Wave Propagation In A Vertical Transversely Isotropic Medium: Field Experiment and Model Study

Summary. The continent-ocean transition adjacent to Hatton Bank was studied using a dense grid of single-ship and two-ship multichannel seismic profiles. The interpretation of the explosive expanding spread profiles (ESPs) which were shot as part of this survey are discussed here in detail. Extensive seaward dipping reflectors are developed in the upper crust across the entire margin. These seaward dipping reflectors continue northwards on the Faeroes and Vøring margins, where they have been shown to be caused by basaltic lavas, as well as on the conjugate margin of East Greenland. The dipping reflectors are an important feature of the rifting history of the margin and show that extensive volcanism was associated with the extension. The ESPs show clear seismic arrivals out to ranges of 100 km. Wide-angle Moho reflections can be seen on all the lines as well as good mid and lower crustal arrivals. The determination of seismic velocity structure was constrained by ray tracing and by amplitude modelling using reflectivity synthetic seismograms. The results from the ESPs show that there is a thick region of lower crustal material beneath the margin with an unusually high crustal velocity of 7.3–7.4 km s⁻¹. This lower crustal material reaches a maximum thickness of 14 km beneath the central part of the margin and is terminated at depth by the Moho. The lower crustal lens of high-velocity material is interpreted as underplated or intruded igneous rocks associated with the large volumes of extrusive basaltic lavas, now seen as dipping reflectors on the margin.     

The Hatton Bank continental margin—I. Shallow structure from two-ship expanding spread seismic profiles

Summary. The Hatton Bank passive continental margin exhibits thick seaward dipping reflector sequences which consist of basalts extruded during rifting between Greenland and Rockall Plateau. Multichannel seismic reflection profiling across the margin reveals three reflector wedges with a maximum thickness near 7 km, extending from beneath the upper continental slope to the deep ocean basin. We present results of the velocity structure within the dipping reflector sequences at eight locations across the margin, interpreted by synthetic seismogram modelling a set of multichannel expanding spread profiles parallel to the margin. At the top of some reflector sequences, we observe a series of 100 m thick high- and low-velocity zones, which are interpreted as basalt flows alternating with sediments or weathered and rubble layers. At the profile locations, the base of the dipping reflectors correlates with P -wave velocities near 6.5 km s⁻¹. However, elsewhere the reflectors appear to extend significantly deeper than the inferred 6.5 km s⁻¹ velocity contour, indicating that the velocity structure may not be controlled solely by lithological boundaries but also by metamorphic effects. Shear-waves were observed on two lines, permitting the calculation of Poisson's ratio. The decrease in Poisson's ratio from 0.28 to near 0.25 in the upper 5 km of crust may also indicate the effect of metamorphism on seismic properties, or alternatively may be explained by crack closure under load.     

Thinning of the Goban Spur continental margin and formation of early oceanic crust: constraints from forward modelling and inversion of marine magnetic anomalies

The deep seismic reflection profile Western Approaches Margin (WAM) cuts across the Goban Spur continental margin, located southwest of Ireland- This non-volcanic margin is characterized by a few tilted blocks parallel to the margin. A volcanic sill has been emplaced on the westernmost tilted block. The shape of the eastern part of this sill is known from seismic data, but neither seismic nor gravity data allow a precise determination of the extent and shape of the volcanic body at depth. Forward modelling and inversion of magnetic data constrain the shape of this volcanic sill and the location of the ocean-continent transition. The volcanic body thickens towards the ocean, and seems to be in direct contact with the oceanic crust. In the contact zone, the volcanic body and the oceanic magnetic layer display approximately the same thickness. The oceanic magnetic layer is anomalously thick immediately west of the volcanic body, and gradually thins to reach more typical values 40 km further to the west. The volcanic sill would therefore represent the very first formation of oceanic crust, just before or at the continental break-up. The ocean-continent transition is limited to a zone 15 km wide. The continental magnetic layer seems to thin gradually oceanwards, as does the continental crust, but no simple relation is observed between their respective thinnings.     

