HR: 1340h
AN: U23A-0859    [Abstracts]
TI: Insight into Impact Crater Structure Using Gravity Data
AU: * Plescia, J
EM: jeffrey.plescia@jhuapl.edu
AF: Applied Physics Laboratory, Johns Hopkins University, Laurel, MD 20723, United States
AB: Impact cratering is one of the most important, often the most important, process occurring on planetary surfaces. On Earth, impact craters are relatively rare (~175 impacts). Almost none are pristine and exposed at the surface; those that are, are simple craters. Complex craters that are exposed are often highly eroded. However, several complex craters, 10s of km diameter, are buried in the subsurface; having been buried shortly after formation preserving their structure in pristine condition. Geophysical techniques are often the only way to assess the structural elements of a buried impact; while drilling provides direct samples; it is expensive. Gravity is a rapid technique that can reveal the major elements (e.g., diameter, presence of a central uplift) of an impact crater and in some cases, provide a reasonable interpretation that a structure is not of impact origin. The Chesapeake Bay structure is ~85 km in diameter. It has an 8 mGal central positive anomaly and a surrounding 10 mGal negative (defining the 35 km inner basin); the block-faulted outer margin does not display an anomaly because it is composed only of rotated sedimentary blocks. The central high is caused by high-density crystalline basement brought up in the central peak; the annular low is caused by low-density breccias. The overall horizontal and vertical dimensions as well as the original recognition of the central uplift were made using gravity data. The Mjolnir structure in the Barents Sea north of Norway is ~40 km in diameter. The structure has a central high (+2.5 mGal) about 14 km in diameter, and a surrounding low (-0.5 to -1.25 mGal); in this case the central high is produced by sedimentary rock with a slightly higher density. The density contrasts within the structure apparently are the result of small density / porosity changes among the sedimentary units involved in the impact. Two features serve as examples of structures that gravity data suggest are not of impact origin: Hackberry Flat, OK and Merna, NE. Hackberry Flat, a ~7 km diameter circular depression, was suggested to be an impact. Gravity data indicate that no anomaly is present. Similarly, a closed depression near Merna was suggested to be a 1.5 km 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.
DE: 0920 Gravity methods (1219)
DE: 5420 Impact phenomena, cratering (6022, 8136)
DE: 6022 Impact phenomena (5420, 8136)
DE: 8136 Impact phenomena (5420, 6022)
SC: Union [U]
MN: 2007 Fall Meeting