Wide-angle Seismic Profiles Research Articles

Crustal structure of the Lofoten–Vesterålen continental margin, off Norway

By compiling wide-angle seismic velocity profiles along the ∼400-km-long Lofoten–Vesterålen continental margin off Norway, and integrating them with an extensive seismic reflection data set and crustal-scale two-dimensional gravity modelling, we outline the crustal margin structure. The structure is illustrated by across-margin regional transects and by contour maps of depth to Moho, thickness of the crystalline crust, and thickness of the 7+ km/s lower crustal body. The data reveal a normal thickness oceanic crust seaward of anomaly 23 and an increase in thickness towards the continent–ocean boundary associated with breakup magmatism. The southern boundary of the Lofoten–Vesterålen margin, the Bivrost Fracture Zone and its landward prolongation, appears as a major across-margin magmatic and structural crustal feature that governed the evolution of the margin. In particular, a steeply dipping and relatively narrow, 10–40-km-wide, Moho-gradient zone exists within a continent–ocean transition, which decreases in width northward along the Lofoten–Vesterålen margin. To the south, the zone continues along the Vøring margin, however it is offset 70–80 km to the northwest along the Bivrost Fracture Zone/Lineament. Here, the Moho-gradient zone corresponds to a distinct, ∼25-km-wide, zone of rapid landward increase in crustal thickness that defines the transition between the Lofoten platform and the Vøring Basin. The continental crust on the Lofoten–Vesterålen margin reaches a thickness of ∼26 km and appears to have experienced only moderate extension, contrasting with the greatly extended crust in the Vøring Basin farther south. There are also distinct differences between the Lofoten and Vesterålen margin segments as revealed by changes in structural style and crustal thickness as well as in the extent of elongate potential-field anomalies. These changes may be related to transfer zones. Gravity modelling shows that the prominent belt of shelf-edge gravity anomalies results from a shallow basement structural relief, while the elongate Lofoten Islands belt requires increased lower crustal densities along the entire area of crustal thinning beneath the islands. Furthermore, gravity modelling offers a robust diagnostic tool for the existence of the lower crustal body. From modelling results and previous studies on- and off-shore mid-Norway, we postulate that the development of a core complex in the middle to lower crust in the Lofoten Islands region, which has been exhumed along detachments during large-scale extension, brought high-grade, lower crustal rocks, possibly including accreted decompressional melts, to shallower levels.

Wide-angle seismic profiling of arc-arc collision zone in the northern Izu-Ogasawara arc –KY0408 cruise–

Wide-angle seismic profiling of arc-arc collision zone in the northern Izu-Ogasawara arc –KY0408 cruise–

Crust–upper mantle seismic velocity structure across Southeastern China

One in-line wide-angle seismic profile was conducted in 1990 in the course of the Southeastern China Continental Dynamics project aimed at the study of the contact between the Cathaysia block and the Yangtze block. This 380-km-long profile extended in NW–SE direction from Tunxi, Anhui Province, to Wenzhou, Zhejiang Province. Five in-line shots were fired and recorded at seismic stations with spacing of about 3 km along the recording line. We have used two-dimensional ray tracing to model P- and S-wave arrivals and provide constraints on the velocity structure of the upper crust, middle crust, lower crust, Moho discontinuity, and the top part of the lithospheric mantle. P-wave velocity, S-wave velocity and V P/ V S ratio are mapped. The crust is 36-km thick on average, albeit it gradually thins from the northwest end to the southeast end (offshore) of the profile. The average crustal velocity is 6.26 km/s for P-waves but 3.6 km/s for S-waves. A relatively narrow low-velocity layer of about 4 km of thickness, with P- and S-wave velocities of 6.2 km/s and 3.5 km/s, respectively, marks the bottom of the middle crust at a depth of 23-km northwest and 17-km southeast. At the crust–mantle transition, the P- and S-wave velocity change quickly from 7.4 to 7.8 km/s (northwest) and 8.0 to 8.2 km/s (southeast) and from 3.9 to 4.2 km/s (northwest) and 3.9 to 4.5 km/s (southeast), respectively. This result implies a lateral contrast in the upper mantle velocity along the 140 km sampled by the profile approximately. The average V P/ V S ratio ranges from 1.68–1.8 for the upper crust to 1.75 for the middle and 1.75–1.85 for lower crust. With the interpretation of the wide-angle seismic data, Jiangshan–Shaoxin fault is considered as the boundary between the Yangtze and the Cathaysia block.

