Deep Seismic Reflection Profiles Research Articles

Seismic imaging of Late Palaeozoic—Early Mesozoic extensional and contractional structures in the Bowen and Surat basins, eastern Australia

The Bowen and Surat basins in eastern Australia form part of a sedimentary basin system that was initiated in the latest Carboniferous or earliest Permian. The basin system developed in a back-arc plate tectonic setting and has had a complex evolutionary history, with early phases of normal and oblique extension being followed by later phases of contraction and transpression. The geometries of the phases are best observed in selected industry seismic data and in long regional BMR deep seismic reflection profiles recorded across the basins in 1984 and 1989. The Denison Trough along the western margin of the Bowen Basin was initiated as a series of extensional half-grabens bounded by north- to north-northwest-striking faults. The polarity of the half-grabens are relatively consistent along strike, with the bounding faults being mainly on the western side of the haif-grabens. Farther east, the Taroom Trough is the main depositional centre. In and around seismic line BMR 84.14, the Taroom Trough also displays a half-graben geometry but only a very limited amount of horizontal extension (3.3 km maximum) can be accommodated on the steep bounding fault at its eastern margin. This trough developed by oblique extension with the bounding fault having a significant strike-slip component. Reactivation of the bounding faults in the Denison Trough in the Late Permian to Middle Triassic led to tectonic inversion and the development of contractional structures. At the same time, to the east of the Denison Trough in the central Bowen Basin, major shortening was accommodated by thin-skinned thrusting on a series of listric thrust faults that dip to the east. These thrusts root in a major east-dipping detachment that appears to flatten in the middle crust. In and around seismic line BMR 84.14, the zone of deformation is extremely localised, confined to near the basin bounding faults, and the structures are interpreted to result from transpression resulting from reactivation of the strike-slip faults. Thus, seismic data from the Bowen and Surat basins provide excellent images of early normal and oblique extensional structures and also of the later contractional structures in the Denison Trough and central Bowen Basin and the transpressional structures associated with the major bounding faults in the east.

Crustal structure of the Kapuskasing uplift from Lithoprobe Near‐vertical/wide‐angle seismic reflection data

Reprocessing of one LITHOPROBE Kapuskasing deep seismic reflection profile discloses significant new information on the structure of the Kapuskasing Uplift (KU). The shallow structure of the Ivanhoe Lake fault zone (ILFZ), along which the high‐grade granulites of the KU were thrust to the surface, is conspicuously imaged on a seismic section as a series of prominent northwest‐dipping reflections with listric geometry. The new images show that the ILFZ is a steep fault (∼50°) near the surface which quickly flattens out at shallow depths. These reflections coincide with bright positive aeromagnetic anomalies over the fault zone. Direct correlation with geological observations indicates that the high reflectivity and high magnetism associated with the fault zone likely originate from mylonites. The reprocessing also reveals a pronounced midcrustal reflector within the Abitibi greenstone belt (AGB), dipping northwest and plunging under the KU. The existence of such a reflector is independently confirmed by wide‐angle reflection data acquired from a cross‐profile. This reflector is apparently also detected on two other reflection profiles crossing the ILFZ about 80 km to the southwest. Its concave‐down shape and broad lateral extent suggest that it represents underthrusting of the AGB beneath the KU. With these new results, a more complete structural cross‐section can be constructed. It shows that, on the northwest side, the KU was overthrust along the northwest‐dipping ILFZ and, on the southeast side, the AGB was underthrust northwesterly. It seems evident that the KU is a product of intraplate collision during the early Proterozoic and the underthrusting of the AGB is primarily responsible for the emplacement of the KU.

Carboniferous magmatism in the Iapetus convergence zone: evidence from deep seismic reflection profiles

An alternative interpretation of the North East Coast (NEC) deep seismic reflection profile is presented. Seismic reflectors in the lower crust of the UK sector of the North Sea, which were formerly attributed to the complex effects of Caledonian continental collision, are shown to be consistent with the inferred processes of Carboniferous igneous intrusion and magmatic underplating in the Iapetus convergence zone. The nature and distribution of Carboniferous magmatism in southern Scotland and northern England are briefly described. ENE-trending intrusions and chains of volcanic centres are widely recognized. Similarly trending aeromagnetic anomalies are used to identify where the offshore continuation of this igneous province intersects the NEC line. As a result, an area of crustal thickening in the northern part of the line is directly related to the presence of Carboniferous igneous intrusions beneath southern Scotland. In the same way, the single deep reflector on the NEC line which was previously identified as the Iapetus suture, is reinterpreted as a lower crustal intrusion associated with the emplacement of the end-Carboniferous Whin sill. The new seismic interpretation is incorporated in a speculative, regional crustal model of Carboniferous magmatism.

