Wide-angle Refraction Experiments Research Articles

The deep seismic structure of the Earth's crust along the Antarctic Peninsula—A summary of the results from Polish geodynamical expeditions

A summary of the results of four Polish geophysical expeditions, which constituted an extensive programme of seismic wide-angle refraction experiments in the northern Antarctic Peninsula region between 1979 and 1991, is analysed here. The results include the interpretation of 20 deep seismic sounding profiles located along the western coast of the Antarctic Peninsula. Additionally, a few shallow seismic profiles in the Deception Island area and a total of 10 reflection profiles from the Bransfield Strait and Drake Passage area were carried out. Crustal velocity models extending across the Antarctic Peninsula continental shelf between the Adelaide Island and the Bransfield Strait show a typical continental crustal structure, with crustal thicknesses of 36–42km near the coast that decreases to 25–28km beneath the outer continental shelf. Farther north in the Bransfield Strait region, the models document a southeastward dip of the Moho discontinuity from a depth of 12km beneath the South Shetland Trench to 40km under the northern tip of the Antarctic Peninsula. Beneath the trough of the Bransfield Strait, a high-velocity body with P-wave velocities exceeding 7.0km/s was detected in a depth range of 6–32km.

Wide-angle refraction experiments in the Daito Ridges region at the northwestern end of the Philippine Sea plate

Three large bathymetric highs (from north to south: the Amami Plateau, the Daito Ridge, and the Oki-Daito Ridge) originating from paleo-island arcs characterize the northwestern end of the Philippine Sea plate. We obtained 10 seismic refraction and multi-channel seismic reflection profiles across and along these bathymetric highs and obtained P wave velocity (Vp) models of the crust and the uppermost mantle. Although there are large variations in the crustal structure throughout this region, these bathymetric highs usually have a middle crust with Vp of 6.3 to 6.8 km/s, a lower crust with Vp of 6.8 to 7.2 km/s, a Pn velocity of 7.6 to 7.8 km/s, and a total crustal thickness of 15 to 25 km. These features are similar to those of the Izu-Ogasawara (Bonin)-Mariana island arc and the Kyushu-Palau Ridge, which are immature paleo-island arcs. However, the crust at the southwestern part of the Oki-Daito Ridge contains a relatively thin middle crust and a smaller total crustal thickness compared with other ridges in this region. In addition, we identified a deep reflector beneath the ridge, with these properties indicating a different origin, such as intraplate volcanism.

The structure of the crust in TESZ area by kriging interpolation

A precise 3D model of the crust is necessary to start any tectonic or geodynamic interpretation. It is also essential for seismic interpretations of structures lying below as well as for correct analysis of shallow structures using reflection seismics. During the last decades, a number of wide-angle refraction experiments were performed on the territory of central and eastern Europe (POLONAISE’97, CELEBRATION 2000, SUDETES 2003), resulting in many high quality 2D models. It is an interesting and complicated transition zone between Precambrian and Palaeozoic Platforms. This paper presents 3D model of the velocity distribution in the crust and upper mantle interpolated from 2D models of the structure along 33 profiles. The obtained model extends to a depth of 50 km and accurately describes the main features of the crustal structures of Poland and surrounding areas. Different interpolation techniques (Kriging, linear) are compared to assure maximum precision. The final model with estimated uncertainty is an interesting reference of the area for other studies.

The fine-scale structure of upper continental lithosphere from seismic waveform methods: insights into Phanerozoic crustal formation processes

SUMMARY Studies of the fine-scale structure of the continental lithosphere using seismic coda, associated with reflections from within the crust and the uppermost mantle, have almost solely been based on the compressional wavefield, observed at near vertical to wide-angle incidence. Here the results from a full waveform analysis of both P- and S-wave coda, obtained during a crustal scale wide-angle refraction experiment, from southwest Ireland are presented. The basic foundation for the study is a 2-D model for the P- and S-wave structure of the crust and its chemical composition, based on traveltime inversion of first arrivals and reflections for the Irish Caledonides. This sector of the Caledonian orogenic belt straddles the boundary between the Laurentian and Avalonian tectonic plates, along the Iapetus Suture Zone. The excellent quality and relatively broad frequency range (e.g. 1–20 Hz) of the P- and S-wave coda permits finer structure in the upper 40 km of lithosphere to be resolved than would be possible using just primary seismic phases. A series of 1-D waveform calculations are performed to systematically build a model for the fine-scale structure of the 32-km-thick crust and the uppermost mantle lithosphere. Petrophysical and geochemical data from felsic lower-crustal xenoliths from Central Ireland place limits on the permitted velocity fluctuations. Additional constraints are imposed by vertical P-wave reflections observed on contiguous offshore seismic reflection profiles. The results indicate that the uppermost mantle to a depth of ca. 40 km consists of a vertically stacked sequence of alternating high (Vp∼ 8.0 km s−1) and low (Vp∼ 7.25 km s−1) velocity layers. Each layer is about 500 m thick with a Vp/Vs ratio ∼1.73. These layers are interpreted as a series of lens-shaped mafic sill complexes, intruded into the subcrustal lithosphere. The fine structure within the lower crust consists of similar seismic velocity fluctuations (Vp∼ 6.4 and 7.1 km s−1) but on a scale of 200–300 m thickness. Velocity variations within the lower crust are consistent with the petrophysical properties of metapelitic, orthogneissic and psammitic lithologies, from lower-crustal xenoliths in Central Ireland, and with similar exhumed sections of lower crust in southern Europe. These results support tectonic theories for the origin of the crust by tectonic accretion of predominantly sedimentary and volcaniclastic material derived from oceanic, island-arc and continental margin sources during the Caledonian orogenic cycle. A conceptual model for crustal accretion and growth involves partial melting of the accreted sedimentary wedge, producing granites and a chemically differentiated crust during the late Caledonian (latest Silurian/early Devonian). The heat source that drove melting of the accreted sediments may have been provided by the interpreted mafic sills intruded into the uppermost mantle during oblique late Caledonian (sinistral) collision of the Laurentian and Avalonian plates.

