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 78 毫秒
1.
Nine vertical electrical soundings of Schlumberger configuration were measured with AB/2 = 1–500 m. Manual and computerized interpretation were done to detect the subsurface stratigraphy of the study area. The results show that the subsurface section consists of alternated units of limestone, clay, marly limestone and dolomitic limestone and the thickness of clay unit ranged from 10 to 40 m. Nine dipole–dipole sections have also been constructed to give a clearer picture of the subsurface at the study area. The length of each dipole–dipole section is 235 m, with a electrode spacing ranging between 5 and 25 m. The Res2Dinv software was used for processing and interpretation of field data. The dipole–dipole sections at the upper plateau display high resistivity values at most parts of the plateau. Twelve shallow seismic refraction profiles are measured at selected locations for the dipole sections to define the interface between the fractured limestone and the upper surface of the clay layer. Each profile consists of 24 geophones with a geophone spacing of 2–3 m. Interpretation of seismic data indicates that the surface layer of the upper plateau consists of fractured limestone with a velocity range of 1.16–1.56 km/s and another layer of compacted clay with a velocity range of 1.38–1.88 km/s. Furthermore, the surface layer of the middle plateau consists of marl and marly limestone with a velocity about 2.1 km/s and its underlying layer consists of massive limestone with a velocity of 4.94 km/s.  相似文献   

2.
The Zeya-Bureya Basin is a part of the East Asian intracontinental riftogenic belt, which includes oil-and-gas bearing and Mesozoic-Cenozoic sedimentary basins perspective for oil and gas (Upper Zeya, Songliao, Liaohe, North Chinese). The basins are characterized by certain geophysical features: reduced thickness of the Earth’s crust and lithosphere, a higher thermal flow and a raised roof of the asthenosphere. The Zeya-Bureya Basin is composed of Mesozoic-Cenozoic sedimentary-volcanic units, with respect to which the deep structure data are absent. In 2010, geoelectric studies were carried out in this territory using the method of magnetotelluric sounding along the profile Blagoveshchensk-Birokan. These works yielded geoelectric sections down to 2 and 200 km depth. The sedimentary cover is characterized by electric resistivity of 20–50 Ohm m and by thickness of 1700 m. In the section, the Khingan-Olonoi volcanogenic trough is distinct for resistivity of 200–300 Ohm m at a background of 500–1000 Ohm m of the basement rocks. The Zeya-Bureya Basin, in terms of its geophysical characteristics, differs from oil-and-gas bearing basins of the riftogenic belt (thickness of the lithosphere is increased up to 120 km, thermal flow is low, 40–47 mW/m2). The structure of mantle underplating is explicitly seen in the section. The geophysical characteristics close to those of the Zeya-Bureya Depression are typical for gold-bearing structures of the Lower Amur ore district. Nevertheless, manifestations of oil-and-gas bearing potential in particular grabens are possible.  相似文献   

3.
Geological and seismic profiling data (more than 25000 km of seismic profiles and about 1000 sediment sampling stations) collected during the last 30 yr by research vessels of the Shirshov Institute of Oceanology, Russian Academy of Sciences are summarized. Seismic records are directly correlated with sediment cores. The distribution map (scale 1 : 500000) of Quaternary lithofacial complexes corresponding to certain stages of the Baltic Sea evolution is compiled. The following four complexes are distinguished (from the base to the top): (I) moraine, with maximum thicknesses 60 and 170 m in valleys and ridges respectively: (II) varved clay of periglacial basins and from the Baltic Ice Lake (BIL), up to 25 m thick in depressions; (III) lacustrinemarine homogeneous clay with a thickness up to 4–8 m in depressions; (IV) marine sediments (mud, aleurite, coarse-grained deposits) accumulated in environments with intense bottom currents activity (thickness 2–4 m in the Gotland Basin, 4–6 m in the Gdansk Basin, and 10–20 m in fans and prodeltas). The Quaternary sequence is cut through by inherited valleys, where the thickest Holocene sediments are noted. Today, these valleys serve as routes of sediment transport to slope bases and central parts of basins. Outblows of deep gas (through faults and fractures) and diagenetic gas (from sediments) to the bottom surface also occur in the valleys. Sedimentation rates are higher in the Gdansk Basin (up to 100–120 cm/ka). Thick sand, aleurite, and mud bodies are accumulated here (about 15–20 m in the Visla River prodelta). The sedimentation rate is slower in the Gotland Basin (up to 50–60 cm/ka), where thin (2–4 m) sections of more fine-grained mud occur  相似文献   

