Abstract
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.
Highlights
Constraining the mechanical properties of the lithosphere is important for understanding how it supports and transmits stress, and the controls on the location and characteristics of deformation
Many of the observable properties of oceanic lithosphere show a general dependence on the age of the lithosphere, and are likely to be related to the thermal structure of the plate
We study the influence of variations in the in-plane force transmitted through the downgoing plate on the recovered elastic thickness
Summary
Constraining the mechanical properties of the lithosphere is important for understanding how it supports and transmits stress, and the controls on the location and characteristics of deformation. Numerous studies have tried to estimate the ‘effective elastic thickness’ (Te ) of oceanic lithosphere (the thickness of the layer within the plate capable of supporting elastic stresses over geological timescales – hereafter referred to as the elastic thickness) from the deflection of the incoming oceanic plate into subduction zones, as observed in either bathymetric or gravitational data Many of the observable properties of oceanic lithosphere show a general dependence on the age of the lithosphere, and are likely to be related to the thermal structure of the plate. The seismogenic thickness is expected to be a similar indicator of the mechanical strength of the plate to the elastic thickness (at least in terms of general trends), as both are strongly dependent on the thermal structure of the lithosphere, regardless of where within the lithosphere the plate-driving forces are supported. Given the direct correspondence seen between plate age and seismogenic thickness (Wiens and Stein, 1983; McKenzie et al, 2005; Craig et al, 2014), a similar age dependence might be expected from estimates of the elastic thickness
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