RomUkrSeis: Seismic model of the crust and upper mantle across the Eastern Carpathians – From the Apuseni Mountains to the Ukrainian Shield
Introduction
This paper presents the results of the RomUkrSeis WARR seismic profile acquired in Romania and Ukraine, in the region from the Apuseni Mountains in the SW, across the Transylvanian Basin and Carpathian Mountains, to the East European platform in the NE (Fig. 1). As such, RomUkrSeis provides an integrated image of a segment of continental crust and upper mantle affected and modified by lithosphere-scale processes from the Archaean until almost the present-day: a sequence including Precambrian cratonisation; continental break-up; Palaeozoic orogenesis and continental accretion leading to Pangaea supercontinent formation followed by large-scale destabilisation until the Mesozoic and ending with subduction, back-arc basin formation and orogenesis in the Cenozoic.
RomUkrSeis uniquely complements a series of WARR-type profiles acquired in Ukraine (including coverage in the neighbouring countries of Poland, Belarus, Hungary and Romania) during the last two decades. The Donbas foldbelt and eastern Black Sea basin were covered by the DOBREfraction’99 (DOBREfraction’99 Working Group, 2003) and DOBRE-2 (Starostenko et al., 2017) profiles. The Carpathian arc in western Ukraine and Hungary was covered by PANCAKE (DOBRE-3; Starostenko et al., 2013a; Verpakhovska et al., 2018). The area from the Pre-Dobrogea Trough across the southern part of the Ukrainian Shield was investigated during the DOBRE-4 experiment (Starostenko et al., 2013b). The Odessa Shelf of the Black Sea and the Crimea Peninsula were covered by the most southern profile, the EW-trending DOBRE-5 profile (Starostenko et al., 2015). The Pripyat-Dnieper-Donets Basin was studied during the EUROBRIDGE’97 (Thybo et al., 2003) and the GEORIFT 2013 (Starostenko et al., 2018) projects.
The RomUkrSeis profile crosses a highly complex crustal transition zone that separates the Baltica proto-continent, comprising the Archaean-Palaeoproterozoic lithosphere of the East European Craton (EEC), and lithospheric domains accreted to it during later, Phanerozoic tectonic episodes (the Palaeozoic Caledonian and Variscan orogenies overprinted by the Mesozoic-Cenozoic Alpine-Carpathian orogeny). This transition zone between Baltica and the subsequently accreted terranes is known as the Teisseyre-Tornquist Zone (TTZ; e.g. Pharaoh, 1999; Pharaoh et al., 2006; Narkiewicz et al., 2015; Grad, 2019) and is located on the inset map of Fig. 2 including its presumed trace beneath the overprinting Alpine-Carpathian geology crossed by RomUkrSeis.
The objective of RomUkrSeis was to investigate the architecture of the sedimentary cover and the structure of the crystalline crust and uppermost mantle of the south-western margin of the EEC (including the adjacent south-western part of the Ukrainian Shield), Carpathian orogen and its foredeep, and the Transylvanian Basin and Apuseni Mountains. The results are to help clarify the spatial extent of the component tectonic units, the evolution and the nature of the boundaries between them and to supply new data for understanding resource potential in this area. A particular interest was crystalline basement affinity and Moho depth along the profile, in particular in the vicinity of the TTZ.
The results, based on state-of-the-art techniques of controlled-source WARR seismic data acquisition and interpretation, add new constraints to the existing geophysical data set of the area and have contributed to a better understanding of the crustal architecture in the lithosphere transition between Ukraine and Romania that is obscured by the overlying accretionary prism of the Outer Carpathians and the Carpathian foredeep.
Section snippets
Geology crossed by the RomUkrSeis profile
The RomUkrSeis profile crosses the following tectonic units from the south-west to the north-east (Fig. 2): the Apuseni Mountains, Transylvanian Basin, Inner Carpathians, Outer Carpathians and the southwestern part of the East European Craton (EEC), adjacent to the southwestern Ukrainian Shield. These Archaean to Cenozoic domains have different tectonic origins and geodynamic histories and the geology of each is briefly described in the following paragraphs.
