Ewald Lüschen

Crustal evolution of the Rhinegraben area. 1. Exploring the lower crust in the Rhinegraben rift by unified geophysical experiments

Abstract Unified geophysical investigations of the lithosphere in the Rhinegraben rift system have revealed new details of the lower crust and its role in the rifting process. The new findings allow an assessment of the compatibility of four different geophysical notions and properties of the lower crust: 1. (1) the lower crust as the layer beneath the Conrad discontinuity with a P-wave velocity of about 6.5 km/s or greater (refraction seismics); 2. (2) the laminated band of reflections, as seen in the near-vertical reflection seismic experiments in many parts of the continents; 3. (3) the ductile part of the crust below the brittle-ductile transition, devoid of earthquakes in seismically active regions; 4. (4) the electrical conductivity of the lower crust indicative of dry or wet conditions, or still unknown conduction phenomena. In the Rhinegraben area the lamination of the lower crust serves as an outstanding marker of deep tectonic activity during the rifting process, in which the crust of the Rhinegraben rift system has been subjected to three different natural dynamic processes: (1) Uplift by 2 to 3 km with subsequent erosion of the Rhinegraben shoulders (Black Forest and Vosges Mountains) caused decompression possibly leading to the formation of a low-velocity/high-electrical-conductivity zone right on top of the laminated lower crust beneath the elevated shoulders of the Black Forest. (2) The brittle crystalline wedge of the graben proper subsided nearly undeformed into the lower crust, which became about 5 to 7 km thinner below the graben than below the shoulders. (3) The deepest hypocentres in the Black Forest (Dinkelberg area), if projected onto the neighbouring reflection profile, would be located 7 to 8 km within the laminated lower crust beneath the southern Black Forest, indicating a discrepancy between the top of the lower crust as defined by the brittle-ductile transition as seen by the deepest earthquakes and by the top of the laminated reflection band. The Rhinegraben rift system reveals the properties and behaviour of the lower crust under a wide variety of tectonic situations.

Shear-wave evidence for an anisotropic lower crust beneath the Black Forest, southwest Germany

Abstract Controlled shear-wave sources (a horizontal vibrator and dynamite) were used in near-vertical reflection studies to explore the nature of the reflective laminated lower crust in southwest Germany. The data reveal strong S-wave reflections from the lower crust. These lower crustal S-reflections were not observed by previous wide-angle surveys in the same region. A laminated crustal model with isotropic layers (and changing Poissons ratio) cannot explain the differing near- and far-offset S-response. By introducing alternating anisotropic and isotropic lamellae, this discrepancy may be resolved. This Seismic model is consistent with a petrological model of deformed amphibolites containing 10–30% of preferentially orientated hornblende. Observed amplitude differences in P- and S-wave reflections from the same depth in the lower crust suggest regions with a changing Poissons ratio.

THE DEEP CRUST OF THE SOUTHERN RHINE GRABEN : REFLECTIVITY AND SEISMICITY AS IMAGES OF DYNAMIC PROCESSES

Abstract The Rhine Graben, part of the European Cenozoic rift system, deserves special attention because of its location in the foreland of the Alpine orogen. The Phanerozoic evolution of the lithosphere in this region is defined by a set of major geodynamic events ranging from the Variscan orogeny, late-orogenic crustal re-equilibration to the interference of rifting and Alpine orogeny in Tertiary times. The Rhine Graben is one of the most detailedly studied continental grabens. Prospecting for hydrocarbons in its sedimentary fill and intensive pre-site studies for the Continental Deep Drilling Program (KTB) on its eastern crystalline flank (Black Forest) provided a comprehensive set of geoscience data. Seismic investigations ranging from a deep seismic network of near-vertical reflection to concurrent wide-angle refraction experiments were accompanied by seismological and geological surveys. Therefore, a direct observational comparison between the two depth definitions of crustal subdivisions, elastic vs. strength/rheological, was possible. In large parts of Western Europe deep seismic sounding (DSS) investigation distinguishes between a crystalline upper crust of comparatively low P-wave velocities (5.9–6.0 km/s) with little and discrete reflectivity and a strongly reflective lower crust with higher P-wave velocities (about 6.4–6.8 km/s). Both are separated by the Conrad discontinuity. In contrast, strength and rheological considerations define a brittle upper crust and a ductile lower crust. The boundary between these two realms is usually referred to as the brittle/ductile transition zone which in seismically active regions is imaged by the deepest crustal hypocentres. The present brittle/ductile transition in the Rhine Graben region, as defined by the deepest hypocentres, does not necessarily coincide with the structural separation in an upper and lower crust as seen in deep seismic studies. This suggests that the laminated lower crust developed during the crustal re-equilibration phase at the end of the Variscan orogeny. The second part of this paper concentrates on evidence of recent tectonic activities related to rifting processes impacting on the Variscan lower crust. A remarkable concentration of deep crustal earthquakes reaching to the top of an asymmetrical thinned lower crust is observed at or near the eastern border fault of the southern Rhine Graben and is possibly induced by the young rifting process. We suggest that this asymmetry could be a consequence of the uppermost mantle detaching from the upper crust in collision with the Alpine orogen. This allows us to propose the distinction between the influence of compression, extension, continental subduction and intra-crustal detachment during graben formation. The dynamic interference of rifting and foreland compression shown by structural and seismological observations leads to the notion of small-scale collisional escape tectonics associated with an intra-continental transform fault at the southern end of the Rhine Graben, the Rhine-Saoˆne Transform zone.