Gravity interpretation of a unified 2-D acoustic image of the central Alpine collision zone

SUMMARY Four individual deep seismic reflection profiles across the eastern and southern Swiss Alps have been combined along the trend of the Bouguer gravity anomalies onto the European Geotraverse. The profiles were migrated ray theoretically with the migration velocity model derived from a smoothed reinterpretation of the seismic wide-angle profiles running parallel to the strike of the Alpine arc. The contours of this velocity model were ray theoretically depth migrated together with the combined digitized line drawings of the seismic reflection profiles. The resulting acoustic image depicts the subduction of parts of the lower crust of the European plate beneath the Adriatic promontory of the African plate at a low angle of 15. Based on the interpretation of the seismic data, orogenic crustal thickening is attributed to the stacking of crystalline nappes onto the upper crust of the European plate and wedging of the European and Adriatic middle and lower crusts the latter being ill-constrained by the seismic evidence alone. The south-vergent thrusting of the Southern Alps can be accounted for by the observed downbending of the Adriatic Moho and the lower crust in conjunction with the inferred wedging at mid-crustal levels. Using the geometric constraints provided by seismic data, gravity modelling of the Alpine lithosphere/asthenosphere system relative to stable central European platform clearly favours a gently inclined subduction zone reaching down to at least 200 km depth. Such a gently dipping subduction zone is at odds with prominent models on Alpine geodynamics which favour a near-vertical orientation of the subducting lithosphere ('Verschluckung'). In agreement with the wedging hypothesis indirectly inferred from the seismic data the short wavelength part of the Alpine gravity anomaly requires a middle crust of anomalously high density in the axial zone of the orogen. Both seismic and gravimetric evidence therefore suggest that late-orogenic lithospheric shortening and crustal thickening was governed by the mechanical decoupling of the upper, middle and lower parts of the crust. The amount of subducted lower crustal material cannot be constrained by gravity modelling since its gravity effect cannot be separated from the effect of the subducting slab.

LISPB revisited: a new look under the Caledonides of northern Britain

SUMMARY The Lithospheric Seismic Profile in Britain (LISPB), shot in 1974, included a 700 km long north-south profile of the crust and upper mantle of northern Britain. The wide-angle seismic data from LISPB was of exceptional quality and coverage making a reinterpretation using current modelling programs worthwhile. This reinterpretation shows that the original crustal model published by Bamford et al. (1978) satisfies the observed first arrival traveltimes, but that further detail of crustal velocity structure may be obtained by model shows strong variations in crustal velocity structure that appear to be correlated with geological and tectonic structures mapped at the surface. The new lithospheric model is compatible with the observed gravity field when converted to appropriate densities, and the variations in loading along the base of the density model show that the lithosphere is close to local isostatic equilibrium. Profiles of the variation of velocity and density with depth from the new model are compared with measurements of Lewisian rocks made at confining pressures up to 8 kbar, allowing the seismic model to be interpreted geologically. A synthetic normal incidence profile of the new model is superimposed on the nearby NEC deep seismic reflection profile, and the features imaged by the two techniques are compared.

Crust and upper mantle velocity structure of the Intermontane belt, southern Canadian Cordillera

