A. W. Brian Jacob

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 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.

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.

The transition between the Erris and the Rockall basins: new evidence from wide-angle seismic data

Abstract The Rockall Trough is a deep-water basin west of Britain and Ireland. The origin of the basin and the nature of the crust beneath it is controversial. A series of seismic wide-angle experiments carried out during 1988 and 1990, help to clarify the crustal and upper mantle structure in the region. Results for the crustal structure from a profile which straddles the shelf break between the basin and the Irish Shelf are discussed here. These data together with the available geological information indicate that the basin probably formed in the late Palaeozoic to early Mesozoic as part of a regionally linked basin assemblage which includes the Hatton Basin and the shallow sea basins surrounding Ireland and Britain. Good data quality has allowed the transition between the relatively unstretched crust of the Irish and British mainland, defined by previous onshore seismic refraction experiments, to be well resolved. The Erris Trough, one of the small late Palaeozoic to early Mesozoic basins which fringe the mainland shelf region north of the Porcupine Basin is a half-graben. Its major bounding fault is located on its western margin and the basin is divided from the Rockall Trough by a narrow horst, the Erris Ridge. The shelf trajectory along the western flank of the horst deepens smoothly towards the trough centre, where the crust thins to 5 km near the trough margin below a sedimentary and water column 8 km thick. Surprisingly, the crustal thickness is slightly greater over a 150 km broad zone at the trough centre (i.e., ca. 6 km). This change in crustal thickness may be due to lateral strain migration to the warmer basin margins as its centre cooled during the deformation. The crustal structure beneath the sediment pile at the trough centre is two layered, as opposed to the three-layered seismic refraction structure found onshore. However, the basic character of the lower crust present in onshore Ireland, in particular the presence of a gradient zone defining the crust/mantle transition, is still preserved in the Trough. This similarity in structure precludes the presence of magmatic underplating. The crustal structure observed in the Rockall Trough can be formed by differential stretching of the lithosphere. In this model the lower ductile crust and mantle lithosphere are stretched over a wide region byβ2 = 2−3. Strain focusing into a much narrower region of brittle upper crust generates severe amounts of crustal thinning (β1 = 8−10), and is responsible for the fusing of the upper and mid-crustal seismic refraction layers found beneath onshore Ireland and Britain. Mediating detachment surfaces, sited at the brittle/ductile transition at any time, served to relay the strain from the lower lithosphere into the upper crust. Syntectonic heat loss plays an important role in controlling the deformation pattern.

The Erris and eastern Rockall Troughs: structural and sedimentological development

Abstract Wide-angle seismic reflection profiles across the western Irish shelf region have resolved the deep crustal structure of the shelf transition into the Rockall Trough. A three-layered sedimentary succession up to 5 km thick is determined within the Rockall Trough. The lowermost layer probably comprises a thin early Mesozoic syn-rift sediment package, while the upper two layers represent Cretaceous to Recent, mostly post-rift, sediments. The seismic response of the crust changes beneath the Erris Trough into the eastern Rockall Trough, as the crust thins from c. 30 km onshore in Ireland to between 5 and 6 km in the deep water region of the Rockall Trough. However, the thinning of seismic layers in response to stretching is not uniform. Differential stretching of the crust can explain seismic observations of both crustal and upper mantle structure. Low upper mantle velocities may be due to partial serpentinization of the upper mantle lithosphere, facilitated by rheological hardening during lithospheric stretching. The model predicts the occurrence of starved deep marine sediments during the late Mesozoic and Cenozoic.

Deep seismic experiments investigate crustal development at continental margin

Important advances have been made in recent years regarding our understanding of the processes involved during continental break-up. Much of this knowledge has arisen from deep seismic profiles of the Atlantic margins. The continent-ocean transition (COT) at many locations along the North Atlantic margin consists of a zone of ‘transitional’ crust that lies between thinned continental crust and unequivocal oceanic crust. The origin of this transitional crust is poorly understood, and yet it is likely to provide key information regarding the break-up process.