Mantle Reflections Research Articles

Linkage between mantle and crustal structures and its bearing on inherited structures in northwestern Scotland

Deep seismic reflection profiles in Scotland reveal mantle structures beneath a crust with a polyphase tectonic history that resulted in several generations of structures. Continuum mechanics suggests that coeval mantle and crustal structures must be kinematically linked. Inherited structures imply relative ages for the reflectors, ages that can be placed into the context of the geological history of the near-surface rocks of northern Scotland. Thus, some mantle reflectors are assigned Triassic ages related to the opening of the West Orkney and related marginal basins of the Atlantic Ocean. Other mantle reflectors are cut by late Caledonian structures associated with the Great Glen Fault Zone and therefore older than c. 400 Ma. Many of these structures also track the late Precambrian margin of Laurentia and may be related to either the opening (900–600 Ma) or closing (500–600 Ma) of the Iapetus Ocean. Some reflective structures may also be attributed to 1800–1700 Ma Laxfordian deformation that was part of a global-scale orogenic belt.

Orogenic Evolution of the Ural Mountains: Results from an Integrated Seismic Experiment

Results of the URSEIS '95 integrated seismic experiment document the lithospheric structure of an intact Paleozoic collisional orogen in the Ural Mountains. Hybrid-source seismic reflection and refraction data provide images of a crustal-scale collisional fabric and a pronounced crustal root preserved since Paleozoic time. Mantle reflections are observed at depths of more than 150 kilometers, possibly representing the base of the lithosphere. The Urals do not conform to existing models of postorogenic evolution involving large-scale extension, which may be a consequence of an incomplete or arrested collisional process that has led to the preservation of the largest continental landmass.

Waveform inversion of deep seismic reflection data: the polarity of mantle reflections

The Flannan and W-reflectors are two prominent mantle features observed on seismic reflection data off the northwest coast of Scotland. They are the brightest, most laterally extensive, intra-mantle reflectors identified on a deep seismic dataset anywhere in the world. Despite extensive study, their physical origin is still the subject of speculation. We present a scheme to determine the polarity of these mantle reflectors, and constrain their upper structure using near-normal-incidence seismic reflection data. The technique exploits the convolutional model of the earth; we use a deterministic source-signature deconvolution to invert the data. We have explored the parameterization of the inversion by testing real and synthetic data. We find that it is critical to the legitimacy of the reflectivity model that many traces are stacked prior to the inversion and that the data have a good signal-to-noise ratio. Furthermore, an accurate estimate of the effective source wavelet is a fundamental requirement for obtaining a valid reflectivity model; in particular we find the deconvolution results are most sensitive to the precise value of the water depth and reflection coefficient used in estimating the sea-bed multiple train. In the case of the Flannan-reflector, the inversion shows unequivocally that it has a positive polarity. Modelling the W-reflector is less straightforward as a result of reduced signal-to-noise ratio. None-the-less, the inversion suggests a positive polarity for the W-reflector, in agreement with observations of post-critical reflections seen on wide-angle seismic data. The near-normal-incidence polarity measurements support the suggestion that both the Flannan and W-reflectors represent an eclogitic slab, presumably a relict oceanic subduction zone, preserved within the continental lithospheric mantle.

