Insight into the mechanical and thermal structure of Mercury's early lithosphere has been obtained from forward modeling of the largest lobate scarp known on the planet. Our modeling indicates the structure overlies a thrust fault that extends deep into Mercury's lithosphere. The best-fitting fault parameters are a depth of faulting of 35 to 40 km, a fault dip of 30° to 35°, and a displacement of ∼2 km. The Discovery Rupes thrust fault probably cut the entire elastic and seismogenic lithosphere when it formed (∼4.0 Gyr ago). On Earth, the maximum depth of faulting is thermally controlled. Assuming the limiting isotherm for Mercury's crust is ∼300° to 600°C and it occurred at a depth of ∼40 km, the corresponding heat flux at the time of faulting was ∼10 to 43 mW m−2. This is less than old terrestrial oceanic lithosphere but greater than the present heat flux on the Moon.