Abstract. For almost 10 years the KTB superdeep drilling project has offered an excellent field laboratory for adapting seismic techniques to crystalline environments and for testing new ideas for interpreting seismic reflections in terms of lithological or textural properties of metamorphic rock units. The seismic investigations culminated in a threedimensional (3-D) reflection survey on a 19 x 19 km area with the drill site at its center. Interpretation of these data resulted in a detailed, structural model of the German Continental Deep Drilling Program (KTB) location with dominant, steep faults in the upper crust. The 3-D reflection survey was part of a suite of seismic experiments, ranging from wide-angle reflection and refraction profiles to standard vertical seismic profiles (VSP) and more sophisticated surface-to-borehole observations. It was predicted that the drill bit would meet the most prominent, steeply dipping, crustal reflector at a depth of about 6500-7000 m, and indeed, the borehole penetrated a major fault zone in the depth interval between 6850 and 7300 m. This reflector offered the rare opportunity to relate logging results, reflective properties, and geology to observed and modeled data. PostVariscan thrusting caused cataclastic deformation, with partial, strong alterations within a steeply dipping reverse fault zone. This process generated impedance contrasts within the fault zone on a lateral scale large enough to cause seismic reflections. This was confirmed by borehole measurements along the whole 9.1 km deep KTB profile. The strongest, reflected signals originated from fluid-filled fractures and cataclastic fracture zones rather than from lithological boundaries (i.e., first-order discontinuities between different rock types) or from texture-and/or foliation-induced anisotropy. During the interpretation of seismic data at KTB several lessons were learned: Conventional processing of twodimensional (2-D) reflection data from a presite survey showed predominantly subhorizontal layering in the upper crust with reflectivity striking in the Variscan direction. Drilling, however, revealed that all rock units are steeply dipping. This confirms that surface common depth point (CDP) seismics strongly enhances subhorizontal reflectivity and may thus produce a very misleading crustal image. Although this was shown for synthetic examples earlier, the KTB provides the experimental proof of how crucial this insight can be.

In den devonischen und vordevonischen Sedimenten des rechtsrheinischen Schiefergebirges erzeugt die Schichtung nur relativ schwache, kurze und unregelm~ifiige Reflexionen. Diese werden von st~irkeren, siidfallenden Reflektoren abgeschnitten, die aufgrund ihrer listrischen Kriimmung und flat/ramp-Geometrie wahrscheinlich als Uberschiebungen zu interpretieren sind. Im Nordteil des Schiefergebirges sind diese f3berschiebungen offenbar blind, werden also nahe der Oberfl~iche dutch Faltung kompensiert. Im Ebbe-Sattel und weiter siidlich lassen sich die meisten der starken, siidfallenden Reflektoren zweifelsfrei mit bekannten Grogiiberschiebungen korrelieren (Ebbe-, Siegen-, Miisen-, Sackpfeife-U., sowie weitere l]berschiebungen in der Lahn-u. Dill-Mulde). Die Basisiiberschiebung der Giessen-Decke wird nur teilweise abgebildet.Zur Tiefe hin zeigen die Oberschiebungen ein zunehmend flacheres Einfallen, und verschwinden in einer relativ transparenten Zone zwischen 3 und 5 s TWT, die im Hangenden und Liegenden durch diinne, stark reflektive Zonen begrenzt ist. Diese transparente Zone entspricht m6glicherweise einer Zone hoher integrierter Leitf~higkeit, die in einem begleitenden magnetotellurischen Experiment nachgewiesen worden ist; es handelt sich entweder um einen Graphit-fiihrenden Phyllit-Horizont oder eine mgchtigere permeable Zone mit Elektrolyt-gefiilltem Porenraum. Die hochreflektiven Ninder iiber und unter der transparenten Zone entsprechen entweder lithologischen Kontrasten oder rheologischen Grenzen, die vermutlich von einer scherenden Verformung iiberpr'~igt worden sind.Die Unterkruste im N-Teil des Profiles enth~ih einen relativ transparenten Bereich, der nach Siiden hin unter dem Nor& tell des Siegener Sattels keilf6rmig ausl~iuft. Ein ~hnliches Bild zeigt der Nordteil des ECORS-Profiles ,,Nord de la France,,. Die transparenten Bereiche beider Profile entsprechen wahrscheinlich einem pr~i-pal~iozoischen kristallinen Basement, das das Brabanter Massif unterlagert und sich rechtsrheinisch fortsetzt. Siidlich des transparenten Keiles wird die Unterkruste zunehmend reflexionsreicher.Die listrisch gekriimmten, an Uberschiebungen gebundenen Reflektoren werden von einem konjugierten System schw~icherer, N-u. S-fallender Refiektoren abgeschnitten, die auf eine jiingere, bruchhafte Verformung durch reine Scherung hindeuten. Der Versatz einiger ~.iterer Reflektoren deutet auf sp{it-oder postvariscische Kompression hin. 524 W. FRANKE et al.In einer ahernativen Interpretation werden nut diese jiingeren Reflektoren als Uberschiebungen gedeutet; die ~iheren, gekriimmten Elemente mtit~ten dann prim~iren lithologischen Grenzen entsprechen.Die Moho steigt von ca. 11 s TWT im N auf 8.5 s TWT unter dem Taunus an. Die Krustenverdiinnung im Siiden geht wahrscheinlich auf Dehnung im Terti~ir zuriick. AbstractThe reflection seismic line DEKORP 2-N reveals an almost complete cross section through the Rhenohercynian Zone, the most external part of the Variscan orogen in Europe.The northern part of DEKORP 2-N and a NE-directed branc...

