Abstract The Iapetus Suture (Solway) line coincides with a magnetic low, which lies between magnetic highs over southwestern Scotland and the Lake District-Isle of Man region. Although topography on deep magnetic basement can account for these long wavelength geophysical variations, an explanation which involves lateral basement magnetization contrasts is preferred on the basis of (a) correlations between inferred magnetization boundaries and major structures delineated from other evidence, and (b) the apparent westward continuation of the Solway low through Ireland and Newfoundland across areas with very different subsidence histories but similar position with respect to the collision of Laurentia and Avalonia. In the preferred model, relatively magnetic continental crust beneath the Southern Uplands and Lake District terranes is separated by a zone of less magnetic crust interpreted as sedimentary rock of Avalonian affinity carried to deeper structural levels within the Iapetus Suture Zone. The magnetic unit beneath the Southern Uplands is bounded to the south by the northward-dipping Iapetus Suture and to the north by a structure which may have been reactivated in late Caledonian times to produce the Moniaive Shear Zone in the overlying rocks; this unit may represent the ‘missing’ arc terrane inferred from provenance studies. Alternatively, the two magnetic basement domains may have originally been part of the same terrane, with that portion beneath the Southern Uplands rifting from the Avalonian continent during its northwards drift and being subsequently trapped in the hanging wall of the Iapetus Suture. The southern margin of the Lake District domain appears as a discontinuity in the magnetic anomaly pattern, with long wavelength anomalies to the south having a southeast ‘Tornquist’ trend.
terrane (ter-rane’) A fault-bounded body of rock of regional extent, characterized by a geological history different from that of contiguous terranes. A terrane is generally considered to be a discrete allochthonous fragment of oceanic or continental material added to a craton at an active margin by accretion. A. G. I. Glossary of Geology . 3rd edition, 1987.
SUMMARY The Lake District Boundary Fault Zone (LDBFZ) lies at the boundary between the Permo-Triassic East Irish Sea Basin and the Lower Palaeozoic Lake District Block. It divides northwards, within west Cumbria, into a network of fault strands that lie within the cover sequence of Upper Palaeozoic and younger rocks and Lower Palaeozoic basement rocks, and terminates at the St Bees Fault Zone. The earliest evidence of movement across the LDBFZ during the Ordovician is drawn from the component Thistleton Fault which has a volcanotectonic origin and bounds blocks of distinctive stratigraphy and structure. Reactivation of the LDBFZ in response to regional tectonic events occurred during the Late Palaeozoic, Mesozoic and Cainozoic. Oblique reverse displacement during the late Carboniferous was associated with Variscan basin inversion. East–west extension and syndepositional normal displacement accompanied the formation of the East Irish Sea Basin during Permian and early Triassic times. Analysis of fracture mineralization phases from the fault zone strands demonstrates repeated fault activity from mid-Triassic to early Cretaceous times. Slickenside data from the Sellafield area indicate these phases of Mesozoic faulting accommodated dip-slip displacement and south-west-directed extension. Regional uplift during the Cainozoic was accompanied by basin inversion; oblique-reverse displacement across the LDBFZ at this time was accompanied by Cainozoic folding of the hanging-wall block. The structural evolution and displacement history noted for the LDBFZ is likely to have been similar to that of other major faults of north-north-west trend in northern England, notably the Pennine Fault.
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.
Boreholes through the Quaternary, Tertiary and Mesozoic sequences of East Anglia provide general evidence that the Palaeozoic basement in the northern part of the UK segment of the London-Brabant Massif occurs at depths of less than 150 m to more than l000 m below sea level. The widely scattered nature of the borehole evidence gives an impression that the basement surface is planar. However, in the northern part of Suffolk, Bouguer gravity anomaly data indicate an elongate basement ridge which also seems to be related to the presence of a concealed Chalk high separating basins of Crag (Plio-Pleistocene) deposits. An investigation of this gravity feature using transient electromagnetic (TEM) soundings confirmed that the gravity high is due to a ridge rising to 140 m below sea level, about l00 m above the surrounding basement. The ridge is believed to be associated with NE- to NNE-trending faults, which also exerted some form of overall structural control on the deposition of the Crag. The feature appears to be comparable with structures affecting Eocene and older rocks reported in the southern North Sea. Part of the success of the TEM method in the area is due to the presence of a very low resistivity zone in the power part of the Chalk which contrasts strongly with the underlying resistive basement. Modelling of the geophysical data indicates that the zone is almost certainly due to the presence of saline formation water in the Chalk. The TEM method therefore has potential applications to the hydrogeological investigation of the Chalk, the major groundwater aquifer in the UK.
