Abstract: We apply three area-balance techniques to a well-constrained experimental sand model of extensional structures. The
bed-length-constant method assumes that bed length and thickness remain constant during deformation, then the extensional
displacement in different stages of deformation can be calculated through flexural-slip restoration. The area-depth-strain method, which
uses the excess/lost areas and depths of several pre-deformation strata to build an area-depth line, can directly give the depth of
detachment and the displacement along it. The area-depth-strain method allows the bed length and thickness to change during
deformation. The area-constant method assumes that the depth of the detachment is known, according to the area balance, and the area
of syn-tectonic sedimentary can be used to calculate the corresponding extensional displacement as well as the total displacement. The
results of the sand model show that the area-depth-strain method and the area-constant method can be well applied to the
pre-deformation strata and syn-tectonic strata, respectively. In addition, the bed-length-constant method can produce extra errors which
may be over 10% and are caused by the shear deformation. The Changchang sag in Qiongdongnan Basin mainly contains the Cenozoic
syn-tectonic strata developed on the Pre-Cenozoic basement. The results of the area-constant-method give the extensional displacements
during the deposition of Lingtou, Yacheng, and Lingshui Formation of 13.8 km, 15 km and 21.4 km, respectively, with the total
amount of displacement and extensional rate of 50.2 km and 42.7%. The results also show that the area-constant method and the
area-depth-strain method are much more reliable than the bed-length-constant method in extensional basins with shear deformation.
The area-depth-strain method can be applied in structures with well constrained pre-deformation strata, and the area-constant method
uses growth strata to calculate the corresponding extensional displacement as well as the total displacement

Key words: bed-length-constant method, area-depth-strain method, area-constant method, extensional amount, Changchang Sag