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1.
Gravity and magnetic data of the Kachchh basin and surrounding regions have delineated major E–W and NW–SE oriented lineaments and faults, which are even extending up to plate boundaries in the north Arabian Sea and western boundary of the Indian plate, respectively. The epicentral zone of Bhuj earthquake and its aftershocks is located over the junction of Rann of Kachchh and median uplifts viz. Kachchh mainland and Wagad uplifts, which are separated by thrust faults. Gravity data with constraints from the results of the seismic studies along a profile suggest that the basement is uplifted towards the north along thrust faults dipping 40–60° south. Similarly gravity and magnetic modeling along a profile across Wagad uplift suggest south dipping (50–60°) basement contacts separating rocks of high susceptibility and density towards the north. One of these contacts coincides with the fault plane of the Bhuj earthquake as inferred from seismological studies and its projection on the surface coincides with the E–W oriented north Wagad thrust fault. A circular gravity high in contact with the fault in northern part of the Wagad uplift along with high amplitude magnetic anomaly suggests plug type mafic intrusive in this region. Several such gravity anomalies are observed over the island belt in the Rann of Kachchh indicating their association with mafic intrusions. The contact of these intrusives with the country rock demarcates shallow crustal inhomogeneities, which provides excellent sites for the accumulation of regional stress. A regional gravity anomaly map based on the concept of isostasy presents two centers of gravity lows of −11 to −13 mGal (10−5 m/s2) representing mass deficiency in the epicentral region. Their best-fit model constrained from the receiver function analysis and seismic refraction studies suggest crustal root of 7–8 km (deep crustal inhomogeneity) under them for a standard density contrast of −400 kg/m3. It is, therefore, suggested that significant amount of stress get concentrated in this region due to (a) buoyant crustal root, (b) regional stress due to plate tectonic forces, and (c) mafic intrusives as stress concentrators and the same might be responsible for the frequent and large magnitude earthquakes in this region including the Bhuj earthquake of January 26, 2001.  相似文献   

2.
The Alleppey Platform is an important morphological feature located in the Kerala-Konkan basin off the southwest coast of India. In the present study, seismic reflection data available in the basin were used to understand the sedimentation history and also to carry out integrated gravity interpretation. Detailed seismic reflection data in the basin reveals that:(1) the Alleppey Platform is associated with a basement high in the west of its present-day geometry(as observed in the time-structure map of the Trap Top(K/T boundary)),(2) the platform subsequently started developing during the Eocene period and attained the present geometry by the Miocene and,(3) both the Alleppey platform and the Vishnu fracture zone have had significant impact on the sedimentation patterns(as shown by the time-structure and the isochron maps of the major sedimentary horizons in the region). The 3-D sediment gravity effect computed from the sedimentary layer geometry was used to construct the crustal Bouguer anomaly map of the region.The 3-D gravity inversion of crustal Bouguer anomaly exhibits a Moho depression below the western border of the platform and a minor rise towards the east which then deepens again below the Indian shield. The 2-D gravity modelling across the Alleppey platform reveals the geometry of crustal extension,in which there are patches of thin and thick crust. The Vishnu Fracture Zone appears as a crustal-scale feature at the western boundary of the Alleppey platform. Based on the gravity model and the seismic reflection data, we suggest that the basement high to the west of the present day Alleppey platform remained as a piece of continental block very close to the mainland with the intervening depression filling up with sediments during the rifting. In order to place the Alleppey platform in the overall perspective of tectonic evolution of the Kerala-Konkan basin, we propose its candidature as a continental fragment.  相似文献   

