Download What is the lithosphere-asthenosphere boundary – a

Solid Earth - Basic Science for the Human Habitat. - ILP’s Second Potsdam Conference 2010
What is the lithosphere-asthenosphere boundary – a quest for information
by
U. Achauer
The lithosphere-asthenosphere boundary (LAB) is the most extensive and active plate boundary on the
Earth. It is inextricably linked to the properties of the underlying low velocity zone, which is of key
importance to the architecture of continental and oceanic lithosphere and to the dynamics of plate
tectonics. However, the LAB beneath the continents is relatively cryptic compared to other first-order
structural subdivisions of Earth. Some aspects of the boundary are attributable to changes in physical
properties along the geotherm, but new results suggest the possible influence of small amounts of
melt, variations in hydration of nominally anhydrous minerals, grain size or in lattice preferred
orientations. Though we face different physical definitions of the LAB in dependence on methods used
to map the boundary, a general understanding “WHAT is the LAB is still missing”.
From the recent DefLAB workshop it may be concluded that while a lithosphere-asthenosphere
boundary is seen by most disciplines in Earth Sciences, the definition, the depth and what it means
from the structural, the rheological and physico-chemical point of view is highly debated. There
seem to be several “boundaries”, namely the LAB-S (seismological), the LAB-M (mechanical), the
LAB-T (thermal), the LAB-C (chemical) and the LAB-E (electromagnetic), all called by the
colleagues from the particular fields in Earth Science “LAB” which differ in depth and thickness
(i.e. whether they are discrete or more like a transitional layer) and most likely will not “define” at
all the same thing!
It is evident that only a multi-disciplinary approach, bringing together all disciplines from Earth
Sciences will help us to shed light on the above question and to better understand and communicate
between the different fields in Earth Sciences, what the lithosphere-asthenosphere boundary is all
about, what it’s origin is and what role it has played and still plays in the evolution of our planet.
In this presentation we shall highlight some new results as well as discuss the differences between
the different disciplines.