Geophysical research on the Kardla impact structure, Hiiumaa Island, Estonia

The 4 km wide and 500 m deep circular Kardla impact structure in Hiiumaa Island, Estonia, of middle Ordovician age (similar to 455 Ma), is buried under Upper-Ordovician and Quaternary sediments. To constrain the geophysical models of the structure, petrophysical properties such as magnetic susceptibility, natural remanent magnetization (NRM), density, electrical conductivity, porosity and P-wave velocity were measured on samples of crystalline and sedimentary rocks collected from drill cores in different parts of the structure and the surrounding area. The results were used to interpret the central gravity anomaly of -3 mGal and the magnetic anomaly of -100 nT and also the surrounding weak positive anomalies revealed by high precision survey data. The unshocked granitic rocks outside the structure have a mean density of similar to 2630 kgm(-3). Their shocked counterparts have densities of similar to 2400 kgm(-3) at a depth of similar to 500 m, increasing up to 2550 kgm(-3) at a depth of 850 m. Porosity and electrical conductivity decrease, but P-wave velocity increases as density increases away from the impact point. Thus, the gradual changes in the physical properties of the rocks as a function of radial distance from the crater centre are consistent with an impact origin for Kardla. As in many other impact structures, the magnetization of the shocked rocks are also clearly lower than those of unshocked target rocks. A new geophysical and geological model of the Kardla structure is presented based on geophysical field measurements and data on gradual changes in petrophysical parameters of the shocked target and overlying rocks, together with structural data from numerous boreholes. An important feature of this model is the lack of an observable geophysical signature of the central uplift observed in drillcores.