NATURE OF SEISMIC REFLECTIONS AND VELOCITIES FROM VSP-EXPERIMENTS AND BOREHOLE MEASUREMENTS AT THE KTB DEEP DRILLING SITE IN SOUTHEAST GERMANY

Abstract The 1989 VSP program at the KTB pilot hole was complemented in 1992 by a standard VSP in the KTB super-deep borehole from 3000 to 6013 m. P-wave velocities oscillate around 5.8 km/s in the upper 3150 m in accordance with sonic log velocities and correlate with paragneisses which prevail in this depth range. At about 3150 m depth, velocities increase to 6.4 km/s correlating with metabasites which dominate in the depth range 3150 to 7500 m. Laboratory measurements and petrophysical modelling provide evidence that the intrinsic velocities were reduced by 5–10% by fracture density and porosity at all depth ranges. Subvertical dip (50–70°) in structures and textures prevail, causing about 10% S-wave anisotropy, indicated by direct observations of S-wave splitting. Pronounced P-wave reflections, accompanied by P- to S-wave conversions and a lack of S-wave reflections, occur in the lower depth range only (3000–6000 m) and correlate with fluid-filled fracture systems. Lithological contrasts (gneiss-amphibolite) play a minor role in generating reflections. The most prominent reflecting elements known from surface profiling and 3D-surveys (i.e. the ‘Franconian Lineament’ reflector at about 7 km, and P-wave reflections at 8.3 km with notably absent associated S-wave reflections) coincide with pronounced anomalies observed in the logging data indicating the presence of major fracture zones.

Shear wave anisotropy of laminated lower crust at the Urach geothermal anomaly

Abstract Deep seismic reflection studies have shown that ‘lamellae’ are a widespread reflectivity pattern of the lower crust of the central European Variscan belt. This pattern has been interpreted, inter alia, as alternating subhorizontal layering of mafic and felsic rocks implying a tectonic process of structural and textural ordering. Consequently, laminated lower crust should be elastically anisotropic. The specific type of anisotropy should provide some insight into the mineral composition and the preferred orientation of minerals in the lower crust. We have investigated this problem in the area of the Urach geothermal anomaly (South Germany) where a ‘classical’ example of lower-crust lamellae is found. A restricted range of subsurface points was probed in a controlled-source expanding spread seismic experiment with two orthogonal azimuths of observation up to 90-km source-geophone offset. Both P- and S-waves were recorded with 3-component geophones at 80–140 m geophone spacing. Based on polarization analysis and traveltime interpretation the following results were obtained: (1) S-wave splitting is observed only for SMS arrivals (not for shallower reflections) implying that the lower crust is anisotropic; (2) the type of anisotropy is quasihexagonal (transversely isotropic) implying that there is no preferred mineral orientation within the horizontal plane; (3) the coefficient of S-wave anisotropy [ (V max − V min ) V min ] is estimated at 6–13% for SV-type waves; the SH-wave velocity shows only small variation with offset; (4) the observed relation between direction and velocity of S-wave propagation can be explained by mafic rocks containing a high amount of orthopyroxene minerals horizontally aligned in the pure shear stress regime.

Fluid reservoir (?) beneath the KTB drillbit indicated by seismic shear-wave observations

Indications for the possible presence of a fluid accumulation below the drillbit of the KTB (German Continental Deep Drilling Project)-Hauptbohrung are provided by comparisons of seismic P-wave and S-wave reflection sections. A bright reflection with negative polarity at approx. 8 km depth is seen only on P-wave sections. It is absent on S-wave sections in contrast to deeper reflections. This observation can be explained by the following geological models or combinations of them: (1) brine accumulation within a strongly fractured rock reservoir, (2) liquid/gas interface within strongly fractured rocks, (3) rock composition with increased quartz content and corresponding low Poissons ratio, and (4) varying symmetry system of seismic anisotropy due to rock foliation. Predictions concerning the nature and location of this reflector are expected to be verified by drilling and direct probing in spring-summer, 1993.

