UC Berkeley
Email:
gung@seismo.berkeley.edu
poster/oral: poster
| We present a degree 8 model of attenuation in the upper mantle ,QRLW8, which is obtained by iterative inversion of three component long period seismic waveforms for elastic and anelastic structure. The main features in QRLW8 can be divided into two depth ranges. The Q distribution above 250 km is generally tectonic-related, which is similar to what is observed in elastic velocity models, with regions of high/low velocity correlated to regions of high/low Q. A notable exception is an elongated zone of low Q in the central Pacific, extending from the south of the equator to Hawaii. Below 250 km, the tectonic-related pattern is gradually replaced by a pattern well correlated with the Vs velocity distribution in the lowermost mantle, with two strong low Q minima centered in the southern Pacific and under Africa. Most surface hot spot positions are above the low Q areas in the upper mantle transition zone. From the depth correlation and cross-section comparison of QRLW8 and velocity models, we observe a striking quasi-vertical correspondence between the upwellings in the lowermost mantle and the low Q structure in the upper mantle transition zone. When they reach the lithosphere, these hot currents deflect horizontally, towards the mid-ocean ridges, which are underlain by shallow-rooted low Q zones. This has implications for the heat flux calculations, which may significantly underestimate the contribution from the bottom of the mantle and the core, as well as on the geochemical signature of ocean island basalt, which may result from mixing of a lower mantle source with progressively more upper mantle material further from the center of the upwellings. |