G.S. Kimbell

Controls on the structure and evolution of the NE Atlantic margin revealed by regional potential field imaging and 3D modelling

A regional three-dimensional model has been constructed for the lithospheric structure of the NE Atlantic margin. Starting from the known bathymetry and an initial sediment thickness estimate and making allowance for thermal effects, the geometry of the crystalline crust was predicted using isostatic and flexural principles. Optimization methods were then used to modify the base sediment and Moho interfaces to improve the fit between observed and calculated gravity anomalies. The method provides new insights into basin morphology and into variations in the thickness of both crystalline continental crust and igneous oceanic crust. When combined with imaging of the gravity and magnetic fields, the model highlights the importance of broadly NW-trending lineaments on the development of post-Caledonian basin architecture. In some cases these lineaments are interpreted as pre-Caledonian structures that were reactivated as transfer zones during phases of Mesozoic extension. Some of the lineaments appear to have influenced the early evolution of the oceanic crust by providing the precursors to transform offsets and possibly also by affecting the pattern of asthenospheric flow. The crustal thickening of the Faroe–Iceland Ridge is clearly imaged and its geometry is interpreted to reflect temporal variations in the enhanced oceanic crustal production rate responsible for this feature, including a Late Eocene minimum which can be correlated with plate reorganization in the north Atlantic region. There is some evidence of Cenozoic deformation linked to transpressive reactivation of the lineaments. However, a deflection in the axis of the North Hatton Anticline across the NW-trending Anton Dohrn lineament is more likely to have been inherited from an offset in an underlying, reactivated basement structure than to have resulted from strike-slip movements at the time of folding.

The nature and age of Cenozoic contractional deformation within the NE Faroe–Shetland Basin

Abstract A number of large, mainly NE-trending, and a few NNE-trending, Cenozoic to Recent anticlines have been identified from commercial seismic reflection data within the NE Faroe–Shetland Basin. Regional seismic markers have enabled the timing of formation of these structures to be ascertained and they appear to have developed mainly during early to mid-Miocene times, but there is also some evidence of activity through to early Pliocene to Recent, and even of pre-Miocene activity. The ages of the Cenozoic folds within the NE Faroe–Shetland Basin partly overlap with those observed on the Norwegian margin and around the Faroe Islands and the Hatton-Rockall areas, although in general, these regions have an important older component of development. Mechanisms for the formation of the folds are controversial, though results from this study indicate that the NE orientations of many of the folds in the NE Faroe–Shetland Basin are compatible with compression approximately orthogonal to the continental margin. The change from NE- to NNE-trending anticlines in the vicinity of the Magnus and Erlend lineaments/transfer zones may suggest that the development of these folds was related to sinistral strike-slip movement along these major structures. Speculatively, Quaternary strike-slip movement along these zones may also have triggered significant slope failure and the formation of mud volcanoes and mud intrusions as evidenced by the close spatial association of the Miller Slide Headwall and the ‘Pilot Whale Diapirs’.

The structure and evolution of the Northumberland Trough from new seismic reflection data and its bearing on modes of continental extension

An 18 km long N–S seismic reflection profile has been acquired across the faulted boundary between the Alston Block and the Northumberland Trough. A NW–SE geological cross-section has been constructed across the trough by integrating this line with other data, and a Tricentrol seismic profile confirms the suitability of this model. The trough has a markedly asymmetric form, with a thickness of more than 4.2 km of Dinantian strata adjacent to its faulted southern margin. The present day surface faults (Stublick, Ninety Fathom) are related to Variscan and later inversion and transpression, and do not everywhere correspond precisely to the earlier syn-depositional normal faults. The basin is believed to have formed in response to reactivation of the Iapetus Suture in a dominantly N–S extensional stress field. Basin evolution has been analysed by fault restorations and subsidence modelling. The fault restorations indicate an upper crustal extension factor of 1.15 to 1.19. Subsidence modelling indicates a whole crustal extension factor of 1.30, with similar sub-crustal lithospheric extension. It is possible that the difference between the inferred extension factors is due to non-uniform (increasing downwards) lithospheric extension, but the uncertainties inherent in both methods are such that this cannot yet be confirmed.

