| A study to map the variations in the thickness and velocity contrast of the Mantle-Crust Transition (MCT) beneath North America is conducted using seismic receiver functions. Receiver functions were calculated from three component broadband seismograms recorded by the CNSN, NCEDC, USNSN, GEOSCOPE, TERRASCOPE, IRIS-IU, IRIS-II, seismic networks for the years 1990 through 1998. The data consist of 731 receiver function stacks, calculated using an iterative time-domain deconvolution applied to teleseismic records at 101 stations distributed in different tectonic provinces (e.g. Shields, Continental Platforms, Orogens, Volcanic Arcs). The diversity of MCT structures encountered reflects the heterogeneous nature of the lower-crust and does not favor any particular crustal growth hypothesis. The observed median MCT thickness does not depend on the plate setting. Variations in MCT structure suggest that its long-term structure is influenced by the character of the sub-continental mantle. The receiver functions were also used to study variations in the average crustal Poisson's ratio, to provide further constraints on the bulk composition of the continental crust. The median Poisson's ratio for the continent (0.27) is consistent with earlier estimates. The largest observed ratios are associated with volcanic arcs and consistent with a relatively mafic composition. The high median values of shields and platforms (0.28) may reflect mafic lower crust; orogenic regions are variable but generally show values lower than average; extended crust regions, have low Poisson's ratio values. |