3.
The structural geometry, kinematics and density structure along the rear of the offshore Taiwan accretionary prism were studied using seismic reflection profiling and gravity modeling. Deformation between the offshore prism and forearc basin at the point of incipient collision, and southward into the region of subduction, has been interpreted as a tectonic wedge, similar to those observed along the front of mountain ranges. This tectonic wedge is bounded by an east-dipping roof thrust and a blind, west-dipping floor thrust. An east-dipping sequence of forearc-basin strata in the hanging wall of the roof thrust reaches a thickness in excess of 4 km near the tip of the interpreted tectonic wedge. Section restoration of the roof sequence yields an estimate of 4 km of shortening, which is small compared with that inferred in the collision area to the north, based on the variation in distance between the apex of the prism and the island arc.Previous studies propose that either high-angle normal faulting or backfolding has exhumed the metamorphic rocks along the eastern flank of the Central Range in the collision zone on land. To better constrain the initial crustal configuration, we tested 350 crustal models to fit the free-air gravity anomaly data in the offshore region to study the density structure along the rear of the accretionary prism in the subduction and initial collision zones before the structures become more complex in the collision zone on land. The gravity anomaly, observed in the region of subduction (20.2°N), can be modeled with the arc basement forming a trenchward-dipping backstop that is overlain by materials with densities in the range of sedimentary rocks. Near the point of incipient collision (20.9°N), however, the free-air gravity anomaly over the rear of the prism is approximately 40 mgal higher, compared with the region of subduction, and requires a significant component of high density crustal rocks within the tectonic wedge. These results suggest that the forearc basement may be deformed along the rear of the prism, associated with the onset of collision, but not in the subduction region further to the south.  相似文献   

4.
New deep reflection seismic, bathymetry, gravity and magnetic data have been acquired in a marine geophysical survey of the southern South China Sea, including the Dangerous Grounds, Northwest Borneo Trough and the Central Luconia Platform. The seismic and bathymetry data map the topography of shallow density interfaces, allowing the application of gravity modeling to delineate the thickness and composition of the deeper crustal layers. Many of the strongest gravity anomalies across the area are accounted for by the basement topography mapped in the seismic data, with substantial basement relief associated with major rift development. The total crustal thickness is however quite constant, with variations only between 25 and 30 km across the Central Luconia Platform and Dangerous Grounds. The Northwest Borneo Trough is underlain by thinned crust (25–20 km total crustal thickness) consistent with the substantial water depths. There is no evidence of any crustal suture associated with the trough, nor any evidence of relict oceanic crust beneath the trough. The crustal thinning also does not extend along the complete length of the trough, with crustal thicknesses of 25 km and more modeled on the most easterly lines to cross the trough. Modeled magnetic field variations are also consistent with the study area being underlain by continental crust, with the magnetic field variations well explained by irregular magnetisations consistent with inhomogeneous continental crust, terminating at the basement unconformity as mapped from the seismic data.  相似文献   

5.
Huge megabreccias occur at the eastern margin of the Cretaceous Apulia Carbonate Platform (Gargano Promontory, southern Italy). Their stratigraphic and genetic meaning are controversial in the debated geological evolution of the Apulia Platform. New stratigraphic analyses have revealed that three distinct megabreccia levels occur within the coarse debrites that were previously interpreted to be the result of repeated collapses of a scalloped platform margin during the late Albian–Cenomanian. Each level has peculiar chronostratigraphic distribution, geometry, composition and genetic features. They are the Posta Manganaro Megabreccias (late early Aptian to late Albian pp. ), Monte S. Angelo Megabreccias (early–middle Cenomanian) and Belvedere di Ruggiano Megabreccias (middle Turonian). These deposits overlie regional, tectonically enhanced unconformities of late early Aptian, late Albian and late Cenomanian age. These megabreccias, which were formed, respectively, during drowning, prograding and exposure events of the Apulia Platform, reflect important turning points in its Cretaceous geodynamic evolution.  相似文献   

