SUMMARY Numerous processes such as metamorphic reactions, fluid and melt transfer and earthquakes occur at a subducting zone, but are still incompletely understood. These processes are affected, or even controlled, by the magnitude and distribution of stress and deformation mechanism. To eventually understand subduction zone processes, we quantify here stresses and deformation mechanisms in and around a subducting lithosphere, surrounded by asthenosphere and overlain by an overriding plate. We use 2-D thermomechanical numerical simulations based on the finite difference and marker-in-cell method and consider a 3200 km wide and 660 km deep numerical domain with a resolution of 1 km by 1 km. We apply a combined visco-elasto-plastic deformation behaviour using a linear combination of diffusion creep, dislocation creep and Peierls creep for the viscous deformation. We consider two end-member subduction scenarios: forced and free subduction. In the forced scenario, horizontal velocities are applied to the lateral boundaries of the plates during the entire simulation. In the free scenario, we set the horizontal boundary velocities to zero once the subducted slab is long enough to generate a slab pull force large enough to maintain subduction without horizontal boundary velocities. A slab pull of at least 1.8 TN m–1 is required to continue subduction in the free scenario. We also quantify along-profile variations of gravitational potential energy (GPE). We evaluate the contributions of topography and density variations to GPE variations across a subduction system. The GPE variations indicate large-scale horizontal compressive forces around the trench region and extension forces on both sides of the trench region. Corresponding vertically averaged differential stresses are between 120 and 170 MPa. Furthermore, we calculate the distribution of the dominant deformation mechanisms. Elastoplastic deformation is the dominant mechanism in the upper region of the lithosphere and subducting slab (from ca. 5 to 60 km depth from the top of the slab). Viscous deformation dominates in the lower region of the lithosphere and in the asthenosphere. Considering elasticity in the calculations has an important impact on the magnitude and distribution of deviatoric stress; hence, simulations with increased shear modulus, in order to reduce elasticity, exhibit considerably different stress fields. Limiting absolute stress magnitudes by decreasing the internal friction angle causes slab detachment so that slab pull cannot be transmitted anymore to the horizontal lithosphere. Applying different boundary conditions shows that forced subduction simulations are stronger affected by the applied boundary conditions than free subduction simulations. We also compare our modelled topography and gravity anomaly with natural data of seafloor bathymetry and free-air gravity anomalies across the Mariana trench. Elasticity and deviatoric stress magnitudes of several hundreds of MPa are required to best fit the natural data. This agreement suggests that the modelled flexural behaviour and density field are compatible with natural data. Moreover, we discuss potential applications of our results to the depth of faulting in a subducting plate and to the generation of petit-spot volcanoes.
The different geodynamic settings with magmatism observed around the world, such as mid-ocean ridges (MORs), volcanic arcs and intraplate volcanism, indicate that asthenospheric melts are extracted under significantly distinct pressure, temperature and rheological conditions (Figure 1a). The main difference between melt extraction at intraplate settings and at MORs is the presence of the lithospheric mantle for the intraplate settings. The geochemical signature of MOR basalt (MORB) presumably depends on magma source composition, melt-extraction and differentiation processes intervening between the magma source and the crust (e.g., Langmuir et al., 1992). MORBs are produced and migrate in the asthenosphere and temperature (T) and pressure (P) variations are,
Temperatures in the root zones of volcanoes play a critical role in the development and persistence of shallow-level magmatic reservoirs in the crust. Here, we present a 1D thermal model allowing evaluation of the thermal impact of magma travelling in conduits to the surface on the root zone of a volcano. This thermal model has been developed to better understand the formation of a vertical intrusion located in the root zone of a dismembered Miocene volcano on Fuerteventura, Canary Archipelago. This intrusion, named PX1, constitutes an almost pure amalgamation of dikes of either clinopyroxenitic or gabbroic composition. Both types of dikes display cumulate textures and are interpreted as resulting from the protracted crystallization of a mafic magma. The formation of clinopyroxenitic, in contrast to gabbroic dikes, requires that the residual melt was extracted at high temperature (>1050°) to avoid plagioclase crystallization. Simulations of multiple dike injections show that the temperature in the root zone increases significantly with the addition of dikes, but the maximum temperature reached in the system depends on the duration of magma flow in the conduits and the time interval between dike injections (i.e., repose period). Active flow is the critical parameter that distinguishes instantaneous dike injection from a magmatic conduit. Without significant magma flow (>1 month), hightemperature conditions (>1000°C) cannot be maintained in the pluton unless dikes are very thick and the repose period is extremely small. On the other hand, magma flow times of one to several months, combined with short time intervals between dike injections (<25 years), which are conditions comparable to those recorded for historical eruptions of oceanic island volcanoes, allow the production and preservation of temperatures above the plagioclase liquidus for significant durations, as required to generate clinopyroxenitic dikes such as those observed in the PX1 pluton. Persistent high temperature in the vicinity of magma conduits limits the differentiation of melts in transit to the surface, providing a potential explanation for why lavas of mafic to intermediate composition predominate in intraplate volcanoes such as Fuerteventura or Fogo Island (Cape Verde
