Summary Positive inversion tectonics involves the reversal of extensional fault movement during contractional tectonics. Basin stratigraphy developed before, during and after extensional fault movements may be described as pre-, syn- and postrift sequences. Growth fault activity may be graphically displayed using thickness changes in stratigraphic intervals from fault footwall to hanging wall. Alternatively, it may be recorded using a hanging wall displacement/distance plot. Contractional reactivation of extensional faults puts progressively older synrift markers into net contraction. The point of change from net extension to net contraction is the null point. Its position in the synrift stratigraphy may be used to quantify the inversion ratio, which is defined as the ratio of contractional to extensional movement. Negative inversion is the reactivation in extension of a significant portion of an existing contractional system. Stratigraphic separation diagrams constructed from geological maps may be used to define the null point of individual faults and to quantify their inversion ratio.
Basement lineaments pre-dating the dominant Mesozoic basin formation of the North Sea are fundamental controlling features within the polyphase tectonic evolution of NW Europe. These inherent zones of weakness were repeatedly reactivated during the evolution of the North Sea rift system. The regional and geometrical constraints imposed by the presence of these pre-existing tectonic grains during the Mesozoic extensional deformation suggest that oblique-slip movement must have occurred along many of the basin-bounding faults in a non-preferential orientation for dip-slip reactivation.While the North Sea can be considered in broad terms as a series of linked, elongated grabens developed in response to dominantly E-W extensional intra-plate stresses, the orientation, distribution and character of pre-existing shear zones ultimately controlled the geometry of many Mesozoic basins and their faulted margins.Four types of inherent basement features are recognized as having influenced subsequent basin development: (a) wide, diffuse zones of NW-SE and NE-SW basement shear, e.'g. in the Central Graben; (b) narrow shear zones oblique to primary Mesozoic extensional faults, e.g. in the East Shetland Basin; (c) E-W-trending discrete fault systems, e.g. on the Western Platform; and (d) massive palaeohighs, e.g. the Halibut Horst.The distribution of these distinct structural features throughout the North Sea was controlled by Early Paleozoic plate tectonic evolution. Repeated Mesozoic and Cenozoic reactivation along fundamental zones of weakness, including oblique-slip displacements and basinal inversion, has led to areas of highly complex structuration. Hence, there is the potential for numerous structural and stratigraphic hydrocarbon trapping mechanisms associated with these underlying basement lineaments.Although the total amount of horizontal crustal movement may be limited, understanding of the role of oblique-slip along pre-existing zones of weakness and the effect of long-lived palaeohighs within the basin is crucial to place Mesozoic and Tertiary hydrocarbon accumulations within a realistic North Sea tectonostratigraphic framework.
The tectonic evolution of SW Dyfed is presented in terms of a model of basin evolution and subsequent destruction by reactivation of the basin bounding faults in contractional mode. Movements along long-lived Palaeozoic extensional faults were reversed during Variscan compression, with the pre-existing faults accommodating the contraction in addition to the development of a thin-skinned thrust and fold system. Basin analysis of the pre-Variscan extensional system uses sedimentological and stratigraphical evidence to constrain the timing of movements of the basin bounding faults, thus allowing division of the stratigraphy into depositional megasequences related to these extensional movements. Field examples on all scales illustrate active extension during the Palaeozoic. Structural cross sections through SW Dyfed demonstrate the interrelationship of the extensional and contractional deformations, with palinspastic restorations of the cross sections illustrating the complexity of the pre-Variscan template.
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