Regional variations in the structure of the crust in the central United States from P-wave spectra

abstract Regional variations in the crustal structure in the central United States have been inferred by the transfer ratio method from an analysis of long-period P waves recorded at SHA, OXF, FLO and MDS, the stations nearly along 89°W longitude. The crustal structure in this region is approximated by a stack of horizontal parallel layers except possibly in the area around FLO, where the structure is rather complicated. The crustal thickness is predominantly controlled by the thick silicic upper crust, whereas the mafic lower crust is about 10 km thick throughout this region. The P-wave velocity of the lower crust is about 6.9 to 7.0 km/sec except probably in the area around FLO, where 7.4 km/sec velocity is more likely. A sedimentary layer with a velocity of about 3.0 km/sec, having a thickness of about 3 km near the coast of the Gulf of Mexico, tapers out to the north within the Gulf Coastal Plain. Deep discontinuities in the crust may be replaced by transitional layers up to 10 km thick. The Moho is about 33 km deep near the coast of the Gulf of Mexico, deepens to about 41 km near an intersection of the Gulf coastal plain and the interior plain, reaches about 47 km or more in the midst of the interior plain, and rises to about 41 km toward an intersection of the interior plain and the superior upland. As for the midst of the interior plain, however, the depth of the Moho reduces by as much as 5 km, if the velocity in the lower crust is about 7.0 km/sec instead of about 7.4 km/sec. In any case, the general trend of the depth of the Moho may match with the topographic feature from the Gulf of Mexico to Lake Superior.