Lower crustal thinning in the Rhinegraben: Implications for recent rifting

Seismic near-vertical and undershooting experiments carried out in the southern upper Rhinegraben area between 1984 and 1991 show pronounced lateral variations of deep crustal properties. Significant differences in the apparent thickness of the reflective lower crust and the transparent upper crust appear to be related to different structural settings. A 12- to 14-km-thick reflective lower crust beneath a 15-km-thick transparent upper crust of the eastern graben shoulder (Black Forest) probably originated during Permo-Carboniferous reequilibration of thickened Variscan orogenic crust. Thinning of this lower crust by about 5 km beneath the graben and rifted domains transitional between graben and shoulder (the Dinkelberg block) is interpreted to be related to Cenozoic extensional faulting. A discrepancy between moderate extension of upper crust and lower crustal geometry indicates mechanical decoupling at depth during extension. Congruent modification of other physical properties is suggested by strong single reflective elements in the topmost parts of the thinned lower crust. In the transitional Dinkelberg block, such an anomalously strong reflector occurs at a depth of 20 km below a pronounced local maximum of earthquake activity; it is interpreted to be the presently active zone of decoupling which in time shifted from the rift axis to the eastern transition area.

Crustal “bright spots” and anisotropy from multi-component P- and S-wave measurements in southern Germany

Abstract Controlled shear-wave experiments have focussed on deep crustal targets that were earlier recognized by DEKORP P-wave surveys in southern Germany (Black Forest, Upper Palatinate). A prominent P-wave reflection (“bright spot”) at 8 km depth at the KTB deep drilling site is not seen on a comparable S-wave reflection section, in contrast to deeper reflections. This behaviour can be explained either by purely compositional effects (layer with anomalously high quartz content) or by fluid accumulation (“reservoir”) in a fracture zone. A 20 km deep, 10 km long prominent P-wave reflection in the southwestern Black Forest is only partially accompanied by corresponding S-wave reflections. The influence of the physical state of the rock e.g. stress-related cracking, in addition to possible compositional variations is suggested to explain the contrasting P- and S-wave reflection strength. Seismic anisotropy of 10% magnitude, revealed by S-wave splitting in vertical seismic profiling data at the KTB to more than 4 km depth, is attributed to sub-vertically dipping and NW-striking foliation.

Hydrological aspects of geophysical borehole measurements in crystalline rocks of the Black Forest

Abstract One of six wells drilled into the crystalline rock of the Black Forest basement during the German Continental Deep Drilling Project (KTB) site survey has been used to detect hydraulically active fractures and to identify their character by a variety of logging tools such as caliper and televiewer logging, vertical seismic profiling (VSP) and electrical and thermal measurements in addition to standard logging recordings. In the 265 m Moosengrund well drilled into the Triberg granite body these methods were applied successfully to discriminate between fractures and breakouts and to identify open and closed fractures and water-consuming or water-expelling fractures, respectively. Fracture strike, dip and width were determined from televiewer data. VSP measurements were used to determine the seismic velocities in the unfractured rock and the fracture zones. Estimates of permeability (10–55 Darcy) and transmissivity (0.1–1.0cm 2 s −1 ) of the fracture zones were obtained by the analysis of P waves and tube waves in the VSP records. The summed values of the transmissivity yield an upper estimate of the overall transmissivity determined by a pumping test.

Gravity and height changes in the ocean-continent transition zone in western Colombia

Abstract Crustal studies in western Colombia, by deep seismic, gravity and geomagnetic surveys, during the last two decades have revealed an extremely anomalous crustal structure as compared to the South American Andes further south. Strong gravity gradients and differences in seismic velocities showed a transition from oceanic to continental character between the Western and Central Andes. Measured gravity and height variations of opposite sign and lengths of 50 to 100 km on three east-west running profiles correlate surprisingly well with the typical positive Bouguer anomaly of the Western Andes which represents an isostatic instability. A gravity decrease of 0.5–1.0 mGal on two profiles and an increase on an intermediate one and corresponding ratios of gravity to apparent height variations of nearly −20 mGal/m are interpreted as consequences of deep-seated density variations. They may be related to collision tectonics and recent obduction processes between aseismic ridges riding on the Pacific Nazca plate and the continent.