4.
This method pertains to oil and gas geology and to geology of sedimentary basins prospective for oil and gas. It includes identifying catagenesis zones in drilled areas within the sedimentary cover of the basin based on assay results for drill cores and cuttings using the standard methods. Analysis is primarily made in order to determine rock catagenesis based on the optical characteristics of vitrinite. A correlation between catagenesis zones and layer seismic velocities obtained from regional and exploration seismic data is made for a drilled area. Both the layer seismic velocities and the degree of rock catagenesis increase with depth under the influence of increasing rock density. Correlations between layer velocities and the degree of rock catagenesis have been established. The following ratios have been determined for the Scotian shelf, Canada, and the Barents shelf, Russia: a protocatagenesis zone (the cap) corresponds to layer seismic velocities (V lay) of 1.5–3.3 km/s, a mesocatagenesis zone (the principal hydrocarbon generation area) corresponds to V lay of 3.3–5.0 km/s, and an apocatagenesis zone (an area with a very low hydrocarbon potential) corresponds to V lay of over 5.0 km/s. An advantage of the new method of identification of catagenesis zones is that it can be used prior to drilling. Its conceptual originality and cost efficiency lie precisely in this.  相似文献   

5.
Gas hydrates have received global attention as a possible alternative non-conventional energy resource. Hence, the detection, characterization and quantification of gas hydrates are very important for evaluating the resource potential. Presence of gas hydrates in sediments above the bottom simulating reflector or BSR is associated with low attenuation or high quality factor (Q), whereas, free gas bearing sediments below the BSR exhibit high attenuation or low seismic Q. Here the logarithm spectral ratio (LSR) method is applied to marine seismic reflection data along two cross lines (18 and 46) in the Krishna-Godavari (KG) basin in eastern Indian margin, where gas hydrates have already been established by drilling/coring. The interval Qs is calculated for three sedimentary layers (A, B, and C) bounded by the seafloor, BSR, one reflector above and another reflector below the BSR at some common depth points (CDPs) to study the attenuation characteristics of sediments across the BSR. The estimated average interval Q (160) for the hydrate bearing sediments (layer B) is much higher than the average interval Q (80) for both the loose clayey sediments (Layer A) and underlying free gas saturated sediments (layer C). This demonstrates that estimation of seismic quality factor Q can be used for characterization of gas hydrate reservoir.  相似文献   

6.
The Rwenzori mountains in western Uganda, with a maximum elevation of more than 5,000 m, are located within the Albertine rift valley. We have deployed a temporary seismic network on the Ugandan side of the mountain range to study the seismic velocity structure of the crust and upper mantle beneath this section of the rift. We present results from a receiver-function study revealing a simple crustal structure along the eastern rift flank with a more or less uniform crustal thickness of about 30 km. The complexity of inner-crustal structures increases drastically within the Rwenzori block. We apply different inversion techniques to obtain reliable results for the thickness of the crust. The observations expose a significantly thinner crust beneath the Rwenzori range with thickness values ranging from about 20–28 km beneath northern and central parts of the mountains. Our study therefore indicates the absence of a crustal root beneath the Rwenzori block. Beneath the Lake Edward and Lake George basins we detect the top of a layer of significantly reduced S-wave velocity at 15 km depth. This low-velocity layer may be attributed to the presence of partial melt beneath a region of recent volcanic activity.  相似文献   

