首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 46 毫秒
1.
The DACIA PLAN (Danube and Carpathian Integrated Action on Process in the Lithosphere and Neotectonics) deep seismic sounding survey was performed in August–September 2001 in south-eastern Romania, at the same time as the regional deep refraction seismic survey VRANCEA 2001. The main goal of the experiment was to obtain new information on the deep structure of the external Carpathians nappes and the architecture of Tertiary/Quaternary basins developed within and adjacent to the seismically-active Vrancea zone, including the Focsani Basin. The seismic reflection line had a WNW–ESE orientation, running from internal East Carpathians units, across the mountainous south-eastern Carpathians, and the foreland Focsani Basin towards the Danube Delta. There were 131 shot points along the profile, with about 1 km spacing, and data were recorded with stand-alone RefTek-125s (also known as “Texans”), supplied by the University Texas at El Paso and the PASSCAL Institute. The entire line was recorded in three deployments, using about 340 receivers in the first deployment and 640 receivers in each of the other two deployments. The resulting deep seismic reflection stacks, processed to 20 s along the entire profile and to 10 s in the eastern Focsani Basin, are presented here. The regional architecture of the latter, interpreted in the context of abundant independent constraint from exploration seismic and subsurface data, is well imaged. Image quality within and beneath the thrust belt is of much poorer quality. Nevertheless, there is good evidence to suggest that a thick (10 km) sedimentary basin having the structure of a graben and of indeterminate age underlies the westernmost part of the Focsani Basin, in the depth range 10–25 km. Most of the crustal depth seismicity observed in the Vrancea zone (as opposed to the more intense upper mantle seismicity) appears to be associated with this sedimentary basin. The sedimentary successions within this basin and other horizons visible further to the west, beneath the Carpathian nappes, suggest that the geometry of the Neogene and recent uplift observed in the Vrancea zone, likely coupled with contemporaneous rapid subsidence in the foreland, is detached from deeper levels of the crust at about 10 km depth. The Moho lies at a depth of about 40 km along the profile, its poor expression in the reflection stack being strengthened by independent estimates from the refraction data. Given the apparent thickness of the (meta)sedimentary supracrustal units, the crystalline crust beneath this area is quite thin (< 20 km) supporting the hypothesis that there may have been delamination of (lower) continental crust in this area involved in the evolution of the seismic Vrancea zone.  相似文献   

2.
The DACIA-PLAN (Danube and Carpathian Integrated Action on Processes in the Lithosphere and Neotectonics) deep seismic reflection survey was performed in August–September 2001, with the objective of obtaining new information on the deep structure of the external Carpathians nappe system and the architecture of the Tertiary/Quaternary basins developed within and adjacent to the Vrancea zone, including the rapidly subsiding Focsani Basin. The DACIA-PLAN profile is about 140 km long, having a roughly WNW–ESE direction, from near the southeast Transylvanian Basin, across the mountainous south-eastern Carpathians and their foreland to near the Danube River. A high resolution 2.5D velocity model of the upper crust along the seismic profile has been determined from a tomographic inversion of the DACIA-PLAN first arrival data. The results show that the data fairly accurately resolve the transition from sediment to crystalline basement beneath the Focsani Basin, where industry seismic data are available for correlation, at depths up to about 10 km. Beneath the external Carpathians nappes, apparent basement (material with velocities above 5.8 km/s) lies at depths as shallow as 3–4 km, which is less than previously surmised on the basis of geological observations. The first arrival travel-time data suggest that there is significant lateral structural heterogeneity on the apparent basement surface in this area, suggesting that the high velocity material may be involved in Carpathian thrusting.  相似文献   

