E. Perchuć

Deep structure of the Earth's crust in the contact zone of the Palaeozoic and Precambrian Platforms in Poland (Tornquist-Teisseyre zone)

Abstract One of the major tectonic problems in Europe concerns the southwest margin of the East European Platform in the region of the so-called Polish-Danish trough. In general, this margin is assumed to be the Tornquist-Teisseyre (T-T) Line, running approximately from northwest to southeast in this part of Europe. Determination of deep crustal structure of the contact zone between the Precambrian Platform and the Palaeozoic Platform was the main aim of the deep seismic sounding (DSS) programme in Poland in 1965–1982. Deep seismic soundings of the Earths crust have been made in the T-T Line zone along nine profiles with a total length of about 2600 km. The results of deep seismic soundings have shown that the crust in the marginal zone of the East European Platform has highly anomalous properties. The width of this zone ranges from 50 km in northwest Poland to about 90 km in southeast Poland. The crustal thickness of the Palaeozoic Platform in Poland is 30–35 km, and of the Precambrian Platform 42–47 km, while in the T-T tectonic zone it varies from 50 to 55 km. Above the Moho boundary, in the T-T zone, at a depth of 40–45 km, there is a seismic discontinuity with P-wave velocities of 7.5–7.7 km/s. Boundary velocities, mean velocities and stratification of the Earths crust vary distinctly along the T-T zone. There are also observed high gravimetric and magnetic anomalies in the T-T zone. The T-T tectonic zone determined in this manner is a deep tectonic trough with rift properties. The deep fractures delineating the T-T tectonic zone are of fundamental importance for the localization of the plate edge of the Precambrian Platform of eastern Europe. In the light of DSS results, the northeastern margin of the T-T tectonic zone is a former plate boundary of the East European Platform.

Seismic reflectivity and magmatic underplating beneath the Kenya Rift

The lower crust around the Kenya Rift is generally reflective in wide-angle seismic sections. Remarkably, high amplitude reflections of low frequency originate from underneath the rift, whereas weaker reflections of high frequency prevail from outside the rift. This indicates thicker layering and larger reflection coefficients in the lower crust beneath the rift than outside it. Petrologically, magmatic intrusions are compatible with the thick layering beneath the rift axis, and the associated large reflection coefficients are indicative of their cumulate layering and fractionation. Hence, the observed thinning of the crust below the rift may be substantially less than the real mechanical thinning due to the addition of intrusive or underplated material.

New map compiled of Europe's gravity field

A gravity anomaly map of Europe was recently compiled that incorporates significant new data, especially from the eastern European countries. The map (Figure 1a), which was developed by a group of geophysicists and geodesists, goes a long way toward establishing a common and reliable database. Geological information contained in the regional gravity field are highlighted in the map, and all known large-scale geological structures are well coordinated with the anomaly patterns. The map offers basic gravity information for geophysical studies related to subsurface geology and large-scale tectonic features in Europe and reinforces or supports previous interpretations of largescale tectonic features.

Tomographic inversion of seismic P- and S-wave velocities from the Baltic Shield based on FENNOLORA data

Abstract Tomographic travel time inversion of seismic compressional (P) and shear (S) wave data from the long-range deep seismic sounding experiment Fennoscandian Long Range (FENNOLORA) reveals the velocity structure of the crust and upper mantle in the Baltic Shield. Pronounced scattering and delay in travel times of seismic P- and S-wave phases together with strong attenuation of S-wave phases beyond ca. 800-km offset are attributed to a low-velocity zone (LVZ) below the 8° discontinuity at a depth of ca. 100 km. Travel time inversion of P- and S-wave first arrivals shows that the 8° discontinuity represents the top of a zone with negative or very small vertical velocity gradients and a Vp/Vs ratio of 1.74–1.77. We observe clear, linear refracted P-wave phases (i.e. the Lehmann refraction) at offsets beyond 1100–1300 km, which suggest that the base of the low-velocity zone is at ca. 150-km depth in the Baltic Shield. No refracted S-wave phases are observed beyond 1200-km offset, which we attribute to strong S-attenuation within the zone below the 8° discontinuity. These features are interpreted by the presence of small amounts of partial melts, almost molten rocks or possibly free fluids, in the 100–150-km depth interval. Local variations in the Vp/Vs ratio of the crust and uppermost mantle correlate with Proterozoic terranes, which collided and were amalgamated during the Precambrian plate tectonic events that led to the assemblage of the Baltic Shield.