7.
The main damage from the July 9, 1997, Cariaco earthquake (Ms=6.8) was concentrated in the town of Cariaco and surrounding villages, which are located in the western part of the Cariaco sedimentary basin, close to the Gulf of Cariaco. Casanay, located at the eastern end of the sedimentary basin, suffered considerably less damage. The El Pilar fault, a right-lateral strike-slip fault that generated the earthquake, runs parallel to the southern border of the valley and crosses both towns. The determination of the velocity structure of the basin is the main objective of this study. Seismic refraction data were recorded along three lines, one of them along-strike and two perpendicular to the valley axis in the northern and southern bedrocks. Beneath Cariaco, approximately 1 km thick Quaternary sediments with seismic velocities of 1.9–2.1 km/s and bedrock velocities of more than 4 km/s were observed. The thickness of the Quaternary sediments varies within the basin, and Pleistocene sediments outcrop beneath Casanay. The increased thickness of the unconsolidated, water-saturated Quaternary sediments, together with the difference in the quality of buildings prior to the earthquake, probably is responsible for the damage pattern of the Cariaco earthquake.  相似文献   

8.
A geophysical survey was undertaken at Wiri area of the Andong in southeastern Korea to delineate subsurface structure and to detect the fault zone, which affected the 1997 mountain–hill subsidence and subsequent road heaving initiated by the intense rainfall. Electrical resistivity methods of dipole–dipole array profiling and Schlumberger array sounding and seismic methods of refraction and reflection profiling were used to map a clay zone, which was regarded as the major factor for the landslide. The clay zone was identified in electrical resistivity and seismic sections as having low electrical resistivity (<100 Ωm) and low seismic velocity (<400 m/s), respectively. The clay zone detected by using geophysical methods is well correlated with its distribution from the trench and drill-core data. The results of the electrical and seismic surveys showed that slope subsidence was associated with the sliding of saturated clay along a fault plane trending NNW–SSE and dipping 10°–20° SW. However, the road heaving was caused by the slope movement of the saturated clay along a sub-vertical NNE-trending fault.  相似文献   

9.
In many parts of the world sedimentary horizons with potential for hydrocarbon are located below flood basalt provinces. However, the presence of high velocity basaltic overburden makes delineation of sediments difficult due to the low velocity layer problem. Electrical and electromagnetic methods have been used in such scenarios because of the good electrical conductivity contrast between basalts and underlying sediments. However, mapping of the target sediments becomes difficult when the layer is thin as the data errors due to inherent noise lead to equivalent solutions. To tackle such difficult situations, a joint inversion scheme incorporating seismic reflection and refraction, magnetotelluric and deep electrical resistivity datasets is presented. Efficacy of the scheme is tested for a model comprising a thin sedimentary layer sandwiched between a thick basalt cover and a granitic basement. The results indicate that the parameters of the target sedimentary layer are either poorly resolved or equivalent solutions are obtained by the inversion of individual datasets. Joint inversions of seismic reflection (RFLS) and refraction (RFRS), or DC and MT dataset pairs provide improved results and the range of equivalent solutions is narrowed down. Combination of any three of the above datasets leads to further narrowing of this range and improvements in mean model estimates. Joint inversion incorporating all the datasets is found to yield good estimates of the structure. Resolution analysis is carried out to appraise estimates of various model parameters obtained by jointly inverting different combinations of datasets.  相似文献   

