A Bound on Heat Flow Below a Double Crossing of the Perovskite-Postperovskite Phase Transition

A lower mantle S-wave triplication with a S cd branch occurring between S and ScS has been recognized for many years with a variety of explanations. It is particularly strong when sampling regions beneath the circum-Pacific lower mantle fast velocity belt seen in global tomographic models where it has been modeled with a 2-3% jump in S-velocity. General properties are; (1) it tends to arrive earlier beneath the fastest anomalies and (2) its rapid changes in strength and timing relative to S. The recently discovered Perovskite (Pv) to Postperovskite (PPv) phase transition [Murakami et al., 2004] is expected to show this change in timing assuming a positive Clapeyron slope (γ) between 3 to 9 MPa/K, however, the predicted velocity jump is about half of the above 1D modeling results. Here, we model the phase boundary height by mapping S-wave tomography into temperature assuming uniform chemistry, or a Mono-Phase-Transition (MPT), and a more complex mapping procedure involving possible changes in Chemistry (CPT). A few adjustable parameters involving reference phase boundary height and velocity jump are determined from comparing synthetic seismogram predictions with densely sampled observations. Particularly strong S cd data are explained by focusing effects caused by small zones of enhanced velocities (slab buckling) seen in some tomographic models. These sharp features in S-wave tomography are confirmed by the behavior of differential PKP (AB-DF) which shows 4 s changes over a distance of 300 km correlating well with the corresponding S-wave models beneath Central America.