Synopsis Results of integrated seismic mapping, within the Firth of Forth in the offshore Midland Valley of Scotland, are presented and illustrate aspects of the subsurface structure and tectonic evolution of the Upper Devonian to Carboniferous succession. Evidence for three main phases of tectonic activity has been recognized: (1) Late Devonian to Dinantian fault-controlled subsidence; (2) basin-wide Silesian subsidence, localized inversion and growth folding; (3) Late Silesian dextral transtensional and transpressional strike-slip faulting. During the first phase of tectonic activity, the NNE-trending Mid Forth Fault is interpreted to represent a Late Devonian to Dinantian extensional fault, with a small, mainly Dinantian depocentre developed in the hangingwall block that has subsequently been inverted during Silesian times. A major Late Devonian to Dinantian depocentre also occurs in the hangingwall block of the NE-trending offshore continuation of the Crossgatehall Fault, although it remains unclear whether this mainly Dinantian depocentre was developed during pull-apart as a result of extension or transtensional strike-slip fault movement. The NNE-trending Leven Syncline and Mid Forth Anticline within the hangingwall block of the Mid Forth Fault are interpreted as Silesian synsedimentary growth folds that formed during the second phase of tectonic activity. The peak of this activity occurred during intra-Westphalian B to Westphalian C times. In the third phase, the ENE-trending Inchkeith Fault Zone is interpreted as a Late Silesian transtensional–transpressional strike-slip fault that dextrally offsets the axial trace of the Leven Syncline. Evidence from the Firth of Forth could provide support for regional tectonic models involving mainly dextral strike-slip fault activity during Devonian–Carboniferous times, or mainly sinistral strike-slip during Devonian to Early Carboniferous followed by dextral strike-slip during Late Carboniferous times, for the development of the Upper Devonian and Carboniferous succession. However, the latter model is preferred as it provides the more convincing explanation for our interpretation that the NNE-trending Mid Forth Fault represents a Late Devonian to Dinantian extensional or transtensional fault that was inverted during Silesian times. This inversion may therefore reflect a major change in the regional stress field.
The simplest models of passive margins would suggest that they are characterized by tectonic quiescence as they experienced gentle thermal subsidence following the extensional events that originally formed them. Analysis of newly acquired and pre-existing 2D seismic data from the Rockall Plateau to the Faroe Shelf, however, has confirmed that the NE Atlantic Margin was the site of significant active deformation. Seismic data have revealed the presence of numerous compression-related Cenozoic folds, such as the Hatton Bank, Alpin, Ymir Ridge and Wyville–Thomson Ridge Anticlines. The distribution, timing of formation and nature of these structures have provided new insights into the controls and effects of contractional deformation in the region. Growth of these compressional features occurred in five main phases: Thanetian, late Ypresian, late Lutetian, Late Eocene (C30) and Early Oligocene. Compression has been linked to hotspot-influenced ridge push, far-field Alpine and Pyrenean compression, asthenospheric upwelling and associated depth-dependent stretching. Regional studies make it clear that compression can have a profound effect on seabed bathymetry and consequent bottom-water current activity. Bottom-water currents have directly formed the early Late Oligocene, late Early Miocene (C20), Late Miocene–Early Pliocene, and late Early Pliocene (C10) unconformities. The present-day Norwegian Sea Overflow (NSO) from the Faroe–Shetland Channel into the Rockall Trough is restricted by the Wyville–Ymir Ridge Complex, and takes place via the syncline (Auðhumla Basin) between the two ridges. The Auðhumla Basin Syncline is now thought to have controlled the path of the NSO into the Rockall Trough and the resulting unconformity formation and sedimentation therein, no later than the Mid Miocene.
R.W. Gatliff, H. Johnson, J.D. Ritchie and K. Hitchen of the British Geological Survey in Edinburgh follow the previous article's assessment of recent drilling by offering some geological evidence . The first well drilled on the UK Atlantic margin was spudded in 1972. Since then, drilling activity has fluctuated (Figure 1) with peaks of activity in 1977 (when the Clair oilfield was discovered), 1984-86 (deep water step-out into the Faroe-Shetland Basin), 1991 (mainly licence round related) and 1994-1996 (related to the Foinaven and Schiehallion discoveries). Initially, exploration tested the major structural highs of the West Shetland and North Rona basins and the Rona Ridge beneath the shallow water shelf areas. Subsequently, exploration stepped out into the deeper water associated with the Faroe-Shetland and Rockall basins. Exploration interest in the area seemed to be waning until the Foinaven and Schiehallion discoveries gave the area renewed impetus. Within the last few years the agreement of the international boundary between the UK and the Faroe Islands has resulted in some renewal of drilling activity in both countries close to the newly agreed median line. This latest phase of exploration seemed to be stalling after a series of disappointing tests across a range of 'play types' in the Faroe-Shetland Basin and the Rockall Basin. However, it remains to be seen whether the Amerada Hess Marjun discovery in Faroese waters (well 6004/16-1Z) and particularly the recent Dooish discovery by Enterprise in Irish waters (well 12/2-1) within the Rockall Basin will lead to another surge in drilling activity. The Marjun discovery, with a 170 m hydrocarbon column, is in a Paleocene structural trap (Smallwood 2002). Details of the Dooish discovery have yet to be released but the Petroleum Affairs Division of Ireland (2002) describes it as a 'significant' hydrocarbon accumulation. It is speculated that the trap may comprise a pre-rift succession within a tilted fault block on the eastern margin of the Rockall Basin. This discovery establishes that a working petroleum system is present within the Rockall Basin and consequently that the potential for further hydrocarbons discoveries is significant. Tilted block structures have already been identified on both east and west flanks of the Rockall Basin (Walsh et al. 1999).
