In southern Chile, the Nazca plate is subducting beneath the South American plate. This region was struck by megathrust earthquakes in 1960 and 2010 and is characterized by the existence of a volcanic chain. In this region, we modeled a three-dimensional thermal structure associated with the subduction of the Nazca plate by using numerical simulations. Based on the obtained temperature distribution, we determined the updip and downdip limit temperatures for the region ruptured by these two megathrust earthquakes. In addition, the distributions of water content and dehydration gradient were calculated by using appropriate phase diagrams and compared with the location of the volcanic chain. As a result, we infer that the coseismic slip of the 2010 Mw8.8 Maule earthquake occurred only at temperatures lower than and around the 350 degrees C isotherm that resembles the beginning of the brittle-ductile transition. We also deduce that the rupture of the 1960 Mw9.5 Valdivia earthquake propagated up to the 450 degrees C isotherm because the magnitude was considerably large and the young hot plate subducted near the Chile Ridge. In addition, the hydrous minerals in the turbidites, MORB and ultramafic rocks released fluids via dehydration reactions, and dehydrated water migrated upward almost vertically, decreasing the melting point of the mantle wedge and contributing to the formation of the volcanic chain.