Terrane boundary reactivation, barriers to lateral fault propagation and reactivated fabrics - Rifting across the Median Batholith Zone, Great South Basin, New Zealand

Prominent pre‐existing structural heterogeneities within the lithosphere may localise or partition deformation during tectonic events. The NE‐trending Great South Basin, offshore New Zealand, formed perpendicular to a series of underlying crustal terranes, including the dominantly granitic Median Batholith Zone, which along with the boundaries between individual terranes, exert a strong control on rift physiography and kinematics. We find that the crustal‐to‐lithospheric scale southern terrane boundary of the Median Batholith Zone is associated with a crustal‐scale shear zone that was reactivated during Late Cretaceous extension between Zealandia and Australia. This reactivated terrane boundary is oriented at a high‐angle to the faults defining the Great South Basin. We identify a large granitic laccolith along the southern margin of the Median Batholith, expressed as sub‐horizontal packages of reflectivity and acoustically transparent areas on seismic reflection data. The presence of this strong granitic body inhibits the lateral south‐westward propagation of NE‐trending faults, which segment into a series of splays that rotate to align along the margin as they approach. Further, we also identify two E‐W and NE‐SW oriented basement fabrics, likely corresponding to prominent foliations, which are exploited by small‐scale faults across the basin. We show that different mechanisms of structural inheritance are able to operate simultaneously, and somewhat independently, within rift systems at different scales of observation. The presence of structural heterogeneities across all scales need to be incorporated into our understanding of the structural evolution of complex rift systems

Evolution of Labrador Sea–Baffin Bay: Plate or Plume Processes?

Breakup between Greenland and Canada resulted in oceanic spreading in the Labrador Sea and Baffin Bay. These ocean basins are connected through the Davis Strait, a bathymetric high comprising primarily continental lithosphere, and the focus of the West Greenland Tertiary volcanic province. It has been suggested that a mantle plume facilitated this breakup and generated the associated magmatism. Plume-driven breakup predicts that the earliest, most extensive rifting, magmatism and initial seafloor spreading starts in the same locality, where the postulated plume impinged. Observations from the Labrador Sea–Baffin Bay area do not accord with these predictions. Thus, the plume hypothesis is not confirmed at this locality unless major ad hoc variants are accepted. A model that fits the observations better involves a thick continental lithospheric keel of orogenic origin beneath the Davis Strait that blocked the northward-propagating Labrador Sea rift resulting in locally enhanced magmatism. The Davis Strait lithosphere was thicker and more resilient to rifting because the adjacent Paleoproterozoic Nagssugtoqidian and Torngat orogenic belts contain structures unfavourably orientated with respect to the extensional stress field at the time

Complex geometry and kinematics of subsidiary faults within a carbonate-hosted relay ramp

Minor fault geometry and kinematics within relay ramps is strongly related to the stress field perturbations that can be produced when two major fault segments overlap and interact. Here we integrate classical fieldwork and interpretation of a virtual outcrop to investigate the geometry and kinematics of subsidiary faults within a relay ramp along the Tre Monti normal fault in the Central Apennines. Although the Tre Monti fault strikes parallel to the regional extension (NE-SW) it shows predominant dip-slip kinematics, suggesting a NW-SE oriented extension acting at sub-regional scale (1–10 km). Conversely, the slickenlines collected on the front segment of the relay ramp highlight right-lateral kinematics. The subsidiary faults in the relay ramp show a complex geometry (variable attitudes) and slickenlines describe multiple kinematics (left-lateral, dip-slip, right-lateral), independently of their orientation. Our fault slip analysis indicates that a local stress field retrieved from the kinematic inversion of the slickenlines collected on the front segment, and likely promoted by the interaction between the overlapping fault segments that bound the relay zone, can explain most of the geometry and kinematics of the subsidiary faults. Further complexity is added by the temporal interaction with both the regional and sub-regional stress fields

New onshore insights into the role of structural inheritance during Mesozoic opening of the Inner Moray Firth Basin, Scotland

The Inner Moray Firth Basin (IMFB) forms the western arm of the North Sea trilete rift system that initiated mainly during the Late Jurassic–Early Cretaceous with the widespread development of major NE–SW-trending dip-slip growth faults. The IMFB is superimposed over the southern part of the older Devonian Orcadian Basin. The potential influence of older rift-related faults on the kinematics of later Mesozoic basin opening has received little attention, partly owing to the poor resolution of offshore seismic reflection data at depth. New field observations augmented by drone photography and photogrammetry, coupled with U–Pb geochronology, have been used to explore the kinematic history of faulting in onshore exposures along the southern IMFB margin. Dip-slip north–south- to NNE–SSW-striking Devonian growth faults are recognized that have undergone later dextral reactivation during NNW–SSE extension. The U–Pb calcite dating of a sample from the synkinematic calcite veins associated with this later episode shows that the age of fault reactivation is 130.99  ±  4.60 Ma (Hauterivian). The recognition of dextral-oblique Early Cretaceous reactivation of faults related to the underlying and older Orcadian Basin highlights the importance of structural inheritance in controlling basin- to sub-basin-scale architectures and how this influences the kinematics of IMFB rifting

