HR: 15:30h
AN: P23A-07    [Abstracts]
TI: The Crustal Dichotomy of Mars: Geological Testing and Constraints on Geophysical Models
AU: * Irwin, R P
EM: irwinr@si.edu
AF: Smithsonian Institution, Center for Earth and Planetary Studies, National Air and Space Museum, 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, Center for Earth and Planetary Studies, National Air and Space Museum, MRC 315, 6th St. at Independence Ave SW, Washington, DC 20013-7012, United States
AB: The term `crustal dichotomy' refers to hemispheric scale differences in crustal thickness, surface age, elevation, and morphology between the southern cratered highlands and northern lowland plains of Mars. Empirical observations of the crustal dichotomy boundary provide important constraints on the timing and mechanisms of early crustal development, which remains poorly understood after 35 years of robotic exploration. Published models include endogenic (long-wavelength mantle convection, perhaps driving plate tectonics) and exogenic (one or more giant impacts) mechanisms. The dichotomy boundary consists of Early Noachian (~4.5 to 3.92 Ga) cratered slopes and a transition zone of Early Hesperian (~3.7 to 3.6 Ga) fretted and knobby terrains between the Noachian highland plateau and Hesperian lowland plains. The old cratered slope predates and influenced the morphometry of Hesperian and younger impact craters, the drainage planform of Late Noachian valley networks, and the erosional modification of Middle to Late Noachian (~3.92 to 3.7 Ga) impact craters. No extensional faults bisect Noachian fresh or degraded craters at higher elevations on the cratered slope, and crater floor deposits are flat rather than tilted, which indicate that the boundary and the lowlands formed very early in the Noachian Period. Formation of the dichotomy in the Early Noachian is consistent with the population of buried impact craters in the lowlands, SNC isotopic data, and the crustal magnetic field data. Late Noachian plateau materials, younger volcanic rocks of the Tharsis province, and Amazonian airfall materials of the Medusae Fossae Formation later buried the old cratered slope. Development of fretted and knobby terrains in the younger plateau materials appears to be structurally controlled, but we have identified no evidence for significant fault displacement. Tectonic and erosional features of these later terrains are not coeval with the crustal dichotomy and do not provide valid model constraints for Early Noachian processes. Many published models of the crustal dichotomy have one or more of the following issues: 1) long- lived processes that extend beyond the Early Noachian Epoch, 2) formation of complex fretted and knobby terrain using one-dimensional tensile stress, 3) use of Hesperian faults and landforms as model constraints, 4) limited applicability beyond the study area, and 5) insufficient testing to reject alternative endogenic hypotheses. Future modeling of the crustal dichotomy should focus on rapid endogenic mechanisms operating within the Early Noachian Epoch.
DE: 5415 Erosion and weathering
DE: 5475 Tectonics (8149)
DE: 6225 Mars
SC: Planetary Sciences [P]
MN: 2007 Joint Assembly