Seismic scattering at the top of the mantle Transition Zone

We document strong seismic scattering from around the top of the mantle Transition Zone in all available high resolution explosion seismic profiles from Siberia and North America. This seismic reflectivity from around the 410 km discontinuity indicates the presence of pronounced heterogeneity in the depth interval between 320 and 450 km in the Earth’s mantle. We model the seismic observations by heterogeneity in the form of random seismic scatterers with typical scale lengths of kilometre size (10–40 km by 2–10 km) in a 100–140 km thick depth interval. The observed heterogeneity may be explained by changes in the depths to the α–β–γ spinel transformations caused by an unexpectedly high iron content at the top of the mantle Transition Zone. The phase transformation of pyroxenes into the garnet mineral majorite probably also contributes to the reflectivity, mainly below a depth of 400 km, whereas we find it unlikely that the presence of water or partial melt is the main cause of the observed strong seismic reflectivity. Subducted oceanic slabs that equilibrated at the top of the Transition Zone may also contribute to the observed reflectivity. If this is the main cause of the reflectivity, a substantial amount of young oceanic lithosphere has been subducted under Siberia and North America during their geologic evolution. Subducted slabs may have initiated metamorphic reactions in the original mantle rocks.

Intraplate earthquakes and a seismically defined lateral transition in the upper mantle

We show that the lateral transition in the upper mantle between the cratonic central/eastern and the tectonically active western regions of North America is narrow and deep-reaching by travel time analysis of Early Rise explosion seismic data. A concentration of enigmatic intraplate earthquakes coincides with the transition. Differential potential energy from density changes across the transition may generate a high compressional stress level which, locally, exceeds the shear strength of the lithosphere. The resultant deformation is ductile in most parts of the lithosphere, but in the brittle upper crust it explains the enigmatic intraplate earthquakes around the lateral transition in the upper mantle.

Two Reflectors in the 400 Km Depth Range Revealed from Peaceful Nuclear Explosion Seismic Sections

Evaluation of reversed, seismic sections from Peaceful Nuclear Explosion shot points reveals two interfaces around depths of 365 and 410 km. The relation in amplitude and reflection character between the two reflections depends on location and shot. Preliminary kinematic and dynamic modelling indicates that the depth range between the two reflectors is characterised by low velocities. These observations identify a complicated velocity structure and an anomalous zone above the transition zone.

Processes of lithosphere evolution: new evidence on the structure of the continental crust and uppermost mantle

We discuss the structure of the continental lithosphere, its physical properties, and the mechanisms that formed and modified it since the early Archean. The structure of the upper mantle and the crust is derived primarily from global and regional seismic tomographystudies ofEurasia andfrom global andregional dataonseismic anisotropy. Thesedata asdocumentedin the papersof this special issue of Tectonophysics are used to illustrate the role of different tectonic processes in the lithospheric evolution since Archean to present. These include, but are not limited to, cratonization, terrane accretion and collision, continental rifting (both passiveandactive),subduction,andlithosphericbasalerosionduetoarelativemotionofcratonickeelsandtheconvectivemantle. D 2002 Elsevier Science B.V. All rights reserved.

The Transition from Cold to Hot Areas of North America Interpreted from Early Rise Seismic Record Sections

The long-range seismic sections of the Early Rise experiment identify a characteristic delay of refractions from below the Lehmann discontinuity on most profiles. The delay coincides with a transition from continuous, homogeneous first arrivals to seismic phases, which are scattered in travel time and amplitude. The Early Rise sections were acquired with a common shot point in Lake Superior within the “cold” North American craton. The delay is observed around the expected EW transition from “cold” to “hot” areas, i.e. from high to low average mantle velocity. The more than 150 km deep transition occurs over a narrow horisontal zone of width less than 100 km. It coincides with high-seismicity zones on all profiles, where observed, indicative of direct influence of uppermost mantle structure on crustal faulting.