The mantle wedge's transient 3‐D flow regime and thermal structure

Abstract

Arc volcanism, volatile cycling, mineralization, and continental crust formation are likely regulated by the mantle wedge's flow regime and thermal structure. Wedge flow is often assumed to follow a regular corner-flow pattern. However, studies that incorporate a hydrated rheology and thermal buoyancy predict internal small-scale-convection (SSC). Here, we systematically explore mantle-wedge dynamics in 3-D simulations. We find that longitudinal ''Richter-rolls'' of SSC (with trench-perpendicular axes) commonly occur if wedge hydration reduces viscosities to Շ1 Á 10 The modulating influence of subback-arc ridges on the subarc system increases with stronger wedge hydration, higher subduction velocity, and thicker upper plates. We find that trenchparallel averages of wedge velocities and temperature are consistent with those predicted in 2-D models. However, lithospheric thinning through SSC is somewhat enhanced in 3-D, thus expanding hydrous melting regions and shifting dehydration boundaries. Subarc Richter-rolls generate time-dependent trench-parallel temperature variations of up to $150 K, which exceed the transient 50-100 K variations predicted in 2-D and may contribute to arc-volcano spacing and the variable seismic velocity structures imaged beneath some arcs.

Is uplift of volcano clusters in the Tohoku Volcanic Arc, Japan, driven by magma accumulation in hot zones? A geodynamic modeling study

“…Assuming that the magma source, i.e., the hot zone, must remain stable at least an equivalent time period, all numerical models in this study are integrated for 1 Myr. This time frame also coincides with the minimum stable period of SSCs modeled by Davies et al [].…”

“…If magmatism, underplating, and dynamic topography are strongly coupled with the long‐lived mantle instability that gives rise to the asthenospheric hot fingers, the current observations should be stable for time frames of 1 Myr or more. In general, in the absence of particularly “fast” phenomena, such as slab retreat or slab tear, the timescale to change the dynamics of the mantle is on the order of several million years [ Honda and Saito , ; Honda et al , ; Davies et al , ]. Therefore, based on their numerical simulations [ Honda and Saito , ; Honda et al , ; Davies et al , ], we expect the “hot finger” suggested by Tamura et al [] to be a stable feature at least on the timescale of 1 Myr.…”

“…Thus, volcanism in the TVA is characterized by trench‐perpendicular variations in magma productivity, as explained by 2‐D subduction zone models [ Pearce and Peate , ], and substantial trench‐parallel variations in magma productivity, thought to originate from the complexity of circulation in the asthenospheric wedge, the development of hot fingers, and the increased flux of magma into the crust that results. Several 2‐D and 3‐D numerical simulations of mantle wedge flow dynamics have revealed the development of small‐scale convection cells (SSC) within the mantle wedge that share many similarities to the proposed “hot fingers” in the TVA [ Honda and Saito , ; Honda et al , ; Davies et al , ]. The simulated cells were found to develop in models of hydrated mantle as gravitational instabilities at the base of the overriding plate; longitudinal SSCs were parallel to the direction of plate motion (i.e., their axes are perpendicular to the trench) and were of similar extents to the proposed hot fingers (100 km long and at least 50 km wide).…”

Water Migration in the Subduction Mantle Wedge: A Two‐Phase Flow Approach

“…As the melts have a much larger shear viscosity (~10 5 ) than water, the compaction length of melt migration is much smaller, explaining why the fluid pathways of the melts may be less prone to being diverted by the high‐pressure zone. The amplitude of the V s anomalies shown in Figure varies along strike (see Halpaap et al, ), reflecting varying water content between the northern and southern subducting slab and the potential effects of 3‐D dynamics (see, e.g., Davies et al, ). Similar observations are made in the Andean subduction zone (Schurr et al, ).…”

Lithospheric Thinning by Mantle Plumes

“…Study the effect, in 2-D and 3-D, of the morphology and onset time of SSC produced from shearing under a moving plate first without, then with, a plume. The behaviour of SSC in 2-D and 3-D is likely to be distinctive: longitudinal roll ('Richter rolls') structures, with their axes aligned parallel to plate motion are favoured in 3-D, whilst transverse roll structures, with their axis perpendicular to plate motion, are the only instabilities that can be simulated in 2-D Agrusta et al 2013;Davies et al, 2016). Also, faster onset time of SSC is expected in 3-D in comparison to 2-D.…”