Cretaceous dykes discovered in the Falkland Islands: implications for regional tectonics in the South Atlantic

New aeromagnetic data resolve the dykes of the Falkland Islands into three swarms. A hitherto unrecognized suite of north–south dykes is established as early Cretaceous by an Ar–Ar date of about 121 Ma. Swarms of NE–SW and east–west dykes are both early Jurassic: the former gives an Ar–Ar age of about 178 Ma, whereas the latter has been previously dated to about 190 Ma. The intrusion of the Cretaceous dykes marks the onset of oceanic crust generation in the South Atlantic and so restricts to mid-Jurassic the microplate rotation envisaged in most models for the Falklands break-out from Gondwana.

Integrated geological and geophysical studies in the SG4 borehole area, Tagil Volcanic Arc, Middle Urals : Location of seismic reflectors and source of the reflectivity

Near-vertical incidence reflection seismic data acquired in the Tagil Volcanic Arc (Middle Urals) show the upper crust to be highly reflective. Two intersecting seismic lines located near the ongoing ∼5400 m deep SG4 borehole show that the main reflectivity strikes approximately N-S and dips ∼35°–55° to the east. Prominent reflections intercept the borehole at ∼1000, ∼1500, 2800–2900, ∼3400, and between ∼4000 and 5400 m, which correspond to intervals of low velocity/low density/low resistivity. The surface projections of these reflections lie parallel to the strike of magnetic anomaly trends. Multioffset vertical seismic profile (VSP) data acquired in the SG4 borehole show a seismic response dominated by P to S reflected converted waves from the moderately east dipping reflectivity and from a set of very steep east dipping reflectors not imaged by the surface data. Modeling of the VSP data constrains the depth at which reflectors intercept the borehole and suggests that the P to S conversions are best explained by low-velocity porous intervals rather than higher-velocity mafic material. The most prominent east dipping reflection on the surface seismic data is only imaged on VSP shots that sample the crust closer to the E-W seismic line. This discrepancy between the VSP and the surface seismic data is attributed to rapid lateral changes in the physical properties of the reflector. Surface and borehole data suggest that the low-velocity/low-density/low-resistivity intervals are the most important source of reflectivity in the SG4 borehole area, although lithological contrasts may also play a role. Drill cores from the these zones contain hydrothermal alteration minerals indicating interaction with fluids. Tectonic criteria suggest that they might represent imbricated fracture zones often bounding different lithologies and/or intrusions. Some of them might also represent high-porosity lava flows or pyroclastic units, common in island arc environments.

Insights into orogenesis: getting to the root of a continent–ocean–continent collision, Southern Urals, Russia

The Ural mountains preserve a late Palaeozoic collision that forms a 2500 km suture in the worlds largest landmass, Eurasia. Several features of the mountain belt, in particular a well-preserved crustal root, are uncharacteristic of other Palaeozoic orogens such as the Appalachians and Caledonides. Previous interpretations of the Southern Uralian root suggested that it is composed of East European Craton crust derived from the west. A new potential field data model, considered in conjunction with published seismic, heat-flow and geological data, indicates that the root is composed mainly of mafic granulite, which we interpret as oceanic arc crust originally accreted from the east, subducted eastward, and metamorphosed. A load caused by crustal lateral density variations, combined with topography, isostatically compensates root buoyancy and is thus the main cause of its preservation.

Rotation of the Falklands microplate reassessed after recognition of discrete Jurassic and Cretaceous dyke swarms

ABSTRACT Recently acquired aeromagnetic data for the Falkland Islands have shown that previous interpretations of the dolerite dyke swarms are inadequate. In particular, most of the dykes previously described from West Falkland as forming a ‘north–south’ swarm of Jurassic age are associated with a set of NE–SW linear magnetic anomalies that are entirely separate from another set of truly N–S anomalies. Very few dykes had been previously reported from East Falkland, but the aeromagnetic survey demonstrates clearly that dykes of both the NE–SW and the N–S swarms are present. Ar–Ar age dating of East Falkland dykes has confirmed the Jurassic age of the NE–SW dykes but has established an early Cretaceous age for the N–S dyke swarm. The Jurassic dykes are generally considered a part of the regional Karoo–Ferrar magmatism linked to the initial break-up of Gondwana. We consider the Cretaceous dykes to be associated with the later opening of the North Falklands Basin during the early development of the South Atlantic Ocean. The Jurassic and Cretaceous dykes must respectively pre-date and post-date the microplate rotation envisaged in most models for the Falklands break-out from Gondwana. The shapes of the aeromagnetic anomalies associated with dykes from each of the swarms support the hypothesis that the early Jurassic dykes have experienced a pre-Cretaceous, clockwise microplate rotation of about 120°.

