Jeroen van hunen
University of Durham, Earth Sciences, Faculty Member
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The Pacific upper mantle structures revealed from recent seismic studies prompt us to study the dynamics of sublithospheric small‐scale convection (SSC) derived from thermal boundary layer instabilities of cooling lithosphere. As oceanic... more
The Pacific upper mantle structures revealed from recent seismic studies prompt us to study the dynamics of sublithospheric small‐scale convection (SSC) derived from thermal boundary layer instabilities of cooling lithosphere. As oceanic lithosphere cools and thickens, its sublayer may go unstable, thus producing SSC in the asthenosphere. By formulating two‐dimensional (2‐D) and three‐dimensional (3‐D) numerical models with realistic mantle rheology, we examine the controls on the onset time of SSC and its dynamic consequences. The onset of SSC is mainly controlled by two parameters: activation energy and asthenospheric viscosity, which can be recast as the Frank‐Kamenetskii parameter θ and a Rayleigh number Rai, respectively. Our models show that the onset time of SSC, τc, scales as Rai−0.68θ0.74, independent of 2‐D or 3‐D geometry. Our scaling coefficient for θ is significantly smaller than that from previous studies, but the weaker dependence on activation energy confirms the res...
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The formation and shape variation of the Hawaiian plume swell is re‐examined numerically. Scaling laws for the plume buoyancy flux and swell width and height help gaining new insight in relationships between swell formation and relevant... more
The formation and shape variation of the Hawaiian plume swell is re‐examined numerically. Scaling laws for the plume buoyancy flux and swell width and height help gaining new insight in relationships between swell formation and relevant model parameters, like plume temperature and size, and mantle rheology. A scaling law for the plume buoyancy F = Aη0−1.2Rp3.5ΔTp2.2 exp(1.3 × 10−8EΔTp), with background mantle viscosity η0, plume radius Rp, plume excess temperature ΔTp, and activation energy E fits numerical flux measurements within 8%. Scaling laws for the swell width and height have similar forms, and their multiplication resembles the buoyancy flux scaling law within 10%. These scaling laws suggest that the background mantle viscosity plays a significant role, and that the increased Hawaiian plume intensity ∼25 Ma ago is due to a plume excess temperature increase of 50%.
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ABSTRACT The Mediterranean area has experienced a complex history of subduction, continental collision and back-arc spreading events since the Eocene. Despite sophisticated reconstructions and high-resolution tomographic results (e.g.,... more
ABSTRACT The Mediterranean area has experienced a complex history of subduction, continental collision and back-arc spreading events since the Eocene. Despite sophisticated reconstructions and high-resolution tomographic results (e.g., Wortel and Spakman, 2000), considerable uncertainty remains about the timing of continental collision at Italy and North-Africa, and the possible occurrence and dynamics of slab detachment in these areas (Faccenna et al., 2004). Here, we present 3-D, fully dynamical numerical models of oceanic subduction and continental collision, and use model results to put new, important constraints on the geodynamical evolution of this area. Seismic tomography clearly shows stagnation of fully developed subducting slabs under the Mediterranean, which indicates that those slabs are largely supported by the mantle phase transition and viscosity increase across 660 km. Freely hanging slabs that are unsupported by the base of the upper mantle experience large tensile stresses, and the break-off of those slabs has been demonstrated before with numerical models (Gerya et al., 2004; Andrews & Billen, 2009). Here, we show that, even for Mediterranean slabs, for which reduced tensile stress is expected, because they rest on the base of the upper mantle, slab break-off is expected to occur. This supports slab detachment scenarios as inferred from post-collisional magmatism (e.g. Davies and von Blanckenburg, 1995) and tomographic studies (Wortel and Spakman, 2000; Spakman et al., 2009). Due to uncertainties in plate reconstructions and limited tomographic resolution, significant uncertainty exists on the timing of continental collision along the North-African margin, and the possible occurrence of slab detachment in this region. Our numerical models provide new constraints for this region. Results indicate that slab age and strength plays a crucial role in the timing of slab break-off and the speed of a propagating slab tear. A strong, old subducting oceanic slab leads to slab break-off at 20-25 Myr after the first onset of continental collision, and subsequently a slab tear migrates more or less horizontally through the slab with a propagation speed of 100-150 mm/yr. In contrast, young or weak slabs show first break-off already 10 Myr after continental collision, and can experience tear migration rates up to 500 mm/yr. Given the old age of the subducted Neo-Tethys ocean at North Africa, these results put tight constraints on collision and break-off timing. Finally, our results illustrate the preference for slab break-off to start in the middle of a slab, and the propagation outwards from there. This result fits well with the postulated slab break-off at the South Apennines (Faccenna et al., 2004) and subsequent propagation northwards and southwards.
