P. W. Readman

The Hatton Basin and continental margin: Crustal structure from wide‐angle seismic and gravity data

Results from a wide-angle seismic and gravity study between the Rockall Bank and the Iceland Basin in the North Atlantic are presented. Crustal and sedimentary structures are resolved in the Hatton Basin and across the Hatton continental margin (HCM) east of magnetic anomaly 24. The structure of the oceanic crust west of the anomaly is also determined. Gravity data support the seismic model in areas of good seismic coverage and are used to control the model where the wide-angle seismic data are poor. A two-layer sedimentary sequence is present both in the Hatton Basin and across the continental margin. The lower layer, with P wave velocity of about 4 km/s, is interpreted as pre-Eocene synrift sediments and is up to 3.5 km thick. A younger and thinner (1–2.5 km) postrift sequence, with a velocity of about 2 km/s, defines a strong velocity contrast, which suggests an erosional unconformity surface. The sedimentary structure is distinctly different from that in the Rockall Trough, where a third intermediate layer (Vp ≈ 3 km/s) occurs. The three-layer crust, characterized by two intracrustal reflections (PiP1 and PiP2) varies from 30 km thick under the Rockall Bank to about 15 km below the Hatton Basin, where it is stretched by a factor of 2 relative to onshore Ireland. The crust is thinnest below the Hatton Bank, where the presence of a single intracrustal reflection indicates that the lower crustal layer thins to below the seismic resolution limit. Below the HCM a region of thick lower crust with anomalously high velocity (Vp ≈ 7.2 km/s) is resolved by the seismic and gravity data. It is connected (west of anomaly 24) to a region of oceanic crust, which is thicker than in the Iceland Basin. These relationships between the thick lower crust below the HCM and the oceanic crust in the Iceland Basin are interpreted as evidence for magmatic underplating, consistent with previous models for the HCM. The inferred unconformity surface between the synrift and postrift layers may be due to regional uplift driven by upwelling of hot asthenosphere before anomaly 24 (early Eocene) time.

Crustal thinning, mantle exhumation and serpentinization in the Porcupine Basin, offshore Ireland: evidence from wide-angle seismic data

New wide-angle seismic data were gathered along a 230 km long profile that runs east–west across a deep structural feature in the Porcupine Basin, offshore Ireland, known as the Porcupine Arch. Ocean bottom seismometers were deployed at 3–4 km intervals and seismic sources fired every 120 m along it. Prominent primary and secondary arrivals indicate that the continental crust is extremely thin (locally less than 2 km) across the basin centre. The sedimentary succession is up to 12 km thick and comprises three distinctive seismic layers. The two uppermost layers are interpreted as mostly a post-rift succession of Cretaceous and Cenozoic strata. The lowest layer thins rapidly towards the basin centre and is interpreted as a succession of predominantly Jurassic synrift sediments. A strong asymmetry in both the geometry of the crust and the sedimentary layers is probably related to a simple shear mode of extension and the subsidence that it induced. Crustal thinning is far greater than in the adjacent Rockall Basin and local exhumation of continental mantle lithosphere may have occurred in parts of the Porcupine Basin. Low Pn velocities beneath the Porcupine Arch are compatible with larger amounts of mantle serpentinization than in the Rockall Basin.

A model for the development of a carbonate mound population in the Rockall Trough based on deep-towed sidescan sonar data

A large carbonate mound population is identified on deep-towed TOBI (towed ocean bottom instrument) sidescan data along the eastern margin of the Rockall Trough, west of Ireland. Individual mounds are circular to elliptical in plan view, varying from 50 to 850 m in width and up to about 200 m in height. Strong NE-flowing contour currents at 800 m water depth are inferred from large-scale sedimentary bedforms. Smaller mound arrays are spatially associated with slope-parallel escarpments produced by mass wastage, while around the largest mounds the sharp escarpments may have been smoothed by the vigorous contour currents. Current streamlining effects control the shape of the mounds, which become more elliptical as their size increases, thereby minimising the hydraulic drag force. The frequency distribution of mound size follows a general power law, which is determined by the growth rate of the framework-building coral species and the rate at which they colonised the substrate. Initially, bottom currents support mound growth until the mounds become so large that hydraulic drag forces retard their growth. A model for the evolution of the population predicts that increased hydraulic drag forces on the larger mounds cause a sharp decrease in their number, in agreement with the observations. The model also allows an age structure for the population to be determined and correlations between the growth of the mound population and palaeoclimatic variations in the NE Atlantic to be attempted.

