Polymodal faulting in rifting settings: strain field and role of pre-existing structures

Normal faults have been typically thought to develop sub-perpendicularly to the extension direction, forming systems of sub-parallel faults. However, a variety of processes may result in the simultaneous development of faults with different strikes (i.e. polymodal faulting), most notably 3D strain fields and influence of pre-existing fabrics. Whilst the classic model on faulting suggests that complex fault patterns should result from polyphase deformation with different extension directions, the concept of polymodal faulting can account for the development of different fault sets under the same stress regime, having possibly a strong impact on the reconstruction of the palaeostress. In the thesis, 3D seismic data were used to assess the occurrence of polymodal faulting in two different extensional tectonic settings: the Barents Sea rift-shear margin (Paper 1), offshore northern Norway, and the Taranaki back-arc rift (Paper 3), offshore New Zealand. Then, analogue models and kinematic analysis were used to investigate the deformation processes. In both settings, polymodal faulting was observed at the 10s of kilometres scale. The occurrence of polymodal faulting at such large scale may affect the previous interpretation of the structural histories of these sedimentary basins, reducing the number of tectonic phases that should be envisaged to explain the observed structures. The tectonic setting appears to have a strong influence on the deformation processes, with polymodal faulting occurring under the control of a 3D strain field in the Barents Sea and of pre-existing basement fabrics in the Taranaki Basin. In the Barents Sea, the onset of a 3D strain field is related to the interaction between the Atlantic and the Arctic rifts, coupled with a characteristic brittle-ductile-brittle mechanical stratigraphy. The analogue models performed in this thesis (Paper 2) highlighted that in 3D strain fields, local fault interactions exert a strong control on the final fault geometries, with the faults forming perpendicular one to the other rather than in orthorhombic symmetry with respect to the principal strain axes as previously thought. In the Taranaki back-arc rift, despite the absence of extensional reactivation of the intra-basement structures, they appear to have exerted a strong control on the distribution and strike of normal faults. The growth history of normal faults highlighted that preferential nucleation/propagation within pre-existing weakness zones and local perturbation of the regional stress field may be effective mechanisms through which pre-existing structures can influence normal faults, even without their direct extensional reactivation. In conclusion, complex fault patterns may not necessarily reflect a complex tectonic history, but can result from the dynamics of deformation processes, which appear to be strongly susceptible to the local influences of developing as well as pre-existing structures.Il modello classico della fagliazione (ovvero la teoria di Mohr-Coulomb) prevede che in un regime estensionale le faglie si formino perpendicolarmente alla direzione di estensione, dando luogo a sistemi di faglie sub-parallele fra loro. Tuttavia, una varietà di meccanismi possono portare allo sviluppo simultaneo di faglie con diverse orientazioni, detto fagliazione polimodale. In particolare, campi di strain 3D e l’influenza da parte di strutture pre-esistenti potrebbero portare allo sviluppo simultaneo di diversi sistemi di faglie in modo pervasivo e su ampia scala. A differenza del modello classico della fagliazione, il concetto di fagliazione polimodale può pertanto spiegare lo sviluppo simultaneo di diversi sistemi di faglie nell’ambito di un unico campo di stress. Adottare un modello o l’altro può dunque avere un impatto drastico sulla ricostruzione dell’evoluzione tettonica di un’area. In questa tesi, si sono usati dati sismici 3D per valutare il presentarsi di fagliazione polimodale in due diversi contesti tettonici estensionali: il Mare di Barents (Articolo 1), un margine di rift-shear al largo della Norvegia Settentrionale, ed il Bacino del Taranaki (Articolo 3), un rift di retro-arco al largo della costa occidentale della Nuova Zelanda. Successivamente, i dati sismici sono stati integrati con modelli analogici e dettagliate ricostruzioni della storia cinematica di specifici piani di faglia al fine di meglio comprendere i meccanismi deformativi. In entrambi i contesti, la fagliazione polimodale è stata osservata alla scala delle decine di chilometri, suggerendo la rilevanza di questo processo in termini tettonici. Il verificarsi di fagliazione polimodale ad ampia scala implicherebbe infatti una riduzione del numero di fasi tettoniche necessarie per giustificare le strutture osservate, modificando l’attuale visione dell’evoluzione strutturale di questi bacini sedimentari. Il confronto fra le due aree suggerisce che il contesto tettono-stratigrafico giochi un ruolo fondamentale sui meccanismi alla base della fagliazione polimodale. Nel caso del Mare di Barents, la fagliazione polimodale risulta essere l’espressione di un campo di strain 3D legato all’interazione fra rifting Artico e Atlantico; sebbene anche il disaccoppiamento fra deformazione superficiale e profonda dovuto ai livelli con reologia duttile sembra essere stato un fattore fondamentale. Nel caso del Bacino del Taranaki, invece, la fagliazione polimodale sembra essere avvenuta sotto il controllo di strutture profonde, ereditate da fasi tettoniche compressive precedenti al rifting. La ricostruzione dei processi deformativi nelle due aree ha portato a rivedere i modelli esistenti della deformazione 3D (nel caso del Barents) e dell’eredità strutturale (nel caso del Taranaki). Da una parte, i modelli analogici della deformazione 3D (Articolo 2) hanno evidenziato come le faglie tendano a svilupparsi perpendicolarmente le une alle altre, piuttosto che con simmetria ortorombica rispetto agli assi della distensione, come previsto dal modello classico di Reches (1978). Questa tesi suggerisce pertanto che le interazioni locali tra faglie siano il principale meccanismo di controllo sulle geometrie finali in campi di strain 3D. Dall’altra parte, lo studio del Taranaki ha posto in luce come strutture profonde del basamento cristallino possano esercitare una notevole influenza sulla distribuzione e orientazione delle faglie normali, malgrado l’assenza di una diretta riattivazione estensionale delle stesse. La nucleazione/propagazione preferenziale delle faglie normali da anisotropie pre-esistenti e perturbazioni locali del campo di stress regionale sembrano essere meccanismi alternativi alla riattivazione estensionale, attraverso cui strutture pre-esistenti possono esercitare una forte influenza sulle faglie normali. In conclusione, questa tesi dimostra che sistemi di faglie complessi non necessariamente sono legati ad una complessa storia strutturale, consistente di molteplici fasi tettoniche. La complessità dei processi deformativi, ed in particolare la suscettibilità di questi a strutture pre-esistenti o in fase di formazione, può infatti spiegare lo sviluppo di sistemi di faglie complessi anche nell’ambito di un'unica fase tettonica

