Abstract
Complex patterns of normal faults with multiple orientations and/or highly curved shapes have been traditionally explained by successive tectonic phases of 2-dimensional deformation. Alternatively, multiple fault sets have been proposed to develop simultaneously and in orthorhombic symmetry during a single phase of 3-dimensional deformation. We use analogue models of normal faults to demonstrate that, without the influence of pre-existing structures, 3D extension is preferentially accommodated by the alternate, rather than simultaneous, development of faults with different trends. By means of stress-driven interactions, 3D deformation can be partitioned into coupled systems of normal faults, which display geometries commonly observed in tectonic settings affected by interacting plate boundaries. Under radial extension, deformation is accommodated by major curvilinear grabens coupled with minor perpendicular faults, resulting in the triple junctions of grabens observed in Afar. On the other hand, the alternate development of perpendicular faults accommodates synchronous bi-directional and mutually perpendicular extension, giving the same fault pattern observed in the Barents Sea rift-shear margin.
Highlights
The deformation of the crust has long been explained by the Andersonian model of faulting[1], which predicts the development of conjugate faults striking in map view perpendicular to the extension direction[2,3,4]
We reproduced symmetrical 3D strain fields, which (i) may be representative of the deformation occurring at symmetric intersections of different plate boundaries and (ii) are suitable to highlight any local perturbation of the stress field induced by fault activity
In order to determine diagnostic fault geometries of true 3D strain fields with respect to polyphasic extension, we reproduced both synchronous bidirectional extension and 2-phase bidirectional extension
Summary
The deformation of the crust has long been explained by the Andersonian model of faulting[1], which predicts the development of conjugate faults striking in map view perpendicular to the extension direction[2,3,4]. In light of seismological studies, a temporally and spatially uniform stress field is unrealistic in an actively deforming system, with post-earthquake variations of the seismicity rates clearly demonstrating an immediate stress-drop following the rupture of a fault[16,17,18]. By perturbing their local stress fields (and the fluid circulation), adjacent faults can influence one another during their development, establishing fault interactions and feeding a self-organisation process of fault patterns (sensu Cowie[19]). A regional 3D strain field can be accommodated by systems of interacting fault sets, which strike perpendicularly to the local extension direction, and in accordance with Anderson’s theory of faulting[1]
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