Analyses of the effective elastic thickness of the Martian lithosphere have been previously used to calculate surface heat flow on Mars at different places and times. In this work, I use elevation differences in a putative Late Hesperian shoreline, named Deuteronilus shoreline, and the relation between thermal state and buoyancy of the lithosphere, in order to estimate the amplitude of the variations of surface heat flow on Mars, probably related to the time in which this feature was formed. The results suggest that, if the Deuteronilus shoreline is a true paleo-equipotential surface, the relative amplitude of surface heat flow variations on the shoreline regions in the Late Hesperian were less than present-day ones in terrestrial continental areas. The results are also roughly valid for the outer contact of the Late Hesperian Vastitas Borealis Formation, if this contact is related to the limits of an ancient ocean. These results could imply that large areas of the Martian lithosphere have been tectonothermally stable since at least that time.