### 1997 Fall AGU Abstract (Marton)

Marton, F. C., C. R. Bina, S. Stein, and D. C. Rubie,
Are all Phi created equal?,
*Eos, Transactions of the American Geophysical Union,
78, Fall Supplement*, F662, 1997.

**T22B-11**

A classic parameterization of subduction zone thermal
structure is that for simple thermal models the depth
to which isotherms penetrate should vary with the
thermal parameter Phi: the product of age
of the subducting lithosphere at the trench,
convergence rate, and dip angle.
Observations of subduction zones show that
although the depth of the deepest seismicity
increases with Phi, it is not a continuous function
of Phi. The deepest earthquakes jump from about 300 km
to >600 km at Phi ~ 5000 km.
In general terms, modeling of subducting slabs suggests
that metastable wedges of olivine, which may provide
conditions necessary for deep seismicity, should grow
increasingly larger and deeper as a function of Phi.
Given the observed complex variation of seismicity with depth,
it is natural to explore further how the thermal and
mineralogical structures of slabs vary with changes
in the constituents of Phi rather than simply with
Phi itself.

We examine the temperature fields of a set of model
subducting slabs of constant Phi and age, but we vary
the convergence rate and dip of the slab, both with and
without the latent heat of transformation of
alpha (olivine) to beta (wadsleyite) or
gamma (ringwoodite), to analyze the roles of these variables.
These models are then compared to the distribution of
seismicity for subduction zones with comparable thermal
parameters. We also examine the shape and size of the
predicted metastable wedge as a function not only of the components
of Phi but also of the uncertainties in both the temperature
models and the kinetic rate laws that govern the olivine
transformations.

Copyright © 1997 American Geophysical Union

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