The DEKORP (Deutsches Kontinentales Reflexionsseismisches Programm) initiative, conducted from 1984 to 1997, was Germany's national reflection seismic program. DEKORP aimed at resolving the deep crustal and upper mantle structure, employing mainly near-vertical incidence seismic surveys that in some cases were complemented by wide-angle seismic studies and accompanied by research projects dedicated to refining and optimizing processing and interpretation methods. The resulting seismic images significantly contributed to the geological interpretation of the crust and upper mantle, e.g., unveiling distinct units of the Central European Variscides in Germany.

We reprocessed several of these DEKORP profiles located within the Federal state of Bavaria and in particular around the German Continental Deep Drillhole (KTB). We employed advanced focusing pre-stack depth imaging techniques in conjunction with newly derived near-surface tomographic velocity models. Our approach yields novel images with improved quality and new structural details in several profile parts. The results provide a comprehensive view of the entire crust and the basis for new geological interpretations and modeling, including a significant enhancement of our understanding of the crustal architecture.

We thank the Bayerisches Landesamt für Geologie for assigning the new processing and providing the seismic data. Particular thanks to J. Großmann and K. Dengler for their support.

The Kraichgau Terrane is a NE-SW elongated high-density body covered by Mesozoic sedimentary rocks and occupies the southwestern parts of the Saxo-Thuringian Zone in the Central European Variscides. Compared to other regions of the Saxo-Thuringian Zone, the Kraichgau Terrane is poorly studied owing to very limited subsurface data. We integrate reprocessed DEKORP-2S seismic reflection profile, filtered Bouguer gravity and total magnetic intensity data and show the regional subsurface litho-structure of the Kraichgau Terrane.

The seismic reflections show a stratified crust with high amplitude, continuous and subhorizontal middle and lower crust in the SE and NW separated by a central zone of medium to low amplitude and transparent area hosting several oppositely dipping high amplitude reflections. This central zone (CZ) is approximately 80 km wide along the DEKORP-2S profile, exhibits a high density anomaly and shows two distinct linear features on the vertical derivatives of the magnetic intensity data. Further NE along the DEKORP-3MVE profile, the CZ is approximately 20 km wide, is denser than surrounding areas, and exhibits a single linear feature on the vertical derivative of the magnetic data. This linear feature is correlated with exposed Early Paleozoic magmatic rocks known as the Vesser units.

In addition to the magnetic and gravity signature of the Vesser units, the tilt and vertical derivatives of the Bouguer gravity anomaly show several NE-SW linear structures in the central and southeastern parts of the Kraichgau Terrane. These linear structures are interpreted as the folded and overthrusted Saxo-Thuringian units during the Variscan tectonics.

We are in the process of making a tectonometamorphic map of the Erzgebirge and present our preliminary state of this work. We follow the scheme of previous studies and distinguish four major allochthonous units in the Erzgebirge: Basal Gneiss Unit, Gneiss-Eclogite Unit, Mica schist-Eclogite Unit, and Phyllite Unit.

In the course of this project we would like to address the following open questions:

1. Where are tectonic boundaries between these units? Which subunits can be distinguished?

2. Do high-pressure/ultrahigh-pressure (HP/UHP) conditions form different clusters? If so, what is the extent of these clusters and is the current scheme of two HP and one UHP cluster accurate?

3. Are high-pressure rocks solitary occurrences in a matrix with different pressure-temperature evolutions or did the units share a common evolution? If units behaved coherently – how do the HP clusters fit into this picture and where are the associated tectonic boundaries?

4. Does the main foliation in the HP units reflect exhumation from eclogite-facies conditions, exhumation from mid-crustal levels or even something else?

5. What explains the overall distribution of metamorphism in the Erzgebirge? While high-pressure and Barrovian conditions seem to fade out towards higher structural levels in the west, the eastern border of the Erzgebirge towards the Elbe Zone is metamorphically abrupt, represents a major structural jump and shows similarities to an extensional detachment fault.

The Elbtalschiefergebirge - the tectonic border between the Lusatian Massiv and the Erzgebirge – is limited by two large regional faults, the West Lusatian fault and the Mid Saxonian fault. In Saxony, both NW-SE striking faults are running mostly parallel. So, a generally cogenetic evolution of both structures was assumed until now.

The paleozoic units of the Elbtalschiefergebirge itself are limited very often also by faults. However, deep drillings are missing until now.

During the still running project „Railway tunnel Dresden-Prague“ several geological units and faults are exposed by numerous drillings up to 400 m depth.

The new results show, that the West Lusatian fault as thrust zone and the Mid Saxonian fault as shear zone are not comparable as well in time as in genesis. The thickness of the Mid Saxonian fault is proved by drillings of at least 900 m. On the other hand, the fault volume of the West Lusatian fault includes only several meter. This is untypical for such a regional fault zone, but the main movement seems to have taken place about 300 m south of the recently mapped fault position.

Instead this, the faults in the Elbtalschiefergebirge are thrusts with considerable displacement.

The drillings of the ongoing project will just be investigate by several research groups of different institutions. Numerous new drillings in special areas are planned in the next years.

In the Saxothuringian Zone a unique assemblage of high to ultra-high pressure and ultra-high temperature metamorphic units is associated to medium-to-low pressure and temperature rocks. The units were studied in a campaign with garnet and monazite petrochronology of gneisses, micaschists and phyllites, and monazite dating in granites. P-T path segments of garnet crystallisation were reconstructed by geothermobarometry and interpreted in terms of monazite stability field, EPMA-Th-U-Pb monazite ages, and garnet Y+HREE zonations (Schulz and Krause 2024). One can recognise (1) Cambrian plutonism (512-503 Ma) with contact metamorphism in the Münchberg Massif. Subordinate monazite populations may indicate a (2) widespread but weak Silurian (444-418 Ma) thermal event. A (3) Devonian (389-360 Ma) high pressure metamorphism prevails in the Münchberg and Frankenberg Massifs. In the ultra-high pressure and high pressure units of the Erzgebirge the predominant (4) Carboniferous (336-327 Ma) monazites crystallised at the decompression paths. In the Saxonian Granulite Massif, prograde-retrograde P-T paths of cordierite-garnet gneisses can be related to monazite ages from 339 to 317 Ma. A (5) local hydrothermal overprint at 313-302 Ma coincides partly with post-tectonic (345-307 Ma) granite intrusions. Such diverse monazite age pattern and P-T-time paths characterise the tectono-metamorphic evolution of each crustal segment involved in the Variscan Orogeny.

Schulz, B., Krause, J. (2024): Electron probe petrochronology of monazite and garnet bearing metamorphic rocks in the Saxothuringian allochthonous domains (Erzgebirge, Granulite and Münchberg Massifs). Geol. Soc. Spec. Publ., 537:249-284. https://doi.org/10.1144/SP537-2022-195.