2007 IUGG Abstract

Bina, C. R., Phase relations in stagnant slabs: Seismic velocity structure and bending moments, Abstracts of the XXIV General Assembly of the International Union of Geodesy and Geophysics (IUGG), Perugia, Italy, JSS012-07, 2007.

Deep subhorizontal extensions of subduction - or "stagnant slabs" - constitute unusual thermobaric environments, in that subducted material may gradually undergo nearly isobaric thermal assimilation. The petrological consequences, in terms of equilibrium or metastable phase relations, should affect both apparent seismic velocity structure and slab morphology [e.g., Bina, 2006].

Evidence for slab stagnation may appear in several forms. Seismic extensions of stagnant slabs may appear as shallowing in apparent dip angles of deep seismicity distributions [e.g., Chen et al. 2004]. Aseismic extensions may appear as subhorizontal deflections of fast velocity anomalies in P-wave and S-wave seismic tomography [e.g., Fukao et al., 2001, Suetsugu et al., 2006]. Lateral depth variations along the 660-km seismic discontinuity in migrated receiver functions [e.g., Kawakatsu and Watada, 2005] may be mapped into thermal anomalies by assuming correspondence to equilibrium deflection of the perovskite-forming transition in ringwoodite.

We have constructed kinematic thermal models [cf. Negredo et al., 2004] of stagnant slabs and undertaken thermodynamic modeling [e.g., Fei et al., 1991, Akaogi et al., 2002] of the consequent thermal perturbation of high-pressure phase transitions in mantle minerals. For such models we have estimated seismic velocity anomalies, as might be imaged by seismic tomography, and corresponding seismic velocity gradients, as might be imaged by boundary-interaction phases. We have also calculated associated thermo-petrological buoyancy forces and bending moments which (along with other factors such as viscosity variations and rollback dynamics [Christensen, 2001]) may contribute to slab deformation and morphology. We have considered effects of variations in depth of stagnation, post-stagnation dip angle, phase transition sharpness, and transition triplication due to multiple intersection of stagnant-slab geotherms with equilibrium phase boundaries. Japan is our primary study area.

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