Mode Of Mantle Convection Research Articles

Dynamics of fault motion and the origin of contrasting tectonic style between Earth and Venus

Plate tectonics is one mode of mantle convection that occurs when the surface layer (the lithosphere) is relatively weak. When plate tectonics operates on a terrestrial planet, substantial exchange of materials occurs between planetary interior and its surface. This is likely a key in maintaining the habitable environment on a planet. Therefore it is essential to understand under which conditions plate tectonics operates on a terrestrial planet. One of the puzzling observations in this context is the fact that plate tectonics occurs on Earth but not on Venus despite their similar size and composition. Factors such as the difference in water content or in grain-size have been invoked, but these models cannot easily explain the contrasting tectonic styles between Earth and Venus. We propose that strong dynamic weakening in friction is a key factor. Fast unstable fault motion is found in cool Earth, while slow and stable fault motion characterizes hot Venus, leading to substantial dynamic weakening on Earth but not on Venus. Consequently, the tectonic plates are weak on Earth allowing for their subduction, while the strong plates on Venus promote the stagnant lid regime of mantle convection.

Origin of geochemical mantle components: Role of spreading ridges and thermal evolution of mantle

AbstractWe explore the element redistribution at mid‐ocean ridges (MOR) using a numerical model to evaluate the role of decompression melting of the mantle in Earth's geochemical cycle, with focus on the formation of the depleted mantle component. Our model uses a trace element mass balance based on an internally consistent thermodynamic‐petrologic computation to explain the composition of MOR basalt (MORB) and residual peridotite. Model results for MORB‐like basalts from 3.5 to 0 Ga indicate a high mantle potential temperature (Tp) of 1650–1500°C during 3.5–1.5 Ga before decreasing gradually to ∼1300°C today. The source mantle composition changed from primitive (PM) to depleted as Tp decreased, but this source mantle is variable with an early depleted reservoir (EDR) mantle periodically present. We examine a two‐stage Sr‐Nd‐Hf‐Pb isotopic evolution of mantle residues from melting of PM or EDR at MORs. At high‐Tp (3.5–1.5 Ga), the MOR process formed extremely depleted DMM. This coincided with formation of the majority of the continental crust, the subcontinental lithospheric mantle, and the enriched mantle components formed at subduction zones and now found in OIB. During cooler mantle conditions (1.5–0 Ga), the MOR process formed most of the modern ocean basin DMM. Changes in the mode of mantle convection from vigorous deep mantle recharge before ∼1.5 Ga to less vigorous afterward is suggested to explain the thermochemical mantle evolution.

Depth-dependent viscosity and mantle stress amplification: implications for the role of the asthenosphere in maintaining plate tectonics

Boundary layer theory is used to derive scaling relationships for plate stresses in a mantle convection system with a low-viscosity asthenosphere. The theory assumes a plate tectonic like mode of mantle convection with flow driven by an active upper boundary layer. The theory predicts that the confinement of horizontal mantle flow within a low-viscosity, sublithospheric channel can lead to an increase in plate stress compared to the case lacking a channel (even if the absolute viscosity of the sublithosphere mantle does not change between the two cases). The theory further predicts increasing shear stress with decreasing low-viscosity channel thickness. If the thickness of tectonic plates is determined dominantly by a dehydrated chemical lithosphere, then the plate normal stress is predicted to also increase with decreasing channel thickness. We use 3-D spherical shell simulations of mantle convection with temperature-, depth- and stress dependent rheology to test scaling trends. The simulations and theoretical scalings demonstrate that a low-viscosity layer (asthenosphere) can amplify convective stresses. If the level of convective stress plays a role in maintaining and/or reactivating plate boundaries, this suggests that a relatively thin low viscosity layer may help to maintain plate tectonics. The numerical simulations support this suggestion as they show that an increase in the thickness of a low viscosity channel can cause the system to transition from an active-lid mode of convection to a stagnant lid state. Collectively, the simulations and theoretical scalings lead to the conclusion that the role of the asthenosphere in maintaining plate tectonics does not come principally from a basal lubrication effect, associated with a low absolute asthenosphere viscosity, but, instead, from a mantle flow channelization effect, associated with a high viscosity contrast from the asthenosphere to the mantle below.

Introduction to Special Section on Physical Processes in Geodynamics

On May 16 and 17, 1992, a meeting was held at Cornell University as part of activities to honor Donald L. Turcotte on the occasion of his 60th birthday (which actually occurred on April 22, 1992). The papers in the following special section are a result of that meeting, which included invited presentations and posters representing the diverse areas of geology and geophysics to which Donald Turcotte has contributed.Three of the papers deal with aspects of mantle convection. A. C. Fowler addresses the problem of initiation of subduction by extending boundary layer theory to a viscoplastic rheology. Turcotte and Oxburgh [1967] pioneered such studies by showing how boundary layer theory could be used to describe the dependence of geophysical observables such as heat flux on distance from a midocean ridge or age of the oceanic crust. D. W. Sparks et al. use a model of three‐dimensional convection beneath a segmented ridge to explore how buoyancy effects and melt production beneath a spreading center influence ridge‐parallel variations in crustal thickness and gravity. Magma migration and the dynamics of midocean ridges have been major focuses of Donald Turcotte's research in papers dating back to 1978 [e.g., Turcotte and Ahern, 1978; Ahern and Turcotte, 1979; Turcotte, 1982a; Kenyon and Turcotte, 1987]. G. A. Glatzmaier and G. Schubert calculate models of three‐dimensional two‐layer mantle convection in a spherical shell and compare with whole layer, spherical shell, three‐dimensional convection models. These models represent the end‐member states of a spectrum of possible modes of mantle convection [Turcotte and Oxburgh, 1972; Oxburgh and Turcotte, 1978; Turcotte, 1979a; Silver et al., 1988].

Ocean bathymetry and mantle convection: 2. Small‐scale flow

Numerical models demonstrate that a boundary layer instability of the lower lithosphere does not stop the lithosphere from subsiding, and so it cannot explain the so‐called flattening of old seafloor. The slower subsidence of the old seafloor is in any case better seen as a perturbation, probably due to plumes. Furthermore, if a boundary layer instability, or any mode of upper mantle convection, transported most of the heat emerging from old seafloor, there would be pervasive, large‐amplitude local bathymetrie, gravity, and geoid anomalies. Such anomalies have never been convincingly differentiated from the effects of the cooling lithosphere and hotspot tracks, except for some low‐amplitude, linear geoid anomalies near spreading centers. The only modes of mantle convection with horizontal scales less than those of plates for which there is reasonably clear evidence are hotspot plumes and some possible near‐ridge boundary layer instabilities. Hotspots transport less than about 10% of the Earth's heat flux, and other small‐scale modes are probably less important.