The extensional deformation of lithosphere can occur by different patterns and results in different structures of extensional basins.According to the deformational characteristics on the cross-section of extensional basins

they can be divided into the following four types:"sag basin"

"faulted-sag basin"

"fault-downthrown basin"

and "sag fault-downthrown basin".The extensional deformation mechanisms of lithosphere include thermal action of the upper mantle

ductile extension of the lower lithosphere

and normal faulting of the upper lithosphere(perhaps it is just shallow crust).The "sag basin" is formed by homogeneous extensional deformation at crust scale;while the "fault-downthrow basin" is shaped by faulting of nonhomogeneous extensional deformation at crust scale.Both "faulted-sag basin" and "sag fault-downthrow basin" are the result of superposition of homogenous flexural deformation and non-homogeneous faulting deformation at crust scale

and their subsidence is controlled partly by extensional faulting and partly by flexural deformation of the extending crust.Although all of the mechanisms work simultaneously

the contribution of each dynamic mechanism varies in different parts of basin and at different periods

causing differences in extensional basin structures at different periods and in diffe-rent parts of the basin at the same period.