Transition zone velocity gradients and the 520‐km discontinuity

Stacks of long‐period Global Digital Seismograph Network (GDSN) seismograms at 110° to 180° epicentral distance reveal precursors to SS that result from underside reflections off upper mantle seismic discontinuities. The 410‐ and 660‐km discontinuities are obvious in these stacks, but identification and modeling of other transition zone discontinuities are complicated by sidelobes from the 410‐ and 660‐km reflections. These sidelobes result from the limited bandwidth of the GDSN instrument responses and the effect of crustal reverberations on the SS reference phase. The crustal effects can be minimized by restricting the records to oceanic bounce points where the ∼6‐km‐thick crust has little effect on the long‐period waveforms. Over 2000 long‐period, transverse‐component seismograms with oceanic SS bounce points recorded by the GDSN from 1976 to 1991 are manually edited, aligned on SS, and then stacked using a new procedure that weights the records by data quality. The resulting image shows a clear reflection from a 520‐km discontinuity that cannot be explained as a sidelobe artifact, confirming earlier results of Shearer [1990, 1991] and Revenaugh and Jordan [1991]. By stacking along the expected travel time curves for discontinuity phases, the time versus range image of the precursor wave field is reduced to a single trace that measures upper mantle reflectivity versus time. The features in this reflectivity profile are sensitive to the brightness and depth of the transition zone discontinuities and to the steepness of the velocity gradients between the interfaces. Using geometrical ray theory and assuming a constant velocity versus density scaling relationship, I fit this reflectivity profile with velocity models of the upper mantle using both forward modeling and direct inversion. The inverse problem is addressed by performing a deconvolution of the profile with the SS reference phase (after a correction for attenuation), followed by a direct mapping of reflectivity versus time into velocity versus depth. Velocity‐depth profiles resulting from these procedures are roughly in agreement with standard upper mantle velocity models, except that the SS precursor data require a minor discontinuity near 520 km and a steeper gradient just below the 660‐km discontinuity. Estimated discontinuity shear impedance changes are 6.7 ± 1.1% at 420 km, 2.9 ± 0.7% at 519 km, and 9.9 ± 1.5% at 663 km. The impedance change near 520 km is consistent with current mineral physics results for the olivine β to γ phase change and places constraints on the fraction of olivine in the transition zone.

Seismic reflections from the mantle represent relict subduction zones within the continental lithosphere

In many areas of the world, the continental lithospheric upper mantle contains bright, continuous, regionally extensive, seismic reflectors. Despite the increasingly common observation of such mantle reflectors on deep seismic reflection profiles, their significance and geologic origin remain obscure. We report results from a series of seismic experiments acquired across two of the brightest of these reflectors and provide new constraints upon the composition and thickness of the reflecting region. These new seismic data reveal regionally extensive dipping and subhorizonal slabs of high-velocity (>8.4 km/s), high-density (>3500 kg/m 3 ) material, several kilometres (>2 km) thick, entrained within otherwise unremarkable upper mantle. The geometry, physical properties, and geologic setting of these mantle reflectors suggest that they represent fragments of eclogitic oceanic crust—a relict of pre-Caledonian oceanic subduction now preserved within the lower continental lithosphere. Such relict subduction zones appear to be widespread within the continental lithosphere and to exert an important influence upon the location and style of subsequent continental deformation.

Crustal and mantle reflectors from Palaeoproterozoic orogens and their relation to arc-continent collisions

Abstract Two prominent geological features characterize the Palaeoproterozoic orogenic belts of Laurentia and Baltica: (1) Archaean cratons form the stable footwall during early stages of convergence and throughout crustal collision; and (2) juvenile, predominantly arc-derived crust was accreted to Archaean cratons through arc-continent collision. Seismic reflection profiling over the Svecofennian (Baltic Shield), Lewisian (British & Irish Isles) and Trans-Hudson (Canadian Shield) segments of an arguably once continuous orogenic belt has provided geometries of reflectors throughout both Palaeoproterozoic and Archaean crustal blocks as well as laterally coherent mantle reflectors. Two BABEL deep seismic reflection profiles within the Baltic Shield revealed structures along an irregularly shaped boundary between the juvenile 2.0–1.8 Ga Svecofennian domain and an Archaean craton (Karelia Province). An important result of the survey is the approximately 100 km horizontal offset between the inferred mantle suture, or palaeo-subduction boundary, and the geochemically-mapped crustal suture between juvenile Proterozoic crust and the Archaean craton. On the other side of the Atlantic, LITHOPROBE reflection profiles across the Trans-Hudson Orogen in central Canada reveal that 1.92–1.83 Ga juvenile arc and oceanic terranes of the Reindeer Zone form an allochthonous carapace about an Archaean basement block. Unexpectedly, the juvenile allochthons dip beneath the bounding Superior and Hearne cratons, defining a crustal-scale culmination in the core of the Orogen. The British and Irish Isles form an important bridge between the Palaeoproterozoic orogens of Laurentia and those of Baltica. Twenty deep reflection profiles on the continental shelf north and west of Scotland have traced a mantle reflector for over 800 km along its reconstructed strike, which is at a high angle to younger structures. This feature dips away from the reconstructed cratonic nucleus of Laurentia and may trace the Palaeoproterozoic (1.7–1.9 Ga) suture between the Archaean Lewisian block and accreted juvenile crust represented by the c. 1.8 Ga Rhinns complex. A common theme that emerges from the integration of all these geological and seismic results is the role played by the Archaean lithosphere during arc-continent collision. In all cases studied, juvenile 1.9–1.8 Ga lithosphere was delaminated and its crustal flakes overrode the Archaean margins. This consistency reflects the relative strength and durability of Archaean crust/lithosphere, and suggests that large parts of the lithosphere underlying detached and flaked Palaeoproterozoic juvenile terranes, such as the Svecofennian or Reindeer Zone terranes, may be Archaean in age but modified during Proterozoic tectonism.

