Searching for Regional Crustal Velocity-Density Relations with the Use of 2-D Gravity Modelling – Central Europe Case

Abstract

In this paper we search for a reference relation between seismic P-wave velocity V and density q ref for the continental crust. Based on the results of modern seismic experiments, we compiled 2-D seismic models into a network of four, each about 1100-1400 km long, continental-scale seismic transects cutting all main tectonic units in Central Europe. The Moho depth (about 52 km beneath the TESZ in SE Poland, to about 25 km beneath the Pannonian Basin) and the crustal structure of this area are characterised by a large variation. This structural variation provides an interesting basis for gravity studies and especially for analysing the difference of the density structure between two major tectonic provinces of distinctive age difference: Precambrian and Phanerozoic. The 2-D gravity modelling applied for crustal cross-sections representing the regional structure, based on a unified gravity anomaly map of the area, allows for a stable determination of some general features of the regional reference velocity-density relation for the continental crust. In general three major seismo-petrological types of rocks can be distinguished: sediments, crystalline crust and mantle. In compacted sediments the reference velocity-density relation is well described by the Gardner or Nafe-Drake model. Calculated gravity anomalies, using unified velocity-density relation for the whole crystalline crust, well describe observed anomalies, with an average difference of 14 mGal. However, calculated gravity anomalies, using separated velocity-density relations for the crystalline crust of Precambrian and Phanerozoic Europe, describe observed anomalies better than for the entire crust, with an average difference 12 mGal. The most important feature of these relations is the large differentiation of the derivative dq ref /dV in the crystalline crust, being about 0.3 g s/m 4 for Precambrian, and about 0.1 g s/m 4 for the Phanerozoic crystalline crust. The modelling suggests a very small density value in the uppermost mantle q = 3.11 g/cm 3 below the younger area, while for the older area it is q = 3.3 g/cm 3 .

Seismic basement in Poland

“…Only these boreholes, which penetrate the crystalline basement into depths of at least several tens of meters, were recognized as representative ones, because of the weathering of the surface of the crystalline basement in ancient times and some other processes leading to a decrease in the density at several meters depths. The top of the Precambrian basement represents the "granitoid type" which consists mostly of gneisses, with an average density of 2.72 g/cm 3 (Krysiński et al 2009). Similar basement density values, c. 2.75 g/cm 3 , were found from 2D gravity modeling along the seismic profile POL-CRUST-01 in SE Poland (Narkiewicz et al 2015).…”

The geophysical characteristic of the lower lithosphere and asthenosphere in the marginal zone of the East European Craton

“…Relationships by Gardner et al (1974) and Brocher (2005) are representative for sedimentary cover and crust. Crystalline crust of the old (Precambrian) platforms and cratons is well-described by relations of Krasovsky (1981), Sobolev and Babeyko (1994) and Krysiński et al (2009). As a comparison, the empirical linear relationship known as Birch's law (for mean atomic weight of rock m = 21 and m = 22) and global data from PREM model are shown (Birch 1961;Dziewonski and Anderson 1981) …”

“…The relationships between density and seismic velocities were investigated using velocity and density measurements under laboratory conditions (e.g., Birch 1961;Krasovsky 1981;Barton 1986;Christensen and Mooney 1995;Schön 1998), as well as in a simultaneous interpretation of seismic experimental data and gravity anomalies (e.g., Kozlovskaya et al 2001Kozlovskaya et al , 2004Krysiński et al 2009). …”

“…The linear relationship the for crystalline Precambrian crust in Central Europe was defined by Krysiński et al (2009): where density ρ is in g/cm 3 , and velocity V p is in km/s. A more complex relationship to convert the velocity to density in the crust was Brocher's empirical formula (Brocher 2005): where ρ is in g/cm 3 , and velocity V p is in km/s.…”

“…For the sediments down to a depth of 6 km, we used Gardner's formula (2) which is widely accepted as being representative for sedimentary cover (Gardner et al 1974;Sheriff and Geldart 1995). For the Precambrian crystalline crust we used a linear relationship (3) defined for Central Europe (Krysiński et al 2009). This relationship describes well the crustal density and is very close to other relations for the old cratons ( Fig.…”