As part of the Lithoprobe Southern Cordillera transect, seismic refraction data were recorded along a 330 km long strike profile in the Intermontane belt. An iterative combination of two-dimensional traveltime inversion and amplitude forward modelling was used to interpret crust and upper mantle P-wave velocity structure. This region is characterized by (i) a thin near-surface layer with large variations in velocity between 2.8 and 5.4 km/s, and low-velocity regions that correlate well with surface expressions of Tertiary sedimentary and volcanic rocks; (ii) an upper and middle crust with low average velocity gradient, possibly a weak low-velocity zone, and lateral velocity variations between 6.0 and 6.4 km/s; (iii) a distinctive lower crust characterized by significantly higher average velocities relative to midcrustal values beginning at 23 km depth, approximately 8 km thick with average velocities of 6.5 and 6.7 km/s at top and base; (iv) a depth to Moho, as defined by wide-angle reflections, that averages 33 km with variations up to 2 km; and (v) a Moho transition zone of depth extent 1–3 km, below which lies the upper mantle with velocities decreasing from 7.9 km/s in the south to 7.7 km/s in the north. Where the refraction line obliquely crosses a Lithoprobe deep seismic-reflection profile, good agreement is obtained between the interpreted reflection section and the derived velocity structure model. In particular, depths to wide-angle reflectors in the upper crust agree with depths to prominent reflection events, and Moho depths agree within 1 km. From this comparison, the upper and middle crust probably comprise the upper part of the Quesnellia terrane. The lower crust from the refraction interpretation does not show the division into two components, parautochthonous and cratonic North America, that is inferred from the reflection data, indicating that their physical properties are not significantly different within the resolution of the refraction data. Based on these interpretations, the lower lithosphere of Quesnellia is absent and presumably was recycled in the mantle. At a depth of ~ 16 km below the Moho, an upper mantle reflector may represent the base of the present lithosphere.

Oceanic crust deep seismic survey

In September 1991, the British Institutions Reflection Profiling Syndicate (BIRPS) collected 578 km of deep seismic reflection profiles over the oceanic crust beneath the Cape Verde abssyal plain in approximately 4900 m of water (Fig. 1). The survey, under the direction of J. H. McBride, was undertaken in response to a proposal made by R. S. White at the 1990 BIRPS open syndicate meeting in Birmingham, England, and was acquired using GECO‐PRAKLA'S M/V Bin Hai 511. The survey consisted of two strike lines parallel to magnetic sea‐floor lineations and nine orthogonal crossing lines oriented parallel to the spreading direction (Fig. 2). Adjacent lines are spaced at 4 km. For the first time, this provides the ability to map oceanic crust in “3D,” since the line spacing is less than or equal to the Fresnel‐zone diameter for the lower crust.

PSV-wave propagation in a medium with local heterogeneities: a hybrid formulation and its application

SUMMARY Hybrid methods, which combine so-called ‘numerical’ and ‘analytical’ methods, are a successful tool for computing synthetic seismograms for media with localized heterogeneous regions. Each of the two methods is applied in that part of the medium in which it can be employed efficiently. This paper deals with a combination of a finite-difference algorithm, applied in the neighbourhood of the heterogeneity, and a frequency–wavenumber filter for the PSV-wave equation applied outside. This method is an alternative to the use of Green's theorem as a wavefield-continuation method (e.g. van den Berg 1984). In the hybrid method presented here, absorption can be incorporated additionally. For different models with first-and second-order discontinuities the hybrid method was tested successfully against the reflectivity method. The hybrid method was applied to study the effects of a heterogeneous lower crust on the wavefield. This application has been inspired by recent deep seismic reflection profiling, which often reveals a highly reflecting lower crust. The scattering models have been divided into three groups: horizontal, vertical and isotropic scatterers. The comparison of the reflection responses of these models shows that thin horizontal or small isotropic scatterers yield high reflected energy, coherent over a few kilometres, in the time window of lower crustal reflections. Vertical structures, characterized by a small correlation length in horizontal direction, have less energy in this time window, but they can generate a complex Moho reflection with a strong Moho coda. It is also shown that the acoustic approximation is mostly not sufficient to describe the wave propagation in such complex media.

Seismic studies in the Limpopo Belt

The structure of the Archaean Limpopo Belt of southern Africa has been investigated by means of a deep seismic reflection profile and the recording of the traveltimes of mining-induced tremors at regional distances. Interpretation of the traveltimes suggests that a seismic low-velocity zone exists in the upper mantle of the northern Kaapvaal Craton and Limpopo Belt, beginning at a depth of about 42 km. The seismic reflection profile provides evidence for a northward dip of the contact between the Kaapvaal Craton and the Southern Marginal Zone of the Limpopo Belt. No mid-crustal decollement zone is detected. The tectonic model of Van Reenen et al. (1990) is in accord with these observations. The lower crust is essentially seismically transparent, and no reflection Moho is observed.

