Numerical Modelling of Melting Processes and Induced Diapirism In the Lower Crust

The processes of partial melting and magmatic diapirism within the lower crust are evaluated using a numerical underplating model. Fully molten basalt ( T = 1200°C) is emplaced at the Moho beneath a solid granite ( T = 750°C) in order that a melt front grows into the granite. If diapirism does not occur, this melt front in the granite reaches a minimal depth in the crust before (like in the molten basalt) crystallization takes place. the density contrast between the partially molten granite layer and the overlying solid granite can lead to a Rayleigh-Taylor instability (RTI) which results in diapiric rise of the partially molten granite. Assuming a binary eutectic system for both the granite and the underplating basalt and a temperature- and stress-dependent rheology for the granite, we numerically solve the governing equations and find (a) that diapirism occurs only within a certain but possibly realistic range of parameters, and (b) that if diapirs occur, they do not rise to levels shallower than 15 or perhaps 12km. the growth rate depends on the degree of melting and the thickness of the partially molten layer, as well as the viscosity of the solid and the partially molten granite. From a comparison of the growth rate with the velocity of a Stefan front it is possible to predict whether a melt front will become unstable and result in diapiric ascent or whether a partially molten layer is created, which remains at depth. We carry out such a comparison using our thermodynamically and thermomechanically consistent model of melting and diapirism.     

On the interaction between small- and large-scale convection and postglacial rebound flow in a power-law mantle

Some consequences arising from the superposition of flows of two different kinds or scales in a non-Newtonian mantle are discussed and applied to the cases mantle convection plus postglacial rebound flow as well as small- plus large-scale mantle convection. If the two flow types have similar magnitude, the apparent rheology of both flows becomes anisotropic and the apparent viscosity for one flow depends on the geometry of the other. If one flow has a magnitude significantly larger than the other, the apparent viscosity for the weak flow is linear but develops direction-dependent variations about a factorn (n being the power exponent of the rheology). For the rebound flow lateral variations of the apparent viscosity about at least 3 are predicted and changes in the flow geometry and relaxation time are possible. On the other hand, rebound flow may weaken the apparent viscosity for convection. Secondary convection under moving plates may be influenced by the apparent anisotropic rheology. Other mechanisms leading to viscous anisotropy during shearing may increase this effect. A linear stability analysis for the onset of convection with anisotropic linear rheology shows that the critical Rayleigh number decreases and the aspect ratio of the movement cells increases for decreasing horizontal shear viscosity (normal viscosity held constant). Applied to the mantle, this model weakens the preference of convection rolls along the direction of plate motion. Under slowly moving plates, rolls perpendicular to the plate motion seem to have a slight preference. These results could be useful for resolving the question of Newtonian versus non-Newtonian or isotropic versus anisotropic mantle rheology.     

Swimming upstream: addressing fossil fuel supply under the UNFCCC

Reducing fossil fuel supply is necessary to meet the Paris Agreement goal to keep warming ‘well below 2°C’, yet the Agreement is silent on the topic of fossil fuels. This article outlines reasons why it is important that Parties to the Agreement find ways to more explicitly address the phasing out of fossil fuel production under the UNFCCC. It describes how countries aiming to keep fossil fuel supply in line with Paris goals could articulate and report their actions within the current architecture of the Agreement. It also outlines specific mechanisms of the Paris Agreement through which issues related to the curtailment of fossil fuel supply can be addressed. Mapping out a transition away from fossil fuels – and facilitating this transition under the auspices of the UNFCCC process – can enhance the ambition and effectiveness of national and international climate mitigation efforts.

Key policy insights

• The international commitment to limit global average temperature increases to ‘well below 2°C’ provides a strong rationale for Parties to the Paris Agreement and the UNFCCC to pursue a phase-down in fossil fuel production, not just consumption.

• Several countries have already made commitments to address fossil fuel supply, by agreeing to phase down coal or oil exploration and production.

• Integrating these commitments into the UNFCCC process would link them to global climate goals, and ensure they form part of a broader global effort to transition away from fossil fuels.

• The Paris Agreement provides a number of new opportunities for Parties to address fossil fuel production.