Basement control on the location of strike-slip shear in the Southern Uplands of Scotland

In the SW part of the Southern Uplands terrane the Moniaive Shear Zone has been superimposed above a geophysically defined basement discontinuity. Deformation was a protracted event, commencing in the early Wenlock and ending with granite intrusion at about 392 Ma. At surface the shear zone separates contrasting suites of granitic plutons. These have age diVerences and isotopic characteristics indicative of a lower crustal and possibly mantle break coincident with the crustal discontinuity deduced from geophysical modelling. It is proposed that sinistral reactivation of the basement structure caused the development of the shear zone within the overlying allochthonous thrust belt. This would require the basement structure to have been established beneath the developing Southern Uplands terrane by the Wenlock.

Regional-scale lateral variation and linkage in ductile thrust architecture: the Oykel Transverse Zone, and mullions, in the Moine Nappe, NW Scotland

Abstract Sharp lateral changes in structural geometry of ductile thrust stacks are not widely reported. A regional-scale lateral culmination wall forms the southern boundary of the Cassley Culmination in Moine rocks in the Caledonides of Sutherland, Northern Scotland. This culmination wall is part of the Oykel Transverse Zone (OTZ), a kilometre-scale shear zone characterized by constrictional finite strain fabrics aligned sub-parallel to the regional WNW-directed thrust transport direction. Main phase folds and fabrics in the transverse zone hanging wall are folded by main phase folds and fabrics in the footwall, thus recording foreland-propagating ductile deformation. South of the Cassley Culmination, shortening occurred uniformly, without development of discrete subsidiary thrusts; distributed deformation (fold development) alternated with localized thrusting within the culmination. The classic ESE-plunging mullions at Oykel Bridge are an integral part of the OTZ and were generated by constriction aligned sub-parallel to the transport direction. Constriction is attributed to differential, transtensional movement across the OTZ during culmination development. Subsequent formation of the underlying Assynt Culmination further accentuated upward-bulging of the Cassley Culmination, amplifying the lateral change across the transverse zone. The OTZ aligns with a pronounced gravity gradient on the south-western side of the Lairg gravity low. Interpretive modelling relates this gradient to a buried basement ramp that possibly controlled the location of the transverse zone.

Regional conductivity data used to reassess Early Palaeozoic structure in the Northern Ireland sector of the Southern Uplands–Down–Longford terrane

Abstract: A high-resolution, airborne conductivity survey has proved spectacularly successful in delineating the zones of carbonaceous mudstone (Moffat Shale Group) that form the structural and stratigraphical base of various sandstone-dominated tracts within the Northern Ireland sector of the Southern Uplands–Down–Longford Ordovician–Silurian accretionary terrane. The anomalies associated with mudstone both at outcrop and concealed allow the major tract boundary faults to be plotted across large areas of poor exposure to reveal a large-scale regional swing in the strike of these faults from their prevalent ENE–WSW trend into a NE–SW orientation. The fault traces defined by the geophysical anomalies cut across those, deduced by extrapolation into an area of sparse bedrock exposure, that are illustrated on the current geological map. A substantial revision of the regional fault pattern is thus required. In places the major tract boundary faults appear to anastomose into strike-slip duplexes, suggesting transpression in the accretionary regime. However, the wholesale realignment of the strike trend is likely to have had a later, post-accretion origin, perhaps involving a releasing bend on a major, strike-parallel fault that controlled emplacement of the Newry granitoid pluton (425 Ma), and might also be associated with Au mineralization in Armagh and Monaghan.