Wojciech Czuba

Transcurrent nature of the Teisseyre–Tornquist Zone in Central Europe: results of the POLCRUST-01 deep reflection seismic profile

Teisseyre–Tornquist Zone (TTZ) corresponds to a crustal boundary between the Precambrian East European Platform (EEP) and the Palaeozoic West European Platform. Although the zone has been controlling Phanerozoic evolution of large parts of Central Europe, its course, geometry and origin are still poorly constrained. Deep reflection seismic profile POLCRUST-01, recently acquired in SE Poland, for the first time allowed a precise comparison of the Ediacaran and later tectonic patterns to the deep crustal features of the TTZ and adjacent areas. The TTZ corresponds to the subvertical Tomaszów Fault separating the Radom–Kraśnik Elevation, composed of the typical EEP crust, from the Biłgoraj–Narol Block (BNB) in the SW, with a thinned crystalline basement showing affinities to the EEP crust. The BNB is a part of the larger Caledonian Łysogóry Terrane as evidenced by its Lower Palaeozoic stratigraphy and gravity data. Thus, for the first time, the proximal Baltican affinity of this unit has been documented unambiguously. The Łysogóry Terrane is delimited from the SW by the subvertical Cieszanów Fault Zone, corresponding to the Holy Cross Suture. The adjacent Małopolska Terrane is characterized by a distinct Early Palaeozoic stratigraphy, and lower-middle crust exhibiting SW-dipping reflective packages interpreted as NE-verging thrust and shear zones of a Neoproterozoic orogen. The observations from the POLCRUST-01 profile and regional comparisons indicate that the TTZ is a major Caledonian transcurrent zone between Poland and East Romania. In central Poland, the TTZ likely forms a narrow subvertical contact between the EEP and a proximal Kuiavia Terrane, as constrained by the deep refraction seismic data. To the NW, the zone extends towards the Pomeranian part of the Caledonide fold-and-thrust belt related to the Avalonia–Baltica collision zone (Thor Suture). South-east of Poland the TTZ corresponds to the Rava Ruska Fault Zone established as a Caledonian suture separating adjacent terrane, probably of a Baltican affinity. The East Romanian part of the TTZ conforms with the Sfântu Gheorghe Fault separating reworked EEP crust of the Pre-Dobrogean Depression from the North Dobrogea unit bearing a strong Variscan and Cimmerian overprint.

Seismic view on the svalbard passive continental margin

Deep seismic sounding measurements were performed in the continent-ocean transition zone of the western Svalbard and Barents Sea margin, during the expeditions in 1985–2008. Seismic energy (airgun and TNT shots) was recorded along several profiles by onshore seismic stations and ocean bottom seismometers, and hydrophone systems. Good quality reflected and refracted P waves provided an excellent data base for a seismic modelling along the profiles. TNT sources were recorded even up to 300 km distances. A minimal depth of about 6 km of the Moho interface was found east of the Molloy Deep. The Moho discontinuity dips down to 28 km beneath the continental part of the northernmost profile and down to maximum 32 km beneath other profiles. The evolution of the region is considered to be within a shearrift tectonic setting. The continent-ocean transition zone along the northernmost profile is mostly dominated by extension; therefore, the last stage of the development of the margin can be classified as rifting. The uplifted Moho interface close to the Molloy Deep can be interpreted as a south-western end of the Molloy Ridge. The margin of the southern Spitsbergen is rather of sheared character while the western Barents Sea margin is of slow to ultraslow spreading type.

Crustal and upper mantle velocity model along the DOBRE-4 profile from North Dobruja to the central region of the Ukrainian Shield: 2. geotectonic interpretation

This part of the paper addresses the geotectonic interpretation of the velocity model obtained from the results of seismic studies under the DOBRE-4 project in Ukraine. The velocity field does not show distinct lateral changes from the Precambrian platform towards the younger tectonic structures in the southwest. Hence, based on the seismic data alone, it is not possible to recognize the tectonic units that are known on the surface. The Moho has an undulating pattern over an interval with a length of ~150 km. The amplitude of the undulations reaches 8 to 17 km. The similar wavelike behavior, although on a shorter spatial scale and lower amplitude, is also typical of the upper crust and upper mantle. The presence of several separate horizons in the folded crust revealed by the velocity model is consistent with the presence of the folded systems which have different extensions on the different depth levels in the Earth’s crust. This situation is believed to be typical of folding on the lithospheric scale and to reflect the rheological stratification of the crust. The DOBRE-4 velocity section of the crust and adjacent part of the mantle promotes a clearer view of the geodynamical model describing the formation of the southwestern part of East European Platform in the Early Precambrian from the plate’s tectonic standpoint.

