Mathematics – Logic
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufm.p44c..02a&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #P44C-02
Mathematics
Logic
5420 Impact Phenomena, Cratering (6022, 8136), 5475 Tectonics (8149), 6225 Mars
Scientific paper
The crust of Mars can be divided into four large-scale physiographic provinces: the southern highlands, northern lowlands, Tharsis, and Arabia Terra. Tharsis is a volcanic province superimposed over the ancient crustal dichotomy. The elliptical shape of the dichotomy boundary and bimodal crustal thickness distribution between the highlands and lowlands suggest that the dichotomy formed in a giant impact, with the northern lowlands representing the ancient Borealis impact basin. This leaves Arabia Terra as the one large-scale physiographic province on Mars whose origin still defies explanation. Arabia Terra has been commonly grouped with the southern highlands because of its ancient cratered surface, though it exhibits lower topography and a thinner crust than typical of the highlands. Arabia Terra is separated from both the highlands and the lowlands by distinct breaks in slope, with the southern edge lying approximately parallel to the northern edge at a distance from the Borealis basin center of approximately 1.57 times the basin radius. A strong topographic similarity to multi-ring structures around other giant impact basins suggests a similar origin for this province. Multi-ring structures can be identified on the basis of either geologic characteristics, such as scarps and massifs, or the long-wavelength topographic structure. For the largest and most ancient basins, only this large-scale topographic signature is preserved. Azimuthally averaged topographic profiles through a number of lunar and martian multi-ring basins reveal a diagnostic structural commonality: an outer ring is expressed as an inwards facing slope at ~1.4 basin radii, which transitions into a gently sloping to concave upwards bench of lower topography before reaching the main basin rim. Based on this simple structural definition, Arabia Terra can be classified as a partial multi-ring structure around the Borealis basin. Preliminary results of finite element models of the collapse of the transient cavity following the impact support the ring-tectonics model of basin ring formation, in which inwards flow of weak deep crustal and mantle material exerts a drag on the brittle lithosphere, leading to tectonic failure at the outer ring scarp. This inwards flow is supported by models of the crustal thickness in the Arabia Terra region, which reveal a northwards (toward the basin center) shift of the dichotomy boundary at the Moho relative to the topographic boundary. If Arabia Terra is indeed a multi-ring structure around the Borealis basin, it will provide a unique opportunity for investigating the structure and formation of multi-ring basins.
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