TSK 11 Göttingen 2006 Wagner et al.
 Structural investigation and
 strain analysis of a polyphase
 flower structure in the Lower
 Saxony Basin, Germany
 Poster
 Lars Wagner1 Tina Lohr2 David
 C. Tanner3 Charlotte M. Krawczyk2
 Onno Oncken2
 Introduction
 The Lower Saxony Basin (LSB) is a part
 of the post-Variscan Central European
 Basin System. We used a 3-D reflection
 seismic dataset in the northern LSB,
 provided by RWE-DEA AG, Hamburg
 (c.f. Lohr et al. submitted) for our in-
 vestigation, which is concerned with the
 detailed structural and kinematic anal-
 ysis of a flower structure within Meso-
 zoic strata. This data is used in turn
 to determine input parameters for fur-
 ther 3-D geometrical retro-deformation.
 The retro-deformation verifies our as-
 sumptions about the structure and tec-
 tonic processes, and gives further infor-
 mation about sub-seismic strain distri-
 bution with respect to the branch faults
 of the flower structure.
 Structural and kinematic analysis
 In a preliminary step, structural anal-
 ysis was carried out to ensure our in-
 terpretation of the investigated flower
 structure was correct. We analysed it
 by using the criteria of Harding (1990).
 The investigated structure shows a pla-
 nar, steeply-dipping main fault zone
 1 Department of Geology, University of
 Gießen, Senkenbergstraße 3, D-35390 Gießen
 2 GeoForschungsZentrum Potsdam, Tele-
 grafenberg, D-14473 Potsdam 3 Department
 of Structural Geology and Geodynamics,
 Geoscience Centre, University of Göttingen,
 Goldschmidtstraße 3, D-37077 Göttingen
 Figure 1: Interpreted 2-D seismic cross-
 section of the flower structure (width of fig-
 ure ca. 4 km, depth from 0–3.5 sTWT). An
 earlier Upper Cretaceous compressive stage
 leads to reduced sedimentation towards the
 central part of the structure. The following
 Paleocene extension led to increased sed-
 imentation and reduction of displacement
 in the deeper parts of the branch faults.
 which indicates wrench tectonics, and
 also divergent branch faults at higher
 levels; both features are characteristics
 of a flower structure (Fig. 1). In ad-
 dition, competing structural interpreta-
 tions could be ruled out. For example,
 the collapsed crest of an anticline would
 not show a deeper main fault zone. Fur-
 thermore, cross-sections at varying dis-
 tances from a nearby Zechstein salt di-
 apir to the SE of this structure show
 no change in structural style, suggesting
 salt tectonics was not the main driving
 force for the tectonic evolution of the
 flower structure. The structure shows
 a polyphase synsedimentary evolution,
 with a compressive stage from Late Cre-
 taceous up to earliest Tertiary, followed
 by extension during the later Tertiary
 (Fig. 1).
 The main goal of the kinematic analy-
 sis was to determine the transport vec-
 tor as one of the input parameters for
 later retro-deformation. In this exam-
 ple, direct determination of the trans-
 port vector was not possible because
 1
Wagner et al. TSK 11 Göttingen 2006
 of the lack of suitable markers, and
 also of the polyphase tectonic history
 of this structure. Instead, minimum
 transport distances are defined on fault
 planes by the cutoff relationships of sed-
 imentary horizons. Additional kine-
 matic indication is given by the topog-
 raphy of the branch fault planes, which
 shows strongly-corrugated morphologies
 (Fig. 2, 3). These asymmetrically-
 shaped corrugations have an amplitude
 of some ten meters and a spacing of
 approximately 1000meters. Their axes,
 which indicate the direction of tectonic
 transport (Needham et al. 1996), show
 less than 20° deviation from the dip
 azimuth of the fault plane (Fig. 3).
 As a consequence of this, the post-
 Paleocene evolution of the flower struc-
 ture seems to have been mainly dom-
 inated by extensional tectonics, since
 lateral transport is negligible. Indica-
 tions for splitting of the strike-slip and
 dip-slip components to different branch
 faults was not found, since all of the
 fault planes show steeply-dipping curva-
 ture lineations.