6.
On September 18, 2004, a 4.6 mbLg earthquake was widely felt in the region around Pamplona, at the western Pyrenees. Preliminary locations reported an epicenter less than 20 km ESE of Pamplona and close to the Itoiz reservoir, which started impounding in January 2004. The area apparently lacks of significant seismic activity in recent times. After the main shock, which was preceded by series of foreshocks reaching magnitudes of 3.3 mbLg, a dense temporal network of 13 seismic stations was deployed there to monitor the aftershocks series and to constrain the hypocentral pattern. Aftershock determinations obtained with a double-difference algorithm define a narrow epicentral zone of less than 10 km2, ESE–WNW oriented. The events are mainly concentrated between 3 and 9 km depth. Focal solutions were computed for the main event and 12 aftershocks including the highest secondary one of 3.8 mbLg. They show mainly normal faulting with some strike-slip component and one of the nodal planes oriented NW–SE and dipping to the NE. Cross-correlation techniques applied to detect and associate events with similar waveforms, provided up to 33 families relating the 67% of the 326 relocated aftershocks. Families show event clusters grouped by periods and migrating from NW to SE. Interestingly, the narrow epicentral zone inferred here is located less than 4 km away from the 111-m high Itoiz dam. These hypocentral results, and the correlation observed between fluctuations of the reservoir water level and the seismic activity, favour the explanation of this foreshock–aftershock series as a rapid response case of reservoir-triggered seismicity, burst by the first impoundment of the Itoiz reservoir. The region is folded and affected by shallow dipping thrusts, and the Itoiz reservoir is located on the hangingwall of a low angle southward verging thrust, which might be a case sensible to water level fluctuations. However, continued seismic monitoring in the coming years is mandatory in this area to infer more reliable seismotectonic and hazard assessments.  相似文献   

7.
We describe three study cases in which we used local earthquake and shot travel-time residuals to investigate the upper crustal structure of three regions in Italy. We inverted for velocity and hypocentral parameters using a damped least-squares technique making use of parameter (velocity and hypocentre) separation. The three studied regions are in Italy, namely (a) the Vulsinian Volcanic Complex (Latium), where there is an active geothermal field; (b) the Irpinia (Campania–Lucania) region, in the Southern Appennines, site of the strongest earthquake in Italy for at least 65 years (November 1980, Ms= 6.9); (c) the Friuli region, in Northeastern Italy, where another strong earthquake (Ms= 6.5) occurred in 1976. The computed shallow velocity models generally correspond with surface geological structures. For the three studied areas, the main results are, respectively: (a) A low-velocity anomaly detected in the centre of the Vulsinian Volcanic Complex at a depth of 5–8 km, probably due to anomalous heat flow caused by a partially molten or cooling intrusive body; (b) the identification of a deep (10 km) discontinuity in the crust beneath the Irpinia fault zone, approximately corresponding with the fault extension at depth; (c) the detection of a wedge of high-velocity, high density material at seismogenic depth (5–10 km) beneath the Friuli region, interpreted as a buried thrust of the metamorphic basement.  相似文献   

8.
Gravity data were integrated with seismic refraction/reflection data, well data and geological investigations to determine a general crustal structure of Tunisia. The gravity data analysis included the construction of a complete Bouguer gravity anomaly map, residual gravity anomaly maps, horizontal gravity gradient maps and a 2.5-D gravity model. Residual gravity anomaly maps illustrate crustal anomalies associated with various structural domains within Tunisia including the Sahel Block, Saharian Flexure, Erg Oriental Basin, Algerian Anticlinorium, Gafsa Trough, Tunisian Trough, Kasserine Platform and the Tell Mountains. Gravity anomalies associated with these features are interpreted to be caused either by thickening or thinning of Palæozoic and younger sediments or by crustal thinning. Analysis of the residual gravity anomaly and horizontal gravity gradient maps also determined a number of anomalies that may be associated with previously unknown structures. A north-south trending gravity model in general indicated similar subsurface bodies as a coincident seismic model. However, thinner Mesozoic sediments within the Tunisian Trough, thinner Palæozoic sediments in the Gafsa Trough, and a greater offset on the Saharian Flexure were required by the gravity data. Additionally, basement uplifts under the Kasserine Platform and Gafsa Trough, not imaged by seismic data, were required by the gravity data. The gravity model revealed two previously unknown basins north and south of the Algerian Anticlinorium (5 km), while the Erg Oriental Basin is composed of at least two sub-basins, each with a depth of 5 km.  相似文献   

