Seismische Untersuchung des Lake Bosumtwi-Impaktkraters, Ghana
The Lake Bosumtwi impact crater has a diameter of 10.5 km, and with its age of 1 Ma it is one of the youngest large craters on Earth. It is one of a relative few craters formed in "hard rock" rather than in a sedimentary target, and it is the source of a tektite strewn field. Seismic reflection and refraction data have been interpreted in an integrated approach, seismic quality Q has been estimated and synthetic seismic data have been modelled. The results yield information on the crater structure, which is buried beneath the lake and the postimpact sediments, and on the properties of the different layers. The identified central uplift has a diameter of almost 2 km and a height of more than 100 m above the annular moat. It is interpreted to consist of fall-back breccia. Within this layer P-wave velocity increases laterally towards the central uplift due to denser material that moved upwards from greater depth. The amount of uplift is estimated to be at least 800 m. The apparent crater depth is 550 m. Seismic velocity increases with increasing depth. The crater floor is not detected and there is no seismic evidence for a melt layer, although a magnetic anomaly suggests a melt body north of the center. Preliminary numeric modelling of the Bosumtwi crater reproduces central uplift and diameter. Postimpact sediments covering the central crater are 180 to 300 m thick. P-wave velocities within the sediment column are low, averaging 1.5 km/s at the top to 1.65 km/s at the bottom. S-wave velocity can only be estimated: It is low at the sediment surface, increasing to a an estimated maximum of 900 m/s. The comparison of the Lake Bosumtwi crater with other terrestrial impact structures underlines its unique structure.