3.
The VRANCEA99 and VRANCEA2001 seismic refraction experiments are part of a multidisciplinary project to study the Eastern Carpathians in Romania. The objectives of these studies are intended to disclose a more detailed picture of the crustal and upper mantle structures above the seismically active Vrancea region. In this paper we provide additional constraints for the upper crustal structures of the area. The 1999 campaign consisted of a 320-km-long N–S profile and a 70-km-long E–W profile. The intersecting 2001 profile extended in E–W direction from the Hungarian border to the Black Sea. In order to enhance the model resolution, first arrival data from local crustal earthquakes were also included.This configuration allowed for the first time to derive a 3-D velocity model for the upper crust of the Romanian Carpathian Orogen, within a 115×235 km wide region, centred over the Vrancea seismic zone. The 3-D model reveals lateral velocity variations, which were not visible on the in-line interpretations. It allows us to distinguish between foreland platform areas, foreland basins and the Carpathian Orogen. Clear velocity differences between the foreland basins south and southeast of the Eastern Carpathians and the Focsani Basin further north indicate different pre-Miocene sedimentary compositions and geological evolutions of these foreland platforms. The involved Moesian and Scythian platforms are separated by the Trotus Fault system, which is observed as a velocity discontinuity. An upper crustal high-velocity zone, above the northern Vrancea seismic zone, could also be identified. This high-velocity zone is explained by a Middle Pliocene to Pleistocene E–W oriented out-of-sequence thrust of the crystalline basement, below the decollement of the flysch nappes.  相似文献   

4.
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   


5.
The large-scale CELEBRATION 2000 seismic experiment investigated the velocity structure of the crust and upper mantle between western portion of the East European Craton (EEC) and the eastern Alps. This area comprises: the Trans-European Suture Zone, the Carpathian Mountains, the Pannonian Basin and the Bohemian Massif. This experiment included 147 chemical shots recorded by 1230 seismic stations during two deployments. Good quality data along 16 main and a few additional profiles were recorded. One of them, profile CEL03, was located in southeastern Poland and was laid out as a prolongation of the TTZ profile performed in 1993. This paper focuses on the joint interpretation of seismic data along the NW–SE trending TTZ–CEL03 transect, located in the central portion of the Trans-European Suture Zone. First arrivals and later phases of waves reflected/refracted in the crust and upper mantle were interpreted using two-dimensional tomographic inversion and ray-tracing techniques. This modelling established a 2-D (quasi 3-D) P-wave velocity lithospheric model. Four crustal units were identified along the transect. From northwest to southeast, thickness of the crust varies from 35 km in the Pomeranian Unit (NW) to 40 km in the Kuiavian Unit, to 50 km in the Radom–Łysogóry Unit and again to 43 km in the Narol Unit (SE). The first two units are thought to be proximal terranes detached from the EEC farther to the southeast and re-accreted to the edge of the EEC during the Early Palaeozoic. The origin of the remaining two units is a matter of dispute: they are either portions of the EEC or other proximal terranes. In the area of the Polish Basin (first two units), the P-wave velocity is very low (Vp < 6.1 km/s) down to depths of 15–20 km indicating that a very thick sedimentary and possibly volcanic rock sequence, whose lower portion may be metamorphosed, is present. The velocity beneath the Moho was found to be rather high, being 8.25 km/s in the northwestern portion of the transect, 8.4 km/s in the central sector, and 8.1 km/s in the southeastern sector.  相似文献   

6.
青藏高原中部古近纪发育伦坡拉盆地、色林错盆地、尼玛盆地,组成伦坡拉-色林错-尼玛沉积凹陷,总体呈近东西走向,长超过250km,宽30~50km;凹陷中心古近系河湖相沉积地层厚度达5~6km,下部为古新统-始新统牛堡组砾岩、砂岩、泥岩、泥灰岩,上部为渐新统丁青湖组泥岩、页岩、粉砂岩夹油页岩,顶部被新近系河湖相沉积不整合覆盖。凹陷南部发育尼玛-色林错逆冲推覆构造,凹陷北侧发育赛布错-扎加藏布逆冲推覆构造,伦坡拉盆地北部发育薄皮推覆构造,伴有不同规模的褶皱变形。地壳深部不同深度发育多重逆冲推覆构造,羌塘地块南部自北向南逆冲推覆,拉萨地块北部自南向北逆冲推覆;两者对冲部位地壳厚度发生显著变化,地表形成古近纪沉积凹陷。根据深地震反射及构造解释,结合Airy均衡分析,表明不同深度逆冲推覆及对冲构造运动导致地壳缩短增厚,增厚地壳均衡隆升及密度差异对古近纪沉积凹陷及盆地演化具有重要控制作用。色林错凹陷及邻区古近纪沉积记录对青藏高原地壳增厚与隆升过程具有重要指示意义。  相似文献   

