The refraction seismic experiment GRANU95 in the Saxothuringian belt, southeastern Germany

Abstract

In this paper we present newly acquired high‐quality wide‐angle seismic data of the GRANU95 project and first models of the crustal structure along two profiles (95‐A and 95‐B) beneath the Saxonian Granulites, a major ellipse‐shaped exposure of lower‐crustal material within the mid‐European Variscan belt in southeastern Germany (Saxony). The crust is subdivided into four layers. The crystalline basement with velocities higher than 6.0 km s−1 is generally reached at shallow depths, with three major sedimentary structures as prominent exceptions where velocities considerably lower than 6.0 km s−1 (as low as 5.1 km s −1 ) reach as deep as 4 km. The highest upper‐crustal velocities (up to 6.5 km s−1 ) are not seen below the exposed granulites themselves, but at shallow depths (4 km) SW of the exposure. These shallow high velocities correlate well in depth with highly reflective zones observed on three seismic‐reflection lines of DEKORP (85‐4N, 95‐01, 95‐02) at their intersection with the seismic‐refraction line 95‐B, where they appear as a set of NW–SE‐trending dome‐shaped reflections. On profile 95‐A this high‐velocity upper‐crustal layer (6.3 km s−1 ) dips from 5 to 9 km beneath the SE margin of the exposed granulites. These results suggest that the granulite dome and its western continuation are widely underlain by a NE‐trending antiformal structure (probably a sheet of metabasic rocks) where the exposed felsic granulites form just a local cap on top. Below the upper‐crustal high velocities, a layer with decreased velocity (6.2–6.25 km s−1 ) extends down to an average depth of 15 km along the Variscan strike (95‐B) and to 11–16 km depth (slightly dipping towards the SE) perpendicular to the terrane boundaries (95‐A). At mid‐crustal levels a weak reflection from a layer with a velocity of 6.4–6.6 km s−1 may indicate the classical Conrad discontinuity. At the depth range 22–24 km the velocity jumps to an average value of 7.0 km s−1, thus defining a prominent high‐velocity layer in the lower crust, which may be viewed as the well‐known laminated lower crust typical of Variscan structures, but with higher average velocity than usually detected. The crust–mantle boundary at about 30–31 km is typical for western Europe and confirms the extensional signature of the West European crust. Below the Moho, poorly constrained upper‐mantle velocities of about 7.9–8.0 km s−1 are derived. The high velocities observed in the lower‐crustal layer would not exclude the possibility of mantle‐derived intrusions, but the lack of any sign of an updoming Moho favours the interpretation of a more passively driven extension.

Differences in the Crustal and Uppermost Mantle Structure of the Bohemian Massif from Teleseismic Receiver Functions

“…In profile GRANU95 (Enderle et al, 1998), as well as profile CEL09 and L-profile made within the CELEBRATION 2000 experiment (Guterch et al, ,2001Málek et al, 2001) we can observe some similar features in P-wave like in S-wave from receiver function. The locations of these profiles are shown in Fig.…”

Eger Rift ICDP: an observatory for study of non-volcanic, mid-crustal earthquake swarms and accompanying phenomena

“…The crustal and upper mantle structure was previously investigated in several active seismic experiments and passive monitoring (DEKORP Research Group, 1994;Behr et al, 1994;Tomek et al, 1997;Enderle et al, 1998;Hrubcová et al, 2005;Hrubcová and Geissler, 2009;Brűckl et al, 2007;Grad et al, 2008). Passive seismic experiments were carried out to study major lithospheric discontinuities using the receiver function approach (Geissler et al, 2005; Heuer et al, 2006).…”

Depth-Recursive Tomography of the Bohemian Massif at the CEL09 Transect—Part B: Interpretation

“…The Berga Anticline was resolved only in Růžek's model. As part of seismic project ' 'GRANU 1995'', Enderle et al (1998 performed ray-tracing modeling along refraction profile 95-B crossing the CEL09 profile at 30 km-see their intersection in Fig. 1 marked as G95.…”

“…The Saxonian Granulite Massif (SGM) was transected by two refraction profiles, GRANU 95A (Enderle et al 1998) and SUDETES 2003S04 profile (Novotný et al 2010). Both the derived velocity models show a deeply established HV anomaly reaching the depth of 5 km beneath the SGM at 6,000 m s -1 , or even *1 km southeast at the intersection with the MVE 90 profile-see fig.…”