Deep seismic investigation of crustal extensional structures in the Danish Basin along the ESTRID-2 profile
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The crust and uppermost mantle in the Danish Basin are investigated by modelling the P-wave velocity distribution along the north–south trending seismic profile ESTRID-2. Seismic tomography and ray inversion modelling demonstrate a variable depth to the top of the crystalline crust, from ∼10 km in the northern part of the profile, to ∼2 km depth in the southernmost part. The crystalline crust shows very high P-wave velocity in the central part of the profile, with ∼6.7 km s−1 at depths as shallow as 12 km, and ∼7.3–7.5 km s−1 in the lowermost crust. These values confirm previous results obtained along the orthogonal ESTRID-1 profile and the Eugeno-S profile 2. This high velocity zone in the middle to lower crust is interpreted as a mafic intrusion, which explains a positive gravity anomaly of ∼50 mGal (Silkeborg Gravity High). The total length of the intrusion is at least 80 km in the east–west direction and ∼25– 35 km in the north–south direction. The estimated thickness of the intrusion, from its top to the Moho level is ∼18–20 km, which gives a total minimum volume of ∼40–50 000 km3. The reflectivity properties of the Moho discontinuity are variable along the profile. Below the intrusion, the PmP signal is very weak, due to the small velocity contrast between the lowermost crust (∼7.4 km s−1) and uppermost mantle (∼7.6–7.7 km s−1). The main Moho reflection has a ‘reverberative’ character to the south of the intrusion. This feature is interpreted by layering at the Moho level, possibly due to magmatic underplating. The occurrence of a large crustal mafic intrusion associated with magmatic underplating may be related to extensional/transtensional tectonism in the Tornquist Fan area in the Late Palaeozoic. The extensional event probably caused the opening of a plumbing system for intrusion of mantle derived magma into the crust. The ascending magma may have been injected at upper-middle crustal levels and, during the late phases of the development, ‘squeezed’ laterally along the Moho.Anticline
Density contrast
Gravimetry
Free-air gravity anomaly
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Basement
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The first degree polynomial order is fitted to the Bouguer gravity anomaly data to produce the first residual gravity anomaly map of Calabar Flank. The residual gravity data was computed by subtracting the regional trend from the Bouguer gravity field. The regional trend in the flank is the first degree surface fit and represents anomaly of long wavelength while the residual component have their origin from short wavelength sources (shallow sources). This quantitative approach is advantageous over the wavelength filtering methods. The result of this low order fitting shows that the residual gravity field is characterized by positive and negative gravity anomalies. This is consistent with the geologic setting and tectonics of the Calabar Flank. (Keywords: Bouguer gravity anomaly data, Calabar Flank, gravity field)
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Free-air gravity anomaly
Density contrast
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This paper presents a number of new concepts concerning the gravity anomaly. First, it identifies a distinct difference between a surface (2-D) gravity anomaly (the difference between actual gravity on one surface and normal gravity on another surface) and a solid (3-D) gravity anomaly defined in the fundamental gravimetric equation. Second, it introduces the ‘no topography’ gravity anomaly (which turns out to be the complete spherical Bouguer anomaly) as a means to generate a quantity that is smooth, thus suitable for gridding, and harmonic, thus suitable for downward continuation. It is understood that the possibility of downward continuing a smooth gravity anomaly would simplify the task of computing an accurate geoid. It is also shown that the planar Bouguer anomaly is not harmonic, and thus cannot be downward continued.
