Franz Hauser

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 LITHOSPHERIC TRACE OF THE IAPETUS SUTURE IN SW IRELAND FROM TELESEISMIC DATA

Abstract The Caledonian Iapetus Suture is a tectonic boundary resulting from the amalgamation in a three-plate configuration of Laurentia and Baltica with Avalonia. In SW Ireland, seismic stations on a wide-angle seismic profile crossing the postulated path of the Iapetus Suture have recorded a set of teleseismic events. The P-wave residual times for an earthquake in the Aleutians (end-on to the line) and a nuclear test in China (nearly orthogonal to the line) show large variations, of about 0.5 s, along this relatively short profile. Heterogeneities in the crustal structure along this profile can account for very little of the variation in residual times and most of it must originate in heterogeneities below the crust. From an analysis of the residual time curves of the two teleseismic events, the probable trace of the Iapetus Suture at depth has been found in the upper mantle, between 30 and 110 km. The suture is defined by a velocity contrast of about 3% and it is located just south of the Shannon River Estuary. The interface dips steeply to the south, indicating that the closure of the Iapetus Ocean may have involved a southward subducted slab. The model obtained in SW Ireland is comparable with a teleseismic tomography model across the Urals. This could indicate a close evolution of these two orogenies since the Palaeozoic.

Basin development and petroleum prospectivity of the Rockall and Hatton region

Abstract The Rockall region, offshore west of Ireland, contains a number of large, virtually unexplored, frontier sedimentary basins: Rockall Trough, Hatton Basin and Hatton Continental Margin. It is suggested that the sedimentary section, up to 6 km thick, is of Late Palaeozoic to Tertiary age. Normal incidence reflection seismic data from the region are typically of poor quality owing to the effects of shallow Tertiary sills. Recent wide-angle reflection seismic profiles have helped to resolve the basin geometry and structural setting. Backstripping (reverse modelling) and forward modelling of the wide-angle model are consistent with development in a complex multi-phase rift setting. Rift events are suggested in the Jurassic and Cretaceous, and possibly locally during the Permo-Triassic. The variation in structural style across the region is interpreted as resulting from the interplay of intraplate stresses, magmatic underplating, thermal doming and thermal subsidence adjacent to the continent/ocean margin, and seafloor spreading dynamics in the region to the west. The basins are predicted to contain a range of petroleum plays. Structural, stratigraphic and combination traps are indicated from a range of data. Reservoirs are predicted at several levels, with source rocks in the Jurassic and lowermost Cretaceous.

Lithospheric structure across the western Eurasian plate from a wide-angle seismic and gravity study: evidence for a regional thermal anomaly

Abstract Crustal structure along a 1100 km seismic profile between Ireland and the Iceland Basin is tested with satellite gravity data and used to isolate the long wavelength gravity field due to sub-crustal sources. Plate cooling models constrain the gravity contribution of the oceanic lithosphere and, together with the crustal seismic geometry, define the thermal structure of the continental lithosphere across the western Eurasian plate. Low density lithosphere is required below much of the continental plate, suggesting that it is thermally perturbed. The thermal anomaly may have had a major impact on Late Mesozoic to Recent magmatism, tectonics and sedimentation across the NE Atlantic.

An overview of the results of the RAPIDS seismic project, North Atlantic

The RAPIDS (Rockall And Porcupine Irish Deep Seismics) project involved the acquisition, processing, interpretation and modelling of a series of wide-angle seismic reflection lines in the North Atlantic west of Ireland. The data were acquired in two cruises during 1988 and 1990. A transverse profile, 1000km long, crossed the Erris Trough, the Rockall Trough, the Hatton Basin, the Hatton Continental Margin and the basement highs between the basins. It extended onto oceanic crust to the west of the Hatton Continental Margin. A 600km long profile was run near the axis of the Rockall Trough and extended southwards onto oceanic crust south of the Charlie Gibbs Fracture Zone. The location of the profiles, together with the transverse profile structure, is shown in Fig. 1, while the axial profile is shown in Shannon et al. (1995; Fig. 3). The objectives of the project were to help resolve long-standing debates concerning the nature of the crust in the region (e.g. Smythe 1989) and to elucidate the thickness, geometry and age of the sedimentary fill in the basins of the region. Ocean bottom seismometers were deployed every 6 to 8km along each line, with 25kg explosive shots fired at 1km intervals at optimum depths producing a main frequency of about 6Hz. Recording distances of up to 180km were obtained, resolving the crust and upper mantle structure. These wide-angle methods overcame some of the problems of poor energy penetration and strong water layer multiples encountered with normal incidence reflection profiling. Data quality along the profiles

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.