The continent‐ocean transition on the northwestern South China Sea

Rifted margins are created as a result of stretching and breakup of continental lithosphere that eventually leads to oceanic spreading and formation of a new oceanic basin. A cornerstone for understanding what processes control the final transition to seafloor spreading is the nature of the continent‐ocean transition (COT). We reprocessed multichannel seismic profiles and use available gravity data to study the structure and variability of the COT along the Northwest subbasin (NWSB) of the South China Sea. We have interpreted the seismic images to discern continental from oceanic domains. The continental‐crust domain is characterized by tilted fault blocks generally overlain by thick syn‐rift sedimentary units, and underlain by fairly continuous Moho reflections typically at 8–10 s twtt. The thickness of the continental crust changes greatly across the basin, from ~20 to 25 km under the shelf and uppermost slope, to ~9–6 km under the lower slope. The oceanic‐crust domain is characterized by a highly reflective top of basement, little faulting, no syntectonic strata and fairly constant thickness (over tens to hundreds of km) of typically 6 km, but ranging from 4 to 8 km. The COT is imaged as a ~5–10 km wide zone where oceanic‐type features directly abut or lap on continental‐type structures. The South China margin continental crust is cut by abundant normal faults. Seismic profiles show an along‐strike variation in the tectonic structure of the continental margin. The NE‐most lines display ~20–40 km wide segments of intense faulting under the slope and associated continental‐crust thinning, giving way to a narrow COT and oceanic crust. Towards the SW, faulting and thinning of the continental crust occurs across a ~100–110 km wide segment with a narrow COT and abutting oceanic crust. We interpret this 3D structural variability and the narrow COT as a consequence of the abrupt termination of continental rifting tectonics by the NE to SW propagation of a spreading centre. We suggest that breakup occurred abruptly by spreading centre propagation rather than by thinning during continental rifting. We propose a kinematic evolution for the oceanic domain of the NWSB consisting of a southward spreading centre propagation followed by a first narrow ridge jump to the north, and then a younger larger jump to the SE, to abandon the NWSB and create the East subbasin of the South China Sea.     

A deep seismic profile of 800 km length recorded in southern Queensland, Australia

Summary. In 1984, the Australian Bureau of Mineral Resources and the Geological Survey of Queensland recorded a regional seismic reflection profile of over 800 km length from the eastern part of the Eromanga Basin to the Beenleigh Block east of the Clarence Moreton Basin. A relatively transparent upper crustal basement with an underlying, more reflective lower crust is characteristic of much of the region. Prominent westerly dipping reflectors occur well below the sediments of the eastern margin of the Clarence Moreton Basin and the adjacent Beenleigh Block, and provide some of the most interesting features of the entire survey. A wide angle reflection/refraction survey of 192 km length and an expanding reflection spread of 25 km length were recorded across the Nebine Ridge. The only clear deep reflectors are interpreted as P-to-SV or SV-to-P converted reflections from a mid-crustal boundary at a depth of about 17 km. The combined Nebine Ridge data provide well-constrained P and S wave velocity models of the upper crust, and suggest a crustal structure quite different from that beneath the adjacent Mesozoic basins.     

Crustal structure beneath exposed accreted terranes of Southern Alaska

Summary. The crustal structure beneath the exposed terranes of southern Alaska has been explored using coincident seismic refraction and reflection profiling. A wide-angle reflector at 8–9 km depth, at the base of an inferred low-velocity zone, underlies the Peninsular and Chugach terranes, appears to truncate their boundary, and may represent a horizontal decollement beneath the terranes. The crust beneath the Chugach terrane is characterized by a series of north-dipping paired layers having low and high velocities that may represent subducted slices of oceanic crust and mantle. This layered series may continue northward under the Peninsular terrane. Earthquake locations in the Wrangell Benioff zone indicate that at least the upper two low-high velocity layer pairs are tectonically inactive and that they appear to have been accreted to the base of the continental crust. The refraction data suggest that the Contact fault between two similar terranes, the Chugach and Prince William terranes, is a deeply penetrating feature that separates lower crust (deeper than 10 km) with paired dipping reflectors, from crust without such reflectors.     

Seismic reflection measurements behind the Hikurangi convergent margin, southern North Island, New Zealand.

