TSK 11 Göttingen 2006 Dresmann et al. A further step toward a ther- mochronological 3-D model of the SE Black Forest Poster Horst Dresmann1 Ina Spottke1 Zoltan Timar-Geng2 Andreas Wetzel1 Bern- hard Fügenschuh3 Fission-track (FT) data always depend on the thermal history of a 3-D geologi- cal complex. Therefore it is expedient to display FT data sets in 3-D mod- els. Such a model in which tectonic, sedimentological and hydrological fea- tures are combined can greatly improve the interpretation of the palaeo-thermal pattern derived from FT analyses. Since 1988 several FT studies have been conducted in the Black Forest (BF) (Michalski 1988, Wyss 2000, Timar- Geng et al. 2004, 2005). Timar-Geng et al. (2006) analyse the crystalline base- ment and the Permian Rotliegend be- neath the Mesozoic units in the Tabular Jura (TJ) east of Basel, Switzerland, us- ing samples taken from the three Nagra boreholes at Kaisten, Riniken and Leug- gern. In particular Timar-Geng et al. (2005, 2006) characterise the thermal history of this pre-Mesozoic basement. For the BF they estimate at least one heating phase during the lower and middle Mesozoic while similar heating could not be ob- served in northern Switzerland. How- ever, the FT-data in both regions show moderate to rapid cooling during the Cretaceous and Lower Eocene, which was followed by an Upper Eocene heat- ing event. 1 Geologisch-Palaeontologisches Institut, Uni- versity of Basel, Switzerland 2 Geologis- ches Institut, University of Freiburg, Germany 3 Institut für Geologie und Palaeontologie, University of Innsbruck, Austria The software package GOCAD (Geolog- ical Objects Computer-Aided Design) was used to build a digital elevation model (DEM), which provide a new de- tailed view of these FT data sets. The model is located about 20 km east of Basel, Switzerland, and extends over an area of about 21 km by 24 km and spans a vertical height difference of about 2km. The data sets described above along with two additional FT analyses from the Buntsandstein which lies directly on the BF crystalline, were compiled and plotted at their topographic heights in the DEM. The FT central-ages (Galbraith & Laslett 1993) of this region range be- tween 25 ± 2Ma and 98 ± 6.5Ma. The topographic positions extend between −1412m at the Borehole Riniken and 960m in the BF with mean sea level as a reference. FT central-age isochron surfaces were drawn in order to visualize the ther- mal evolution within the model range. Because the FT central-ages also corre- spond to a closure temperature, these surfaces can also be considered an isotherm. The FT closure temperature of apatite is about 90± 30°C (Laslett et al. 1987). Therefore each surface shows the position and shape of the ca. 90°C isotherm of a specific age. This 3-D model points out an impor- tant difference in the thermal evolution of the BF und the TJ. The vertical dis- tance between the isothermal surfaces increases from north to south. Between 90Ma and 60Ma the ca. 90°C isotherm drops at the Kaisten borehole by 1000m while in the same time span in the BF a lowering of the same isotherm by 300m can be observed. In the eastern part of the model this feature is not as marked 1 Dresmann et al. TSK 11 Göttingen 2006 as in the west but nevertheless it is ob- servable. To explain this entirely different ther- mal evolution it is necessary to turn to the tectonic and other geological fea- tures of the region. South of the ex- posed BF crystalline and beneath the TJ there lies an old Variscan struc- ture: the Permo-Carboniferous trough (PCT). This trough strikes in WSW- ENE direction and extends from Lake Constance to the Bresse Graben and contains up to 6000m of Palaeozoic sed- iments. Additionally, some Variscan fault structures strike in WNW–ESE di- rection and cut both the BF an the PCT, for example the Eggberg Fault and the Vorwald Fault. Beside the tec- tonic structures the hydrological charac- teristics played an important role during the palaeo-thermal evolution. Circu- lating hot fluids controlled the thermal pattern. Variscan faults were often reac- tivated during the Mesozoic (e.g. Wet- zel et al. 2003) and also during the formation of the Upper Rhine Graben. (e.g. Illies 1967) These faults are the major water-conducting features in the crystalline basement of the BF, joints and fracture networks are tributaries. Below the aquifers within the Mesozoic of the TJ, the PCT trough sediments predominantly act as an aquitard. Only the border faults of the trough were important pathways for fluids (Thury 1994). Considering the Mesozoic sedimentolog- ical history of the region it is unlikely that fault movements are responsible for the different palaeo-thermal pattern of the BF and the TJ. Only differ- ent magnitudes of heat flow caused by hydrothermal circulating fluids can ex- plain the ‘warm’ BF crystalline in com- parison to the ‘cold’ basement of the TJ at the transition between the Mesozoic and Tertiary. References Galbraith RF & Laslett GM (1993) Statistical models for mixed fission track ages. Nucl. Tracks 21, 459–70 Illies JH (1967) Development and tetonic pat- tern of the Rhinegraben. Thr Rhinegraben progress report 1967 Laslett GM, Green PF, Duddy IR & Gleadow AJW (1987) Thermal annealing of fission tracks in apatite 2. A quantitative analy- sis. Chemical Geology (Isotope Geoscience Section) 65:1–13 Michalski I (1987) Apatit-Spaltspuren- Datierungen des Grundgebirges von Schwarzwald und Vogesen: Die post- variszische Entwicklung. Unpubl. doctoral dissertation, Heidelberg, pp 125 Thury M, Gautschi A, Mazurek M, Müller WH, Naef H, Pearson FJ, Vomvoris S & Wilson W (1994) Geology and hydrogeology of the crystalline basement of Northern Switzer- land. Nagra Technischer Bericht, NTB 93– 01, Baden Timar-Geng Z, Fügenschuh B, Schaltegger U & Wetzel A (2004) The impact of the Juras- sic hydrothermal activity on zircon fission track data from the southern Upper Rhine Graben area. Schweizerische Mineralogische und Petrographische Mitteilungen 84, 257– 269 Timar-Geng Z, Fügenschuh B, Wetzel A & Dresmann H (2005) Low-temperature ther- mochronology of the flanks of the southern Upper Rhine Graben. International Journal of Earth Sciences (in press) Timar-Geng Z, Fügenschuh B, Wetzel A & Dresmann H (2006): The low-temperature thermal history of northern Switzerland as revealed by fission track analysis and inverse thermal modelling. Eclogæ Geologicæ Hel- vetiæ. (submitted) Wetzel A, Allenbach R & Allia V (2003) Reacti- vated basement structures affecting the sed- imentary facies in a tectonically ‘quiescent’ epicontinental basin: an example from NW Switzerland. Sedimentary Geology 157:153– 172 2 TSK 11 Göttingen 2006 Dresmann et al. Wyss A, (2001) Apatit Spaltspur Unter- suchungen in der Vorwaldscholle (SW- Deutschland). Unpubl. diploma thesis, Univ. Basel, pp 69 3