9.
汶川5月12日8.0级地震在构造上起因于印度板块与欧亚板块以每年约5 cm的速度聚敛,并因此而引起青藏高原的地壳物质向四川盆地及中国东南大陆运移.主震震源及余震活动集中于以龙门山为中轴的一条长约350 km、宽约100 km的地震活动带.震源深度一般分布丁地壳脆性-韧性转换边界以上约10～20 km区间的地壳震源层之中,属浅源构造地震.主要震源机制与龙门山构造运动方式密切相关,以其地壳厚度向西急剧加厚、重力梯度带、高波速比(Vp/Vs～2.2)等深部异常及逆冲断层兼具走滑性质的地质构造为特征.在震源辐射、路径传播和场地效应研究的基础上,分别计算并比较了岩石和土壤条件下的地震响应谱,特别强调了土壤条件下的场地放大效应;同时对与地震安全性有关的一些问题如地质灾害、地震频谱设计、地震早期预警系统及中、长期至短期地震预报等进行了探讨;特别提供了一个由加权平均计算、以岩石条件下震波衰减模式为基础的地震频谱设计参考实例.地震构造与动力学研究可融人工程地质与环境工程等学科发展.经历汶川地震考验的一些新近设计和建设的工程项目可为今后改进工程建筑规范与标准提供重要而有益的参考.地震预报是当今一大难题,但需探索研究,不可懈怠.地震减灾与预防足目前比较切合实际的安全举措.  相似文献   

10.
The granite‐greenstone terranes of the Eastern Goldfields Province, Yilgarn Craton, Western Australia, are a major Australian and world gold and nickel source. The Kalgoorlie region, in particular, hosts several world‐class gold deposits. To attempt to understand why these deposits occur where they do, it is important to understand the crustal architecture in the region and how the major mineral systems operate in this architecture. One way to understand these relationships is to develop a detailed 3–D geological model for the region. The best method to map the 3–D geometry of major geological structures is by acquisition and interpretation of seismic‐reflection profiles. To contribute to this aim, a grid of deep seismic‐reflection traverses was acquired in 1999 to examine the 3–D geometry of the region in an area including the Kalgoorlie mineral region and mineral fields to the north and west. This grid was tied to the 1991 regional deep seismic traverse and 1997 high‐resolution seismic profiles in the same region. The grid covers an area measuring approximately 50 km wide by 50 km long and extended to a depth of approximately 50 km (below the base of the crust in this region). The resulting 3–D geological model was further constrained by both surface geological data and geophysical interpretations, with the seismic interpretations themselves also constrained by gravity and magnetic modelling. The 3–D model was used to investigate the geometric relationships between the major faults and shear zones in the area, the relationship between the granite‐greenstone succession and the basement, and the spatial relationships between the greenstones and the granites. Interpretation of the grid of seismic lines and construction of the 3–D geological model confirmed the existence of the detachment surface and led to the recognition that the granite‐greenstone contact usually occurs at a much shallower level than the detachment. Also, west‐dipping faults in the vicinity of the Golden Mile, including the Abattoir Shear through to Boulder‐Lefroy Fault, appear to be more important than previously thought in controlling the structure of that area. An antiformal thrust stack occurs beneath a triangle zone centred on the Golden Mile. The Black Flag Group was deposited in a probable extensional setting, and late extension was also probably more important than previously thought. The granite‐gneiss domes were uplifted by the formation of antiformal thrust stacks at depth beneath them.  相似文献   

