Continental magmatism, volatile recycling, and a heterogeneous mantle caused by lithospheric gravitational instabilities

Abstract

[1] Removal of the lower lithosphere (mantle lithosphere with or without portions of the crust) through ductile gravitational instabilities can produce magma under continents. Using numerical experiments approximating the rheology of continental crust and lithosphere and underlying asthenosphere and using phase equilibria from the literature, I investigate the topographic and magmatic results of lithospheric gravitational instabilities, along with the fate of the sinking material. Lithospheric removal, commonly referred to as delamination regardless of the mechanism, may allow asthenospheric material to rise and to melt adiabatically, and this asthenosphere can conductively heat portions of the lithosphere previously at lower temperatures. The size and rheology of the sinking material greatly influence the resulting surface topography as well as whether or not melting occurs. The sinking material may devolatilize as it reaches higher temperatures and pressures, just as a subducting slab does, triggering further melting. Gravitational instabilities are possible causes of nonmagmatic basins, continental magmatism of varying volume and composition in the absence of subduction, areas of high heat flow and uplift, and creation of an upper mantle heterogeneous in major and trace elements and volatiles. Magmatism can be simultaneous with topographic subsidence and possibly with subsequent uplift. In those cases that produce magma, melting can occur over a depth range from $30 to 200 km, though anomalously hot mantle is required to reach the volume of flood basalts.

Crustal deformation induced by mantle dynamics: insights from models of gravitational lithosphere removal

“…These results are compatible with earlier studies that investigate the dynamics of weak crust above foundering mantle lithosphere (e.g. Neil & Houseman 1999;Pysklywec & Shahnas 2003;Pysklywec & Beaumont 2004;Elkins-Tanton 2007;Molnar & Houseman 2013;Wang et al 2014;Molnar & Houseman 2015;Wang et al 2015).…”

“…As a result, the effective viscosity of the crust varies with temperature, strain rate, and possibly pressure. Studies that consider a Newtonian crust with a viscosity that decreases with increasing depth (temperature) show that topographic uplift decreases as the viscosity variation in the crust increases (Elkins-Tanton 2007;Molnar & Houseman 2015). Only the lowermost crust is able to flow, which limits the amount of thickening and uplift.…”

“…There have been a few studies that address the dimensional effects using 2-D axisymmetric models (e.g. Hoogenboom & Houseman 2006;Elkins-Tanton 2007) and full 3-D models (e.g. Moresi & Lenardic 1999).…”

“…Neil & Houseman 1999;Pysklywec & Shahnas 2003;Hoogenboom & Houseman 2006) or variable viscosity (e.g. Elkins-Tanton 2007;Gemmer & Houseman 2007;Gögüş & Pysklywec 2008), or a two-layer crust with a variable viscosity (e.g. Wang et al 2014Wang et al , 2015.…”

Yin and yang of continental crust creation and destruction by plate tectonic processes

“…In this circumstance, crustal refinement is accomplished by fractionation and anatectic melting to yield granodioritic upper crust as well as residual and cumulate gabbroic lower crust accompanied by loss of dense, pyroxene-and garnetrich subcrustal material. Foundering can lead to new magmatic episodes and thus crust formation (Gao et al 1998;Bédard 2006;Elkins-Tanton 2007). In spite of the fact that these processes are related, it is useful to separate these processes in order to better articulate them, as done below.…”