Gravity gradients and upper-crustal tectonic fabrics, Ireland

Gravity gradients derived from an updated Bouguer anomaly map of Ireland reveal large-scale lineaments, many of which can be related to geological structures and tectonic development. First horizontal derivatives of the anomaly calculated at various azimuths accentuate linear trends in different directions, while the second derivative helps resolve near surface structure. The dominant NE–SW Caledonian-trending lineaments are interpreted as due to fault boundaries and folds in pre-Carboniferous basement and Caledonian fault reactivation during the Variscan Orogeny. Subsidiary NNE–SSW–trending lineaments are present, as well as weaker NW–SE-trending lineaments. The largest negative anomalies are produced by late Caledonian granites and widespread buried granitic rock in the middle to upper crust is proposed. Both exposed and inferred granites appear to be spatially related to intersections of NE–SW- with NNE–SSW- and NW–SE-trending lineaments. The orientation of the dominant gravity trend changes from NE–SW towards ENE–WSW going westwards across Ireland and forms part of a more regional change in the ‘Caledonian’ gravity fabric recognised throughout the northeast Atlantic region. ‘Variscan’ E–W gravity trends in the south of Ireland overlay the ‘Caledonian’ trends. Relationships between gravity lineaments and anomalies support a ‘thin-skinned’ model for the Variscan cover.

Structural setting, geological development and basin modelling in the Rockall Trough

Evidence from wide-angle seismic data in the Irish sector of the Rockall Trough suggests that the basin is underlain by thinned continental crust which has undergone differential stretching. The upper crust has been thinned by a stretching factor of 8–10 while the middle and lower crust (and probably also the lithospheric mantle) was stretched by a factor of 2–3. The latter figure is suggested as being representative of the overall lithospheric stretching. Crustal modelling fails to demonstrate any significant effect of the Iceland plume on the development of the basin. The Rockall Trough contains up to 6 km of sedimentary strata. Well and seismic data from adjacent basins indicate that the succession in the basin is likely to be of Late Palaeozoic to Recent age. The pre-Cretaccous faeies are suggested to be broadly similar to other basins of the Atlantic borderlands. Cretaceous and Tertiary strata show progressive clastic starvation as thermal subsidence outstripped sedimentation in the basin, Sandy facies are likely to be concentrated towards the faulted basin margins and the re-entrant regions where the basin margin changes orientation. Basin modelling, based on normal incidence and wide-angle seismic profiles from a number of areas in the Irish sector of the basin, demonstrates that the observed seismic geometries and subsidence patterns cannot be explained by a single rift episode in the Cretaceous, The best fit of possible models suggests that the basin developed in response to discrete rift episodes in the Triassic, Late Jurassic and Early Cretaceous.

Analysis and tectonic interpretation of gravity data over the Variscides of southwest Ireland

The gravity data set for southwest Ireland comprising 5500 stations with a spacing of about 1–3 km has been filtered using standard wavenumber domain techniques. A band pass filtered map, emphasizing gravity anomalies originating to a maximum depth of 3–4 km, correlates well with surface geology. It shows major faults and density contrasts among six lithostratigraphical units, the most important being between the Old Red Sandstone and underlying basement. A low pass filtered map, emphasizing gravity anomalies from a maximum depth of 10 12 km shows major negative gravity anomalies at Killarney and Fermoy and positive anomalies beneath the Shannon estuary, on the Dingle peninsula and along the south coast. These data suggest that Caledonian basement was involved in Variscan deformation and any detachment lies at mid-crustal levels. Two north-south gravity profiles are modelled along Eastings 100 and 200, constrained by surface geology and published seismic refraction data. On Line 100 the source of the negative anomaly at Killarney is modelled as either a granite within basement or a deformed Devonian basin. The source of the Shannon positive anomaly is modelled as a large dense body within the crust whose southern limit coincides with the Iapetus Suture Zone. On Line 200, the Fermoy low is modelled as a laccolithic granite within basement. The positive anomaly along the south coast is a consequence of Mesozoic crustal thinning.

Slope instability and sediment redistribution in the Rockall Trough: constraints from GLORIA

Abstract A recent GLORIA (Geological LOng Range Inclined Asdic) sidescan survey covered 200 000 km2 of the sea bed in the Irish Rockall Trough. It revealed a range of sedimentary features on the trough floor and its steep (>6°) margins. The western margin is characterized by large-scale (of the order of hundreds of kilometres in length) downslope mass movement. Smaller-scale slides and slumps (tens of kilometres across) occur on the eastern margin, but they are subordinate to canyon, channel and fan systems. The western and central parts of the trough floor contain the Feni Sediment Ridge, a 600 km long contourite sediment build-up covered by large sediment waves trending sub-parallel to the dominant modern current pattern. Strong, northward-flowing bottom currents are thought to have eroded the base of the slope in the east and redeposited the sediments on the western margin and the trough floor. Mass wasting and terrigenous sediment input through canyons is regarded as the primary source of sediment in the region. The increase in the degree and frequency of canyon incision along the NE margin of the trough reflects increased terrigenous input from the Irish mainland and a possible glacial influence on the basin margin. The GLORIA images reflect a broad interplay of alongslope and downslope sediment transport processes in the Rockall Trough with sediments sourced from the NE margin and redistributed by currents along the western margin. Although alongslope and downslope processes are the major controlling factors, basin subsidence, Quaternary glaciations and glacio-eustatic sea-level fluctuations have also influenced the pattern of sedimentation in the Rockall Trough.