Scales of structure in the lithosphere — images of processes

Advances in Deep-Seismic Sounding observations of the lithosphere on continents allow to identify various scales of heterogeneities in the crust-mantle transition. In this paper a fresh look is taken at the dynamic properties of seismic waves propagating through this zone ranging from the lower crust well into the lithospheric mantle as observed in near-vertical reflection and wide-angle refraction experiments. The abrupt termination of near-vertical reflections at the Moho and the coincident presence of a strong coda, accompanying the supercritical PMP reflection phase in record sections of continental seismic wide-angle refraction experiments, reveal a new property of the crust-mantle boundary. The Moho forms a sandwiched mix of crust-mantle material between lower crust and topmost mantle in combination with a step in mean velocity, a significant change in scale of the structural dimensions and of velocity variance. This view is strongly supported by the recognition of the subcrustal lithosphere as a high-frequency (5–10 Hz) scattering wave-guide for long-distance propagation of the Pn phase over more than 3000 km, detected during seismic experiments in Russia using Peaceful Nuclear Explosions. Evidence is brought forward that this high-frequency wave-guide is likely to be present also in the Phanerozoic upper mantle of western Europe.It is furthermore hypothesized that the global propagation of teleseismic Sn or corresponding Pn waves observed on continents and oceans combined with the recording of strong PMP-codas for wave paths on continents points to a world-wide change of scale of structure and velocities near the Moho. A change in scale of heterogeneities under almost all physical and chemical conditions indicates possibly a differential horizontal movement or decoupling of crust and upper mantle at the level of the Moho. The global presence of the scattering Pn wave-guide with a preferred horizontal elongation is hypothetically described as a seismic image of processes in the lithospheric mantle. Locations of present-day tectonic activity where horizontal decoupling between crust and mantle are likely to occur (e.g., subduction at continent-continent collision zones, change of tectonic regimes and continental transform faults) are discussed.

The deep crust of the Southern Rhine Graben: reflectivity and seismicity as images of dynamic processes

The Rhine Graben, part of the European Cenozoic rift system, deserves special attention because of its location in the foreland of the Alpine orogen. The Phanerozoic evolution of the lithosphere in this region is defined by a set of major geodynamic events ranging from the Variscan orogeny, late-orogenic crustal re-equilibration to the interference of rifting and Alpine orogeny in Tertiary times.The Rhine Graben is one of the most detailedly studied continental grabens. Prospecting for hydrocarbons in its sedimentary fill and intensive pre-site studies for the Continental Deep Drilling Program (KTB) on its eastern crystalline flank (Black Forest) provided a comprehensive set of geoscience data. Seismic investigations ranging from a deep seismic network of near-vertical reflection to concurrent wide-angle refraction experiments were accompanied by seismological and geological surveys. Therefore, a direct observational comparison between the two depth definitions of crustal subdivisions, elastic vs. strength/rheological, was possible. In large parts of Western Europe deep seismic sounding (DSS) investigation distinguishes between a crystalline upper crust of comparatively low P-wave velocities (5.9–6.0 km/s) with little and discrete reflectivity and a strongly reflective lower crust with higher P-wave velocities (about 6.4–6.8 km/s). Both are separated by the Conrad discontinuity. In contrast, strength and rheological considerations define a brittle upper crust and a ductile lower crust. The boundary between these two realms is usually referred to as the brittle/ductile transition zone which in seismically active regions is imaged by the deepest crustal hypocentres. The present brittle/ductile transition in the Rhine Graben region, as defined by the deepest hypocentres, does not necessarily coincide with the structural separation in an upper and lower crust as seen in deep seismic studies. This suggests that the laminated lower crust developed during the crustal re-equilibration phase at the end of the Variscan orogeny.The second part of this paper concentrates on evidence of recent tectonic activities related to rifting processes impacting on the Variscan lower crust. A remarkable concentration of deep crustal earthquakes reaching to the top of an asymmetrical thinned lower crust is observed at or near the eastern border fault of the southern Rhine Graben and is possibly induced by the young rifting process.We suggest that this asymmetry could be a consequence of the uppermost mantle detaching from the upper crust in collision with the Alpine orogen. This allows us to propose the distinction between the influence of compression, extension, continental subduction and intra-crustal detachment during graben formation. The dynamic interference of rifting and foreland compression shown by structural and seismological observations leads to the notion of small-scale collisional escape tectonics associated with an intra-continental transform fault at the southern end of the Rhine Graben, the Rhine-Saoˆne Transform zone.