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The Thor Suture Zone: from Subduction to Intraplate Basin Setting

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The Thor Suture Zone: from Subduction to Intraplate Basin Setting The Thor suture zone: From subduction to intraplate basin setting Jeroen Smit1, Jan-Diederik van Wees1,2, and Sierd Cloetingh1 1Department of Earth Sciences, Faculty of Geosciences, Utrecht University, PO Box 80.021, 3508 TA Utrecht, Netherlands 2TNO Energy, PO Box 80015, 3508 TA Utrecht, Netherlands ABSTRACT deep crustal structures and their boundaries, such The crustal seismic velocity structure of northwestern Europe shows a low P-wave veloc- as the northwest European Caledonian suture ity zone (LVZ) in the lower crust along the Caledonian Thor suture zone (TSZ) that cannot zones (e.g., Barton, 1992; MONA LISA Work- be easily attributed to Avalonia or Baltica plates abutting the TSZ. The LVZ appears to cor- ing Group, 1997; Pharaoh, 1999; England, 2000; respond to a hitherto unrecognized crustal segment (accretionary complex) that separates Fig. 1; Fig. DR1 in the GSA Data Repository1). Avalonia from Baltica, explaining well the absence of Avalonia further east. Consequently, Reflection seismic profiles in the southern North the northern boundary of Avalonia is shifted ~150 km southward. Our interpretation, based Sea and the North German Basin generally show on analysis of deep seismic profiles, places the LVZ in a consistent crustal domain inter- poor resolution at deeper crustal levels due to pretation. A comparison with present-day examples of the Kuril and Cascadia subduction the presence of evaporites, one of the reasons to zones suggests that the LVZ separating Avalonia from Baltica is composed of remnants of record refraction seismic profiles (e.g., Rabbel the Caledonian accretionary complex. If so, the present-day geometry probably originates et al., 1995; Krawczyk et al., 2008). In general, from pre-Variscan extension and eduction during Devonian–Carboniferous backarc exten- defining terranes on the basis of seismic veloci- sion. The reinterpretation of deep crustal zonation provides a crustal framework in which ties alone is not always trivial, as seismic velocity the northern limit of Avalonia corresponds to the southern limit of the deep North German distributions can be affected by tectonic events Basin and the northern limit of prolific gas reservoirs and late Mesozoic inversion structures. following their amalgamation. Consequently, it is not always clear in how far current lower INTRODUCTION demand precise outlines of terranes and conti- crustal velocities still represent a property of the Basin analysis, including quantitative model- nents, their margin geometry, and accretion and original terranes. In the case of the Thor suture ing of active basins, is critically dependent on a postaccretion histories. This is quite challenging zone (TSZ), however, there is a systematic, con- priori assumptions on deeper crustal and litho- when the basement is currently in the middle of sistent, and direct correlation between this struc- spheric structure and composition. This applies continents and covered by deep basins. In such ture and a low-velocity zone (LVZ) in the lower in particular to studies that aim at quantitative settings, marked by limited direct observations, crust detected from a set of five parallel deep assessments of basin maturation (e.g., Van Wees identification of crustal domains strongly relies et al., 2009), in-depth understanding of long- on available seismic and potential field data. In 1 GSA Data Repository item 2016229, Figures lived and repeatedly active fault zones and the past decades significant progress has been DR1–DR5 (classic interpretation of tectonic setting, (upper) crustal segmentation (e.g., Cloetingh et made in the resolution and velocity interpretation comparisons of classic and new interpretation, and extent of upper surface of LVZ), is available online al., 2010), and precise paleogeographic recon- of the deep crust from refraction and reflection at www.geosociety.org/pubs/ft2016.htm, or on request structions (e.g., Torsvik et al., 2012). These all seismic profiles, allowing the identification of from [email protected]. 0° 5° 10° 15° A 55° C B 200 km A B C Figure 1. Three deep seismic refraction profiles across the Thor suture zone of northwestern Europe (locations in inset and in Fig. 2). Blue areas in inset mark location of main basins deeper than 1000 m. A: MONA LISA 3 (profile ML-3) across the North Sea Central Graben (modified from Lyngsie and Thybo, 2007). RFH—Ringkøbing-Fyn high; M—Moho. B: Combined European GeoTraverse subprofiles EUGEMI and EUGENO-S 1, showing relations between Thor suture zone, North German Basin, and northern Avalonian margin (modified from Aichroth et al., 1992; Thybo, 2001). C: LT-7 profile across Baltica margin, east of Rheic suture (from Guterch and Grad, 2006). TESZ—Trans-European suture zone. GEOLOGY, September 2016; v. 44; no. 9; p. 707–710 | Data Repository item 2016229 | doi:10.1130/G37958.1 | Published online 22 July 2016 GEOLOGY© 2016 The Authors.