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

Abstract

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 décollement to move up to the roof décollement, and the roof décollement becomes inactive after the passage of the BTH, and then the décollement moves down to the base. As the active décollement position changes, the sequences of underthrust sediments and uplifted imbricate thrusts are sandwiched between the décollements 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 the “shadow” 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 décollements. 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.

Accretion Cycles, Structural Evolution, and Thrust Activity in Accretionary Wedges With Various Décollement Configurations: Insights From Sandbox Analog Modeling

“…The discontinuous basal décollement in the type 3 and 4 models produced similar geometric and kinematic features as those caused by seamount/ridge subduction, although our sand mound was neither rigid nor fixed to the base (cf., Dominguez et al., 2000; Lallemand et al., 1992; Miyakawa et al., 2022; Noda et al., 2020; Okuma et al., 2022; Ruh et al., 2016). For the type 3 model, when the sand mound starts to be subducted at the deformation front, the wedge enters a compressively unstable regime in order to adjust to the higher basal friction until the wedge reaches a new equilibrium with a higher taper angle (Figure 14a).…”

“…Sun et al, 2020). When a high-friction patch on the plate interface works as a barrier to stress transmission on the basal décollement, the wedge deformation includes indentation or slope failure on the surface topography, widening of the brittle-damaged zone on the plate interface, and promotion of the underthrusting of subducted sediments beneath the wedge (Dominguez et al, 2000;Miyakawa et al, 2022;Morgan & Bangs, 2017;Noda et al, 2020;Okuma et al, 2022;Wang & Bilek, 2011). In summary, these previous studies have shown that accretion is a cyclical process, including frontal accretion and underthrusting, and that the décollement configurations affect the wedge architecture.…”

“…In terms of the continuity of the décollement layer, previous studies have investigated the subduction of a rough plate interface or a high‐friction patch that interrupts the basal décollement layer (Bangs et al., 2004; Bangs et al., 2006; Dominguez et al., 1998, 2000; Koge et al., 2018; Kukowski et al., 1994; Lallemand & Le Pichon, 1987; Lallemand et al., 1992, 1994; Miyakawa et al., 2022; Morgan & Bangs, 2017; Noda et al., 2020; Okuma et al., 2022; Ruh et al., 2013; T. Sun et al., 2020). When a high‐friction patch on the plate interface works as a barrier to stress transmission on the basal décollement, the wedge deformation includes indentation or slope failure on the surface topography, widening of the brittle‐damaged zone on the plate interface, and promotion of the underthrusting of subducted sediments beneath the wedge (Dominguez et al., 2000; Miyakawa et al., 2022; Morgan & Bangs, 2017; Noda et al., 2020; Okuma et al., 2022; Wang & Bilek, 2011).…”

“…In terms of the continuity of the décollement layer, previous studies have investigated the subduction of a rough plate interface or a high-friction patch interrupting the basal décollement layer (Bangs et al, 2004(Bangs et al, , 2006Dominguez et al, 1998Dominguez et al, , 2000Koge et al, 2018;Kukowski et al, 1994;Lallemand & Le Pichon, 1987;Lallemand et al, 1992Lallemand et al, , 1994Miyakawa et al, 2022;Morgan & Bangs, 2017;Noda et al, 2020;Okuma et al, 2022;Ruh et al, 2013;T. Sun et al, 2020).…”

Strain and Stress Accumulation in Viscoelastic Splay Fault and Subducting Oceanic Crust

“…Miyakawa et al. (2022) construct a numerical model as a function of temperature and investigate a model in which splay faults develop and dislocate on geological timescales. Conin et al.…”