The basin form continues in existence as long as basin strength is able to balance the stresses and strains which accumulate in the depositional prisms. If the balance of strength fails, a new phase (orogeny) of the basin tectonic cycle occurs, and the cycle commonly ends as it began, in epeirogeny. The part of the basin which remains uncompressed during the new phase may be called the epeirogen, in antithesis to the orogen. If the balance of strength does not fail, the continuing initial phase of epeirogeny proceeds through a subsidiary cycle of algebraic summation or net subsidence of basement.

Using subsidence of basement as the key parameter, the major large-area epeirogens and basins (many intracratonic) are divided into three classes: 10,000 ft of subsidence, 15,000 ft of subsidence, and greater than 15,000 ft of subsidence. Area totals, area percentages, and other considerations suggest that basement subsidence of significant basin sectors below 15,000 ft is a special condition herein termed hypersubsidence. The term is defined and then examined, first as a geological condition and second as a background to petroleum accumulation.