Free thermal convection and subsidence of the Panxi (intracontinental) (rift) (basin)

The large-scale thick volcanic rocks with well-developed fractures and fissures occur in the Panxi rift basin. The underlying abnormal mantle heat gave rise to the free thermal convection, which in turn caused the heat to be lossed. The time duration of the heat loss that may show a maximum period of 80 Ma depends upon the thickness of the overlying sedimentary rocks and heat-transfer capacity. The free thermal convection may be divided into several stages, including (1) the opening of the fractural and fissure systems in the upper continental crust influenced by the heat and tectonic stress; (2) the production of the free thermal convection; (3) the closure of the fractural and fissure systems because of the variations in temperature and pressure and the precipitation of the minerals; (4) the cease or deceleration of the free thermal convection, and (5) the occurrence of the abnormal heat flows on the surface. The formation of the free thermal convection elements is interpreted to be responsible for the nonuniform subsidence and uplift of the crust, and of the "graben-horst" framework. The lateral migration of the free thermal convection elements permits the asymmetric basin to be developed in the study area.