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a) Two principal transport mechanisms: Rayleigh-Taylor instability (RTI) and slab-parallel advection in melt-rich layers on dipping slabs. α ⁎ denotes the critical slab-dip angle for their transition. b) Laboratory models showing a transition of RTI-to advection-dominated structures in buoyant source layers with increase in slab-dip angle (α) from 20° to 30°. The layers initially had a uniform thickness (10 mm). R: viscosity ratio between the overburden and the source-layer fluids.
Source publication
We show slab-parallel advection and Rayleigh-Taylor instability (RTI) as two competing gravity-driven flow mechanisms in the melt-rich layer atop a subducting slab. Scaled laboratory model results, supported by CFD simulations, indicate a transition of the RTI to advection mechanism at a threshold slab-dip angle (α) between 20° and 30°. The advecti...
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... slab-dip angles (α = 0° to 20°) produced RTIs in the entire buoyant layer, forming regular wave instabilities, which amplified vertically to form a series of plumes down the slab-dip (Fig. 1a, b-i). The instabilities grew more or less equally when α ≤ 10°. With an increase in α, the growing waves had a tendency to drift in the updip direction, suggesting the onset of a slab-parallel advection with the RTI. However, the waves ultimately grew in amplitude to keep the RTI mechanism active. This mechanism ceased to operate as α ≥ ...
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... with the RTI. However, the waves ultimately grew in amplitude to keep the RTI mechanism active. This mechanism ceased to operate as α ≥ 30°, leaving the buoyant layer almost flat, showing no sign of wavy undulations at its interface with the denser overburden. The buoyant materials advected updip to produce a single plume at the upper end (Fig. 1a, b-ii). The experiments indicate a transition of the RTI to advection mechanism at a threshold α value between 20° and 30° , irrespective of R > or < 1. (10 mm). R: viscosity ratio between the overburden and the source-layer ...
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... would be a minimum at α = 0, i.e., in case of horizontal layers. The graphical plots of σ with k also reveal reducing growth rates of the instability with an increase in α (Fig. 3b). Now, we need to account for the slab-parallel advection process to show the existence of a threshold α for instability observed in our analogue and numerical models (Fig. 1b, 2a). The real part of ω represents the advection velocity (V 0 ) of the wave for a uniform source layer thickness h 0 , where, (12) In Eq. (12) V 0 is proportional to α, and h 0 . An increase in α would thus promote the advection process in the buoyant layer (Fig. 3d), and facilitate the material to accumulate at the upper edge of the ...
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... critical slab-dip angle (α * ) in Eq. (18) is consistent with the analogue and numerical model results as well as natural data (Figs. 1, 2; Fig. S2, see supplementary for further discussion). Considering different possible parametric values for natural subduction systems, α * can however range from 20° to 28° (Fig. ...
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... analysis presented in this article indicates that the RTIs in a buoyant layer can occur only when its inclination is less than ~28°. Inclinations exceeding this threshold value replace the RTI mechanism with a slab-parallel advection. This theoretical prediction applies well to our experimental observation as well as numerical simula tions ( Fig. 1, 2). We will now explore how far this criticality analysis can be extended to natural subduction ...
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... the central part of TMVB, the active volcanoes are not spatially scattered, but distributed roughly along a trench-parallel linear trend. This pattern suggests melt focusing to the updip region in the subduction zone (Fig. 4b-i), which we can relate to the slab-parallel advection mechanism in our laboratory experiments (Fig. 1). In contrast, the northeastern Cocos plate with a gentle α beneath Guerrero causes a global RTI in the melt-rich layer to produce spatially scattered active volcanoes in TMVB (Fig. 4b-ii). Our theoretical solution shows a critical α of ~28°, where α > 28° would suppress the RTI to facilitate the advection mechanism. This theoretical ...
Citations
... Our experiments suggest that the trench geometry, offset by a transform boundary ( Supplementary Fig. 1a), is required for the Table 2. maturation and long-term stability of mode-III VT within a homogenous subducting plate. For the reference model, with 40 Ma plate age and strong strain weakening, the 200 km trench offset enables the intact plate continuously be segmented into two subducting plate sections (Fig. 2a, Supplementary Fig. 3a-d). In contrast, mode-III VT did not appear in experiments with a 100 km trench offset (Fig. 2b, Supplementary Fig. 3e-h). ...
... In the plate age phase diagram (top group), the friction coefficient is 0.6-0, representing a strong strain weakening. In the strain weakening simulations (bottom group), plate ages are set to 40 Ma. Each of the circles refers to one of the numerical experiments, and their color indicates different modes: oblique subduction mode (blue), transition mode (orange), and tearing mode (green). ...
... However, we suggest them as potential origins because of the lack of natural examples in modern subduction systems. Another and more common origination of trench offset geometry is the inherited geometry from previous lateral non-uniform subduction induced by the subduction of lithospheric anomalies (buoyant terranes 19,20 or weak zones combined with lateral heterogeneity 25,39,40 ). One natural instance is the subduction of the buoyant aseismic Caroline Island Ridge. ...