11.
Deep crustal reflection data that are critical for the interpretation of Laramide structure have been obtained by the Consortium for Continental Reflection Profiling (COCORP). The Laramide orogeny, which occurred from the late Cretaceous to early Eocene, is characterized in Wyoming by large uplifts of Precambrian basement, commonly flanked by reverse faults. The attitude of these faults at depth has been a major tectonic problem and is very important for deciding whether horizontal or vertical crustal movements were primarily responsible for the basement uplifts. COCORP has run 158 km of deep seismic reflection profiles (recording to 20-sec two-way travel time) across the southeastern end of the Wind River Mountains, the largest of these Laramide uplifts. Reflections from the thrust fault flanking the Wind River uplift can be clearly traced on the profiles to at least 24-km depth and possibly as deep as about 36 km with a fairly uniform apparent dip of 30°–35°. Other reflection events subparallel to the main Wind River thrust are present in the seismic profiles and may represent other faults. There is at least 21 km of crustal shortening along the thrust. There is no evidence in the reflection profiles for large-scale folding of the basement; the Wind River Mountains were formed predominantly by thrust movements. Gravity anomalies in the Wind River Mountains can be modeled by a thrust that displaces dense material in the lower crust. If the thrust ever cut the Moho, the effect is not observed in the gravity today. A proposed model for the presence of uplifted basement in Wyoming invokes a shallowly dipping, subducted Farallon plate beneath the North American continent; drag between the two plates localized compressional stresses in an area over 800 km into the North American plate causing large thrusts to develop. The earth's crust seems to have fractured as a fairly rigid plate  相似文献   

12.
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   

13.
The Southern Granulite Terrain with exposed Archean lower crustal rocks is studied using various geophysical tools. The crustal structure derived from seismic reflection and refraction/wide-angle reflection studies is used to understand the tectonic evolution of the region. Deep seismic reflection section along the Kolattur–Palani segment shows an oppositely dipping reflection fabric near the Moyar–Bhavani shear zone, which is interpreted as a signature of collision between the Dharwar craton and another crustal block in the south. The thickened crust due to collision was delaminated during the orogenic collapse and modified the central part, covering the Cauvery Shear Zone system, located between the Moyar–Bhavani and Karur–Oddanchatram shear zones. The delaminated lower crust is altered by magmatic underplating as evidenced by the high velocity layer just above the Moho. The velocity model of the region indicates crustal thickening at the boundary of the Dharwar craton and Moyar–Bhavani shear zone and thinning further south. Back-scattered seismic wave field with negative moveout and the Moho-offset indicate the spatial location and strike-slip nature of the shear zones. Present study suggests that the late Archean collision and suturing of the Dharwar craton with the southern crustal block at the Moyar–Bhavani shear zone may be responsible for the evolution of late Archean granulites. Late Neoproterozoic rifting is observed along the paleo-fault zones. The seismic studies constrained by gravity, magnetic and magnetotelluric data suggest that the Moyar–Bhavani and Karur–Oddanchatram shear zones of the Cauvery Shear Zone system mark terrane boundaries/suture zones.  相似文献   

14.
A ～400 km long deep crustal reflection seismic survey was acquired in central Victoria, Australia, in 2006. It has provided information on crustal architecture across the western Lachlan Orogen and has greatly added to the understanding of the tectonic evolution. The east-dipping Moyston Fault is confirmed as the suture between the Delamerian and western Lachlan Orogens, and is shown to extend down to the Moho. The Avoca Fault, the boundary between the Stawell and Bendigo Zones, is a west-dipping listric reverse fault that intersects the Moyston Fault at a depth of about 22 km, forming a V-shaped geometry. Both the Stawell and Bendigo Zones can be divided broadly into a lower crustal region of interlayered and imbricated metavolcanic and metasedimentary rocks and an upper crustal region of tightly folded metasedimentary rocks. The Stawell Zone was probably part of a Cambrian accretionary system along the eastern Gondwanaland margin, and mafic rocks may have been partly consumed by Cambrian subduction. Much of the Early Cambrian oceanic crust beneath the Bendigo Zone was not subducted, and is preserved as a crustal-scale imbricate thrust stack. The seismic data have shown that a thin-skinned structural model appears to be valid for much of the Melbourne Zone, whereas the Stawell and Bendigo Zones have a thick-skinned structural style. Internal faults in the Stawell and Bendigo Zones are mostly west-dipping listric faults, which extend from the surface to near the base of the crust. The Heathcote Fault Zone, the boundary between the Bendigo and Melbourne Zones, extends to at least 20 km, and possibly to the Moho. A striking feature in the seismic data is the markedly different seismic character of the mid to lower crust of the Melbourne Zone. The deep seismic reflection data for the Melbourne Zone have revealed a multilayered crustal structure that supports the Selwyn Block model.  相似文献   

