SHEAR-WAVE SPLITTING ACROSS THE ROCKY-MOUNTAIN FRONT

We search for evidence of seismic anisotropy by determining shear-wave splitting parameters using teleseismic phases from stations placed ac ross the Rocky Mountain Front. Three features are striking: 1) fast po larization orientations are consistent within small geographic regions , but vary rapidly across the network, 2) a large number of high quali ty ''null'' measurements indicate that little transverse anisotropy wi th horizontal symmetry axis is present, and 3) stations with well-cons trained but inconsistent parameters for rays from different sources im ply that a single layer of anisotropic material with a horizontal symm etry axis is an inadequate parameterization. The general pattern sugge sts fast axes pointing toward (or away from) a central region of littl e anisotropy. A model of asthenospheric flow converging on or divergin g from the central uplifted region is postulated. Within the southern Rocky Mountains, parameters are similar to those in the nearby norther n Rio Grande Rift, suggesting that a similar mechanism causes the anis otropy in both regions.