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Tomographic images of the mantle below the Hercynian belt of Brittany display strong anomalies of P-wave traveltimes that likely represent variations in composition, and whose trends change above and below 130 km. A quantitative strain analysis of surface geological structures has shown that the crust in the area was affected by regional-scale distributed simple shear. The change of trends of tomographic anomalies below and above 130 km is largely removed by extending the simple shear model down to 130 km. From this, we infer lithospheric-scale wrenching, and therefore deep-seated horizontal decoupling, possibly at the lithosphere-asthenosphere boundary. Simple shear restoration further provides arguments for the occurrence of a steeply dipping slab. Geological data support its interpretation as a remnant of oceanic lithosphere subducted prior to the Hercynian collision. Beyond regional implications, the study underlines the interest of combining geology and tomography for the understanding of lithosphere deformation. Keywords: Hercynian lithosphere, mantle deformation, seismic tomography, shear belt, central Brittany.

Automatic methods used in geosciences to interpolate between orientation data have often limited applicability and strength, in particular where large ranges of orientations occur. In this paper, we show that geostatistical methods yield rather strong and powerful results when applied to directional data. The procedure involves the calculation of variograms, followed by a kriging interpolation of the data. In order to free from the circular property of directional data, the treatment of initial angular data sets is performed using scalar values provided by the direction cosines of double-angle values. The strength and application of the method are demonstrated by the analysis of theoretical and natural data sets. Natural examples are focused on the calculation and the analysis of cleavage trajectory maps. [Copyright &y& Elsevier]