10.
The Upper San Pedro River Basin in Mexico and the United States is an important riparian corridor that is coming under increasing pressure from growing populations and the associated increase in groundwater withdrawal. Several studies have produced three-dimensional maps of the basin fill sediments in the US portion of the basin but little work has been done in the Mexican portion of the basin. Here, the results of a ground-based transient electromagnetic (TEM) survey in the Upper San Pedro Basin, Mexico are presented. These basin fill sediments are characterized by a 10–40 m deep unsaturated surficial zone which is composed primarily of sands and gravels. In the central portion of the basin this unsaturated zone is usually underlain by a shallow clay layer 20–50 m thick. Beneath this may be more clay, as is usually the case near the San Pedro River, or interbedded sand, silt, and clay to a depth of 200–250 m. As you move away from the river, the upper clay layer disappears and the amount of sand in the sediments increases. At 1–2 km away from the river, sands can occupy up to 50% of the upper 200–250 m of the sediment fill. Below this, clays are always present except where bedrock highs are observed. This lower clay layer begins at a depth of about 200 m in the central portion of the basin (250 m or more at distances greater than 1–2 km from the river) and extends to the bottom of most profiles to depths of 400 m. While the depth of the top of this lower clay layer is probably accurate, its thickness observed in the models may be overestimated due to the relatively low magnetic moment of the TEM system used in this study. The inversion routine used for interpretation is based on a one-dimensional geologic model. This is a layer based model that is isotropic in both the x and y directions. Several survey soundings did not meet this requirement which invalidates the inversion process and the resulting interpretation at these locations. The results from these locations were rejected.  相似文献   

11.
The problems of poor data quality and statics in seismic surveys have been attributed to lack of proper understanding of the low velocity layer characteristics of the area from which such data was acquired. Downhole seismic refraction survey was therefore conducted at twenty (20) borehole locations within parts of Niger Delta, Nigeria to determine the low velocity layer characteristics of the area using the Geometrics Stratavisor NZ11 instrument. The data was processed using Udysys software with spatial representation of the results presented. Static corrections were carried out on reflection seismic data acquired from the study area using Geoscribe II software to determine the effects of the low velocity layer on reflection seismic data in the area. Results of the study revealed that the velocity of the low velocity layer ranged between 144 and 996m/s with a regional average of 407m/s. The thickness of the low velocity layer varied between 3.0 and 9.6m with a mean value of 5.0m. Similarly, the velocity of the consolidated layer ranged between 1449 and 1812m/s with a mean value of 1738m/s. Results of the static correction carried out on the seismic reflection data revealed a substantial improvement in the resolution of the data after static correction. Based on these findings, it is therefore, recommended that shots for reflection seismic survey should be located at a minimum depth of 9.6m in the area to eliminate the effects associated with the low velocity layer.  相似文献   

12.
First arrival times from P-wave refraction and reflection seismic surveys along Bear Creek Valley on the Oak Ridge Reservation, Tennessee, were inverted to produce refraction tomographic velocity images showing seismic velocity variations within thinly mantled karstic bedrock to a depth of approximately 20 m. Inverted velocities are consistent with two distinct bedrock groups: the Nolichucky Shale (2,730–5,150 m/s) and Maynardville Limestone (3,940–7,575 m/s). Low-velocity zones (2,700–4,000 m/s) in the tomographic images correspond to previously inferred cross-valley strike-slip faults; in places, these faults create permeability barriers that offset or block groundwater flowing along Bear Creek Valley. These faults may also force groundwater contaminants, such as dense non-aqueous phase liquids, to migrate laterally or downward, spreading contamination throughout the groundwater system. Other, previously unmapped cross-valley faults may also be visible in the tomographic images. Borehole logs suggest the low-velocity values are caused by low rigidity fractured and vuggy rock, water zones, cavities and collapse features. Surface streams, including Bear Creek, tend to lie directly above these low-velocity zones, suggesting fault and fracture control of surface drainage, in addition to the subsurface flow system. In some cases, fault zones are also associated with bedrock depressions and thicker accumulations of unconsolidated sediment.  相似文献   