Seismic image of the Ivanhoe Lake Fault Zone in the Kapuskasing Uplift of the Canadian Shield

The Kapuskasing uplift, located in the central Canadian shield, represents an oblique exposure of the Archean middle to lower crust. The Ivanhoe Lake fault zone, believed to be the basal thrust carrying the high‐grade rocks of the Kapuskasing zone over the low‐grade Abitibi greenstone belt, holds the key to understanding the nature and evolution of the Kapuskasing uplift. Despite numerous geological and geophysical studies, including LITHOPROBE deep seismic reflection profiles, and because of very limited bedrock exposure in the area, the shallow structure of the Ivanhoe Lake fault zone remains obscure. Here we present results obtained by reprocessing data from a LITHOPROBE seismic reflection profile across the fault zone. For the first time, the Ivanhoe Lake fault zone is clearly imaged on the seismic section as a series of west‐dipping reflectors with an average dip of 20°, which can be traced to the surface. The results support the conclusion that fault zones form good reflectors.

Seismic evidence for a low-viscosity lower crust beneath the continental deep drilling location, Oberpfalz, Germany

A network of six deep seismic reflection profiles was recorded in the Oberpfalz as part of the pre-site investigations for the German deep drilling programme. In addition, this network is crossed by a longer regional profile. A striking observation is the highly variable reflectivity of the lower crystalline crust. Based on the relationship between (temperature-dependent) viscosity and crustal reflectivity it seems possible to relate the changes encountered here to the tectono-thermal regime. We propose that processes forming the Eger Graben also thermally influence the crust beneath the Oberpfalz. These mechanisms should have a considerable impact on the maximum depth of the deep borehole. The results of the pilot hole give strong support to our interpretation: the temperatures are higher than the maximum estimates from near-surface heat flow determinations and the maximum depth for the borehole has been reduced from 14 to 10 km.

Thermal uplift and erosion across the continent-ocean transform boundary of the southern Exmouth Plateau

Thermal evolution of the continental lithosphere at a continent-ocean transform margin is examined using a two-dimensional heat conduction model. All heating is assumed to result from the emplacement of new oceanic ridge against the continent. The assumed initial continental temperature gradient is probably best applied to regions having experienced finite-duration periods of rifting lasting tens of millions of years. A deep seismic reflection profile across the southern paleo-transform margin of the Exmouth Plateau (northwest Australia) tests the predictions of the model. Using this reflection profile, it is estimated that up to 3.5 km of sediments have been eroded from the continental rim, diminishing to almost no erosion at 60 km from the continent-ocean transform boundary. This trend and these values for erosion can be matched approximately using the model under the condition of local isostasy. A finite-difference scheme is employed, where the surface elevation is the result of the competing (1) thermal uplift (up), (2) surficial erosion (down), and (3) local isostatic rebound (up) in response to the erosion. The model predicts that most of the erosion ceases by 40 Ma after ridge emplacement and that of the order of 1000 km 3 eroded sediments are shed for every 10 km of transform length.

On the structure of the Ivrea‐Verbano Zone (northern Italy) and its implications for present‐day lower continental crust geometry

ABSTRACTThe Ivrea‐Verbano Zone in northern Italy represents a section through the lower continental crust which has been tilted and emplaced into its present position during the Alpine orogeny. Recent and on‐going structurally‐oriented geological mapping in this region is providing new information about the geometry of the complex. The central part of the zone is dominated by a large basic complex (the ‘mafic formation') which is intrusive into the surrounding gneisses. The foliation within the envelope of gneisses is deflected around the intrusive complex as if by ballooning, but in the region south‐west of Monte Capio both units are folded together into a tight to isoclinal steeply plunging fold with an amplitude of c. 10 km. This fold locally inverts the stratigraphy of the layered basic group of the complex, and is thought to be the result of gravitational collapse following intrusion and inflation of a large magma body into the lower crust.Several high‐temperature shear zones have now been traced within the country rock for distances up to 20 km. The geometry of these, and their relationship to the basic complex suggests that at least some of the extensional collapse of the mafic body is related to uplift caused by intrusion of this body.Close parallels can be drawn between the observed structure in the Ivrea‐Verbano Zone (after removing the effects of late, low‐temperature faulting and folding related to emplacement of the rocks into their present position), and those inferred from deep seismic reflection profiling in areas of current extension such as parts of the US Basin and Range province.

