Extensional Deformation in the Pacific Oceanic Plateaux

Oceanic plateaux are large igneous provinces that are embedded into or extruded onto ocean lithosphere, with a plateau then drifting on an oceanic plate towards a subduction boundary. The Pacific plate hosts several of the major oceanic plateaux, such as the Shatsky Rise, Hess Rise, Manihiki, and Ontong Java. Significantly, these plateaux show episodes of extensional deformation and related magmatism after their emplacement and prior to collision at a subduction boundary. It has been suggested that thermal subsidence or collision-related lithospheric buckling are mechanisms for such tectono-magmatic events. However, these theories are singular events either after the plateau emplacement or at the time of arrival to subduction trench. Geological and geophysical evidence, on the other hand, suggests that these extensional episodes and magmatism cover a wide range of time from emplacement to collision, and not a single, short tectonic event.

Here, we conducted numerical geodynamic experiments to understand the mechanics of extensional deformation of oceanic plateaux during plate motion (e.g., 'syn-drift' tectonics). Our results demonstrate that a "subduction pulley" slab-pull force can be transmitted over a thousand kilometres from the subduction trench, and deform plateaux at their distant locations from plate boundaries. Extensional deformation in our models develops continuously from the start to end of the plateau drift phase. Moreover, according to our experiments, the magnitude of extension is higher on the trench side of the plateaux, which is consistent with interpretations from the available seismic reflection profiles that are collected from the Shatsky Rise and Manihiki plateaux. Essentially, oceanic plateaux represent rather weaker regions of oceanic plate and may not resist tensile stress that is originated from subducting ocean slabs. Oceanic lithosphere, therefore, could undergo extensional intra-plate deformation at the location of oceanic plateaux.