Upper mantle reflector structure and origin beneath the Scottish Caledonides

The mantle reflector pattern beneath the Scottish and Irish Caledonides is perhaps the most widely studied individual structure in the uppermost continental mantle. Previous investigations interpreted the reflector as a thrust related to the Caledonian orogeny (Silurian‐Early Devonian) or as a normal fault or shear zone active during post‐Caledonian (e.g., Devonian or Permo‐Triassic) extension. This study uses all available deep seismic reflection data to map mantle reflector structure along the Caledonian orogen and to constrain better its geometrical and tectonic relation to regional geology. The distribution of the mantle reflector shows that it is not spatially related to either Caledonian compressional or subsequent basin extensional structures in a simple way. A structural contour map shows two distinct discontinuous surfaces consisting of a steeply plunging antiform‐synform pair beneath the West Orkney basin (north of the Scottish mainland) and the Færoe‐Shetland basin west of Shetland, respectively, separated by an inferred zone of structural disruption. A working hypothesis for this study is that these two surfaces may have once been part of a continuous, more or less planar surface that has been offset and deformed by either (or perhaps a combination) of two processes: (1) differential extension of crust that was at least partially coupled to the uppermost mantle beneath the West Orkney and Færoe‐Shetland basins; (2) a broad, approximately orthogonal (i.e., east‐west trending) zone of simple shear corresponding to the disrupted zone that was largely confined to the upper mantle but reactivated to deform the crust. Our results also suggest that the mantle reflector structure may be inherited from Neoproterozoic events associated with rifting of the Laurentia‐Baltica supercontinent.

Lithospheric structure in the southern Canadian Cordillera from a network of seismic refraction lines

Lithospheric velocity structure and its relationship to regional tectonics and development of the southern Canadian Cordillera are derived from a synthesis of interpretations from nine in-line seismic refraction–wide-angle reflection profiles and broadside data recorded during the Lithoprobe Southern Cordillera Refraction Experiment (SCoRE) and other refraction experiments across southern British Columbia, and one profile in northwestern Washington. Consistency of the SCoRE two-dimensional models at their intersection positions is achieved through application of a simultaneous inversion of all relevant traveltime data. The cross-sectional and map presentations demonstrate the strong degree of three-dimensional heterogeneity within the crust and upper mantle. A first-order characteristic is the continuous increase in crustal velocities westward from the Foreland belt to the Insular belt. The variations do not correlate with the morphogeological belts; they do correspond with large-scale geological and (or) tectonic features and seismic reflection results. Crustal thickness varies from 30 to 48 km; a lack of comparable variation in Bouguer gravity anomalies requires significant density changes in the crust. Variations in the seismic parameters do not correlate well with variations in crustal resistivity or heat flow, suggesting that generalizations relating low resistivities, high temperatures, and low seismic velocities must be treated with caution. Seismic heterogeneities are due primarily to lithological and (or) structural variations and are superimposed on the generally low velocities attributed to the thermal regime. An upper mantle reflector beneath the mainland Cordillera is inferred to be the top of a shallow asthenosphere. Westward flow in the warm asthenosphere interacts with the cold lithosphere of the subducting Juan de Fuca plate below the central Coast belt, forming a "sink" that could provide a driving mechanism for the flow.

Does the Great Glen fault really disrupt Moho and upper mantle structure?