New onshore insights into the role of structural inheritance during Mesozoic opening of the Inner Moray Firth Basin, Scotland

The Inner Moray Firth Basin (IMFB) forms the western arm of the North Sea trilete rift system that initiated mainly during the Late Jurassic–Early Cretaceous with the widespread development of major NE–SW-trending dip-slip growth faults. The IMFB is superimposed over the southern part of the older Devonian Orcadian Basin. The potential influence of older rift-related faults on the kinematics of later Mesozoic basin opening has received little attention, partly owing to the poor resolution of offshore seismic reflection data at depth. New field observations augmented by drone photography and photogrammetry, coupled with U–Pb geochronology, have been used to explore the kinematic history of faulting in onshore exposures along the southern IMFB margin. Dip-slip north–south- to NNE–SSW-striking Devonian growth faults are recognized that have undergone later dextral reactivation during NNW–SSE extension. The U–Pb calcite dating of a sample from the synkinematic calcite veins associated with this later episode shows that the age of fault reactivation is 130.99  ±  4.60 Ma (Hauterivian). The recognition of dextral-oblique Early Cretaceous reactivation of faults related to the underlying and older Orcadian Basin highlights the importance of structural inheritance in controlling basin- to sub-basin-scale architectures and how this influences the kinematics of IMFB rifting

The Iceland Microcontinent and a continental Greenland-Iceland-Faroe Ridge

The breakup of Laurasia to form the Northeast Atlantic Realm was the culmination of a long period of tectonic unrest extending back to the Late Palaeozoic. Breakup was prolonged and complex and disintegrated an inhomogeneous collage of cratons sutured by cross-cutting orogens. Volcanic rifted margins formed, which are blanketed by lavas and underlain variously by magma-inflated, extended continental crust and mafic high-velocity lower crust of ambiguous and probably partly continental provenance. New rifts formed by diachronous propagation along old zones of weakness. North of the Greenland-Iceland-Faroe Ridge the newly forming rift propagated south along the Caledonian suture. South of the Greenland-Iceland-Faroe Ridge it propagated north through the North Atlantic Craton along an axis displaced ~ 150 km to the west of the northern rift. Both propagators stalled where the confluence of the Nagssugtoqidian and Caledonian orogens formed a transverse barrier. Thereafter, the ~ 400-km-wide latitudinal zone between the stalled rift tips extended in a distributed, unstable manner along multiple axes of extension that frequently migrated or jumped laterally with shearing occurring between them in diffuse transfer zones. This style of deformation continues to the present day. It is the surface expression of underlying magma-assisted stretching of ductile mid- and lower continental crust which comprises the Icelandic-type lower crust that underlies the Greenland-Iceland-Faroe Ridge. This, and probably also one or more full-crustal-thickness microcontinents incorporated in the Ridge, are capped by surface lavas. The Greenland-Iceland-Faroe Ridge thus has a similar structure to some zones of seaward-dipping reflectors. The contemporaneous melt layer corresponds to the 3–10 km thick Icelandic-type upper crust plus magma emplaced in the ~ 10–30-km-thick Icelandic-type lower crust. This model can account for seismic and gravity data that are inconsistent with a gabbroic composition for Icelandic-type lower crust, and petrological data that show no reasonable temperature or source composition could generate the full ~ 40-km thickness of Icelandic-type crust observed. Numerical modeling confirms that extension of the continental crust can continue for many tens of Myr by lower-crustal flow from beneath the adjacent continents. Petrological estimates of the maximum potential temperature of the source of Icelandic lavas are up to 1450 °C, no more than ~ 100 °C hotter than MORB source. The geochemistry is compatible with a source comprising hydrous peridotite/pyroxenite with a component of continental mid- and lower crust. The fusible petrology, high source volatile contents, and frequent formation of new rifts can account for the true ~ 15–20 km melt thickness at the moderate temperatures observed. A continuous swathe of magma-inflated continental material beneath the 1200-km-wide Greenland-Iceland-Faroe Ridge implies that full continental breakup has not yet occurred at this latitude. Ongoing tectonic instability on the Ridge is manifest in long-term tectonic disequilibrium on the adjacent rifted margins and on the Reykjanes Ridge, where southerly migrating propagators that initiate at Iceland are associated with diachronous swathes of unusually thick oceanic crust. Magmatic volumes in the NE Atlantic Realm have likely been overestimated and the concept of a monogenetic North Atlantic Igneous Province needs to be reappraised. A model of complex, piecemeal breakup controlled by pre-existing structures that produces anomalous volcanism at barriers to rift propagation and distributes continental material in the growing oceans fits other oceanic regions including the Davis Strait and the South Atlantic and West Indian oceans

