LITHOSPHERIC STRENGTH AND INTRAPLATE SEISMICITY IN THE NEW MADRID SEISMIC ZONE

A simple hypothesis is proposed to explain the occurrence of localized zones of tectonic deformation and seismicity within intraplate region s subjected to relatively uniform far-field tectonic stresses. In most intraplate regions (especially continental shield areas and old ocean ic basins), temperatures in the lower crust and upper mantle are quite low so that the upper mantle is cold and strong. In these regions, si gnificant lithospheric deformation does not occur because the cumulati ve strength of the lithosphere far exceeds the magnitude of plate-driv ing forces. If lower crust and upper mantle temperatures are relativel y high, however, plate-driving forces are largely supported by the upp er crust because the lower crust and upper mantle are relatively weak. In this case, the regions can deform relatively rapidly because the c umulative strength of the lithosphere is comparable in magnitude to th at of the forces acting on the lithosphere. In this paper, we apply th is hypothesis to the New Madrid seismic zone and the surrounding centr al and eastern United States. Within the seismic zone, the heat flow a ppears to be slightly elevated (about 60 mW/m(2)) relative to the back ground regional value of 45 mW/m(2). Calculated crustal geotherms and laboratory-derived ductile flow laws suggest that the lower crust and upper mantle are sufficiently weak within the seismic zone that intrap late stresses are largely transmitted through the upper crust and defo rmation can occur at relatively rapid rates for this intraplate area. In marked contrast, in the surrounding area where the heat flow is rel atively low, cumulative lithospheric strength appears to far exceed th e plate-driving force, and the tectonic stress is carried in both the crust and upper mantle. Thus the marked contrast in seismicity between the seismic zone and the surrounding area appears largely because of heat flow and whether or not the lower crust and upper mantle support an appreciable fraction of the plate-driving forces.