The evolution of the layered lower crust and the Moho through geological time in Western Europe: contribution of deep seismic reflection profiles

ABSTRACTDeep seismic reflection images from a set of profiles shot in Western Europe have been reviewed and compared, and tentative conclusions have been proposed concerning the evolution of the layered lower crust and the Moho. The disappearance of Variscan mountain roots is related to the set‐up of a new Moho at a typical 30‐km depth and the creation of seismic layering in the lower crust. Deep seismic profiles suggest that these processes resulted, at least in part, from magmatic intrusion, partial crustal melting and metamorphism of deep crustal rocks into eclogite. On the other hand, the layered lower crust is greatly attenuated beneath Cretaceous basins and Tertiary rifts in relation to prominent Moho upwellings. The unusual amplitude of the Moho reflection and the presence of anomalously high seismic velocities in the lowermost crust beneath the Tertiary rifts suggest that the Moho and part of the layering are comparatively young features related to interactions between crust and mantle. Beneath Triassic‐Jurassic basins, the layered lower crust was not affected by the subsidence of the basement, with the whole crustal thinning being entirely concentrated in the upper crust. This indicates that the layered lower crust and the Moho were formed or restored during or after the main rifting phase. Seismic data reveal constraints on the processes that affect the crust‐mantle transition and seem to restore the Moho to its typical depth after any mechanical deformation of the lithosphere.

Some observations on basin evolution and dynamics

Sedimentary basin evolution is intimately bound in with continental lithosphere dynamics. Deep seismic reflection profiling has revealed images of the crust and upper mantle that suggest linkages between the shape and structure of sedimentary basins as they evolve with the rheological responses to tectonic stress of upper and lower crust and upper mantle components of the lithosphere. This is well illustrated by the European Geotraverse lithosphere profile from North Cape, Norway to Tunisia. The Alpine mountain belt has a lithospheric root as well as a crustal root. By virtue of its asymmetric disposition, this may be the cause of compressive and tensile forces in close association in the upper crust, which have created mountain uplift and basin subsidence, extension and thrusting, all in juxtaposition. Evidence for an elastic upper layer of crust above a plastic substratum can be used as a basis for modelling basin geometry, structure and stratigraphy built upon the flexural nature of the upper crust. Such models go a long way towards simulating the evolution of basins such as the North Sea Viking Graben. In more detail, analogue structural models can reveal the geometry of basin fill in relationship to crustal movement, and the relationships between structure and sedimentation patterns. However, sedimentary basin modelling needs to begin with the quantitative simulation of depositional environments in relation to such factors as tectonics, sea level variations, sediment supply and erosion, in order to match sedimentary architecture. Only then can the range of deposits be placed in space and time within a basin environment, as building blocks within a dynamically evolving structural framework, to create predictive models for comparison with geological reality.

A deep seismic reflection profile across the Nova Scotia continental margin, offshore eastern Canada

Results from two deep seismic reflection lines are presented. When combined, these lines span the rifted continental margin off Nova Scotia, from crust unaltered by rifting to the ocean basin. These data provide crustal and upper mantle reflection geometry to depths of over 50 km and elucidate the rifting process on this margin which occurred during the Mesozoic breakup of Pangaea. The continental crust below the continental shelf and slope becomes progressively thinner toward the ocean–continent boundary. In the upper crust, normal faults accommodated Mesozoic extension, and these flatten and terminate at 5–6 s (two-way time). In the lower crust and upper mantle Mesozoic rifting may be reflected in dipping events, which are interpreted to be normal faults. All Mesozoic extensional faulting could be controlled by the preexisting fabric of the crust, which in this region would be related to Appalachian compression within the Meguma Terrane. Below the continental rise, there is some evidence for magmatic underplating of the thinned continental crust, but the presence of synrift diapiric salt prevents clear definition of deeper structure. The extreme seaward end of the profile lies in a region interpreted in most other, earlier studies to be oceanic in nature. However, the seismic profile described here shows that relief on basement is associated with listric normal faults, which flatten in decollement, and that linear, landward-dipping intrabasement reflections characterize the area. These features can be explained in either a continental or an oceanic context.

