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Subduction zones play a pivotal role in the mechanics of plate tectonics by providing the driving force through slab pull and weak megathrusts that facilitate the relative motion between tectonic plates. The initiation of subduction zones is intricately linked to the accumulation of slab pull and development of weakness at plate boundaries and, by consequence, the largest changes in the energetics of mantle convection. However, the transient nature of subduction initiation accompanied by intense subsequent tectonic activity, leaves critical evidence poorly preserved and making subduction initiation difficult to constrain. We overcome these limitations through a comprehensive analysis focused on Puysegur, a well-constrained extant example of subduction initiation offshore South Island, New Zealand. Through time-dependent, three-dimensional thermo-mechanical computations and quantitative comparison to new geophysical and geological observations, including topography, stratigraphy, and seismicity, we demonstrate that subduction initiation develops with a fast strain weakening described with a small characteristic displacement ( ${\mathrm{\Delta }}_s\approx$4 to 8 km). Potential physical mechanisms contributing to the strain weakening are explored and we find that the observed fast weakening may arise through a combination of grain-size reduction within the lower lithosphere and fluid pressurization at shallower depths. With the shared commonality in the underlying physics of tectonic processes, the rapid strain weakening constrained at Puysegur offers insights into the formation of the first subduction during early Earth and the onset of plate tectonics. 

Variable viscosity in Earth’s mantle exerts a fundamental control on mantle convection and plate tectonics, yet rigorously constraining the underlying parameters has remained a challenge. Inverse methods have not been sufficiently robust to handle the severe viscosity gradients and nonlinearities (arising from dislocation creep and plastic failure) while simultaneously resolving the megathrust and bending slabs globally. Using global plate motions as constraints, we overcome these challenges by combining a scalable nonlinear Stokes solver that resolves the key tectonic features with an adjoint-based Bayesian approach. Assuming plate cooling, variations in the thickness of continental lithosphere, slabs, and broad scale lower mantle structure as well as a constant grain size through the bulk of the upper mantle, a good fit to global plate motions is found with a nonlinear upper mantle stress exponent of 2.43 $\pm$0.25 (mean $\pm$SD). A relatively low yield stress of 151 $\pm$19 MPa is required for slabs to bend during subduction and transmit a slab pull that generates asymmetrical subduction. The recovered long-term strength of megathrusts (plate interfaces) varies between different subduction zones, with South America having a larger strength and Vanuatu and Central America having lower values with important implications for the stresses driving megathrust earthquakes. 

Abstract Hadean zircons provide a potential record of Earth's earliest subduction 4.3 billion years ago. It remains enigmatic how subduction could be initiated so soon after the presumably Moon‐forming giant impact (MGI). Earlier studies found an increase in Earth's core‐mantle boundary (CMB) temperature due to the accumulation of the impactor's core, and our recent work shows Earth's lower mantle remains largely solid, with some of the impactor's mantle potentially surviving as the large low‐shear velocity provinces (LLSVPs). Here, we show that a hot post‐impact CMB drives the initiation of strong mantle plumes that can induce subduction initiation ∼200 Myr after the MGI. 2D and 3D thermomechanical computations show that a high CMB temperature is the primary factor triggering early subduction, with enrichment of heat‐producing elements in LLSVPs as another potential factor. The models link the earliest subduction to the MGI with implications for understanding the diverse tectonic regimes of rocky planets. 

Abstract The slow motion of tectonic plates over thousands of kilometers is intermittently interrupted by great earthquakes with sudden slips localized near convergent plate boundaries. We developed a subduction model that self‐consistently integrates buoyancy forces, diffusion and dislocation creep, and inter‐plate friction. From the nonlinear dynamics emerge long‐term plate motions that achieve velocities of cm/year, effective viscosities of Pas below plates, and sudden slips up to m repeating every several hundred years. Along‐strike resistance arising from long‐wavelength variation of coseismic slip is naturally incorporated with a rupture length scale, . Computations with km generate events with . When decreases, there is a commensurate decrease in the effective moment of rupture events. Predicted long‐term plate velocities, mantle viscosities, cycles of stress loading and release, and rupture event size and magnitude all show good agreement with observations. 

