Paleostress Analysis Applied to Fault-Slip Data from the Southern Margin of the Central European Basin System (CEBS) J

Home , Fracture (geology), Inversion (geology), Paleostress, Stress field

DGMK/ÖGEW-Frühjahrstagung 2007, Fachbereich Aufsuchung und Gewinnung, Celle

Paleostress Analysis Applied to Fault-Slip Data from the Southern Margin of the Central European Basin System (CEBS) J. Sippel*, M. Scheck-Wenderoth*, K. Reicherter**, S. Mazur***, *GFZ Potsdam, **RWTH Aachen, ***University of Wroclaw, Poland

Abstract We investigate the paleostress fields which controlled the post-Variscan evolution of the Central European Basin System (CEBS). Therefore, field studies are carried out in the marginal areas of the CEBS where Late Palaeozoic and Mesozoic rocks of the basin fill are present in outcrops bearing the imprints of several deformation phases that affected the basin system since the latest Carboniferous. Field studies including structural analysis, measurement of fault-slip data and careful collection of kinematic indicators provide the data base for this study. In the case of polyphase tectonics, the chronology of successive events is deduced and the total fault population from each site is qualitatively divided into different subsets, each being consistent with one specific stress regime. Since the stratigraphy and evolution of the CEBS are well known, temporal and spatial correlations of paleostress orientations are possible. Beside cross-cutting relationships derived from outcrops, we apply different graphical and numerical methods to separate the faults into homogeneous subsets. Depending on (1) the nature of faults (i.e. neoformed or reactivated), (2) the distribution of fault-slip data and (3) the deformation style, the deviatoric stress tensor is calculated for each subset using either the Numeric Dynamic Analysis (Spang, 1972; Sperner et al., 1993) or the Multiple Inverse Method (Yamaji 2000). The results are obtained in terms of a reduced stress tensor, consisting of (1) the orientations of the three principal stress axes σ1, σ2 and σ3 with σ1≥σ2≥σ3 and (2) the ratio of principal stress differences, R=(σ2−σ3)/(σ1−σ3) with 1≥Φ≥0.

Introduction The Central European Basin System (CEBS) covers an area extending from the southern North Sea across Denmark, The Netherlands and northern Germany to Poland (Fig. 1). Studies that investigate the structural evolution of the CEBS on a basin scale (Scheck- Wenderoth & Lamarche, 2005) or on the scale of sub-basins (e.g. Clausen & Pedersen, 1999; Scheck & Bayer, 1999; Hansen et al., 2000; Baldschuhn et al., 2001; Scheck et al., 2002a,b; Evans et al., 2003; Lamarche, et al. 2003) argue for a recurrent change of the stress field through time in this area. Whereas the present day stress field of the CEBS is well known (Roth & Fleckenstein, 2001), studies on the evolution of paleostresses in northwestern Europe are limited to certain regions or time-slices (e.g. Vandycke, 1997, 2002; Lamarche et al., 1999, 2002). For this study we analyse the structural pattern, in particular mesoscale faults, from outcrops distributed along the southern margin of the CEBS. Tectonic stress, regardless if recent or paleostress cannot be measured directly but must be derived from strain. We use slickensides that document the movement along fault planes (in many cases even the sense of movement) as kinematic indicators (Doblas, 1998). A fault-slip datum is composed of fault orientation (dip direction, dip), slip orientation (azimuth, plunge), and the sense of slip (reverse, normal, dextral, or sinistral). Knowing the fault-slip attitudes of numerous faults, it is possible to calculate the reduced stress tensor which consists of (1) the orientations of the

DGMK-Tagungsbericht 2007-1, ISBN 978-3-936418-65-1

three principal stress axes σ1, σ2, and σ3 with σ1≥σ2≥σ3 and (2) the ratio of principal stress differences R = (σ2-σ3) / (σ1-σ3) (e.g. Angelier, 1979, 1984; Etchecopar et al. 1981). Relating fault kinematics to stress states implies that all faults considered have slipped in response to the same deviatoric stress. For this reason, heterogeneous fault-slip data must be separated into homogeneous subsets before stress inversion. If field information on kinematically inconsistent structures is insufficient we use the graphical P-B-T method (Sperner et al., 1993) and the numerical Multiple Inverse Method (Yamaji, 2000) for separation of faults and stress states. A diligent separation of inconsistent subsets is an essential for data from the CEBS that experienced several deformation phases.