7.
Seismic refraction surveys conducted in 1976 and 1979 over the broken ice surface of the Arctic Ocean, reveal distinctly different crustal structures for the Fram, Makarov and Canada basins. The Canada Basin, characterized by a 2–4 km thick sedimentary layer and a distinct oceanic layer 3B of 7.5 km/s velocity has the thickest crust and is undoubtedly the oldest of the three. The crust of the Makarov Basin has a thin sedimentary layer of less than 1 km and is about 9 km in total thickness. The Fram Basin has a similarly thin sedimentary layer but is 3–4 km thicker than the Makarov as it approaches the Lomonosov Ridge near the North Pole. The ridge itself is cored by material with a velocity of 6.6 km/s and may be a metagabbro similar to oceanic layer 3A. This ridge root material extends to a depth of about 27 km, where a change occurs to upper-mantle material with a velocity of 8.3 km/s. The core is overlain by up to 6 km of material with a velocity of about 4.7 km/s which could be oceanic layer 2A basalts or continental crystalline rocks with some sedimentary material.The Fram Basin probably began to open contemporaneously with the North Atlantic about 70 m.y. ago, by spreading along the Nansen-Gakkel Ridge. Although not yet dated, the Makarov Basin is probably no older than the initiation of the Fram Basin and may be much younger. The Alpha Ridge may once have been part of the Lomonosov Ridge, splitting off to form the Makarov Basin between 70 and 25 m.y. ago and possibly contributing to the Eurekan Orogeny of 25 m.y. ago, evident on Ellesmere Island. In contrast, the likely age of the Canada Basin lies in the 125–190 m.y. range and may have been formed by the counter-clockwise rotation of Alaska and the Northwind Ridge away from the Canadian Arctic Islands. The Lomonosov Ridge emerges from this scenario as a block resulting from a strike-slip shear zone on the European continental shelf, related to the opening of the Canada basin (180-120 my) and then becomes an entity broken from this shelf by the opening of the Eurasia Basin (70-0 m.y.).  相似文献   

8.
The origin of regional sedimentary basins is being investigated by the ESTRID project (Explosion Seismic Transects around a Rift In Denmark). This project investigates the mechanisms of the formation of wide, regional basins and their interrelation to previous rifting processes in the Danish–Norwegian Basin in the North Sea region. In May 2004 a 143 km long refraction seismic profile was acquired along the strike direction of a suspected major mafic intrusion in the crust in central Denmark. The data confirms the presence of a body with high seismic velocity (> 6.5 km/s) extending from a depth of  10–12 km depth into the lower crust. There is a remarkable Moho relief between 27 and 34 km depth along this new along-strike profile as based on ray-tracing modelling of PmP reflections. The lack of PmP reflections at a zone of very high velocity in the lowest crust (7.3–7.5 km/s) suggests a possible location of a feeder channel to the batholith. The presence of volcanic rocks of Carboniferous–Permian age above the intrusion (mafic batholith) suggests a similar age of the intrusion. An older obliquely crossing profile and two new fan profiles deployed perpendicular to the main ESTRID profile, show that the batholith is about 30–40 km wide. The existence of this large mafic batholith supports the hypothesis that the origin of the Danish–Norwegian Basin is related to cooling and contraction after intrusion of large amounts of mafic melts into the crust during the late Carboniferous and early Permian. The data and interpretations from project ESTRID will form the basis for subsidence modelling. Tentatively, we interpret the formation of the Danish–Norwegian Basin as a thermal subsidence basin, which developed after widespread rifting of the region.  相似文献   