Many previous interpretations of deep seismic reflection profiles across the Great Glen strike‐slip fault system have postulated that the fault penetrates the crust and upper mantle as a vertical plane that sharply offsets the Moho discontinuity. Such an interpretation has become a general feature of more recent regional syntheses of the deep geology of the British Isles. Reprocessing of portions of four profiles across the fault improves the resolution of lower crustal and upper mantle structure and calls into question the “Moho step” interpretation. Diffraction analysis and seismic migration applied to reflection data north of Shetland across the northward continuation of the Great Glen fault (Walls Boundary fault) indicate a narrow synform or “keel” developed on the Moho directly beneath the fault. This keel is itself underlain by a possible mantle reflection that is cut off by the downward projection of the fault. North and west of Ireland, analyses of amplitudes, frequencies, and the geometrical behavior of reflections upon migration show that structures previously interpreted as Moho steps may be better explained as distinct packages of reflectivity that are uppermost mantle or possibly out of the plane in origin. In one location north of Ireland where some three‐dimensional control exists, the vertically downward projection of the Great Glen fault intersects, without disrupting, dipping structure in the upper mantle. This observation leads to a model for displacement on the fault system in which motion is laterally transferred along a dipping ramp (or blind thrust) in the uppermost mantle, somewhat analogous to models developed for the San Andreas fault that indicate displacement along the fault to be laterally offset within the middle crust. One of the principal conclusions of this study, that major vertical steps on the Moho beneath the Great Glen fault are difficult to support from the available seismic data, is consistent with Theologically based studies which predict that Moho “topography” such as vertical steps is unlikely to be preserved over long periods of geologic time.

東北日本上部マントルの部分溶融域と低周波微小地震・地殻内反射面との対応について

東北日本上部マントルの部分溶融域と低周波微小地震・地殻内反射面との対応について

Two-dimensional modelling and interpretation of seismic wide-angle data from the Western Gulf of Bothnia

In late 1989, wide-angle reflection and refraction profiles were shot in the Baltic Sea, the Bothnian Sea and the Gulf of Bothnia in order to study the lithospheric structure of Archaean and Proterozoic domains. The present study concerns the crustal structure and velocity variations beneath the Gulf of Bothnia area. Data collected at six land stations, recording marine airgun shots fired along BABEL line 2 in the Gulf of Bothnia, have been interpreted. The results from traveltime inversion show an upper crustal thickness of 13–22 km with P- and S-wave velocities varying from 5.5 to 6.5 and 3.3 to 3.7 km/s, respectively. The middle crust has a thickness of 13–15 km, with P-wave velocity varying from 6.3 to 6.9 km/s and S-wave velocity from 3.6 to 3.9 km/s. The lower crustal layer with thickness ranging from 9 to 20 km shows a high P- and S-wave velocity of approximately 7.5 and 4.4 km/s, respectively. The P-and S-wave velocities for the upper mantle are about 8.0 and 4.5 km/s. Calculated Poisson's ratio ranges from 0.20 to 0.24 for the upper crust and 0.25 to 0.28 for the middle and lower crust, and 0.29-0.31 for the upper mantle, respectively. The Moho depth varies between 42 and 55 km along the profile. The Moho topography shows a smooth undulation underneath the central part of the Gulf of Bothnia. A steep offset of about 8 km occurs beneath the southernmost part of the profile. Northward-dipping reflectors have been recorded in the lower crust. These lower crustal reflections line up perfectly with the northward-dipping upper mantle reflectors displayed in the vertical reflection section from the same line.

