Evolution of the geological structure and mechanical properties due to the collision of multiple basement topographic highs in a forearc accretionary wedge: insights from numerical simulations

We propose a conceptual geological model for the collision of multiple basement topographic highs (BTHs; e.g., seamounts, ridges, and horsts) with a forearc accretionary wedge. Even though there are many BTHs on an oceanic plate, there are few examples of modeling the collision of multiple BTHs. We conducted numerical simulations using the discrete element method to examine the effects of three BTH collisions with forearcs. The typical geological structure associated with a BTH collision was reproduced during the collision of the first BTH, and multiple BTH collisions create a cycle of formation of BTH collisional structures. Each BTH forces the basal decollement to move up to the roof decollement, and the roof decollement becomes inactive after the passage of the BTH, and then the decollement moves down to the base. As the active decollement position changes, the sequences of underthrust sediments and uplifted imbricate thrusts are sandwiched between the decollements and incorporated into the wedge. At a low horizontal compressive stress, a shadow zone is formed behind (i.e., seaward of) the BTH. When the next BTH collides, the horizontal compressive stress increases and tectonic compaction progresses, which reduce the porosity in the underthrust sediments. Heterogeneous evolution of the geological and porosity structure can generate a distinctive pore pressure pattern. The underthrust sediments retain fluid in theshadow of the BTH. Under the strong horizontal compressive stresses associated with the next BTH collision, pore pressure increases along with a rapid reduction of porosity in the underthrust sediments. The distinctive structural features observed in our model are comparable to the large faults in the Kumano transect of the Nankai Trough, Japan, where a splay fault branches from the plate boundary and there are old and active decollements. A low-velocity and high-pore-pressure zone is located at the bottom of the accretionary wedge and in front (i.e., landward) of the subducting ridge in the Kumano transect. This suggests that strong horizontal compressive stresses associated with the current BTH collision has increased the pore pressure within the underthrust sediments associated with previous BTHs.

Automated summary

This study proposes a conceptual geological model for the collision of multiple basement topographic highs (BTHs) with a forearc accretionary wedge. Numerical simulations show that each BTH collision can create distinct geological structures and variations in porosity that influence the distribution of pore pressure. The strong horizontal compressive stresses associated with BTH collisions can significantly impact the pore pressure within underthrust sediments.