Bina, C. R., Slab mineralogy in deep subduction, Abstracts of the Workshop on Structure and Tectonics of Convergent Plate Margins, Castle of Zahrádky, Czech Republic, 17-18, 2002.
Subducting lithospheric slabs are more than thermomechanical entities; their chemical identities also contribute significantly to subduction-related phenomena. The resulting mineralogical behavior of subducting slabs has important implications for understanding the seismogenic and geodynamic consequences of density variations, the distribution of local thermal anomalies, and the seismic wavespeed signatures associated with subducting slab material.
The temperature gradients within subduction zones are reflected in laterally varying depths of mineralogical phase transitions, whose juxtaposition gives rise to density contrasts. Such density anomalies generate buoyancy forces whose resulting contributions to slab stress fields exhibit striking correlations with patterns of observed deep seismicity. Furthermore, temporal evolution of such buoyancy anomalies has the potential to significantly affect subduction velocities, both in the simple case of equilibrium phase relations and in the more hypothetical case of disequilibrium persistence of metastable phases. In extreme cases, related buoyancy anomalies may affect slab morphology, perhaps in the form of subhorizontal slab deflection.
Laterally varying phase relations should also be expressed in variations in thermal structure within subducting slabs. Latent heats associated with phase changes significantly perturb local geotherms even in the equilibrium case, but such perturbations should be greatly magnified in the case of delayed disequilibrium transitions. The resulting ``local superheating'' may play a role in the types of adiabatic instabilities or thermal runaway processes that have been postulated as seismogenic mechanisms.
The pattern of seismic wavespeeds discerned within subduction zones should be significantly affected by the laterally varying phase relations associated with slab thermal structures. This is particularly the case with regard to upper mantle slab anomalies, where equilibrium phase changes contribute fine structure to the overall fast anomalies expected from thermal effects alone. Any disequilibrium phase relations may also contribute more subtle effects to wavespeed structures. Furthermore, lateral variations in slab phase relations are attended by lateral variations in the properties of associated seismic reflectors, potentially contributing to non-uniformity in the visibility of and detectable topography on seismic ``discontinuities''. Finally, lower mantle wavespeed anomalies and seismic scatterers may, in some cases, manifest chemical contrasts in petrologically layered slab material subject to deep subduction.
Copyright © 2002 Craig R. Bina