TSK 11 Göttingen 2006 Tanner et al. Kinematic 3D Retro- Deformation of Fault Blocks Picked from 3D Seismics Poster David C. Tanner1 Tina Lohr2 Char- lotte M. Krawczyk2 Onno Oncken2 Heike Endres3 Ramin Samiee3 Hen- ning Trappe3 Peter A. Kukla4 Introduction Movement on fault planes causes a large amount of smaller-scale deformation, ductile or brittle, in the area surround- ing the fault. Much of this deforma- tion is below the resolution of reflec- tion seismics (i.e. sub-seismic, <10m displacement), but it is important to de- termine this deformation, since it can make up a large portion of the total bulk strain, for instance in a develop- ing sedimentary basin. Calculation of the amount of sub-seismic strain around a fault by 3-D geometrical kinematic retro-deformation can also be used to predict the orientation and magnitude of these smaller-scale structures. However, firstly a 3-D model of the fault and its faulted horizons must be con- structed at a high enough resolution to be able to preserve fault and hori- zon morphology with a grid spacing of less than 10m. Secondly, the kinemat- ics of the fault need to be determined, and thirdly a suitable deformation al- gorithm chosen to fit the deformation style. Then by restoring the faulted horizons to their pre-deformation state 1 Geowissenschaftliches Zentrum der Georg-August-Universität Göttingen, Abteilung Strukturgeologie und Geody- namik, Goldschmidtstr. 3, D-37077 Göttingen 2 GFZ Potsdam, Telegrafenberg, D-14473 Potsdam 3 TEEC, Burgwedelerstr. 89, D-30916 Isernhagen 4 RWTH Aachen, Geol. Institut, Wüllnerstr. 2, D-52056 Aachen (a ‘regional’), the moved horizons can be interrogated as to the strain they underwent. Since strain is commuta- tive, the deformation demonstrated dur- ing this retro-deformation is equivalent to that during the natural, forward de- formation. Working area Our working area is located within the northern part of the Lower Sax- ony Basin. This structural unit is a part of the post-Variscan Central European Basin System. We use 3- D, depth-converted, reflection seismics provided by RWE-DEA AG, Hamburg for this investigation. The seismic database covers an area of ca. 15 × 10 km2 and extends down to 7.5 km depth. This is deep enough to reveal the subcrop of Upper Carboniferous and stratigraphically-higher strata. Construction of a 3-D model Our particular interest is the Rotliegend strata of this area. The Rotliegend mainly consists of aeolian and fluviatile sandstones, which form an onshore hy- drocarbon play. The Top Rotliegend surface lies between 4.5 and 5.7 km depth in the studied area. First, the Top Rotliegend surface was picked at a high resolution, as were fault surfaces which displaced Top Rotliegend and lower strata. Displacement of the Top Rotliegend varies from 0–250m, mainly caused by normal faulting. These sur- faces were then gridded in GoCAD, and then transferred to the Midland Val- ley software package 3DMove for retro- deformation. 1 Tanner et al. TSK 11 Göttingen 2006 Figure 1: Left: Map view of the hanging-wall of a Rotliegend thrust after retro- deformation, colour-coded for magnitude of the major strain (e1). Lighter greys are higher strain. Right: Map view of the hanging-wall of the thrust after retro-deformation, showing trajectories (and dip and strike data) of the e2–e3 plane, and thus the expected orientation of extensional fractures, caused by thrusting. Insert shows lower hemisphere, equal-area stereonet of poles to the e2–e3 planes. Retro-deformation For retro-deformation modelling we used the algorithms ‘inclined shear’ for normal faults and ‘fault-parallel flow’ for reverse faults, respectively. In the former, the hanging-wall volume of a fault is sheared at an oblique angle to the fault surface, and thus strain is di- rectly attributable to fault curvature. In the latter method, all horizon surface nodes move parallel to the fault surface (as the name suggests), and therefore strain is also related to fault curvature. Figure 1 demonstrates an example of retro-deformation (using the ‘fault- parallel flow’ algorithm) of a thrust within the Rotliegend, and shows the strain tensor values and orientations which occurred in the surrounding strata. We propose the values of the strain tensor, and thus the magnitude of the strain, are equivalent after retro- or forward deformation. For this ex- ample, the e1-magnitude ranges from 0–20%, but the highest strain intensi- ties are constrained to within 700m of the fault generally, and up to 1 km lo- cally (Fig. 1). We propose that the e1- 2 TSK 11 Göttingen 2006 Tanner et al. magnitude can be correlated to the den- sity, and that the e2–e3 can be corre- lated to the orientation, of small-scale structures. 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. 3