Bina, C. R., and H. Kawakatsu, Seismological constraints on the thermal structure of the stagnant slab beneath Japan, Abstracts of the 5th Biennial Workshop on Subduction Processes in the Japan-Kurile-Kamchatka-Aleutian Arcs (JKASP-5), Sapporo, Japan, P513, 96, 2006.
Recent images derived from migrated receiver functions yield high-resolution constraints on the lateral depth variations of the 660-km seismic discontinuity beneath Japan, along an apparently stagnant slab as imaged by P-wave tomography. Assuming that this discontinuity topography represents equilibrium deflection of the perovskite-forming transition in ringwoodite, we have converted these depth variations to local thermal anomalies via a Clapeyron slope. The observed topography suggests a large but relatively narrow cold anomaly where the slab first intersects the lower mantle.
To explore the possible origins of such a thermal structure, we have constructed kinematic thermal models of both stagnant and penetrative subducting slabs, along with corresponding models of equilibrium phase relations. Penetrative slabs exhibit deep depression of the rw → pv+mw transition below 660 km, with swift rebound back to 660 km depth. Simple models of a stagnant slab, on the other hand, exhibit moderate depression of the rw → pv+mw transition, with spatially delayed rebound back to 660 km depth. Stagnant slab models more closely resemble the receiver function images for increasingly large stagnation depths, but such large depths are not consistent with tomographic images.
A good fit to both receiver functions and tomography simultaneously is obtained at shallower stagnation depths if the transition is moderately sharp and the stagnant slab initially sags below the horizontal by a few degrees. In this case, a deep and narrow depression of the rw → pv+mw transition is obtained, while the corresponding P-wave velocity anomaly is largely concentrated above 700 km depth. Such a scenario also results in a triplication of the rw → pv+mw transition downstream of the stagnation point, as the geotherm passing through the stagnant slab multiply intersects the equilibrium phase boundary. The bending moments imposed by thermo-petrological buoyancy forces likely contribute to the dynamics of slab stagnation.
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