Generation and driving forces of plate-like motion and asymmetric subduction in dynamical models of an integrated mantle-litho sphere system

The dynamical effects of an asymmetric subduction structure on the generation of plate-like motion were investigated using two-dimensional numerical models of the integrated lithosphere-mantle system. To dynamically generate the plate boundary, we introduce a history-dependent rheology in which the yield strength is determined by past fractures. Only the buoyancy due to the internal density contrast consistently drives convective flow, including the motion of the viscous lithosphere, without imposed boundary conditions. We first investigate the effects of plate yield strength, friction at the plate boundary, and plate age on the emergence of plate-like motion with asymmetric subduction. Plate-like motion is generated when maximum plate strength is as high as that estimated by experimental rheology studies. The reason for this is that asymmetric subduction requires a plate-bending force much less than that for symmetric subduction because the plate gently bends when one-sided subduction occurs. In contrast, the strength of the plate boundary has to be very small for emergence of subduction, as several previous studies on the numerical convection and subduction modeling have pointed out. Development of the subducted slab is also controlled by the age of the plate. In the early stages of subduction, older plates increase their velocities faster because of their larger negative buoyancy. After the slab develops, the plate stiffness, that is, both the yield strength and the plate thickness, control plate velocity so that an older plate subducts more sluggishly. We next explore effects of viscosity layering in the underlying mantle, focusing on the mechanism in which the asthenosphere promotes plate motion. The low viscosity under the lithosphere enhances a mantle drag force that drives the plate, not only concentrating the convective flow beneath the plate but also enlarging its aspect ratio. We also examine longevity of the plate-like motion using the convection models with asymmetric subduction. The asymmetrically

Dynamical mechanisms controlling formation and avalnche of a stagnant slab

We performed a numerical study to understand the dynamical mechanism controlling the formation and avalanche of a stagnant slab using two-dimensional dynamical models of the integrated plate-mantle system with freely movable subducting and overriding plates. We examined slab rheology as a mechanism for producing various styles of stagnating or penetrating slabs that interact with the 410-km and 660-km phase transitions. The simulated results with the systematically changed rheological parameters are interpreted using a simple stability analysis that includes the forces acting on the stagnant slab. Slab plasticity that memorizes the shape produced by past deformation generates slab stagnation at various depths around the 660-km phase transition. The slab stagnates even beneath the 660-km phase boundary, with a gentle Clapeyron slope. Feedbacks between trench backward migration and slab deformation promote each other during the slab stagnation stage. Slab viscosity also determines the final state of the subducted slab, that is, it continues stagnation or initiates penetration. A low viscosity slab can finally penetrate into the lower mantle because the growth time of the Rayleigh–Taylor instability is shorter. After the avalanche, the direction of the trench migration changes depending on the lower mantle slab viscosity

Specific brain processing of facial expressions in people with alexithymia: an H215O‐PET study

Alexithymia is a personal trait characterized by a reduced ability to identify and describe one’s own feelings and is known to contribute to a variety of physical and behavioural disorders. To elucidate the pathogenesis of stress‐related disorders and the normal functions of emotion, it is important to investigate the neurobiology of alexithymia. Although several neurological models of alexithymia have been proposed, there is very little direct evidence for the neural correlates of alexithymia. Using PET, we studied brain activity in subjects with alexithymia when viewing a range of emotional face expressions. Twelve alexithymic and 12 non‐alexithymic volunteers (all right‐handed males) were selected from 247 applicants on the basis of the 20‐item Toronto Alexithymia Scale (TAS‐20). Regional cerebral blood flow (rCBF) was measured with H215O‐PET while the subjects looked at angry, sad and happy faces with varying emotional intensity, as well as neutral faces. Brain response in the subjects with alexithymia significantly differed from that in the subjects without alexithymia. The alexithymics exhibited lower rCBF in the inferior and middle frontal cortex, orbitofrontal cortex, inferior parietal cortex and occipital cortex in the right hemisphere than the non‐alexithymics. Additionally, the alexithymics showed higher rCBF in the superior frontal cortex, inferior parietal cortex and cerebellum in the left hemisphere when compared with the non‐alexithymics. A covariance analysis revealed that rCBF in the inferior and superior frontal cortex, orbitofrontal cortex and parietal cortex in the right hemisphere correlated negatively with individual TAS‐20 scores when viewing angry and sad facial expressions, and that no rCBF correlated positively with TAS‐20 scores. Moreover, the anterior cingulate cortex and insula were less activated in the alexithymics’ response to angry faces than their response to neutral faces. These results suggest that people with alexithymia process facial expressions differently from people without alexithymia, and that this difference may account for the disorder of affect regulation and consequent peculiar behaviour in people with alexithymia