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ABSTRACT Cratons are the ancient continental cores, and contain the oldest and thickest(180~250km) lithosphere on Earth. Several mechanism have been proposed to explain the longevity and stability, including the excess buoyancy, a large... more
ABSTRACT Cratons are the ancient continental cores, and contain the oldest and thickest(180~250km) lithosphere on Earth. Several mechanism have been proposed to explain the longevity and stability, including the excess buoyancy, a large viscosity contrast with the underlying asthenosphere, and a relatively high brittle yield stress of cratonic lithosphere(Lenardic et al.2003), which can be all attributed to the unique chemical composition and low temperature of cratonic lithosphere. Cratons have highly melt-depleted and dry mantle roots which make them more buoyant and insusceptible than surrounding lithosphere. Even though cratons are often considered stable and indestructible, there are at several examples that underwent significant thinning. Seismic evidence shows that the current thickness of eastern part of North China Craton(NCC) is now at most 80km, while kimberlitic evidence implies the previous existence of a thick(~200km) and refractory mantle lithosphere in the middle Ordovician (Gao et al.2008). Significant thinning is also suggested for the Wyoming Craton (Lee et al.2011). Lithospheric delamination and thermal erosion from the bottom of lithosphere are the two of most discussed mechanisms for the thinning (or destruction)of cratons, with water and water relative processes playing an important role in both mechanisms. Some other cratons may also have experienced different geodynamic thinning processes. For example, a mantle plume may have affected the South Africa craton and contributed to its high topography(Brown, 2006). In this study, we compare numerical results of cratonic lithospheric thinning processes to some of the discussed observations. We show how lithospheric delamination can lead to rapid thinning(about 80km) over a timescale of 5~30 Myrs, and illustrate how it can shed light on the thinning of the NCC or its surrounding lithosphere . In addition, we discuss some preliminary results of spontaneous lithospheric thinning by small-scale convective instabilities.
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Spatial patterns in geochemistry at hotspots follow mixing trajectories in compositional diagrams that offer clues regarding the mantle feeding the hotspot. At many hotspots, the observations can only be explained with three or more... more
Spatial patterns in geochemistry at hotspots follow mixing trajectories in compositional diagrams that offer clues regarding the mantle feeding the hotspot. At many hotspots, the observations can only be explained with three or more lithologic components in the mantle; the distribution of these components in the source is unknown but is important for understanding the scales and styles (layered versus non-layered) of heterogeneity in the deep source. For example, 208Pb/204Pb vs. 206Pb/204Pb data at Hawaii follow many mixing trajectories, which when interpreted as two-component binary mixing, suggest that a volcano samples many different components over its lifetime, and that the Kea and Loa sub-chains do not share at least one component. We use a 3D geodynamic model of a hot, veined mantle plume interacting with a lithospheric plate to explore the spatial patterns in magma geochemistry that result from upper mantle dynamics and melting of a mantle with small-scale heterogeneities. We assume that there are three compositional components in the mantle each with different mass fractions, solidi, and trace element and isotope compositions. The deeper melting components have wider melting zones, and thus can be sampled proportionally more heavily at the edges of the hotspot compared to near the center where the shallower melting components are prominently expressed. Also, plume-lithosphere interaction offsets the melting zones, which causes the predicted geochemical pattern at the surface to be asymmetric in the direction of plate motion. In general, the magma composition at the surface is very different from the initial average solid composition in the mantle. Models predict that in the lifetime of a volcano passing over the hotspot, magma composition follow pseudo-binary mixing trajectories that in some cases do not appear to share the same end-member compositions even though all components are present everywhere in the mantle. An interesting result is that predicted 208Pb/204Pb vs. 206Pb/204Pb mixing trajectories erupted by single volcano are not always linear or those that are linear have different slopes. These results are augmented if the mass fraction of one of the veined components increases with proximity to the center of the plume. The results illustrate that complicated, nonlinear mixing trajectories can result from variable melting of a relatively simple veined mantle with little or know regional scale zoning in compositions.
