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Spatio-temporal variability in arc geochemistry and the conditions recorded by exhumed rocks suggest subduction zone thermal structure evolves in time and along-strike. Although much effort has been dedicated to studying subduction zone thermal structure, we lack an understanding of spatio-temporal temperature variability during time-dependent subduction. We model 3-D, dynamic subduction and examine the time evolution of the along-strike temperature difference of the slab’s upper surface (‘slab-top’) at the centre relative to the edge of the subduction zone. We examine this slab-top temperature variability for subduction systems of different widths and with different plate mobilities (i.e. fixed versus free subducting and overriding plates). In all of our models, the main control on slab-top temperature is convergence rate; either by simply controlling the rate of slab sinking or via the effect it has on the decoupling depth (DD). In the early stages of subduction, more rapid convergence at the plate centre produces a cooler slab relative to warmer slab edges. For mature subduction, this flips; a shallower DD at the slab centre produces warmer temperatures with respect to the edge. Importantly, our maximum along-strike temperature changes are reduced (≤50 °C) relative to previous kinematically driven modelling studies, due to a reduced role for slab-top heating via toroidal flow. Our dynamic subduction models, therefore, point towards a strong time dependence in the sense of along-strike temperature variation, but with relatively low absolute values in geometrically simple subduction zones.

 

Abstract Plate reconstruction models are constructed to fit constraints such as magnetic anomalies, fracture zones, paleomagnetic poles, geological observations and seismic tomography. However, these models do not consider the physical equations of plate driving forces when reconstructing plate motion. This can potentially result in geodynamically-implausible plate motions, which has implications for a range of work based on plate reconstruction models. We present a new algorithm that calculates time-dependent slab pull, ridge push (GPE force) and mantle drag resistance for any topologically closed reconstruction, and evaluates the residuals—or missing components—required for torques to balance given our assumed plate driving force relationships. In all analyzed models, residual torques for the present-day are three orders of magnitude smaller than the typical driving torques for oceanic plates, but can be of the same order of magnitude back in time—particularly from 90 to 50 Ma. Using the Pacific plate as an example, we show how our algorithm can be used to identify areas and times with high residual torques, where either plate reconstructions have a high degree of geodynamic implausibility or our understanding of the underlying geodynamic forces is incomplete. We suggest strategies for plate model improvements and also identify times when other forces such as active mantle flow were likely important contributors. Our algorithm is intended as a tool to help assess and improve plate reconstruction models based on a transparent and expandable set of a priori dynamic constraints. 

Subduction zones are fundamental features of Earth's mantle convection and plate tectonics, but mantle flow and pressure around slabs are poorly understood because of the lack of direct observational constraints on subsurface flow. To characterize the linkages between slabs and mantle flow, we integrate high‐resolution representations of Earth's lithosphere and slabs into a suite of global mantle convection models to produce physically plausible present‐day flow fields for Earth's mantle. We find that subduction zones containing wide, thick, and long slabs dominate regional mantle flow in the neighboring regions and this flow conforms to patterns predicted by simpler regional subduction models. These subduction zones, such as Kuril‐Japan‐Izu‐Bonin‐Mariana, feature prismatic poloidal flow coupled to the downgoing slab that rotates toward toroidal slab‐parallel flow near the slab edge. However, other subduction zones, such as Sumatra, deviate from this pattern because of the competing influence of other slabs or longer‐wavelength mantle flow, showing that upper mantle flow can link separate subduction zones and how flow at subduction zones is influenced by broader scale mantle flow. We find that the non‐linear dislocation creep reduces the coupling between slab motion and asthenospheric flow and increases the occurrence of non‐ideal flow, in line with inferences derived from seismological constraints on mantle anisotropy.

