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1.
K. Hinz 《Tectonophysics》1973,20(1-4):295-302
Within the frame of the German-French project ANNA-1970, two long refraction profiles were investigated north and south of the island of Majorca.

For the southern Balearic Basin an oceanic crust can be derived from the travel-time curves consisting of a 4.0 km thick Cenozoic sedimentary layer with: Vp = 2.35 (km/sec) + 0.35 (sec−1) × Z (km) and a 5 km thick layer with: Vp = 4.0 (km/sec) + 0.28 (sec−1) × Z (km)

The transition to the upper mantle takes place at a depth of 12 km. Directly south of Majorca a crustal thickening was measured which may be caused by the process of crustal shortening. P]In the northern Balearic Basin a faulted transitional type of crust has been observed indicating probably an embryonic and juvenile ocean expansion.  相似文献   


2.
The POLONAISE'97 (POlish Lithospheric ONset—An International Seismic Experiment, 1997) seismic experiment in Poland targeted the deep structure of the Trans-European Suture Zone (TESZ) and the complex series of upper crustal features around the Polish Basin. One of the seismic profiles was the 300-km-long profile P2 in northwestern Poland across the TESZ. Results of 2D modelling show that the crustal thickness varies considerably along the profile: 29 km below the Palaeozoic Platform; 35–47 km at the crustal keel at the Teisseyre–Tornquist Zone (TTZ), slightly displaced to the northeast of the geologic inversion zone; and 42 km below the Precambrian Craton. In the Polish Basin and further to the south, the depth down to the consolidated basement is 6–14 km, as characterised by a velocity of 5.8–5.9 km/s. The low basement velocities, less than 6.0 km/s, extend to a depth of 16–22 km. In the middle crust, with a thickness of ca. 4–14 km, the velocity changes from 6.2 km/s in the southwestern to 6.8 km/s in the northeastern parts of the profile. The lower crust also differs between the southwestern and northeastern parts of the profile: from 8 km thickness, with a velocity of 6.8–7.0 km/s at a depth of 22 km, to ca.12 km thickness with a velocity of 7.0–7.2 km/s at a depth of 30 km. In the lowermost crust, a body with a velocity of 7.20–7.25 km/s was found above Moho at a depth of 33–45 km in the central part of the profile. Sub-Moho velocities are 8.2–8.3 km/s beneath the Palaeozoic Platform and TTZ, and about 8.1 km/s beneath the Precambrian Platform. Seismic reflectors in the upper mantle were interpreted at 45-km depth beneath the Palaeozoic Platform and 55-km depth beneath the TTZ.

The Polish Basin is an up to 14-km-thick asymmetric graben feature. The basement beneath the Palaeozoic Platform in the southwest is similar to other areas that were subject to Caledonian deformation (Avalonia) such that the Variscan basement has only been imaged at a shallow depth along the profile. At northeastern end of the profile, the velocity structure is comparable to the crustal structure found in other portions of the East European Craton (EEC). The crustal keel may be related to the geologic inversion processes or to magmatic underplating during the Carboniferous–Permian extension and volcanic activity.  相似文献   


3.
The detailed velocity structure of a 70 by 35 km area of 6–10 Ma old crust on the flank of the mid Atlantic Ridge at 24°N was studied using 358 explosive charges and several hundred 16.4-1 airgun shots fired into an array of eight ocean bottom hydrophones. Inversion of the first arrival refracted travel times shows that the crust comprises a normal oceanic section about 5 km thick with a steep velocity gradient in the upper crust increasing from about 3.5 km/sec at the seafloor overlying a typical oceanic layer 3 and a probably anisotropic mantle. Delay time function mapping using two datasets containing arrivals from layer 3 and from the mantle show that lateral variability is generally low over most of the survey area, with a small region of high delay times in the northwest corner caused by the presence of abnormal crust probably associated with a minor fracture zone. We find that the topography of the base of layer 2 is similar to that of the top, indicating that the normal faulting which occurs along the margins of the median valley extends down at least into layer 3. Our observations from mantle arrivals are consistent with a much flatter Moho which constrains possible models of crustal formation at the spreading centre.  相似文献   

