Lithospheric-age control on the migrations of oceanic convergent margins

Abstract

Global compilations have revealed a wide range of trench margin motions, from advancing to retreating, a complexity for which there is as yet no single dynamic explanation. While the buoyancy force of the sinking slabs in the mantle can solely account for retreating trench motions, additional forces are required to explain trench advance. Here, the role of gravitational sliding of oceanic lithosphere from the ridge rise to the trench, which is in opposition to the hinge retreat, is considered. Because lithospheric ageing and thickening have a first-order control on the driving forces of slab buoyancy and ridge push, controls on trench migrations should be age-dependent. A force balance model is used to show that gravitational sliding remains smaller than hinge retreat for a young (<30–40M.years) trench; with predicted trench motions in slow retreat due to the low buoyancy of young lithosphere. Only when older lithosphere is subducted does gravitational sliding become larger than hinge retreat, opposing the natural tendency of the hinge to rollback, and trench advance occurs. This model can explain the range of observed trench motions, from advance to retreat, and their anti-correlation with trench age along most of the convergent margins.