DEEP TEXTURE OF THE APPALACHIAN TERRANES

The Appalachian Orogen is traditionally divided into terranes with different origin and tectonic history. Individual terranes are defined on the basis of rocks presently exposed on the surface. The geometry, nature and depth extent of the boundaries between terranes remain the subject of ongoing research.

Coherent deformation of rocks making up the Earth's lithosphere imprints characteristic fabrics which may be detected and characterized by seismological observations sensitive to directional dependence (anisotropy) of seismic wave speed. In this presentation we seek to relate observations of anisotropic seismic properties to the surface geology of the Northern Appalachians.

We carry out a systematic mapping of the anisotropic properties in New England and Maritime Canada. We employ a popular shear wave splitting method based on records of core-refracted waves from earthquakes at distances 85-180 degrees from the region. The method is able to detect anisotropic velocity of waves passing through distinct rock boundaries in the upper mantle. Our study takes advantage of numerous long-operating observatories in the region. For each location we seek to build a maximally detailed characterization of the shear wave splitting pattern of rocks containing anisotropic properties using multiple core-refracted phases (SKS, PKS, SKKS) and paying particular attention to data from areas with less frequent seismicity. We use multiple measurement methods simultaneously, both to ensure the fidelity of our observations and to develop additional attributes that may help in interpretation.

Our results provide a first-order characterization of different terranes of the Appalachian Orogen in terms of their anisotropic properties. It can be used for qualitative interpretation, e.g. on the basis of distinct patterns of splitting, and also for the development of vertically and laterally variable models of anisotropic seismic velocity beneath the Northern Appalachians.