4.
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   

5.
The lithospheric structure of the western part of the Mediterranean Sea is shown by means of S-velocity maps, for depths ranging from 0 to 35 km, determined from Rayleigh-wave analysis. The traces of 55 earthquakes, which occurred from 2001 to 2003 in and around the study area have been used to obtain Rayleigh-wave dispersion. These earthquakes were registered by 10 broadband stations located on Iberia and the Balearic Islands. The dispersion curves were obtained for periods between 1 and 45 s, by digital filtering with a combination of MFT and TVF filtering techniques. After that, all seismic events were grouped in source zones to obtain a dispersion curve for each source-station path. These dispersion curves were regionalized and after inverted according to the generalized inversion theory, to obtain shear-wave velocity models for rectangular blocks with a size of 1° × 1°. The shear velocity structure obtained through this procedure is shown in the S-velocity maps plotted for several depths. These maps show the existence of lateral and vertical heterogeneity. In these maps is possible to distinguish several types of crust with an average S-wave velocity ranging from 2.6 to 3.9 km/s. The South Balearic Basin (SBB) is more characteristic of oceanic crust than the rest of the western Mediterranean region, as it is demonstrated by the crustal thickness. We also find a similar S-wave velocity (ranging from 2.6 km/s at the surface to 3.2 km/s at 10 km depth) for the Iberian Peninsula coast to Ibiza Island, the North Balearic Basin (NBB) and Mallorca Island. In the lower crust, the shear velocity reaches a value of 3.9 km/s. The base of the Moho is estimated from 15 to 20 km under Iberian Peninsula coast to Ibiza Island, continues towards NBB and increases to 20–25 km beneath Mallorca Island. While, the SBB is characterized by a thinner crust that ranges from 10 to 15 km, and a faster velocity. A gradual increase in velocity from the north to the south (especially in the upper 25 km) is obtained for the western part of the Mediterranean Sea. The base of the crust has a shear-wave velocity value around of 3.9 km/s for the western Mediterranean Sea area. This area is characterized by a thin crust in comparison with the crustal thickness of the eastern Mediterranean Sea area. This thin crust is related with the distensive tectonics that exists in this area. The low S-wave velocities obtained in the upper mantle might be an indication of a serpentinized mantle. The obtained results agree well with the geology and other geophysical results previously obtained. The shear velocity generally increases with depth for all paths analyzed in the study area.  相似文献   

6.
P. Giese  C. Morelli  L. Steinmetz   《Tectonophysics》1973,20(1-4):367-379
During the past two decades deep seismic sounding measurements have been carried out in western and southern Europe, mainly using the refraction method. These investigations were performed partly on a national basis but as well within international cooperative programs under the sponsorship of the European Seismological Commission.

In France, a systematic study has been executed to determine the main feature of deep structures under the Central Massif and the Paris Basin. In the Forez and Margeride regions, the sub-crustal velocity is lower (7.2 km/sec) than the normal value (8.0 km/sec) observed in the adjacent areas.

The central and southern part of Western Germany is covered by an extensive network of refraction profiles. The crustal thickness varies, similarly to France, from 25 to 35 km. A great amount of deep reflection data was obtained by commercial and special reflection work. The crust beneath the Rhinegraben area shows the typical “rift system” structure with a low subcrustal velocity (7.4–7.7 km/sec).

Very intensive refraction work has been carried out in the Alpine area. The maximum crustal thickness found near the axis of the negative gravity anomaly is about 55–60 km. Furthermore, a clear lowvelocity layer at a depth between 10 and 30 km has been detected. A key position with regard to the geotectonic structure of the Alps is held by the zone of Ivrea characterized by a pronounced gravity high. From the refraction work it may be concluded that there material of the lower crust and the upper mantle (7.2–7.5 km/sec) is overlying a layer of extremely low velocity (5.0 km/sec) which is interpreted as sialic crust.