Mantle discontinuity structure beneath China

Mantle reflectivity profiles obtained from long‐period recordings of multiple‐ScS reverberations from intermediate and deep‐focus earthquakes surrounding China reveal remarkably sharp images of China deep structure. The Tibetan Plateau and fold belts to the north are underlain by a midlithospheric, low‐velocity zone (LVZ) evidenced by coincident G (lid‐LVZ boundary) and L' (base of the LVZ) reflectors and a thickened lithosphere consistent with uniform thickening as the dominant mode of north‐south deformation. The mantle beneath northeastern China, Mongolia and southeastern Russia has a typically cratonic reflectivity signature with H (Hales) and deep L (Lehmann) discontinuities. The Sino‐Korean Paraplatform of eastern China is a region of active extensional tectonics, as expressed by an “oceanic” reflectivity pattern at shallow depths (a G discontinuity at ∼80 km), but a more continental structure below. The southern Sea of Japan, Korean Peninsula, Yellow Sea and bordering regions of the China mainland are underlain by a ∼5.4% impedance decrease near 330 km depth that we interpret as the upper limit of dense silicate melt atop the 410‐km discontinuity produced at the subduction zones to the east. Regional inversion of the reflection coefficient of the 410‐km discontinuity, R(410), reveals a pronounced high beneath Mongolia and north to northwestern China which is best explained by increased upper mantle olivine content, signifying extensive basalt depletion in a tectosphere that extends below the 410‐km discontinuity. A low value of R(660) associated with topographic depression of the 660‐km discontinuity beneath eastern Asia is consistent with a horizontally inclined slab atop the phase transition marking the seismic discontinuity. Reflection defocusing is an alternative but requires ≥38 km of intermediate‐scale topography within the depressed region, exceeding the amplitude of the depression by a factor of 1.5–2. Either mechanism requires strong heterogeneity far west of seismically active slabs. Two lower mantle reflectors (one beneath southern Honshu, Kyushu, and the Sea of Japan; the other below Shantung province (China) and the Yellow Sea) correlate with horizontally elongated velocity anomalies in western Pacific seismic tomography models. We speculate that they may represent slabs stagnating below the 660‐km discontinuity.

Synthetic seismogram images of upper mantle structure: No evidence for a 520‐km discontinuity

Seismological data used by Shearer (1990, 1991) to infer the existence of a seismic discontinuity at 520 km depth are compared with complete long‐period body wave seismograms calculated for the International Association of Seismology and Physics of the Earth's Interior 1991 (Iaspei91) Earth model. The Iaspei91 model does not contain a seismic discontinuity at or near 520 km depth. The observed P and SH multiples caused by topside reflections and SS precursors caused by underside reflections from the 410‐km and 660‐km discontinuities are well reproduced by the synthetic stacks. The synthetics exhibit clear “phase extrema” between these reflections that correlate well with similar features in the observed waveform stacks. Shearer interpreted these “phase extrema” as separate reflections from a seismic discontinuity at 520 km depth. Cross‐correlation analysis of synthetic seismograms gives an apparent discontinuity depth of about 520 km for P and SH multiples as well as SS precursors. Similarly, amplitude analysis of synthetic upper mantle reflections is in reasonable agreement with the observations reported by Shearer (1991). The phase extrema seen in the synthetics are the result of an extended, multicycle wavelet which is composed of depth phases and other structural phases such as reverberations from the crustal layer, convolved with the instrument response of long‐period stations of the Global Seismograph Network. The comparison of observational data, presented by Shearer (1990, 1991), with the synthetic seismogram stacks of this paper shows that the claim of good seismological evidence for a 520‐km seismic discontinuity as a global feature is not compelling.

The polarity of deep seismic reflections from the lithospheric mantle: Evidence for a relict subduction zone

BIRPS offshore deep seismic reflection profiles to the north of Scotland have revealed two bright continuous reflectors in the continental lithospheric upper mantle, the dipping Flannan and sub-horizontal W reflectors. The polarity of reflections from the Flannan has been determined from normal-incidence reflection data, using the far-field source wavelet as a starting model. The effective far-field wavelet was calculated by adding a receiver ghost and the effects of recording filters, attenuation during transmission and sea bottom multiples to the source wavelet. Reflection polarity was determined in a blind test by comparing the modelled wavelet with stacks of the Flannan reflection. In eight out of ten stacks, the reflection was picked as positive polarity, two out of ten were unknown. To verify this result, the test was repeated for Moho reflections; in this case nine out of ten stacks were picked as positive. The modelling also demonstrated that the Flannan reflection is apparently from a simple interface; complex interlayering is not required to explain the waveforms, and they are not consistent with reflections from a single thin layer. Seismic data acquired at wide-angle across the W mantle reflector show sub-crustal high-amplitude arrivals, which can only be explained as post-critical reflections. Only a high-velocity eclogitic layer, contained within normal mantle, with a sharp upper boundary and a diffuse base can explain all our observations. We suggest that the Flannan reflector represents the top of a relict oceanic and eclogitic component of a pre-Caledonian subduction zone within the lithospheric mantle.

