Seismic Structure of the Crust in the Northern Appalachians and the Easternmost Grenville Province

Abstract:

Eastern Canada and the northeastern USA were shaped by a series of collisional and accretionary events since ~1 Ga. Having access to data from relatively dense arrays of broad-band seismic stations enables us to apply an ambient noise tomography technique across the southeastern Proterozoic Grenville Province and the northern Phanerozoic Appalachians. Our datasets include over two years of vertical component time series from 2013 to 2015, recorded by 69 stations belonging to 7 different networks. The resulting stacked cross- correlograms gave rise to clear fundamental-mode Rayleigh waves at periods between 4 and 60 seconds. The observed asymmetry in the resulting waveforms suggests a non-uniform azimuthal and temporal distribution of ambient seismic noise in the study area. This motivated us to perform directionality and seasonality analysis on our datasets, and the results indicate different dominant noise source locations at different period ranges. Next, we used a recently developed approach to measure the regional average and inter-station phase velocities at different periods in the frequency domain. The resulting inter-station phase velocity dispersion curves are input to a tomographic inversion for phase velocity and anisotropy at periods sensitive to crustal structure. The relatively dense path coverage across the study area allows us to distinguish small-scale structures within the regional tectonic domains. Finally, we invert our phase velocity models for shear wave structure and use these models along with evidence from previous studies to discuss tectonic implications.