The basin interior is covered by the Caloris interior plains (CIP) that have been deformed by complex tectonic features (Denevi et al., 2018), including concentric ridges and tensional grabens that exhibit both radial and tangential strikes (Byrne et al., 2018). Both the northern and southern parts of the basin floor have been significantly uplifted to heights that exceed the basin rim (Figure 1c), and the two mountain belts have a trend that is more-or-less parallel to the east-northeast to west-southwest direction (Byrne et al., 2014).

We investigated the relaxation states of 11 large impact basins on Mercury based on an updated crustal thickness map, finding that the pre-Tolstojan basins have comparable instead of varied relaxation states, suggesting that Moho temperature (Temp Moho) did not decrease substantially from ∼4.2 to 3.8 Ga. At the same time, mantle uplift beneath the Caloris basin is the least degraded, therefore implying a sharp decrease of Temp Moho ∼ 3.8 Ga. These findings contrast with our thermal evolution models that predict a fast decrease of Temp Moho between ∼4.2 and 3.8 Ga. Therefore, the discrepancies in the cooling rate suggest that the relatively elevated bombardment history between ∼4.2 and 3.8 Ga might have input additional energy to Mercury and substantially decreased the cooling rate. Plain Language Summary Formation of large impact basins on Mercury, a terrestrial rocky planet, was accompanied with a rebound of the floor, which resulted in an uplift of the mantle plug beneath it. These structures are likely to subside or relax, a process influenced by the thermal structure. The crustal thickness model of Mercury reveals the geometric shape of mantle plugs under large impact basins, indicating variation in the degree of relaxation, thus differences in the temperatures on Mercury lithosphere when the basins formed. Therefore, the present-day shape of mantle plugs under the age-determined basins provides information about the thermal history of Mercury. The discrepancy between the thermal history as inferred from age-determined basin relaxation measurements and an analytical model of planetary cooling suggests that impact bombardments might have input additional energy to Mercury during the period of basin formation.

The thin elastic shell lithospheric model is widely used in the geophysical studies of terrestrial bodies (Turcotte et al., 1981). This model assumes that the planetary lithosphere is an elastic shell that deflects when mass-related loads are imposed. The magnitude of deflection depends on the applied loads and the physical properties of the lithosphere (such as elastic thickness). If this deflection causes a change in the relief on a density interface, it will produce a gravity anomaly. Therefore, deflection measurements can be made using gravity and topography data. The elastic thickness can be used to characterize the physical properties of the lithosphere (Watts, 2001), which are mainly influenced by the temperature structure and are therefore closely related to the evolution of the entire planet.The interpretation of gravity and topography data is usually performed in the spherical harmonic (SH) domain. Degree-dependent correlation and admittance (transfer function) between gravity and topography contain information about loads in the lithosphere. Various types of loading models have been proposed. The simplest loading