SHEAR-WAVE SPLITTING IN THE APPALACHIANS AND THE PYRENEES - IMPORTANCE OF THE INHERITED TECTONIC FABRIC OF THE LITHOSPHERE

Splitting of teleseismic shear waves has been measured in the Appalach ians (eastern USA) and the Pyrenees (western Europe) using data record ed by permanent and portable stations. From a comparison of the result s, it appears that an interpretation of the recorded seismic anisotrop y in terms of geodynamics is not straightforward. Successive geodynami c events have generated structures that may have resulted in a similar pattern of mantle flow and that therefore may have contributed in the development of the recorded anisotropy. Combining geological and geop hysical arguments, it appears that the mantle anisotropy measured acro ss the Appalachians and the Pyrenees may not be systematically Appalac hian or Pyrenean in age but may be mainly due to a lithospheric struct ure formed during earlier major tectonic events, i.e. the Grenvillian and the Hercynian orogenies, respectively. We suggest that during majo r episodes of continent assembly, a pervasive tectonic fabric is devel oped in the lithospheric mantle. In the subsequent evolution of the co ntinent, this fabric may induce a significant mechanical anisotropy th at will drastically influence the mechanical behaviour of the lithosph ere when submitted to new tectonic events.