The New Madrid seismic zone of the Central United States - Intraplate Earthquakes - page 168

Figure 7.4 Late Precambrian-early Paleozoic transform faults (dashed lines), rifted continental mar-
gin (parallel straight lines), Cambrian basement fault systems (curved lines), and late Paleozoic
orogenic belts and cratonic structures (modified from Thomas, 1988 ) .

of Kentucky, Missouri, Arkansas, and Tennessee ( Figure 7.4 ) (Erwin and McGinnis, 1975 ;
Hildenbrand, 1985 ; Nelson and Zhang, 1991 ; Dart and Swolfs, 1998 ; Thomas, 2006 ;
Csontos et al ., 2008 ) . Superposition of the northwest- and northeast-trending faults resulted
in the Reelfoot Rift consisting of sub-basin blocks.
A composite east-west COCORP deep reflection profile across the Reelfoot Rift 50 km

north of Memphis, Tennessee, reveals inward-dipping listric normal faults that form half-
grabens along the southeast and northwest boundaries of the rift ( Figure 7.5 ) (Nelson
and Zhang, 1991 ) . Inward-dipping reflections to 40 km depth indicate that extensional
faulting may exist through the crust to the Moho. Extension across the rift is only 17%;

however, the amount of subsequent inversion shortening due to Appalachian/Ouachita
orogeny compression is not known. Nelson and Zhang ( 1991 ) also identified northeast-
trending faults within the rift: a steep east-dipping fault that projects up-section to the
western margin of Crowley's Ridge, the steep west-dipping Axial fault (Cottonwood Grove