Northwest-Trending Basement Fracture Zones in the Eastern United States and their Role in Controlling Neotectonic Movement and Earthquakes

Abstract

A few northwest-trending fault zones have long been known in the United States east of the Rocky Mountains, and numerous ones have been revealed by geomorphic, geophysical, LANDSAT, side-looking-radar, and detailed geologic data during the past 15 years. Many are grouped to form long fracture zones which appear to form major zones with smaller zones between them. Enough major zones are now known to suggest that they form a basic crustal framework. Their ages range from Precambrian to post-Cretaceous. Many of the younger fracture zones were probably once buried and later reactivated, as indicated by geophysical expressions larger than can be accounted for by surface offsets.The northwest-trending fracture zones appear spatially related to the offshore northwest-trending transform fracture zones in the North Atlantic and Gulf of Mexico basins, although many of the former are older. Movement along transform fracture zones may have been transferred to the nearest existing onshore zone or, more likely, the onshore zones controlled the initial position of the offshore zones as the Atlantic basin began to open in the Jurassic period.Movement along fracture zones is known to have affected Paleozoic sedimentation in places, and both these zones and offshore transform fracture zones have controlled northwest-trending late Cretaceous and Tertiary basins, referred to as embayments, in the coastal plain deposits. Much of the present-day movement, as shown by earthquakes, is occurring locally along the fracture zones, especially where they cross northeast-trending belts undergoing vertical movement. The earthquake activity is commonly located at structural intersections with northeast- or north-trending fault zones. For example, the activity at New Madrid, Missouri, the site of the greatest earthquakes in United States history, is located where a northwest-trending fracture zone crosses a northeast-trending basement graben. Some of the north-trending faults are known to be extensional and may have moved in conjunction with right-lateral movement along fracture zones. This appears to be the case in the south-central United States, where some Holocene faulting has occurred along fracture zones, and may perhaps be the case farther east, where no Holocene faulting has been proven.KeywordsFault ZoneFracture ZoneUnited States Geological SurveyNuclear Regulatory CommissionOffshore ZoneThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.