The Thor suture zone: From subduction to intraplate basin setting

Abstract

The crustal seismic velocity structure of northwestern Europe shows a low P-wave velocity zone (LVZ) in the lower crust along the Caledonian Thor suture zone (TSZ) that cannot be easily attributed to Avalonia or Baltica plates abutting the TSZ. The LVZ appears to correspond to a hitherto unrecognized crustal segment (accretionary complex) that separates Avalonia from Baltica, explaining well the absence of Avalonia further east. Consequently, the northern boundary of Avalonia is shifted ~150 km southward. Our interpretation, based on analysis of deep seismic profiles, places the LVZ in a consistent crustal domain interpretation. A comparison with present-day examples of the Kuril and Cascadia subduction zones suggests that the LVZ separating Avalonia from Baltica is composed of remnants of the Caledonian accretionary complex. If so, the present-day geometry probably originates from pre-Variscan extension and eduction during Devonian-Carboniferous backarc extension. The reinterpretation of deep crustal zonation provides a crustal framework in which the northern limit of Avalonia corresponds to the southern limit of the deep North German Basin and the northern limit of prolific gas reservoirs and late Mesozoic inversion structures.

Deep structure of the northern North Sea and southwestern Norway based on 3D density and magnetic modelling

“…The outcome of the 3D density modelling supports previous results (Lyngsie et al, 2006;Fichler et al, 2011;Smit et al, 2016) showing that the crystalline crust of the study area is not only heterogeneous in a vertical direction but it can be also subdivided into several horizontal blocks. From the west to the east, the modelled middle crystalline crust consists of four crustal domains, two of which are represented by middle crust of Laurentia and Avalonia (layer 15) and middle crust of Baltica (layer 16) with similar densities of 2746-2761 and 2747 kg/m 3 (for the 'heavy' model 2780-2813 and 2801 kg/m 3 ), respectively (Table 3).…”

“…17C) cannot be differentiated due to the impossibility of distinguishing between the lower high-density crusts of Baltica, Laurentia and Avalonia, and a part of this layer beneath the Viking Graben may thus have at least a partial Baltican origin. On the other hand, the extension of Baltican lower crust beneath the southwestern flank of the Central Graben is not obvious according to Smit et al (2016) who suggested that there were remnants of the Caledonian accretionary complex between Avalonia and Baltica based on the presence of the low P-wave velocity zone in the lower crust of the southwestern flank of the Central Graben. The latter is reflected in part by a thinning of the high-density lower-crustal layer beneath the southwestern flank of the Central Graben in the case of our model (Figs.…”

“…Actually, the middle crust of the continent Laurentia and the microcontinent Avalonia has to be further divided into two different blocks due to the reason that Avalonia and Laurentia joined the continent of Baltica at different times. Initially, Avalonia accreted to Baltica during Ordovician-Silurian closure of the Thor Ocean-Tornquist Sea, and only after that Laurentia joined the combined continent of Baltica-Avalonia during closure of the Iapetus Ocean in the Late Silurian (e.g., Torsvik & Rehnström, 2003;Cocks & Torsvik, 2006;Smit et al, 2016). However, our model area covers only a small piece of Avalonia in the southwestern ness and heat flow without involving magmatic processes which would have led to an increase of density and thickness of the lower crust during the Late CarboniferousEarly Permian event.…”

Contrasting sources of Late Paleozoic rhyolite magma in the Polish Lowlands: evidence from U–Pb ages and Hf and O isotope composition in zircon

“…The composition of the pre-Permian basement is also constrained by geophysical studies (Grad et al 2003;Dadlez et al 2005;Guterch and Grad 2010) and drill core data showing the presence of Ordovician-to-Carboniferous sediments (Podhalańska Fig. 1 a Plate tectonic sketch of West and Central Europe after Smit et al 2016; TTZ Teisseyreornquist Zone, L Laurentia; b map of northwestern Poland indicating the location of the sampling sites of PermoCarboniferous volcanic rocks. Numbers indicate the thickness of drilled volcanic rocks in meters (data from Dadlez 2006;Geiβler et al 2008).…”

“…Our data include Secondary Ion Mass Spectrometry (SIMS) U-Pb ages and O isotope analyses followed by Hf isotope analyses using Laser Ablation Inductively Coupled Mass Spectrometry (LA-ICP-MS). The goal of the present contribution is to constrain the sources of rhyolite magma in the Polish Lowlands and to fit this information into current models of continental crust and plate tectonic evolution in the region (Mazur et al 2015;Smit et al 2016;Żelaźniewicz et al 2016). In addition, we address the extent to which rhyolite magmas were the product of remelted crustal granitic bodies.…”