Geoscience Reference
In-Depth Information
Figure 7.5 Two interpretations of a COCORP reflection line across the Reelfoot Rift located
50 km
north of Memphis, Tennessee. In (A) the 7.5 km/s velocity (black) at the base of the crust is
interpreted to be a Cambrian magmatic underplate, whereas in (B) the 7.5 km/s velocity is interpreted
to be magmatic sills injected into the pre-existing crust with no underplate (from Nelson and Zhang,
was deposited only within the down-dropped rift, and the detailed characteristics of these
rocks remain a mystery because they have only been penetrated by a few oil exploration
wells.
Subsidence of the Reelfoot Rift ceased by the end of deposition of the regionally
extensive Knox Group, as revealed in the fact that the top of the Knox (470Ma) is essentially
flat across the rift. The NMSZ region experienced far-field effects of the Appalachian
Taconic orogeny (480-450 Ma) and the Acadian orogeny (417-329 Ma) (Clendenin and
the NMSZ region. However, the Appalachian Allegheny orogeny (320-250 Ma) directly
affected the NMSZ region. This orogeny resulted in the collision of North America with
Europe, Africa, South America, and microcontinents to form the Appalachian-Ouachita
mountain system. Appalachian Mountain thrust belt rocks continue westward beneath
the Mississippi River valley and merge with the contiguous Ouachita Mountains. While
formation of the Appalachian-Ouachita systemwas applying stress from the east and south,
the Ancestral Rockies were also rising and thus applying stress from the west during the
Carboniferous. The southern portion of the Reelfoot Rift was overthrust by Ouachita thrust
sheets, while sediments accumulated over the rift in the Arkoma/Black Warrior foreland
basin.
The Paleozoic closed with the Appalachian-Ouachita Mountain system forming a weld
in the assembly of Pangea. This huge mountain system traversed the interior of Pangea
resulting in drainage across the future United States flowing north and west off of the