Abstract A series of three‐dimensional models has been constructed for the structure of the crust and upper mantle over a large region spanning the NE Atlantic passive margin. These incorporate isostatic and flexural principles, together with gravity modelling and integration with seismic interpretations. An initial isostatic model was based on known bathymetric/topographic variations, an estimate of the thickness and density of the sedimentary cover, and upper mantle densities based on thermal modelling. The thickness of the crystalline crust in this model was adjusted to equalise the load at a compensation depth lying below the zone of lateral mantle density variations. Flexural backstripping was used to derive alternative models which tested the effect of varying the strength of the lithosphere during sediment loading. The models were analysed by comparing calculated and observed gravity fields and by calibrating the predicted geometries against independent (primarily seismic) evidence. Further models were generated in which the thickness of the sedimentary layer and the crystalline crust were modified in order to improve the fit to observed gravity anomalies. The potential effects of igneous underplating and variable upper mantle depletion were explored by a series of sensitivity trials. The results provide a new regional lithospheric framework for the margin and a means of setting more detailed, local investigations in their regional context. The flexural modelling suggests lateral variations in the strength of the lithosphere, with much of the margin being relatively weak but areas such as the Porcupine Basin and parts of the Rockall Basin having greater strength. Observed differences between the model Moho and seismic Moho along the continental margin can be interpreted in terms of underplating. A Moho discrepancy to the northwest of Scotland is ascribed to uplift caused by a region of upper mantle with anomalously low density, which may be associated with depletion or with a temperature anomaly.
Abstract Decades of oil and gas exploration across the North Sea have led to a detailed understanding of its Cenozoic–Mesozoic structure. However, the deeper basin architecture of Paleozoic petroleum systems has been less well defined by seismic data. This regional structural overview of the Devono-Carboniferous petroleum systems incorporates interpretations from more than 85 000 line-kilometres of 2D seismic data and 50 3D seismic volumes, plus a gravity, density and magnetic study, from the Central Silverpit Basin to the East Orkney Basin. A complex picture of previously unmapped or poorly known basins emerges on an inherited basement fabric, with numerous granite-cored blocks. These basins are controlled by Devono-Carboniferous normal, strike-slip and reverse faults. The main basins across Quadrants 29–44 trend NW–SE, influenced by the Tornquist trend inherited from the Caledonian basement. North of Quadrants 27 and 28, and the presumed Iapetus suture, the major depocentres are NE–SW (e.g. the Forth Approaches and Inner Moray Firth basins) to east–west (e.g. the Caithness Graben), and WNW–ESE trending (e.g. the East Orkney Basin), reflecting the basement structural inheritance. From seismic interpretation, there are indications of an older north–south fault trend in the Inner Moray Firth that is difficult to image, since it has been dissected by subsequent Permo-Carboniferous and Mesozoic faulting and rifting.
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.
SUMMARY Potential field modelling has been used to test structural models of the Permo-Triassic cover sequence in the Sellafield area. An analysis of rock density data indicated that the main density interfaces occur at the base of the Calder Sandstone and the base of the St Bees Sandstone, and these horizons were employed in 3D gravity stripping. In addition, 2.5D gravity and magnetic modelling was conducted along selected profiles, with structures constrained by boreholes and good quality seismic reflection data, but subject to modification elsewhere. These studies highlighted shortcomings in earlier models for the margins of the Ravenglass Sub-basin and have led to new versions which agree better with available data. Subsequent drilling has confirmed the validity of the re-interpretation. Gravity data provide strong evidence of a north-west-trending fault zone which bounds the north-eastern corner of the Ravenglass Sub-basin. The sharp change in the strike of the basin margin represented by this fault zone probably arose because of the influence of pre-existing basement structures (granites and lines of weakness) on basin evolution. Relatively magnetic rocks within the Borrowdale Volcanic Group can be traced beneath Permo-Triassic cover using aeromagnetic data; the magnetic signature relating to the displacement of these rocks at the margins of the Ravenglass Sub-basin provides corroboration of models based on gravity data.
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