Description: We studied the effect of increased water content on the dynamics of the lithosphere-asthenosphere boundary in a postsubduction setting. Results from numerical mantle convection models show that the resultant decrease in mantle viscosity and the peridotite solidus produce small-scale convection at the lithosphere-asthenosphere boundary and magmatism that follows the spatially and temporally scattered style and volumes typical for collision magmatism, such as the late Cenozoic volcanism of the Turkish-Iranian Plateau. An inherent feature in small-scale convection is its chaotic nature that can lead to temporally isolated volcanic centers tens of millions of years after initial continental collision, without evident tectonic cause. We also conclude that water input into the upper mantle during and after subduction under the circum-Mediterranean area and the Tibetan Plateau can account for the observed magmatism in these areas. Only fractions (200–600 ppm) of the water input need to be retained after subduction to induce small-scale convection and magmatism on the scale of those observed from the Turkish-Iranian Plateau.

Description: The formation and evolution of a backarc basin are linked to the dynamics of the subduction system. The opening of the central Mediterranean basins is a well-documented example of backarc extension characterized by short-lived episodes of spreading. The underlying reasons for this episodicity are obscured by the complexity of this subduction system, in which multiple continental blocks enter the subduction zone. We present results from three-dimensional numerical models of laterally varying subduction to explain the mechanism of backarc basin opening and the episodic style of spreading. Our results show that efficient backarc extension can be obtained with an along-trench variation in slab buoyancy that produces localized deformation within the overriding plate. We observe peaks in the trench retreating velocity corresponding first to the opening of the backarc basin, and later to the formation of slab windows. We suggest that the observed episodic trench retreat behavior in the central Mediterranean is caused by the formation of slab windows.

Description: Depleted gas reservoirs are potential sites for CO 2 storage; therefore, it is important to evaluate their storage capacity. Historically, there have been difficulties in identifying the reservoir drive mechanism of gas reservoirs using traditional P / z plots, having direct impacts for the estimation of the original gas in place (OGIP) and dependent parameters for both theoretical and effective CO 2 storage capacity estimation. Cole plots have previously provided an alternative method of characterization, being derived from the gas material balance equation. We use production data to evaluate the reservoir drive mechanism in four depleted gas reservoirs (Hewett Lower Bunter, Hewett Upper Bunter, and North and South Morecambe) on the UK Continental Shelf. Cole plots suggest that the North Morecambe and Hewett Upper Bunter reservoirs experience moderate water drive. Accounting for cumulative water influx into these reservoirs, the OGIP decreases by up to 20% compared with estimates from P / z plots. The revised OGIP values increase recovery factors within these reservoirs; hence, geometrically based theoretical storage capacity estimates for the North Morecambe and Hewett Upper Bunter reservoirs increase by 4 and 30%, respectively. Material balance approaches yield more conservative estimates. Effective storage capacity estimates are between 64 and 86% of theoretical estimates within the depletion drive reservoirs, and are 53 – 79% within the water drive reservoirs. Supplementary material: A more detailed description of the aquifer modelling is available at https://doi.org/10.6084/m9.figshare.c.3803770.v1

Description: Comparison of modeling results with observed subsidence patterns from the West Siberian Basin provides new insight into the origin of the Siberian Traps, and constrains the temperature, size, and depth of an impacting mantle plume head during and after the eruption of the Siberian Traps at the Permian-Triassic boundary (250 Ma). We compare subsidence patterns from one-dimensional conductive heat flow models to observed subsidence from backstripping studies of wells in the basin. This results in a best-fit scenario with a 50-km-thick initial plume head with a temperature of 1500 °C situated 50 km below the surface, and an initial regional crustal thickness of 34 km, in agreement with published values. Backstripping and modeling results agree very well, including a 60–90 m.y. delay between the rifting phase and the first regional sedimentation. Regional subsidence patterns indicate that the plume head was present across a minimum area of ~2.5 x 10 6 km 2 . These results re-emphasize the viability of a mantle plume origin for the Siberian Traps, provide important constraints on the dynamics of mantle plume heads, and suggest a thermal control for the subsidence of the West Siberian Basin.