 Restoration and strain analysis
 We perform 3-D geometrical retro-
 deformation of the fault displacement
 using the Midland Valley 3DMove soft-
 ware. For this, input parameters such
 as heave distance, transport direction,
 and inclined shear vector were obtained
 from the previous structural and kine-
 matic analysis. Iterative tests of some of
 the parameters and the use of different
 restoration algorithms provide an indi-
 cation of the reliability of our modelling
 and give some insights on their effect
 on the distribution of sub-seismic strain
 distribution (Fig. 4).
 The 3D retro-deformation points to a
 strong correlation between fault sur-
 Figure 2: Branch fault planes, showing a
 strongly-corrugated morphology.
 Figure 3: Stereonet-diagram with dip az-
 imuth (crosses) and corrugation axis direc-
 tion (circles) of branch fault planes (great
 circles). Calculation of the displacements
 and dipping angles of the branch fault
 planes, typically between 40–50°, allows to
 calculate the minimum extension amount
 of 240m, or 10%, with respect to Base Ter-
 tiary level’s width of 2400m.
 face morphology and strain distribution
 in the hanging wall, especially above
 ramp structures and with respect to
 fault surface corrugations. Further-
 more, the strain distribution also de-
 pends strongly on the angular differ-
 ences between the orientation of cor-
 rugation axes and transport direction.
 The modelling shows that deviations of
 more than 20–30°between transport di-
 rection and corrugation axis may lead to
 2
TSK 11 Göttingen 2006 Wagner et al.
 Figure 4: Greyscale map showing the rela-
 tionship between strain accumulation (light
 grey — highest strain (15%) in the hanging
 wall of the Base Tertiary horizon and the
 fault surface morphology. View from top,
 fault plane dips toward upper right.
 significant higher strain amounts. This
 is in agreement with the modelling re-
 sults of Needham et al. (1996), in their
 analysis of North Sea faults. The use
 of different restoration algorithms shows
 only few differences (and therefore the
 results shown here are representative) in
 amount and distribution of sub-seismic
 strain in our models. That is, we pre-
 dict strains of 10–15% (e1 magnitude)
 within 200m of the faults, extending to
 400m above ramp structures.
 Conclusions
 Using the criteria of Harding (1990), we
 established the flower structure nature
 of the investigated structure. Our inves-
 tigation demonstrates that this flower-
 structure has undergone a polyphase
 evolution, with compression in the Up-
 per Cretaceous and predominately ex-
 tension during Paleocene to Eocene
 times. The minimum amount of this
 extension is 240m, with respect to the
 Base Tertiary level. There is no evi-
 dence for wrench tectonics in the lat-
 ter tectonic stages. The 3D retro-
 deformation points to a strong correla-
 tion between fault surface topography
 and strain distribution, but the strain
 value also depends strongly on the an-
 gular difference between the corrugation
 axes of fault planes and transport di-
 rections. In our models, the use of dif-
 ferent restoration algorithms shows only
 few differences in amount and distribu-
 tion of sub-seismic strain. The mod-
 elling show that deviations of more than
 20–30° between transport direction and
 fault plane corrugation axis may lead to
 significant higher strain amounts in the
 hanging-wall.
 Acknowledgements We thank
 RWE-DEA AG, Hamburg for allowing
 us to use their 3D seismic database
 and borehole data for this project. The
 project is part of the DFG SPP 1135;
 we acknowledge DFG grants TA 427/1
 and KR 2073/1.
 References
 Harding TP (1990) Identification of wrench
 faults using subsurface structural data: cri-
 teria and pitfalls. AAPG Bull 74, 10
 Lohr T, Krawczyk CM, Tanner DC, Samiee R,
 Endres H, Trappe H, Oncken O & Kukla PA
 (under review) Structural evolution of the
 NW German Basin over time from 3-D reflec-
 tion seismic data — a case study at the bor-
 der between Lower Saxony Basin and Pom-
 peckj Block. Submitted to Journal of Struc-
 tural Geology
 Needham DT, Yielding, G & Freeman B (1996)
 Analysis of fault geometry and displacement
 patterns In: Buchnan, PG & Nieuwland, DA
 (eds) Interpretation, Validation and Mod-
 elling. Geol Soc Sec Publ 99
 3