Igneous sills record far-field and near-field stress interactions during volcano construction: Isle of Mull, Scotland

Sill emplacement is typically associated with horizontally mechanically layered host rocks in a near-hydrostatic far-field stress state, where contrasting mechanical properties across the layers promote transitions from dykes, or inclined sheets, to sills. We used detailed field observations from the Loch Scridain Sill Complex (Isle of Mull, UK), and mechanical models to show that layering is not always the dominant control on sill emplacement. The studied sills have consistently shallow dips (1◦–25◦) and cut vertically bedded and foliated metamorphic basement rocks, and horizontally bedded cover sedimentary rocks and lavas. Horizontal and shallowly-dipping fractures in the host rock were intruded with vertical opening in all cases, whilst steeply-dipping discontinuities within the sequence (i.e. vertical fractures and foliation in the basement, and vertical polygonal joints in the lavas) were not intruded during sill emplacement. Mechanical models of slip tendency, dilation tendency, and fracture susceptibility for local and overall sill geometry data, support a radial horizontal compression during sill emplacement. Our models show that dykes and sills across Mull were emplaced during NW–SE horizontal shortening, related to a far-field tectonic stress state. The dykes generally accommodated phases of NE–SW horizontal tectonic extension, whereas the sills record the superposition of the far-field stress with a near-field stress state, imposed by emplacement of the Mull Central Volcano. We show that through detailed geometric characterisation coupled with mechanical modelling, sills may be used as an indication of fluctuations in the paleostress state

Fracture attribute scaling and connectivity in the Devonian Orcadian Basin with implications for geologically equivalent sub-surface fractured reservoirs

Fracture attribute scaling and connectivity datasets from analogue systems are widely used to inform sub-surface fractured reservoir models in a range of geological settings. However, significant uncertainties are associated with the determination of reliable scaling parameters in surface outcrops. This has limited our ability to upscale key parameters that control fluid flow at reservoir to basin scales. In this study, we present nine 1D-transect (scanline) fault and fracture attribute datasets from Middle Devonian sandstones in Caithness (Scotland) that are used as an onshore analogue for nearby sub-surface reservoirs such as the Clair field, west of Shetland. By taking account of truncation and censoring effects in individual datasets, our multiscale analysis shows a preference for power-law scaling of fracture length over 8 orders of magnitude (10−4 to 104 m) and kinematic aperture over 4 orders of magnitude (10−6 to 10−2 m). Our assessment of the spatial organization (clustering and topology) provides a new basis for up-scaling fracture attributes collected in outcrop- to regional-scale analogues. We show how these relationships may inform knowledge of geologically equivalent sub-surface fractured reservoirs

The influence of crustal strength on rift geometry and development – insights from 3D numerical modelling

The lateral distribution of strength within the crust is non-uniform, dictated by crustal lithology and the presence and distribution of heterogeneities within it. During continental extension, areas of crust with distinct lithological and rheological properties manifest strain differently, influencing the structural style, geometry, and evolution of the developing rift system. Here, we use 3D thermo-mechanical models of continental extension to explore how pre-rift upper-crustal strength variations influence rift physiography. We model a  km volume containing 125 km wide domains of mechanically “strong” and “weak” upper crust along with two reference domains, based upon geological observations of the Great South Basin, New Zealand, where extension occurs parallel to the boundaries between distinct geological terranes. Crustal strength is represented by varying the initial strength of 5 km3 blocks. Extension is oriented parallel to the domain boundaries such that each domain is subject to the same 5 mm yr−1 extension rate. Our modelling results show that strain initially localises in the weak domain, with faults initially following the distribution of initial plastic strain before reorganising to produce a well-established network, all occurring in the initial 100 kyr. In contrast, little to no localisation occurs in the strong domain, which is characterised by uniform strain. We find that although faults in the weak domain are initially inhibited at the terrane boundaries, they eventually propagate through and “seed” faults in the relatively strong adjacent domains. We show characteristic structural styles associated with strong and weak crust and relate our observations to rift systems developed across laterally heterogeneous crust worldwide, such as the Great South Basin, New Zealand, and the Tanganyika Rift, East Africa