<p>The Lithosphere-Asthenosphere boundary (LAB) is a conceptual zone decoupling cold and rigid lithosphere from the hot and weak asthenosphere. It is marked by the changes in many physical properties which are measured with different geophysical techniques. Seismic anomalies associated with the LAB are observed at 45-80 km depth across the old Pacific plate<sup>1</sup>. These are interpreted as the presence of melt, but their locations are significantly shallower than the LAB depth predicted by the thermal model. The extraction of MORB at mid-ocean ridges will produce a residual mantle relatively poor in volatiles, which questions the origin of magmas observed under the Pacific plate, Tharimena<sup>1</sup> and co-authors recognize this issue and suggest that the observed seismic anomalies are associated with the migration of magma from the asthenosphere which accumulates at the base of the lithosphere. Many studies on intraplate magmatism suggested that primary partial melts are produced in the asthenosphere followed by differentiation in the crust. Melt-rock interactions during melt transport across the LAB and lithosphere are often neglected or overly simplified given that it is likely that melt will react with the surrounding mantle and cool as it passes through this zone.</p> <p>&#160;</p> <p>To understand how to stabilize melt at the P-T conditions of observed anomalies and to understand what is the transport mechanism associated with the migration of magmas into and across LAB as well as the geochemical and geophysical implications of such transport, we have developed a thermo-hydro-mechanical-chemical (THMC) model<sup> </sup>for reactive melt transport using the finite difference method. Our first model&#160;<sup> </sup>considered melt migration by reactive porosity waves in 1D within a simplified forsterite-fayalite-silica chemical system. This numerical model is based on solving the differential equations for the conservation of mass, conservation of fluid, and solid momentum, including a nonlinear relation between porosity and permeability. With our initial model, we have shown that the single porosity wave has only a minor impact on the chemical evolution of the lithosphere. With this new ongoing model development, we have greatly increased compositional complexity by using Thermolab<sup>2</sup>, which is a versatile Gibbs energy minimizer that uses local thermodynamic equilibrium and permits multicomponent thermodynamic calculations. The model confirmed by thermodynamic calculations that at the corresponding P-T conditions we will first start to crystalize anhydrous cumulates followed by hydrous cumulates at lower temperatures and depths. The new model is in 2D and therefore allows us to explore the channeling effect of porosity wave on melt evolution.<br />Additionally, we were able to produce spontaneous initiation of subsequent pulses of porosity which are following the already established channel of the previous pulse. This stabilization of porosity waves could progressively increase the chemical evolution of even highly compatible elements. This leads to the conclusion that the formation of intraplate volcanism is not a simple process driving melt across the lithosphere, but requires percolation, differentiation, and reaction probably occurring in multiple stages. These mechanisms are important to consider when making geophysical interpretations of the asthenosphere-lithosphere boundary.</p> <ul> <li>Tharimena et al., 2017, J.Geophys.Res.Solid Earth, 122</li> <li>Vrijmoed & Podladchikov, 2022, <em>G<sup>3</sup></em><em>, <strong>23</strong>,</em> <em>e2021GC010303</em></li> </ul>
<p>The lithosphere and the asthenosphere are characterized by different heat transport mechanisms, conductive for the lithosphere, convective for the asthenosphere. The zone associated with the transition between these two distinct mechanisms is known as the "Thermal Boundary Layer" (TBL). How the melt is transported across this zone is an important question regarding intraplate magmatism and for the nature of the seismic <em>Low-Velocity Zone.</em> Numerous studies and models suggest that primary magmas from intraplate volcanos are the product of low degree partial melting in the asthenosphere, while the differentiation process takes place in the crust or shallow lithospheric mantle. The question is how low degree melt ascends through the TBL and the lithospheric mantle. The thermal structure of the lithosphere is characterized by a high geothermal gradient, which questions the ability of melt to cross the lithospheric mantle without cooling and crystallizing. Since the base of the lithosphere is ductile, the possible modes of magma transport are porous flow or porosity waves. For these reasons, we would like to understand how melt is transported and what are the implications on the evolution of primitive melt, going from the convective part of the geotherm to the conductive part of the geotherm and further across the lithosphere.</p><p>We present the results of a thermo-hydro-mechanical-chemical (THMC) model<sup>1 </sup>for reactive melt transport using the finite difference method. This model considers melt migration by porosity waves and a chemical system of forsterite-fayalite-silica. Variables, such as solid and melt densities or MgO and SiO<sub>2</sub> mass concentrations, are functions of pressure, temperature, and total silica mass fraction (<em>C</em><sub>t</sub><sup>SiO2</sup>). These variables are pre-computed with Gibbs energy minimization and their variations with evolving <em>P</em>, <em>T, </em>and <em>C</em><sub>t</sub><sup>SiO2</sup> are implemented in the THMC model. We consider <em>P</em> and <em>T </em>conditions relevant across the TBL. With input parameters characteristic for alkaline melt and conditions at the base of the lithosphere, we obtain velocities between 1 to 150 m yr<sup>-1</sup>,<sup></sup>which is a velocity similar to melt rising at mid-ocean ridges<sup>2</sup>. This implies the inability of primary melts to cross the lithosphere. However, melt addition to the base of the lithosphere is important to understand mantle metasomatism, and could, to some extent, contribute to physical properties of the <em>Lithosphere-Asthenosphere Boundary</em> and <em>Mid Lithosphere Discontinuity</em> observed with geophysical methods. We suggest that the appearance of alkaline magmas at the surface requires multiple stage processes as melts rising in the lithosphere progressively modify the geotherm allowing new melts to propagate to the surface. Our earlier modeling results<sup>1</sup> demonstrated that a single porosity wave has a minor impact on chemical evolution. In this study, we search for a mechanism responsible for stabilizing porosity wave motion to some lateral location forcing consecutive waves to follow the same ascent path. The passage of a large number of quickly rising porosity waves over a long time through the same path would accumulate large melt to rock ratios and cause significant chemical evolution.</p><p>&#160;</p><ul><li>Bessat et at., 2022, <em>G<sup>3</sup></em>, <em>in press</em></li> <li>Connolly et al. 2009, <em>Nature</em> 462, 209-212.</li> </ul>
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