Crustal kinematics and deep structure of the northern Rhine Graben, Germany

Surface and subsurface geological data, unpublished information gleaned from industrial seismic surveys, and results from a deep reflection seismic profile across the Tertiary northern Rhine Graben indicate that crustal extension in excess of 6 km was achieved by asymmetric detachment of the upper crust at a depth of about 15 km. Comparison of a simple geometric‐kinematic cross‐sectional model and its depth extrapolation with the DEKORP‐9‐N deep crustal reflection data suggests that extension below the graben was achieved by slip along a gently westward dipping shear zone which truncated and offset lower crustal planar fabrics and which continues into the uppermost mantle. The planar lower crustal reflectors probably originated mainly during late Variscan (Permocarboniferous) crustal extension and/or magmatic underplating.

Collision deformation across the Iapetus Suture Zone observed from deep-seismic reflection profiles

Collision deformation across the Iapetus Suture Zone observed from deep-seismic reflection profiles

The BMR regional seismic line across the Amadeus Basin, central Australia: implications for the tectonics of the Basin and for hydrocarbon exploration

In 1985 the Bureau of Mineral Resources undertook deep seismic reflection and refraction profiling experiments across the Early Proterozoic to Late Palaeozoic Amadeus Basin and Arunta Block in central Australia. The experiments were designed to test various models of intracratonic basin formation which involved tectonic interaction with the surrounding basement. The seismic reflection results show the thrust belt at the northern margin of the Amadeus Basin to be dominated by a major, south-directed, basement thrust feature, the Redbank Thrust Zone, which was imaged to mantle depths. Other, more southerly, basement-cored thrusts appear to splay southwards from the Redbank Thrust Zone towards the Amadeus Basin. The seismic reflection results support a 'thick-skinned' Laramide or Wind River style of overthrusting for the final compressive Late Devonian-Carboniferous event, the Alice Springs Orogeny. It was the accumulation of thick, synorogenic conglomeratic sediments in a foreland-like setting at the time of thrusting that appears to have been the dominant control on maturation of the underlying Early Ordovician source rocks. In contrast to the northern margin, in the central and southern parts of the Amadeus Basin, the seismic reflection results indicate north-directed, 'thin-skinned' overthrusting on shallow detachments that were accompanied by Jura-style box folding. The detachments sole out in a salt horizon near the base of the succession. Major repetition of part of the succession has occurred, particularly at the leading northern edge of the overthrust region. Basement involvement at the southern basin margin is enigmatic, but appears to involve both Late Proterozoic and Devonian-Carboniferous tectonism.

Crustal structure of the central and southern North Sea from BIRPS deep seismic reflection profiling

Through the generous gift of shiptime on M/V Mobil Search in June 1987, BIRPS (British Institutions Reflection Profiling Syndicate) was able to acquire nearly 1500 km of deep seismic profiles recorded to 16s TWT in a regional survey across the central and southern North Sea. Reflective lower crust beneath the southern North Sea basin gives way laterally to a zone of mid-crustal reflections within the London–Brabant platform. These mid-crustal reflectors are interpreted as due to crustal shearing, although whether related to stretching during basin formation or to some earlier event remains enigmatic. Sub-crustal, SW-dipping reflections are observed at the edge of the London–Brabant platform. Across the mid-North Sea High, N–S profiles show distinctive patterns of lower-crustal reflectivity that indicate lateral variability in structure and character on a scale of 30–50 km. These are interpreted as crustal units relating to the Iapetus suture developed during the Caledonian orogeny. They may have been modified, however, during Carboniferous basin development, or subsequently. The absence of any distinctive sub-crustal reflectors associated with the Iapetus Suture, where a Caledonian subduction zone should have existed, tends to suggest that sub-crustal reflections observed elsewhere around Britian are not subduction related but are more likely to be from younger faults or shear zones relating to lithosphere extension and basin formation. Onshore wide-angle recording in northern England of the marine seismic source produced good quality data. The high resolution obtained has revealed details of mid-crust (PcP) and Moho (PmP) reflections, from around 18 and 30 km depth respectively, not observed before in this area. Both reflectors show events of laterally variable reflectivity that appear as bands of discontinuous reflection segments.