in the depth range of 50–90 km. A seismic reflector appears as a general feature at around 10 km depth below Moho in the area, independent of the actual depth to the Moho and sub-Moho seismic velocity. “Ringing reflections” are explained by relatively small-scale heterogeneities beneath the depth interval from 90 to 110 km. Qualitative interpretation of the observed wave field shows a differentiation of the reflectivity in the lower lithosphere. The seismic reflectivity of the uppermost mantle is stronger beneath the Palaeozoic Platform and TESZ than the East European Platform. The deepest interpreted seismic reflector with zone of high reflectivity may mark a change in upper mantle structure from an upper zone characterised by seismic scatterers of small vertical dimension to a lower zone with vertically larger seismic scatterers, possible caused by inclusions of partial melt.  相似文献   

17.
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   

18.
Abyssal variations beneath the Baikal rift zone are revealed in an irregular seismic stratification of the crust, the presence of an intracrust waveguide and by the vast (> 200,000 km2) underlying area of anomalously low velocity (Pn = 7.6−7.8 km/sec) uppermost mantle. In its abyssal structure the Baikal rift is heterogeneous along the strike, with sharp changes in crustal thickness (35–50 km).Comparison of first-arrival seismic-velocity curves and also the respective velocity columns reveals the essential similarity of upper-mantle seismic cross-sections for all continental rift zones. The anomalous upper layer of the mantle (ca. 7.7 km/sec) is relatively thin (15-13 km) and can be linked with the mantle waveguide only locally.  相似文献   

19.
Crustal structure across the passive continental margin of the northeastern South China Sea (SCS) is presented based on a deep seismic survey cooperated between Taiwan and China in August 2001. Reflection data collected from a 48-hydrophone streamer and the vertical component of refraction/reflection data recorded at 11 ocean-bottom seismometers along a NW–SE profile are integrated to image the upper (1.6–2.4 km/s), lower (2.5–2.9 km/s), and compacted (3–4.5 km/s) sediment, the upper (4.5–5.5 km/s), middle (5.5–6.5 km/s) and lower (6.5–7.5 km/s) crystalline crust successively. The velocity model shows that the thickness (0.5–3 km) and the basement of the compacted sediment are strongly varied due to intrusion of the magma and igneous rocks after seafloor spreading of the SCS. Furthermore, several volcanoes and igneous rocks in the upper/middle crust (7–10 km thick) and a high velocity layer (0–5 km thick) in the lower crust of the model are identified as the ocean–continent transition (OCT) below the lower slope in the northeastern margin of the SCS. A thin continent NW of the OCT and a thick oceanic crust SE of the OCT in the continental margin of the northeastern SCS are also imaged, but these transitional crusts cannot be classified as the OCT due to their crustal thickness and the limited amount of the volcano, the magma and the high velocity layer. The extended continent, next to the gravity low and a sag zone extended from the SW Taiwan Basin, may have resulted from subduction of the Eurasian Plate beneath the Manila Trench whereas the thick oceanic crust may have been due to the excess volcanism and the late magmatic underplating in the oceanic crust after seafloor spreading of the SCS.  相似文献   

20.
The Hidaka Collision Zone (HCZ), central Hokkaido, Japan, is a good target for studies of crustal evolution and deformation processes associated with an arc–arc collision. The collision of the Kuril Arc (KA) with the Northeast Japan Arc (NJA), which started in the middle Miocene, is considered to be a controlling factor for the formation of the Hidaka Mountains, the westward obduction of middle/lower crustal rocks of the KA (the Hidaka Metamorphic Belt (HMB)) and the development of the foreland fold-and-thrust belt on the NJA side. The “Hokkaido Transect” project undertaken from 1998 to 2000 was a multidisciplinary effort intended to reveal structural heterogeneity across this collision zone by integrated geophysical/geological research including seismic refraction/reflection surveys and earthquake observations. An E–W trending 227 km-long refraction/wide-angle reflection profile found a complicated structural variation from the KA to the NJA across the HCZ. In the east of the HCZ, the hinterland region is covered with 4–4.5 km thick highly undulated Neogene sedimentary layers, beneath which two eastward dipping reflectors were imaged in a depth range of 10–25 km, probably representing the layer boundaries of the obducting middle/lower crust of the KA. The HMB crops out on the westward extension of these reflectors with relatively high Vp (>6.0 km/s) and Vp/Vs (>1.80) consistent with middle/lower crustal rocks. Beneath these reflectors, more flat and westward dipping reflector sequences are situated at the 25–27 km depth, forming a wedge-like geometry. This distribution pattern indicates that the KA crust has been delaminated into more than two segments under our profile. In the western part of the transect, the structure of the fold-and-thrust belt is characterized by a very thick (5–8 km) sedimentary package with a velocity of 2.5–4.8 km/s. This package exhibits one or two velocity reversals in Paleogene sedimentary layers, probably formed by imbrication associated with the collision process. From the horizontal distribution of these velocity reversals and other geophysical/geological data, the rate of crustal shortening in this area is estimated to be greater than 3–4 mm/year, which corresponds to 40–50% of the total convergence rate between the NJA and the Eurasian Plate. This means that the fold-and-thrust belt west of the HCZ is absorbing a large amount of crustal deformation associated with plate interaction across Hokkaido Island.  相似文献   

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