P. Środa

P- and S-wave velocity model of the southwestern margin of the Precambrian East European Craton; POLONAISE'97, profile P3

The large-scale seismic experiment POLONAISE’97 investigated the seismic structure of the crust and the uppermost mantle in the Polish region of the Trans-European Suture Zone. This paper covers the interpretation of seismic data along the 300-km-long P3 profile, which is located in the Precambrian East European Craton ( EEC ) parallel to the Teisseyre‐Tornquist Zone. The recordings were of high quality, with seismic energy detectable out to 300 km oVsets. The crustal model developed by two-dimensional raytracing forward modelling and waveform analysis is characterized by a subhorizontal uniform structure, although the boundaries of several large basement features are crossed. The crystalline crust consists of three parts: the upper, middle and lower crust with P-wave velocities of 6.1‐ 6.4 km/s, 6.55‐6.7 km/s and 7.05‐7.15 km/s respectively. The P m P wave can usually be correlated at distances beginning at about 100 km, but it has variable reflection character. Moho depth increases from 38 km at the NW end of the profile to 44 km at the SE end. The P n velocity of 8.05‐8.1 km/s is less than that previously found in neighbouring areas of the EEC. The V P /V S ratio was determined separately for the upper/middle and lower crust to be 1.67 and 1.77 respectively. Following the P n wave, another phase with an apparent velocity of about 8.3 km/ si s interpreted as a weak reflection from a low-contrast discontinuity in the uppermost mantle.

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.

Moho Depth along the Antarctic Peninsula and Crustal Structure across the Landward Projection of the Hero Fracture Zone

Results of deep seismic soundings collected during four Polish Geodynamical Expeditions to West Antarctica between 1979 and 1991 were synthesised to produce a map of Moho depth beneath the NW coast of the Antarctic Peninsula. In this paper, we present a new interpretation of the deep landward projection of the Hero Fracture Zone based on two seismic transects. On each transect we found high velocity bodies with Vp>7.2 km s−1, similar to ones we detected previously in Bransfield Strait. However, these bodies are not continuous; they are separated by a zone of lower velocities located SW of Deception Island. In Bransfield Strait an asymmetric “mushroom”-shaped high velocity body was found at a depth interval from 13–18 km down to the Moho boundary at a depth of ca. 30 km. As a summary of results, we present a map of Moho depth along the coast of the Antarctic Peninsula, prepared using previous seismic 2D models. The map shows variations in crustal thickness from 38–42 km along the Antarctic Peninsula shelf in the southern part of the study area to 12–18 km beneath the South Shetland Trench.

Lithospheric structure of Central Europe: Puzzle pieces from Pannonian Basin to Trans‐European Suture Zone resolved by geophysical‐petrological modeling

We have analyzed the thermochemical structure of the mantle in Central Europe, a complex area with a highly heterogeneous lithospheric structure reflecting the interplay of contraction, strike slip, subduction, and extension tectonics. Our modeling is based on an integrative 3-D approach (LitMod) that combines in a self-consistent manner concepts and data from thermodynamics, mineral physics, geochemistry, petrology, and solid Earth geophysics. This approach minimizes uncertainties of the estimates derived from modeling of various data sets separately. To further constrain our 3-D model we have made use of the vast geophysical and geological data (2-D and 3-D, shallow/crustal versus deep lithospheric experiments) based on experiments performed in Central Europe in the past decades. Given the amount and the different nature/resolution of the available constraints, one of the most challenging tasks of this study was to consistently combine them, finding a trade-off between all local and regional data sets available in a way that (i) preserves as many structural details as possible and (ii) summarizes those data sets into a single robust regional model. The resulting P/T-dependent mantle densities are in LitMod 3-D calculated based on a given mineralogical composition. They therefore provide more reliable estimates compared to pure gravity models, which enhance modeling of the crustal structures. Our results clearly indicate presence of several lithospheric domains characterized by distinct features, Pannonian Basin being one of the most outstanding ones. It has the thinnest crust and lithosphere in the area modeled, characterized by relatively fertile composition.

Comment on the Seismic Method Depth-Recursive Tomography on Grid (DRTG) Developed by Miroslav Novotný and Recently Published in Three Papers in Surveys in Geophysics

The comment addresses three papers published recently in Surveys in Geophysics. These papers are related, using the same seismic tomography approach developed by the same first author. They deal with modelling of seismic refraction crustal data in the Bohemian Massif and their geological interpretation. Novotny ´ (2011) presents a P-wave velocity model based on tomography along the refraction profile CEL09 of the CELEBRATION 2000 experiment; Novotny ´ (2012) presents a geological interpretation of this model.

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.