Abstract The Porcupine Basin is characterized by a large central free air gravity anomaly high (+ 55 mGal) flanked by local lows. In contrast, the Procupine Seabight Basin has low-amplitude anomalies in its centre, flanked by edge anomalies. Two transects, one in each of these basins, have been modelled using satellite gravity data; the upper parts of the transects are constrained by interpretation of recent commercial seismic reflection data and two wells. Results from the modelling suggest that the Porcupine Basin is not in isostatic equilibrium. In contrast, the essentially zero free air anomaly over the centre of the Porcupine Seabight Basin suggests that this basin is isostatically compensated. The difference in isostatic compensation between the two basins may reflect a fundamental contrast between the strength of the crust; the crust underlying the Porcupine Basin possesses the greater strength. The Clare Lineament may represent a fundamental boundary within the ‘Avalonian Terrane’ that juxtaposes basement blocks of differing rheologies.
Well data and seismic reflection profiles have been used to pinpoint three Jurassic volcanic centres in the Central North Sea: the Fisher Bank and Glenn centres are closely juxtaposed and form part of the Forties volcanic province, and the Puffin centre occurs separately on the northern flanks of the Mid North Sea High. A fourth centre, Ivanhoe, has been tentatively located in the Outer Moray Firth. The thickness, distribution, age and stratigraphic relationships of the associated volcanic rocks are described with the aid of maps and seismic sections. In a speculative regional synthesis, it is suggested that various intra-Jurassic unconformities can be related to the growth and development of the volcanic centres. The onset of volcanism at the Fisher Bank centre may have occurred during the late Early Jurassic; the Puffin centre was active during the Mid-Jurassic, and the Glenn centre may be predominantly Callovian in age. As a result, the relationship between the volcanic rocks and the fluvio-deltaic sediments of the Pentland Formation varies across the area. The occurrence of widespread volcanic-related uplift favours models involving a thermal perturbation in the mantle, rather than a period of lithospheric stretching, to account for Jurassic volcanism in the Central North Sea.
Synopsis Caledonian isotopic ages from the southern section of the Scotland–Norway Basement Ridge effectively separated the Archaean–Proterozoic Lewisian Complex and the Lofoten-Vesterålen Complex forming the SW and NE ends of the ridge. The distribution of Caledonian and Lewisian age data to the north and west of Shetland confirm suggestions that the Moine Thrust merges generally within the region of the west margin of the Caledonian mobile belt, as seen on east Greenland. The Moine Thrust between Orkney and Shetland is offset in a dextral sense by c. 25 km. The Walls Boundary Fault intersects the West Shetland Platform edge to the north of Shetland, with the basement displaying an apparent c. 15 km sinistral offset.
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.
The nature and age of the Cenozoic compressional/transpressional deformation within the NE Faroe–Shetland Basin, the Wyville–Thomson Ridge and Hatton Bank areas have been investigated, primarily using seismic reflection data. In all three areas, the folds reach approximately 2 to 4k min amplitude and 40k min wavelength. Early and mid-Eocene compressional/transpressional deformation affected the Hatton Bank and Wyville–Thomson Ridge areas, and folding was locally active even earlier, during Paleocene/Cretaceous times. However, the main Cenozoic compressional/transpressional tectonism that affected the Hatton Bank area was coeval with development of the regional Late Eocene Unconformity (C30), and with changes in spreading geometries and a phase of accelerated subsidence in the Rockall Basin. Within the NE Atlantic margin, WNW-to NW-trending lineaments/transfer zones and associated oceanic fracture zones facilitate significant structural segmentation. Offsets in the continent–ocean boundary along Hatton Bank probably reflect inherited basin architecture, and many Cenozoic folds in the Hatton Bank, Wyville–Thomson Ridge and NE Faroe–Shetland Basin areas are considered to mainly reflect compressional buttressing against pre-existing structures. However, relatively small lateral displacements probably occurred along some reactivated transfer zones following continental break-up. Paleocene–Eocene compressional/transpressional deformation may have affected parts of the Faroe–Shetland Basin, but seismic resolution of this is largely masked by pervasive polygonal faulting. Significant, early to mid-Miocene compressional/transpressional deformation is recorded in the NE Faroe–Shetland Basin, and may also have exerted a major influence on the Wyville–Thomson Ridge and surrounding area. In particular, mid-Miocene growth of the Faroe Bank Channel syncline may have resulted in major changes in northern hemisphere deep-ocean circulation with associated impact on global climate. Compressional/transpressional deformation appears to have continued into Pliocene– ?Recent times and resulted in the development of features such as the Pilot Whale Anticline and associated mud volcanoes/diapirs.
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