* Astronomy

Title: Structural modelling of the Vichada impact structure from interpreted ground gravity and magnetic anomalies.
Authors: Orlando Hernández; Sait Khurama; Gretta C. Alexander

A prominent positive free-air gravity anomaly mapped over a roughly 50-km diameter basin is consistent with a mascon centred on (4°30'N, -69°15'W) in the Vichada Department, Colombia, South America. Ground follow up gravity and magnetic anomalies were modelled confirming the regional free air gravity anomalies. These potential field anomalies infer a hidden complex impact basin structure filled with tertiary sedimentary rocks and recent quaternary deposits. Negative Bougueranomalies of 8 mGals to 15 mGals amplitude are associated with a concentric sedimentary basin with a varying thickness from 100m to 500 m in the outer rings to 700m to 1000m at the center of the impact crater basin. Strong positive magnetic anomalies of 100 nT to 300 nT amplitude infer the presence of a local Precambrian crystalline basement that was affected by intensive faulting producing tectonic blocks dipping to the center of the structure, showing a typical "domino structure" of impact craters such as that of Sudbury, Ontario, Canada. Basic to intermediate mineralised veins and dikes with contrasting density and magnetic susceptibility properties could be emplaced along these faulting zones, as inferred from local gravity and magnetic highs. The geologic mapping of the area is limited by the flat topography and absence of outcrops/ geomorphologic units. Nevertheless, local normal faults along the inner ring together with radially sparse irregular blocks over flat terrains can be associated with terraced rims or collapse of the inner crater structure and ejecta blanket, respectively. A detailed airborne electromagnetic survey is recommended to confirm the gravity and magnetic anomalies together with a seismic program to evaluate the economic implications for energy and mineral exploration of the Vichada impact structure.

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