 

Subduction zones are associated with spatially heterogeneous pressure fields that, depending on location, push/pull on Earth's surface producing dynamic topography (DT). Despite this, subduction zones, and associated pressure fields, are typically over‐simplified within global mantle flow models. Here, I use subduction models within a global domain to probe mantle pressure build‐up beneath subducting plates (SPs) and the resulting DT. Positive pressure develops beneath the SP in most subduction models. This produces positive DT (≤450 m) and tilts the SP upwards toward the trench (≤0.25 m/km). As SP size increases, the pressure magnitude increases producing greater topography/tilting. At a global scale, I find potential evidence for the modeled tilting in published residual topography. I argue that the rigorous incorporation of subduction zones into mantle flow models, and hence the inclusion of this signal, is needed to continue to bring future DT predictions and observational estimates into closer alignment.

 

SUMMARY Tectonic plate motions predominantly result from a balance between the potential energy change of the subducting slab and viscous dissipation in the mantle, bending lithosphere and slab–upper plate interface. A wide range of observations from active subduction zones and exhumed rocks suggest that subduction interface shear zone rheology is sensitive to the composition of subducting crustal material—for example, sediments versus mafic igneous oceanic crust. Here we use 2-D numerical models of dynamically consistent subduction to systematically investigate how subduction interface viscosity influences large-scale subduction kinematics and dynamics. Our model consists of an oceanic slab subducting beneath an overriding continental plate. The slab includes an oceanic crustal/weak layer that controls the rheology of the interface. We implement a range of slab and interface strengths and explore how the kinematics respond for an initial upper mantle slab stage, and subsequent quasi-steady-state ponding near a viscosity jump at the 660-km-discontinuity. If material properties are suitably averaged, our results confirm the effect of interface strength on plate motions as based on simplified viscous dissipation analysis: a ∼2 order of magnitude increase in interface viscosity can decrease convergence speeds by ∼1 order of magnitude. However, the full dynamic solutions show a range of interesting behaviour including an interplay between interface strength and overriding plate topography and an end-member weak interface-weak slab case that results in slab break-off/tearing. Additionally, for models with a spatially limited, weak sediment strip embedded in regular interface material, as might be expected for the subduction of different types of oceanic materials through Earth’s history, the transient response of enhanced rollback and subduction velocity is different for strong and weak slabs. Our work substantiates earlier suggestions as to the importance of the plate interface, and expands the range of quantifiable links between plate reorganizations, the nature of the incoming and overriding plate and the potential geological record. 

The importance of slab–slab interactions is manifested in the kinematics and geometry of the Philippine Sea Plate and western Pacific subduction zones, and such interactions offer a dynamic basis for the first-order observations in this complex subduction setting. The westward subduction of the Pacific Sea Plate changes, along-strike, from single slab subduction beneath Japan, to a double-subduction setting where Pacific subduction beneath the Philippine Sea Plate occurs in tandem with westward subduction of the Philippine Sea Plate beneath Eurasia. Our 3-D numerical models show that there are fundamental differences between single slab systems and double slab systems where both subduction systems have the same vergence. We find that the observed kinematics and slab geometry of the Pacific–Philippine subduction can be understood by considering an along-strike transition from single to double subduction, and is largely independent from the detailed geometry of the Philippine Sea Plate. Important first order features include the relatively shallow slab dip, retreating/stationary trenches, and rapid subduction for single slab systems (Pacific Plate subducting under Japan), and front slabs within a double slab system (Philippine Sea Plate subducting at Ryukyu). In contrast, steep to overturned slab dips, advancing trench motion, and slower subduction occurs for rear slabs in a double slab setting (Pacific subducting at the Izu–Bonin–Mariana). This happens because of a relative build-up of pressure in the asthenosphere beneath the Philippine Sea Plate, where the asthenosphere is constrained between the converging Ryukyu and Izu–Bonin–Mariana slabs. When weak back-arc regions are included, slab–slab convergence rates slow and the middle (Philippine) plate extends, which leads to reduced pressure build up and reduced slab–slab coupling. Models without back-arcs, or with back-arc viscosities that are reduced by a factor of five, produce kinematics compatible with present-day observations.