The Lake Bosumtwi meteorite impact structure, Ghana—Where is the magnetic source?

Abstract

Abstract-The Bosumtwi impact structure (Ghana) is a young and well-preserved structure where a vast amount of information is available to constrain any geophysical model. Previous analysis of the airborne magnetic data and results of numerical simulation of impact predicted a strongly magnetic impact-melt body underneath the lake. Recent drilling through the structure did not penetrate such an expected impact-melt rock magnetic source. A new 3-D magnetic model for the structure was constructed based on a newly acquired higher-resolution marine magnetic data set, with consideration of the observed gravity data on the lake, previous seismic models, and the magnetic properties and lithology identified in the two International Continental Scientific Drilling Program (ICDP) deep boreholes. The new model contains highly magnetic bodies located in the northeast sector of the structure, not centered onto the drilling sites. As in previous models, higher magnetization than that measured in outcropping impactites had to be assigned to the unexposed source bodies. Integration of the new model with the borehole petrophysics and published geology indicates that these bodies likely correspond to an extension to the south of the Kumasi batholith, which outcrops to the northeast of the structure. The possibility that these source bodies are related to the seismically identified central uplift or to an unmapped impact-melt sheet predicted by previous models of the structure is not supported. Detailed magnetic scanning of the Kumasi batholith to the north, and the Bansu intrusion to the south, would provide a test for this interpretation.

The Lake Bosumtwi meteorite impact structure, Ghana—A magnetic image from a third observational level

“…1 of Ugalde et al 2007). The crater is occupied by Lake Bosumtwi and has a rim-torim diameter of 10.5 km (Koeberl and Reimold 2005).…”

“…The crater was formed by a meteorite impact about 1.07 Myr ago when a bolide hit lower greenschist facies supracrustal rocks of the 2.1-2.2 Gyr Birimian Supergroup, which are intruded by various intrusive crystalline rocks Koeberl and Reimold 2005; Fig. 2 of Ugalde et al 2007).…”

“…This survey collected total magnetic field, electromagnetic field, and gamma radiation data. In 2001, the Kwame Nkrumah University of Science and Technology (KNUST, Kumasi, Ghana) collected a marine magnetic data set to complement the existing airborne data (Danuor 2004;Ugalde et al 2007). This new data set had a wider spacing of 800 m, and the magnetometer was towed behind the marine platform used for that purpose.…”

“…The shorter distance to the sources and enhanced sampling rate allows a better representation of the magnetic anomalies, which cannot be achieved by pure downward analytic continuation from the much higher observation surface of the airplane data to the lake level. Interpretation and modeling of these data sets are discussed by Ugalde et al (2007), and we will therefore concentrate on the new data here.…”

“…Spacing between stations varies from 100-125 m on the northern edge of the lake to 500-600 m along the north-south traverses on the east side. A finer sampling was chosen on the north edge in order to have a better control on the northern-positive part of the main magnetic anomaly (Plado et al 2000;Ugalde et al 2007). …”