9.
Mineral exploration drillholes and geoelectric prospecting provide for the first time evidence for thrusting of the South Carpathian Paleozoic basement over northerly adjacent Middle Miocene sediments. Investigations were carried out in two locations, 30 km apart, along the northern margin of the Poiana Rusca Mountains, Romania, southwestern Carpathians. Drill holes in both locations encountered weakly consolidated Middle Miocene clay, sand, and fine gravel below Paleozoic low-grade metamorphic rocks. Intersections from various drill holes demonstrate the presence of low-angle thrusting. Kinematic indicators are so far lacking, but with a thrust direction oriented roughly normal to strike of the Poiana Rusca Mountains, minimum displacement is 1–1.4 km in northwestern or northern direction, respectively. Thrusting occurred most likely during the Late Miocene–Pliocene, whereafter Quaternary regional uplift dissected the thrust plane. In the tectonic framework of Neogene dextral translation of the Tisza–Dacia Block against the southerly adjacent Moesian Platform, transtension appears responsible for Middle Miocene basin formation along the northern margin of the Poiana Rusca region. Proceeding collision of the Tisza–Dacia Block with the East European Craton introduced stronger impingement of the Tisza–Dacia Block against the Moesian Platform, leading to a Late Miocene–Pliocene transpressional regime, in which the northern Poiana Rusca basement was thrust over its adjacent Middle Miocene sediments.  相似文献   

10.
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   

11.
The Vrancea zone of Romania constitutes one of the most active seismic zones in Europe, where intermediate-depth (70–200 km) earthquakes of magnitude in excess of Mw = 7.0 occur with relative frequency in a geographically restricted area within the 110° bend region of the southeastern Carpathian orogen. Geologically, the Vrancea zone is characterized by (a) a laterally restricted, steeply NW-dipping seismogenic volume (30 × 70 × 200 km), situated beneath (b) thickened continental crust within the highly arcuate bend region of the Carpathian orocline, and (c) miscorrelation of hypocenters with the position of known or inferred suture zones in the Carpathian orogenic system. Geologic data from petroleum exploration in the Eastern Carpathians, published palinspastic reconstructions, and reprocessing of industry seismic data from the Carpathian foreland indicate that (1) crust of continental affinity extends significantly westward beneath the external thrust nappes (Sub-Carpathian, Marginal Folds, and Tarcau) of the Eastern Carpathians, (2) Cretaceous to Miocene strata of continental affinity can be reconstructed westward to a position now occupied by the Transylvanian basin, and (3) geologic structure in the Carpathian foreland (including the Moho) is sub-horizontal directly to the east and above the Vrancea seismogenic zone. Taken together, these geologic relationships imply that the Vrancea zone occupies a region overlain by continental crust and upper mantle, and does not appear to originate from a subducted oceanic slab along the length of the Carpathian orogen. Accordingly, the Vrancea zone appears to potentially be an important place to establish evidence for active lithospheric delamination.  相似文献   

12.
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   

13.
The 1370 km long 4-AR reference profile crosses the North Barents Basin, the northern end of the Novaya Zemlya Rise, and the North Kara Basin. Integrated geophysical studies including common deep point (CDP) survey and deep seismic sounding (DSS) were carried out along the profiles. The DSS was performed using autonomous bottom seismic stations (ABSS) spaced 10–20 km apart and a powerful air gun producing seismic signals with a step size of 250 m. As a result, detailed P- and S-wave velocity structures of the crust and upper mantle were studied. The basic method was ray-tracing modeling. The Earth’s crust along the entire profile is typically continental with compressional wave velocities of 5.8–7.2 km/s in the consolidated part. Crustal thickness increases from 30 km near the islands of Franz Josef Land to 35 km beneath the North Barents Basin, 50 km beneath the Novaya Zemlya Rise, and 40 km beneath the North Kara Basin. The North Barents Basin 15 km deep is characterized by unusually low velocities in the consolidated crust: The upper crust layer with velocities of 5.8–6.4 km/s has a thickness of about 15 km beneath the basin (usually, this layer wedges beneath deep sedimentary basins). Another special property of the crust in the North Barents Basin is the destroyed structure of the Moho.  相似文献   

