Basement fractures in North Wales: their recognition and control on Caledonian deformation
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Abstract A detailed survey of the patterns of regional deformation in North Wales has revealed the presence of several discrete, linear zones of anomalous deformation. These zones are distinguished by atypical finite strain values, intense tight to isoclinal folding, non-axial planar cleavages and high metamorphic gradients. A history of prolonged tectonic activity along these zones is indicated by their influence on sedimentation and volcanism. It is proposed that these zones reflect the deformation of the Lower Palaeozoic cover above pre-existing basement fractures. These basement fractures form a framework which divides North Wales into a series of blocks and sub-blocks. The end-Caledonian deformation is explained by a transpression model dominated by SE-directed shear with minor components of strike-slip localized in these zones. The application of this model to other parts of the Welsh Basin is briefly considered.Keywords:
Transpression
Basement
Décollement
Basement
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The represented method allows to create three-dimensional geological models of collectors of paleozoic basement, which provides a significant economic effect in the subsequent deposit explorations for typical russian companies - subsoil users, having a limited amount of data. In geological modeling of the collectors of paleozoic basement, the application of the method of dual porosity (double medium) is most relevant. The created approach allows to refine the geological model with an increase of geological reserves by 30 % in reservoirs with natural fracturing.
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Subsoil
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In Spain, the superficial part of the Hercynian basement, under the Permo-triassic cover, appears in two quite different ways : - when the Paleozoic basement is directly covered by the Buntsandstein sandstones, it is altered and rubefied on one or some ten meters of thickness ; - when the Paleozoic basement is covered by the Permian detrital formations, it is not altered. Therefore, a comparative study of clay minerals was performed in the Paleozoic basement and in its altered fades, as well as in the Permian and Triassic sediments.
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Geological structure and petroleum potential of Paleozoic in the West-Siberian oil and gas basin remains unstated and underestimated despite research projects conducted and hydrocarbon deposits discovered in Paleozoic sediments. This is attributed firstly to exploration works focused on Mesozoic oil and gas complex due to its high potential, and secondly to exiting tectonic model of the region where Paleozoic complex is included in folded basement or foundation. According to sediment migration theory of hydrocarbon generation, the basement of the basin is not a source for hydrocarbon fluids, and therefore cannot be considered as an independent oil and gas complex with its own hydrocarbon source rocks and reservoirs. However, oil and gas deposits discovered in the basement of Western Siberia show inconsistency of currently existing hypotheses and models. Among them there are two major versions for hydrocarbon origin in Paleozoic organic and inorganic theories. Geological structure and reservoir distribution in Paleozoic complexes is very specific and require a thorough analysis of all geological, geophysical and geochemical materials.
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Seven first-level combination types are extracted according to the combination forms of detachment-fault and deformation-structure,which are the the type of basal decollement thrust-anti-transformation in north of Subei-South Yellow Sea (Ⅰ),the type of uplift in South Yellow Sea centre(Ⅱ),the type of thrust-anti-transformation in Suzhong and south-central of South Yellow Sea(Ⅲ ),the type of siluric decollement nappe in Sunan and south of South Yellow Sea (Ⅳ),the type of basal decollement thrust in Wannan-Zhexi-Wunansha (Ⅴ),the type of thick-skinned deformation in Jiangnan uplift (Ⅵ),and the type of thick-skinned deformation in Zhangbaling-Qianliyan thick-skinned deformation (Ⅶ).Five second-level combination types are extracted according to the deformation combination characteristic of structure layers above and below detachment-fault,which are the type Ⅰ that upper palaeozoic accord with lower palaeozoic,the type Ⅱ that both upper palaeozoic and lower palaeozoic are simple,the typeⅢ that upper palaeozoic is simple and lower palaeozoic is complex,the type Ⅳ that upper palaeozoic is complex and lower palaeozoic is simple,the type Ⅴ that both upper palaeozoic and lower palaeozoic are complex.According to the three level division rule,lower Yangtze area is divied into seven first-level divisions,seven second-level divisions,and ten third-level divisions.
Décollement
Thrust fault
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Transpression
Strain partitioning
Lineation
continental collision
Collision zone
Overprinting
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