Lateral non-uniform subduction is impacted by continuous plate segmentation owing to vertical tearing of the subducting plate. However, the dynamics and physical controls of vertical tearing remain controversial. Here, we employed 3D numerical models to investigate the effects of trench geometry (offset by a transform boundary) and plate rheology (plate age and the magnitude of brittle/plastic strain weakening) on the evolution of shear stress-controlled vertical tearing within a homogenous subducting oceanic plate. Numerical results suggest that the trench offset geometry could result in self-sustained vertical tearing as a narrow shear zone within the intact subducting oceanic plate, and that this process of tearing could operate throughout the entire subduction process. Further, the critical trench offset length for the maturation of vertical tearing is impacted by plate rheology. Comparison between numerical modelling results and natural observations suggests that vertical tearing attributed to trench offset geometry is broadly developed in modern subduction and collision systems worldwide.
... Our focus is on instability of the layer once subduction has ceased. A full analysis of this problem is beyond the scope of this paper; here we indicate only how such instabilities may arise, and estimate approximate time scales; see Ghosh et al. (2020) for further discussion. For simplicity, we consider a Newtonian fluid. ...
A thermal and mechanical framework is presented for analysis of pressure‐temperature (P‐T) data and structural observations from high‐pressure‐low‐temperature (HPLT) terrains. P‐T data from 281 HPLT rocks exhibit two regimes separated at a pressure of ∼1.5 GPa, which corresponds to the modal maximum depth of thrust faulting in subduction zones. At pressures ≲1.5 GPa, interpreted as recording conditions on the plate interface, temperatures increase at about 350°C/GPa and are consistent with conditions calculated for shear stresses of ∼30–100 MPa on the interface. Such shear stresses are high enough to carry several kilometers' thickness of sediment at least to the base of the plate interface. Burial of material on plate interfaces occurs predominantly during large‐to‐great earthquakes; the exhumation phase involves contrasts in ascent rates of adjacent units, because of their differing buoyancies and strengths. In consequence, juxtaposition of unrelated rock types is expected to be ubiquitous, during both descent and ascent. The scarcity of temperatures higher than ∼650°C at pressures ≳1.5 GPa may reflect loss of material from the wedge‐slab interface by buoyant ascent. Exhumation of rocks in the subduction interface requires substantial reduction in shear stress, most plausibly by (near‐)cessation of subduction. During prograde metamorphism temperatures increase smoothly with depth in the plate interface, with almost isothermal compression in the wedge‐slab interface. Following cessation of subduction, rocks rising along the wedge‐slab interface are likely to heat slightly during decompression. Within the plate interface, temperatures drop following the cessation of shear heating, and rocks follow counter‐clockwise hairpin PT paths.
... When the amplitude is equal to the wavelength, the disturbance begins to become asymmetric, and then grows nonlinearly, and finally enters the turbulent mixing stage [1]. The RT instability is a typical and fundamental interfacial instability that plays an important role in many fields, such as inertial confinement fusion [2][3][4], supernova explosions [5][6][7], meteorology [8,9], and geophysics [10][11][12]. Therefore, the study of RT instability has important theoretical significance and application value, and has aroused considerable interest and been developed rapidly. ...
Rayleigh-Taylor (RT) instability widely exists in nature and engineering fields. How to better understand the physical mechanism of RT instability is of great theoretical significance and practical value. At present, abundant results of RT instability have been obtained by traditional macroscopic methods. However, research on the thermodynamic non-equilibrium (TNE) effects in the process of system evolution is relatively scarce. In this paper, the discrete Boltzmann method based on non-equilibrium statistical physics is utilized to study the effects of the specific heat ratio on compressible RT instability. The evolution process of the compressible RT system with different specific heat ratios can be analyzed by the temperature gradient and the proportion of the non-equilibrium region. Firstly, as a result of the competition between the macroscopic magnitude gradient and the non-equilibrium region, the average TNE intensity first increases and then reduces, and it increases with the specific heat ratio decreasing; the specific heat ratio has the same effect on the global strength of the viscous stress tensor. Secondly, the moment when the total temperature gradient in y direction deviates from the fixed value can be regarded as a physical criterion for judging the formation of the vortex structure. Thirdly, under the competition between the temperature gradients and the contact area of the two fluids, the average intensity of the non-equilibrium quantity related to the heat flux shows diversity, and the influence of the specific heat ratio is also quite remarkable.
... Moreover, the code has been used to simulate a wide range of large-scale geodynamic processes such as the mid-oceanic ridge system (Montési and Behn, 2007), subduction zone modeling (Rodríguez-González et al., 2012;Dasgupta and Mandal, 2018), magma upwelling [Flesch et al., 2001;Liu and Yang, 2003;Copley and McKenzie, 2007;Warren et al., 2008;Li and Gerya, 2009] Frontiers in Earth Science | www.frontiersin.org February 2021 | Volume 9 | Article 632806 (Shahraki and Schmeling, 2012), mantle convection (He, 2014), Rayleigh-Taylor instabilities (Ghosh et al., 2020), continental deformations (Bischoff and Flesch, 2019). We have employed Arbitrary Lagrangian and Eulerian method (ALE) (Fullsack, 1995;Dasgupta and Mandal, 2018) in our timedependent model runs to advect the model upper surface with the flow and to measure the development of model topography. ...
