Evidence for a large-scale remnant of subducted lithosphere beneath Fiji

Local slab penetration into lower mantle controls deep-focus seismicity and Changbaishan volcanism in northeast China

Intraplate volcanism in the Changbaishan region of northeast China is underlain by a deep subduction-related earthquake cluster in the mantle transition zone, but the underlying physical connections between them remain elusive. Here we use teleseismic double-difference tomography to obtain high-resolution images of the subducting Pacific slab to provide better constraints on slab morphology and potential relationships with volcanism and deep-focus seismicity. Our results reveal a narrow slab tongue penetrating the lower mantle, flanked to the north and south by slab stagnating in the mantle transition zone. This geometry creates a gap for sub-slab hot materials to ascend, providing a deep mantle source for Changbaishan volcanism. Additionally, the slab tongue is warped, which spatially coincides with a cluster of deep earthquakes, implying that an active deformation zone causes the clustered deep earthquakes. Our findings demonstrate that Changbaishan volcanism and deep-focus earthquakes are fundamentally controlled by the subducting slab's dynamic morphology beneath northeast China.

Numerical modeling the process of deep slab dehydration and magmatism

This study uses a 2D high-resolution thermo-mechanical coupled model to investigate the dynamic processes of deep plate hydration, dehydration, and subsequent magmatic activity in ocean-continent subduction zones. We reveal the pathways and temporal evolution of water transport to the deep mantle during the subduction process. Plate dehydration plays a critical role in triggering partial melting of the deep mantle and related magmatic activity. Our study shows significant differences in the volumes of melt produced at different depths, with dehydration reactions in deeper regions being weaker compared to shallower ones. It takes a longer time to reach the suitable P-T conditions for hydrous melting in the deep mantle. The results highlight the geophysical significance of water transport in deep subduction zones and its role in magmatic processes, particularly in the formation of magma chambers beneath continental plates.

Phase-transformation-induced lubrication of earthquake sliding

Frictional failure is not possible at depth in Earth, hence earthquakes deeper than 30-50 km cannot initiate by overcoming dry friction. Moreover, the frequency distribution of earthquakes with depth is bimodal, suggesting another change of mechanism at about 350 km. Here I suggest that the change at 30-50 km is from overcoming dry friction to reduction of effective stress by dehydration embrittlement and that the change at 350 km is due to desiccation of slabs and initiation by phase-transformation-induced faulting. High-speed friction experiments at low pressure indicate that exceeding dry friction provokes shear heating that leads to endothermic reactions and pronounced weakening. Higher-pressure studies show nanocrystalline gouge accompanying dehydration and the highest pressure experiments initiate by exothermic polymorphic phase transformation. Here I discuss the characteristic nanostructures of experiments on high-speed friction and high-pressure faulting and show that all simulated earthquake systems yield very weak transformation-induced lubrication, most commonly nanometric gouge or melt. I also show that phase-transformation-induced faulting of olivine to spinel can propagate into material previously transformed to spinel, apparently by triggering melting analogous to high-speed friction studies at low pressure. These experiments taken as a whole suggest that earthquakes at all depths slide at low frictional resistance by a self-healing pulse mechanism with rapid strength recovery.This article is part of the themed issue 'Faulting, friction and weakening: from slow to fast motion'.

Upper- and mid-mantle interaction between the Samoan plume and the Tonga-Kermadec slabs

Mantle plumes are thought to play a key role in transferring heat from the core-mantle boundary to the lithosphere, where it can significantly influence plate tectonics. On impinging on the lithosphere at spreading ridges or in intra-plate settings, mantle plumes may generate hotspots, large igneous provinces and hence considerable dynamic topography. However, the active role of mantle plumes on subducting slabs remains poorly understood. Here we show that the stagnation at 660 km and fastest trench retreat of the Tonga slab in Southwestern Pacific are consistent with an interaction with the Samoan plume and the Hikurangi plateau. Our findings are based on comparisons between 3D anisotropic tomography images and 3D petrological-thermo-mechanical models, which self-consistently explain several unique features of the Fiji-Tonga region. We identify four possible slip systems of bridgmanite in the lower mantle that reconcile the observed seismic anisotropy beneath the Tonga slab (V(SH)>V(SV)) with thermo-mechanical calculations.