Seismic stratigraphy of the southern Rockall Basin: a comparison between wide-angle seismic and normal incidence reflection data

A wide-angle reflection/refraction profile from the RAPIDS 3 (Rockall and Porcupine Irish Deep Seismic) experiment is compared to a coincident high-quality normal incidence reflection line, both of which traverse the southern part of the Rockall Basin on the Atlantic continental margin, offshore Ireland. The positions and geometries of the independently-derived layers show a generally close comparison between the two data sets. The normal incidence seismic reflection data show a greater degree of detail on seismic facies while the wide-angle seismic profile displays greater penetration of the entire sedimentary succession and of the crust. The wide-angle geometries and the normal incidence seismic stratigraphy provide complementary data that help constrain the geology and evolution of the lightly explored Rockall region. A seismic stratigraphy is presented, based on correlation between the wide-angle and normal incidence data, shallow seismic site surveys, shallow borehole cores and tectonostratigraphic studies. Within the central part of the Rockall Basin, the sedimentary layers are generally flat-lying and have a maximum thickness of 7 km. The succession is interpreted as Jurassic to Recent in age. Some of the layers are confined to the basin whilst others extend onto the Rockall and Porcupine highs. Marginal basins at the western (Conall Basin) and eastern (Macdara Basin) edges of the Rockall Basin contain up to 4 km of pre-Tertiary sedimentary rocks, some possibly as old as Permo-Triassic.

The crustal structure of the NW Moroccan continental margin from wide-angle and reflection seismic data

SUMMARY The Atlantic margin off Morocco with its neighbouring Jurassic oceanic crust is one of the oldest on earth. It is conjugate to the Nova Scotia margin of North America. The SISMAR marine seismic survey acquired deep reflection seismic data as well as wide-angle seismic profiles in order to image the deep structure of the margin, characterize the nature of the crust in the transitional domain and define the geometry of the synrift basins. We present results from the combined interpretation of the reflection seismic, wide-angle seismic and gravity data along a 440-km-long profile perpendicular to the margin at 33‐34 ◦ N, extending from nearly normal oceanic crust in the vicinity of Coral Patch seamount to the coast at El Jadida and approximately 130 km inland. The shallow structure is well imaged by the reflection seismic data and shows a thick sedimentary cover that is locally perturbed by salt tectonics and reverse faulting. The sedimentary basin thickens from 1.5 km on the normal oceanic crust to a maximum thickness of 6 km at the base of the continental slope. Multichannel seismic (MCS) data image basement structures including a few tilted fault blocks and a transition zone to a thin crust. A strong discontinuous reflection at 12 s two-way travel-time (TWT) is interpreted as the Moho discontinuity. As a result of the good data quality, the deep crustal structure (depth and velocity field) is well constrained through the wide-angle seismic modelling. The crust thins from 35 km underneath the continent to approximately 7 km at the western end of the profile. The transitional region has a width of 150 km. Crustal velocities are lowest at the continental slope, probably as a result of faulting and fracturing of the upper crust. Uppermantle velocities could be well defined from the ocean bottom seismometer (OBS) and land station data throughout the model.

