Jorge Sanhueza

The migration of fluids released during slab-dehydration in deep subduction environments is strongly controlled by deformation and lithological discontinuities such as serpentinized shear zones. However, the geological meaning of geochemical fingerprints and how they relate to transport mechanisms and spatial scales of fluid flux in deformed serpentinite at depth remain poorly understood. We herein focus on three subduction-related mantle sections: an intra-slab serpentinized shear zone (Monviso, Italian Alps), an underplated ultramafic sliver (Zagros suture zone, Iran) and the former base of a mantle wedge (Polar Urals, Russia). Most major and trace element signatures of serpentinites appear rather homogeneous along the transects. In contrast, boron isotopic signatures (δ11B) show systematic variations at several hundred meters scale approaching the main structural boundaries for each locality. A decrease is observed in the Monviso and Urals localities (from c. 25‰ to c. 7‰, and from c. 16‰ to c. 0‰, respectively), while the Zagros section shows an increase from c. 1‰, up to c. 9‰ in the most sheared and serpentinized samples. These variations reflect complex fluid-rock interactions processes including B loss or B addition associated with protolith and fluid variability. This demonstrates that major shear zones exert a first-order control on the serpentinite boron isotopic signature. We combine boron isotopic data with Darcy-based flux models to quantify the volume of rock influenced by paleo-fluid fluxes in deep ultramafic settings. These combined petrostructural and isotopic constraints highlight the importance of fracture-controlled fluid flow in slab-top serpentinites, and yield a time-integrated permeability of the (partly) serpentinized base of the mantle wedge in the range of 10-19 to 10-18 m².

Subduction zone geodynamics have been a primary area of focus since the early days of plate tectonics theory. Initial approaches using analytical solutions sought to understand the fundamental driving forces, moment balances, and energetics governing oceanic plate subduction. Despite the limitations of scarce geophysical imaging, limited geological sampling, and emerging numerical techniques, these studies provided the foundations of modern geodynamics. As numerical techniques improved, power-law rheologies and complex geological processes were integrated into various codes. These provided more realistic simulations to understand a wide variety of regions on Earth, but they are rarely presented in the context of classical physical approaches. Nowadays, a unique opportunity exists to revisit these classical frameworks, aided by improved subducting slab imaging, an excellent geological record, and a deeper understanding of slab dynamics across active and ancient subduction zones.In this work, a simplified but physically transparent framework is developed to revisit the force/moment balance, energetic conditions, and dissipation analyses governing slab dynamics. For this purpose, three representative slab geometries (steep, normal, and flat) were selected based on their dip below 40 km depth, where slab behavior is dominated by internal negative buoyancy and viscous lifting forces from the mantle. Slab pull was estimated by varying density contrasts and thickness, while viscous forces were derived from semi-analytical Stokes flow solutions to resolve pressure distributions along the slab surface. Gravitational potential energy changes were calculated to test whether internal density variations (e.g., eclogitization) and crustal thickening from oceanic plateaus drive changes in geometry.Results show that mantle wedge suction can counteract slab weight in a flat subduction setting, while increased density from eclogitization destabilizes flat slabs and promotes steepening, linking moment balance and buoyancy to the global diversity of slab dips. Total energy dissipation is mainly controlled by mantle wedge flow, with low-angle and flat subduction representing the most dissipative configurations. Once moment balance allows, these tend to evolve toward steeper, more energetically stable states. Slab flattening occurs with increased buoyancy, higher convergence velocities, and greater mantle viscosities, producing flattening within 10–30 Myr. Conversely, reduced buoyancy, slower convergence, and lower viscosities favor steepening and slab rollback on comparable or shorter timescales of 5–10 Myr. This integrated and physically transparent analysis is consistent with the development of arc magmas on western margins of the Americas and provides a clear perspective to explain the diverse slab morphologies observed on Earth.

We consider how regional processes associated with plate subduction and lithospheric geometry can lead to horizontal forces sufficient to produce the crustal compression and thickening needed to support high mountains and plateaus. A basic assumption is that the shallowest 200-300 km part of the mantle (the asthenosphere) is supplied by mantle plumes. This plume fed asthenosphere is assumed to be slightly lower in density (<1%) than the deeper mantle due to being hotter and/or more depleted. We show that thinning back arc asthenosphere can lead to compressional force sufficient to push up high mountains and plateau. Asthenospheric thinning may occur when back-arc asthenosphere accretes to the top of a cold subducting slab that carries it into the deeper mantle. Such thinning can only happen in places where asthenospheic flow into the back arc area is blocked by thick continental cratonic roots, and its timing is consistent with the lifespan of subduction at convergent margins. Analytic, physical and simple numerical models demonstrate the plausibility of this simple model to explain the force needed for orogenesis. This model also accounts for rapid post-orogenic collapse, such as the Basin and Range Province, that can occur after regional ridge subduction allows flow of asthenosphere in to the region below the plateau.

