Abstract
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.
Highlights
Compared with subduction-related magmatism, mantle-derived collision zone magmatism is still poorly understood
We studied the dynamics of the postsubduction syncollisional mantle with the hypothesis that the upper mantle on the overriding plate side has been hydrated, leading to instability of the lithosphere-asthenosphere boundary, sublithospheric small-scale convection (Hernlund et al, 2008), and consequent melting
We suggest that the irregularity of the long-term syncollisional magmatism can be explained by small-scale sublithospheric convection, effectively a form of repeated and localized lithosphere delamination or dripping, induced by the viscosity- and solidus-lowering effect of water added to the upper mantle by the precollision subduction, and possibly enhanced by asthenospheric stirring triggered by slab break-off
Summary
Compared with subduction-related magmatism, mantle-derived collision zone magmatism is still poorly understood. We studied the dynamics of the postsubduction syncollisional mantle with the hypothesis that the upper mantle on the overriding plate side has been hydrated, leading to instability of the lithosphere-asthenosphere boundary, sublithospheric small-scale convection (Hernlund et al, 2008), and consequent melting.
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