Patrick M. Shannon

Gas hydrate crystals may help build reefs

During a recent cruise in the Porcupine Basin, off southwest Ireland, we discovered two extensive and hitherto largely unsuspected deep-water reef provinces, including a giant cluster of hundreds of buried mounds. The ring shapes of many reefs suggest that they are caused by an axial fluid expulsion at the sea bed, a transient flow well confined in space and time. We are exploring various hypotheses, but a stimulating avenue for research is opened by a glacially controlled growth pulse and subsequent decay of a shallow layer of gas hydrates as a methane buffer and probably indirectly as a ground for overlying biological communities.

The lithosphere below the Rockall Trough: wide-angle seismic evidence for extensive serpentinisation

Abstract The longer-range data from wide-angle seismic reflection experiments along an axial and transverse profile determined the seismic properties of the shallow mantle lithosphere beneath the Rockall Trough in the North Atlantic. Two subcrustal P-and S-wave reflections are observed. The first defines a layer 3–10 km thick below the Moho where P-wave velocities vary from 7.5 to 7.8 km/s. V p V s ratios increase within this layer from 1.80 in the north to 1.83 in the south of the basin along the axial profile. The second reflection from approximately 34 km depth identifies a layer 15–20 km thick, with a Vp velocity of approximately 8.1 km/s and a V p V s ratio of 1.73. These values are typical for normal mantle peridotites. Both P- and S-wave velocities and V p V s ratios constrain the possible composition of the first layer which is interpreted as a zone of partially serpentinised peridotites below the Moho. About 15% volume alteration of the parent mantle peridotite is required to produce the observed seismic properties. This degree of alteration accounts for a systematic deficit in total tectonic subsidence when compared with that predicted from the variation in bulk crustal stretching along the axis of the basin. Syntectonic cooling occurred during differential lithospheric stretching, as the upper to mid-crust became more extended over a narrower region than the mantle lithosphere. This served to rheologically couple the lower crust to the mantle near the final stage of deformation as the primary brittle/ductile transition zone in the crust migrated downwards into the mantle lithosphere. The resultant fracturing generated the permeability necessary to facilitate the seawater circulation which hydrated the cold mantle.

The development of Irish offshore sedimentary basins

A variety of ensialic Mesozoic and Cenozoic sedimentary basins surround Ireland. Three main periods of basin development occurred. The first, in the Permo-Triassic, produced a series of small rift basins which nucleated along reactivated, generally Caledonian, structures. The second, in the Mid- and Late Jurassic, represents a period of crustal extension and rifting. The third phase reflects thermal subsidence of Cretaceous and Tertiary age, and coincides with seafloor spreading in the North Atlantic. The basins developed in response to general east-west extension. Simple extensional structures, accompanied by crustal thinning, dominated in the north-south-trending Porcupine Basin. The NE-SW-oriented basins in the Celtic Sea, Slyne-Erris-Donegal and Rockall areas developed in a transtensional setting until Early Cretaceous times. The Goban Spur basins, at the confluence of regional structural trends, and adjacent to the continent-ocean boundary, are dominated by transtensional structures. The effects of Early Tertiary Alpine compression are seen especially in the Celtic Sea basins. The basins, whilst having some broad geological similarities which reflect eustatic sea-level variations, differ in their detailed evolution. Each basin experienced unique facets of local structural control, tectonic history and facies distribution.

Neogene evolution of the Atlantic continental margin of NW Europe (Lofoten Islands to SW Ireland): anything but passive

A regional stratigraphic framework for the Neogene succession along and across the NW European margin is presented, based on a regional seismic and sample database. The stratigraphy provides constraints on the timing and nature of the mid- to late Cenozoic differential tectonic movements that have drivenmajor changes in sediment supply, oceanographic circulation and climate (culminating in continental glaciation). The overall context for Neogene deposition on the margin was established in the mid-Cenozoic, when rapid, km-scale differential subsidence (sagging) created the present-day deep-water basins. The Neogene is subdivided into lower (Miocene–lower Pliocene) and upper (lower Pliocene–Holocene) intervals. The lower Neogene contains evidence of early to mid-Miocene compressive tectonism, including inversion anticlines and multiple unconformities that record uplift and erosion of basin margins, as well as changes in deep-water currents. These movements culminated in a major expansion of contourite drifts in the mid-Miocene, argued to reflect enhanced deep-water exchange across the Wyville-Thomson Ridge Complex, via the Faroe Conduit. The distribution and amplitude of the intra-Miocene movements are consistent with deformation and basin margin flexure in response to enhanced intra-plate compressive stresses during a local plate reorganization (transfer of the Jan Mayen Ridge from Greenland to Europe). The upper Neogene records a seaward tilting (

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.

