Abstract

Cretaceous and Cenozoic faults in eastern North America provide evidence of modest intraplate tectonic activity in this region. Thrust and gravity faults are found; strike-slip movement is not demonstrable but could have occurred in some cases. Fault movements are dated by offset Cretaceous, Tertiary, and Quaternary sediments, Cretaceous kimberlite, and Pleistocene glacial stria. Cretaceous and Cenozoic displacements range from about 1 mm to several tens of metres. The faults are found in the central Mississippi River valley, the southeastern and northeastern United States, and eastern Canada. Correlations among historic seismicity, fault plane solutions, and the Cretaceous and Cenozoic faulting in the central Mississippi River valley and the southeastern United States suggest a close causal relationship. The quality of the correlation between historic seismicity and Cretaceous and Cenozoic faulting in the northeastern United States and eastern Canada varies considerably from area to area; hence the relation there between Cretaceous and Cenozoic faulting and contemporary seismotectonics is uncertain.

Sharp offsets in loosely consolidated sediments, the presence of slickensides and fault gouge, and the involvement of Precambrian or Paleozoic bed rock provide the best evidence that faulting was associated with tectonic earthquakes. The displacement of near-surface deposits implies that surface faulting has occurred, although it has not been documented for modern earthquakes in eastern North America. At least some of the displacements occurred along pre-existing faults. The number and amounts of Cretaceous and Cenozoic movements on each fault have not been determined, but on one fault a displacement of 0.6 m during a short time span suggests that an earthquake of about magnitude 6 was associated with the displacement. The largest Cretaceous and Cenozoic displacements must represent more than one movement.

The presence of thrust and gravity faults and the inconsistent trends of the faults imply that stresses that vary spatially and (or) temporally, rather than a single stress field acting steadily throughout the entire North American plate, are responsible for causing this intraplate tectonic activity.

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