Three years ago, a systematic study of crustal structure of the Italian peninsula has been started. Reversed profiles were observed on Sicily, in Calabria, and in Puglia. On Sicily, the structure is very complicated; the crust of the western part looks like a transition between a continental and oceanic structure whereas the eastern side shows a continental-type crust. In Calabria and Puglia, the crustal thickness has been determined to be about 25–35 km.  相似文献   


7.
Seismic refraction profiles completed in the past twenty years reveal that the top of the basement complex generally lies near sea level in East Antarctica but typically 2 or 3 km below sea level in West Antarctica. Throughout much of East Antarctica the thickness of the layer overlying the basement complex is less than half a kilometer, although a Phanerozoic sequence more than 1 km thick probably underlies the ice at the South Pole. Throughout central West Antarctica, on the other hand, a section one to several kilometers thick generally overlies the basement complex. The observed sedimentary section is no more than one half kilometer thick on either side of the Transantarctic Mountains. Rocks with high seismic velocities typical of the lower continental crust occur within a few kilometers of the surface on both sides of the Transantarctic Mountains. This occurrence lends support to the hypothesis of an abrupt increase in crustal thickness between West and East Antarctica.

In 1969, deep seismic soundings were carried out by the 14th Soviet Antarctic Expedition near the coast of Queen Maud Land. The crustal thickness was found to be about 40 km near the mountains, decreasing to about 30 km near the coast. In the top 15 km of the crust there is a gradual downward increase in P-wave velocity from 6.0 to 6.3 km/sec. The average velocity through the crust is 6.4 km/sec and the measured velocity below the M-discontinuity is 7.9 km/sec.

At the southwestern margin of the Ronne Ice Shelf, near-vertical reflections from the M-discontinuity have been recorded. A mean P-wave velocity of 6 km/sec in the crust was measured, leading to an estimated depth to M of 24 km below sea level.

Seismic surface wave dispersion studies indicate a mean crustal thickness of about 30 km in West Antarctica and about 40 km in East Antarctica. The dispersion data also show that group velocities across East Antarctica are much closer to those along average continental paths than to those across the Canadian shield. The results thus support other indications that central East Antarctica is not a simple crystalline shield.

P′P′-reflections beneath the continent support the existence of a low-velocity channel for P-waves, but show no significant difference in deep structure between Antarctica and other continents.  相似文献   


8.
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   

9.
A combined gravity map over the Indian Peninsular Shield (IPS) and adjoining oceans brings out well the inter-relationships between the older tectonic features of the continent and the adjoining younger oceanic features. The NW–SE, NE–SW and N–S Precambrian trends of the IPS are reflected in the structural trends of the Arabian Sea and the Bay of Bengal suggesting their probable reactivation. The Simple Bouguer anomaly map shows consistent increase in gravity value from the continent to the deep ocean basins, which is attributed to isostatic compensation due to variations in the crustal thickness. A crustal density model computed along a profile across this region suggests a thick crust of 35–40 km under the continent, which reduces to 22/20–24 km under the Bay of Bengal with thick sediments of 8–10 km underlain by crustal layers of density 2720 and 2900/2840 kg/m3. Large crustal thickness and trends of the gravity anomalies may suggest a transitional crust in the Bay of Bengal up to 150–200 km from the east coast. The crustal thickness under the Laxmi ridge and east of it in the Arabian Sea is 20 and 14 km, respectively, with 5–6 km thick Tertiary and Mesozoic sediments separated by a thin layer of Deccan Trap. Crustal layers of densities 2750 and 2950 kg/m3 underlie sediments. The crustal density model in this part of the Arabian Sea (east of Laxmi ridge) and the structural trends similar to the Indian Peninsular Shield suggest a continent–ocean transitional crust (COTC). The COTC may represent down dropped and submerged parts of the Indian crust evolved at the time of break-up along the west coast of India and passage of Reunion hotspot over India during late Cretaceous. The crustal model under this part also shows an underplated lower crust and a low density upper mantle, extending over the continent across the west coast of India, which appears to be related to the Deccan volcanism. The crustal thickness under the western Arabian Sea (west of the Laxmi ridge) reduces to 8–9 km with crustal layers of densities 2650 and 2870 kg/m3 representing an oceanic crust.  相似文献   