Crustal structure of seismic velocity in southern tibet and east-westward escape of the crustal material

The reflecting events from Moho and other interfaces within the crust are recognized from the wavefield characteristics of P- and S-wave for the 480km long wide-angle seismic profile between Peigu Tso and Pumoyong Tso. Then, seismic crustal structures of P- and S-wave velocities and Poisson ratio under the nearly east-west profile in southern Tibet are interpreted by fitting the observed traveltimes with the calculated ones by forward modelling. Our interpreting results demonstrate that the crustal thickness varies remarkably in the east-west direction, showing a pattern that the crust could be divided into three parts bounded by the west of Dingri and the east of Dinggye, respectively, where the depth of Moho is about 71km for the western part, about 76km for the middle and about 74km for the eastern. There is one lower velocity layer (LVL) at the bottom of the upper crust with depth of 20—30 km. One of the distinct features is that the thickness of LVL abruptly thins from 24 km on the west to 6km on the east. The other is that the velocity variation in the crust along east-west direction for both P- and S-wave displays a feature as quasi-periodic variation. The lower velocity (compared to the average value for the continent of the globe) in the lower crust and three sets of north-southward active normal faults are probably attributed to the coupling process of material delamination in the lower crust, crustal thicking and east-westward escape of the crustal material accompanied with the continental collision between India and Eurasia Plate.

Image the Zhefang‐Binchuan and Monglian‐Malong Wide‐Angle Seismic Profiles in Yunnan Province

AbstractBased on the finite difference inversion and ray inversion method, the crust and upper mantle structure along the Zhefang–Binchuan and Menglian–Malong wide–angle seismic profiles, both of which are located in Yunnan province, are imaged using the geophysical data of travel‐time, amplitude ratio and Bouguer gravity anomalies. Thus some new recognitions about the geodynamics and the seismotectonic environment are derived. The crust thickness along the Zhefang–Binchuan profile is 35~46km, and that of the Menglian–Malong profile is 33~44.5km. The geometry of the Moho interface looks like that given by Hu H X et al. The P wave velocity on the top of the mantle beneath some places is relatively low and the variation range of the velocity is very large, which may indicates that the Yunnan region is a typical area of active tectonics. Moreover, there exits a prominent consistency between the shallow and deep structures, which implies that the activity of shallow material always has a deep background. The velocity structure of Zhefang–Binchuan profile indicates that there exists a large low velocity anomaly which penetrates the whole crust to the east of the Nujiang fault exposed on the ground, and this possibly associated with the upwelling of the deep material . The characteristic of the large faults which are the borders between some first order tectonic units can also be derived from the both velocity structure profiles: the Red River fault is a hyper‐fault that cuts the lithosphere, the Nujiang fault cuts deeply in the crust and even stretches downward into the top of the upper mantle, yet the Changning–Shuangjiang fault has a listric shape with a small dip angle, which may suggest that its incision depth is not very deep. The large earthquakes took place in Yunnan province always have a relationship with the large and deep faults that stretches into the upper mantle. Some earthquakes with shallow focal depths are generally located at the converge positions of different faults within the upper‐middle crust or the places where the velocity contours on the belt between the high and low velocity anomaly blocks are crooked. It is estimated that these locations are favorable to the accumulation of energy and regional stress.

Segment-scale variations in the crustal structure of 150-300 kyr old fast spreading oceanic crust (East Pacific Rise, ) from wide-angle seismic refraction profiles

