11 Understanding Earth's Interior: Geophysics in the Eastern United States from the Near-Surface to the Mantle

Thursday, 29 March 2007
1:40 PM - 3:40 PM

STRUCTURE OF THE CRUST AND UPPER MANTLE IN THE SOUTHERN APPALACHIANS FROM RECEIVER FUNCTION ANALYSIS

Paper number: 11-4
Program: SYMPOSIA SESSIONS
Time: 2:40 PM – 3:00 PM

Authors

Abstract

Using two permanent broadband seismic stations from the US National Network (MYNC in Murphey, NC and GOGA in Godfrey, GA), new estimates of crustal thickness and average composition were calculated using the receiver function method. By grouping teleseismic wave arrivals into azimuthal ranges, lateral variations around each station were investigated yielding multiple estimates of thickness and average VP/VS for each station. In addition to arrivals from the conversion at the Moho and within the crust, clear upper mantles arrivals were present in the data as well.

Using seismic velocities found with wide–angle refraction surveys, the receiver function results show a trend of decreasing crustal thickness from the NW to SE for each individual station as well as over the entire region. The crustal thickness beneath MYNC is approximately 48–52 km with a variable average VP/VS ratio between 1.742 and 1.852. For GOGA, the crustal thickness is 39–42 km and a slightly lower average VP/VS ratio between 1.720 – 1.777. The VP/VS ratios suggest the presence of rocks with a granitic to intermediate average compostion, but with the crust beneath GOGA having a slightly more granitic average composition. The difference in crustal thickness between the two stations is consistent with previous reflection and refraction studies that suggest westward crustal thickening beneath the Blue Ridge Mountains and portions of the Valley and Ridge. Three arrivals associated with upper mantle conversions are present in the receiver functions. Using a simple velocity model, the depths to these interfaces are approximately 160 km, 410 km, and 660 km, corresponding with the familiar upper mantle discontinuities.