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Formation and development of complex impact craters can be efficiently studied only by combining both geological-geophysical observations and numerical modelling. Kärdla crater is the smallest (4-km in diameter) complex craters on Earth. It is very well preserved and rather well pre-investigated, which makes it ideal for such complex studies. Drillings at Kärdla show a small central-uplift. With detailed reflection seismic work we will find out the location and dimensions of the central uplift, and to what extent has the central uplift collapsed, if at all. With seismic method we will also study the gullies in the crater rim in the north and south that were created by the back-surging sea water. In addition, we will investigate erosional features in the western part of the rim that might be created by non-channelized flow, which has not been documented elsewhere. The data are of very great value for modelling of oblique impacts into the shallow sea. The third topic where observations in Kärdla can provide valuable data for numerical models is related to impact heating and crater cooling. Impact rocks at Kärdla are believed to be melt-free. However, the shock alteration level and first numerical models suggest that the pressures were high enough for melting. This might mean that (a) the melt production and dispersion mechanism working in water/sea covered targets is substantially different from continental counterparts; or (b) we face a total recrystallization of dispersed melt parts and bodies. Another important problem to study is the crater cooling accompanied by hydrothermal processes, their geological-mineralogical and possible biological consequences. Results of both reflection seismic and mineralogical studies will be used for calibration of impact models. We simulate (a) excavation and modification stages of formation of the Kärdla crater and (b) erosion/sedimentation by the re-surging sea and (c) thermal imprint of the impact and development of the hydrothermal systems.
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