The spatial-temporal evolution of volcanic arcs provides valuable insights into the deep melting processes occurring in the mantle wedge. The dehydration of the subducting slab is key because these fluids directly affect the melting temperatures of the mantle wedge. Fluids in this region (partial melts and released fluids from the slab) migrate to the corner of the wedge, where pressure/temperature conditions are optimal for magma production. Changes in the locus of the volcanic arc can be thus related to the position or changes in the physicochemical properties of the mantle wedge at depths, which is drastically dependent on subduction dynamics in time. The dip of the subducting slab is one of the key factors affecting the relative location of the mantle wedge, which can migrate the volcanic arc several hundreds of kilometers into the continent during flat slabs periods. However, the transition to a normal subduction angle or even processes such as slab break-off will migrate the mantle wedge, and the volcanic arc, to the trench and potentially generating large magmatic provinces in the lifespan of an active margin.The scope of this preliminary study is to track the location of the magmatic arc in time driven by different subduction styles (e.g., low/high angle subduction, slab break-off) and the generation of magmatic provinces in the continent. We conducted a series of 2D geodynamics models using the code I2ELVIS feeded with ad hoc thermodynamic pseudosection modelling with the Perple_X software, to reproduce different subduction angles and the transition between them. The timings and mechanisms of the arc migration is applied to the well-documented exposure of Jurassic igneous rocks along the Antarctic Peninsula and Patagonia in the Chon Aike magmatic province. Recent debate postulates an active margin origin of these rocks, which is supported by geochemical signatures of typical slab-dehydration reactions and a mixed magmatic source that resided in the continental crust. Even though, the subduction dynamics are not constrained, the location and age of these rocks suggest several episodes of arc migration during the Jurassic, making this an exceptional study case to understanding the mechanisms of arc migration and the role of subduction dynamics.Preliminary results of our modelling tracked the position of the mantle wedge by the presence of partial melts and the maximum depth of dehydration of the subducting slab. Explored scenarios consisted on periods of flat slab subduction triggered by the subduction of aseismic ridges and the return to a normal subduction. During the flat slab period, we also tested the generation of slab break-off, which induced local mantle upwelling and melting. Finally, we expect to reproduce the magmatic history of Antarctic Peninsula and Patagonia in the Jurassic to support the active margin hypothesis for the generation of the Chon Aike magmatic province.

A combination of the geochemical and isotopic compositions of the Jurassic igneous rocks exposed along the Antarctic Peninsula and Patagonia suggest that they may have formed in a continental arc setting without a significant input of a mantle plume, as presented in Bastias et al. (2021). This interpretation revises previous interpretations (e.g. Pankhurst et al., 2000; Riley et al., 2001) that suggested the early deposits of the Chon Aike magmatic province in eastern and central Patagonia formed by the coupled action of an active margin and the peripheral thermal effect of the Karoo mantle plume. We present a tectono-magmatic evolution for the Chon Aike magmatic province following the argument of Bastias et al. (2021). Furthermore, we argue that a supra-subduction zone origin is supported by: (i) enriched LILE and LREE, with negative Nb, Ta, Sr and Ti anomalies in all of the Jurassic igneous rocks, which are typical of slab-dehydration reactions and thus active margins, (ii) the trace element compositions of Jurassic igneous rocks in Patagonia and the Antarctic Peninsula are indistinguishable throughout the region, and thus it is likely that they evolved via the same magmatic processes, (iii) the whole rock Nd and Sr, and zircon Hf isotopic compositions of Late Triassic-Jurassic igneous rocks show that the magmas formed from mixed sources that resided within the continental crust and (iv) numerical modelling. Therefore, unlike the igneous units of the Karoo and Ferrar LIPs, the involvement of a mantle plume is not necessary to generate any of the chronological, geochemical or isotopic characteristics of the Chon Aike magmatic province. This contribution serves as further information on the debate searching for the responsible mechanism generating the Chon Aike magmatic province: active margin vs. mantle plume.

The generation of a slab window impacts the spatio-temporal evolution of subduction zones and promote complex mantle flow pattern where slabs once descended. The origin of slab windows is attributed to processes such as mid-ocean ridge subduction, slab tearing and/or break-off. The interaction between mid-ocean ridges and trenches is a common process affecting the geodynamic history of the margins around the Pacific, at least, during the Cenozoic and generated several modern slab windows. These intriguing features have notable effects on the upper mantle where temperature anomalies develop due to the asthenospheric upwelling and complex toroidal flow patterns through and around slab windows. There are profound effects on the overriding plate for the surface heat flow, geochemistry and spatial distribution of magmatic activity, seismicity and topographic relief. However, these manifestations evolve through space and time depending on the ridge axis-trench geometry, inducing the continuous slab window opening during its subduction.In this contribution, we derived a simplified expression for the slab window angle and then conducted 3D geodynamic modeling to link slab windows dynamics with geochemical and geophysical observables. The numerical models were conducted with fixed geometries in steady-state (using finite elements), compared with time-dependent solutions (using the I3ELVIS code) and then compared with observations from modern slab windows along the eastern Pacific. The analytical solution for the plan projection of the slab window depends on three parameters: the ratio between the half-spreading rate to the velocity of the overriding plate, the subduction angle and the obliquity of the ridge axis respect to the trench.Fast spreading or slow plate convergence promotes a wide (> 90°) slab window while slow spreading or fast convergence narrows this gap (< 90°). The slab dip and ridge obliquity have a second order control on the plan projection of the slab window but affect the existence of a steady-state solution. The implementation of this geometry into 3D steady-state models was used to generate a novel methodology to estimate mantle/melt upwelling and temperature anomalies in the upper mantle for a wide range of tectonic settings. Preliminary results on 3D time-dependent models reproduce a self-consistent opening of the slab window by only imposing spreading at the mid-ocean ridge and a subduction velocity with respect to the overriding plate. The ratio and absolute magnitude of these velocities controls the timing of the opening as well as the lateral and depth extent of the subducting plates. This timing also influences the development of upwelling and toroidal flow patterns around the slab edges. Finally, observations in modern slab windows along the eastern Pacific are consistent with the temperature and velocity field of the models. Variations in temperatures in the upper mantle are consistent with mantle shear wave speeds anomalies, while the flow field is correlated with the azimuthal anisotropy. In terms of magmatism, variables degrees of melting are consistent with the generation of tholeiitic to alkaline magmas in backarc areas.