14.
The present study was undertaken with the objective of deriving constraints from available geological and geophysical data for understanding the tectonic setting and processes controlling the evolution of the southern margin of the East European Craton (EEC). The study area includes the inverted southernmost part of the intracratonic Dnieper-Donets Basin (DDB)–Donbas Foldbelt (DF), its southeastern prolongation along the margin of the EEC–the sedimentary succession of the Karpinsky Swell (KS), the southwestern part of the Peri-Caspian Basin (PCB), and the Scythian Plate (SP). These structures are adjacent to a zone, along which the crust was reworked and/or accreted to the EEC since the late Palaeozoic. In the Bouguer gravity field, the southern margin of the EEC is marked by an arc of gravity highs, correlating with uplifted Palaeozoic rocks covered by thin Mesozoic and younger sediments. A three-dimensional (3D) gravity analysis has been carried out to investigate further the crustal structure of this area. The sedimentary succession has been modelled as two heterogeneous layers—Mesozoic–Cenozoic and Palaeozoic—in the analysis. The base of the sedimentary succession (top of the crystalline Precambrian basement) lies at a depth up to 22 km in the PCB and DF–KS areas. The residual gravity field, obtained by subtracting the gravitational effect of the sedimentary succession from the observed gravity field, reveals a distinct elongate zone of positive anomalies along the axis of the DF–KS with amplitudes of 100–140 mGal and an anomaly of 180 mGal in the PCB. These anomalies are interpreted to reflect a heterogeneous lithosphere structure below the supracrustal, sedimentary layers: i.e., Moho topography and/or the existence of high-density material in the crystalline crust and uppermost mantle. Previously published data support the existence of a high-density body in the crystalline crust along the DDB axis, including the DF, caused by an intrusion of mafic and ultramafic rocks during Late Palaeozoic rifting. A reinterpretation of existing Deep Seismic Sounding (DSS) data on a profile crossing the central KS suggests that the nature of a high-velocity/density layer in the lower crust (crust–mantle transition zone) is not the same as that of below the DF. Rather than being a prolongation of the DDB–DF intracratonic rift zone, the present analysis suggests that the KS comprises, at least in part, an accretionary zone between the EEC and the SP formed after the Palaeozoic.  相似文献   

15.
S.B. Lyngsie  H. Thybo   《Tectonophysics》2007,429(3-4):201-227
We present a new model for the lithospheric structure of the transitions between Laurentia, Avalonia and Baltica in the North Sea, northwestern Europe based on 2¾D potential field modelling of MONA LISA profile 3 across the Central Graben, with constraints from seismic P-wave velocity models and the crustal normal incidence reflection section along the profile. The model shows evidence for the presence of upper-and lower Palaeozoic sedimentary rocks as well as differences in crustal structure between the palaeo-continents Laurentia, Avalonia and Baltica. Our new model, together with previous results from transformations of the gravity and magnetic fields, demonstrates correlation between crustal magnetic domains along the profile and the terrane affinity of the crust. This integrated interpretation indicates that a 150 km wide zone, characterized by low-grade metamorphosis and oblique thrusting of Avalonia crust over Baltica lower crust, is characteristic for the central North Sea area. The magnetic susceptibility and the density across the Coffee Soil Fault range from almost zero and 2715 kg/m3 in Avalonia crust to 0.05 SI and 2775 kg/m3 in Baltica crust. The model of MONA LISA profile 3 indicates that the transition between Avalonia and Baltica is located beneath the Central Graben with a ramp–flat–ramp geometry. Our results indicate that the initial rifting of the Central Graben and the Viking Graben was controlled by the location of the Caledonian collisional suture, located at the Coffee Soil Fault, and that the deep crustal part of Baltica extends further to the west than hitherto believed.  相似文献   

