P. Schmoldt, PhD - MTNet - DIAS

10.2. Inversion for mantle structures
Features of the inversion models shown in Figures 10.14 and 10.15 should not be overinterpreted
as they are solely the result of an initial inversion sequence in order to examine
contributions of the two modes. However, through this initial inversion sequence insights
about the characteristics of each modes are already gained. These insights can be used
to guide subsequent isotropic 2D combined-mode inversion steps; namely, (a) that TE
mode data are potentially distorted by crustal or off-profile features and should be downweighted,
and (b) an enhanced starting model based on the TM mode inversion model
suggesting an eLAB in the depth range 100 – 150 km.
Estimation of lithosphere-asthenosphere boundary depth
In its original, rheological, definition the lithosphere–asthenosphere boundary (LAB) denotes
the transition from a rigid to a viscously deforming region within the Earth’s mantle;
i.e. a transition from a mechanical strong to a weak region 3 . The LAB depth is therefore of
major importance for understanding tectonic processes in the study area. Moreover, due to
the strong temperature-dependence of mantle material viscocity, a correlation of the mechanical
lithosphere (or mechanical boundary layer, MBL) and the thermal LAB (tLAB,
also referred to as thermal boundary layer, TBL) has been proposed [e.g. Artemieva,
2009]. The tLAB indicates the change from conductive to convective heat transport and
its depth is commonly defined using an isotherm; with the isotherm value depending on
the choice of the investigator, usually within the range 1200 – 1350°C [e.g. Tejero and
Ruiz, 2002; Artemieva, 2006; Tesauro et al., 2009b]. The thermal estimate of the LAB is
derived through thermal modelling using surface heat flow measurements with a range of
assumptions regarding heat conduction and production within the lithosphere [e.g. Tejero
and Ruiz, 2002] or by deducing thermal conditions from results of other methods like seismology
[e.g. Artemieva, 2006; Tesauro et al., 2009b]. The LAB depth has in most cases a
strong impact on the geologic setting of the regions above, as it significantly affects local
T-P condition.
Owing to its great depth only few methods are capable of probing the LAB location,
namely seismology and magnetotellurics. These methods do not measure rheological
properties directly, but yield LAB estimates in terms of their respective properties under
investigation. Accordingly, these LAB estimates may be referred to as seismic LAB
(sLAB), electric (eLAB). A range of different seismic approaches are used to determine
parameters that are used as indicators for the LAB; i.e. change of seismic anisotropy direction
or magnitude (sLABa), reduction of surface wave velocity (sLABsw), and signals
in receiver functions data (sLABrf). Since these seismic approaches use different parameters
as LAB indicator, depth estimates can vary between the approaches [e.g. Eaton et al.,
2009, and references within] and may even be refer to features that at not related to the
LAB; i.e. variation in magnitude or direction of seismic anisotropy, the top of an older
solidified melt layer, the spinel-garnet transition, or presence of water or carbon dioxide
3 Over time different, and sometimes misguiding, use has been made of the term LAB by different authors
and disciplines; see Section 5.2.2 and the reviews by Eaton et al. [2009] Artemieva [2009] for details.
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