The tectonic evolution of the region
Potential field and heat flow data in the vicinity of the RomUkrSeis profile
The main tectonic units crossed by the RomUkrSeis profile correspond in varying degrees with the potential field (gravity and magnetic) patterns along the RomUkrSeis profile (Fig. 3). The main feature of the regional Bouguer gravity field (taken from EG-99 data base; Wybraniec et al., 1998) is the strongly negative anomaly (less than −120 mGal) associated with the Carpathian belt and its foredeep. This is caused by the thick flysch complexes of the Eastern Carpathian nappes and presumably
Field experiment and seismic data
RomUkrSeis data were acquired in August 2014. The field programme was carried out by an international consortium involving institutions and personnel from Romania, Ukraine, Poland and Scotland. Recording instruments included 230 “DSS Cubes” provided by the Geophysical Instrument Pool of the Deutsches GeoForschungsZentrum (GFZ), Potsdam, and by the Institute of Geophysics of the Polish Academy of Sciences (deployed in Romania) and 118 RefTek “Texans” from the institutes of Geophysics of the
Ray-tracing modelling strategy
The acquired seismic data and obtained travel-time picks (Fig. 4a,b) served as a basis for forward modelling the crustal and upper mantle structure beneath the RomUkrSeis transect. Modelling was done by trial-and-error using the 2-D ray-tracing SEIS83 package (Červený and Pšenčík, 1984) with the graphical user interfaces MODEL (Komminaho, 1998) and ZPLOT (Zelt, 1994). SEIS83 is based on a high frequency approximation of the wave equation for computation of ray paths, travel-times and synthetic
Interpretation and discussion of the RomUkrSeis velocity model
The velocity structure of the crust and uppermost mantle on the RomUkrSeis profile reveals three main domains, which also correspond to the character of the gravity, magnetic and heat flow anomalies along the profile (Fig. 5). These domains from simplest to most complex are 1) the East European Craton (EEC) and its margin, comprising the south-western part of the Ukrainian Shield and the Volyn-Podolsk Monocline; 2) the southern Apuseni Mountains and the Transylvanian Basin, both thought to have
Precambrian-early Palaeozoic
The EEC segment of the crust and upper mantle lithosphere imaged by RomUkrSeis was accreted and assembled primarily in the Archaean with Palaeoproterozoic tectonic overprinting. By sometime in the late Mesoproterozoic or Neoproterozoic, after one or more earlier global cycles of proto-continental assembly and break-up, it formed part of the supercontinent Rodinia, which itself broke apart in the late Neoproterozoic, leaving behind proto-continent Baltica (the conjugate margin of which is
Summary and conclusions
WARR seismic profile RomUkrSeis is 675 km long and crosses from NE to SW three distinct lithospheric domains of different structure and tectonic evolution: the Archaean old cratonic domain of the East European Craton (EEC; southwestern part of the Ukrainian Shield and the cratonic margin), the Eastern Carpathians and the Teisseyre-Tornquist Zone (TTZ), and the younger composite Tisza-Dacia terrane with the Transylvanian Basin and Apuseni Mountains. The structure of the crust and upper mantle
Authorship credit statement
All listed are authors associated with the institutions that undertook acquisition of the original field data and all were directly engaged in the processing and analyses of these data, including modelling to produce crustal and upper mantle velocity models, and the writing of materials appearing in this manuscript and preparation of all graphical materials that are in it. The senior four authors were those mainly involved in the conceptualisation of the field programme and leading its planning
Declaration of Competing Interest
The authors declare that they have no conflict of interest, financial or otherwise, as authors of this paper.
Acknowledgements
RomUkrSeis was carried out by a consortium of organisations, the Faculty of Geology and Geophysics and the Doctoral School of Geology of the University of Bucharest (Romania), the Subbotin Institute of Geophysics of the National Academy of Sciences of Ukraine (Kiev), the Institute of Geophysics of the Polish Academy of Sciences (Warsaw), the Deutsches GeoForschungsZentrum (Potsdam) and the School of Geosciences of the University of Aberdeen (Scotland). Financial support for drilling/shooting
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