Fig. 1: The depth to top pre-Permian illustrates the major structural elements of the Central European Basin System (Scheck-Wenderoth & Lamarche, 2005).

Methods The P-B-T method (PBT) is a kinematic analysis that in the strict sense yields strain axes rather than stress axes (Turner, 1953; Sperner et al., 1993): For each individual fault-slip datum, theoretical contraction (P), neutral (B), and extension axis (T) are constructed in a lower-hemisphere plot (Fig. 2). According to the Mohr-Coulomb fracture criterion, the method adopts a defined fracture angle Θ between P and the fault plane (we use Θ=30°). After PBT calculation for all faults of an outcrop, kinematically inconsistent faults can be separated by clusters of strain axes in the plot. After data separation, the reduced stress tensor is calculated for each homogeneous subset applying the Numeric Dynamic Analysis (NDA;

Sperner et. al., 1993) which is based on the assumption that σ1, σ2, and σ3 lie in those directions where the strain axes P, B, and T, respectively, concentrate.

Fig. 2: P-B-T method applied to one fault-slip datum. The contraction axis P and the extension axis T lie in a plane given by the shear plane normal and the slip line. P is constructed with a distance of Θ=30° from the slip line in a direction opposite to that indicated by the slip line arrow. The neutral B axis lies within the fault plane; P, B, and T are mutually perpendicular.

We additionally apply the Multiple Inverse Method (MIM; Yamaji, 2000). MIM is a modification of the Direct Inverse Method (Angelier, 1979) assuming that slip is parallel to the maximum resolved shear stress; it is specially developed to identify all significant stress states inherent in a heterogeneous fault-slip data set. The whole data set is divided into k- element subsets (we use k=4) to which the inverse technique is applied. The reduced stress tensor for each artificial k-fault-subset is plotted with σ1 and σ3 in two lower-hemisphere plots and the stress ratio R colour coded (Fig. 3). Significant stress states can be identified as clusters of agreeing symbols in the plot.

Fig. 3: MIM applied to fault-slip data from Lienen quarry (Upper Cretaceous limestones, Osning Thrust area). Stress states calculated for all 4-fault-subsets created from the whole data set of 35 faults. σ1 axes are plotted in the left and σ3 axes in the right stereonet (lower hemisphere, equal-area plots). Symbols’ heads represent the directions of stress axes; the tail of a σ1 symbol indicates the direction and plunge angle of the corresponding σ3 axis, and vice versa. The value of R is expressed by the colour of each symbol.

Results Both techniques, the combined usage of PBT and NDA on the one and MIM on the other hand, allow an unambiguous separation of heterogeneous data sets. In most cases the results of both techniques are the same, not only in terms of separation but also in terms of subsequent stress calculation. From Upper Cretaceous rocks located along the southern margin of the NW German basin we can derive a stress state corresponding to NNE-SSW compression which can clearly be assigned to the Late Cretaceous inversion phase. This stress state can also be inferred from Upper Jurassic, Middle Triassic, and Upper Carboniferous rocks throughout the entire southern margin of the CEBS. Local deviations from this general trend can be found in Upper Cretaceous rocks where uprising salt or the influence of a major zone of structural weakness could be hold responsible for faulting. At many locations, the inversion phase is followed by a weaker NNE-SSW extensional phase. Stress states that preceded the Late Cretaceous inversion do not show such a directional consistency throughout the basin. Their temporal classification is the main aspect of our current and future work.

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