Origin of the Martian global dichotomy by crustal thinning in the late Noachian or early Hesperian
The marked dichotomy in topography, surface age, and crustal thickness between the northern lowland (NL) and southern upland of Mars has been explained as due to an initially inhomogeneous crust, a single megaimpact event, several overlapping large basin impacts, and first-order convective overtum of the Martian mantle. All of these hypotheses propose that the dichotomy was formed before the end of the primordial heavy bombardment. Geological data indicate episodes of fracturing and faulting in the late Noachian and the early Hesperian, within the NL and along the lowland/highland boundary. Igneous activity also peaked in the late Noachian and early Hesperian. These data suggest a tectonic event near the Noachian/Hesperian boundary characterized by enhanced heat loss and extensive fracturing, including formation of the faults that define much of the highland/lowland boundary. It is argued that the major result of this tectonic event was formation of the dichotomy by thinning of the crust above a large convection cell or plume.