Search

The catastrophic 2004 Sumatra‐Andaman Mw9.1 earthquake associated with destructive tsunamis has characterized Sumatra as one of the most dangerous convergence zones. Nevertheless, the effects of the thermohydrous state on the strongly coupled megathrust of the incoming plate remain enigmatic. By using a three‐dimensional (3‐D) thermomechanical model to compute the temperature variation and the complicated phase transition process of the water‐bearing descending plate which generates unstable thrust slips, we find that Sumatran earthquakes at varying depths are likely under the control of the inter or intraplate hydrothermal regime, which occurs in or close to the petrological metamorphism transition area. The slab dehydration of the water‐rich mid‐ocean‐ridge basalts (MORB) and the ultramafic rocks in the oceanic lithosphere releases a large amount of fluid to the continental wedge and further facilitates arc magmatism. The fluids are prone to upwelling following the subduction channel along the plate interface and thus contribute to the clustering of earthquakes updip of the dehydration front beneath offshore Sumatra. Brittle failure and dehydration embrittlement at depth, along with the temperature differences caused by variant slab geometry, are conjectured to greatly influence the occurrence of fast and slow earthquakes in Sumatra. Key Points: Sumatran earthquakes are likely under control, and they occur in or close to the area transitioning from petrological metamorphismSlab dehydration of the water‐rich rocks in the oceanic lithosphere release a large amount of fluid to the continental wedgeBrittle failure and dehydration embrittlement have greatly influenced the occurrence of fast to slow earthquakes in Sumatra [ABSTRACT FROM AUTHOR]

We investigated the relationships among slab geometry, obliquity, and the thermal regime associated with the subduction of oceanic plates using a three-dimensional (3D) parallelepiped thermal convection model. Various models with convex and concave slab shapes were constructed in the numerical simulation, and the temperature and mantle flow distributions were calculated. The results revealed that when the slab dip angle increases, or the obliquity of subduction becomes steeper, the interplate temperature decreases remarkably. Cooler (warmer) zones on the plate interface were identified from the modeling where there was a larger (smaller) subduction angle. Consequently, the interplate temperature distribution is partly controlled by the true subduction angle (TSA), which is a function of the slab dip angle and the obliquity of subduction. The rate of change of the interface temperature for the TSA was 10–50 °C (10°< TSA < 20°) at depths ranging from (TSA – 10) × 5 km to 60 + (TSA – 10) × 5 km for a flat slab after a subduction history of 7 Myrs. The along-arc slab curvature affects the variation in TSA. The slab radius also appeared to influence the radius of induced mantle flow. [ABSTRACT FROM AUTHOR]

Abstract: Global seismic tomography has recently revealed horizontally lying slabs near the upper and lower mantle boundary beneath the Northwestern Pacific region. Although physical mechanisms that could produce such slab stagnation have been proposed based on numerical simulations, there has been little research into what occurs after slab stagnation. We proposed trench advance and trench jumps as effective mechanisms related to the fall of stagnant slabs into the lower mantle, and our numerical simulations of temperature and fluid flow associated with slab subduction in a 2-D box model confirmed these mechanisms. Our results indicate that a supply of slab material associated with further slab subduction after slab stagnation plays an important role in differentiating further slab stagnation from the falling of slabs into the lower mantle. A shortage of material supply would produce extended slab stagnation near the 660-km boundary for ringwoodite to perovskite+magnesiowüstite phase transformation, whereas downward force due to further slab subduction on a stagnant slab would enhance its fall into the lower mantle. The behaviors of falling stagnant slabs were not affected by Clapeyron slope values associated with phase equilibrium transformation within the range from −3.0 to 0.0MPa/K. Compared with models of normal mantle viscosity, a high-viscosity lower mantle played a role in hindering the fall of slabs into the lower mantle, resulting in complicated shapes and slow falling velocities. Lower mantle viscosity structure also affected slab behavior. Slabs tended to stagnate when a low-viscosity zone (LVZ) existed just below a depth of 660km because friction between the slab and the LVZ was weak there. Slab stagnation around a depth of 660km also occurred when a high-viscosity zone existed below a depth of 1200km and acted as a resistive force against a slab, even if the slab existed in the lower part of the upper mantle. [ABSTRACT FROM AUTHOR]

Abstract: Recent seismic tomography has revealed various morphologies in the subducted lithosphere. In particular, significant flattening and stagnation of slabs around the 660-km boundary are seen in some areas beneath the northwestern Pacific subduction zones. We examined the cause of slab stagnation in terms of the Clapeyron slope of the phase transformation from ringwoodite to perovskite+magnesiowüstite, trench retreat velocity, dip angles, and high viscosity of the lower mantle based on two-dimensional (2-D) numerical simulations of thermal convection. In particular, we examined the conditions necessary for slab stagnation assuming a very small absolute value of the Clapeyron slope, which were proposed based on recent high-pressure, high-temperature (high P–T) experiments. Our calculations show that slabs tend to stagnate above the 660-km boundary with an increasing absolute value of the Clapeyron slope, viscosity jump at the boundary, and trench retreat velocity and a decreasing initial dip angle. Stagnant slabs could be obtained numerically for a realistic range of parameters obtained from high P–T experiments and other geophysical observations combining buoyancy, high lower-mantle viscosity, and trench retreat. We found that a low dip angle of a descending slab at the bottom of the upper mantle plays an important role in slab stagnation. Two main regimes underlie slab stagnation: buoyancy-dominated and viscosity-dominated regimes. In the viscosity-dominated regime, it is possible for slabs to stagnate above the 660-km boundary, even when the value of the Clapeyron slope is 0 MPa/K. [Copyright &y& Elsevier]