Summary. The active Australian-Pacific plate boundary passes through New Zealand. In the north, the Pacific plate subducts beneath the Australian plate with an accretionary wedge forming the eastern continental (Hikurangi) margin of the North Island. The structure of the region behind the Hikurangi margin changes from the extensional back-arc basin under central North Island to a postulated crustal downwarp under the southern North Island. A 100 km long multichannel seismic reflection profile was recorded across the region of crustal downwarp. The data show discontinuous coherent reflectors dipping westwards at the east end of the profile, and east dipping reflectors at the west end, from depths of 9 to 15 s two way time. Simple hand migration of these events indicate that the east dipping reflectors, interpreted as the base of the Australian plate crust, abut against the west dipping reflectors which are interpreted as marking the top of the subducted Pacific plate. Detailed earthquake hypocentre locations in the area show a dipping zone of high seismicity, the top of which coincides closely with the west dipping events, thus supporting this interpretation.     

The transition from subduction to continental collision: crustal structure in the North Canterbury region, New Zealand

The North Canterbury region marks the transition from Pacific plate subduction to continental collision in the South Island of New Zealand. Details of the seismicity, structure and tectonics of this region have been revealed by an 11-week microearthquake survey using 24 portable digital seismographs. Arrival time data from a well-recorded subset of microearthquakes have been combined with those from three explosions at the corners of the microearthquake network in a simultaneous inversion for both hypocentres and velocity structure. The velocity structure is consistent with the crust in North Canterbury being an extension of the converging Chatham Rise. The crust is about 27 km thick, and consists of an 11 km thick seismic upper crust and 7 km thick seismic lower crust, with the middle part of the crust being relatively aseismic. Seismic velocities are consistent with the upper and middle crust being composed of greywacke and schist respectively, while several lines of evidence suggest that the lower crust is the lower part of the old oceanic crust on which the overlying rocks were originally deposited.
The distribution of relocated earthquakes deeper than 15 km indicates that the seismic lower crust changes dip markedly near 43S. To the south-west it is subhorizontal, while to the north-east it dips north-west at about 10. Fault-plane solutions for these earthquakes also change near 43S. For events to the south, P -axes trend approximately normal to the plate boundary (reflecting continental collision), while for events to the north, T -axes are aligned down the dip of the subducted plate (reflecting slab pull). While lithospheric subduction is continuous across the transition, it is not clear whether the lower crust near 43S is flexed or torn.     

Evidence for underthrusting beneath the Queen Charlotte Margin, British Columbia, from teleseismic receiver function analysis

The Queen Charlotte Fault zone is the transpressive boundary between the North America and Pacific Plates along the northwestern margin of British Columbia. Two models have been suggested for the accommodation of the ∼20 mm yr⁻¹ of convergence along the fault boundary: (1) underthrusting; (2) internal crustal deformation. Strong evidence supporting an underthrusting model is provided by a detailed teleseismic receiver function analysis that defines the underthrusting slab. Forward and inverse modelling techniques were applied to receiver function data calculated at two permanent and four temporary seismic stations within the Queen Charlotte Islands. The modelling reveals a ∼10 km thick low-velocity zone dipping eastward at 28° interpreted to be underthrusting oceanic crust. The oceanic crust is located beneath a thin (28 km) eastward thickening (10°) continental crust.     

Pre-drilling reflection survey of the Black Forest, SW Germany

Summary. The unified seismic exploration program, consisting of 345 km of deep reflection profiling, a 200 km refraction profile, an expanding spread profile and near-surface high resolution reflection meaasurements, revealed a strongly differentiated crust beneath the Black Forest. The highly reflective lower crust contains numerous horizontal and dipping reflectors at depths of 13-14 km down to the crust-mantle boundary (Moho). The Moho appears as a flat horizontal first order discontinuity at a relatively shallow level of 25–27 km above a transparent upper mantle. From modelling of synthetic near-vertical and wide-angle seismograms using the reflectivity method the lower crust is supposed to be composed of laminae with an average thickness of about 100 m and velocity differences of greater than 10% increasing from top to bottom. The upper crust is characterised by mostly dipping reflectors, associated with bivergent underthrusting and accretion tectonics of Variscan age and with extensional faults of Mesozoic age. A bright spot at 9.5 km depth is characterised by low velocity material suggesting a fluid trap. It appears on all of the three profiles in the centre of the intersection region. The upper crust seems to be decoupled from the lowest crust by a relatively transparent zone which is' also identified as a low-velocity zone. This low velocity channel is situated directly above the laminated lower crust. The laminae in the Rhinegraben area are displaced vertically to greater depths indicating an origin before Tertiary rift formation and a subsidence of the whole graben wedge.     

