Geoid and topography over subduction zones: The effect of phase transformations

Abstract

[1] The association between local maxima in the geoid and subduction zones is examined. While it is well known that subduction zones are associated with broad local maxima in the geoid at spherical harmonic degrees 4-9, there is an impressive correlation between back arc geoid maxima at the 5000 km length scale (i.e., spherical harmonic degrees >9) and subduction zones with the exception of the Scotia and Caribbean arcs. Geoid maxima are also observed in the forearc regions of the Aleutian and Central American subduction zones. Numerical modeling of compressible convection with multiple phase transformations indicates that an increase in viscosity at or near the 660 km phase transformation is necessary to explain the pattern of geoid maxima associated with subduction zones. The addition of an endothermic phase transformation at 660 km depth does not provide sufficient resistance to the slab to eliminate the need for a viscosity increase at or near 660 km. Short-wavelength geoid and topography profiles might provide an additional constraint on the deformation at subduction zones; however, it may be necessary to incorporate geologic history into subduction calculations to further improve the results presented here.

Subduction-transition zone interaction: A review

“…An increase in viscosity at the base of the transition zone by a factor 20-50 provides sufficient resistance to sinking to induce slab buckling and/or trench retreat. Such an increase in viscosity also explains the dominance of downdip compression below 300 km depth seen in Benioff seismicity globally (Vassiliou and Hager, 1988;Alpert et al, 2010), as well as the local geoid signature around subduction zones (Chen and King, 1998;King, 2002; Goes et al | Subduction-transition zone review GEOSPHERE | Volume 13 | Number 3 Tosi et al, 2009). It does not, however, sufficiently stagnate slabs to accumulate flat slabs of up to 2000 km long (Garel et al, 2014).…”

“…Furthermore, experiments show that bridgmanite, which comprises ~80% of the lower-mantle composition, is a relatively strong phase (Yamazaki and Karato, 2001). Additionally, geoid modeling of the signature above subduction zones indicates that the resistive effect of an endothermic phase transition has a low signal and an increase in viscosity at the boundary, by a factor 30-100, is required to explain smaller-scale geoid structure (King, 2002;Tosi et al, 2009), where it should be borne in mind that dynamically, a strong viscosity-depth gradient will have a similar effect as a sharp jump.…”

“…Models predict significant buckling and thereby thickening when the viscosity contrast encountered by the slabs at the base of the transition zone is at least a factor of 20 or 30 (Gurnis and Hager, 1988;Gaherty and Hager, 1994;King, 2002;Běhounková and Čížková, 2008;Lee and King, 2011;Čížková and Bina, 2013) The increase in slab pull due to a shallowing olivine to wadsleyite phase transition in the slab ~400 km depth may enhance buckling (Běhounková and Čížková, 2008). Finally, some amount of transition-zone slab buckling is consistent with focal mechanisms (e.g., Myhill, 2012), as well as with high seismic strain rates inferred for the Tonga slab (Giardini and Woodhouse, 1984;Bevis, 1988;Holt, 1995).…”

Geophysical implications of Izu–Bonin mantle wedge hydration from chemical geodynamic modeling

“…The basalt to eclogite transition within the slab is accounted for by a 6% density increase within the AOC layer of the down‐going plate. Despite evidence that including the effects of phase transformations near the transition zone in the calculation of the dynamic topography and the geoid is insignificant considering the overwhelming effect of the radial viscosity structure (King 2002), we include these effects as part of a comprehensive approach.…”

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.…”