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Identifying Basement Heterogeneities and Evaluating Their Influence on Normal Fault Characteristics

Abstract

Compositional and structural variations within crystalline basement may influence the nucleation and growth of normal faults and hence, the overall rift geometry. However, the degree to which basement heterogeneities control normal fault evolution has not been studied in detail yet and remains a fundamental research theme given that large volumes of hydrocarbons remain in rifts that have evolved above heterogeneous basement. Furthermore, limitations in the depth of conventional seismic imaging, the commonly acoustically transparent nature of basement, minimal well penetrations, and complex overprinting of multiple tectonic events often complicate investigations of the crust beneath sedimentary basins. This study integrates 3D seismic reflection, borehole, and potential field data to identify variations in basement composition and to examine their influence on normal fault growth and rift structural style, offshore Mid-Norway; covering an area of approximately 3000 km2. Intra-basement structures are well-imaged on seismic and potential field data in this area due to relatively shallow burial of the basement beneath a rather thin sedimentary cover (<3.5 km). We aim to determine the offshore continuation of Precambrian basement, Caledonian nappes and Devonian basins which are well documented on the Norwegian mainland performing extensive 3D mapping of intra-basement reflections and seismic facies. Additionally, 2D gravity and magnetic data modelling provide further constraints on the upper and lower crustal configuration. Variations in basement composition along the Mid-Norwegian margin have been interpreted involving Western Gneiss cored anticlines and the presence of basement folds and mylonite zones. Preliminary results from quantitative fault analysis indicate a distinct change in fault characteristics and strain distribution on faults from S to N. This change in fault properties can be correlated with the suspected variations in basement composition and structure. Therefore, we propose that the observed changes in structural style and fault characteristics may be influenced by variations in basement composition and pre-existing weaknesses. The results of this study not only provide a fundamental understanding of normal fault growth above heterogeneous basement but also show the potential of integrating different geophysical datasets for determining the structural and compositional nature of crystalline basement in the absence of sufficient borehole control.