Magnetic and gravity model of the Morokweng impact structure

The geophysical characteristics of the 145-Ma-old Morokweng impact structure in South Africa have been studied by modeling combined gravity and magnetic data along a 100-km cross section. Rock physical properties to guide the modeling were measured on samples from three drill cores in the central part of the structure. Based on the resultant model, the current crater diameter is estimated at 65–70 km, and a central uplift of approximately 4 km is derived. This estimated diameter is in good agreement with a value of about 70 km recently obtained from analysis of a deep borehole from just outside of the envisaged crater rim. The Morokweng impact structure has almost no gravity expression, but has a distinct, irregular magnetic anomaly of 25–35 km diameter at its center. This anomaly is surrounded by an annular region with discontinued regional anomaly features. Such magnetic patterns are typical for a number of other impact structures and have been related to impactinduced changes in magnetization properties of target rocks and the occurrence of impact melt and suevite bodies. At Morokweng, the most likely cause for the central magnetic anomaly pattern is the presence of remnants of a central impact melt body and impact-induced variation in remagnetization of the crystalline basement rocks. D 2002 Elsevier Science B.V. All rights reserved.