Extension parallel to the rift zone during segmented fault growth: application to the evolution of the NE Atlantic
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Abstract. The mechanical interaction of propagating normal faults is known to influence the linkage geometry of first-order faults, and the development of second-order faults and fractures, which transfer displacement within relay zones. Here we use natural examples of growth faults from two active volcanic rift zones (Koa`e, island of Hawai`i, and Krafla, northern Iceland) to illustrate the importance of horizontal-plane extension (heave) gradients, and associated vertical axis rotations, in evolving continental rift systems. Second-order extension and extensional-shear faults within the relay zones variably resolve components of regional extension, and components of extension and/or shortening parallel to the rift zone, to accommodate the inherently three-dimensional (3-D) strains associated with relay zone development and rotation. Such a configuration involves volume increase, which is accommodated at the surface by open fractures; in the subsurface this may be accommodated by veins or dikes oriented obliquely and normal to the rift axis. To consider the scalability of the effects of relay zone rotations, we compare the geometry and kinematics of fault and fracture sets in the Koa`e and Krafla rift zones with data from exhumed contemporaneous fault and dike systems developed within a > 5×104 km2 relay system that developed during formation of the NE Atlantic margins. Based on the findings presented here we propose a new conceptual model for the evolution of segmented continental rift basins on the NE Atlantic margins.Keywords:
Echelon formation
Dike
Rift zone
Dike
Rift zone
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Newly available, 2D Bouguer gravity anomaly data from the Baikal Rift zone, Siberia, indicate that this discrete, intracontinental rift system is regionally compensated by an elastic plate ∼50 km thick. However, spectral and spatial domain analyses and isostatic anomaly calculations show that simple elastic plate theory does not offer an adequate explanation for compensation in the rift zone, probably because of significant lateral variations in plate strength and the presence of subsurface loads. Our results and other geophysical observations support the interpretation that the Baikal Rift zone is colder than either the East African or Rio Grande rift.
Rift zone
Anomaly (physics)
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Echelon formation
Rift zone
Dike
Rift valley
Seafloor Spreading
Half-graben
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East African Rift
Half-graben
Echelon formation
Rift zone
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The active East African Rift System traverses the east African plateau over 4500km from the Afar depression to the Zambezi river. It is divisible into the eastern and western rifts. The rift system consists of some graben structures arranged en echelon: each graben also consists of en echelon normal faults.Geological phenomena in the areas of rift valleys suggest that the fracturing of the African continent had occurred under the stress field of tensional tectonics: the rifting area is apparently confined in the narrow belt along the rift valleys. We can consider the belt as a fractured region occurred between two rigid plates.The geometrical analysis of a strain ellipse had led to a conclusion that the geometrical interrelationship of en echelon fracturing depends on the angle θ between the trend of the fractured zone and the direction of horizontal extention. We can find the latter direction by using a formula α=45°-θ/2, where α is the angle between an element of en echelon fractures and its row or array: α is defined as RE-angle. The method was applied to the determination of the direction of the horizontal extension along the rift valley area (Fig. 1).The eastern rift as well as the western one is considered as an element of en echelon structure and the region which involves both their elements is regarded as the first-order structural unit in Africa. It is supposed that some deep lineament in the lower part of the lithosphere might have controlled the trend and space of the structural unit since 2000m.y.
Echelon formation
East African Rift
Half-graben
Rift valley
Rift zone
Lineament
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Rift zone
Shield volcano
East African Rift
Rift valley
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Abstract Large volcanic edifices are often shaped by the coalescence of adjacent volcanoes as well as intrusive rift zones and gravitational spreading. To better understand the structure of such volcanoes we designed analogue experiments simulating gravitational spreading of an edifice made by overlapping cones of different age, and examined the formation of rift zones. The results allow distinction of two main rift geometries. (i) Spreading edifices of similar age that partly overlap, tend to develop a rift zone approximately perpendicular to the boundary of both volcanoes. Such a rift zone causes two volcanoes to grow together and form an elongated topographic ridge. (ii) Partly overlapping volcanoes of different age are spreading at different rates and thus form a rift zone parallel to the boundary of both volcanoes. Such a rift zone causes two volcanoes to structurally separate. The results are widely applicable for large volcanoes subject to rifting and flank spreading, which we demonstrate for Réunion Island and for southern Hawaii.
Rift zone
Rift valley
Shield volcano
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Half-graben
Echelon formation
Rift zone
East African Rift
Horst and graben
Lineament
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Clockwise
Rift valley
Transform fault
Rift zone
Echelon formation
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