Abstract Plate tectonics distinguishes Earth from the other terrestrial planets but its initiation mechanism and onset time are debated. We propose plate tectonics was initiated by the deposition of magnetite‐rich banded iron formations (BIFs) through biogeochemical iron cycling in Neoarchean oceans. In the photic zone of proto‐continental margins, photoferrotrophic bacteria efficiently oxidized the dissolved Fe(II) and induced massive precipitation of ferric oxyhydroxide, which would rapidly react with Fe(II)‐rich hydrothermal fluids from coeval vigorous volcanism in Neoarchean oceans to produce magnetite‐rich BIFs. Mechanical models demonstrate that the localization of high‐density BIF deposition near proto‐continents induces collapse of the lithosphere and can initiate the earliest subduction. The peak deposition of BIFs in 2.75–2.40 Ga provides a time constraint on the inception of plate tectonics. 

Plate tectonic reconstructions of three of the best-defined Cenozoic subduction initiation (SI) events in the western Pacific, Izu-Bonin-Mariana, Vanuatu, and Puysegur subduction zones, show substantial components of strike-slip motion before and during the subduction initiation. Using computational models, we show that strike-slip motion has a large influence on the effective strength of incipient margins and the ease of subduction initiation. The parameter space associated with visco-elasto-plastic rheologies, plate weakening, and plate forces and kinematics is explored and we show that subduction initiates more easily with a higher force, a faster weakening, or greater strike-slip motion. With the analytical solution, we demonstrate that the effect of strike-slip motion can be equivalently represented by a modified weakening rate. Along transpressive margins, we show that a block of oceanic crust can become trapped between a new thrust fault and the antecedent strike-slip fault and is consistent with structural reconstructions and gravity models of the Puysegur margin. Together, models and observations suggest that subduction initiation can be triggered when margins become progressively weakened to the point that the resisting forces become smaller than the driving forces, and as the negative buoyancy builds up, the intraplate stress eventually turns from compressional into extensional. The analytical formulation of the initiation time,t_SI, marking the moment when intraplate stress flips sign, is validated with a computational models. The analytical solution shows thatt_SIis dominated by convergence velocity, while the plate age, strike-slip velocity, and weakening rate all have a smaller but still important effect on the time scale of subduction initiation. 

SUMMARY The initiation and development of subduction zones are associated with substantial stress changes both within plates and at plate boundaries. We formulate a simple analytical model based on the force balance equation of a subduction zone, and validate it with numerical calculations of highly non-linear, coupled thermomechanical system. With two kinds of boundary conditions with either fixed velocity or fixed force in the far-field, we quantitatively analyse the role of each component in the force balance equation, including slab pull, interplate friction, plate bending and basal traction, on the kinematics and stress state of a subducting plate. Based on the numerical and analytical models, we discuss the evolution of plate curvature, the role of plastic yielding and elasticity, and how different factors affect the timing of subduction initiation. We demonstrate with the presence of plastic yielding for a plate of thickness, H, that the bending force is proportional to H2, instead of H3 as previously thought. Although elasticity increases the force required to start nucleating subduction it does not substantially change the total work required to initiate a subduction zone when the yielding stress is small. The analytical model provides an excellent fit to the total work and time to initiate subduction and the force and velocity as a function of convergence and time. Plate convergence and weakening rate during nucleation are the dominant factors influencing the force balance of the plate, and 200 km of plate convergence is typically required to bring a nascent subduction zone into a self-sustaining state. The closed-form solution now provides a framework to better interpret even more complex, time-dependent systems in three dimensions. 

Abstract Growth of the Andes has been attributed to Cenozoic subduction. Although climatic and tectonic processes have been proposed to be first-order mechanisms, their interaction and respective contributions remain largely unclear. Here, we apply three-dimensional, fully-dynamic subduction models to investigate the effect of trench-axial sediment transport and subduction on Andean growth, a mechanism that involves both climatic and tectonic processes. We find that the thickness of trench-fill sediments, a proxy of plate coupling (with less sediments causing stronger coupling), exerts an important influence on the pattern of crustal shortening along the Andes. The southward migrating Juan Fernandez Ridge acts as a barrier to the northward flowing trench sediments, thus expanding the zone of plate coupling southward through time. Consequently, the predicted history of Andean shortening is consistent with observations. Southward expanding crustal shortening matches the kinematic history of inferred compression. These results demonstrate the importance of climate-tectonic interaction on mountain building.