15.
This paper combines geological knowledge and geophysical imagery at the crustal scale to model the 3D geometry of a segment of the Hercynian suture zone of western Europe in the Champtoceaux area (Brittany, France). The Champtoceaux complex consists of a stack of metamorphic nappes of gneisses and micaschists, with eclogite-bearing units. The exhumation of the complex, during early Carboniferous times, was accompanied by deformation during regional dextral strike–slip associated with a major Hercynian shear zone (the South Armorican Shear Zone, SASZ). Dextral shearing produced a km-scale antiformal structure with a steeply dipping axial plane and a steeply eastward plunging axis. Armor 2 deep seismic profile shows that the regional structure was cut by a set of faults with northward thrusting components. Based on the seismic constraint, direct 2D crustal-scale modelling was performed throughout the Champtoceaux fold on seven radial gravity profiles, also using geological data, and density measurements from field and drill-hole samples. The 3D integration of the cross-sections, the digitised geological map, and the structural information (foliation dips) insure the geometrical and topological consistency of all sources of data. The 2D information is interpolated to the whole 3D space using a geostatistical analysis. Finally, the 3D gravity contribution of the resulting model is computed taking into account densities for each modelled geological body and compared to the Bouguer anomaly. The final 3D model is thus compatible with the seismic and gravity data, as well as with geological data. Main geological results derived from the modelling are (i) the overall 3D geometry of the south dipping thrust system interpreted on the seismic profile emphasises northward thrusting and folding of the Champtoceaux complex which was coeval with strike–slip along the South Armorican Shear Zone; (ii) the gravity modelling suggests the presence of a relatively dense body below the Champtoceaux complex that could be interpreted as a result of relative uplift of midcrustal material during thrusting along the E–W trending wrench–thrust system; (iii) the northern limb of the Champtoceaux anticline is a relatively shallow feature; and (iv) Vigneux synkinematic granitic body is a laccolith sheared and rooted along the southern branch of the SASZ and spreads away from the strike–slip zone within weak country-rocks.  相似文献   

16.
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   

17.
Modelling of gravity and airborne magnetic data integrated with seismic studies suggest that the linear gravity and magnetic anomalies associated with Moyar Bhavani Shear Zone (MBSZ) and Palghat Cauvery Shear Zone (PCSZ) are caused by high density and high susceptibility rocks in upper crust which may represent mafic lower crustal rocks. This along with thick crust (44–45 km) under the Southern Granulite Terrain (SGT) indicates collision of Dharwar craton towards north and SGT towards south with N–S directed compression during 2.6–2.5 Ga. This collision may be related to contemporary collision northwards between Eastern Madagascar–Western Dharwar Craton (WDC) and Eastern Dharwar Craton (EDC). Arcuate shaped N and S-verging thrusts, MBSZ-Mettur Shear and PCSZ-Gangavalli Shear, respectively across Cauvery Shear zone system (CSZ) in SGT also suggest that the WDC, EDC and SGT might have collided almost simultaneously during 2.6–2.5 Ga due to NW–SE directed compressional forces with CSZ as central core complex in plate tectonics paradigm preserving rocks of oceanic affinity. Gravity anomalies of schist belts of WDC suggest marginal and intra arc basin setting.The gravity highs of EGFB along east coast of India and regional gravity low over East Antarctica are attributed to thrusted high-density lower crustal/upper mantle rocks at a depth of 5–6 km along W-verging thrust, which is supported by high seismic velocity and crustal thickening, respectively. It may represent a collision zone at about 1.0 Ga between India and East Antarctica. Paired gravity anomalies in the central part of Sri Lanka related to high density intrusives under western margin of Highland Complex and crustal thickening (40 km) along eastern margin of Highland Complex with several arc type magmatic rocks of about 1.0 Ga in Vijayan Complex towards the east may represent collision between them with W-verging thrust as in case of EGFB. The gravity high of Sri Lanka in the central part falls in line with that of EGFB, in case it is fitted in Gulf of Mannar and may represent the extension of this orogeny in Sri Lanka.  相似文献   

