THERMAL-STRESSES AND THE BIMODAL DISTRIBUTION OF ELASTIC THICKNESS ESTIMATES OF THE OCEANIC LITHOSPHERE

Observations of plate flexure beneath seamounts and at deep-sea trenches suggest that the mechanical strength of the oceanic lithosphere increases with age. The mechanical thickness (H) seems to follow the depth to a particular isotherm in the cooling plate model. A detailed review of available estimates of elastic thickness (T(e)) shows that the strength deduced from studies at seamounts is significantly different from the strength inferred at trenches and fracture zones. Whereas Te beneath seamounts approximately follows the 400-degrees-C isotherm, H at trenches follows the depth to the 700-degrees-800-degrees-C isotherms. The cause of this bimodal distribution is not fully understood. Thermal bending stresses due to cooling of the lithosphere have been suggested to play an important role in the flexural deformation at transform faults and fracture zones, and their distribution with depth is consistent with intraplate seismicity. This paper evaluates the effects of thermal stresses on deformation beneath seamounts and at trenches and how their interaction with flexural stresses may modify plate strength. The results indicate that thermal bending stresses will bias the moment-curvature relationship such that a plate of a certain strength will appear relatively weaker beneath a seamount (positive curvature) but relatively stronger at a trench (negative curvature). This systematic bias appears to be in qualitative agreement with published T(e) estimates. For old lithosphere the theory predicts a diminishing amount of bias. The general lack of sufficient T(e) determinations from old ocean basins and the uncertainties associated with modeling of trenches prohibit a detailed analysis of this part of the T(e) versus age relationship. I conclude that the scatter in the elastic thickness estimates appears to be caused by a combination of lithospheric reheating during seamount emplacement, bias introduced by thermal stresses, and variable amount of yielding of the plate beneath seamounts in proportion to seamount size and lithospheric age at the time of loading.