Abstract We have devised a new absolute Late Jurassic‐Cretaceous Pacific plate model using a fixed hot spot approach coupled with paleomagnetic data from Pacific large igneous provinces (LIPs) while simultaneously minimizing plate velocity and net lithosphere rotation (NR). This study was motivated because published Pacific plate models for the 83.5‐ to 150‐Ma time interval are variably flawed, and their use affects modeling of the entire Pacific‐Panthalassic Ocean and interpretation of its margin evolution. These flaws could be corrected, but the revised models would imply unrealistically high plate velocities and NR. We have developed three new Pacific realm models with varying degrees of complexity, but we focus on the one that we consider most realistic. This model reproduces many of the Pacific volcanic paths, modeled paleomagnetic latitudes fit well with direct observations, plate velocities and NR resulting from the model are low, and all reconstructed Pacific LIPs align along the surface‐projected margin of the Pacific large low shear wave velocity province. The emplacement of the Shatsky Rise LIP at ~144 Ma probably caused a major plate boundary reorganization as indicated by a major jump of the Pacific‐Izanagi‐Farallon triple junction and a noteworthy change of the Pacific‐Izanagi seafloor spreading direction at around chron M20 time.
Although mantle slabs ultimately drive plate motions, the mechanism by which they do so remains unclear. A detached slab descending through the mantle will excite mantle flow that exerts shear tractions on the base of the surface plates. This “slab suction” force drives subducting and overriding plates symmetrically toward subduction zones. Alternatively, cold, strong slabs may effectively transmit stresses to subducting surface plates, exerting a direct “slab pull” force on these plates, drawing them rapidly toward subduction zones. This motion induces mantle flow that pushes overriding plates away from subduction zones. We constrain the relative importance of slab suction and slab pull by comparing Cenozoic plate motions to model predictions that include viscous mantle flow and a proxy for slab strength. We find that slab pull from upper mantle slabs combined with slab suction from lower mantle slabs explains the observation that subducting plates currently move ∼4 times faster than nonsubducting plates. This implies that upper mantle slabs are strong enough to support their own weight. Slab suction and slab pull presently account for about 40 and 60% of the forces on plates, but slab suction only ∼30% if a low‐viscosity asthenosphere decouples plates from mantle flow. The importance slab pull has been increasing steadily through the Cenozoic because the mass and length of upper mantle slabs has been increasing. This causes subducting plates to double their speed relative to nonsubducting plates during this time period. Our model explains this temporal evolution of plate motions for the first time.
SUMMARY Cratons are the oldest parts of the lithosphere, some of them surviving since Archean. Their long-term survival has sometimes been attributed to high viscosity and low density. In our study, we use a numerical model to examine how shear tractions exerted by mantle convection work to deform cratons by convective shearing. We find that although tractions at the base of the lithosphere increase with increasing lithosphere thickness, the associated strain-rates decrease. This inverse relationship between stress and strain-rate results from lateral viscosity variations along with the model’s free-slip condition imposed at the Earth’s surface, which enables strain to accumulate along weak zones at plate boundaries. Additionally, we show that resistance to lithosphere deformation by means of convective shearing, which we express as an apparent viscosity, scales with the square of lithosphere thickness. This suggests that the enhanced thickness of the cratons protects them from convective shear and allows them to survive as the least deformed areas of the lithosphere. Indeed, we show that the combination of a smaller asthenospheric viscosity drop and a larger cratonic viscosity, together with the excess thickness of cratons compared to the surrounding lithosphere, can explain their survival since Archean time.
The feedback between plate tectonics and mantle convection controls the Earth's thermal evolution via the seafloor age distribution. We therefore designed the MACMA model to simulate time‐dependent plate tectonics in a 2D cylindrical geometry with evolutive plate boundaries, based on multiagent systems that express thermal and mechanical interactions. We compute plate velocities using a local force balance and use explicit parameterizations to treat tectonic processes such as trench migration, subduction initiation, continental breakup and plate suturing. These implementations allow the model to update its geometry and thermal state at all times. Our approach has two goals: (1) to test how empirically‐ and analytically‐determined rules for surface processes affect mantle and plate dynamics, and (2) to investigate how plate tectonics impact the thermal regime. Our predictions for driving forces, plate velocities and heat flux are in agreement with independent observations. Two time scales arise for the evolution of the heat flux: a linear long‐term decrease and high‐amplitude short‐term fluctuations due to surface tectonics. We also obtain a plausible thermal history, with mantle temperature decreasing by less than 200 K over the last 3 Gyr. In addition, we show that on the long term, mantle viscosity is less thermally influential than tectonic processes such as continental breakup or subduction initiation, because Earth's cooling rate depends mainly on its ability to replace old insulating seafloor by young thin oceanic lithosphere. We infer that simple convective considerations alone cannot account for the nature of mantle heat loss and that tectonic processes dictate the thermal evolution of the Earth.
Abstract First‐order variations in sea level exhibit amplitudes of ∼200 m over periods that coincide with those of supercontinental cycles (∼300–500 Myr). Proposed mechanisms for this sea level change include processes that change the container volume of the ocean basins and the relative elevation of continents. Here we investigate how unbalanced rates of water exchange between Earth's surface and mantle interior, resulting from fluctuations in tectonic rates, can cause sea level changes. Previous modeling studies of subduction water fluxes suggest that the amount of water that reaches sub‐arc depths is well correlated with the velocity and age of the subducting plate. We use these models to calibrate a parameterization of the deep subduction water flux, which we together with a parameterization of mid‐ocean ridge outgassing, then apply to reconstructions of Earth's tectonic history. This allows us to estimate the global water fluxes between the oceans and mantle for the past 230 Myr and compute the associated sea level change. Our model suggests that a sea level drop of up to 130 m is possible over this period and that it was partly caused by the ∼150Ma rift pulse that opened the Atlantic and forced rapid subduction of old oceanic lithosphere. This indicates that deep water cycling may be one of the more important sea level changing mechanisms on supercontinental time scales and provides a more complete picture of the dynamic interplay between tectonics and sea level change.
Olivine lattice preferred orientation (LPO), or texture, forms in relation to deformation mechanisms such as dislocation creep and can be observed in the upper mantle as seismic anisotropy. Olivine is also mechanically anisotropic, meaning that it responds to stresses differently depending on the direction of the stress. Understanding the interplay between anisotropic viscosity (AV) and LPO, and their role in deformation, is necessary for relating seismic anisotropy to mantle flow patterns. In this project, we employ three methods to predict olivine texture (D-Rex, MDM, and MDM+AV) in a shear box model and a subduction model. D-Rex and MDM are two representative texture development methods that have been compared before, and our results are in line with previous studies showing that textures computed by D-Rex develop faster and are stronger and more point-like than textures calculated with MDM. MDM+AV uses the same no-AV mantle deformation as D-Rex and MDM but includes the effect of AV for texture predictions. MDM+AV predicts a texture similar to MDM with a distinct girdle-like orientation in simple shear deformation or at low strain. At larger strains, MDM+AV’s textures are more point-like and stronger compared to the other two methods. The effective viscosity for MDM+AV drops by up to 40% in a shear box model, while the anisotropic viscosity can be both smaller and larger relative to the isotropic viscosity in different regions of a subduction model. Our results emphasize the significant role of AV in olivine texture development, which could substantially affect geodynamic processes in the upper mantle.
