FABRIC OF THE LITHOSPHERE IN NORTHEASTERN NORTH AMERICA

A complex mosaic of tectonic units in northeastern North America is likely to be reflected in the internal structure of the lithospheric mantle beneath them. Most studies looking for internal layering of the lithospheric mantle in the region identify clear seismic boundaries between the depths of 50 to 150 km. The nature of these boundaries, their lateral extent, and their relationship to the geologically-defined terrane boundaries are all subjects of ongoing studies.

We present results of a systematic survey of lithospheric layering that is associated with the directional variation (anisotropy) of the seismic wave speed. We use data from the Earthscope Transportable Array and its successors to develop site-specific vertical profiles of directionally-dependent seismic wave speed. To accomplish that, we use a newly developed version of receiver function methodology that takes advantage of the directional changes in the nature of P-to-S converted waves from distant earthquakes.

Long-term, continuous records of seismic waves from teleseismic distances are essential for the success of our study. We focus on sites that operated for 3 or more years in the region including the Adirondack Mountains, New England, and parts of Canadian Maritimes. At most sites we identify clear sources of converted waves residing deeper than the crust-mantle boundary that have directional variations consistent with either anisotropic velocity or inclined interfaces or both. In many instances these features do not have obvious directionally-independent components, and thus would not be visible on more traditional converted wave images that are sensitive to the impedance contrasts only.

Our survey is intended to complement the set of other constraints on the lateral variations in the lithospheric properties of the region, such as the distribution of seismic wave speed, the vertical extents of the crust and the lithosphere, and the nature of the integrated signature of seismic anisotropy in the mantle. Taken together, these constraints are the best guide to the regional divisions in the mantle lithosphere of eastern North America, and a record of its complex evolution.