Tomography of the subducting Pacific slab and the 2015 Bonin deepest earthquake (Mw 7.9)

Subduction age and stress state control on seismicity in the NW Pacific subducting plate

Intermediate depth (70-300 km) and deep (> 300 km) earthquakes have always been puzzling Earth scientists: their occurrence is a paradox, since the ductile behavior of rocks and the high confining pressure with increasing depths would theoretically preclude brittle failure and frictional sliding. The mechanisms proposed to explain deep earthquakes, mainly depending on the subducting plate age and stress state, are generally expressed by single parameters, unsuitable to comprehensively account for differences among distinct subduction zones or within the same slab. We analyze the Kurile and Izu-Bonin intraslab seismicity and detail the Gutenberg-Richter b-value along the subducted planes, interpreting its variation in terms of stress state, analogously to what usually done for shallow earthquakes. We demonstrate that, despite the slabs different properties (e.g., lithospheric age, stress state, dehydration rate), in both cases deep earthquakes are restricted to depths characterized by equal age from subduction initiation and are driven by stress regimes affected by the persistence of the metastable olivine wedge.

Polymorphism of feldspars above 10 GPa

Feldspars are rock-forming minerals that make up most of the Earth’s crust. Along the mantle geotherm, feldspars are stable at pressures up to 3 GPa and may persist metastably at higher pressures under cold conditions. Previous structural studies of feldspars are limited to ~10 GPa, and have shown that the dominant mechanism of pressure-induced deformation is the tilting of AlO₄ and SiO₄ tetrahedra in a tetrahedral framework. Herein, based on results of in situ single-crystal X-ray diffraction studies up to 27 GPa, we report the discovery of new high-pressure polymorphs of the feldspars anorthite (CaSi₂Al₂O₈), albite (NaAlSi₃O₈)_, and microcline (KAlSi₃O₈). The phase transitions are induced by severe tetrahedral distortions, resulting in an increase in the Al and/or Si coordination number. High-pressure phases derived from feldspars could persist at depths corresponding to the Earth upper mantle and could possibly influence the dynamics and fate of cold subducting slabs.

Feldspars are stable at pressures up to 3 GPa along the mantle geotherm, but they can persist metastably at higher pressures at colder conditions. Here, above 10 GPa the authors find  new high-pressure polymorphs of feldspars that could persist at depths corresponding to the Earth’s upper mantle, potentially influencing the dynamics and fate of cold subducting slabs.

Slab morphology and deformation beneath Izu-Bonin

Seismic tomography provides unique constraints on the morphology, the deformation, and (indirectly) the rheology of subducting slabs. We use teleseismic double-difference P-wave tomography to image with unprecedented clarity the structural complexity of the Izu-Bonin slab. We resolve a tear in the slab in the mantle transition zone (MTZ) between 26.5° N and 28° N. North of the tear, the slab is folded in the MTZ. Immediately above the fold hinge, a zone of reduced P-wavespeed may result from viscous dissipation within an incipient shear zone. To the south of the tear, the slab overturns and lies flat at the base of the MTZ. The ~680 km deep 2015 Bonin earthquake (Mw~7.9) is located at the northernmost edge of the overturning part of the slab. The localised tearing, shearing and buckling of the Izu-Bonin slab indicates that it remains highly viscous throughout the upper mantle and transition zone.

In the 1000 km long Izu-Bonin subduction zone to the south of Tokyo, the Pacific Plate descends beneath the Philippine Sea Plate. Here the authors use teleseismic double-difference tomography to image the complex morphology of the Izu-Bonin slab, especially in the mantle transition zone.

Horizontal mantle flow controls subduction dynamics

It is generally accepted that subduction is driven by downgoing-plate negative buoyancy. Yet plate age –the main control on buoyancy– exhibits little correlation with most of the present-day subduction velocities and slab dips. “West”-directed subduction zones are on average steeper (~65°) than “East”-directed (~27°). Also, a “westerly”-directed net rotation of the lithosphere relative to the mantle has been detected in the hotspot reference frame. Thus, the existence of an “easterly”-directed horizontal mantle wind could explain this subduction asymmetry, favouring steepening or lifting of slab dip angles. Here we test this hypothesis using high-resolution two-dimensional numerical thermomechanical models of oceanic plate subduction interacting with a mantle flow. Results show that when subduction polarity is opposite to that of the mantle flow, the descending slab dips subvertically and the hinge retreats, thus leading to the development of a back-arc basin. In contrast, concordance between mantle flow and subduction polarity results in shallow dipping subduction, hinge advance and pronounced topography of the overriding plate, regardless of their age-dependent negative buoyancy. Our results are consistent with seismicity data and tomographic images of subduction zones. Thus, our models may explain why subduction asymmetry is a common feature of convergent margins on Earth.