Voyage of the Indian subcontinent since Pangea breakup and driving force of supercontinent cycles: Insights on dynamics from numerical modeling

Recent advances in three-dimensional numerical simulations of mantle convection have aided in approximately reproducing continental movement since the Pangea breakup at 200 Ma. These have also led to a better understanding of the thermal and mechanical coupling between mantle convection and surface plate motion and predictions of the configuration of the next supercontinent. The simulations of mantle convection from 200 Ma to the present reveals that the development of large-scale cold mantle downwellings in the North Tethys Ocean at the earlier stage of the Pangea breakup triggered the northward movement of the Indian subcontinent. The model of high temperature anomaly region beneath Pangea resulting from the thermal insulation effect support the breakup of Pangea in the real Earth time scale, as also suggested in previous geological and geodynamic models. However, considering the low radioactive heat generation rate of the depleted upper mantle, the high temperature anomaly region might have been generated by upwelling plumes with contribution of deep subducted TTG(tonalite-trondhjemite-granite) materials enriched in radiogenic elements. Integrating the numerical results of mantle convection from 200 Ma to the present, and from the present to the future, it is considered that the mantle drag force acting on the base of continents may be comparable to the slab pull force, which implies that convection in the shallower part of the mantle is strongly coupled with surface plate motion.

Mantle convection modeling of the supercontinent cycle: Introversion,extroversion, or a combination?

The periodic assembly and dispersal of continental fragments, referred to as the supercontinent cycle, bear close relation to the evolution of mantle convection and plate tectonics. Supercontinent formation involves complex processes of ＂introversion＂ （closure of interior oceans）, ＂extroversion＂ （closure of exterior oceans）, or a combination of these processes in uniting dispersed continental fragments, Recent developments in numerical modeling and advancements in computation techniques enable us to simulate Earth＇s mantle convection with drifting continents under realistic convection vigor and rheology in Earth-like geometry （i.e., 3D spherical-shell）. We report a numerical simulation of 3D mantle convection, incorporating drifting deformable continents, to evaluate supercontinent processes in a realistic mantle convection regime. Our results show that supercontinents are assembled by a combi- nation of introversion and extroversion processes. Small-scale thermal heterogeneity dominates deep mantle convection during the supercontinent cycle, although large-scale upwelling plumes intermit- tently originate under the drifting continents and/or the supercontinent.

Conjecture with water and rheological control for subducting slab in the mantle transition zone

Seismic observations have shown structural variation near the base of the mantle transition zone （MTZ） where subducted cold slabs, as visualized with high seismic speed anomalies （HSSAs）, flatten to form stagnant slabs or sink further into the lower mantle. The different slab behaviors were also accompanied by variation of the ＂660 kin＂ discontinuity depths and low viscosity layers （LVLs） beneath the MTZ that are suggested by geoid inversion studies. We address that deep water transport by subducted slabs and dehydration from hydrous slabs could affect the physical properties of mantle minerals and govern slab dynamics. A systematic series of three-dimensional numerical simulation has been conducted to examine the effects of viscosity reduction or contrast between slab materials on slab behaviors near the base of the MTZ. We found that the viscosity reduction of subducted crustal material leads to a sepa- ration of crustal material from the slab main body and its transient stagnation in the MTZ. The once trapped crustal materials in the MTZ eventually sink into the lower mantle within 20 30 My from the start of the plate subduction. The results suggest crustal material recycle in the whole mantle that is consistent with evidence from mantle geochemistry as opposed to a two-layer mantle convection model. Because of the smaller capacity of water content in lower mantle minerals than in MTZ minerals, dehydration should occur at the phase transformation depth, ~660 kin. The variation of the disconti- nuity depths and highly localized low seismic speed anomaly （LSSA） zones observed from seismic P waveforms in a relatively high frequency band （~ 1 Hz） support the hypothesis of dehydration from hydrous slabs at the phase boundary. The LSSAs which correspond to dehydration induced fluids are likely to be very local, given very small hydrogen （H＋） diffusivity associated with subducted slabs. The image of such local LSSA zones embedded in HSSAs may not be necessarily captured in tomography studies. The high electrical conductivity in the MTZ beneath the northwestern Pacific subduction zone does not necessarily require a broad range of high water content homogeneously.

Plate and plume tectonics:Numerical simulation and seismic tomography

About three decades after the establishment of the plate tec- tonics theory in the late 1960s, Maruyama （1994） proposed the ＂plume tectonics＂ theory based on whole-mantle seismic tomogra- phy image （Fukao, 1992; Fukao et al., 1994）. According to this the- ory, the earth＇s interior is divided into three regimes： the earth＇s surface region governed by lateral motions of tectonic plates, the mantle governed by vertical motions of ＂superplumes＂ （i.e., large- scale mantle upwelling/downwelling plumes）, and the core, whose convection style is probably controlled by superplumes in the mantle. With the rapid progress in earth science after the birth of the plume tectonics theory, it is now widely accepted that various geological phenomena observed in the earth＇s surface are closely linked to the fluid motion in the deep mantle （e.g., Davies, 2011）.

Energetics of the Solid Earth:An integrated perspective

The internal energy of the Solid Earth is mainly transferred through thermal convection of the mantle.Here,we discuss the 10^(20)-J-scale energy relating to the dynamics of the Solid Earth’s interior.The energy released from the interior of the present-day Earth to outer space per year is estimated as E_(earth)=1.4×10^(21) J yr^(-1) based on the recent dataset of globally observed crustal heat flow,which is a factor of two or three times larger than the annual energy consumption of the total population of the world,5.7×10^(20) J yr^(-1).Of the energy from global crustal heat flow,the energy released by all of the major hotspot plumes in volcanic vents per year is estimated as E_(plume)=7.2×10^(19) J yr^(-1),which is approximately only 6%of the Eearth.We propose that a large number of mantle plumes have not emerged as hotspots on the Earth’s surface,with the possibility that Eplume is larger than expected if the energy released from small seamounts of the Earth is considered.Considering the heat(energy)budget of the Earth,the heat production by the decay of radioactive isotopes in the mantle and crust is nearly comparable to the heat released by the secular cooling of the Earth.Of Eearth,the annual energy released by the secular cooling of the Earth is estimated as 6.6×10_(20) J yr^(-1).This energy is closely related to the geothermal energy from our planet.