13.
A 54-m long core was raised from the bed of the Nal Sarovar, a large shallow lake located in the middle of the low-lying region linking the Gulfs of Kachchh and Khambhat, in western India. A three-layer sequence comprising: Zone-1 (top 3 m), predominantly silty-clay/clayey; Zone-2 (3–18 m), sandy; and Zone-3 (18–54 m), dominated by sticky silty-clay/clayey-silt with occasional thin sand layers and basalt fragments was identified. Smectite and illite are the dominant clay minerals with minor amounts of kaolinite and chlorite. Very high content of smectite (53–97%) in the clays of the lowermost zone (18–54 m) and the geomorphic features of the surrounding region suggested that the sediments were derived from the basaltic terrain of Saurashtra and/or via the Gulf of Khambhat. The clay content in the middle zone (3–18 m), dominantly sandy, is very low. Therefore, provenance for this zone was derived using heavy minerals in the sand fraction. The heavy mineral species in this zone suggested the mixed metamorphic and igneous terrain of Aravallis as the major source. The grain-size distribution of this zone closely matched with the sediments underlying the modern Sabarmati riverbed at Ahmedabad, suggesting fluvial depositional environment. Clays also dominate sediments of the topmost (0–3 m) zone with illite as the dominant (74–81%) specie followed by smectite suggesting derivation from the mixed metamorphic and igneous terrain of Aravallis.  相似文献   

14.
Authors collected 38 sedimentary boreholes and numerous seismic profiles from previous publications to delineate the Holocene sedimentation rate of six major depositional sinks in the middle-lower Changjiang River basins and its river coast. The results demonstrate that the highest sedimentation rate of ca. 15 m/ka occurred in the mono-depositional sink of the former Changjiang River mouth during 10 000–8000 aBP, when post-glacial transgression happened and the Changjiang water level remained at lower stand. With the rising of the Changjiang water level in response to sea level rise, Jianghan Basin of the middle Changjiang River becomes the other important depositional sink with highest sedimentation rate of 10 m/ka since 7000 aBP. As Jianghan Basin was mostly filled up at ca. 4000 aBP, Dongting Basin and the lower Changjiang valley trapped sediments in great amounts like in the river mouth. A considerable amount of Changjiang sediments has been delivered, both eastward and southward, to the inner continental shelf of the East China Sea, especially after 2000 aBP. This indicates reduced sediment storage capacity of the middle-lower Changjiang valley and the river mouth. In total, ca. 1307.4 billion tons of sediment have deposited in the middle-lower Changjiang floodplain since 7000 aBP. In the meantime, ca. 947 billion tons of sediment have been deposited in the river coast to form the Changjiang subaqueous delta and the Zhejiang-Fujian along-shelf mud wedge. Our result also reveals two time stages with lower sedimentation rates(< 4 m/ka) in all basins during 8000–7000 aBP and in the estuarine area during 4000–2000 aBP, probably owing to stengthened chemical weathering of decline of monsoon precipitation. __________ Translated from Journal of Palaeogeography, 2007, 9(4):419–429 [译自: 古地理学报]  相似文献   

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

16.
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   

17.
New data on the deep structure of the White Sea have been obtained. An interpretation of traverses 510 km in total length is presented. It has been found that the area of the Black Sea that was surveyed with the traverses is characterized by a consolidated crust consisting of two layers. In the velocity sections, interleaving of horsts and grabens is sharply identified. In the central part of the sea, an isometric trough is found with sediments up to 7–8 km thick. The trough is surrounded by east- and northeast-striking ledges and faults. Rocks with anomalously increased velocities are found in the lower part of the sedimentary cover. The thickness of the upper crust is 5–7 km. The lower crust is of a complicated structure and is 30 km thick; it forms a large fold surrounded by rocks with decreased velocitys.  相似文献   

