Buoyancy rather than rheology controls the thickness of the overriding mechanical lithosphere at subduction zones

Abstract

Abstract. The thickness of Earth's mechanical lithosphere is poorly defined. To investigate whether rheology controls the thickness of the overriding plate's mechanical lithosphere in subduction zones, the thermal structure was modelled numerically assuming a temperature dependent mantle viscosity. It was found that the overriding lithosphere was ablated such that very high temperatures reached close to the surface near the apex of the wedge comer, leading to unrealistiCally high heat flow. Since temperature dependent rheology clearly does not control the thickness of the mechanical lithosphere, we suggest that it is instead controlled by buoyancy. The source of buoyancy we assume is compositional, e.g. buoyant crust. Two end-member models with crustal thickness of 10 and 70 km respectively were then undertaken, these had lower heat flow. This work sUppOrts the assumption of some earlier workers (e.g. Plank and Langmuir, 1988) who equated the mechanical lithosphere with the crust of the overriding plate.

On the decompression melting structure at volcanic arcs and back‐arc spreading centers

“…Different formulations have been used in variable‐viscosity numerical models to decouple the overriding and subducting plates, with differing results. Variable‐viscosity models that assume the two plates are completely viscously coupled show a dramatic amount of ablation of the overriding plate, with hot, upwelling asthenosphere kept from reaching the surface only because of the assigned boundary conditions, leading to dramatic over‐predictions of heat flow in the forearc and arc [ Rowland and Davies , 1999; Eberle et al , 2002]. Dynamic models often incorporate weak (low‐viscosity) nodes at the subduction‐overriding plate interface to keep the two plates from simply thickening, or underplating each other [e.g., Kincaid and Sacks , 1997].…”

Depths and temperatures of <10.5 Ma mantle melting and the lithosphere‐asthenosphere boundary below southern Oregon and northern California

“…Their mantle models incorporating temperature‐dependent viscosity and widely accepted values for activation energy and asthenospheric viscosity are able to produce temperature‐depth relationships consistent with the petrologic estimates, including those from this study. Similarly, the geodynamic models of Rowland and Davies [] find that mechanical lithosphere thickness at subduction zones is controlled by compositional buoyancy and therefore closely related to the thickness of the crust. Plank and Langmuir [] also find a correlation between crustal thickness and the major element composition of parental magma at subduction zones and suggest that this observation can be explained if crustal thickness controls the temperature and depth of asthenospheric melting, as observed here.…”

Lithosphere erosion and crustal growth in subduction zones: Insights from initiation of the nascent East Philippine Arc

“…In an attempt to place further constraints on the thickness of EPA crust, which in mature arcs is believed to equate with the thickness of the lithosphere (Rowland and Davies, 1999), ∆(Dy/Yb)/∆SiO 2 is compared to Na 6.0 , which has been noted to correlate positively with 139 crustal thickness (Plank and Langmuir, 1988). Na 6.0 is the Na 2 O content that would have been present in a melt containing 6 wt.% MgO.…”

“…The scarce and valuable insights available from real margins have been complemented by increasingly sophisticated numerical models of where and how initiation occurs. One of the clearest predictions of most subduction initiation models is large scale, and possibly rapid, thinning of the overriding plate during the earliest stages of subduction (Andrews and Sleep, 1974;Hall et al, 2003;Gurnis et al, 2004;Arcay et al, 2006) to produce mature margins in which arc lithosphere consists largely of largely of crust (Rowland and Davies, 1999). Testing this prediction and determining timescales for the processes involved is difficult because much of the geology in fossilized nascent margins has been obscured by subsequent plate motions and volcanism.…”