The New Madrid seismic zone of the Central United States - Intraplate Earthquakes - page 178

7.2 ) . An obvious question is: what is controlling this structural pattern? It appears that
these stepover zones are related to basement faults of the Reelfoot Rift. The Reelfoot fault
extends between the rift margins, is divided into two segments by theAxial fault, and appears
to overlie the Grand River tectonic zone (Csontos, 2007 ) . Sikeston Ridge originates at the
intersection of Reelfoot fault (Grand River tectonic zone) and the northwestern Reelfoot
Rift margin; Joiner Ridge appears to be related to the intersection of the southeastern
Reelfoot Rift margin and the Bolivar Mansfield tectonic zone; and the southern portion of
Crowley's Ridge extends across the entire Reelfoot Rift, wherein the northern end originates
at the Bolivar Mansfield tectonic zone/northwest margin of the Reelfoot Rift intersection
and its southern end originates at the White River fault zone/southeastern margin of the
Reelfoot Rift intersection. The relatively uniform spacing, parallelism, and the fact that
the stepovers terminate at basement fault intersections suggest that the basement fault
intersections are controlling the positions and orientations of the compressional stepovers
(Talwani, 1999 ; Hildenbrand et al ., 2001 ; Gangopadhyay and Talwani, 2005 ; Van Arsdale,
2009 ) .

7.3.3 Reelfoot fault segments

The Reelfoot fault consists of two fault segments with different strikes and dips based on
earthquake foci locations. Between the depths of 4 and 14 km earthquake foci illuminate
the Reelfoot North fault (167 SW)

(Csontos and Van Arsdale, 2008 ) . In the upper 1 km, both faults have been imaged with
seismic reflection data (Purser and Van Arsdale, 1998 ; Van Arsdale et al ., 1998 ) and dip
73

° ,30 ° SW) and the Reelfoot South fault (150 ° ,44 °

SW dip to a depth of approximately
4km( Figure 7.8 ) (Purser and Van Arsdale, 1998 ; Csontos and Van Arsdale, 2008 ) . These
two fault segments also differ at the ground surface. Whereas the Reelfoot North fault has
a 10 m high monoclinal scarp, there is no surface scarp along the Reelfoot South fault
(Van Arsdale et al ., 1999 ) . Although there are subtle geological indicators that the Reelfoot
South fault moved in 1812 (Van Arsdale et al ., 1999 ) , the Pleistocene Hatchie River terraces
of the Obion River (a west-flowing tributary of the Mississippi River) are essentially flat
where they overlie the subsurface Reelfoot South fault. It should also be noted that the
Reelfoot North fault monocline has its maximum height of 10 m at its mid-point near
its intersection with the Mississippi River and the scarp height diminishes to zero north
at New Madrid and south at the southeastern margin of Reelfoot Lake (Csontos and Van
Arsdale, 2008 ) . The uplift that has been occurring on the Reelfoot North fault over the
past 2,600 years appears to be truncated by the New Madrid North fault (Western Reelfoot
Rift margin) at its northern end and the Axial fault at its southern end, with no significant
surface displacement on the Reelfoot South fault. A possible explanation for the absence of
a scarp along the Reelfoot South fault may be that the blind reverse fault underlies a 50 m
higher landscape and the fault has simply not propagated high enough to warp the ground
surface (Van Arsdale et al ., 1999 ) . It is also possible that the two faults are contiguous and
are acting semi-independently or they are not continuous.

° SW. It is believed that both faults retain the 73 °