Crustal and upper mantle velocity model along the DOBRE-4 profile from North Dobruja to the central region of the Ukrainian Shield: 1. seismic data

For studying the structure of the lithosphere in southern Ukraine, wide-angle seismic studies that recorded the reflected and refracted waves were carried out under the DOBRE-4 project. The field works were conducted in October 2009. Thirteen chemical shot points spaced 35–50 km apart from each other were implemented with a charge weight varying from 600 to 1000 kg. Overall 230 recording stations with an interval of 2.5 km between them were used. The high quality of the obtained data allowed us to model the velocity section along the profile for P- and S-waves. Seismic modeling was carried out by two methods. Initially, trial-and-error ray tracing using the arrival times of the main reflected and refracted P- and S-phases was conducted. Next, the amplitudes of the recorded phases were analyzed by the finite-difference full waveform method. The resulting velocity model demonstrates a fairly homogeneous structure from the middle to lower crust both in the vertical and horizontal directions. A drastically different situation is observed in the upper crust, where the Vp velocities decrease upwards along the section from 6.35 km/s at a depth of 15–20 km to 5.9–5.8 km/s on the surface of the crystalline basement; in the Neoproterozoic and Paleozoic deposits, it diminishes from 5.15 to 3.80 km/s, and in the Mesozoic layers, it decreases from 2.70 to 2.30 km/s. The subcrustal Vp gradually increases downwards from 6.50 to 6.7–6.8 km/s at the crustal base, which complicates the problem of separating the middle and lower crust. The Vp velocities above 6.80 km/s have not been revealed even in the lowermost part of the crust, in contrast to the similar profiles in the East European Platform. The Moho is clearly delineated by the velocity contrast of 1.3–1.7 km/s. The alternating pattern of the changes in the Moho depths corresponding to Moho undulations with a wavelength of about 150 km and the amplitude reaching 8 to 17 km is a peculiarity of the velocity model.

Various Approaches to Forward and Inverse Wide-Angle Seismic Modelling Tested on Data from DOBRE-4 Experiment

The interpretation of seismic refraction and wide angle reflection data usually involves the creation of a velocity model based on an inverse or forward modelling of the travel times of crustal and mantle phases using the ray theory approach. The modelling codes differ in terms of model parameterization, data used for modelling, regularization of the result, etc. It is helpful to know the capabilities, advantages and limitations of the code used compared to others.This work compares some popular 2D seismic modelling codes using the dataset collected along the seismic wide-angle profile DOBRE-4, where quite peculiar/uncommon reflected phases were observed in the wavefield.The ~505 km long profile was realized in southern Ukraine in 2009, using 13 shot points and 230 recording stations. Double PMP phases with a different reduced time (7.5–11 s) and a different apparent velocity, intersecting each other, are observed in the seismic wavefield. This is the most striking feature of the data. They are interpreted as reflections from strongly dipping Moho segments with an opposite dip. Two steps were used for the modelling. In the previous work by Starostenko et al. (2013), the trial-and-error forward model based on refracted and reflected phases (SEIS83 code) was published. The interesting feature is the high-amplitude (8–17 km) variability of the Moho depth in the form of downward and upward bends. This model is compared with results from other seismic inversion methods: the first arrivals tomography package FAST based on first arrivals; the JIVE3D code, which can also use later refracted arrivals and reflections; and the forward and inversion code RAYINVR using both refracted and reflected phases. Modelling with all the codes tested showed substantial variability of the Moho depth along the DOBRE-4 profile. However, SEIS83 and RAYINVR packages seem to give the most coincident results.

Continental Passive Margin West of Svalbard and Barents Sea in Polish Arctic Seismic Studies

Deep seismic sounding measurements were performed in the continent-ocean transition zone of the western Svalbard and Barents Sea margin, during the Polish–international expeditions in 1976–2008. Seismic energy (airgun and TNT shots) was recorded along several profiles by onshore seismic stations, ocean bottom seismometers (OBS) and hydrophone systems (OBH). Good quality reflected and refracted P waves provided an excellent data base for a seismic modelling along the profiles. TNT sources were recorded up to 300 km distances. A minimal depth of about 6 km of the Moho interface was found east of the Molloy Deep. The Moho discontinuity dips down to 28 km beneath the continental part of the northernmost profile and down to maximum 32 km beneath other profiles. The evolution of the region is considered to be within a shear-rift tectonic setting.