Geophysical update of the diameter of the Zhamanshin impact structure, Kazakhstan

Abstract The rim of the 0.9‐Ma‐old Zhamanshin impact crater in Kazakhstan is considered to be almost completely exposed via a series of hills surrounding a central flat area, indicating an apparent diameter of 14 km. This would make Zhamanshin the second largest impact crater of the Quaternary period. However, the overall geometry of these hills is not circular, and there is no visible central uplift, raising doubts about this diameter estimate. In this work, we used a set of geophysical methods (magnetic and gravity field mapping, and passive seismic measurements) to better characterize the geometry of the impact structure. The new potential field data reveal residual anomaly maps with significant concentric features extending only 2.5 to 3 km away from the structure center. The passive seismic data were used to derive H/V spectra. Combined with previous drilling information and surface geology, it allows us to delineate the depth of the interface between the fractured bedrock and the post‐impact sediments, which reveals a buried crater with a small central uplift. These new field geophysical data were combined with laboratory petrophysical measurements to constrain a model of the impact structure. The modeled complex impact structure has a 5 to 6 km diameter and is totally buried by sediments. This burial has occurred in <0.9 Ma, which is challenging but not incompatible considering the erosion rates in the area and the pre‐existing neighboring topography.