Role of temperature-dependent viscosity and surface plates in spherical shell models of mantle convection

Layered viscosity, temperature-dependent viscosity, and surface plates have an important effect on the scale and morphology of structure in spherical models of mantle convection. We find that long-wavelength structures can be produced either by a layered viscosity with a weak upper mantle or tempera ture-dependent viscosity even in the absence of surface plates, corroborati ng earlier studies, However,combining the layered viscosity structure with a temperature-dependent viscosity results in structure with significantly s horter wavelengths. Our models show that the scale of convection is mainly controlled by the surface plates, supporting the previous two-dimensional s tudies. Our models with surface plates: layered and temperature-dependent v iscosity, and internal heating explain mantle structures inferred from seis mic tomography. The models show that hot upwellings initiate at the core-ma ntle boundary (CMB) with linear structures, and as they depart from CMB, th e linear upwellings quickly change into quasi-cylindrical plumes that dynam ically interact with the ambient mantle and surface plates while ascending through the mantle. A linear upwelling structure is generated again at shal low depths (<200 km) in the vicinity of diverging plate margins because of the surface plates. At shallow depths, cold downwelling sheets form at conv erging plate margins. The evolution of downwelling sheets depends on the ma ntle rheology. The temperature-dependent viscosity strengthens the downwell ing sheets so that the sheet structure can be maintained throughout the man tle. The tendency for linear upwelling and downwelling structures to break into plume-like structures is stronger at higher Rayleigh numbers. Our mode ls also show that downwellings tp first-order control surface plate motions and the locations and horizontal motion of upwellings. Upwellings tend to form at stagnation points predicted solely from the buoyancy forces of down wellings. Temperature-dependent viscosity greatly enhances tb: ascending ve locity of developed upwelling plumes, and this may reduce the influence of global mantle flow on the motion of plumes. Our results can explain the ant icorrelation between hotspot distribution and fast seismic wave speed anoma lies in the lower mantle and may also have significant implications to the observed stationarity of hotspots.