Mathematics – Logic
Scientific paper
Dec 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufm.p34a..01p&link_type=abstract
American Geophysical Union, Fall Meeting 2011, abstract #P34A-01
Mathematics
Logic
[5420] Planetary Sciences: Solid Surface Planets / Impact Phenomena, Cratering
Scientific paper
Impact cratering is one of the most important geologic process occurring on planetary surfaces. On Earth, craters are rare; there are only ~175 identified impact structures. Understanding the detailed structural geology and impact mechanics requires understanding how the crust has responded to the impact. Mapping the surface geology provides information in two dimensions that can be extrapolated for short distances into the third dimension (depth). Geophysical techniques and drilling are required to understand details of the third dimension. Gravity studies are a powerful technique to understand the structure at depth, particularly as it can be done quickly and cheaply. Gravity measurements essentially map the density distribution of the crust with near-surface density contrasts producing the largest short-wavelength anomalies. The central uplift of an impact structure is one of the most deformed parts wherein material is uplifted from relatively deep levels, folded and faulted. Gravity studies allow the dimensions of the uplift to be determined (e.g., diameter, height). The bulk density of the central peak provides constraints on bulk shock effects (compression or dilation). The gravity signature of a central peak, compared with the adjacent crater floor and the exterior provides insight into the nature of deeper deformation under the central peak. For example, the amount of uplift within the central peak may decrease with depth until a point is reached at which the deformation is zero. Alternatively, the deformation, as well deformation on the bounding faults, may sole into a decollement at depth; deformation will end abruptly at a particularly structural level. The boundary between a sedimentary section and crystalline basement or a major strength-contrast in a sediments may act as a decollement. Depending upon which scenario occurs, the central uplift gravity anomaly will differ. The Chesapeake Bay structure (85 km) serves as an example of how gravity can be used to define details of a central uplift. CBIS 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. 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 margins of the structure do not exhibit gravity anomalies as the bounding faults simply displace blocks of the sedimentary section against each other and there are no resulting density contrasts. The bounding faults also do not exhibit a gravity anomaly as they do not significantly displace the basement, hence no density anomalies are created.
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