Summary We have simultaneously inverted seismic refraction and wide-angle Moho reflection traveltimes for the 2-D crustal thickness and velocity structure of 150–300 kyr old crust along the East Pacific Rise (EPR) between the Siqueiros and Clipperton fracture zones (FZs). Our results show a strong correlation between ridge segmentation and upper- and mid-crustal seismic velocities, with higher velocities near segment centres and lower velocities near segment ends. Low crustal velocities at the Clipperton and Siqueiros FZs are interpreted as fracturing resulting from brittle deformation of the crust in the transform domain. A relict overlap basin left on the Pacific Plate by the 9°03′N overlapping spreading centre (OSC) as it propagated southward is associated with a large (∼1 km s−1), negative upper- and mid-crustal velocity anomaly. This anomaly is consistent with the presence of an unusually thick extrusive section within the basin and with tectonic alteration, fracturing and shearing arising from rotation of the basin as it was formed. The discordant zone left by this OSC on the Cocos Plate is characterized by moderately low crustal velocities, probably because of crustal fracturing as the OSC propagated into older crust. Higher crustal velocities near segment centres may reflect a higher ratio of dikes to extrusives in the upper crust, and lower-intensity tectonic alteration of the crust, than near segment ends. The mean crustal thickness along the EPR between the Siqueiros and Clipperton FZs is 6.7–6.8 km. The thickest crust is found beneath the Lamont seamounts (∼9 km), and in a southward-pointing, V-shaped band located just north of the off-axis trace of the 9°03′N OSC (7.3–7.8 km). The thinnest crust (<6 km) is found proximal to the Clipperton and Siqueiros FZs. The crust associated with the off-axis trace of the 9°03′N OSC is not anomalously thin, suggesting that magma supply beneath the OSC is similar to that of the northern and southern segments. We see a similar pattern of crustal thickness variation to that determined using multichannel reflection data, including a gradual thickening of the crust from north to south along the northern ridge segment, and the location of the thickest crust just north of the 9°03′N OSC. However, the magnitude of the along-axis crustal thickness variation we observe along the northern ridge segment between 9°50′N and 9°15′N(∼1.3–1.8 km, excluding the Lamont seamounts) is significantly less than the 2.3 km of variation previously reported, weakening the case for the existence of a low-density mantle diapir at 9°50′N inferred from gravity data. The band of thick crust located just north of the off-axis trace of the 9°03′N OSC suggests a close genetic link between this feature and the OSC. Thus we attribute the pattern of crustal thickness variations along the northern segment to the kinematics of the southward-propagating 9°03′N OSC over the past 0.5 Myr, and not to along-axis melt migration away from a mantle diapir as previously proposed.

East-west crustal structure and “down-bowing” Moho under the northern Tibet revealed by wide-angle seismic profile

We recognized 6 sets of reflecting P- and S-wave events from Moho and other interfaces within the crust, respectively, with the wide-angle seismic data acquired from 510 km-long Selincuo-Ya’anduo profile in the northern Tibet, fitted the observed events with forward modeling, and interpreted crustal structure of P- and S-wave velocities and Poisson’s ratio under the profile. The results demonstrate that the crustal structure between Yarlungzangbo and Bangong-Nujiang sutures changes abruptly, and the crust is the thickest at the middle part of the profile with thickness of 80 km or more. The “down-bowing” Moho is the striking feature for the crustal variation along the west-east direction. The Moho uplifts with steps, and the uplifting rate westward is greater than that eastward. The heterogeneity of P- and S-wave velocities exists both vertically and horizontally, and one lower velocity layer (LVL) exists with the depth range of 27–34 km and the thickness range of 5–7 km. For the upper crust, Poisson’s ratio is the lowest at the middle part of the profile; for the lower crust, the Poisson’s ratio at the east segment is lower than that at west segment, which means that the crustal rigidity for the upper crust is different from the lower crust, and the lower crust under the east segment of the profile is more ductile. We infer that the substance in the lower crust endured eastward flow along with the collision between Eurasian and Indian plates, and the “down-bowing” Moho is attributable to the multi-phase E-W tectonic processes.