Simple-shear deformation of the Skagerrak lithosphere during the formation of the Oslo Rift

Simple-shear deformation of the entire lithosphere has been postulated by Wernicke (1985) and others, but up to now unequivocal seismic evidence in support of this hypotheses has been lacking. Here we describe well-defined seismic reflectors below the Skagerrak Sea, one of which is interpreted as a low-angle fault underlying the Skagerrak Graben segment of the Permian Oslo Rift. This reflective lineament can be traced from the mid-crust through the lower crust, offsetting Moho and continuing downwards to ca. 50 km depth (16 s). A separate mantle reflection beneath the graben may be associated with an earlier period of thrusting. The 1730 km of deep seismic reflection data in Skagerrak indicate that the crust and mantle inherited a pronounced structural fabric from the Proterozoic Grenvillian-Sveconorwegian orogeny. During formation of the Oslo Rift, reactivation of these implied weak zones as localized detachment planes would explain the extensional deformation style of the non-magmatic Skagerrak Graben.

Global mapping of upper mantle reflectors from long-period SS precursors

SUMMARY Long-period precursors to SS resulting from underside reflections off upper mantle discontinuities (SdS where d is the discontinuity depth) can be used to map the global distribution and depth of these reflectors. We analyse 5,884 long-period seismograms from the Global Digital Seismograph Network (1976-1987, shallow sources, transverse component) in order to identify SdS arrivals. Corrections for velocity dispersion, topography and crustal thickness at the SS bounce point, and lateral variation in mantle velocity are critical for obtaining accurate estimates of discontinuity depths. The 410 and 660 km discontinuities are observed at average depths of 413 and 653 km, and exhibit large-scale coherent patterns of topography with depth variations up to 40 km. These patterns are roughly correlated with recent tomographic models, with fast anomalies in the transition zone associated with highs in the 410 km discontinuity and lows in the 660 km discontinuity, a result consistent with laboratory measurements of Clapeyron slopes for the appropriate phase changes. The best resolved feature in these maps is a trough in the 660km discontinuity in the northwest Pacific, which appears to be associated with the subduction zones in this region. Amplitude variations in SdS arrivals are not correlated with discontinuity depths and probably result from focusing and defocusing effects along the ray paths. The SdS arrivals suggest the presence of regional reflectors in the upper mantle above 400 km. However, only the strongest of these features are above probable noise levels due to sampling inadequacies.

Laterally varying reflector at the top ofD″beneath northern Siberia

SUMMARY We analyse P-wave data from the French 'Laboratoire de DCtection et GCophysique' (L.D.G) network of digital short-period seismic stations. Whereas our previous paper (Houard & Nataf 1992) showed a few promising Kurd events and depicted the methods we proposed to apply, an extensive analysis of 32 earthquakes from the KuriVKamchatka region has now been carried out. By using the deconvolution technique previously described (Houard & Nataf 1992), clear and quantitative evidence of intermediate arrivals between the P and PcP waves is shown, in the 75-82 distance range. Our approach is complementary to previous studies in the same region, which used broad-band data recorded at a dense narrow-aperture array (Weber & Davis 1990; Weber 1993) or long-period data recorded at the WWSSN stations (Gaherty & Lay 1992). For each station, record sections of deconvolved signals from different earthquakes are built, and lead us to eliminate crustal reverberations beneath L.D.G stations as a major explanation for these secondary arrivals. A compilation of the most conclusive signals has been made. It is perhaps one of the most complete PdP-wave record sections, since the move out with distance can be followed clearly with a good sampling over more than 5. The move out with distance of this extra 'PdP' arrival, its residual slowness value, eliminates source complexities, diffraction, or multipathing through the Kuril subduction slab as possible explanations. The extra arrivals indicate the existence of a lower mantle reflector. The structure of the 'Lay discontinuity' is investigated, using TpdP-Tp residual traveltimes. Two different residual time versus distance tendencies are found, which correlate to different geographical bounce point regions, indicating the existence of lateral variations of the structure. The D region is also sampled further south at distances well beyond the theoretical triplication cross-over by analysing events from the Honshu/Ryukyu Islands region, but only one P-wave branch is observed. The hodochrones and p-A curves of the first P arrival are used as a complementary analysis. The global PREM model (Dziewonski & Anderson 1981), predicts correct P wave slownesses, though its smooth lower mantle structure does not account for PdP observations. This may be an indication that the 'Lay discontinuity' is a global feature. However, the hodochrones of the first P arrival do not allow us to test the existence of the discontinuity in the 82-85' cross-over distance range.

