Abstract:
The contrast in crustal thickness, surface age, elevation, and morphology between the southern cratered highlands and northern lowland plains of Mars is termed the crustal dichotomy. The oldest exposed sections of the crustal dichotomy boundary are ancient cratered slopes, which influenced post-Noachian fresh crater morphometry, Late Noachian valley network planform, and the degradation patterns of Middle to Late Noachian (similar to 3.92-3.7 Ga) impact craters. Noachian visible and topographically defined impact craters at the top of the cratered slope show no evidence of flexure-induced normal faulting. These observations and published geophysical data collectively require an Early to Pre-Noachian age for the crustal dichotomy, prior to the largest recognized impact basins. Late Noachian plateau deposits and more prolonged Tharsis volcanism appear to have buried parts of the old cratered slope, and fretted terrain developed in this transition zone during the Early Hesperian Epoch (similar to 3.7-3.6 Ga). Fretted/knobby terrains, lowland plains, and most visible structures (wrinkle ridges, fractures, and normal faults) postdate Noachian crater modification and are several hundred million years younger than the cratered slope of the crustal dichotomy, so they provide no valid basis or constraint for models of its formation. Long-wavelength topography in cratered terrain dates to Early to Pre-Noachian time and provides a useful model constraint. Geological and geophysical observations are thus reconciled around an early age and relatively rapid development of the Martian crustal dichotomy.
