Seismic, Structural and Petrological Models of the Subcrustal Lithosphere in Southern Germany: A Quantitative Revaluation

Abstract

Anisotropy in the subcontinental lithosphere becomes increasingly important, because it is observed in many seismic studies especially for P n -waves. Typical rocks of the uppermost mantle are peridotites, which predominantly exhibit a pronounced elastic anisotropy. This anisotropy is mainly caused by the anisotropic elastic properties and the lattice preferred orientation (here referred to as texture) of olivine. To evaluate the elastic anisotropy of peridotites from the subcontinental lithosphere, specimens of the Northern Hessian Depression (Germany) and the Balmuccia Ultramafic Massif (Northern Italy) have been used. They comprise four olivine texture types, which are characteristic for olivine textures observed worldwide. The bulk rock elastic properties have been calculated using olivine and orthopyroxene textures, their single-crystal elastic constants at ambient pressure/temperature conditions and their volume fraction. Clinopyroxene and spinel are assumed to be randomly distributed. The effect of four different orientations of the foliation within the uppermost mantle has been evaluated, since this orientation is usually unknown.Two of the olivine textures have a pronounced azimuthal dependence of compressional waves when a horizontal foliation within the uppermost mantle is presumed. These variations cause significant azimuthal variations of the P-wave reflections coefficients at the Moho. Primarily, we predict a significant azimuthal dependence of the critical points where the reflected amplitude increases from approximately 15% to 95%. Possibly, these azimuthal variations can be detected by seismic reflection measurements carried out at earth surface.The remaining two texture types only manifest a small directional dependence. When anisotropy of compressional waves is observed in seismic studies, these latter types can only be of subordinate importance. However, all of the peridotites investigated are able to explain the seismically observed azimuthal variations of compressional waves when a vertical foliation is proposed. This ambiguity can be substantially reduced when shear waves (S-waves) are considered. The directional distribution of S-wave velocities and of the S-wave splitting exhibits characteristic patterns for the different olivine texture types. This could be used to discriminate between different texture types and orientations of the foliation within the uppermost mantle. A fundamental requirement for a more comprehensive interpretation is the availability of detailed S-wave observations. The maximum S-wave splitting in the peridotites investigated coincides with the maximum of the faster (leading) S-wave. This may be of importance to detect S-wave splitting in future seismic studies.

Evidence for preservation of crustal root beneath the Proterozoic Lapland‐Kola orogen (northern Fennoscandian shield) derived from P and S wave velocity models of POLAR and HUKKA wide‐angle reflection and refraction profiles and FIRE4 reflection transect

“…However, this model of anisotropy is not compatible with the recent studies of upper mantle anisotropy in the Fennoscandian shield [ Plomerová et al , 2001; Vecsey et al , 2007], which show that the fast velocity axis is usually inclined in anisotropic structures beneath the shield. In such a case all major types of olivine textures would manifest azimuthal dependence of Pn wave propagation [ Weiss et al , 1999a].…”

The evolution of crystal‐poor rhyolite in the middle Okinawa Trough and its implications for the state of magma chamber

“…Besides, the anisotropy of Fe‐Mg interdiffusion has been determined: = 1.7 * = 3.5 * (Chen et al, 2023; Dohmen et al, 2016; Krimer & Costa, 2017). Fortunately, outstanding crystal habit have been observed in the selected orthopyroxene (Figure 10a), which allowed us to run a nearly a ‐axis diffusion model (Krimer & Costa, 2017; Weiss et al, 1999). According to the sigmoid 1‐D diffusion model (Morgan et al, 2004), the diffusion time of Fe‐Mg in orthopyroxene is 660 years (Figure 10c).…”

Joint inversion of multiple data types with the use of multiobjective optimization: problem formulation and application to the seismic anisotropy investigations

“…Under the assumption that the elastic constants of peridotite aggregate are known from petrophysical studies (see, e.g. Babuška et al 1993; Ben Ismaïl & Mainprice 1998; Weiss et al 1999), this requires two (hexagonal) or three (orthorhombic) spherical angles for unique determination of the orientation of the symmetry axes (Šílený & Plomerová 1996).…”