### 2006 JKASP Abstract

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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