18.
The aim of this study is to define the Vp and Vp/Vs structure of the fault zone ruptured by the ML 5.1 earthquake of October 15, 1996 which occurred near Reggio Emilia (central-northern Italy). A 1-month-long seismic sequence followed the mainshock and occurred in a small region along the outer border of the northern Apenninic belt, at depth ranging between 10 and 17 km. P- and S-wave arrival times from 304 aftershocks recorded by two local dense seismic arrays installed in the epicentral region have been inverted to obtain one- and three-dimensional velocity models by using state of the art local earthquake tomographic techniques. Velocity models and aftershock relocation help us to infer the seismotectonic of the region. Earthquakes originated along a NW-dipping backthrust of a NE-trending main thrust, composing the western part of the broad Ferrara Arc. A main high Vp and high Vp/Vs region delineates a pop-up structure in the center of the area. The high Vp/Vs within the pop-up structure supports the presence of a zone with increased pore pressure. The hypocentral depth of both mainshock and aftershocks is greater than those usually found for the main seismogenic regions of the Apenninic belt. P-wave velocity values in the seismogenic area, obtained by tomography, are compatible with rocks of the Mesozoic cover and suggest that seismicity occurred within the Mesozoic units stack at present by compressional tectonics.  相似文献   

19.
The influence of deep crustal processes on basin formation and evolution and its relation to current morphology is not well understood yet. A key feature to unravel these issues is a detailed seismic image of the crust. A part of the data recorded by the hydrocarbon industry in the late 1970s and 1980s in the North German Basin were released to the public recently. The seismic reflection data were recorded down to 15 s two-way travel time. The mean Common Midpoint fold of about 20 is relatively low compared to contemporary seismic acquisitions. The processing of the 1980s focussed on the sedimentary structures to explore the hydrocarbon potential of this area. We applied the Common Reflection Surface stack technique to the data sets, which is well suited for low-fold data. The reprocessing was focussed on the imaging of the subsedimentary crustal range. The reprocessed images show enhanced reflections, especially in the mid and lower crustal part. Also, the image of the salt structures in the graben area was improved. Furthermore, the reprocessed images indicate an almost flat Moho topography in the area of the Glückstadt Graben and an additional lower crustal structure, which can be correlated with a high-density body found in recent gravity modeling studies.  相似文献   

20.
Understanding how the Australian continent came together requires an understanding of structure in all levels of the lithosphere. Deep seismic reflection profiles across several Proterozoic orogens have revealed entirely buried tectonic elements, termed seismic provinces. Although undoubtedly important, the nature of these seismic provinces is typically not well characterised. The Capricorn Orogen is one such region, where the upper crust is relatively well known from geological and geophysical studies, but much of the deep crust is buried beneath Proterozoic basins. Here we combine geophysical datasets, including active and passive source seismic data and gravity data, to image the density, seismic velocity and compositional structure of the deep crust of the Capricorn Orogen. Crustal structure interpreted from deep seismic reflection studies is re-scaled using velocity information from receiver function studies. This modified geometry is used to construct a density model that satisfies Bouguer gravity data. Finally, after correcting for temperature and pressure dependencies, the velocity and density information is used to generate a compositional model of the orogen. This model indicates a varied structure with at least four distinct blocks between the Yilgarn and Pilbara cratons, bounded by major shear zones. We suggest that this variation is linked to multiple accretion events during the amalgamation of the West Australian Craton.  相似文献   

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