The crustal structure and regional development of the Irish Atlantic margin region

During the past decade, a suite of wide-angle seismic reflection/refraction profiles has been shot in the Porcupine, Rockall and Hatton basins, as well as across the Hatton continental margin. Integration of the wide-angle seismic data with normal-incidence reflection profiles and with gravity and magnetic data reveals a clear picture of the regional crustal and upper mantle structure, and of the large-scale sedimentary geometry of the Late Palaeozoic–Cenozoic basins in the area. The region contains a set of large sedimentary basins resting upon variably thinned continental crust. The crust beneath the basin-bounding structural highs is typically around 25km thick, with the crust beneath the basins being as thin as 6km. The Porcupine Basin contains up to 10 km of sediments. Up to 7km of sedimentary strata are indicated in the Rockall Basin, with 4km of sediment in the Hatton Basin. Several seismically distinct layers, with P-wave velocities of 1.8–5.3 kms −1 , were identified above the basement in the sedimentary basins. A significant variability in thickness of the sedimentary layers is seen on the wide-angle data. These reflect the presence of a number of older Mesozoic rift sedimentary basins. The wide-angle data reveal marked topographic irregularity at the base of the sedimentary succession in the Rockall Basin, interpreted as the result of extensive multi-phase Mesozoic rifting. The Moho beneath the Rockall Basin shows a clear asymmetry, with a steeper gradient along the eastern margin of the basin. The continent–ocean boundary west of the Hatton High has a significant underplated and volcanic component, while that at the southern margin of the Rockall Basin shows evidence of a greater transpressive component. The nature of the lower crust appears to exert a controlling influence on the location of crustal separation. In areas where thinning is accommodated by upper and middle crustal attenuation, the strength of the upper crust (and underlying brittle mantle) has served to preserve the integrity of the continental crust and inhibit seafloor spreading. However, where the lower crust is thinned significantly, as beneath the Hatton continental margin, crustal separation has taken place even though the upper and middle crust remains relatively thick.

The deep structure of the Porcupine Basin, offshore Ireland, from gravity and magnetic studies

Marine gravity and magnetic surveys around Ireland, together with satellite gravity data from the deeper ocean, are used to investigate the large-scale crustal structure of the Porcupine Basin, offshore west of Ireland. The structure of the syn-rift to post-rift sedimentary successions, derived from vertical incidence seismic reflection data, is used to isolate the gravity and magnetic responses of the crust and mantle. The crustal structure derived from wide-angle seismic data in the region is used to control the interpretation and modelling. The results of gravity modelling in the southern part of the Porcupine Basin suggest a crustal thickening in the centre of the basin. Comparison with the Rockall Basin, where axial crustal thickening is seismically defined, suggests a similar pattern of crustal extension. The amount of extension is less across the narrower northern sector of the Porcupine Basin, where a north–south-trending axial gravity high is present resulting from anomalous density variations in the lower crust/upper mantle. A new model for the large-scale structural development of the Porcupine Basin is presented that explains the along-axis variations in crustal structure in terms of changes in the kinematics of crustal extension across NW–SE transfer zones. This model involves tectonic unroofing of the mantle lithosphere and serpentinization of the exhumed mantle peridotites.

Slope failure features on the margins of the Rockall Trough

Abstract A TOBI sidescan sonar survey in the Irish sector of the Rockall Trough reveals the presence of a range of slope failure features of various sizes and extent along both the eastern and western margins. A number of different types are identified. These include incipient cuspate slides, slab failures and evolved slides, and debris flows. It is suggested that the incipient cuspate slides, slab failures and evolved slides represent slope failure of muddy sediments whereas the failures that gave rise to debris flows lie on steeper slopes and may be of less muddy composition. Many of the slope failure features are relatively recent (probably <15 ka), although some evidence points towards either a prolonged period of movement or a number of phases of slope movement locally along the margins. A comprehensive understanding of the nature, distribution, age and controls on the formation of the slope failure features will be necessary in planning the likely location of sea-bed structures in the event of petroleum development in the region.