The Agulhas Plateau: structure and evolution of a Large Igneous Province

Large Igneous Provinces (LIP) are of great interest due to their role in crustal generation, magmatic processes and environmental impact. The Agulhas Plateau in the southwest Indian Ocean off South Africa has played a controversial role in this discussion due to unclear evidence for its continental or oceanic crustal affinity. With new geophysical data from seismic refraction and reflection profiling, we are able to present improved evidence for its crustal structure and composition. The velocity–depth model reveals a mean crustal thickness of 20 km with a maximum of 24 km, where three major units can be identified in the crust. In our seismic reflection records, evidence for volcanic flows on the Agulhas Plateau can be observed. The middle crust is thickened by magmatic intrusions. The up to 10 km thick lower crustal body is characterized by high seismic velocities of 7.0–7.6 km s⁻¹. The velocity–depth distribution suggests that the plateau consists of overthickened oceanic crust similar to other oceanic LIPs such as the Ontong-Java Plateau or the northern Kerguelen Plateau. The total volume of the Agulhas Plateau was estimated to be 4 × 10⁶ km³ of which about 10 per cent consists of extruded igneous material. We use this information to obtain a first estimate on carbon dioxide and sulphur dioxide emission caused by degassing from this material. The Agulhas Plateau was formed as part of a larger LIP consisting of the Agulhas Plateau itself, Northeast Georgia Rise and Maud Rise. The formation time of this LIP can be estimated between 100 and 94 (± 5) Ma.     

The seismic structure of thinned continental crust in the northern Bay of Biscay

Summary. The stretching and thinning of the continental crust, which occurs during the formation of passive continental margins, may cause important changes in the velocity structure of such crust. Further, crust attenuated to a few kilometres' thickness, can be found underlying 'oceanic' water depths. This paper poses the question of whether thinned continental crust can be distinguished seismically from normal oceanic crust of about the same thickness. A single seismic refraction line shot over thinned continental crust as part of the North Biscay margin transect in 1979 was studied in detail. Tau— p inversion suggested that there are differences between oceanic and continental crust in the lower crustal structure. This was confirmed when synthetic seismograms were calculated. The thinned continental crust (β± 7.0) exhibits a two-gradient structure in the non-sedimentary crust with velocities between 5.9 and 7.4 km s⁻¹; an upper 0.8 s⁻¹ layer overlies a 0.4 s⁻¹ layer. No layer comparable to oceanic layer 3 was detected. The uppermost mantle also contains a low-velocity zone.     

Lithoprobe seismic reflection profiling across Vancouver Island: results from reprocessing

Summary. Multichannel seismic reflection sections recorded across Vancouver Island have revealed two extensive zones of deep seismic reflections that dip gently to the northeast, and a number of moderate northeasterly dipping reflections that can be traced to the surface where major faults are exposed. Based on an integrated interpretation of these data with information from gravity, heat flow, seismicity, seismic refraction, magnetotelluric and geological studies it is concluded that the lower zone of gently dipping reflections is due to underplated oceanic sediments and igneous rocks associated with the current subduction of the Juan de Fuca plate, and that the upper zone represents a similar sequence of accreted rocks associated with an earlier episode of subduction. The high density/high velocity material between the two reflection zones is either an underplated slab of oceanic lithosphere or an imbricated package of mafic rocks. Reprocessing of data from two of the seismic lines has produced a remarkable image of the terrane bounding Leech River fault, with its dip undulating from >60° near the surface to 20° at 3 km depth and ∼38° at 6 km depth.     

