HIGH-RESOLUTION CRUSTAL STRUCTURE NEAR AND OF THE MID-CONTINENT RIFT SYSTEM IN NORTH AMERICA USING P-WAVE RECEIVER FUNCTIONS

The more than 1000-km-long main branch of the mid-continent rift system (MRS) near Lake Superior and the Minnesota-Wisconsin border represents a billion-year old failed attempt to rift North America. Magmatic events and subsidence- related sedimentary processes changed the structure of the crust across and along the MRS. To map this crust and lateral changes therein, we analyzed P-wave receiver functions (RFs) from 82 SPREE (Superior Province Rifting EarthScope Experiment) broadband seismic stations and 37 EarthScope Transportable Array (TA) stations. For each station the RFs are derived from the deconvolution of the radial component with the vertical component in the time domain (Ammon et al., 1991). To constrain the thickness of the crust and the average α/β ratio we applied H-κ stacking and waveform fitting to these RFs for each station. We used the results from the same waveform fitting exercise to estimate the thickness of the sedimentary strata. Next, a Kirchhoff migration was employed to image additional discontinuities. The most prominent feature of the RFs for stations outside of the MRS is the P-to-S wave converted at the Moho, which reflects the impedance contrast in rocks between the lower crust and upper mantle. Inside the MRS, the RFs are more ambiguous and heterogeneous, suggesting the presence of layer between the crust and lithospheric mantle of intermediate seismic velocities.