Thinning of continental backarc lithosphere by flow-induced gravitational instability

Many continental backarcs have thin (∼ 60 km) lithosphere for 100s of km behind the volcanic arc, even where there has been no significant extension. One mechanism to produce thin backarc lithosphere is through gravitational thinning of normal thickness lithosphere by subduction-related mantle flow. The stability of backarc mantle lithosphere is examined using thermal-mechanical models of subduction of an oceanic plate beneath continental lithosphere with an initial thickness of 120 km and a thermal structure similar to average Phanerozoic continental lithosphere. Subduction-induced mantle flow shears the base of the backarc lithosphere, producing lateral density perturbations. Owing to the non-Newtonian lithosphere rheology, shearing also reduces the effective viscosity of the lowermost lithosphere, enabling the density perturbations to become gravitationally unstable. The perturbations develop into rapidly-growing downwellings which result in removal of lower backarc lithosphere on timescales of 5-10 Ma. Conductive heating and shearing of the remaining lithosphere results in a second, more muted, phase of gravitational instability and thinning. Lower lithosphere instability is enhanced by higher subduction rates, weaker intrinsic rheology, higher compositional density, and hotter initial thermal structure. The numerical model results are in good agreement with a buoyancy stability analysis, which includes the vertical gradients in temperature and strain rate through the lithosphere. As both rheology and density depend on lithosphere composition, significant thinning may be restricted to continental mantle lithosphere that is fertile and contains a small amount of water. To produce a final thickness comparable to that observed without lithospheric contraction or extension, it is necessary to have a weak lithosphere rheology, an initial thermal structure hotter than average Phanerozoic continental geotherms, or a combination of the two. The region of thin lithosphere at the northern Cascadia backarc coincides with terranes that were accreted to the North American craton. The known fertile composition of the backarc lithosphere may allow it to be thinned, while the drier, more refractory craton lithosphere is resistant to thinning.