Importance of initial buoyancy field on evolution of mantle thermal structure: Implications of surface boundary conditions

Abstract

a b s t r a c tAlthough there has been significant progress in the seismic imaging of mantle heterogeneity, the outstanding issue that remains to be resolved is the unknown distribution of mantle temperature anomalies in the distant geological past that give rise to the present-day anomalies inferred by global tomography models. To address this question, we present 3-D convection models in compressible and self-gravitating mantle initialised by different hypothetical temperature patterns. A notable feature of our forward convection modelling is the use of self-consistent coupling of the motion of surface tectonic plates to the underlying mantle flow, without imposing prescribed surface velocities (i.e., plate-like boundary condition). As an approximation for the surface mechanical conditions before plate tectonics began to operate we employ the no-slip (rigid) boundary condition. A rigid boundary condition demonstrates that the initial thermally-dominated structure is preserved, and its geographical location is fixed during the evolution of mantle flow. Considering the impact of different assumed surface boundary conditions (rigid and plate-like) on the evolution of thermal heterogeneity in the mantle we suggest that the intrinsic buoyancy of seven superplumes is most-likely resolved in the tomographic images of present-day mantle thermal structure. Our convection simulations with a plate-like boundary condition reveal that the evolution of an initial cold anomaly beneath the Java-Indonesian trench system yields a long-term, stable pattern of thermal heterogeneity in the lowermost mantle that resembles the presentday Large Low Shear Velocity Provinces (LLSVPs), especially below the Pacific. The evolution of subduction zones may be, however, influenced by the mantle-wide flow driven by deeply-rooted and longlived superplumes since Archean times. These convection models also detect the intrinsic buoyancy of the Perm Anomaly that has been identified as a unique slow feature distinct from the two principal LLSVPs. We find there is no need for dense chemical 'piles' in the lower mantle to generate a stable distribution of temperature anomalies that are correlated to the LLSVPs and the Perm Anomaly. Our tomography-based convection simulations also demonstrate that intraplate volcanism in the south-east Pacific may be interpreted in terms of shallow small-scale convection triggered by a superplume beneath the East Pacific Rise.

Kinematics and dynamics of the East Pacific Rise linked to a stable, deep-mantle upwelling

“…The dynamical importance of deep-mantle upwellings, extracting up to 20 TW from CMB ( 44 , 50 ), is compatible with recent estimates of high CMB heat flux drawn from the core that are derived from mineral physical arguments ( 16 , 17 ). High heat transport across the CMB would also account for up to approximately 50% of the heat flux at the top of the mantle ( 12 ).…”

“…From the perspective of the predicted plate motions, the main difference between the V1 and V2 viscosities is that the latter has a thicker, higher viscosity lithosphere, which reduces the plate velocities (reflected in a reduced fit; see Table 1). From the perspective of deeper flow dynamics, the V2 profile has a higher viscosity in the bottom half of the lower mantle, yielding a more stable deep-mantle flow that has been extensively explored in the past numerical simulations of very long-term mantle convection ( 44 , 50 , 51 ). Unless otherwise specified, all mantle flow predictions discussed below use the V2 profile.…”

“…This global flow model has also been tested in direct comparisons of time-dependent topography predictions with detailed geological constraints on the variable uplift of the U.S. Atlantic Coastal Plain since the mid-Pliocene ( 57 ). Finally, this flow model has also served as a starting point for very long time simulations of thermal convection, which reveal the longevity and geographic fixity of a number of mantle-wide thermal upwellings that correlate with surface hot spot locations and are maintained over hundreds of millions of years by high CMB heat flux ( 44 , 50 ). On the basis of these multiple comparisons of model predictions and observation of the global mantle flow, we now return our focus to the specific predictions related to the EPR.…”

Variations in grain size and viscosity based on vacancy diffusion in minerals, seismic tomography, and geodynamically inferred mantle rheology

“…The absolute viscosity at middle lower mantle depths (Figure c) is higher than anywhere else in the mantle, and the lateral variations are mainly characterized by local zones of relatively reduced viscosity. These regions are related to hotter‐than‐average upwellings beneath West Africa, South Africa, West Pacific, and South‐East Pacific that are detected in tomography‐based convection models [ Glišović et al , ; Glišović and Forte , ]. Our calculations also show two large‐scale regions of low‐viscosity ∼10 21 Pa s at the top of the D " layer that are associated with LLSVPs (Figure d).…”

Joint modeling of lithosphere and mantle dynamics: Evaluation of constraints from global tomography models

“…Three types of surface boundary conditions (free‐slip, no‐slip, or plate velocity boundary conditions) can be assumed. Previous studies that have investigated these three types of surface boundary conditions have suggested that free‐slip boundary conditions provide the best fit to the geoid in comparison with the other two types [ Glišović and Forte , ; Thoraval and Richards , ]. Using the HC code, we use free‐slip boundary conditions at the surface and core‐mantle boundary and compute instantaneous flow associated with the present‐day inferred mantle density distributions.…”