Orthorhombic faults system at the onset of the Late Mesozoic-Cenozoic Barents Sea rifting

The structures of the Late Mesozoic/Cenozoic Barents Sea rifting have been investigated with multichannel 3D seismics, covering an area of 7700 sqKm in the Hoop Fault Complex, a transitional area between the platform and the marginal basins. The main structural lineaments have been mapped in a time domain 3D surface and their activity ranges have been constrained through the sin-sedimentary thickness variations detected in time-thickness maps. Two main fault systems have been identified: an orthorhombic fault system consisting of two fault sets trending almost perpendicularly one to the other (WNW-ESE and NNE-SSW) and a graben/half-graben system, elongated approximately N-S in the central part of the study area. While the graben/half-graben system can be explained through the theory of Anderson, this landmark theory fails to explain the simultaneous activity of the two fault sets of the orthorhombic system. So far, the models that can better explain orthorhombic fault arrangements are the slip model by Reches (Reches, 1978; Reches, 1983; Reches and Dieterich, 1983) and the odd-axis model by Krantz (Krantz, 1988). However, these models are not definitive and a strong quest to better understand polymodal faulting is actual (Healy et al., 2015). In the study area, the presence of both a classical Andersonian and an orthorhombic system indicates that these models are not alternative but are both effective and necessary to explain faulting in different circumstances. Indeed, the Andersonian plain strain and the orthorhombic deformation have affected different part of the succession during different phases of the rifting. In particular, the orthorhombic system has affected only the Late Mesozoic-Cenozoic interval of the succession and it was the main active system during the initial phase of the rifting. On the other hand, the graben/half-graben system has affected the whole sedimentary succession, with an increasing activity during the development of the rifting. It has been observed that, in the upper part of the succession, devoid of pre-existing discontinuities and detached from the lower part of the succession by the Upper Triassic shales, the deformation has been accommodated by the newly-formed orthorhombic system; while, in the deeper part of the succession, likely to host pre-existing weakness zones, the deformation has been accommodated through the graben/half-graben system. Hence, during the Late Mesozoic/Cenozoic Barents Sea rifting it seems that the absence of pre-existing discontinuities played a key-role in the development of an orthorhombic fault arrangement in the upper part of the succession rather than a classical plain strain system. Indeed pre-existing discontinuities in the lower part of the succession can focus the deformation, preventing the formation of new faults and in this case favouring a plain strain mode. Furthermore, the Upper Triassic detachment limited the influence of deep structures on the upper part of the succession, allowing initially for the development of an entirely new fault system. As the rifting proceeded, the deep reactivated structures propagated towards the surface and, finally, their activity became predominant on the activity of the orthorhombic system, as indicated by time-thickness maps

Onset of N-Atlantic rifting in the Hoop Fault Complex (SW Barents Sea): An orthorhombic dominated faulting?

