Subducted slabs and the geoid: Constraints on mantle rheology and flow

Abstract

Geoid anomalies are primarily the result of the density contrasts driving mantle convection and plate motion. The total geoid anomaly resulting from a given density contrast in a convecting earth is affected by the mass anomalies associated with the flow‐induced deformation of the upper surface and internal compositional boundaries as well as by the density contrast itself. These boundary deformations, and hence the total gravity field, depend on the radial distribution of effective viscosity. If the internal density contrasts can be estimated, as is the case for subducted slabs, useful constraints can be placed on the depth and on the variation of viscosity with depth of the convecting system. The degree 4–9 components of the observed long‐wavelength geoid are highly correlated with those predicted by a density model for seismically active subducted slabs. The (positive) sign of the correlation requires that the effective viscosity increases with depth by a factor of 30 or more. The amplitude of the correlation cannot be explained by the density contrasts associated with just the seismically active parts of subducted slabs, however. The amplitude can be explained if the density contrasts associated with subduction extend into the lower mantle or if old lithosphere is piled up at the base of the upper mantle beneath subduction zones to a thickness in excess of 350 km over horizontal distances of thousands of kilometers. Mantlewide convection in a mantle that has a viscosity increasing with depth provides a simple explanation of the long‐wavelength geoid anomalies over subduction zones.

Mapping convection in the mantle

“…1 and Nakanishi and Anderson, 1984). Hager [1984] found an excellent correlation between the observed geoid and the 'slab geoid' for£= 4, 5 and 7-9 and weak correlation at£= 6. The slab geoid does not explain the geoid lows in Siberia, Canadian shield -western Atlantic, and the South Atlantic or the geoid highs in the south Indian Ocean, the Red Sea, and the North Atlantic.…”

“…Dense regions of the mantle that are in isostatic equilibrium also generate geoid lows [Haxby and Turcotte, 1978;Hager, 1983]. Dense regions in dynamic equilibrium generate geoid lows if the viscosity does not increase too rapidly with depth [Hager, 1984]. High density and high velocity are both consistent with cold mantle.…”

“…These are highvelocity and low-velocity regions, respectively. The slab-geoid correlation can be understood in terms of a large amount of dynamically supported dense material either in the upper mantle or in both the upper and lower mantles in the vicinity of active subduction zones [Hager, 1984]. The surface wave -geoid correlation, which apparently explains the missing £ = 6 component of the slab geoid, however, must be due, at least in part, to an anomalous mass distribution in the shallow mantle.…”

Postglacial variations in the level of the sea: Implications for climate dynamics and solid‐Earth geophysics

“…Section 4 will focus upon solidEarth geophysics, whereas section 5 will consider issues in climate dynamics. the convection timescale constraints provided by observed nonhydrostatic geoid anomalies [Hager, 1984;Richards and Hager, 1984]. This question is clearly related to the issue of whether the rheology of the mantle is Newtonian or non-Newtonian.…”

Lithosphere structure and upper mantle characteristics below the Bay of Bengal

“…3F). Subduction zones in general are characterized by long-wavelength (n=4-9) positive geoid anomalies due to the cold upper mantle slabs, particularly in the case of subduction zones that are associated with the deeper penetration of subducted slabs to the lower mantle, and an effective mantle viscosity change (Hager, 1984;King, 2002). The Andaman-Sumatra arc forms part of the Sunda subduction zone in the eastern Indian Ocean, along which the Indian and Australian plates together subduct below the Eurasian plate.…”