The marked dichotomy in topography, surface age, and crustal thickness between the northern lowland and southern upland of Mars has been explained as due to an initially inhomogeneous crust, a single mega-impact event, several overlapping large basin impacts, and first-order convective overturn of the martian mantle. All of the published hypotheses propose that the dichotomy was formed early in martian history; before the end of the primordial heavy bombardment. A primordial origin is inherent in the initial crustal inhomogeneity hypothesis, and required for both impact hypotheses. Endogenic hypotheses are not so constrained. Geological data indicate episodes of fracturing and faulting in the late Noachian and the early Hesperian. This fracturing and faulting occurred primarily within the northern lowland and along the boundary between lowland and highland. 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. We argue 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. ¿ American Geophysical Union 1990