The Hoop Fault Complex is one of the main fault systems in the south-western Barents Sea. This platform underwent a long extensional history under the influence of both the Atlantic and the Arctic rifts, which culminated in the Atlantic break-up in the Cenozoic. The object of this paper is the structural analysis of the late Mesozoic rifting in the Hoop Fault Complex area, based on a 10,000 km2 3D seismic volume. We constrained the intervals of activity of the main fault systems during the late Mesozoic rifting through the synsedimentary thickness variations, reconstructing the evolution of the strain field. In order to clarify the relationship between the strain field and the rheological layering, we compared the structures at different depths, highlighting a decoupling of shallow and deep deformations along the Triassic ductile clay-rich layers. A transition froman orthorhombic faulting, corresponding to a 3D strain field, to an Andersonian faulting, related to a planar strain field,was observed. The change of the strain field could be driven by the evolution of the regional stress field or, alternatively, by the reactivation of deep structures. In this latter case, the structural evolution of the Hoop Fault Complex could potentially represent a general process to be extended to other rifting settings with a similar mechanical stratigraphy

Pre-existing basement thrusts influence rifting in the Taranaki Basin, New Zealand

Discrete structures (e.g. faults) or pervasive fabrics (e.g. foliation), which may occur in pre-rift sedimentary and/or crystalline basement rock, can control the growth and geometry of rift-related normal fault arrays. Previous studies examining how such structures/fabrics affect rift geometry typically rely only on plan-view correlations between the strike and dip of observed or, in some cases, inferred pre- and syn-kinematic structures. Three-dimensional relationships between and kinematic evolution of, pre-existing structures/fabrics and rift-related normal faults remains poorly constrained because: (i) outcrop patterns rarely expose both rift-related normal faults and the underlying rock units that could host pre-kinematic structures; (ii) discrete structures or pervasive fabrics are often poorly imaged in seismic reflection data; and (iii) it is difficult to quantitatively assess how pre-kinematic structures/fabrics influence normal fault nucleation and growth. Here, we use 3D seismic reflection data from the Taranaki Basin, offshore western New Zealand to study the kinematic history of a Cenozoic, rift-related, NE-SW striking normal fault array developed above a suite of N-S striking, intra-basement reflections interpreted as Palaeozoic thrusts. Only six of the 16 mapped rift-related normal faults mirror the strike of and appear physically linked to, the basement thrusts for at least 50% of their strike length; this spatial relationship would typically be inferred to reflect reactivation and upward propagation of the basement thrusts during rifting. However, fault throw analysis reveals the normal faults nucleated in the sedimentary cover ~1–2 km above the unconformity marking the top basement. We show the rift-related normal faults propagated downwards to either intersect NE-SW striking thrust segments or twisted to become aligned with the local strike of the basement structures. We propose that the presence and subtle reactivation of basement thrusts during Cenozoic extension locally reoriented the principal stress axes within the sedimentary basin, causing rift-related normal faults to deviate from their dominant NE strike. Despite having superficially similar strikes, rift-related normal faults may not simply form due to reactivation and upward propagation of basement structures

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

How do intra-basement fabrics influence normal fault growth? Insights from the Taranaki Basin, offshore New Zealand

Pre-existing intra-basement structures can have a strong influence on the evolution of rift basins. Although 3D geometric relationships provide some insight into how intra-basement structures determine the broad geometry and spatial development (e.g. strike and dip) of rift-related faults, little is known about the impact of the former on the detailed kinematics (i.e. nucleation and tip propagation) of the latter. 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 thrusts related to Mesozoic subduction and basement terrane accretion. These geometric relationships indicate pre-existing, intra-basement fabrics 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) thrusts and folds resulting from Late Miocene reactivation and upward propagation of intra-basement structures acted as nucleation sites for Plio-Pleistocene rift-related faults; and (iii) despite playing an important role during rifting, intra-basement structures do not appear to have been significantly extensionally reactivated. Our analysis shows how km-wide, intra-basement structures can have a temporally and spatially far-reaching influence over the nucleation and development of newly formed normal faults, principally due to 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 spatial or geometric relationship between structures developed at multiple structural levels