18.
The Didwana playa, the second largest playa in the eastern part of the Thar desert, is 5.6 km long and 2.4 km wide and supports commercial salt production. The thickness of lake sediment package is reported to be 20 m and comprises fine grained clays and silts, with abundant calcite, gypsum, and halite, associated with hypersaline water. Isolated hills of graphitic phyllite and quartzite are seen on the western side of the lake. During the course of investigations for uranium in surficial environment of semi-arid terrain of Rajasthan, ground water sampling defined a NE-SW trending uranium halo encompassing the Didwana playa. Subsequent sampling of unlined dug wells, up to water table in central part of the playa, indicated uranium values up to 190 ppm and 2072 ppb in lake sediments and brine respectively. These values are of the order of 21 ppm and 192 ppb towards the southwestern periphery of the lake. The average uranium content, as inferred from 12 samples in the central part of the lake, is around 60 ppm over a thickness of 5 m. It appears that the uranium is loosely bonded to the sediments in amorphous form and is, hence, easily leachable. Samples of brine (n=10), from both the central and southwestern portions of the lake, analysed high (1,67,500–3,00,000 mg/l) TDS, HCO3− (1128–8395 mg/l), and SO4 (30,536–88,000 mg/l). These are of alkaline (pH: 7.2–9.3) and reducing (Eh: −200 to −340 mV) nature. Under these Eh-pH conditions below the groundwater table, and for such uranium bearing groundwater, precipitation of primary uranium is expected. It is, therefore, modelled that uranium in lake sediment package above water table is concentrated by evaporation process and by chemical reduction below the water table.  相似文献   

19.
Two NE-SW trending wide-angle seismic profiles were surveyed across the Chinese side Two NE-SW trending wide-angle seismic profiles were surveyed across the Chinese side of the Yinggehai (莺歌海) basin (YGHB) with ocean bottom hydrophones (OBHs) and piggyback recorded by onshore stations located on the Hainan (海南)Island.Detailed velocity-depth models were obtained through traveltime modeling and partially constrained by amplitude calculations.More than 15 km Tertiary sedimentary infill within the YGHB can be divided in to three layers with distinct velocity-depth distribution.Overall,the upper layer has a high velocity gradient with 3.8-4.1 km/s at its bottom,consistent with progressive compaction and diagenesls.Its thickness increases gradually towards the basin center,reaching 4.5 km along the southern profile.The middle layer is characterized in its most part as a pronounced low velocity zone (LVZ) with average velocity as low as 3.0 km/s.Its thickness increases from 3.0 to over 4.5 km from NW towards SE.The primary causes of the velocity inversion are high accumulation rate and subsequent under-compaction of sediments.The velocity at the top of the lower layer is estimated at about 4.5 km/s.Despite strong energy source used (4 x 12L airgun array),no reflections can be observed from deeper levels within the basin.Towards NE the basin is bounded sharply by a clear and deep basement fault (Fault No.1),which seems to cut through the entire crust.A typical continental crust with low-velocity middle crust is found beneath the coast of the western Hainan Island.Its thickness is determined to be 28 km and shows no sign of crustal thinning towards the basin.The sharp change in crustal structure across Fault No.1 indicates that the fault is a strike-slip fault.The crustal structure obtained in this study clearly favors the hypothesis that the YGHB is a narrow pull-apart basin formed by strike-slip faulting of the Red River fault zone.  相似文献   

20.
The deep crustal structure of eastern Dharwar craton has been investigated through τ-p extremal inversion of P-wave travel times from a network of seismographs recording quarry blasts. Travel times have been observed in the distance range 30–250 km in a laterally homogeneous lithospheric segment Main features of the inferred velocity-depth relationship include: (a) 29 km thick combined upper and middle crust velocity varying from 6 km/s to 7 km/s, with no observable velocity discontinuity in this depth range; (b) a lower crust (∼ 29–41 km) with velocity increasing from 7.0 to 7.3 km/s; (c) an average upper mantle velocity of 8.1 km/s; and (d) presence of a 12 km thick high velocity crustal layer (7.4 – 7.8 km/s) in the depth range 41–53 km, with a distinct velocity gradient marking a velocity increase of 0.4 km/s. The anomalous 53 km thick crust is viewed as a consequence of magmatic underplating at the base of the crust in the process of cratonization of the eastern Dharwar craton during late Archaean. The underplated material reflects here with the velocity of 7–3 to 7–8 km/s below the depth of 40 km. Our proposition of magmatic underplating is also supported by the presence of large scale I-granitoid, a product of partial melting of the upper mantle material.  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