Crustal Structure Beneath Lofoten, N. Norway, From Vertical Incidence and Wide-Angle Seismic Data

Summary A high-quality multichannel seismic reflection line was acquired in 1987 along a 175 km long profile across the continental shelf off Lofoten, northern Norway. A seismic wide-angle experiment was performed in 1988 along the same profile, using seven three-component Ocean Bottom Seismographs (OBS) with 20-25 km spacing and shotpoint intervals of 240 m. The study of the data has shown that the combination of the multichannel reflection and the wide-angle (OBS) technique provides information about the crustal structure beneath the Lofoten shelf that could not have been achieved using only one of the techniques. the multichannel reflection data provide a detailed image of the shallow (Cretaceous) structures, which represents an important basis for inversion of the OBS data. the lower crust and the Moho are also well mapped in some parts of the area with the multichannel reflection technique. The OBS data reveal that significant amounts of pre-Cretaceous sediments exist along almost the entire profile, with a maximum thickness of about 5 km in the Vestfjorden Basin. From the OBS data the thickness of the lower crust is inferred to decrease from about 11.5 km under the Rost High to about 2 km below the Lofoten Ridge. the OBS data indicate further that the Moho position under the Vestfjorden Basin is considerably deeper than can be inferred from the reflection data. About 10km below Moho a strong dipping event is observed in the OBS data. This upper mantle reflection might be related to a possible seaward dipping master fault, and/or presence of layers of partially hydrated peridotite.

Seismic anisotropy inferred from wide-angle reflections off Lofoten, Norway, indicative of shear-aligned minerals in the upper mantle

Data from six three-component Ocean Bottom Seismographs situated on a 175 km long profile on the continental shelf off Lofoten, northern Norway, have been modelled using 2-D seismic ray-tracing, and P- and S-wave velocity models for the upper mantle are presented. A dipping structure about 10 km below the Moho causes strong P and S reflections as well as P-to-S conversions. This upper mantle reflection might be related to a possible seaward dipping master fault, and/or the presence of partially hydrated peridotite. The modelling of these arrivals reveals significant seismic anisotropy in the upper mantle. The maximum P-wave anisotropy is inferred to be in excess of 15%, the maximum S-wave anisotropy in the order of 5–10%, and the axis of the fast velocity is inferred to be inclined at least 45° from the horizontal. The strong anisotropy for P-waves indicates that its cause is related to alignment of minerals. We postulate that the inferred steep inclination of anisotropic minerals might be a result of shearing along faults and fractures during post-Caledonian extensional episodes. It must be emphasized, however, that it cannot be excluded that more complicated anisotropic models with other geological implications can also explain the observations satisfactorily. The study demonstrates the importance of incorporating S-waves in analysis of seismic data, because the strong P-wave anisotropy would not have been discovered without the constraints given by the S-waves.

Evidence for extensional shear zones in the mantle, offshore Britain, and their implications for the extension of the continental lithosphere

Deep seismic profiles recorded in the seas around the British Isles have revealed bands of seismic reflections in the uppermost mantle. On the basis of their appearance, geophysical characteristics, and association with major crustal faults, these features are best interpreted as mantle shear zones. Furthermore, in the North Sea, the mantle reflectors appear to follow an extensional trend and, when interpreted as extensional shear zones, help explain the regional stratigraphy of the overlying basin. The crosscutting relationships between mantle reflections to the west of Scotland and a major strike‐slip fault, the Great Glen Fault (GGF), suggest that the mantle structure postdates the GGF, and hence is probably related to subsequent extension. Furthermore, kinematic analysis of lithospheric deformation to the north of Scotland is consistent with the structure observed on the seismic data if the Flannan mantle reflection is an extensional shear zone. Reconstruction of the lithosphere prior to Permian extension restores the Flannan to the edge of the Rockall Trough, a rift probably initiated in the Carboniferous. Thus the Flannan Fault may have developed during Carboniferous rifting. It is concluded that most of the mantle reflections are some form of extensional shear zone in the mantle, and the role of such structures during lithospheric extension is considered.