The Martian dichotomy is a global feature separating the cratered southern highlands and smooth northern lowlands. Parts of the boundary are defined by steep slopes with elevation differences of 2--6 km. Here we use constraints from the well-preserved section of the dichotomy in the Ismenius Region. We study the relaxation of the boundary using both finite element and semianalytical techniques to better constrain the thermomechanical conditions in early Mars. We argue that the steep slopes are the product of faulting resulting from relaxation of the edge of the boundary by lower crustal flow. This relaxation requires that the lower crust must be at least 10 km thick and have a viscosity of ~1021 Pa s during the first 100--300 Myr of relaxation. We show that a wet crustal rheology and a thermal gradient of ~15--20 K/km at 4 Ga allows for both the partial relaxation of the Martian dichotomy and preservation of long wavelength topography as observed on Mars. This result is generally consistent with a stagnant-lid model of cooling of Martian interior. The thermal gradient at 4 Ga is also consistent with the estimated elastic thickness on early Mars. Our modeling constrains the original slope of the boundary to approximately 2¿. We cannot constrain the crustal thickness more tightly than the current estimates of 50--100 km. On the basis of our semianalytical modeling, the relaxation of the boundary can contribute to the general north-south slope observed on Mars.