IMAGING LOWER CRUSTAL FLOW USING HARMONIC DECOMPOSITION OF RECEIVER FUNCTIONS BENEATH A DENSE SEISMIC PROFILE IN EASTERN MASSACHUSETTS

Ganderia and the Southeastern New England Avalon terrane are both terranes that rifted from Gondwana and accreted to North America in the early to mid-Paleozoic. Accretion of the Avalon terrane was accompanied by plutonism, deformation, and metamorphism including partial melting within the Nashoba terrane, the trailing edge of Ganderia, and may be interpreted as indicators for mid- to lower-crustal channel flow. Channel flow describes the flow of weak, partially molten material between more competent crust as a result of pressure gradients in the mid- to lower crustal levels. Such flow should typically result in seismic anisotropy due to the crystallographic preferred orientations of minerals and shape preferred orientations at various scales. Here, we present first results for the crustal anisotropic structure beneath the Nashoba terrane that were produced with a newly developed approach from currently collected data in the region.

To investigate the hypothesis of crustal flow during the orogenic history of Southeastern New England, we deployed a dense profile of 6 broadband seismic stations crossing the Nashoba terrane. We analyze the harmonic variation of amplitudes in teleseismic P-Receiver Functions (RFs) to identify interfaces of isotropic and anisotropic contrasts within the crust. In the case of particularly prominent anisotropic features that have significantly larger amplitudes than other signals, it is feasible to derive quantitative constraints on the strength and orientation of the anisotropy. However, with growing complexity, a classical forward modelling or grid search approach becomes unfeasible.

These difficulties can be mitigated by applying Bayesian inversion, which infers values of model parameters from a probabilistic perspective. Applying a Bayesian inversion to the harmonically decomposed RFs has the potential to infer complex anisotropic seismic structures. We find evidence for two crustal anisotropic layers with confined properties to the geologic units of the Ganderia, Nashoba and Avalon terranes that might be related to episodes of lower crustal flow. In addition, we identify anisotropy at shallow mantle levels beneath the Nashoba and Avalon Terranes possibly indicating the upper interface of a shallow asthenosphere in the region.