Robert Gatliff

Cenozoic exhumation of the southern British Isles

Rocks that crop out across southern Britain were exhumed from depths of as much as 2.5 km during Cenozoic time. This has been widely attributed to Paleocene regional uplift resulting from igneous underplating related to the Iceland mantle plume. Our compilation of paleothermal and compaction data reveals spatial and temporal patterns of exhumation showing little correspondence with the postulated influence of underplating, instead being dominated by kilometer-scale variations across Cenozoic compressional structures, which in several basins are demonstrably of Neogene age. We propose that crustal compression, due to plate boundary forces transmitted into the plate interior, was the major cause of Cenozoic uplift in southern Britain, witnessing a high strength crust in western Europe.

Early Tertiary magmatism in the offshore NW UK margin and surrounds

The Iceland Plume, activated perhaps as early as Late Cretaceous time and extending through the Tertiary, was responsible for the generation of large volumes of igneous rocks, covering an area of roughly 2000 km in diameter, encompassing Greenland. the Voring Basin, the Faeroes, the Faeroe–Shetland Basin, the Rockall Trough and NW Scotland. Within this large area, magmatism is mainly manifested as lava fields, sediment–sill complexes, and central or strato/shield type volcano complexes. Towards the continental margin of NW Europe, extrusive rocks occur m two distinct geographical areas. SW of the Munkagrunnur Ridge of the Faeroe Platform, lavas are mainly extruded radially from strato/shield volcano complexes. To the NE, in contrast, seismic evidence suggests that the tipper part of the Faeroe Plateau Lava Group extrusive originate from the same source that formed the Seaward Dipping Reflector Sequences NW of the Faeroes. Towards the SE of the Faeroes, the distribution of the Faeroe Plateau Lava Group (Upper. Middle and Lower Series) lavas is controlled by pre-existing structure and sea-level fluctuations, Seismic and well evidence confirms that, generally, the Upper Series extends furthest into the Faeroe–Shetland Basin while basaltic progradational units form the SE limit of the Lower Series. Similar structures of possibly the same age have been identified within the NE Rockall Trough. There appear to have been two major phases or pulses of Early Tertiary magmatism within the North Atlantic Igneous Province; an initial outburst at around 62 Ma and a later phase at around 55 Ma.

The geological evolution of the Falkland Islands continental shelf

Abstract The Falkland Islands are surrounded by four major sedimentary basins: the Falkland Plateau Basin to the east, the South Falkland Basin to the south, the Malvinas Basin to the west, and the North Falkland Basin to the north. The four main basins appear to have formed initially as Triassic through earliest Cretaceous extensional rifts associated with the break-up of Gondwana. A ?Valanginian end to rifting was followed by thermal sag. There is evidence of Cenozoic uplift in at least the North Falkland Basin, possibly coincident with Andean compression and the development of overthrusting along the plate boundary to the south of the islands resulting from opening of the Scotia Sea. There is no evidence from offshore seismic and gravity-magnetic data to support interpretations that the Falkland Islands have rotated clockwise through up to 180° during Gondwana separation. With the exception of the South Falkland Basin all the major basins probably underwent initially, more or less east-west extension, and had a similar orientation to adjacent South American and western southern African basins. The Falkland basins probably shared a similar geological history with the offshore southern African and South American basins.

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 Origin of Compression in Passive Margins

Increasingly, researchers have reported that passive margins do not show a simple uninterrupted thermal sag pattern of post-rift subsidence following continental separation. Rather, the structural and stratigraphic development of such margins may record evidence of complex phases of differential subsidence, exhumation and fold development. Some of the fold structures observed on passive continental margins appear to be related to regional stresses transmitted through basement rocks, whereas others are related to gravitational sliding and toe-thrusting. This special publication concentrates on the first of these categories. The morphology and distribution of such folds, together with potential mechanisms for generation of regional stress, are described in a series of papers by authorities in the field. As well as being an enigmatic feature of passive margin geology, the compressive folds have significance in the exploration for petroleum.

The effects of Cenozoic compression within the Faroe-Shetland Basin and adjacent areas

Abstract The effects of Cenozoic compression within the Faroe–Shetland Basin and surrounding areas are mainly manifested in the form of growth folds. The scale and orientation of the folds varies significantly, with axial trace lengths ranging between less than 10 to over 250 km and trends including east, NE-, NNE-, ENW-, NNW- and WNW. The NE-trending features are the most numerous, though they are mainly restricted to the NE Faroe–Shetland Basin where an inherited Caledonian structural grain is most prevalent. Limited evidence exists for late Paleocene and early Eocene activity along the Wyville Thomson Ridge, whereas mid–late Eocene and Oligocene fold growth is more common in the SW Faroe–Shetland Basin. Although the effects of well-defined early–mid Miocene deformation appear to be mainly constrained to the NE Faroe–Shetland Basin, this phase of activity is also inferred to have been responsible for major growth of the Wyville Thomson Ridge. Early Pliocene fold growth is observed within the Faroe–Shetland Basin and adjacent areas, with raised seabed profiles over some of the anticlinal features suggesting that the effects of compressional stress continue at the present day. Despite the variation in trend and size of growth folds, there is, we believe, similarity in their local mechanism of emplacement, with buttressing of sedimentary successions against pre-existing basement architecture and igneous intrusions being of particular significance. However, the lack of obvious spatial or temporal pattern to fold growth development within the NE Atlantic margin as a whole mitigates against a single regional driving mechanism being able to explain the current distribution, orientation and timing of the folds.

Aspects of the structure of the Porcupine and Porcupine Seabight basins as revealed from gravity modelling of regional seismic transects

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.

Seabed mapping for the 21st century : the Marine Environmental Mapping Programme (MAREMAP): preface

During the 1970s and 1980s, the British Geological Survey (BGS) carried out systematic surveys of the seabed and subseabed around Britain, as part of a Government-funded programme to map the UK Continental Shelf (UKCS). Using an array of sampling, coring and seismic profiling equipment, the programme resulted in a series of geological maps and scientific publications which described the sediments at the seafloor, the sediments deposited during the Quaternary glaciations, and the older sedimentary, igneous and metamorphic rocks. These systematic surveys of the UKCS were followed during the 1990s by BGS and industry co-funded surveys in the deep-water areas to the west of the UK, such as the Rockall Trough and Plateau; although here, the density of sampling, shallow boreholes and seismic surveys was significantly reduced.

Processing Low-frequency Data From the Faroe-Shetland Basin For Sub-basalt Imaging

Figure 1: Basalt covered areas of the North-East Atlantic Margin and associated igneous centres. Penetrating boreholes and hydrocarbon discoveries near the Shetlands are also shown. Following the findings that one has to use low frequency data to image beneath a multi-layered basalt sequence (Mack, 1997), Veritas DGC and Texaco UK re-shot a profile in August 2001 using a solid streamer and an additional low-frequency setting (Ziolkowski et al., 2001). Comparing the new low-frequency data with the old data shows not only a superior data quality due to the solid streamer used but also improved low-frequency response from beneath the basalt.