2:00 PM - 2:15 PM
[SIT19-02] Mantle dynamics beneath and surrounding the Philippine Sea Plate: Constraints from multi-scale global convection models
Keywords:Philippine Sea Plate, Mantle dynamics, subduction zone, Expanding mantle region under the PSP
The Philippine Sea Plate (PSP) is highly unique from a subduction geodynamics and large-scale tectonics perspective: It is both at the center of a ring of regional and highly populated subduction zones (e.g., Ryukyu-Nankai and Izu-Bonin) and is wedged between the globally spanning Pacific and Eurasian Plates. We therefore develop multi-scale numerical models, which properly include both the regional and global scales, to investigate the geodynamic processes responsible for its current geophysical and geological state. In particular, as the PSP subducts under the Eurasian continent, converging trench motions on either side of the PSP have led previous authors to suggest the PSP is shrinking. However, how does mantle flow organize beneath the potentially shrinking PSP? And how is this mantle flow affected by, and vice versa, the complex network of slabs surrounding the PSP?
To address these questions, we develop 3-D global mantle convection models, focused on the present-day tectonic geometries, with adaptive mesh resolution using the ASPECT finite element code. We embed realistically shaped slabs, weak zones along plate boundaries in these models and increase the model resolution within a spherical chunk centered on the Western Pacific. To assess the influence of slab geometry and rheology of slab and weak zones on mantle flow, we run a series of model tests with, e.g., variable slab and plate interface strengths. By comparing these models with near-surface observations, such as plate and trench velocities, we identified a reference model that best matches the observations. We use this reference model to analyze the mantle flow pattern beneath the PSP and quantify the volumetric flow budget of the mantle region beneath the PSP and bounded laterally by the subducting slabs.
Our models reveal the development of strong positive dynamic pressure beneath the subducting Pacific Plate and negative dynamic pressure beneath the PSP and Eurasian continent. Driven by these dynamic pressure differences and the retreating motions of the Manila, Ryukyu, and Philippine trenches, outward toroidal flows emerge on the western side of the PSP, while inward toroidal flow occurs along its southern edge near the Mariana Trench. By analyzing the total volume of mantle flow into/out of the sub-PSP region, our results show the inward flow around the Mariana Trench dominates the lateral mantle flux, hence producing net lateral in-flow between the subducting slabs beneath the PSP. However, this in-flow is mostly balanced by vertical downward flow beneath the system (i.e., outflow towards the lower mantle). The remaining inflowing mantle material is accommodated by the space created by the retreating Manila Trench.
Overall, our results suggest that despite the shrinking surface plate, the mantle region beneath the PSP is actually expanding, driven by the pressure differences from double subductions and lateral inflow. The increasing mantle volume is mainly accommodated by an increasing distance between the laterally bounding slabs at deeper depths (~225-425 km).
To address these questions, we develop 3-D global mantle convection models, focused on the present-day tectonic geometries, with adaptive mesh resolution using the ASPECT finite element code. We embed realistically shaped slabs, weak zones along plate boundaries in these models and increase the model resolution within a spherical chunk centered on the Western Pacific. To assess the influence of slab geometry and rheology of slab and weak zones on mantle flow, we run a series of model tests with, e.g., variable slab and plate interface strengths. By comparing these models with near-surface observations, such as plate and trench velocities, we identified a reference model that best matches the observations. We use this reference model to analyze the mantle flow pattern beneath the PSP and quantify the volumetric flow budget of the mantle region beneath the PSP and bounded laterally by the subducting slabs.
Our models reveal the development of strong positive dynamic pressure beneath the subducting Pacific Plate and negative dynamic pressure beneath the PSP and Eurasian continent. Driven by these dynamic pressure differences and the retreating motions of the Manila, Ryukyu, and Philippine trenches, outward toroidal flows emerge on the western side of the PSP, while inward toroidal flow occurs along its southern edge near the Mariana Trench. By analyzing the total volume of mantle flow into/out of the sub-PSP region, our results show the inward flow around the Mariana Trench dominates the lateral mantle flux, hence producing net lateral in-flow between the subducting slabs beneath the PSP. However, this in-flow is mostly balanced by vertical downward flow beneath the system (i.e., outflow towards the lower mantle). The remaining inflowing mantle material is accommodated by the space created by the retreating Manila Trench.
Overall, our results suggest that despite the shrinking surface plate, the mantle region beneath the PSP is actually expanding, driven by the pressure differences from double subductions and lateral inflow. The increasing mantle volume is mainly accommodated by an increasing distance between the laterally bounding slabs at deeper depths (~225-425 km).