An explanation for the age independence of oceanic elastic thickness estimates from flexural profiles at subduction zones, and implications for continental rheology

- We compare elastic and elastic-plastic plate models for flexure in subduction zones.
- Elastic plate models can underestimate the mechanical thickness of the plate.
- Outer-rise seismicity demonstrates that permanent deformation takes place.
- Age-independence likely results from variations in in-plane force and plate rheology.

Most properties of oceanic lithosphere are widely observed to be dependent on the age of the plate, such as water depth, heat flow, and seismogenic thickness. However, estimates of the 'effective elastic thickness' of oceanic lithosphere based on the deflection of the plate as it enters a subduction zone show little correlation with the age of the incoming lithosphere. This paradox requires reconciliation if we are to gain a full understanding of the structure, rheology, and behaviour of oceanic lithosphere. Here, we show that the permanent deformation of the plate due to outer-rise faulting, combined with uncertainties in the yield stress of the lithosphere, the in-plane forces transmitted through subduction zones, and the levels of noise in bathymetric and gravity data, prevents simple elastic plate modelling from accurately capturing the underlying rheological structure of the incoming plate. The age-independent estimates of effective elastic thickness obtained by purely elastic plate modelling are therefore not likely to represent the true rheology of the plate, and hence are not expected to correspond to the plate age. Similar effects may apply to estimates of elastic thickness from continental forelands, with implications for our understanding of continental rheology.