Ubiquitous lower-mantle anisotropy beneath subduction zones

Abstract

Seismic anisotropy provides key information to map the trajectories of mantle flow and understand the evolution of our planet. While the presence of anisotropy in the uppermost mantle is well-established, the existence and nature of anisotropy in the transition zone and uppermost lower mantle are still debated. Here we use 3-D global seismic tomography images based on a large data set sensitive to this region to show the ubiquitous presence of anisotropy in the lower mantle beneath subduction zones. Whereas above the 660-km seismic discontinuity slabs are associated with faster SV anomalies up to about 3%, in the lower mantle faster SH anomalies of about 2% persist near slabs down to about 1,000-1,200 km. These observations are consistent with 3-D 1 numerical models of deformation from subducting slabs and the associated lattice preferred orientation of bridgmanite produced in the dislocation creep regime in areas subjected to high stresses. This study provides evidence that dislocation creep may be active in the Earth's lower mantle, providing new constraints on the debated nature of deformation in this key but inaccessible component of the deep Earth.The Earth's upper and lower mantle have quite distinct physical properties, with the characteristics of material exchange between them being a long-debated issue. Progress in global seismic tomography in the 1990s 1,2 showed that the upper and lower mantle interact mainly via subducting slabs and mantle plumes, albeit subject to the presence of strong resistance along the upper-lower mantle boundary at ∼660 km depth. More recently, enhanced tomography images showed that amongst the slabs that penetrate into the lower mantle, many of them stagnate down to about ∼1,000 km depth 3 . Conversely, mantle plumes rising from the deep lower mantle seem to deflect laterally when they reach this region 4 . However, the uppermost lower mantle, located at depths of ∼660-1,000 km, remains an enigmatic part of the Earth. It has been suggested that compositional layering 5,6 or a viscosity increase 7,8 may cause flow stagnation in this region, but its rheology and role in mantle convection are poorly understood.The stagnation of subducting slabs at ∼660 km depth and their penetration into the lower mantle lead to intense strain and deformation around the slabs, which in turn can align mineral aggregates. Since the most abundant lower mantle mineral (bridgmanite) is anisotropic, observable seismic anisotropy should develop when considering a dislocation creep deformation mechanism 9,10,11 . However, apart from the D" region in the lowermost mantle 12 , the presence of seismic anisotropy in the lower mantle is uncertain and debated 13,14,15 , with most previous seismological models suggesting that the bulk of the uppermost lower mantle is radially isotropic in shear wavespeed 16 . In order to resolve this paradox, it has been proposed that the dominant

On the Origin of Radial Anisotropy Near Subducted Slabs in the Midmantle

“…The use of huge data sets and, notably, a large number of surface wave overtone dispersion measurements has led to an improved agreement between models. Chang et al () quantitatively compared recent models and found (i) an improved correlation between SGLOBE‐rani (Chang et al, ) and savani (Auer et al, ) of 0.5; this is an encouraging improvement compared to previous studies, which do not show correlations larger than 0.3; (ii) fast‐SV radial anisotropy anomalies in the transition zone in SGLOBE‐rani near subducted slabs, which also appear in some other models (e.g., savani and SEMUCB‐WM1, French & Romanowicz, ); and (iii) fast‐SH radial anisotropy anomalies in the ULM in SGLOBE‐rani near subducted slabs (Ferreira et al, ), which also appear in savani and seem consistent with shear‐wave splitting analyses. Given this increasing volume of observations, it is timely to investigate the origin of anisotropy in the mid‐mantle with geodynamic simulations and mantle fabrics calculations, which is the focus of this study.…”

“…It has been reported that bridgmanite has a single‐crystal V S anisotropy of 33% at ambient conditions (Yeganeh‐Haeri, ), but that it decreases to 8% at 1,000‐km depth when using the extrapolated temperature and pressure derivatives of Wentzcovitch et al (). Such estimates of bridgmanite single‐crystal V S anisotropy have been shown to be even larger when considering elastic properties calculated by Zhang et al () (Ferreira et al, ). Ferropericlase, constituting 16% of the lower mantle, remains nearly isotropic at 660‐km depth, but its single‐crystal V S anisotropy increases considerably with pressure, becoming up to 40% in the D” region (Marquardt et al, ).…”

“…The interpretation of radial seismic anisotropy is not straightforward and requires a coordinated effort with other scientific disciplines, such as mineral physics and geodynamics (e.g., Chang & Ferreira, ; Chang et al, ; Ferreira et al, ). For example, LPO calculations depend on temperature, deviatoric stress, dominant creep mechanism, the slip systems of anisotropic minerals, their elastic constants, and water content (see, e.g., Karato et al, ; Mainprice, ).…”

“…3‐D petrological‐thermomechanical flow models of dynamic subduction by Faccenda () explored mid‐mantle anisotropy produced by strain‐induced LPO, indicating that high‐deviatoric stresses associated with subduction deform transition zone and lower mantle aggregates by dislocation creep. This approach has been subsequently used to interpret seismic observations of anisotropy in the ULM (Ferreira et al, ) as well as anomalies associated with a deep plume‐slab interaction (Chang et al, ). The investigation of mantle flow and seismic anisotropy near subducted slabs in the mid‐mantle is particularly difficult because complex 3‐D patterns of mantle convection can develop (e.g., Faccenda & Capitanio, ; Faccenda, ).…”

“…We consider a variety of potential slip systems of bridgmanite and take into account possible contributions of mantle anisotropy from estimates of grain‐scale SPO. Recently, Ferreira et al () used a similar approach to interpret observations of radial anisotropy in the ULM, but here, we examine the whole mid‐mantle. Moreover, we consider a greater range of subduction styles and modeling parameters, notably by introducing a slab penetrating deeply into the lower mantle and by using a variety of possible slip systems.…”