A gas hydrate site with bi-verging folding in the outer wedge offshore SW Taiwan: Results from kinematic structural modeling and finite strain analysis

摘要

Understanding the structural evolution of complex convergent plate boundaries could contribute to linking the anticipated fluid production and transportation at depth to the measured amounts of fluid stored in hydrate methane. To better understand fluid behavior within a complex convergent boundary, we propose an evolution model for a set of doubly plunging, oppositely-verging structures referred to as Ridge A and the Four-Way Closure Ridge (FWCR), located offshore southwestern Taiwan. The structures exhibit 1) initial deformation along a decollement forming a seaward (westward)-verging detachment fold, followed by 2) a landward (eastward)-verging fault propagation fold (trishear) about 8 km west of the detachment fold, and 3) a westward-verging low-angle thrusting modifying the previous structures. Furthermore, finite strain analyses based on the kinematic model suggest high pore space reduction between the detachment and fault propagation folds. The volume of methane possibly expelled during the pore space reduction is not enough to explain the high hydrocarbon concentration necessary for hydrate formation. Kinematic modeling along with finite strain analyses support the possibility of deep sourced fluid migration along such bi-vergent structures at this hydrate-rich site.