16.
The Pine Creek Orogen, located on the exposed northern periphery of the North Australian Craton, comprises a thick succession of variably metamorphosed Palaeoproterozoic siliciclastic and carbonate sedimentary and volcanic rocks, which were extensively intruded by mafic and granitic rocks. Exposed Neoarchean basement is rare in the Pine Creek Orogen and the North Australian Craton in general. However, recent field mapping, in conjunction with new SHRIMP U–Pb zircon data for six granitic gneiss samples, have identified previously unrecognised Neoarchean crystalline crust in the Nimbuwah Domain, the eastern-most region of the Pine Creek Orogen. Four samples from the Myra Falls and Caramal Inliers, the Cobourg Peninsula, and the Kakadu region have magmatic crystallisation ages in the range 2527–2510 Ma. An additional sample, from northeast Myra Falls Inlier, yielded a magmatic crystallisation age of 2671 ± 3 Ma, the oldest exposed Archean basement yet recognised in the North Australian Craton. These results are consistent with previously determined magmatic ages for known outcropping and subcropping crystalline basement some 200 km to the west. A sixth sample yielded a magmatic crystallisation age of 2640 ± 4 Ma. The ca. 2670 Ma and ca. 2640 Ma samples have ca. 2500 Ma metamorphic zircon rims, consistent with metamorphism broadly coeval with emplacement of the volumetrically dominant ca. 2530–2510 Ma granites and granitic gneisses. Neoarchean zircon detritus, particularly in the ca. 2530–2510 Ma and ca. 2670–2640 Ma age span, are an almost ubiquitous feature of detrital zircon spectra of unconformably overlying metamorphosed Palaeoproterozoic strata of the Pine Creek Orogen, and of local post-tectonic Proterozoic sequences, consistent with this local provenance. Neoarchean zircon is also a common detrital component in Palaeoproterozoic sedimentary units across much of the North Australian Craton suggesting the existence of an extensive, if not contiguous, Neoarchean crystalline basement underlying not only a large part of the Pine Creek Orogen, but also much of the North Australian Craton.  相似文献   

17.
New deep seismic reflection data provide images of the crust and uppermost mantle underlying the eastern Middle Urals and adjacent West Siberian Basin. Distinct truncations of reflections delineate the late-orogenic strike-slip Sisert Fault extending vertically to ∼28 km depth, and two gently E-dipping reflection zones, traceable to 15–18 km depth, probably represent normal faults associated with the opening of the West Siberian Basin. A possible remnant Palaeozoic subduction zone in the lower crust under the West Siberian Basin is visible as a gently SW-dipping zone of pronounced reflectivity truncated by the Moho. Continuity of shallow to intermediate-depth reflections suggest that Palaeozoic accreted island-arc terranes and overlying molasse sequences exposed in the hinterland of the Urals form the basement for Triassic and younger deposits in the West Siberian Basin. A highly reflective lower crust overlies a transparent mantle at about 43 km depth along the entire 100 km long seismic reflection section, suggesting that the lower crust and Moho below the eastern Middle Urals and West Siberian Basin have the same origin.  相似文献   

18.
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   


19.
The Apuseni Mountains are located between the Pannonian Basin and the Transylvanian Basin along a direction of SE convergence with the Carpathian belt. A flexural model based on the cylindrical bending of a semi-infinite, isostatically supported, thin elastic plate is here examined with the Apuseni playing the role of flexural bulge, and under the assumption that the plate is deforming under the action of a vertical shear force and a bending moment applied at the end of the plate, beneath the Carpathians. The model yields estimates of the plate thickness ranging between 13 and 14.5 km, depending on the assumed density contrast between crust/sediments and mantle providing buoyancy. The vertical shear force which is necessary to bend the plate is in the range between 60 and 300 × 1011 N m− 1, depending on the assumed density contrast. This force is shown to be modelled by a gravitational ‘slab pull’ force, using model parameters derived from seismic tomography. If the height of the flexural bulge, after correction for erosion, is allowed to increase, the model yields an estimate of the horizontal strain rate at the top of the bulge. For example, 5 mm/yr vertical change of the flexural bulge of a 14 km thick plate results in a horizontal deformation rate of approximately 7 nanostrain/yr at the top of the bulge, a value which is at the threshold of sensitivity of continuous GPS measurements. Different vertical rates will change the horizontal strain rate almost proportionally.  相似文献   

20.
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   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号