02/2A18

Guilhem Barruol*
Paul G. SiIver**
Alain Vauchez*

* Laboratoire de Tectonophysique, CNRS, Universíté de Montpellier II 34095 Montpellier, France.
** Carnegie Institution of Washington, 5241 Broad Branch Rd, NW. 20015 Washington, D.C., USA.

A compilation of splitting measurements of teleseismic shear wave (SKS. SKKS) recorded by - 40 portable and permanent stations in the eastern US is presented to discuss the upper mantle structure.

Appalachians stations are typically characterized by delay times 8t = Is and fast wave polarization plane 0 trending N50- 70<'E in the south and central regions and N30-40T in the north, closely following the trend of the belt. The transition between north and central Appalachians is characterized by Si = 1 - 1.3 s and by EW-trending. The anisotropy on the North American craton, is characterized by io trending N40-70*E and by 8t increasing, from 0.6 u at the periphery to 1.7 s in the middle of the craton.

The large-scale pattern of anisotropy together with the small-scale variations is hardly consistent with that predicted for simple asthenospheric flow beneath the plate.

Despite splitting along the southern and eastern margins of the continent are consistent with that expected from Grenvillian deformation, a model of forced asthenospheric flow around the North American craton acting as a keel cannot be ruled out. This model implies that a flow of low viscosity asthenosphere beneath the eastern US is deviated around the more viscous cratonic upper mantle.

However, a low velocity anomaly seen by tomography in the northern Atlantic down to 250 km depth (Van der Lee 1995) does not favor the interpretation of flowing asthenosphere: The good correlation of this anomaly with the New England seamounts suggests that it could be the fossil trail of the hotspot that generated these seamounts, and therefore, that no differential motion occurred in this upper mantle thíckness since - 100 Ma.

Finally, on the cratonic core, the observed & are maximum where the high velocity root is imaged by tomographies at least down to 300 km (Grand, 1994, Van der Lee, 1995). suggesting an anisotropy frozen in the cratonic upper mantle. The absence of low velocity zone beneath these high velocity roots may indicate that no mechanical asthenosphere is present beneath the craton. This wouId imply that the whole upper mantle beneath the North American craton could move coherently and therefore, that this "tectosphere" (as defined by Jordan, 1975) could be dírectly coupled with the lower mantle.