Geophysical characteristics of the continental crust along the Lithoprobe Eastern Canadian Shield Onshore–Offshore Transect (ECSOOT): a review

The Eastern Canadian Shield Onshore–Offshore Transect (ECSOOT) of the Lithoprobe program included 1200 km of normal-incidence seismic profiles and seven wide-angle seismic profiles across Archean and Proterozoic rocks of Labrador, northern Quebec, and the surrounding marine areas. Archean crust is 33–44 km thick. P-wave velocity increases downwards from 6.0 to 6.9 km/s. There is moderate crustal reflectivity, but the reflection Moho is unclear. Archean crust that stabilized in the Proterozoic is similar except for greater reflectivity and a well-defined Moho. Proterozoic crust has similar or greater thickness, variable lower crustal velocities, and strong crustal reflectivity. Geodynamic processes of Paleoproterozoic growth of the Canadian Shield are similar to those observed in modern collisional orogens. The suturing of the Archean Core Zone and Superior provinces involved whole-crustal shearing (top to west) in the Core Zone, linked to thin-skinned deformation in the New Quebec Orogen. The Torngat Orogen sutures the Nain Province to the Core Zone and reveals a crustal root, in which Moho descends to 55 km. It formed by transpression and survived because of the lack of postorogenic heating. Accretion of the Makkovik Province to the Nain Province involves delamination at the Moho and distributed strain in the juvenile arcs. Delamination within the lower crust characterizes the accretion of Labradorian crust in the southeastern Grenville Province. Thinning of the crust northwards across the Grenville Front is accentuated by Mesozoic extension that reactivates Proterozoic shear zones. The intrusion of the Mesoproterozoic Nain Plutonic Suite is attributed to a mantle plume ponding at the base of the crust.

Tarim underthrust beneath western Kunlun: evidence from wide-angle seismic sounding

This paper describes the results of a wide-angle seismic profile, over 700km long, across the southern margin of the Tarim Basin. A crustal structure that best fits the observed data, that tests and verifies the earlier discovery that the crystalline basement dips southward from the area of the petroleum industry seismic reflection profiling and geological investigation is presented. The profile also reveals that the Moho dips southward beneath the southern Tarim, at an angle in concordance to that of the crystalline basement. This result is thought to be the first evidence for the underthrusting of the Tarim crust beneath the western Kunlun. Additional evidence of underthrusting is that the relatively weak western Kunlun crust has contracted, and expressed by a lower crust, which is more than 20km thick, with an uplift of the Kunlun basement, accompanied by northward-directed thrusts.

Crustal structure of Ascension Island from wide-angle seismic data: implications for the formation of near-ridge volcanic islands

The study of the internal structure of volcanic islands is important for understanding how such islands form and how the lithosphere deforms beneath them. Studies to date have focused on very large volcanic edifices (e.g., Hawaiian Islands, Marquesas), but less attention has been paid to smaller islands, which are more common. Ascension Island, a 4-km high volcanic edifice with a basal diameter of 60 km, is located in the equatorial Atlantic (8°S), 90 km west of the Mid-Atlantic Ridge on 7 Ma oceanic lithosphere. We present results of a wide-angle seismic profile crossing the island revealing a crustal thickness of 12–13 km, an overthickened layer 3 (7 km thick) and little evidence of lithospheric flexure. Together these results suggest Ascension Island may be older than previously assumed and may have begun forming at an on-axis position around 6–7 Ma. This hypothesis is further supported by the presence of a young 1.4-km high edifice directly adjacent to the Mid-Atlantic Ridge with a volume about 1/7 that of Ascension Island, possibly representing the earliest stages of seamount formation. Excess magmatism appears to be related to the tectonic setting at the ridge–fracture zone intersection.

