3D extension at plate boundaries accommodated by interacting fault systems

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

3D Extension at Plate Boundaries Accommodated by Interacting Fault Systems

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

How important are intrabasement structures in controlling the geometry of sedimentary basins? Insights from the Taranaki Basin, offshore New Zealand

Intrabasement structures often are envisaged to have acted as a structural template for normal fault growth in the overlying sedimentary cover during rifting (e.g. Barents Sea; Egersund Basin, offshore southern Norway). However, in other settings, the geometry of rift-related faults was apparently unaffected by the pre-existing basement fabric (e.g. Mal\uf8y Slope, offshore western Norway). Understanding the nucleation and propagation of normal faults in the presence of basement structures may elucidate how and under what conditions basement fabric can exert an influence on rifting. This study is based on borehole constrained 3D seismic data from an area of the Taranaki Basin, offshore New Zealand, situated at the boundary between two basement terranes generated during the Mesozoic convergence along the margin of Gondwana. The relatively shallow basement (<3.5 km) is overlain by a late Paleocene to Pleistocene sedimentary cover scarcely affected by the late Miocene inversion and Pliocene rifting, resulting in excellent imaging of basement structures. We mapped the 3D geometry and distribution of throw on the fault planes for clarifying the relationships between basement and cover structures and the kinematic history of the faults. Our analysis has highlighted two types of intrabasement structures. In the northern part of the survey, a N\u2013S-striking, west-dipping lineament marks the transition between two basement units, characterized by different seismic facies. This lineament was reactivated during the late Miocene inversion. In addition, a network of arcuate, N\u2013S-elongated, west-dipping high-amplitude reflectors cut through a largely homogenous low-amplitude basement throughout the whole study area and is only partly reactivated during the inversion phase. Two classes of normal fault segments affected different intervals of the sedimentary cover. The lower fault segments are hard-linked with the intrabasement structures and nucleated within few hundreds of metres from the basement-cover interface. They are blind and swing from NW-SE to NNE-SSW trends. We document different styles of interaction between them and the overlying faults. The segments diverging from the regional NNE-SSW trend are confined in the lower 500 m of the sedimentary succession, whilst the aligned ones are connected with the upper segments. The upper fault segments mostly strike according to the regional NNE-SSW trend; they nucleated within the late Miocene strata and were active during the Pliocene. Above the N\u2013S-striking basement lineament, the upper fault segments strike parallel to it and are systematically hard-linked with the lower ones, generating a single fault zone affecting the whole sedimentary cover. Conversely, away from this lineament, deep and shallow fault planes are only occasionally linked, with some shallow faults totally lacking any connection to basement features. Our study suggests that basement fabric can effectively constrain the geometry of later normal faults in the proximity of the top basement and at the transition between basement units, whilst elsewhere the deformation seems to respond to the regional stress field. The interplay between intrabasement structures and regional stress generates complex geometric relationships between structures at different levels of the sedimentary cover

Normal fault growth influenced by basement fabrics: The importance of preferential nucleation from pre-existing structures

Reactivation of pre-existing intra-basement structures can influence the evolution of rift basins, yet the detailed kinematic relationship between these structures and overlying rift-related faults remains poorly understood. Understanding the kinematic as well as geometric relationship between intra-basement structures and rift-related fault networks is important, with the extension direction in many rifted provinces typically thought to lie normal to fault strike. We here investigate this problem using a borehole-constrained, 3D seismic reflection dataset from the Taranaki Basin, offshore New Zealand. Excellent imaging of intra-basement structures and a relatively weakly-deformed, stratigraphically simple sedimentary cover allow us to: (i) identify a range of interaction styles between intra-basement structures and overlying, Plio-Pleistocene rift-related normal faults; and (ii) examine the cover fault kinematics associated with each interaction style. Some of the normal faults parallel and are physically connected to intra-basement reflections, which are interpreted as mylonitic reverse faults formed during Mesozoic subduction and basement terrane accretion. These geometric relationships indicate pre-existing intra-basement structures locally controlled the position and attitude of Plio-Pleistocene rift-related normal faults. However, through detailed 3D kinematic analysis of selected normal faults, we show that: (i) normal faults only nucleated above intra-basement structures that experienced late Miocene compressional reactivation; (ii) despite playing an important role during subsequent rifting, intra-basement structures have not been significantly extensionally reactivated; and (iii) preferential nucleation and propagation of normal faults within late Miocene reverse faults and folds appears to be the key genetic relationship between contractionally reactivated intra-basement structures and rift-related normal faults. Our analysis shows that km-scale, intra-basement structures can control the nucleation and development of newly formed, rift-related normal faults, most likely due to a local perturbation of the regional stress field. Because of this, simply inverting fault strike for causal extension direction may be incorrect, especially in provinces where pre-existing, intra-basement structures occur. We also show that a detailed kinematic analysis is key to deciphering the temporal as well as simply the spatial or geometric relationship between structures developed at multiple structural levels

Structural inheritance in the North Atlantic

The North Atlantic, extending from the Charlie Gibbs Fracture Zone to the north Norway-Greenland-Svalbard margins, is regarded as both a classic case of structural inheritance and an exemplar for the Wilson-cycle concept. This paper examines different aspects of structural inheritance in the Circum-North Atlantic region: 1) as a function of rejuvenation from lithospheric to crustal scales, and 2) in terms of sequential rifting and opening of the ocean and its margins, including a series of failed rift systems. We summarise and evaluate the role of fundamental lithospheric structures such as mantle fabric and composition, lower crustal inhomogeneities, orogenic belts, and major strike-slip faults during breakup. We relate these to the development and shaping of the NE Atlantic rifted margins, localisation of magmatism, and microcontinent release. We show that, although inheritance is common on multiple scales, the Wilson Cycle is at best an imperfect model for the Circum-North Atlantic region. Observations from the NE Atlantic suggest depth dependency in inheritance (surface, crust, mantle) with selective rejuvenation depending on time-scales, stress field orientations and thermal regime. Specifically, post-Caledonian reactivation to form the North Atlantic rift systems essentially followed pre-existing orogenic crustal structures, while eventual breakup reflected a change in stress field and exploitation of a deeper-seated, lithospheric-scale shear fabrics. We infer that, although collapse of an orogenic belt and eventual transition to a new ocean does occur, it is by no means inevitable