UNDERSTANDING IMPACT STRUCTURES WITH GRAVITY

Impact craters are common on planetary surfaces, yet are rare on the Earth (~175 known impacts); the lack of a significant number of terrestrial craters being due to the relative youth of the Earth's surface. Few impacts are young and well exposed such as Meteor Crater; most are highly eroded and many are partly to completely buried. Geophysical techniques are often the only way to assess the structural elements of a proposed impact. Gravity is a rapid technique that can reveal the major structural elements (e.g., diameter, presence of a central uplift) and in some cases, provide a reasonable interpretation that a structure is not of impact origin. Impacts produce large density contrasts among the various structural elements creating measurable anomalies. Typically, simple craters have a negative anomaly and complex craters have a central positive and surrounding negative anomalies. The Chesapeake Bay structure is ~85 km in diameter. It has an 8 mGal central positive anomaly and a surrounding 10 mGal negative which define the 35 km diameter inner basin; the block-faulted outer margin does not display an anomaly because it is simply composed of rotated sedimentary blocks. At Kelly West, Northern Territory Australia, only the 2 km central uplift is exposed. Gravity data indicate the structure is 6.6 km in diameter and the central uplift has complex structure. At Mt. Toondina, South Australia, again only the central uplift is exposed; gravity data indicate it is ~4 km in diameter. Examples of structures that gravity data suggest are not impacts include Hackberry Flat and Merna. Hackberry Flat OK, a ~7 km diameter circular depression, was suggested to be an impact. A gravity survey indicated no anomaly. Similarly, a closed depression near Merna NE was suggested to be a 1.5 km diameter young crater. Both features would be expected to exhibit a significant negative gravity anomaly; the absence of such an anomaly indicates they are not impact craters.