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Sinking Mafic Body in a Reactivated Lower Crust: A Mechanism for Stress Concentration at the New Madrid Seismic Zone

U.S. Geological Survey
345 Middlefield Road, MS 977
Menlo Park, California 94025
fpollitz{at}usgs.gov
(F.F.P.)

Department of Geology
1 Shields Ave.
University of California, Davis
Davis, California 95616
(L.K.)

Department of Geology and Geophysics
McCone Hall
University of California, Berkeley
Berkeley, California 94720
(R.B.)

We propose a geodynamic model for stress concentration in the New Madrid seismic zone (NMSZ). The model postulates that a high-density (mafic) body situated in the deep crust directly beneath the most seismically active part of the NMSZ began sinking several thousands of years ago when the lower crust was suddenly weakened. Based on the fact that deformation rates in the NMSZ have accelerated over the past 9 k.y., we envision the source of this perturbation to be related to the last North American deglaciation. Excess mass of the mafic body exerts a downward pull on the elastic upper crust, leading to a cycle of primary thrust faulting with secondary strike-slip faulting, after which continued sinking of the mafic body reloads the upper crust and renews the process. This model is consistent with the youth of activity, the generation of a sequence of earthquakes, and the velocity evolution during interseismic periods, which depend upon the density contrast of the mafic body with respect to the surrounding crust, its volume, and the viscosity of the lower crust.