The New Madrid seismic zone of the Central United States - Intraplate Earthquakes - page 167

traced primarily by gravity and magnetic data southerly through the United States, and the
province's southwestern continuation has been mapped in Texas. However, it is not known
where Grenville rocks exist beneath the southern United States, and in particular between
southern Tennessee and its exposure in the Llano Uplift of central Texas, because there is
insufficient drill-hole data in the southern United States. Culotta et al . ( 1990 ) have proposed
that Grenville rocks underlie the NMSZ and Nelson and Zhang ( 1991 ) speculate that the
western margin of the Grenville Front underlies and is responsible for the location of the
Cambrian Reelfoot Rift. As the above survey of the literature indicates, the true nature of
the Precambrian crust beneath the NMSZ is still an open question.
At the close of the Grenville orogeny, the Grenville Mountains extended along what is
today the eastern seaboard and wrapped around the southern margin of the United States,
just like the much younger 300 Ma old Appalachian-Ouachita Mountains (Van Arsdale,
2009 ; Bartholomew and Hatcher, 2010 ) . Formation of the Grenville Mountains was the
culmination of the collision of Laurentia (North America), Baltica (Europe), Amazonia
(South America), and Kalahari (Africa) to form the supercontinent of Rodinia. Thus, during
Rodinia time, the southeastern United States was a topographically high area located in the
interior of Rodinia.
The Precambrian closed with the disassembly of Rodinia and the formation of a passive
margin along the southern margin of the United States. More specifically, Thomas ( 1991 ,
1993) argues that the southern margin of the United States near the NMSZ area was formed
by the Alabama-Oklahoma transform fault ( Figure 7.4 ) . If true, the continental shelf was
sharply truncated, and immediately south of this transform fault (perhaps within as little as
25 km) was very deep water above an abyssal plain.

7.2.2 Paleozoic

Present-day NMSZ seismicity is occurring along reactivated faults of the Reelfoot Rift
( Figures 7.1 , 7.2 , and Table 7.1 ) . This rift formed during the Early or Middle Cambrian
(520-500 Ma) as part of the Reelfoot Rift-Rough Creek graben-Rome Trough ( Figure 7.4 )
in an abortive attempt to pull off a corner of Laurentia (United States) during the disassembly
of Rodinia (Thomas, 1991 ) . The southern end of the Reelfoot Rift was apparently the shelf
edge at the Alabama-Oklahoma transform fault ( Figure 7.4 ) .
The Reelfoot Rift consists of fault-bounded blocks that were created by the intersection
of Proterozoic (N west-

trending faults that displace Paleozoic strata in southeastern Missouri (Anderson, 1979 )
and northeastern Arkansas (Haley et al ., 1993 ) are interpreted to be reactivated faults
of the Mazatzal terrane (Central Plains orogen) (McCracken, 1971 ) . These reactivated
Proterozoic faults (Cox, 1988 ) have been proposed to pass beneath the Mississippi River
valley into western Tennessee ( Figure 7.2 ) (Stark, 1997 ; Csontos et al ., 2008 ) . For example,
the Grand River Tectonic Zone appears to continue beneath the Mississippi River valley as
the Reelfoot fault (Csontos et al ., 2008 ) . During Cambrian rifting of Rodinia the N50

55 ° W) and Cambrian (N50 ° E) faults ( Figure 7.2 ) . North 55 °

E-
trending Reelfoot Rift formed and now underlies the Mississippi River valley in portions

°