The Karla impact structure (Russia) explored by potential-field investigations

With no significant topographic expression and limited bedrock exposure, the similar to 10 km diameter Karla impact structure (Tatarstan, Russia) is poorly known. The age of the impact is also poorly constrained stratigraphically to between 4 and 60 Ma, even if an upper Miocene age is more likely. Targeted gravity and magnetic field surveys were conducted over Karla to explore its size and structure in 2019. Bouguer gravity anomaly data reveal a central positive (+2 mGal) peak similar to 2 km in diameter surrounded by a concentric negative (-1 mGal) anomaly extending to similar to 3 km radius; a more irregular, outward-decreasing (+1 to -1 mGal) positive anomaly extends to 6-8 km radius. A complex impact structure with diameter of 8-10 km is consistent with the Bouguer anomalies. Magnetic field data show 1 to several km-wavelength anomalies with amplitude variation from +150 to -150 nT and little concentric structure, although the impact feature broadly corresponds to a magnetic low with a weak central high. A 2-D numerical model of the structure was built using these potential-field data and petrophysical properties measured on collected samples. It confirms a central uplift composed of Paleozoic sediments and Archean crystalline basement up to 1 km of depth. A 500 m deep collapsed disruption cavity filled by breccia and lacustrine deposits accounts for the Bouguer negative ring. The reversely polarized and weak central magnetic anomalies are controlled by the geometry of the crystalline basement associated with the deformation during the central uplift.