Download Lithological Moho boundary in Precambrian shields

Lithological Moho boundary in Precambrian shields: constraints from
P- and S-wave velocity models and gravity modelling
E. Kozlovskaya (1), J. Yliniemi (1), T. Janik (2) M. Grad (3), T.Tiira (4), G.
Karatayev (5)
(1) Sodankylä Geophysical Observatory/Oulu Unit, POB 3000, FIN-90014, University of
Oulu, Finland ([email protected]),
(2) Institute of Geophysics, Polish Academy of Sciences, Ks. Janusza 64, 01-452
Warsaw, Poland,
(3) Institute of Geophysics, University of Warsaw, Pasteura 7, 02-093, Warsaw, Poland,
(4) Insitute of Seismology, POB 68, FIN-90014, University of Helsinki, Finland,
(5) Institute of Geological Sciences, Zhodinskaya str. 7, Minsk, 220141, Belarus
Controlled source seismic experiments in the Fennoscandian Shield and in the western
part of the East European Craton (EEC) demonstrated that the Moho boundary is not
always easily detectable by the methods based upon interpretation of P-waves (e.g.
reflection profiling and wide-angle reflection and refraction experiments). Generally, the
Moho appears to be reflective in the areas where the depth to the Moho is of about 38-45
km, while in the areas of deeper Moho (>50 km) it appears as a weakly reflective
boundary or transition zone. In this case the position of the lithological crust-mantle
boundary cannot be easily estimated. However, in shield areas the quality of S-waves in
wide-angle reflection and refraction data is comparable with that of P-waves due to
absence of thick sediments. This opens possibility for developing independent S-wave
velocity models of the crust and estimating the Vp/Vs ratio in various crustal layers and
geological units. Analysis of S-wave recordings of several wide-angle profiles in the
Fennoscandian and Ukrainian Shields showed that in contrast to P-waves, the recordings
of S-waves demonstrate clear reflections from the Moho boundary (SmS), both in the
areas of thick and thin crust (Fig. 1 and Fig. 2) (Luosto et al., 1990, Yliniemi et al., 1996,
FENNIA Working Group, 1998, Thybo et al., 2003). The reflections can be explained by
strong contrast of S-wave velocity at the Moho, which agrees with the results obtained by
teleseismic receiver function studies based on analysis of P- to SV conversions (Alinaghi
et al., 2003). This suggests that the Moho obtained from S-waves indicates the presentday position of the lithological crust-mantle boundary. The depth to this boundary varies
in a wide range (e.g. 38-65 km), indicating that the present-day crust-mantle boundary
was formed at different time by a variety of tectonic processes.
In addition, the S-wave velocity models can be used to calculate density models of the
crust. For this purpose we used a method of gravity data inversion, in which the density
model is parameterised by the relationship connecting density to both P- and S-wave
velocity models. Such a parameterisation makes it possible to obtain not only the density
model, but also the relationship between density and seismic velocities. Using this
technique, we obtained and analysed relationships between density and seismic velocities
(Vp and Vs) for selected geological units of the Fennoscandian and Ukrainian Shields.
Generally, all these relationships are close to linear. However, they are scattered, differ
from each other and deviate from the corresponding relationships for anhydrous
magmatic rocks with averaged chemical composition selected as reference curves
(Sobolev and Babeyko, 1994). Comparison of these relationships to the petrophysical
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data (Markwick and Downes, 2000, Markwik et al., 2001) suggests that deviations of
density-velocity curved from reference density-Vp and density-Vs curves can be
explained by specific mineral composition of rocks, resulting from different age and
conditions of crust formation. Therefore, the analysis of density-velocity diagrams can be
used to restrict the composition of the crust and, in particular, the composition and
metamorphic grade of the lower crust. Thus, if the density-velocity relationships are close
to the reference curve in the range of velocities corresponding to the lower crust and the
density values are less than 3.0 g/cm3, it may indicate that the lower crust is composed of
rocks of mafic composition and granulite metamorphic grade. The density-Vp
relationship shifted to the higher velocities from the reference curves indicates the high
plagioclase content and absence of amphibole, which is typical for igneous rocks. On the
contrary, the shift from the reference curve to the lower velocity values may indicate the
high content of amphibolite facies rocks. The density values in excess of 3.0 g/cm3
indicate that the lower crust was metamorphosed under high pressure conditions and
contains high propotion of garnet-bearing rocks, i.e. mafic garnet granulites and/or
eclogites.
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Fig. 1. Record section of SP A of BALTIC profile, demonstrating clear PmP and SmS
phases.
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Fig. 2. Record section of SP F of FENNIA profile, demonstrating weak PmP reflection
from the Moho boundary, but very clear SmS reflection from the Moho.
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