Probing into the Diapirism in Southwest Taiwan by Numerical Simulation

Mud diapirs are widely observed offshore southwestern Taiwan and are generally interpreted to originate from the overpressured Gutingkeng Formation, which experienced rapid sedimentation. Fluid enrichment within this formation reduces its bulk density relative to the surrounding strata, allowing buoyancy forces to drive upward migration and diapir formation. However, recent gravity analyses challenge this classical diapirism model by indicating a positive density contrast associated with the observed diapirs. This apparent contradiction raises the question of whether buoyant diapirism can coexist with a positive density anomaly.

In this study, we use numerical simulations incorporating visco–elasto–plastic rheology to investigate the formation mechanisms of mud diapirs under varying physical conditions. The models explore the effects of viscosity, elastic moduli, and density contrasts between diapiric material and the overlying sedimentary layers. Our results demonstrate that diapiric structures with a positive density contrast can be successfully reproduced. We further show that diapirism is systematically accompanied by the development of sedimentary basins filled with unconsolidated sediments, which introduce a strong negative density contrast relative to surrounding rocks. Gravity forward modeling indicates that a sedimentary basin with a thickness of approximately 500 m is sufficient to generate a gravity anomaly of ~5 mGal, consistent with observed data. These results suggest that the presence of positive-density diapirs does not preclude buoyancy-driven ascent and can be reconciled through the combined effects of diapirism and syn-deformational sedimentation.