Description: We studied the effect of increased water content on the dynamics of the lithosphere-asthenosphere boundary in a postsubduction setting. Results from numerical mantle convection models show that the resultant decrease in mantle viscosity and the peridotite solidus produce small-scale convection at the lithosphere-asthenosphere boundary and magmatism that follows the spatially and temporally scattered style and volumes typical for collision magmatism, such as the late Cenozoic volcanism of the Turkish-Iranian Plateau. An inherent feature in small-scale convection is its chaotic nature that can lead to temporally isolated volcanic centers tens of millions of years after initial continental collision, without evident tectonic cause. We also conclude that water input into the upper mantle during and after subduction under the circum-Mediterranean area and the Tibetan Plateau can account for the observed magmatism in these areas. Only fractions (200–600 ppm) of the water input need to be retained after subduction to induce small-scale convection and magmatism on the scale of those observed from the Turkish-Iranian Plateau.

Description: The formation and evolution of a backarc basin are linked to the dynamics of the subduction system. The opening of the central Mediterranean basins is a well-documented example of backarc extension characterized by short-lived episodes of spreading. The underlying reasons for this episodicity are obscured by the complexity of this subduction system, in which multiple continental blocks enter the subduction zone. We present results from three-dimensional numerical models of laterally varying subduction to explain the mechanism of backarc basin opening and the episodic style of spreading. Our results show that efficient backarc extension can be obtained with an along-trench variation in slab buoyancy that produces localized deformation within the overriding plate. We observe peaks in the trench retreating velocity corresponding first to the opening of the backarc basin, and later to the formation of slab windows. We suggest that the observed episodic trench retreat behavior in the central Mediterranean is caused by the formation of slab windows.

Description: Comparison of modeling results with observed subsidence patterns from the West Siberian Basin provides new insight into the origin of the Siberian Traps, and constrains the temperature, size, and depth of an impacting mantle plume head during and after the eruption of the Siberian Traps at the Permian-Triassic boundary (250 Ma). We compare subsidence patterns from one-dimensional conductive heat flow models to observed subsidence from backstripping studies of wells in the basin. This results in a best-fit scenario with a 50-km-thick initial plume head with a temperature of 1500 °C situated 50 km below the surface, and an initial regional crustal thickness of 34 km, in agreement with published values. Backstripping and modeling results agree very well, including a 60–90 m.y. delay between the rifting phase and the first regional sedimentation. Regional subsidence patterns indicate that the plume head was present across a minimum area of ~2.5 x 10 6 km 2 . These results re-emphasize the viability of a mantle plume origin for the Siberian Traps, provide important constraints on the dynamics of mantle plume heads, and suggest a thermal control for the subsidence of the West Siberian Basin.

Description: As subducting plates reach the base of the upper mantle, some appear to flatten and stagnate, while others seemingly go through unimpeded. This variable resistance to slab sinking has been proposed to affect long-term thermal and chemical mantle circulation. A review of observational constraints and dynamic models highlights that neither the increase in viscosity between upper and lower mantle (likely by a factor 20–50) nor the coincident endothermic phase transition in the main mantle silicates (with a likely Clapeyron slope of –1 to –2 MPa/K) suffice to stagnate slabs. However, together the two provide enough resistance to temporarily stagnate subducting plates, if they subduct accompanied by significant trench retreat. Older, stronger plates are more capable of inducing trench retreat, explaining why backarc spreading and flat slabs tend to be associated with old-plate subduction. Slab viscosities that are ~2 orders of magnitude higher than background mantle (effective yield stresses of 100–300 MPa) lead to similar styles of deformation as those revealed by seismic tomography and slab earthquakes. None of the current transition-zone slabs seem to have stagnated there more than 60 m.y. Since modeled slab destabilization takes more than 100 m.y., lower-mantle entry is apparently usually triggered (e.g., by changes in plate buoyancy). Many of the complex morphologies of lower-mantle slabs can be the result of sinking and subsequent deformation of originally stagnated slabs, which can retain flat morphologies in the top of the lower mantle, fold as they sink deeper, and eventually form bulky shapes in the deep mantle.