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W artykule przedstawiono probe wyprowadzenia związku korelacyjnego miedzy grubością skorupy a anomaliami Bouguera dla obszaru Polski. Stwierdzono, ze związek ten mozna opisac bardzo prostą formulą dla calej Polski, z wyjątkiem obszaru regionalnego wyzu grawimetrycznego, polozonego w poludniowo-wschodniej cześci kraju. Omowiono mozliwośc wykorzystania wyprowadzonego związku korelacyjnego dla badania niejednorodności budowy skorupy calego kraju. Przedstawiono wstepną interpretacje anomalnej budowy skorupy w obrebie obszarow wspomnianego wyzu grawimetrycznego. Zwrocono uwage na nie calkowicie jasną sytuacje tego obszaru w stosunku do przyjmowanych dziś wielkich jednostek tektonicznych. ANOMALIES OF GRAVITY AND EARTH CRUST STRUCTURE IN POLAND Recent developments in the studies on correlations between Earth crust thickness and Bouguer anomalies on the world, continent and region scale are discussed. Graphic comparison of such correlations are given. Moreover, some of the correlations are discussed ,in detail, similarly as differences in density parameters for the contact zone between the crust and the Upper Mantle determined by them. Moreover, there are discussed the causes of marked discrepancies between the estimations of thickness of the Earth Crust in Poland, based on results of deep seismic soundings, and the thickness data obtained by Z. Fajklewicz using the formulae proposed by R. M. Demienicka and G. P. Woollard (fide Z. Fajklewicz, 1964, 1973) and ,presented on ,two different versions of the Earth crust thickness. A correlative equation for the thickness of the Earth crust In Poland was established on the basis of data from small-scale Bouguer anomaly maps and the results of deep seismic soundings. This relationship is valid fur almost the whole country except for the areas of anomaly thick crust (Lublin and presumably Świetokrzyski gravimetric highs). The proposed formula for estimating Earth crust thickness is characterized by a mean error of the order of ±4.6 km for the case of single entrance values. The differences between values for the above mentioned areas of anomaly thick crust and values obtained using this formula markedly exceed the incertitude error (i.e. 3 times standard deviation). Attention is paid to previously noted role of the Lublin gravimetric high which cannot be explained by its shallow geological structure. An attempt was made to relate the Lublin high to the presence of large of rocks heavier than the neighbouring ones located below the top of the consolidated basement. Attention is also paid to the validity of the correlative relationship between the Earth crust thickness and Bouguer anomalies for areas differing in genesis and age (Carpathian Mts, Paleozoic and Precambrian Platforms, Sudety Mts), and its relatively high accuracy. It is further proposed to use the relationship for calculating in gravimetry. The would be defined by some normalized structure of the crust with density contrast at the Moho boundary equal 0.11 g/cm 3 , also obtained using the relationship. The normal field defined in this way would reflect the effects of denivellatios in the zone of the Moho discontinuity whilst the differences between the recorded values of Bouguer anomalies and normal field values would indirectly characterize the structure and homogeneity of the crust. Two versions of normal field determined in this way, as well as estimations of amplitude of the anomalies connected with Earth crust structure in eastern part of the Lublin gravimetric high are given. A possible model of masses is given in order to illustrate their size. In addition, preliminary geological interpretation of the model is given. The attention is, however, drawn to the fact that there is no direct evidence for any connections between the area with anormal structure and currently accepted boundaries of main tectonic units of the country. On the other hand the size and location of the area implicate a necessity of redefinition of the tectonic units. In concluding some attention is paid to the effectiveness of a complex use of the results of gravimetric and deep seismic surveys for searching for local anomalies lies in the structure of the crust even though their relations to the tectonics are still unclear. The hazard of overestimation of the significance of the data concerning local morphology of Moho discontinuity for delineating even large tectonic units is emphasized.
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An isostatic residual gravity map of Brazil has been computed by removing from a 0.5° × 0.5° Bouguer anomaly grid a regional gravity field calculated for compensating masses of surface topography. The coherence function, a statistical measure of the correlation between Bouguer anomaly and topography, was first computed in order to constrain the compensation mechanism within Brazil. Similar to results for North America and Australia, the coherence function of South America has a broad transition between high and low coherence values, suggesting a combination of tectonic provinces with different flexural rigidities and/or loading processes. In view of this result, we have considered, as a first approximation, a model in which the surface topography is the only load acting on a nonrigid lithosphere. A regional gravity field has been computed assuming Airy‐Heiskanen isostasy with compensation at the crust‐mantle boundary. The residual gravity map, which was obtained by removing the computed regional gravity field from the observed Bouguer anomaly, shows a long‐wavelength N‐S trending negative anomaly over most of Brazil. This gravity feature of approximately 3000 km width is the southern continuation of the western North Atlantic negative geoid/gravity anomaly and reaches at least ∼15 mGal in the northern portion of Brazil. Using the upward continued isostatic residual gravity field at 300 km, this long‐wavelength component, which may be dynamically induced, has been removed to first approximation. The final isostatic residual gravity anomaly map depicts anomalies with wavelengths between 100 and 1000 km which correlate with major tectonic provinces. Negative anomalies occur mainly over Paleozoic intracratonic and Cretaceous rift‐type sedimentary basins, and granitic intrusions and along Proterozoic thrust belts. Positive residual anomalies are generally observed over regions affected by igneous activity and volcanism such as in the Amazon basin and the Paraná flood basalt province. Positive anomalies are also associated with overthrust crustal plates which define a suture zone in central Brazil and over sub‐Andean Tertiary foreland basins.