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ABSTRACT Plate reconstruction and geodetic data reveals that the motion of the trench at convergent margin is equally partitioned on the Earth's surface between advancing and retreating modes with respect to the upper plate, and... more
ABSTRACT Plate reconstruction and geodetic data reveals that the motion of the trench at convergent margin is equally partitioned on the Earth's surface between advancing and retreating modes with respect to the upper plate, and is susceptible to abrupt changes in velocity and direction. Here, we explore the idea that trenches' motion represents the surface manifestation of the dynamic of subduction. To gain understanding of the trench dynamics, we performed numerical model calculations. Numerical models allow a quantitative investigation of a wide range of parameters and the assessment of derived quantities such as the amount of energy dissipated during the subduction process. The model is calibrated with laboratory model results. We performed 2- and 3-D numerical models to investigate the influence of different geometrical and rheological parameters on the subduction process. Slabs with different geometries and rheological properties interact differently with the 660-km discontinuity, which is simulated in this case as an impermeable barrier. The energy dissipated in the system varies during the several phases of subduction. During the initial slab sinking, the energy dissipated by the lithosphere is similar for all model calculations. In the subsequent steady state phase, differences become much larger. In both advancing and retreating (rollback) modes, most of the total initial energy is dissipated by the mantle, ~60% and ~70%, respectively. The extra 10% in the retreating mode is caused by a more vigorous backward toroidal flow compared with forward toroidal flow in the advancing style. Furthermore, the rollback motion is the favorable configuration from an energy point of view. In the intermediate situation, plates fold and pile up on the discontinuity. In this mode, the plate dissipates ~60-65% of the total energy, with only minor motion of the surrounding mantle. We conclude that the potential energy provided by a subducting plate is mainly dissipated by the mantle viscous flow, and only a minor part is used to bend and deform the lithosphere.
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Observations at ridges migrating away from hotspots such as the Mid-Atlantic Ridge at Iceland and the Galapagos Spreading Center display repeat jumps of the axis toward the hotspot. The mechanisms that control the initiation of jumps and... more
Observations at ridges migrating away from hotspots such as the Mid-Atlantic Ridge at Iceland and the Galapagos Spreading Center display repeat jumps of the axis toward the hotspot. The mechanisms that control the initiation of jumps and repeat jumps are poorly understood, but likely include the effects of off-axis plume-lithosphere interaction and lithospheric heating caused by magma penetration. We use
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<p>Water from abandoned, flooded mines can form an excellent low-enthalpy, renewable long-term heat source, provided it is managed properly. Its sustainability, however, is only as good as its proper management. The poor... more
<p>Water from abandoned, flooded mines can form an excellent low-enthalpy, renewable long-term heat source, provided it is managed properly. Its sustainability, however, is only as good as its proper management. The poor understanding of the condition of the mine, post-closure makes the investment in these projects risky compared to other alternatives. Our modelling allows us to explore uncertainties and reduce a variety of project risks.</p> <p>By combining numerical and analytical methods with digitised legacy mine data, we developed a tool to estimate the variations in the abstraction water temperature over the lifetime of a project. We couple the heat transfer approximation method originally proposed by Rodriguez and Diaz (2009) to that of flow in a pipe network as described by Todini and Pilati (1987). We refine the original heat transfer approximation by accounting for a flow regime specific heat transfer coefficient between the rock mass and the water, as prescribed by Loredo et al. (2017). We also develop a novel weighting function to account for the interference between adjacent mine galleries.