Seismic Exploration of the Deep Continental Crust

Part of the series Pure and Applied Geophysics(PAGEOPH) pp 139-155

Realistic Models of Anisotropic Laminated Lower Crust

  • Melanie PohlAffiliated withGeophysical Institute, Karlsruhe University
  • , Friedemann WenzelAffiliated withGeophysical Institute, Karlsruhe University
  • , Thomas WeissAffiliated withInstitute of Geology and Dynamics of the Lithosphere, Göttingen University
  • , Siegfried SiegesmundAffiliated withInstitute of Geology and Dynamics of the Lithosphere, Göttingen University
  • , Thomas BohlenAffiliated withGeophysical Institute, Kiel University
  • , Wolfgang RabbelAffiliated withGeophysical Institute, Kiel University

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The genesis of the laminated lower crust has been attributed to extensional processes leading to structural and textural ordering. This implies that the lower crust might be anisotropic. Laboratory measurements of lower crustal rock samples and xenolithes show evidence of anisotropy in these rocks due to oriented structure.

In this paper we investigate the seismic shear-wave response of realistic anisotropic lower crustal models using the anisotropie reflectivity method. Our models are based on representative petrophysical data obtained from exposed lower crustal sections in Calabria (South Italy), Val Strona and Val Sesia (Ivrea Zone, Northern Italy). The models consist of stacks of anisotropie layers characterized by quantified elastic tensors derived from representative rock samples which provide alternating high and low velocity layers.

The seismic signature of the data is comparable to seismic observations of in situ lower crust. For the models based on the Calabria and Val Strona sequences shear-wave splitting occurs for the Moho reflection at offsets beyond 70 km with travel-time delays up to 300 and 500 ms, respectively. The leading shear wave is predominantly horizontally polarized and followed by a predominantly vertically polarized shear wave. Contrastingly, the Val Sesia model shows no clear evidence of birefringence. Isotropic versus anisotropie modelling demonstrates that the shear-wave splitting is clearly related to the intrinsic anisotropy of the lower crustal rocks for the Val Strona sequence. No evidence of birefringence caused by thin layering is found.

Key words

Anisotropy reflectivity