HR: 1340h
AN: H33C-1400    [Abstracts]
TI: Geomorphology of the Martian Crustal Dichotomy Boundary: Implications for Age and Origin
AU: * Irwin, R P
EM: irwinr@si.edu
AF: Smithsonian Institution, National Air and Space Museum CEPS, MRC 315 6th St at Independence Ave SW, Washington, DC 20013-7012 United States
AU: Watters, T R
EM: watterst@si.edu
AF: Smithsonian Institution, National Air and Space Museum CEPS, MRC 315 6th St at Independence Ave SW, Washington, DC 20013-7012 United States
AB: The fairly abrupt decline in surface age, elevation, and crustal thickness from the southern highlands to the northern lowlands of Mars is termed the crustal dichotomy. The cause of northern crustal thinning remains among the major unresolved issues in Mars research. Most geophysical models have invoked degree-1 mantle convection, whereas others suggest plate tectonism or one or more giant impacts in the northern hemisphere. Other studies have focused on modifications to the highland/lowland dichotomy boundary, which may or may not be relevant to the issue of origin. These hypotheses include erosion, extensional faulting, lateral crustal flow, and flexure of an elastic lithosphere by loading of the lowlands. The geomorphology of the highland/lowland boundary region provides important controls for these geophysical models of early crustal development. Geologic observations suggest that the thin lowland crust and dichotomy boundary originated in the Early Noachian Epoch (>4 Ga). The slope within ancient cratered terrain along the boundary influenced post-Noachian fresh crater morphology, Late Noachian valley network planform, and the degradation of Middle to Late Noachian (~3.95-3.7 Ga) impact craters. For fresh (post-Noachian) craters emplaced on the dichotomy slope, the plane of the rim crest inclines subparallel to the exterior slope, although the interior cavity and central peak are oriented vertically, consistent with impacts on a precursor slope. Valleys converge down modern slopes, so the cratered dichotomy slope predates Late Noachian valley development. Degraded craters within the north-sloping cratered and dissected units have floors that are flat or slightly concave, so tilting did not occur after the crater floors were emplaced in Middle Noachian to Early Hesperian time. Most crater floors along the dichotomy boundary are not dissected, as one would expect from an erosional response to Late Noachian tilting, and the few craters with dissected floors have obvious outlet breaches and/or are interspersed with flat-floored craters along the steepest cratered slopes. Crater rim-lowering and infilling on the cratered slope often produced flat-floored amphitheaters that open to the north, which is a natural consequence of long-term erosion and infilling of craters that formed on slopes. Lithospheric extension has not disrupted fresh or degraded craters on north-sloping cratered terrain, which would occur if the convex slope formed at any time after the Early Noachian. The highlands and crustal dichotomy are isostatically compensated, whereas the younger Tharsis and Elysium regions are not, suggesting that the dichotomy formed during an earlier time of higher heat flow. Finally, the thin crust of the lowlands extends well beyond and therefore predates the Utopia basin, which formed in the Early Noachian as shown by its superimposed quasi-circular depressions (buried craters). These observations constrain the development of the lowland crust and therefore the crustal dichotomy to the Early Noachian, although a precise age is not yet available within this ~600 Ma epoch. Fretted terrain is confined within a superimposed Late Noachian plateau unit, which was extensively eroded during the Early Hesperian (~3.7-3.6 Ga), followed by lowland plains emplacement. Despite some erosion, the general form of the ancient dichotomy boundary is still preserved in cratered terrain, allowing geophysical models of crustal development to be tested. The transition zone and lowland terrains are >300 Ma younger and not relevant to the dichotomy origin.
DE: 5415 Erosion and weathering
DE: 5419 Hydrology and fluvial processes
DE: 5455 Origin and evolution
DE: 6225 Mars
SC: Hydrology [H]
MN: Fall Meeting 2005