Upper mantle structure around the Trans-European Suture Zone obtained by teleseismic tomography

Abstract

Abstract. The presented study aims to resolve the upper mantle structure around the Trans-European Suture Zone (TESZ) which is the major tectonic boundary in Europe. The data of 183 temporary and permanent seismic stations operated during the period of the PASsive Seismic Experiment PASSEQ 2006–2008 within the study area from Germany to Lithuania was used to compile the dataset of manually picked 6008 top quality arrivals of P waves from teleseismic earthquakes. We used the non-linear teleseismic tomography algorithm TELINV to perform the inversions. As a result, we obtain a model of P wave velocity variations up to about ±3% compared to the IASP91 velocity model in the upper mantle around the TESZ. The higher velocities to the east of the TESZ correspond to the older East European Craton (EEC), while the lower velocities to the west of the TESZ correspond to younger Western Europe. We find that the seismic lithosphere-asthenosphere boundary (LAB) is more distinct beneath the Phanerozoic part of Europe than beneath the Precambrian part. To the west of the TESZ beneath the eastern part of the Bohemian Massif, the Sudetes Mountains and the Eger Rift the negative anomalies are observed from the depth of at least 70 km, while under the Variscides the average depth of the seismic LAB is about 100 km. We do not observe the seismic LAB beneath the EEC, but beneath Lithuania we find the thickest lithosphere of about 300 km or more. Beneath the TESZ the asthenosphere is at a depth of 150–180 km, which is an intermediate value between that of the EEC and Western Europe. The results imply that the seismic LAB in the northern part of the TESZ is of a shape of a ramp dipping to the NE direction. In the southern part of the TESZ the LAB is shallower, most probably due to younger tectonic settings. In the northern part of the TESZ we do not recognize any clear contact between Phanerozoic and Proterozoic Europe, but further to the south we may refer to a sharp and steep contact on the eastern edge of the TESZ. Moreover, beneath Lithuania at the depth of 120–150 km we observe the lower velocity area following the boundary of the proposed palaeosubduction zone.

Upper mantle structure around the Trans-European Suture Zone obtained by teleseismic tomography

“…9c). This general finding coincides with the results by Koulakov et al (2009) who reported the sharp transition along the TESZ from the negative amplitudes, characterizing the young tectonic features of central-western Europe, to positive ones beneath the old EEC. Moreover, a sharp transition from low to high shear-wave velocities between the Phanerozoic Europe and EEC, respectively, was observed from waveform inversion of both body and surface waves by Zielhuis and Nolet (1994).…”

“…The observed velocity perturbations down to about 120 km beneath Poland are about 2 to 3 % higher with respect to the IASP91 velocity model, while going deeper the variations are slightly smaller, which most likely indicates some effects due to the applied crustal TT corrections. The higher velocity values in this area are observed down to about 200 km, which coincides well with the studies by WildePiórko et al (2010), Majorowicz et al (2003) and Koulakov et al (2009). Legendre et al (2012) found the highest velocity values in the mantle of the EEC at about 150 km depth.…”

“…These data are sparse compared to the study area, and the spatial resolution is questionable; however, the thick seismic lithosphere reported by was also found in the area during other studies. Koulakov et al (2009) observed the positive P wave velocity anomaly beneath the EEC down to at least 300 km, which indicates even thicker lithosphere compared to . Legendre et al (2012) find no indications of a deep cratonic root below about 330 km for the EEC, while Geissler et al (2010) do not observe any clear indications of deep seismic LAB beneath the EEC either.…”

“…Geissler et al (2010) indicate the lithosphere thickness of about 115-130 km in the vicinity of the TESZ, while the LAB beneath the southwestern part of the Variscan Bohemian Massif is estimated at a depth of 115 km, and the thin lithosphere of only about 75 km is reported beneath some parts of the Pannonian Basin. Beneath the Bohemian Massif, an extensive low-velocity heterogeneity in the upper mantle is found (Koulakov et al, 2009;Karousova et al, 2013), while the high-resolution tomography studies indicate the most distinct low-velocity perturbations along the Eger Rift down to about 200 km (Karousova et al, 2013). Plomerova et al (2007) interpret the broad low-velocity anomaly beneath the Eger Rift as an upwelling of the LAB.…”

“…While it was found that the cratonic lithosphere extends much deeper than that of the younger continental regions (e.g., Eaton et al, 2009;Shomali et al, 2006;Gregersen et al, 2010), the studies revealed that the structure of the lithosphere and the LAB differs a lot on both sides of the TESZ (e.g., Zielhuis and Nolet, 1994;Majorowicz et al, 2003;Koulakov et al, 2009;WildePiórko et al, 2010). Regarding different physical properties and geophysical techniques, the LAB has different practical definitions: (1) the seismic LAB defines the transition between the solid outer layer of the Earth, which is characterized by higher seismic velocity values, and its interior, which is characterized by lower seismic velocity values; (2) the thermal LAB defines the transition between the outer layer with dominating conductive heat transfer above the convective mantle that usually coincides with a depth of a constant isotherm of about 1300 • C (McKenzie, 1967); (3) the electrical LAB is a transition between the generally electrically resistive outer layer of the Earth and the conductive layer in the upper mantle.…”