Summary The Sunda plate has shaped itself in a complex tectonic framework, driven by the interactions of multiple subduction zones in its history. Using thermo-mechanical computational fluid dynamic models we show in this paper how the in-dip double-subduction dynamics has controlled the first-order 3D topography of this plate, currently bounded by two major N-S trending active trenches: Andaman-Sumatra-Java and Philippines on its western and eastern margins, respectively. We consider six E-W transects to account for an along-trench variation of the subduction parameters: subduction rate (Vc), shallow-depth (200–300 km) slab dip (α) and inter-trench distance (λ) in our 2D numerical experiments. The deviatoric stress fields and the topographic patterns are found to strongly depend on λ. For large inter-trench distances (λ = 2000 to 3000 km), the overriding plate develops dominantly tensile stresses in its central zone, forming low topographic elevations. Decreasing λ results in a transition from extensional to contractional deformation, and promotes topographic uplift in the southern part. We explain these effects of λ in terms of the sub-lithospheric flow vortex patterns produced by the subducting slabs. Large λ (> 2000 km) generates non-interacting flow vortices, located close to the two trenches, leaving the mantle region beneath the overriding plate weakly perturbed. In contrast, small λ results in their strong interaction to produce a single upwelling zone, which facilitates the overriding plate to gain a higher topographic elevation. The stress field predicted from our model is validated with the observed stress patterns. We also interpolate a three-dimensional topographic surface and vertical uplift rates from the serial model sections, and compare them with the observed surface topography of the Sunda plate.
Summary By combining scaled laboratory experiments and numerical simulations, this study presents a quantitative analysis of the bending radius (RB) of subducting slabs within the upper mantle, taking into account the effects of age (A). Based on a half-space cooling model, we constrain the density (ρ), viscosity (η) and thickness (h) of slabs as a function of A, and develop representative models to estimate RB for different A. Laboratory subduction models produce visually contrasting bending curvatures for young (A = 10 Ma), intermediate (A = 70 Ma) and old (A = 120 Ma) slabs. Young slabs undergo rollback, resulting in a small bending radius (scaled up RB ∼ 150 km), whereas old slabs subduct along a uniformly dipping trajectory with large bending radius (RB ∼ 500 km). Equivalent real scale computational fluid dynamic (CFD) simulations reproduce similar bending patterns of the subducting slabs, and yield RB versus A relations fairly in agreement with the laboratory results. We balance the buoyancy driven bending, flexural-resistive moments and viscous flow induced suction moment to theoretically evaluate the rate of slab bending. The analytical solution suggests an inverse relation of the bending rate with A, which supports our experimental findings. Finally, slab geometries of selected natural subduction zones, derived from high-resolution seismic tomographic images have been compiled to validate the experimental RB versus A regression. We also discuss the subduction settings in which this regression no longer remains valid.
<p>Magmatic overpressure in shallow- and mid-crustal magma chambers (MC) can deform the crustal host rocks. Stress field produced by such deformation often control the nucleation and subsequent crack formation for magma emplacement. A direction of physical volcanology is concerned with determination of the volcanotectonic ground surface displacements that can aid in monitoring and sometimes forecasting magmatic eruptions. The existing Mogi Model can analytically calculate surface displacements due to overpressure in a single MC by considering elastic deformation of a finite crustal section. Many geological and geophysical studies report that magma plumbing systems represent an array of randomly placed interconnected MCs, and there is a need of theoretical estimation of their ground surface displacement. In this study we present a new analytical formulation to estimate surface displacement in terms of both vertical as well as horizontal directions above a dual MC setting. Our analytical solution finds support from finite element (FE) models performed with the same set of geometrical and physical parameters. The off-axis chambers considered in our model are separated along both vertical and horizontal directions. The present study suggests that with increasing horizontal chamber separation (<em>S</em><em><sub>h</sub></em>) the vertical ground displacement above the two chambers gradually changes from a single peak into an indistinct double-peak, and finally two prominent independent, high-amplitude peaks. On the other hand, on increasing the vertical separation (<em>S</em><em><sub>v</sub></em>) between two off-axis chambers we observed that the initial double peaks merged to produce a single peak situated roughly above the middle of the two chambers. Stress map obtained from the FE models shows that the deformation of two MCs can only interact when located within a critical distance, else their deformation remains independent. Interestingly, our study suggests that the magnitude of stress field strongly depends on the strength of the mechanical interaction between two neighboring chambers.</p>
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