     

Numerical modelling of rise and fall of a dense layer in salt diapirs

Numerical models are used to study the entrainment of a dense anhydrite layer by a diapir. The anhydrite layer is initially horizontally embedded within a viscous salt layer. The diapir is down-built by aggradation of non-Newtonian sediments ( n = 4, constant temperature) placed on the top of the salt layer. Several parameters (sedimentation rate, salt viscosity, perturbation width and stratigraphic position of the anhydrite layer) are studied systematically to understand their role in governing the entrainment of the anhydrite layer. High sedimentation rates during the early stages of the diapir evolution bury the initial perturbation and, thus, no diapir forms. The anhydrite layer sinks within the buried salt layer. For the same sedimentation rate, increasing viscosity of the salt layer decreases the rise rate of the diapir and reduces the amount (volume) of the anhydrite layer transported into the diapir. Model results show that viscous salt is capable of carrying separate blocks of the anhydrite layer to relatively higher stratigraphic levels. Varying the width of the initial perturbation (in our calculations 400–800 m), from which a diapir triggers, shows that wider diapirs can more easily entrain an embedded anhydrite layer than the narrower diapirs. The anhydrite layer is entrained as long as rise rate of the diapir exceeds the descent rate of the denser anhydrite layer. We conclude that the four parameters mentioned above govern the ability of a salt diapir to entrain an embedded dense layer. However, the model results show that the entrained blocks inevitably sink back if the rise rate of the diapir is less than the rate of descent of the anhydrite layer or the diapir is permanently covered by a stiff overburden in case of high sedimentation rates.     

On the orientation of tensile dikes in a ridge-centered plume

We present three-dimensional numerical models of the ascent of a plume under a spreading ridge and the concomitant melt generation and crust formation using a mantle viscosity which depends on pressure, temperature, melt content, and, optionally, water content. From the velocity field of these convection models we compute the viscous stress tensor in the mantle. Assuming that melt-rich channels or melt-filled dikes are oriented parallel to the maximum compressional stress, we calculate the orientation of such dikes in the partially molten zone of the plume head and beneath the ridge. For the central part of the plume we find dike orientations parallel to the ridge in shallow parts of the plume head, while they are ridge-perpendicular at greater depth. The boundary between these two regimes is shifted downwards for a water-rich plume which dehydrates upon melting. The two regimes of different dike or melt channel orientations seem to be in good agreement with seismic observations on depth-dependent seismic anisotropy, indicating a preference of our wet model. We find a modest focusing of melt towards the spreading center beneath normal ridge in both models, but also in the fully dehydrated parts of the plume head in the model where water was included; those are regions where upwelling is essentially passive. In contrast, the plume in the water-free model and the only partly dehydrated parts of the plume in the water-bearing model tend to develop a dike orientation which moves melt away from the spreading center; these regions are characterized by active upwelling. The defocusing of melt in actively upwelling plume mantle has radial symmetry about the plume axis, i.e. an outward orientation of the dikes is not only visible in planes perpendicular to the ridge, but also along the ridge.     

Rift induced delamination of mantle lithosphere and crustal uplift: a new mechanism for explaining Rwenzori Mountains’ extreme elevation?

With heights of 4–5 km, the topography of Rwenzori Mountains, a large horst of old crustal rocks located inside a young passive rift system, poses the question “Why are the Rwenzori Mountains so high?”. The Cenozoic Western Rift branch of the East African Rift System is situated within the Late Proterozoic mobile belts between the Archean Tanzania Craton and Congo Craton. The special geological setting of the massif at a rift node encircled by the ends of the northern Western Rift segments of Lake Albert and Lake Edward suggests that the mechanism responsible for the high elevation of the Rwenzoris is related to the rifting process. Our hypothesis is based on the propagation of the rift tips, surrounding the stiff old lithosphere at Rwenzori region, thereby triggering the delamination of the cold and dense mantle lithosphere (ML) root by reducing viscosity and strength of the undermost lower crust. As a result, this unloading induces fast isostatic pop-up of the less dense crustal Rwenzori block. We term this RID—“rift induced delamination of Mantle Lithosphere”. The physical consistency of the RID hypothesis is tested numerically. Viscous flow of 2D models is approximated by a Finite Difference Method with markers in an Eulerian formulation. The equations of conservation of mass, momentum and energy are solved for a multi-component system. Based on laboratory data of appropriate rock samples, a temperature-, pressure- and stress-dependent rheology is assumed. Assuming a simple starting model with a locally heated ML, the ML block between the weakened zones becomes unstable and sinks into the asthenosphere, while the overlying continental crust rises up. Thus, RID seems to be a viable mechanism to explain geodynamically the extreme uplift. Important conditions are a thermal anomaly within the ML, a ductile lower crust with visco-plastic rheology allowing significant strength reduction and lateral density variations. The special situation of a two-sided rifting or offset rift segments to decouple the ML laterally from the surrounding continental lithosphere seems to be most decisive. Further support for the RID mechanism may come from additional crustal thickness and an extensive stress field. Some parameters, such as the excess temperature and yield stress, are very sensitive, small changes determine whether delamination takes place or not.