Upper Mantle Discontinuity Topography from Thermal and Chemical Heterogeneity
Nicholas Schmerr* and
Edward J. Garnero
Using high-resolution stacks of precursors to the seismic phase
SS, we investigated seismic discontinuities associated with
mineralogical phase changes approximately 410 and 660 kilometers
(km) deep within Earth beneath South America and the surrounding
oceans. Detailed maps of phase boundary topography revealed
deep 410- and 660-km discontinuities in the down-dip direction
of subduction, inconsistent with purely isochemical olivine
phase transformation in response to lowered temperatures. Mechanisms
invoking chemical heterogeneity within the mantle transition
zone were explored to explain this feature. In some regions,
multiple reflections from the discontinuities were detected,
consistent with partial melt near 410-km depth and/or additional
phase changes near 660-km depth. Thus, the origin of upper mantle
heterogeneity has both chemical and thermal contributions and
is associated with deeply rooted tectonic processes.
* To whom correspondence should be addressed. E-mail: nschmer{at}asu.edu
