Some constraints on the form of the Welsh Basin from regional gravity and aeromagnetic data, with particular reference to Central Wales
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Abstract Gravity and aeromagnetic data sets provide systematic coverage over the whole of the Welsh Basin. Physical property information shows considerable overlap in densities within the Lower Palaeozoic and Precambrian formations, with relatively few units of the exposed succession being strongly magnetic. While the geophysical anomalies cannot be interpreted unambiguously in terms of these sources, the data can still be used to test different geological models and to indicate the range of possible solutions. The major fault zones are seen to influence the anomaly patterns in a variety of ways, reflecting a combination of differing near-surface contrasts and effects within the underlying basement. The general increase in gravity values towards Cardigan Bay may originate from the lower crust, and the large amplitude aeromagnetic low seen over Cardigan Bay indicates that a distinctive change in the character of the crust occurs here. Two-dimensional modelling in central Wales shows that a westward thickening of the Welsh Basin, to a maximum of about 10 km, may also be significant. It is necessary to postulate a separation of the base of denser, Lower Palaeozoic rocks from the underlying, more magnetic basement in order to account for offsets in anomaly source locations. This implies the presence of less dense Cambrian sediments and/or non-magnetic components of the Precambrian basement.Keywords:
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Abstract Abstract The central Lockne area is located in the Caledonian thrust front in Jämtland. Though most of the Caledonian orogenesis is thought to have influenced only the Palaeozoic sedimentary rocks that were deformed in a décollement-style, there is clear evidence that at least some Palaeozoic deformation of the Precambrian basement has taken place. In the described area the basement is brecciated within a NW-SE trending zone. This brecciation has occurred during at least two episodes, one occurring in pre-Aseri time; another is not older than of Dalby age. They are perhaps connected with Palaeozoic reactivation of Precambrian (transform?) faults. In a new quarry in the vicinity of Lockne church, crushed Precambrian crystalline rocks overlie Cambrian alum shales. This suggests that Caledonian thrusting may influence the basement in this part of Jämtland. Key Words: Lockne areaJämtlandSwedenbrecciaPrecambrian basementsedimentary dykesalum shaleasphaltiteoverthrust granite-gneiss
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The Luzong basin is located in the Zongyang-Lujiang anomaly area of the Huaining-Lujiang anomaly zone which exhibits regional high gravitational and magnetic anomalies.In this paper,we obtained density and magnetic information at different depth,and the boundary of gravity and magnetic anomalies in the Lu-Zong Basin based on multi-scale edge detection.Integrating with the gravity and magnetic anomaly distribution,we inferred the tectonic framework,divided the basement uplift areas,and constructed the tectonic framework of the Luzong basin.We consider that there are four directions of basement faults in the Luzong basin,and the main direction is north-east.The basin includes four basement uplift areas and one basement remnant uplift area.Considering the relationship between ore concentration areas and tectonic framework of the Luzong basin,it is suggested that the S shape gravity-high anomaly zone is favorable for searching for deep and concealed ore deposits.
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Skaly prekambryjskie tworzą podloze calego kraju, lecz zostaly szczegolniej wyroznione i zbadane w trzech regionach: na prekambryjskiej platformie wschodnioeuropejskiej i w Karpatach, gdzie tworzą fundament krystaliczny ukryty pod skalami pieter osadowych, oraz na Dolnym Śląsku, gdzie są wlączone w system faldow waryscyjskich, wypietrzone i odsloniete. Skaly prekambryjskie roznią sie nie tylko polozeniem, ale takze budową tektoniczną i strukturalną, metamorfizmem regionalnym, wiekiem i stylem przebudowy. Odmiennie ksztaltowaly sie dzieje ich badan i niejednolity jest stopien rozpoznania. THE PRESENT STATE AND DIRECTIONS OF STUDIES ON THE PRECAMBRIAN ROCKS OF POLAND Although the whole basement of Poland is built of Precambrian rocks, these rocks were recorded and recognized in three regions only. They form crystalline basement overlain by rocks of sedimentary stages in the Precambrian Platform and the Carpathians, being incorporated in the Variscan fold system, uplifted and exposed in the Lower Silesia. Precambrian rocks of these three regions differ in location as well as in tectonic and structural units, regional metamorphism, age and style of reactivation. The history of studies of these rocks is also different, similarly as the degree of knowledge. In Poland, these rocks built crystalline basement on which rocks of Paleozoic and Meso-Cenozoic sedimentary stages were originating and undergoing structural rebuilding. The development of the latter was determined by movements of crystalline basement blocks, responsible for regional and tectonic differentiation of the sedimentary covers. The studies of crystalline basement of the East-European Platform were initiated in 1954 by making three drillings preceded by geophysical surveys. In the next years, there took place intense development in there studies, determined by search for metal ore deposits and bitumens in the sedimentary cover of the Platform. The geological studies on the crystalline basement are concentrated in the Geological Institute which makes possible appropriate planning of the works and synchronization of geophysical, drilling and mapping programs. On the basis of results of petrographic, mineralogical, geochemical, geochronological and deposit studies of core material from almost 200 boreholes, and rich geophysical data (including those from magnetic and gravimetric semi-detailed and detailed mapping), it was possible to prepare petrological map of the top of crystalline basement in the scale 1:500 000 and, subsequently, the map in the scale 1:200 000 in 34 sheets. The tectonic model and stratigraphic scheme of rocks of the crystalline basement were also prepared. The studies have been crowned with discovery of vanadium-bearing titanomagnetite deposit in anorthosites in the Suwalki area and demonstration of its resources. Moreover, chemogenic hematite taconites were found at Łochow near Warsaw at 2400 m depth, and concentrations of rare earth elements – in nepheline syenites and carbonatites in the areas of Elk and Tajno. The history of studies on the Precambrian basement of the Carpathians and southern Poland began when crystalline rocks were found by a drilling at Rzeszotary in 1909. However, at that time it was not certain whether the drilling entered an exotic or autochtoneous crystalline massif. This question has not been solved before the end of the sixties, when deep drillings penetrated the Carpathian Flysch and entered crystalline basement. The knowledge of crystalline basement in this part of Poland is unsatisfactory as petrological maps, tectonic model and stratigraphic scheme are still missing. The available data show that the basement is strongly eroded ill the west, where the Carpathian Flysch rests on amphibolites fades metamorphic rocks, whereas in the east the Flysch rests on strongly diagenesed or weakly metamorphic rocks of the uppermost Precambrian. After the world war II, the Geological Institute began studies on crystalline rocks of the Lower Silesia, with detailed geological mapping of areas for which maps were still missing. Up to the present, 100 sheets of geological map in the scale 1: 25000 are completed and the remaining ones are in preparation. To carry out programs of stratigraphic, lithological, petrographic and tectonic studies and search for deposits, there has been called into existence the Lower Silesian Branch of the Geological Institute in Wroclaw. Along with time, the Lower Silesian Branch of the Geological Institute started introducing modern research techniques, as e.g. mesoscopic analysis in studies on tectonics, petrotectonic studies, geochemical analysis, analysis of radiogenic age, physical studies of rocks, geophysical studies, and biostratigraphic studies of “silent rock series”. The developments in the studies were accompanied by widening their range by deep drillings, especially in the forefield of the Sudety Mts. The results of these studies markedly changed views on stratigraphy, lithological development and metamophic alterations of the Precambrian.
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