Fine scale heterogeneity in the earth’s crust and mantle

Category Geophysic
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Thybo, Hans
Holding Date 23 September 2008

New high-resolution seismic techniques provide evidence for pronounced fine scale heterogeneity in the Earth’s crust and mantle. Whereas other depth intervals appear transparent in the frequency band of 0.5-15 Hz, fine scale heterogeneity has mainly been demonstrated in four distinct depth intervals:
(1) The Mantle Low-Velocity Zone (LVZ) below a depth of 100±20 km is globally observed from a pronounced seismic coda, which shows that the zone is highly heterogeneous at characteristic scale lengths of 5-15 by 2-6 km. We interpret that the rocks in the LVZ have a temperature close to the solidus or even may contain small fractions of partial melt.
(2) Significant scattering from around the top of the Mantle Transition Zone indicates the presence of pronounced heterogeneity at scale lengths of 8-20 by 3-8 km in the depth range of 320-450 km. This observation probably requires significant chemical heterogeneity.
(3) The deepest ever controlled-source seismic reflections from above the Core-Mantle Boundary image a heterogeneous zone, which we ascribe to a very high percentage of partial melts.
(4) Multiple underside reflections from lower crustal heterogeneity fully explain the enigmatic Teleseismic Pn Wave, which cannot be ascribed to the uppermost mantle as previously interpreted by other authors.
The scale lengths and velocity contrasts of the mantle heterogeneity are statistically represented in our 2D Finite Difference simulations of seismic wave propagation. This technique does not allow direct detection of structure, but the heterogeneous structure of zones in the mantle is now well demonstrated, probably caused by different petrologic and thermal processes.