| Volume Gold 44 |Open Number Access: 9 | www.gsapubs.orgThis paper is published under the terms of the CC-BY license. 707 seismic refraction lines (locations are given in crustal P-wave velocities (Vp 6.9–7.2 km/s; profile (Reichert, 1993) are the only profiles to Fig. 2 and Fig. DR1) that cannot be easily attrib- EUGENO-S Working Group, 1988; Thybo, document the transition from the LVZ to lower uted to typical Avalonia or Baltica crustal signa- 2001) (Fig. 1). The high-velocity lower crust crustal velocities of 6.6–6.8 km/s of Avalonian tures (e.g., MONA LISA Working Group, 1997; continues southward, forming a wedge under crustal signature. Thybo, 2001). This LVZ, located to the south the South Permian Basin until it thins out at the Located ~200 km east of the EGT EUGEMI of the Elbe-Odra line (EOL), has received little EOL (e.g., Thybo, 2001) (Fig. 1). It probably profile, profile LT-7 (e.g., Guterch and Grad, attention, contrary to the high-velocity lower represents basement rocks of the thinned pas- 2006) images the crust east of the Rheic suture crust of the Baltica margin north of the EOL sive margin of Baltica. Well data (e.g., Ziegler, (Fig. 1). The Baltica margin is wider and lay- (Fig. 1; Figs. DR2 and DR3) (e.g., Rabbel et 1990; well locations in Fig. 2) show low-grade ers are more tabular along profile LT-7, but its al., 1995; Thybo, 2001; Krawczyk et al., 2008). metasediments of Cambrian–Ordovician age overall velocity structure is similar to the Baltica In this study we place the lower crustal LVZ with Vp ~5.2–6.0 km/s overlying the high-veloc- crust in the EGT profile. South of the high-veloc- in a consistent crustal domain interpretation. We ity lower crust (Fig. 1). Devonian–Carbonifer- ity Baltica crust, a two-layer crust with lower propose that the LVZ corresponds to the exis- ous and younger rocks with Vp ~4.5–5.2 km/s crustal velocities Vp 6.4–6.6 km/s is typical of tence of a hitherto unrecognized crustal segment overlie these rocks. The available deep seismic Variscan crust as imaged by other refraction pro- that separates Avalonia from Baltica. Compari- refraction lines that image the deep structure of files (e.g., Guterch and Grad, 2006). With both son with the active Kuril and Cascadia subduc- the TSZ image the LVZ through low Vp 6.3–6.4 profiles located at either side of the Rheic suture, tion zones and with the Rhodope metamorphic km/s in the lower crust from the North Sea Cen- this suture appears to be the eastern limit of both core complex yields an explanation for the LVZ tral Graben to the Rheic suture. This LVZ abuts Avalonia and the LVZ (Fig. 2). and a scenario for the postsubduction history and partly covers the high-velocity lower crust of the TSZ. of Baltica. Therefore, and in accordance with Thor Suture LVZ as New Crustal Domain: regional tectonic models (e.g., Pharaoh, 1999), it Implications for the Northeastern Limit of GEOLOGICAL SETTING is generally ascribed to Avalonia, although these Avalonia The TSZ separates Baltica from Avalonia velocities are uncommon in Phanerozoic lower The transition from the LVZ (Vp ~6.3–6.4 and represents the northwestern segment of the crust (see Figs. DR2 and DR3 for the classic km/s) to normal (Vp ~6.6–6.8 km/s) P-wave Trans-European suture zone (TESZ). It formed interpretation). High-pressure laboratory mea- velocities in the lower crust as imaged by the during Ordovician–Silurian closure of the Thor surements indicate that these P-wave velocities EGT EUGEMI (Fig. 1; Fig. DR3) and NORD- Ocean–Tornquist Sea by southward subduction of 6.3–6.4 km/s normally represent serpentinites DEUTSCHLAND 1975/76 profiles suggests that of Baltica under Avalonia, prior to the formation or mid-crustal granites and gneisses instead the LVZ along the TSZ is not typical for Avalon- of Laurussia by closure of the Iapetus Ocean (Christensen and Mooney, 1995). ian crust as generally assumed. Instead, Avalonia between Baltica-Avalonia and Laurentia. From To the west, the LISPB (Lithospheric Seismic most likely has a regular Phanerozoic velocity the North Sea to the Polish Basin, the TSZ is Profile in Britain) profiles (e.g., Barton, 1992; structure with a lower crustal P-wave velocity of deeply buried below basins with late Paleo- Maguire et al., 2011) transect Avalonia from the 6.6–6.8 km/s as found to the west (e.g., Barton, zoic–Holocene sedimentary thicknesses of up Iapetus suture zone to the Rheic suture (see Fig. 1992). Consequently, the LVZ forms a separate, to ~15 km (e.g., Thybo, 2001; Doornenbal and DR1 for location). A low-velocity lower crust 50–100-km-wide crustal domain that is charac- Stevenson, 2010). as along the TSZ is absent.
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