</p> <p>This method is applied to investigate the scenario in which multiple users will extract heat from the same mine water block. We investigate the interference resulting from heat extraction at multiple locations, using a mine system from the North East of England as a study case. The results of this study provide constraints on the maximum mine water extraction rates and proximity of the different users. The poorly constrained connectivity (through mine shafts, connecting roadways or porous flow) between mine workings from different coal seams is shown to be one of the most significant uncertainties in assessing the feasibility of a mine system as a sustainable heat source.</p> <p>References:</p> <ul> <li>Loredo C, Banks D, Roqueñí N. Evaluation of analytical models for heat transfer in mine tunnels. Geothermics 2017; 69; 153-164.</li> <li>Rodriguez R and Díaz M. Analysis of the utilization of mine galleries as geothermal heat exchangers by means a semi-empirical prediction method. Renewable Energy 2009; 34(7), 1716-1725.</li> <li>Todini E and Pilati S. A gradient method for the analysis of pipe networks. Computer app. In water supply 1987; 1-20, v1.</li> </ul>
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Geodynamic models can aid understanding the evolution of rifting in North China and other rift systems. The North China Craton (NCC) formed by the collision of two Archean blocks in the Paleoproterozoic resulting in a broad collision zone... more
Geodynamic models can aid understanding the evolution of rifting in North China and other rift systems. The North China Craton (NCC) formed by the collision of two Archean blocks in the Paleoproterozoic resulting in a broad collision zone known as Trans-North China Orogen. The NCC shows two different modes of extension that are separated by space and time. Wide, distributed rifts formed during the Paleogene above the Eastern NCC, in the Neogene migrated to the Western NCC forming narrow, localised rifts near the Paleoproterozoic orogens. However, the mechanism that led to development of these fundamentally different rifts and the migration of rifting remains debated. Here we use the geodynamical tool ASPECT to perform 2D thermo-mechanical modelling to explain the role of variable lithospheric strength and inherited lithospheric weaknesses in the development of rift systems. We found that a wide, distributed rift develops over non-cratonic lithosphere, while the adjacent cratonic lithosphere will accommodate little strain. To explain rift migration in North China we require 1.) a period of tectonic quiescence that strengthens the lithosphere following distributed initial rifting 2.) a specific range of relative lithospheric thickness variations and 3) presence of a lithosphere scale weak zone, i.e., an inherited feature. Our results show how lithospheric thickness and strength variations as well as discrete zones of lithospheric weaknesses can influence the style of rifting and facilitate the breakup of an ancient craton. These results are applicable to other multiphase rift systems around the world such as the North Atlantic.
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The longevity of the cratonic lithosphere is controlled by its buoyancy, strength, and the viscosity contrast with that of the underlying sub-lithospheric mantle. A number of geodynamic models show that the style and characteristic of... more
The longevity of the cratonic lithosphere is controlled by its buoyancy, strength, and the viscosity contrast with that of the underlying sub-lithospheric mantle. A number of geodynamic models show that the style and characteristic of lithospheric removal/thinning mechanisms over cratons (i.e. whether delamination, drip, or hydration weakening) are accounted by their geological history and geodynamic evolution. For example, the question of which process(es) control lithospheric removal from beneath the Wyoming and North China cratons still enigmatic. To address this problem, we are using 2D numerical models to investigate how lithospheric mantle of the North China Block has been thinned in which geological, geophysical and petrological studies refers the areas as key example of cratonic destruction/removal that occurred (120-80 Ma). Considering the geological evolution of North China region, the main focus of the study is to investigate the effects of a set of parameters (e.g., viscosity, buoyancy and thickness) for the base of cratons which is likely weakened by fluids released from the subducting oceanic plate. Our preliminary results show that movement of the subducting plate is sensitive to the parameters affecting the stability of the lithosphere whereas overriding plate is mainly affected by viscosity. If the base of the cratonic lithospheric mantle is dense, thick and relatively less viscous, it forces oceanic slab to rollback, else the overlying plate slides through the base of the cratonic mantle. The model results with stagnated oceanic plate at the transition zone with low viscosity cratonic base is responsible for the deformation of the cratonic roots.