Crustal structure across the Grand Banks–Newfoundland Basin Continental Margin – II. Results from a seismic reflection profile

New multichannel seismic reflection data were collected over a 565 km transect covering the non-volcanic rifted margin of the central eastern Grand Banks and the Newfoundland Basin in the northwestern Atlantic. Three major crustal zones are interpreted from west to east over the seaward 350 km of the profile: (1) continental crust; (2) transitional basement and (3) oceanic crust. Continental crust thins over a wide zone (∼160 km) by forming a large rift basin (Carson Basin) and seaward fault block, together with a series of smaller fault blocks eastwards beneath the Salar and Newfoundland basins. Analysis of selected previous reflection profiles (Lithoprobe 85-4, 85-2 and Conrad NB-1) indicates that prominent landward-dipping reflections observed under the continental slope are a regional phenomenon. They define the landward edge of a deep serpentinized mantle layer, which underlies both extended continental crust and transitional basement. The 80-km-wide transitional basement is defined landwards by a basement high that may consist of serpentinized peridotite and seawards by a pair of basement highs of unknown crustal origin. Flat and unreflective transitional basement most likely is exhumed, serpentinized mantle, although our results do not exclude the possibility of anomalously thinned oceanic crust. A Moho reflection below interpreted oceanic crust is first observed landwards of magnetic anomaly M4, 230 km from the shelf break. Extrapolation of ages from chron M0 to the edge of interpreted oceanic crust suggests that the onset of seafloor spreading was ∼138 Ma (Valanginian) in the south (southern Newfoundland Basin) to ∼125 Ma (Barremian–Aptian boundary) in the north (Flemish Cap), comparable to those proposed for the conjugate margins.     

A wide-angle seismic traverse through the Variscan of southwest Ireland

A wide-angle seismic profile across the western peninsulas of SW Ireland was performed. This region corresponds to the northernmost Variscan thrust and fold deformation. The dense set of 13 shots and 109 stations along the 120  km long profile provides a detailed velocity model of the crust.
  The seismic velocity model, obtained by forward and inverse modelling, defines a five-layer crust. A sedimentary layer, 5–8  km thick, is underlain by an upper-crustal layer of variable thickness, with a base generally at a depth of 10–12  km. Two mid-crustal layers are defined, and a lower-crustal layer below 22  km. The Moho lies at a depth of 30–32  km. A low-velocity zone, which coincides with a well-defined gravity low, is observed in the central part of the region and is modelled as a Caledonian granite which intruded upper-crustal basement. The granite may have acted as a buffer to northward-directed Variscan thrusting. The Dingle–Dungarvan Line (DDL) marks a major change in sedimentary and crustal velocity and structure. It lies immediately to the north of the velocity and gravity low, and shows thickness and velocity differences in many of the underlying crustal layers and even in the Moho. This suggests a deep, pre-Variscan control of the structural development of this area. The model is compatible with thin-skinned tectonics, which terminated at the DDL and which incorporated thrusts involving the sedimentary and upper-crustal layers.     

Crustal structure of Atlantic fracture zones–II. The Vema fracture zone and transverse ridge

Summary. The crustal structure beneath the Vema fracture zone and its flanking transverse ridge was determined from seismic refraction profiles along the fracture zone valley and across the ridge. Relatively normal oceanic crust, but with an upwarped seismic Moho, was found under the transverse ridge. We suggest that the transverse ridge represents a portion of tectonically uplifted crust without a major root or zone of serpentinite diapirism beneath it. A region of anomalous crust associated with the fracture zone itself extends about 20 km to either side of the central fault, gradually decreasing in thickness as the fracture zone is approached. There is evidence to suggest that the thinnest crust is found beneath the edges of the 20 km wide fracture zone valley. Under the fracture zone valley the crust is generally thinner than normal oceanic crust and is also highly anomalous in its velocity structure. Seismic layer 3 is absent, and the seismic velocities are lower than normal. The absence of layer 3 indicates that normal magmatic accretionary processes are considerably modified in the vicinity of the transform fault. The low velocities are probably caused by the accumulation of rubble and talus and by the extensive faulting and fracturing associated with the transform fault. This same fracturing allows water to penetrate through the crust, and the apparently somewhat thicker crust beneath the central part of the fracture zone valley may be explained by the resultant serpentinization having depressed the seismic Moho below its original depth.     