Geological Aims, Planning, Field Investigations Purpose of the InvestigationsThe seismic lines discussed in this paper explore a central part of the Variscan Belt of Europe. This belt is more than 1000 km wide and can be subdivided -in the Central European segment -into several zones, which were first recognized by Kossmat (1927). The zonal boundaries are now regarded as sutures which originated from the closure of several Paleozoic basins. First results on the geophysical-gelogical investigations in the Oberpfalz area are found in DEKORP Research Group (1988).The DEKORP 4 and 4-Q lines, as well as the closely related KTB 8501 to 8506 lines, were intended to scan the border zone between the Moldanubian Zone and the northward adjacent Saxothuringian Zone (Fig. 1). The Saxothuringian (ST) represents the infill of a basin which opened during the Cambro-Ordovician beyond the rift stage at least into a narrow ocean, and the Moldanubian (MN) contains blocks of pre-Variscan crust (and their Paleozoic cover) which were reactivated by the Variscan orogeny and thrusted toward the NW over the Saxothuringian foreland. Tectonic deformation and co-related metamorphism are polyphase and have produced a highly complex crustal structure For more detailed information see Franke (this Vol.). Special emphasis has been put on the area S of Erbendorf which was under consideration and has since been selected as the site of the Continental Deep Drilling Project KTB.The long DEKORP 4 profile was intended as a general cross-section from the Teuschnitz syncline in the N to the MN rocks in the S. The KTB 8501 to 8506 lines were arranged in form of a grid to explore the ST/MN boundary region near Erbendorf (i.e., around the KTB drilling site). This array of lines (Fig. 1) represents a first step in obtaining a 3-D view of the target area.The DEKORP 4-Q line is positioned at a right angle with respect to the DEKORP 4 line near its southern end. It was intended to investigate important tectonic structures such as the Bavarian "Pfahl" and the R.

No abstract

Within the DEKORP project (DEKORP: Deutsches Kontinentales Reflexionsseismisches Programm) a joint deep seismic reflection venture with the BELCORP (Belgian Continental Reflection Seismic Programme) group of the Belgian Geological Survey was carried out in 1987 across the Rhenish Massif, a part of the mid-European Variscides. This orogenic belt developed in the Upper Devonian/Carboniferous. Mostly Devonian rocks crop out at the surface. The Rhenish Massif is bordered by two sedimentary troughs: the sub-Variscan Foredeep in the north and the Permo-Carboniferous Saar-Nahe Basin in the south. In the east-west direction it is subdivided by the axial depression of the Eifel Nord-Siid Zone.The aim of the survey which totals almost 220 km of seismic profiling, was to investigate the crustal structure of the western part of the Rhenish Massif and to compare it with the line DEKORP 2-N which crosses the eastern portion of the massif. The results indicate the presence of NW-vergent tectonics of various styles that can often be traced down to deep parts of the crust. Horizontal Variscan compression plays a dominant role in the northern part while post-Variscan extension seems to dominate in the Saar-Nahe Basin, although even there traces of Variscan compression seem to be preserved in the middle crust. Common characteristics of the pre-Palaeozoic basement, differences between the western and eastern parts of the Rhenish Massif, and the deep extension of the Aachen Thrust (Faille du Midi) have been clearly observed. This prominent thrust in the north with its characteristic ramp and flat structure has been followed over 100 km length down 'Now at: Forschungsanstalt der Bw. fur Wasserschall-und Geophysik, Kiel, Germany 203 204 DEKORP Research Group I to 15 km depth. It contrasts sharply with the very complex deep fault system in the south, separating the post-orogenic Saar-Nahe Basin from the Hunsruck mountains. The integration of accompanying geophysical and geological studies provides a new synoptic picture of the evolution and tectonics of this key area of Central Europe.