Marek Grad

Complex lithospheric structure under the central Baltic Shield from surface wave tomography

Complex lithospheric structure under the central Baltic Shield from surface wave tomography

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.

EUROBRIDGE: New insight into the geodynamic evolution of the East European Craton

Abstract The Palaeoproterozoic crust and upper mantle in the region between the Ukrainian and Baltic shields of the East European Craton were built up finally during collision of the previously independent Fennoscandian and Sarmatian crustal segments at c. 1.8-1.7 Ga. EUROBRIDGE seismic profiling and geophysical modelling across the southwestern part of the Craton suggest that the Central Belarus Suture Zone is the junction between the two colliding segments. This junction is marked by strong deformation of the crust and the presence of a metamorphic core complex. At 1.80-1.74 Ga, major late to post-collisional extension and magmatism affected the part of Sarmatia adjoining the Central Belarus Zone and generated a high-velocity layer at the base of the crust. Other sutures separating terranes of different ages are found within Sarmatia and in the Polish-Lithuanian part of Fennoscandia. While Fennoscandia and Sarmatia were still a long distance apart, orogeny was dominantly accretionary. The accreted Palaeoproterozoic terranes in the Baltic-Belarus region of Fennoscandia are all younger than 2.0 Ga (2.0-1.9, 1.90-1.85 and 1.84-1.82 Ga), whereas those in Sarmatia have ages of c. 2.2-2.1 and 2.0-1.95 Ga. Lithospheric deformation and magmatism at c. 1.50-1.45 Ga, and Devonian rifting, are also defined by the EUROBRIDGE seismic and gravity models.

Heat flow models across the Trans-European Suture Zone in the area of the POLONAISE’97 seismic experiment

Abstract Heat flow data from the Polish basin show a sharp change in the transition from the East European Craton (EEC) and Teisseyre–Tornquist Zone (TTZ) in the north-east to the accreted terranes in the south west (Paleozoic Platform). The analysis of this data and numerical modelling of the crustal temperatures show evidence of extensive crustal–mantle warming in the area between the Sudetes to the south and the Trans-European Suture Zone to the north. The change in heat flow is 100% when compared with values for the EEC. Heat flow in the anomalous zone is also higher than in the Sudetes. The axis of the anomaly is aligned with the Dolsk Fault and Variscan deformation front. Low crustal/mantle temperatures derived from the relationship between temperature and P n velocities (more than 8.2 and as high as 8.4 km/s) are at odds with high crustal temperatures calculated from surface heat flow, seismic velocity based heat generation models and thermal conductivity. High heat flow (Variscan platform) and related high temperatures of the crust coincide with small crustal thickness (30–35 km). The opposite is the case for the low heat flow EEC (45–50 km). High heat flow above thin crust and low heat flow above thick crust with no major variation in elevation is supported by a simple isostatic balance model. Crustal heat generation explains part of the high heat flow within the zone with thick meta-sediments reaching down to 20 km depth, however, it is far from explaining high heat flow in Variscan crust and in the transition zone into a cold EEC. 2D numerical models of heat flow based on new seismic data require a contrast of 15 mW/m 2 in mantle heat flow. High mantle heat flow (35–40 mW/m 2 ) is likely to occur in the high heat flow zone while cold crust and cold and high-density mantle (mantle heat flow of 20–30 mW/m 2 ) is typical of the EEC. Thermal lithosphere thickness for the craton is 200 km while it is only 100 km in the accreted terranes to the southwest of the TTZ. The TTZ in Poland appears as a relatively cold area.

ALP 2002 seismic experiment

The ALP 2002 was organized as an international seismic experiment whose scientific objective is to further scientific understanding of the structure and evolution of the lithosphere in the Eastern Alps and surrounding areas. The ALP 2002 experiment included passive seismic monitoring and an active source seismic refraction experiment. Furthermore, local high-density deployments were carried out in Austria and Hungary to investigate local geologic problems. All data will be integrated with the goal of better understanding the geodynamic processes currently at work and the complex tectonic history of this region.

Seismic refraction evidence for crustal structure in the central part of the Trans-European Suture Zone in Poland

Abstract The results of seismic investigations obtained for the Trans-European Suture Zone (TESZ) show the presence of relatively low velocity rocks (Vp < 6.1 kms−1), of sedimentary, metamorphic or volcanic origin, down to a depth of 20 km; high velocity (Vp = 6.8–7.3 kms−1) lower crust, the Moho at a depth of approximately 30–33 km; and a high-velocity (Vp > 8.3 kms −1) uppermost mantle. The transition of the crustal structure is seen across a 200 km wide zone. The three-layered crystalline crust of Baltica changes over this distance into the two-layered crust of Palaeozoic (Variscan) Europe, due to the disappearance of the lowest layer (Vp ∼ 7.1 kms−1) and tapering off of the Baltican/cratonic wedge. The seismic profiles suggest that the lower crust (Vp ∼ 7.1 kms−1) in the transition zone represents the attenuated Baltica margin underthrust towards the SW beneath the Avalonian accretionary wedge. The latter corresponds to the low-velocity upper crust (Vp < 6.1 kms−1) characteristic of the German-Polish Caledonides. Consequently, the high-velocity reflective lower crust of Baltica affinity extends approximately 200 km to the SW of the Teisseyre-Tornquist Zone within the basement of the Palaeozoic Platform. The Avalonian upper/middle crust is confined in the SW against the WNW-ESE trending Dolsk Fault. To the SW of the Odra Fault, a typical Variscan crust is detected which shows two-layer structure and relatively low P-wave velocities. The WNW-ESE Odra Fault, approximately parallel to the Dolsk Fault, splits the Variscan domain into the Variscan externides buried beneath the Palaeozoic Platform in the NE and the Variscan internides of the Sudetes in the SW. We interpret both the Odra and Dolsk Faults as dextral strike-slip features that cross cut the NE termination of the Variscan Orogen parallel to the Teisseyre-Tornquist Zone. In a relatively small area, they juxtapose three crustal domains representing, successively, the Variscan internides, externides and the Variscan foreland.

