SEISMIC ANISOTROPY IN THE EASTERN UNITED-STATES - DEEP-STRUCTURE OF ACOMPLEX CONTINENTAL PLATE

We have analyzed shear wave splitting recorded by portable and permane nt broadband and long-period stations located in the eastern United St ates. Teleseismic shear waves (SKS, SKKS, and PKS) were used to retrie ve the splitting parameters: the orientation of the fast wave polariza tion plane phi and the delay time delta t. In total, 120 seismic event s were processed, allowing for more than 600 splitting measurements. W ithin the Appalachians, stations located in the western (external) par t are characterized by delta t approximate to 1s and phi trending N50 degrees-70 degrees E in the south and central regions and N30 degrees- 40 degrees E in the north, closely following the trend of the orogenic belt in these areas. The transition region between north and central is characterized by delta t approximate to 1 - 1.3 s and by E-W trendi ng phi that are at a high angle to the regional geologic trend. Measur ements at two stations located in the eastern (internal) part of the b elt indicate very weak anisotropy. The large-scale pattern of anisotro py is not consistent with that predicted for simple asthenospheric flo w beneath the plate. Splitting along the southern and eastern margins of the continent is consistent with that expected for Grenvillian defo rmation, an alternative model of asthenospheric flow around the craton ic keel cannot be ruled out. Within the cratonic core, the correlation between delta t and lithospheric thickness suggests a lithospheric an isotropy. Smaller-length-scale variations also argue for a significant contribution of lithospheric structures. The fabric responsible for s hear wave splitting may have formed during tectonic episodes that affe cted the eastern United States, i.e., the Grenville and Appalachian or ogenies and the subsequent rifting of the North Atlantic Ocean. Our ob servations in the western Appalachians suggest that the anisotropy may be preserved since the Grenvillian orogeny. The absence of detectable splitting in the two stations in the eastern Appalachians is attribut ed to the igneous intrusions related to the Atlantic rifting. The meas urements in the transition between the northern and central southern A ppalachians, constitute an intriguing anomaly, whose E-W phi have litt le obvious relation to the regional surface geology. We suggest two po ssible causes: (1) the local dominance of asthenospheric flow, motivat ed by the proximity of a pervasive low-velocity anomaly and (2) lithos pheric deformation in a transcontinental strike-slip fault zone active during the Appalachian collision.