Subduction dynamics in the Western Pacific: Slab dips, plate velocities, and mantle pressure

While it is well established that the “slab pull” of dense oceanic plates is the primary driving force of subduction, the dynamic details of the subduction process have proved difficult to constrain. In this study, we use the Philippine Sea Plate region as a site to explore links between kinematic observables and the dynamics of subduction systems (e.g. mantle flow, mantle pressure). Here, the unique plate boundary configuration presents the opportunity to explore subduction dynamics in a setting where two nearby slabs may interact mechanically: the strength of such interactions having strong implications for the geodynamic properties of subduction zones. By combining regional, numerical subduction models with kinematic observations (plate velocities, slab dips, etc.), we propose that pressure build-up occurs in the asthenosphere that is trapped between the two slabs subducting on either side of the Philippine Plate, and that this is responsible for producing most of the observed subduction kinematics. While these regional subduction models, with small plates, are able to explain the most of the observed kinematics, we find that the modeled subducting slabs dip too steeply. We are able to resolve this discrepancy using semi-analytical subduction models that include realistically large plate geometries (i.e. Pacific Plate length > 10000 km). This latter part of the study suggests that global plate geometries exert an important control on even local subduction behavior.