Evidence from episodic seamount volcanism for pulsing of the Iceland plume in the past 70 Myr.

The North Atlantic volcanic province has been attributed to continental rifting about 60 Myr ago over an Iceland plume head with a diameter of 1,000–2,000 km (refs 1, 2). But evidence from a few igneous centres has been used to infer that earlier plume activity occurred in this region. The three seamounts in the Rockall trough off the Atlantic coast of Scotland are among the few accessible remnants of such early plume activity. Here we present 40Ar–39Ar incremental-heating ages of samples from these seamounts, which show that volcanism began there in the late Cretaceous period (70 ± 1 Myr ago), and then continued for the next 30 Myr in at least four discrete phases: 62, 52, 47 and 42 Myr ago. We relate this activity to pulsing of large masses (∼10 8 km3) of hot Iceland plume material on timescales of 5–10 Myr. This significantly extends the time span for Iceland plume activity both backwards and forwards in time, and provides a possible alternative to the ‘plume head’ models for the formation of continental flood basalts.

The crustal structure of the Rockall Trough: Differential stretching without underplating

The crustal structure along the axis of the Rockall Trough, in the North Atlantic, has been studied along a 600-km refraction/wide-angle reflection transect, containing three lines each 200 to 250 km long, using explosives and ocean bottom seismometers. One-dimensional inversions of each section were made using the τ – p method and forward modeling of the observed travel times. In the next stage, travel times and amplitudes were modeled using ray tracing techniques through two-dimensional heterogeneous structures. The results indicate that there are three sedimentary layers with velocities ranging from 2 km/s to 4.5 km/s. The whole sedimentary section is up to 6 km thick and interpreted as late Paleozoic to Tertiary in age. A two-layer continental crust, 5 to 7 km thick, occurs along the length of the profile. The upper crust (6.0–6.3 km/s), is circa 2 km thick and the lower crust (6.6–6.9 km/s), is circa 3 km thick. A Moho transition zone, approximately 1 km thick, lies at the base of the crust. Velocities in this transition zone increase from 6.9 km/s up to 7.8 km/s along the profile. The underlying upper mantle has a laterally variable velocity between 7.6 and 7.8 km/s. Unstretched crust onshore in Ireland comprises a three-layered crust, with each layer approximately 10 km thick, and a Moho transition zone, which is about 3 km thick. The two upper layers in the onshore region are interpreted as corresponding to the upper crust in the Rockall Trough and indicate a stretching factor (β) of 8–10. The velocity pattern in the lower crust in the Rockall Trough and under Ireland are similar, suggesting significantly less stretching (β = 2 - 3). The differential stretching model is supported by the presence of the Moho transition zone which is stretched by a similar factor to the lower crust. The bulk stretching factor for the crust as a whole is in the range of 4–6. If this represents the lithospheric stretching factor, significant underplating would be expected. However, if the stretching factor for the lower crust in the differential stretching model is more representative of overall lithospheric stretching, little or no underplating is predicted. The velocity patterns observed in the Rockall Trough indicate the absence of any significant underplating at the base of the crust, such as that observed at the continental margin west of the Hatton Basin.

Cretaceous and Tertiary basin development west of Ireland

The Porcupine, Rockall and Hatton–Rockall basins lie on the continental seaboard west of Ireland. Up to 9 km of Cretaceous and Tertiary strata are preserved in the Porcupine Basin, in contrast to 2–3 km in the Rockall and Hatton–Rockall basins. Differences in seismic stratigraphic sequences through the region are attributed to an interplay between the developing North Atlantic Ocean, post-rift thermal subsidence and sea-level changes. Localized Ryazanian fault-controlled alluvial fan clastic rocks in the Porcupine Basin are succeeded by Valanginian to Barremian marine deposits. Sea-level lowstands in Albian and Paleocene–Eocene times, interpreted as rift and ridge-push effects respectively, resulted in delta and submarine fan progradation. Rapid thermal subsidence in Early Oligocene times led to marine deposits and to the onset of geostrophic currents. Miocene slumping triggered shale-dominant turbidite development, followed by tranquil deep marine sediments as subsidence outstripped sedimentation. The Cretaceous sequence in the Rockall Trough is thought to contain extensive igneous bodies overlain by an Upper Cretaceous muddy chalk sequence which is cut by abundant Tertiary sills and dykes. A number of shale-prone Upper Tertiary sequences occur. The Upper Cretaceous to Lower Tertiary succession is thin and frequently absent in the Hatton–Rockall Basin.

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.