Implications of oceanic intraplate seismicity for plate stresses, driving forces and rheology

Recent studies of earthquakes occurring within the oceanic lithosphere provide valuable information about stresses in the lithosphere, plate tectonic driving forces, and the rheology of the lithosphere, asthenosphere and mantle. Focal mechanisms indicate that oceanic lithosphere older than 35 million years is almost entirely in deviatoric compression. Both compressional and extensional events occur in younger lithosphere. In young lithosphere mechanisms generally indicate compressive stress in the spreading direction or extension oblique to the spreading direction; extension in the spreading direction is not observed. Using the constraint of compression in the spreading direction in old lithosphere, models of the stresses produced by a combination of ridge push and basal drag forces require the magnitude of the drag be less than a few bars for rapidly moving plates and a few tens of bars for slow moving plates. Assuming that basal drag results from mantle return flow, the upper limits on drag can be converted to constraints on the viscosity structure of the asthenosphere and mantle using simple two-dimensional models. If return flow occurs in a mantle of viscosity 10²² poise, comparable to the results from glacial rebound studies, the predicted basal drag is too high unless a thin asthenosphere with viscosity less than 10¹⁹ to 10²⁰ poise (depending on flow depth) is present. The intraplate stress data thus are consistent with the idea of oceanic plates largely decoupled from the underlying mantle. The strength of the lithosphere is constrained by the maximum depth of oceanic intraplate seismicity, which increases with lithospheric age and appears to be bounded by a 700°-800°C isotherm. This limiting depth is approximately equal to the flexural elastic thickness of the lithosphere and is consistent with experimental olivine rheologies which predict rapid weakening at high temperatures. Similar phenomena are important for estimating the fraction of seismic and aseismic slip on transform faults and in determining the extent of rupture for large trench normal faulting events.