Abstract
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.
Highlights
The primary regional-scale segmentation of extensional terranes is controlled by the development of networks of normal fault systems and the partitioning of strain across them
Fault growth models have been derived using natural examples and numerical, or scaled-analogue modelling techniques, in which normal faults grow through stages in which discontinuous segments interact and link across relay zones to form composite structures with fault displacement deficits initially accommodated by soft-linkage rotation and/or material folding (e.g. Trudgill and Cartwright, 1994; Gupta and Scholz, 2000; Peacock, 2002; Long and Imber, 2010)
Mechanical interaction between discontinuous fault segments can have an important influence on fault system evolution, including the geometry of first-order faults and the development and distribution of second-order faults and fractures within developing interfault zones
Summary
The primary regional-scale segmentation of extensional terranes is controlled by the development of networks of normal fault systems and the partitioning of strain across them. Fault growth models have been derived using natural examples and numerical, or scaled-analogue modelling techniques, in which normal faults grow through stages in which discontinuous segments interact and link across relay zones to form composite structures with fault displacement deficits initially accommodated by soft-linkage rotation and/or material folding Displacement (throw) gradients on adjacent normal faults are commonly accommodated by relay structures Peacock and Sanderson, 1991; Childs et al, 1995; Long and Imber, 2010), requiring horizontal axis bending of the host layering (Fig. 1). The bounding faults of a relay zone exhibit opposing horizontal displacement (heave) gradients, which requires a component of vertical axis rotation to maintain the connection between the hanging wall and footwall It cannot be accommodated by layer-parallel or flexural slip between layers (unless layering is vertical) and requires the material to bend or stretch within the layer plane
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