Crustal structure of central Tibet as derived from project INDEPTH wide-angle seismic data

Summary In the summer of 1998, project INDEPTH recorded a 400 km long NNW–SSE wide-angle seismic profile in central Tibet, from the Lhasa terrane across the Banggong-Nujiang suture (BNS) at about 89.5°E and into the Qiangtang terrane. Analysis of the P-wave data reveals that (1) the crustal thickness is 65 ± 5 km beneath the line; (2) there is no 20 km step in the Moho in the vicinity of the BNS, as has been suggested to exist along-strike to the east based on prior fan profiling; (3) a thick high-velocity lower crustal layer is evident along the length of the profile (20–35 km thick, 6.5–7.3 km s−1); and (4) in contrast to the southern Lhasa terrane, there is no obvious evidence of a mid-crustal low-velocity layer in the P-wave data, although the data do not negate the possibility of such a layer of modest proportions. Combining the results from the INDEPTH III wide-angle profile with other seismic results allows a cross-section of Moho depths to be constructed across Tibet. This cross-section shows that crustal thickness tends to decrease from south to north, with values of 70–80 km south of the middle of the Lhasa terrane, 60–70 km in the northern part of the Lhasa terrane and the Qiangtang terrane, and less than 60 km in the Qaidam basin. The overall northward thinning of the crust evident in the combined seismic observations, coupled with the essentially uniform surface elevation of the plateau south of the Qaidam basin, is supportive of the inference that northern Tibet until the Qaidam basin is underlain by somewhat thinner crust, which is isostatically supported by relatively low-density, hot upper mantle with respect to southern Tibet.

Interpretation of transverse gravity lineaments in the Rockall Basin

Abstract A number of regional transverse gravity lineaments crosscutting the Rockall Basin are interpreted from satellite gravity data. Euler deconvolution carried out on gravity data along a wide-angle seismic profile indicates that a major NW-SE-trending lineament within the basin reflects pronounced variations in crustal structure and sedimentary thickness. These thickness variations are interpreted as the result of cross-basin faulting along a zone defined by this lineament. Transverse gravity lineaments to the north of this feature are similarly interpreted as major cross-basin fault zones.

VARNET-96: influence of the Variscan and Caledonian orogenies on crustal structure in SW Ireland

VARNET, an international multidisciplinary project, was designed to examine the ‘Variscan Front’ in the southwest of Ireland. As part of the seismic experiment a 200 km long wide-angle seismic profile (Line A) was recorded from the Old Head of Kinsale in the south of Ireland to Galway Bay on the west coast. Along Line A, 170 seismic stations were deployed at approximately 1 km intervals. Results from ray trace modelling indicate a multilayered crust. The upper crust, extending to a depth of about 14 km, is laterally variable. South of the Shannon Estuary sedimentary basins alternate with uplifted basement (5.8-6.0 km s−1). This correlates well with the surface geology. The larger sedimentary basin in the south reveals a shallowly south-dipping structural feature that coincides with the Killarney-Mallow Fault Zone. North of the Shannon Estuary surprisingly high velocities (6.4 km s−1) at shallow depths were observed. Between the upper and middle crust there is a transition layer about 2-4 km thick. The middle crust thickens towards the Shannon Estuary. These structural changes coincide with the proposed location of the Iapetus Suture Zone. The middle crust is underlain by a lower crust with a variable velocity structure, and the total crustal thickness varies from 29–32 km. Detailed traveltime investigations have also revealed a reflector in the upper mantle at depths between 39 and 44 km. The seismic model shows that there are no deep crustal variations beneath the Killarney-Mallow Fault Zone, indicating a thin-skinned mechanism of deformation.

Wide-angle seismic control on the development of the Munster Basin, SW Ireland

Abstract Three wide-angle seismic profiles were acquired from onshore southern Ireland during VARNET-96. These data provide the only subsurface seismic images of a series of predominantly Late Palaeozoic sedimentary basins. The Dingle Basin is a broadly symmetrical rift on the Dingle peninsula, but assumes a half-graben geometry to the east. The southern syn-rift basin margin is defined by the north-dipping Dingle Bay-Galtee Mountains Line. A south-dipping normal fault at Kerry Head marks the northern basin margin in the west, with displacement on this structure decreasing towards the east. The Dingle Basin is separated from the Munster Basin to the south by a basement horst. Seismic data strongly suggest that the Killarney-Mallow Fault Zone, located on the southern margin of this positive feature, is the Munster Basin northern syn-rift marginal fault. The preserved extensional displacement on this detachment decreases from c. 2.5 km in northern Iveragh, to c. 1 km at Mallow, suggesting that the fault tips out to the east of Mallow. A tilted fault block, possibly cored by shallow granite, is imaged in the northern Munster Basin. This fault terrace is bounded to the south by the Cork-Kenmare Line. The combined Munster Basin and South Munster Basin succession thickens across the Cork-Kenmare Line to a maximum thickness of 8 km on the Beara peninsula. A tectonic thickness of 6 km of sediment is modelled at the Old Head of Kinsale, increasing to 7 km in the southwest.

