Electrical and seismic estimates of the depth to the LAB

Alan G. Jones, Jaroslava Plomerova and Toivo Korja

The lithosphere is a rigid mechanical boundary layer at the Earth's surface. It is underlain by a weak layer (the asthenosphere) characterized by pervasive plastic deformation on time scales of tens of thousands of years. The rheological concepts of lithosphere and asthenosphere originated from studies of post-glacial rebound (Barrell, 1914), but have since been incorporated into the plate-tectonic paradigm as fundamental components. In this paradigm, the lithosphere is composed of discrete plates and the lithosphere-asthenosphere boundary (LAB) separates each plate from the underlying convecting mantle. The LAB thus comprises the most extensive type of plate boundary on the planet.

Although the LAB is an active plate boundary, beneath the continents it is relatively cryptic compared to other first-order structural subdivisions of Earth. Specimens of the LAB, in the form of xenoliths and xenocrysts brought to the surface, are sparsely and unevenly distributed. Furthermore, since the differential motion between lithosphere and asthenosphere is accommodated aseismically, i.e., no earthquakes, there is no seismological technique to map the LAB interface directly. These limitations have spawned many indirect proxies for the LAB that depend on the type of measurement made. These proxies are derived from disparate types of observations, including petrologic, geochemical, thermal, seismic-velocity, seismic-anisotropy and electrical-conductivity characteristics of the regions directly above and below the LAB. Thus, we need to use all of our geoscientific tools in a co-ordinated manner focused on definition of the LAB if we are to have a hope of understanding this boundary and its spatial and temporal variation.

It must be kept in mind that the original definition of the lithosphere is mechanical, not seismological or electrical. It still remains to be shown whether the seismological, electrical and the mechanical lithospheres are identical. This can only be evaluated if sufficient information about the topography of the LAB is available. Estimates of the nature of, and depth to, the LAB from seismological and electromagetic techniques yield information of the LAB of today. Such information is valuable for providing input to geodynamic models.

This paper will review the current state of knowledge of the depth to the LAB from seismological and electromagnetic methods, focussing on results from central Europe.