Description: Preliminary dynamic modelling, using TOUGH2/ECO2N, has been carried out to assess the suitability of a site in the UK North Sea for sequestering CO 2 . The potential storage site is a previously unused saline formation within the Permian Rotliegend sandstone. Data regarding the site are limited. Therefore, additional input parameters for the model have been taken from the literature and nearby analogues. The sensitivity of the model to a range of parameters has been tested. Results indicate that the site can sustain an injection rate of around 2.5 Mt a –1 of CO 2 for 20 years. The main control on pressure build-up in the model is the permeability of the unit directly beneath the Rotliegend in the location of the proposed storage site. The plume diameter is primarily controlled by the porosity and permeability of the site. A comparison between static, analytical and dynamic modelling highlights the advantages of dynamic modelling for a study such as this. Further data collection and modelling are required to improve predictions of pressure build-up and CO 2 migration. Despite uncertainties in the input data, the use of a full three-dimensional (3D) numerical simulation has been extremely useful for identifying and prioritizing factors that need further investigation.

Description: Continental collision is commonly accompanied by a sequence of several plate–mantle interactions, including accretion of buoyant features, pulses of slab rollback, slab break-off, formation of slab windows, and lithosphere delamination. Using the combined insight from seismic and dynamical modelling studies, we illustrate how these processes and their characteristic rates and timescales played an important role in shaping the Mediterranean and how they dominated the closure of the Tethyan oceans. Older collisions, such as the one that formed the Norwegian Caledonites, probably experienced similarly complex plate–mantle interaction, even though direct evidence of the associated mantle dynamics is absent.

Description: As subducting plates reach the base of the upper mantle, some appear to flatten and stagnate, while others seemingly go through unimpeded. This variable resistance to slab sinking has been proposed to affect long-term thermal and chemical mantle circulation. A review of observational constraints and dynamic models highlights that neither the increase in viscosity between upper and lower mantle (likely by a factor 20–50) nor the coincident endothermic phase transition in the main mantle silicates (with a likely Clapeyron slope of –1 to –2 MPa/K) suffice to stagnate slabs. However, together the two provide enough resistance to temporarily stagnate subducting plates, if they subduct accompanied by significant trench retreat. Older, stronger plates are more capable of inducing trench retreat, explaining why backarc spreading and flat slabs tend to be associated with old-plate subduction. Slab viscosities that are ~2 orders of magnitude higher than background mantle (effective yield stresses of 100–300 MPa) lead to similar styles of deformation as those revealed by seismic tomography and slab earthquakes. None of the current transition-zone slabs seem to have stagnated there more than 60 m.y. Since modeled slab destabilization takes more than 100 m.y., lower-mantle entry is apparently usually triggered (e.g., by changes in plate buoyancy). Many of the complex morphologies of lower-mantle slabs can be the result of sinking and subsequent deformation of originally stagnated slabs, which can retain flat morphologies in the top of the lower mantle, fold as they sink deeper, and eventually form bulky shapes in the deep mantle.

Description: Numerically modelling the dynamics of a self-consistently subducting lithosphere is a challenging task because of the decoupling problems of the slab from the free surface. We address this problem with a benchmark comparison between various numerical codes (Eulerian and Lagrangian, Finite Element and Finite Difference, with and without markers) as well as a laboratory experiment. The benchmark test consists of three prescribed setups of viscous flow, driven by compositional buoyancy, and with a low viscosity, zero-density top layer to approximate a free surface. Alternatively, a fully free surface is assumed. Our results with a weak top layer indicate that the convergence of the subduction behaviour with increasing resolution strongly depends on the averaging scheme for viscosity near moving rheological boundaries. Harmonic means result in fastest subduction, arithmetic means produces slow subduction and geometric mean results in intermediate behaviour. A few cases with the infinite norm scheme have been tested and result in convergence behaviour between that of arithmetic and geometric averaging. Satisfactory convergence of results is only reached in one case with a very strong slab, while for the other cases complete convergence appears mostly beyond presently feasible grid resolution. Analysing the behaviour of the weak zero-density top layer reveals that this problem is caused by the entrainment of the weak material into a lubrication layer on top of the subducting slab whose thickness turns out to be smaller than even the finest grid resolution. Agreement between the free surface runs and the weak top layer models is satisfactory only if both approaches use high resolution. Comparison of numerical models with a free surface laboratory experiment shows that (1) Lagrangian-based free surface numerical models can closely reproduce the laboratory experiments provided that sufficient numerical resolution is employed and (2) Eulerian-based codes with a weak surface layer reproduce the experiment if harmonic or geometric averaging of viscosity is used. The harmonic mean is also preferred if circular high viscosity bodies with or without a lubrication layer are considered. We conclude that modelling the free surface of subduction by a weak zero-density layer gives good results for highest resolutions, but otherwise care has to be taken in (1) handling the associated entrainment and formation of a lubrication layer and (2) choosing the appropriate averaging scheme for viscosity at rheological boundaries.