Velocity structure of the upper crust under the San Francisco volcanic field, Arizona

AbstractIn addition to traditional in-line wide-angle seismic profiles, the 1989 Pacific to Arizona Crustal Experiment (PACE-1989) included some off-line shots and seismic recorders, which, along with the data from the main profiles, provide tomographic coverage of the late Cenozoic San Francisco volcanic field. This major volcanic center is interesting because of its location on the margin of the Colorado plateau and because of the diversity of the composition of the volcanics. We employed arrival times for the Pg phase to undertake a tomographic analysis of the structure of the upper crust. The tomographic image indicates high-velocity zones at upper crustal depths under the San Francisco Mountain region. The P-wave velocities within these zones are 6% higher than the surrounding basement rocks. Although other interpretations are possible, we believe that these high-velocity regions most likely represent igneous intrusions associated with the volcanic centers that formed the San Francisco volcanic field. Geologic studies indicate that these intrusions would be of intermediate to mafic composition. Gravity data in the San Francisco volcanic field area also indicate that these high-velocity regions are coincident with high-density bodies at upper crustal depths.

The lithospheric trace of the Iapetus Suture in SW Ireland from teleseismic data

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.

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.

Crustal structure from the Ligurian Sea to the Northern Apennines — a wide angle seismic transect

A new wide-angle seismic reflection profile, 250 km long, was shot along a line extending from Parma in the Apennines to the deepest part of the Ligurian Sea. Thirty seismic land stations (LOBS) and 25 ocean bottom seismographs (OBS) recorded 54 shots fired at sea. The resulting 55 record sections allowed the definition of the transition from continental to oceanic crust in a geologically complex, and hitherto poorly understood, region. The subducted continental crust beneath the Northern Apennines has a thickness of 40 km, consisting of 30 km of igneous crust overlain by 10 km of sediments. Two sedimentary layers are identified with v P-velocities in the range from 2.8 to 5.5 km/s. The igneous portion of continental crust thins gradually to approx. 11 km across a 120-km-wide zone extending from the southwestern part of the Apennines (some 20 km inland) into the Ligurian Sea. An easterly-thinning low-velocity body (7.2–7.5 km/s) immediately beneath the base of the crust is interpreted as an underplated unit, emplaced prior to the onset of seafloor spreading. This may have been initiated by magmatic activity along the incipient continental–oceanic boundary. A basement high coincides with the northeastern edge of the Ligurian Sea and is interpreted as part of the Alpine ophiolite belt, indicating strike-slip motion. The two sedimentary layers above the transition zone are typically 5 km thick and have slightly lower velocities than those further to the northeast. A sharp continent–ocean break is recognised in the Ligurian Sea. Oceanic crust is 6–7 km thick and is overlain by a two-layered sedimentary succession approx. 5 km thick. Mantle velocities in the region range from 7.9 km/s beneath the thinned crust to 8.1 km/s beneath the thick crust. The ratio δ c/o of continental to oceanic crustal thickness at the continent–ocean transition is approx. 1.5 and the overall crust stretching factor ( β) at that boundary is approx. 3.5. These values are similar to other continent–ocean boundaries and indicate that the oceanic crust in the Ligurian Sea formed largely by stretching, with a small pull-apart component.