The crustal dichotomy between the ancient highlands of the Southern Hemisphere and the young-appearing northern lowlands is a defining feature of Mars. In the Eastern Hemisphere the dichotomy boundary is marked by a prominent scarp and extensional and compressional tectonic features. Topographic data across the boundary returned from the Mars Global Surveyor indicate lithospheric flexure of the southern highlands. The topography of the boundary can be fit by a universal lithospheric deflection profile that corresponds to an elastic thickness of ∼31–36 km. Flexure of the southern highlands may be due to late Noachian–early Hesperian vertical loading of the northern lowlands. Fracturing and normal faulting along the boundary may be in response to bending stresses, while thrust faulting may result from a combination of stresses due to flexure, erosion, and global contraction.

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