The New Madrid seismic zone of the Central United States - Intraplate Earthquakes - page 165

valley reveal 6 km of Mississippi Embayment sediments and Reelfoot rift-fill strata

overlying basement (Ginzburg et al ., 1983 ) . Precambrian igneous and metamorphic rocks
(6.2 km/s, 2.74 g/cm 3 ) of the upper crust extend from 6 to 17 km in depth. The lower
crust varies in thickness and extends to depths of 39-46 km. Composition of the lower
crust is speculative, but it is interpreted to be crystalline rocks of intermediate composition
(6.6 km/s, 2.95 g/cm 3 ) (Stuart et al ., 1997 ) . The variable thickness of the lower crust is
due to a rift pillow at the base of the lower crust (Mooney et al ., 1983 ; Hildenbrand, 1985 ;
Hildenbrand and Hendricks, 1995 ) . Although not well constrained, the rift pillow rocks
(7.3 km/s, 3.1 g/cm 3 ) are interpreted to be a quartz tholeiite intrusion or underplating that
has been metamorphosed to garnet granulite. The pillow is proposed to have formed by
mantle melt intrusions in the deep crust by a mantle plume during Cambrian rifting (Mooney
et al ., 1983 ; Hildenbrand, 1985 ; Pollitz et al ., 2001 ) , Appalachian orogeny (Hildenbrand
and Hendricks, 1995 ) , or during the Middle Cretaceous superplume event (Csontos, 2007 ) .
The top of the mantle (8 km/s, 3.3 g/cm 3 ) is at depths of 39-46 km.
Crustal rocks of the southeastern United States formed as a complex of microplates
and island arcs that were accreted during the Archean and Proterozoic ( Figure 7.3 ) . The
major geological events of these times were the formation of the Superior Province (3.5-
2.7 Ga), Penokean Province (1.89-1.83 Ga), Yavapai Province (1.78-1.72 Ga), Mazatzal
Province (1.65 Ga), Eastern Granite-Rhyolite Province (1.47 Ga), Southern Granite-
Rhyolite Province (1.37 Ga), and Midcontinent Rift system (1.1 Ga), and these provinces
are bordered on the east and south by the Grenville Province (1.2-1.0 Ga) (Atekwana, 1996 ;
Van Schmus et al ., 1996 , 2007; Tollo et al ., 2004 ; Holm et al ., 2007 ; Van Arsdale, 2009 ) .
Rocks of the Yavapai and Mazatzal Provinces (Geon 16 and 17 terranes of Figure 7.3 ) have
a southwesterly grain and consist of metasedimentary, metaigneous, and basalt-rhyolite
volcanic suites that were subsequently intruded by anorogenic granitic rocks of the South-
ern and Eastern Granite-Rhyolite provinces. The Mazatzal Province is bound on the east
by the Eastern Granite-Rhyolite Province (EGRP), and the EGRP is generally accepted
to be the basement rock of the NMSZ region. Seismic reflection data indicate widespread
sub-horizontal reflectors, suggesting an interlayered sequence of rhyolites, granites, and
mafic igneous rocks and/or sedimentary rocks within the EGRP (Van Schmus et al ., 1993 ) .
Gravity and magnetic data suggest that the Mazatzal Province rocks may continue beneath
the EGRP (Atekwana, 1996 ) , whereas isotopic data (Van Schmus et al ., 2007 ) indicate that
Precambrian rocks younger than 1.6 Ga lie below the EGRP southeast of the heavy dashed
line in Figure 7.3 .
The Grenville Province bounds the eastern and southern flanks of the EGRP ( Figure
7.3 ) . This province formed by the collision of the North America craton first with island
arcs and then with a continent to the east that is now buried beneath the Appalachian
Mountains or was carried away during the late Proterozoic and early Paleozoic opening
of the Iapetus Ocean (Rankin et al ., 1993 ; Atekwana, 1996 ; Bartholomew and Hatcher,
2010 ) . The Grenville orogeny culminated in the formation of a huge mountain range
comparable to the Himalayas of today and also in the formation of the supercontinent of
Rodinia. The western boundary of the Grenville Province, the Grenville Front, has been