Crustal structure of the Whipple Mountains, southeastern California: a refraction study across a region of large continental extension

Summary. Closely spaced refraction profiling across the Whipple Mountains metamorphic core complex in southeastern California yields a complex picture of crustal structure in this region of large continental extension. A NE-directed profile, parallel to the extension direction, reveals a high-velocity mid-crustal layer (6.6–6.8 km s⁻¹) at 16-18 km depth, bounded above and below by laterally discontinuous low-velocity zones (<6.0 km s⁻¹). In marked contrast, a NW-directed profile shows a more uniform 6.0 km s⁻¹ crust down to the crust-mantle boundary. The apparent contrast between these two perpendicular profiles may be related not only to a more complex geologic structure in the NW-SE direction, but also to velocity anisotropy associated with mid-crustal mylonites. Despite the differences between the two refraction profiles, both define a flat Moho at 26-27 km depth with an associated upper mantle-velocity of 7.8 km s⁻¹. This observation is significant as it suggests that, although the amount of extension has been highly variable regionally, the crust is no thinner beneath the Whipple Mountains (where extension has been extreme) than the surrounding mountain ranges. Such an observation requires either that the crust was considerably thicker prior to extension, or that lateral flow in the lower crust and/or inflation of the crust via magmatism occurred contemporaneous with extension.     

Expanding spread profile at the northern Jan Mayen Ridge

An expanding spread seismic profile at the central northern Jan Mayen Ridge, ESP-5, has yielded a crustal seismic velocity distribution which is similar to observations from the thinned continental crust at the Norwegian continental margin. The profile reveals a post-early Eocene sedimentary sequence, about 1. 5 km thick, overlying 1 km of volcanic extrusives and interbedded sediments. Below, there are about 3 km of pre-opening sediments above the seismic basement. The results indicate that the main ridge block is underlain by a thinned crust, possibly only 13.5 km thick. The results are compatible with a continental nature for the main ridge complex.     

Seismic refraction profiles between Cyprus and Israel and their interpretation

Summary. A long seismic refraction profile was carried out between southern Israel and Cyprus. The seismic energy was generated by 33 sea shots each of 0.8 t explosives and was recorded by land stations in Israel and Cyprus and by ocean bottom seismographs deployed along the profile.
The results showed that the continental crust of southern Israel thins towards the Mediterranean underneath a northward thickening sedimentary cover. Cyprus is underlain by a 35 km thick continental crust thinning south-wards and extending to Mt Eratosthenes. Between Mt Eratosthenes and the Israel continental shelf the crystalline crust is composed of high velocity (6.5 km s^-1)material and is about 8 km thick. It is covered by 12–14 km of sediments and may represent a fossil oceanic crust.     

Crustal structure of the French Guiana margin, West Equatorial Atlantic

Geophysical data from the Amazon Cone Experiment are used to determine the structure and evolution of the French Guiana and Northeast Brazil continental margin, and to better understand the origin and development of along-margin segmentation. A 427-km-long combined multichannel reflection and wide-angle refraction seismic profile acquired across the southern French Guiana margin is interpreted, where plate reconstructions suggest a rift-type setting.
The resulting model shows a crustal structure in which 35–37-km-thick pre-rift continental crust is thinned by a factor of 6.4 over a distance of ∼70  km associated with continental break-up and the initiation and establishment of seafloor spreading. The ocean–continent boundary is a transition zone up to 45  km in width, in which the two-layered oceanic-type crustal structure develops. Although relatively thin at 3.5–5.0  km, such thin oceanic crust appears characteristic of the margin as a whole.
There is no evidence of rift-related magmatism, either as seaward-dipping sequences in the reflection data or as a high velocity region in the lower crust in the P -wave velocity model, and as a such the margin is identified as non-volcanic in type. However, there is also no evidence of the rotated fault block and graben structures characteristic of rifted margins. Consequently, the thin oceanic crust, the rapidity of continental crustal thinning and the absence of characteristic rift-related structures leads to the conclusion that the southern French Guiana margin has instead developed in an oblique rift setting, in which transform motion also played a significant role in the evolution of the resulting crustal structure and along-margin segmentation in structural style.