USING SEISMIC RECEIVER FUNCTIONS TO STUDY THE DEEP CRUST BENEATH NORTHERN APPALACHIAN TERRANES

Over the last forty years, continental geology has been revolutionized by the understanding that the continents have been assembled over time by accretion of terranes that may have traveled thousands of kilometers to reach their present position. The New England Appalachians provide an excellent example of this process with multiple orogenic episodes identified in the rock record. While geological observations record events spanning billions of years, geophysical measurements record current conditions in the subsurface. The question remains whether orogenic activity during Appalachian mountain building is preserved as relict structures within the deep crust and upper mantle. A recent study (J. Schuh, 2014) correlated arrivals on receiver function (RF) seismograms with surface geological structures in central New England. In this paper, we extend that study to include data from the IRIS Earthscope Transportable Array stations as well as permanent stations operated in New York and New England. We focus on stations located along the 1988 USGS ONYNEX seismic refraction survey, and use RFs calculated by the EARS project hosted at the IRIS DMC. An important point is that RF data sample the earth outboard of the station. The Moho, for example, is sampled ~7-12 km from the station in the direction of the incoming seismic waves (dependent on distance to the earthquake). By using events at a range of azimuths, the crust can be sampled over an area ~350 km² surrounding the station. Our procedure is to calculate synthetic RFs and compare these directly to observed RFs for high-quality individual events and to RF stacks in restricted back azimuth/distance bins. Seismic models at each station are taken from the ONYNEX refraction line nearest the station. We illustrate the procedure at the Lebanon, NH, station (LBNH) located near the boundary between the Bronson Hill anticlinorium and the Central Maine Basin. LBNH is a US National station with a long set of observations, and is located approximately 9 km SW of ONYNEX shot point SP06. Based on RF data, the crust shows considerable variation around LBNH, with thicker crust (~43 km) from SW to NW. The crust thins to ~36 km to the S and E. The RF Moho beneath the ONYNEX profile (to the N and NE of LBNH) is complex with two prominent arrivals. This complex Moho is not predicted by the ONYNEX refraction model.