Seismic evidence of negligible water carried below 400-km depth in subducting lithosphere

Strong evidence exists that water is carried from the surface into the upper mantle by hydrous minerals in the uppermost 10-12 km of subducting lithosphere, and more water may be added as the lithosphere bends and goes downwards. Significant amounts of that water are released as the lithosphere heats up, triggering earthquakes and fluxing arc volcanism. In addition, there is experimental evidence for high solubility of water in olivine, the most abundant mineral in the upper mantle, for even higher solubility in olivine's high-pressure polymorphs, wadsleyite and ringwoodite, and for the existence of dense hydrous magnesium silicates that potentially could carry water well into the lower mantle (deeper than 1,000 km). Here we compare experimental and seismic evidence to test whether patterns of seismicity and the stabilities of these potentially relevant hydrous phases are consistent with a wet lithosphere. We show that there is nearly a one-to-one correlation between dehydration of minerals and seismicity at depths less than about 250 km, and conclude that the dehydration of minerals is the trigger of instability that leads to seismicity. At greater depths, however, we find no correlation between occurrences of earthquakes and depths where breakdown of hydrous phases is expected. Lastly, we note that there is compelling evidence for the existence of metastable olivine (which, if present, can explain the distribution of deep-focus earthquakes) west of and within the subducting Tonga slab and also in three other subduction zones, despite metastable olivine being incompatible with even extremely small amounts of water (of the order of 100 p.p.m. by weight). We conclude that subducting slabs are essentially dry at depths below 400 km and thus do not provide a pathway for significant amounts of water to enter the mantle transition zone or the lower mantle.

Pervasive Seismic Wave Reflectivity and Metasomatism of the Tonga Mantle Wedge

Subduction zones play critical roles in the recycling of oceanic lithosphere and the generation of continental crust. Seismic imaging can reveal structures associated with key dynamic processes occurring in the upper-mantle wedge above the sinking oceanic slab. Three-dimensional images of reflecting interfaces throughout the upper-mantle wedge above the subducting Tonga slab were obtained by migration of teleseismic recordings of underside P- and S-wave reflections. Laterally continuous weak reflectors with tens of kilometers of topography were detected at depths near 90, 125, 200, 250, 300, 330, 390, 410, and 450 kilometers. P- and S-wave impedances decreased at the 330-kilometer and 450-kilometer reflectors, and S-wave impedance decreased near 200 kilometers in the vicinity of the slab and near 390 kilometers, just above the global 410-kilometer increase. The pervasive seismic reflectivity results from phase transitions and compositional zonation associated with extensive metasomatism involving slab-derived fluids rising through the wedge.

Shearing instabilities accompanying high-pressure phase transformations and the mechanics of deep earthquakes

Deep earthquakes have been a paradox since their discovery in the 1920s. The combined increase of pressure and temperature with depth precludes brittle failure or frictional sliding beyond a few tens of kilometers, yet earthquakes occur continually in subduction zones to approximately 700 km. The expected healing effects of pressure and temperature and growing amounts of seismic and experimental data suggest that earthquakes at depth probably represent self-organized failure analogous to, but different from, brittle failure. The only high-pressure shearing instabilities identified by experiment require generation in situ of a small fraction of very weak material differing significantly in density from the parent material. This "fluid" spontaneously forms mode I microcracks or microanticracks that self-organize via the elastic strain fields at their tips, leading to shear failure. Growing evidence suggests that the great majority of subduction zone earthquakes shallower than 400 km are initiated by breakdown of hydrous phases and that deeper ones probably initiate as a shearing instability associated with breakdown of metastable olivine to its higher-pressure polymorphs. In either case, fault propagation could be enhanced by shear heating, just as is sometimes the case with frictional sliding in the crust. Extensive seismological interrogation of the region of the Tonga subduction zone in the southwest Pacific Ocean provides evidence suggesting significant metastable olivine, with implication for its presence in other regions of deep seismicity. If metastable olivine is confirmed, either current thermal models of subducting slabs are too warm or published kinetics of olivine breakdown reactions are too fast.

Remote triggering of deep earthquakes in the 2002 Tonga sequences

It is well established that an earthquake in the Earth's crust can trigger subsequent earthquakes, but such triggering has not been documented for deeper earthquakes. Models for shallow fault interactions suggest that static (permanent) stress changes can trigger nearby earthquakes, within a few fault lengths from the causative earthquake, whereas dynamic (transient) stresses carried by seismic waves may trigger earthquakes both nearby and at remote distances. Here we present a detailed analysis of the 19 August 2002 Tonga deep earthquake sequences and show evidence for both static and dynamic triggering. Seven minutes after a magnitude 7.6 earthquake occurred at a depth of 598 km, a magnitude 7.7 earthquake (664 km depth) occurred 300 km away, in a previously aseismic region. We found that nearby aftershocks of the first mainshock are preferentially located in regions where static stresses are predicted to have been enhanced by the mainshock. But the second mainshock and other triggered events are located at larger distances where static stress increases should be negligible, thus suggesting dynamic triggering. The origin times of the triggered events do not correspond to arrival times of the main seismic waves from the mainshocks and the dynamically triggered earthquakes frequently occur in aseismic regions below or adjacent to the seismic zone. We propose that these events are triggered by transient effects in regions near criticality, but where earthquakes have difficulty nucleating without external influences.