Upper crustal structure of Deception Island area, Bransfield Strait, West Antarctica

This paper describes the results of seismic refraction investigations of the upper crustal structure in the area of Deception Island, West Antarctica, which were made during the Polish Antarctic Geodynamical Expeditions in 1979-80 and 1987-88. In the caldera and immediate vicinity of Deception Island a layer of unconsolidated and poorly consolidated young sediments of 1.9-2.2 km s −1 P-wave velocity was found. Velocities of 4.1-4.3 km s −1 were found in the depth interval from 0.6-1.3 to about 3 km. Lateral differences in upper crustal structure between the south-eastern and western sectors were identified

Seismic velocities and Q-factors in the uppermost crust beneath the SVEKA profile in Finland

The SVEKA deep seismic sounding profile in Central Finland was shot in 1981 to study the crustal structure of the Fennoscandian shield in the contact zone between the Archaean and Svecokarelian provinces. Apart from P-and S-waves, well developed Rayleigh surface waves (R-waves) with periods of 0.5–2.0 s were recorded from all the shot points. In the frequency band 0.5–2.5 Hz the R-waves were clearly observed on the entire 325 km long profile. Amplitudes of P-, S- and R-waves corrected for charge size were used to study the attenuation of waves and Q-factors. From the records of body and R-waves the distributions of seismic velocities and quality factors were determined using the dynamic ray-tracing method (2-D modelling) and the reflectivity method (1-D modelling). In the 2-D model of the uppermost crust, the P-wave velocities vary from 5.8–5.9 km/s at the top to 6.2–6.3 km/s at a depth of 6 km. The velocity ratio solVpVs varies with depth and distance from 1.67 to 1.74, reaching its maximum value of 1.8–2.0 in the uppermost 200–500 m and values of 1.76–1.80 in the uppermost 1 km. The quality factors Qp and Qs vary in the uppermost 1 km from 20 to 120, reaching a higher value of 140 in the marginal zone of the Archaean basement. In the depth range of 1–2 km Q-values of 80–400 were found. In the depth range 2–6 km Q-values of 300–800 were found for the Svecokarelian province and 200–400 for the Archaean. The low values of Vs and Q in the uppermost 1 km are thought to be the result of rock fracturing. The crack density, ϵ, was estimated for the uppermost crust along the profile. For the SVEKA profile we found ϵ-values of 0.35–0.54 in the uppermost 200 m, 0.10–0.32 at a depth of 0.2-1.0 km and ϵ < 0.1 for depths of 1–4 km. The comparison of our seismic data with results of theoretical and laboratory investigations implies that the uppermost crustal rocks are saturated. The zones of strong attenuation of R-waves coincide with faults and schist zones which are well known from surface geology.

Seismic studies of the crustal structure in West Antarctica 1979–1980—Preliminary results

Abstract The Polish Geophysical Expedition to West Antarctica in the summer of 1979–1980 was organized by the Institute of Geophysics of the Polish Academy of Sciences. The purpose of the expedition was to carry out studies of deep structures of the Earths crust by reflection, refraction and deep seismic sounding methods. Special attention was paid to tectonically active zones and to the contact zones between the blocks of the Earths crust and the lithospheric plates. Geophysical measurements were carried out in the area extending between 61° and 65°S and between 56° and 66°W. The measurements covered the southern Shetlands, the Antarctic Peninsula, the Bransfield Strait, the Drake Passage, the Palmer Archipelago, the Gerlache Strait and the Bismarck Strait towards the southern Pacific. Deep seismic soundings were made along profiles with a total length of about 2000 km. Seismic reflection measurements were made along profiles about 1100 km long. A detailed analysis of the seismic wave field shows that the structure of the Earths crust in this part of West Antarctica is very complex. Numerous deep fractures divide the Earths crust into blocks of different physical properties. The thickness of the Earths crust changes from 32 km in the region of the South Shetland Islands to 40–45 km in the region of the Antarctic Peninsula. A preliminary geodynamical model of this part of West Antarctica is presented.

Crustal seismic velocity structure of southern Poland: preserved memory of a pre-Devonian terrane accretion at the East European Platform margin

The updated geological and potential fields data on the East European Platform margin in SE Poland confirm the existence of several regional units differing in Ediacaran to Silurian development: the Upper Silesian Block, Malopolska Block and Łysogory Block. All the blocks are characterized by a distinct crustal structure seen in Vp velocity models obtained from the seismic refraction data of the CELEBRATION 2000 Programme. The first two units are interpreted as exotic terranes initially derived from Avalonia-type crust and ultimately accreted before the late Early Devonian. The Łysogory Block is probably a proximal terrane displaced dextrally along the Baltica margin. The sutures between the terranes do not precisely match lateral gradients in Vp models. This is partly explained by a limited resolution of refraction seismic data (20 km wide interpretative window). Most of the difference is related, however, to a post-accretionary tectonism, mainly Variscan transtension–transpression. The latter processes took advantage of lithospheric memory recorded earlier as zones of rheological weakness along the former suture zones. The course of the East European Platform margin (= Teisseyre–Tornquist Zone) corresponds most likely to the Nowe Miasto–Zawichost Fault marking the NE boundary of the proximal Łysogory Terrane.