Reelfoot Rift
Location
See text below and figures 1 and 6
References
(selected) Ervin and McGinnis 1975, McKeown and Pakiser 1982, Schwalb 1982, Houseknecht and Weaverling 1983, Gori and Hays 1984, Howe and Thompson 1984, Buschbach 1986, COCORP 1988, Heigold 1991, Kolata and Nelson 1991a
Description
The existence of a buried rift zone in the northern Mississippi Embayment was postulated by Burke and Dewey (1973) and elaborated by Ervin and McGinnis (1975), who named the feature Reelfoot Rift. The name is taken from Reelfoot Lake that formed when an area in northwestern Tennessee near the Mississippi River sank during the New Madrid earthquakes of 1811 and 1812.
The rift is indicated by geophysical data including seismic reflection and refraction profiles as well as gravity and magnetic surveys. Several deep oil test holes have entered the rift-filling sedimentary rocks, but none has penetrated completely. These data indicate a northeast-trending graben about 40 miles (65 km) wide and more than 200 miles (320 km) long. It is bordered by normal faults that, in places, may have more than 10,000 feet (3,000 m) of displacement (Howe and Thompson 1984, COCORP 1988). The northwest border runs from northeastern Arkansas across the bootheel of Missouri into southernmost Illinois. The Lusk Creek Fault Zone appears to coincide with part of the northwest border fault. The southeast border of the Reelfoot Rift crosses westernmost Tennessee and Kentucky. At its northeastern end the Reelfoot Rift connects with the east-trending Rough Creek Graben, a fault-bounded trench of similar dimensions.
The Reelfoot Rift (and Rough Creek Graben) contains 10,000 feet (3,000 m) or more of layered rocks older than the Mt. Simon Sandstone (Upper Cambrian). Deep well penetrations indicate arkoses and basinal shales (Houseknecht and Weaverling 1983), but layered volcanic rocks, common in rift settings, may be present below. Upper Cambrian and Lower Ordovician rocks thicken markedly into the rift but do not appear to be displaced by the boundary faults (Howe and Thompson 1984). Thus, the graben is interpreted as having formed in Early Cambrian time, while the rest of Illinois and adjacent areas were stable uplands (Kolata and Nelson 1991a). Thickening of the Mt. Simon and younger strata signify continued subsidence of the rift after cessation of active faulting. A zone of deformation termed the axial fault zone is expressed seismically along the centerline of the Reelfoot Rift, and it is interpreted as a product of post-Pennsylvanian compressional and strike-slip faulting (Howe and Thompson 1984). The same compressional stresses may have been responsible for uplift of the Pascola Arch, which is centered within the rift. Post-Pennsylvanian reverse faulting and later normal faulting are documented along the Lusk Creek Fault Zone (Nelson 1986). Other northeast-trending faults in the Fluorspar Area Fault Complex are probably related to extensional reactivation of the northern part of the Reelfoot Rift. The junction of the Reelfoot Rift and Rough Creek Graben was the scene of intense post-Pennsylvanian faulting, igneous activity, and mineralization (see FLUORSPAR AREA FAULT COMPLEX). A series of large, approximately circular magnetic and gravity anomalies along both borders of the rift are interpreted to be igneous intrusions (Mc Keown and Pakiser 1982). One of these is situated in Alexander County, Illinois; it has not been confirmed by drilling.
The area of the Reelfoot Rift began to subside again in the Cretaceous Period and became the Mississippi Embayment. Subsidence continued through the Holocene Epoch. Earthquake activity is concentrated in a narrow northeast-trending zone along the axis of the rift, offset by a short northwest-trending segment near New Madrid, Missouri. Earthquakes in the New Madrid Seismic Zone are believed to be the result of slippage along faults reactivated in the present compressional stress field (McKeown and Pakiser 1982, Gori and Hayes 1984).
See also ROUGH CREEK GRABEN, ST. LOUIS ARM (discarded), and SOUTHERN INDIANA ARM (discarded).