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Free-air gravity anomaly
Anomaly (physics)
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One of the most difficult problems in gravity interpretation is the separation of regional and residual gravity anomalies from the Bouguer gravity anomaly. This study discusses the application of the minimum‐curvature method to determine the regional and residual gravity anomalies.
Free-air gravity anomaly
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Separation (statistics)
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Using the newly ultrahigh order gravity field model EIGEN-6C2,the global DEM model topo_15.1.img and the crust model CRUST1.0,we calculate three kinds of static gravity fields(Free-air gravity anomaly,Bouguer anomaly and residue gravity anomaly)of Sichuan-Yunnan region,and study the relationship between these gravity fields and regional earthquakes.As a whole,the Free-air gravity anomaly has a clear mirror-image relation with regional topography,which means that the regional topography has a big effect on Free-air gravity anomaly.By subtracting the topography gravity effect from Free-air gravity anomaly,we get the Bouguer anomaly of Sichuan-Yunnan region,which presents a characteristic of the lower northwest higher southeast;And the Bouguer anomaly basically reflect the crustal thickness changes of SichuanYunnan region,which be superposed of the intensity of various range of local anomalies.Based on the Parker method we calculate the gravity effect of regional Moho discontinuity,by subtracting it from regional Bouguer anomaly,we get the residue gravity anomaly, which can reflect the density difference of crust in Sichuan-Yunnan region.By comparing the patterns of these gravity fields with the earthquakes source mechanism which occurred after the year 2000,we found that,the thrust or normal type of earthquakes are likely to break on the high gradient zone of three kinds of static gravity fields,and the strike-slip type of earthquakes are likely to break on the smooth and steady area static of gravity fields,which may be concerned with the nature of the faults and the structural setting of the crust.
Free-air gravity anomaly
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Discontinuity (linguistics)
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The gravity anomalies of the Jurassic and deep structures were obtained by stripping the gravity effect of Cretaceous and Tertiary formations from the available Bouguer gravity map in central and south Iraq. The gravity effect of the stripped layers was determined depending on the density log or the density density obtained from the sonic log. The density relation with the seismic velocity of Gardner et al (1974) was used to obtain density from sonic logs in case of a lack of density log. The average density of the Cretaceous and Tertiary formation were determined then the density contrast of these formations was obtained. The density contrast and thickness of all stratigraphic formations in the area between the sea level to the top of Jurassic formations were used to determine the gravity effect of these layers. The gravity anomaly map of the stripped formation was determined. The gravity anomaly map of the stripped formation was subtracted from the Bouguer gravity map, and the gravity anomaly map of deep structures was obtained. The regional and residual maps (3rd order polynomial ) were determined for the gravity anomaly maps before and after stripping. The regional gravity map before stripping shows one positive anomaly located at the western part of the study area and west Abu-Jir and Euphrates faults. The regional gravity map after stripping shows a positive anomaly located along an axis extended from Kut toward Najaf. This positive anomaly map divided the sedimentary basin into two sub-basins. The positive gravity residual anomaly of the Bouguer map before stripping shows regionally three structural axes trending NW-SE. These axes are Baghdad-Kut axis, northwest Karbala axis and west Samawa- Nasiriyah axis. The positive residual anomaly map after stripping shows two important anomaly areas. The first area is located between Kut and Karbala-Najaf . and the second is located northwest Karbala by about 100-120 km. These two areas may be prospective areas for hydrocarbon. The stripping method application in the study area shows good result; therefor, it can be used to enhance the gravity data to investigate deep structures in other areas.
Density contrast
Anomaly (physics)
Free-air gravity anomaly
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