SHEAR-WAVE SPLITTING AROUND THE NORTHERN ATLANTIC - FROZEN PANGAEAN LITHOSPHERIC ANISOTROPY

Although the number of teleseismic shear wave splitting measurements h as considerably increased world-wide during the last decade, there is no consensus on a unique and universal interpretation of these data. S plitting might primarily originate in the lithospheric and/or asthenos pheric mantle, but the technique has no vertical resolution and theref ore leaves this question of anisotropy location unresolved, Generally, the consequences of the two possibilities are used to infer which is more likely. If splitting occurs in the lithosphere, the splitting par ameters (the orientation of the fast split shear wave phi and the dela y time delta t) should be related to frozen or active deformation. On the other hand, if the splitting occurs in the asthenosphere, the obse rved splitting parameters should be related to the present-day flow in duced by the differential motion between the tectonic plates and the u nderlying mantle, The aim of this paper is to test these hypotheses ag ainst shear wave splitting observations from western Europe and the ea stern U.S. Indeed, except in a few places-splitting on either side of the Atlantic, on Hercynian, Caledonian or Grenvillian structures do no t appear to have been deeply and pervasively affected by the opening o f this ocean. This provides us the opportunity to place these results in the Pangaean reference frame of the Hercynian-Caledonian belt befor e the opening of the Atlantic, In many places the close parallelism be tween lithospheric structure and the absolute plate motion (APM) imped es a reliable identification of the origin of the anisotropy. Especial ly, an asthenospheric flow channelled around the deeply rooted North A merican craton may be compatible with the large-scale anisotropy patte rn in the eastern U.S. Despite this ambiguity, fast polarization direc tions (phi) observed on or near some major lithospheric structures cor relate better with the trend of the local tectonic structures than wit h the expected asthenospheric flow, Therefore, we suggest that these l ithospheric structures: developed before the opening of the Atlantic, remain frozen since Hercynian, Caledonian or Grenvillian times and are responsible for the observed splitting at these places.