DAMAGED CRUST AND MODERN INTRAPLATE SEISMICITY

No clear relationship links modern intraplate seismicity to any of the various known tectonic fabric elements in the basement rocks of eastern North America. Three examples suggest that damaged (attenuated, dilated) crust has been localized through a variety of tectonic processes and ultimately has served to concentrate modern intraplate seismicity in eastern North America.

During assembly of supercontinent Rodinia, the continental suture of proto-Laurentia and Amazonia included large-scale sinistral transform motion (1200–1150 Ma). The New York–Alabama magnetic lineament (NYAML) shown on aeromagnetic maps is interpreted as the trace of the transform suture. An abrupt southeastwardly concave bend of the trace of the NYAML to the south from Tennessee defines a releasing bend with respect to sinistral strike slip. Earthquake hypocenters in the East Tennessee seismic zone are concentrated along the southeast side of the NYAML inside the releasing bend. Extension within the releasing bend damaged the crust, leaving it prone to seismicity in the modern stress field. The less abundant earthquakes northwest of the NYAML are compatible with damaged crust along secondary conjugate shear fractures, which propagated into proto-Laurentian crust adjacent to the transform.

The New Madrid seismic zone is at the corner between intracratonic fault systems—the rift-parallel Mississippi Valley graben and transform-parallel Rough Creek graben of Iapetan rifting and breakup of Rodinia (765–530 Ma). Pull-apart releasing bends along strike-slip faults within the Rough Creek graben suggest a larger scale releasing bend between the nearly orthogonal fault systems where extension damaged the crust. Subsequent Appalachian compression further deformed the damaged crust, as indicated by thin-skinned shortening of the cover strata. The net result of pull-apart extension and subsequent contraction is damaged crust, which concentrates seismicity within the modern stress field.

Charlevoix is at the site of an impact crater overprinted on Iapetan rift-margin extensional faults. The Iapetan rift faults alone may not have concentrated modern seismicity. The shattering effects of the impact event locally damaged the crust within the impact zone and enhanced damage along the older Iapetan rift faults.