Low-viscosity channel flow, originating from a melt-weakened mid-crustal layer, is one of the most popular tectonic models to explain the exhumation of deep-seated rocks in the Greater Himalayan Sequence (GHS). The driving mechanism of such channel flow, generally attributed to focused erosion in the mountain front, is still debated, and yet to be resolved. Moreover, the channel flow model cannot explain eclogites in the GHS. In this study, we present a new two-dimensional thermo-mechanical numerical model, based on lubrication dynamics to demonstrate the exhumation process of deep crustal rocks in GHS. The model suggests that a dynamic-pressure drop in the Himalayan wedge, following a large reduction in the India-Asia convergence velocity (15 cm/yr at 50 Ma to nearly 5 cm/yr at ∼22 Ma) localized a fully developed extrusion zone, which controlled the pressure-temperature-time (P-T-t) path of GHS rocks. We show that the wedge extrusion, originated in the lower crust (∼60 km), was initially bounded by two oppositely directed ductile shear zones: the South Tibetan Detachment systems (STDS) at the top and the Higher Himalayan Discontinuity (HHD) at the bottom. With time the bottom shear boundary of the extrusion zone underwent a southward migration, forming the Main Central Thrust (MCT) at ∼17 Ma. Our model successfully reproduces two apparently major paradoxical observations in the Himalaya: syn-convergence extension and inverted metamorphic isograds. Model peak P (10–17 kb) and T (700–820°C) and the exhumation P-T-t path estimated from several Lagrangian points, traveling through the extrusion zone, are largely compatible with the petrological observations in the GHS. The model results account for the observed asymmetric P-T distribution between the MCT and STDS, showing peak P-T values close to the MCT. The lubrication dynamics proposed in this article sheds light on the fast exhumation event (>1 cm/yr) in the most active phase of crustal extrusion (22-17 Ma), followed by a slowed-down event. Finally, our model explains why the extrusion zone became weak in the last 8-10 Ma in the history of India-Asia collision.
It is a well-accepted hypothesis that deep-mantle primary plumes originate from a buoyant source layer at the core-mantle boundary (CMB), where Rayleigh–Taylor (RT) instabilities play a key role in the plume initiation process. Previous studies have characterized their growth rates mainly in terms of the density, viscosity and layer-thickness ratios between the denser overburden and the source layer. The RT instabilities, however, develop in the presence of global flows in the overlying mantle, which can act as an additional factor in the plume mechanics. Combining 2D computational fluid dynamic (CFD) model simulations and a linear stability analysis, this article explores the influence of a horizontal global mantle flow in the instability dynamics. Both the CFD simulation results and analytical solutions reveal that the global flow is a dampening factor in reducing the instability growth rate. At a threshold value of the normalized global flow velocity, short as well as long wavelength instabilities are completely suppressed, allowing the entire system to advect in the horizontal direction. Using a series of real-scale numerical simulations this article also investigates the growth rate as a function of the density contrast, expressed in Atwood number = (- )/ (+), and the viscosity ratio, where and are densities and viscosities of the overburden mantle and source-layer, respectively. It is found that increase in either or promotes the growth rate of a plume. In addition, the stability analysis predicts a nonlinearly increasing RT instability wavelength with increasing global flow velocity, implying that the resulting plumes widen their spacing preferentially in the flow direction of kinematically active mantle regions. The theory accounts for additional physical parameters: source-layer viscosity and thickness in the analysis of the dominant wavelengths and their corresponding growth rates. The article finally discusses the problem of unusually large inter-hotspot spacing, providing a new conceptual framework for the origin of sporadically distributed hotspots of deep-mantle sources.
Rayleigh-Taylor (RT) instability widely exists in nature and engineering fields. How to better understand the physical mechanism of RT instability is of great theoretical significance and practical value. At present, abundant results of RT instability have been obtained by traditional macroscopic methods. However, research on the thermodynamic non-equilibrium (TNE) effects in the process of system evolution is relatively scarce. In this paper, the discrete Boltzmann method based on non-equilibrium statistical physics is utilized to study the effects of the specific heat ratio on compressible RT instability. The evolution process of the compressible RT system with different specific heat ratios can be analyzed by the temperature gradient and the proportion of the non-equilibrium region. Firstly, as a result of the competition between the macroscopic magnitude gradient and the non-equilibrium region, the average TNE intensity first increases and then reduces, and it increases with the specific heat ratio decreasing; the specific heat ratio has the same effect on the global strength of the viscous stress tensor. Secondly, the moment when the total temperature gradient in y direction deviates from the fixed value can be regarded as a physical criterion for judging the formation of the vortex structure. Thirdly, under the competition between the temperature gradients and the contact area of the two fluids, the average intensity of the non-equilibrium quantity related to the heat flux shows diversity, and the influence of the specific heat ratio is also quite remarkable.
