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

Abstract

[1] Regional seismic tomography provides valuable information on the structure of shields, thereby gaining insight to the formation and stabilization of old continents. Fennoscandia (known as the Baltic Shield for its exposed part) is a composite shield for which the last recorded tectonic event is the intrusion of the Rapakivi granitoids around 1.6 Ga. A seismic experiment carried out as part of the European project Svecofennian-Karelia-Lapland-Kola (SVEKALAPKO) was designed to study the upper mantle of the Finnish part of the Baltic Shield, especially the boundary between Archean and Proterozoic domains. We invert the fundamental mode Rayleigh waves to obtain a three-dimensional shear wave velocity model using a ray-based method accounting for the curvature of wave fronts. The experiment geometry allows an evaluation of lateral variations in velocities down to 150 km depth. The obtained model exhibits variations of up to ±3% in S wave velocities. As the thermal variations beneath Finland are very small, these lateral variations must be caused by different rock compositions. The lithospheres beneath the Archean and Proterozoic domains are not noticeably different in the S wave velocity maps. A classification of the velocity profiles with depth yields four main families and five intermediate regions that can be correlated with surface features. The comparison of these profiles with composition-based shear wave velocities implies both lateral and vertical variations of the mineralogy.

DynaQlim – Upper Mantle Dynamics and Quaternary Climate in Cratonic Areas

“…Existing data on experimentally studied lower crustal and mantle composition and 3-D structure derived from xenolith data, lithospheric thermal models Hieronymus et al, 2007) and seismic studies (Bruneton et al, 2004;Sandoval et al, 2004;Yliniemi et al, 2004;Hjelt et al, 2006;Pedersen et al, 2006;Plomerova et al, 2006;Gregersen et al, 2006;Janik et al, 2007;Olsson et al, 2007) should be utilized for forward rheological modelling of the lithosphere and for testing of dynamic uplift models. The presence and volume of fluids in the upper mantle and the influence of fluids on the mantle rheology is an open question.…”

The elusive lithosphere–asthenosphere boundary (LAB) beneath cratons

“…• changes in the nature of the lateral velocity heterogeneity across an array confined to a shield region (Bruneton et al, 2004b);…”

“…The scope of surface-wave based models ranges from relatively low-resolution 3- Three-dimensional shear velocity modelling carried out by Bruneton et al (2004b) showed significant lateral heterogeneity across the region, interpreted to arise from compositional variations within the lithospheric mantle. The authors noted that it may be possible to identify the LAB by a pronounced reduction of the lateral heterogeneity across a study region; however, the lateral resolution of the SVEKALAPKO study was insufficient below 150 km to test this hypothesis.…”

Traces of the crustal units and the upper-mantle structure in the southwestern part of the East European Craton

“…Thick lithosphere was previously reported for other cratonic areas, i.e. the Fennoscandian Shield , but no evidence of the seismic LAB was found anywhere within the depth of 300 km beneath the Fennoscandian Shield (Bruneton et al, 2004). The shear-wave studies of Legendre et al (2012) showed no deep cratonic roots below about 330 km in the EEC.…”

“…A number of studies (e.g. Bruneton et al, 2004;Beller et al, 2013) showed that the upper mantle beneath anorogenic granitoid massifs inside cratonic crust is different from that of the surrounding cratonic mantle. There is another anorogenic granitoid massif, the Riga Pluton (Fig.…”