The mechanisms sustaining basaltic continental intraplate volcanism remain controversial. Continental intraplate volcanism is often geographically associated with slab stagnation in the mantle transition zone (MTZ), for example, in eastern Asia, central Europe, and western North America. Using 2‐D geodynamic models, we here explore the role of the stagnation of a slab and an associated hydrous layer in the MTZ on the formation and evolution of intraplate volcanism. Due to the intrinsic buoyancy of the hydrous layer atop the stagnant slab, upwellings develop within a few million years and rise to ~410‐km depth. At these depths, they partly lose their intrinsic buoyancy due to dehydration and stall intermittently. However, they are readily entrained by sublithospheric small‐scale convection to reach the base of lithosphere, sustaining mantle melting and intraplate volcanism. Water contents of >0.3 wt.‐% in a ≥ 60‐km‐thick layer atop the slab are sufficient for an early (<~20 Myr) onset of melting to account for volcanism, for example, in NE China. Thus, significant amounts of hydrous materials are not expected to remain stable in the MTZ for geological timescales, consistent with geophysical estimates. To explain the geochemical signatures of the Cenozoic basaltic volcanism in northern China, a mixed composition of the hydrous layer, including an enriched mantle‐type and a hybrid depleted mid‐ocean ridge basalts mantle/high μ‐type component, is required.
The Taiwan Island is the product of convergence and collision between the Eurasian and the Philippine Sea plates, where the geological structure is complex, seismicity is high and deformation strong. In order to quantitatively study characteristics of crustal deformation and to understand its geodynamic mechanisms of this area, we calculated the strain rate field in and around Taiwan by using the finite element method (FEM), utilizing GPS data from 1995 to 2005 as boundary constraints in simulation. The results show that contractions are predominant in central Taiwan, while extension exists in northeast and south, respectively. The largest deformation appears on the Coast Range and the adjacent waters in eastern Taiwan. The calculated slip rates on the Longitudinal Valley Fault (LVF) are 13.81∼23.48 mm/yr, meaning part of the convergence is absorbed by LVF, thus deformation to the west of LVF decays rapidly westwards and northwestwards. Moreover, the results show that the calculated velocities and GPS vectors are in good agreement, and calculated orientations of principal stresses are consistent with in‐situ stress measurements and focal mechanism solutions, demonstrating the finite element model established in this paper is reasonable. In addition, the calculated results imply that the general framework of present‐day deformation in Taiwan results from interactions by many factors such as plate collision between the Eurasian and the Philippine Sea plates, geometry of plate boundaries, faulting and rifting, opening of the Okinawa trough and retreat of the Ryukyu trench.
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