10.
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   

11.
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   

12.
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   

13.
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   

14.
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   

15.
A.C. Ries 《Earth》1978,14(1):35-63
The anticlockwise rotation of the Iberian Peninsula away from south Brittany during the Mesozoic has been demonstrated from palaeomagnetic data. Marine magnetic, seismic and gravity surveys have indicated that there is oceanic crust in the Bay of Biscay. Initial rifting began in the Triassic but the major part of the rotation of the Iberian Peninsula occurred during the Late Cretaceous. Two mechanisms for the opening of the Bay of Biscay have been proposed, firstly a simple anticlockwise rotation of the Iberian plate and secondly, a 350 km sinistral displacement of the Iberian plate along transform faults about a pole of rotation near Paris. Geological data, partly new, favours the first mechanism but with a pole of rotation approximately 100 km east of the west end of the Pyrenees rather than in the southeast corner of the Bay of Biscay. Geophysical data indicates a further small rotation of the Iberian Peninsula during the Late Eocene which resulted in the formation of the Pyrenees.  相似文献   

16.
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   

17.
Abdullh M.S. Al-Amri   《Tectonophysics》1998,290(3-4):271-283
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   

18.
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   


19.
During 1973–1977, as part of the International Geodynamic Project, some seismic investigations of the Earth's crust have been carried out by geotraverses of the Tien Shan—Pamirs—Karakorum—Himalayas. The seismic data obtained together with other geophysical information, allow the construction and interpretation of the lithospheric section through the Pamirs-Himalayas structure. This section includes thick crust with complex layering, supra-asthenospheric and asthenospheric layers of the upper mantle. The thickness of the Earth's crust increases from 50–55 km in the north, in the Ferghana depression (Tien Shan), to 70–75 km in the south, near the Karakul Lake (Northern Pamir). It varies within 60–65 km for the Central and Southern Pamir, Karakorum and the Inner Himalayas. Its thickness is least (35–37 km) in the south, under the outer margin of the Himalayan foredeep. Extreme gravity minima and depressions on the geoid surface correspond to the regions with maximum thickness of the Earth's crust. The centers of the disturbing masses on the geoid surface are located in the vicinity of the asthenosphere's upper layer; this determines the effect of the whole lithospheric layer, including its asthenospheric layer, at intense changes of gravity anomalies. The asthenospheric upper layer is recorded at a depth of about 120 km, its base at a depth of 200 km, in the northern and southern regions, and 300 km in its central part (Southern Pamir, Karakorum). In the middle asthenospheric layer, wave velocities decrease to 7.5 km/sec, under the base they increase to 8.4 km/sec and reach 9.4 km/sec at a depth of about 400 km. In the supra-asthenospheric layer of the upper mantle, longitudinal and shear wave-velocities slightly increase (by less than 0.1 km/sec) towards its base.  相似文献   

20.
We present velocity models determined by inverting refracted and reflected arrivals along two active source lines in the Changbaishan volcanic region, NE China. We resolve a prominent low-velocity zone (LVZ) in the crust, with velocities as low as 5.4 km/s. Away from the LVZ, the velocity gradients in the crust are relatively smooth, with average P-wave velocities of about 6.0–6.5 km/s. The Moho is at about 35 km depth, thickening to about 40 km under the Tianchi volcano, and thinning to about 30 km under the LVZ. The LVZ is located about 30–60 km to the north of the summit of the Tianchi volcano (the most recently active volcano in the region), is about 30–75 km in north–south extent, is at most 35 km in east–west extent, and is in the depth range of about 10–25 km below the surface. We use these results to constrain receiver function inversions, and show that the receiver functions in the region are compatible with our findings. With these data alone, the significance of the LVZ in non-unique, although we do not see any evidence to support the presence of partial melt in the crust, and favor the interpretation that the LVZ indicates a residual crustal magma chamber.  相似文献   

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