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mechanism has been active in the deep geological past of the Reelfoot Rift. I now consider
the history of the Reelfoot North fault.
There has been 16 m of Quaternary displacement, 15 m of post-Wilcox/pre-Quaternary
displacement, 11 m of Wilcox displacement, 21 m of Midway displacement, and 10 m
of Late Cretaceous displacement on the Reelfoot North fault (Van Arsdale, 2000 ) . Based
on these displacements and estimates of their respective time frames one can calculate a
Late Cretaceous slip rate of 0.0007 mm/yr, Paleocene slip rate of 0.002 mm/yr, Wilcox
Group slip rate of 0.001 mm/yr, post-Wilcox/pre-Holocene slip rate of 0.0003 mm/yr, and
a Holocene slip rate of 1.8 mm/yr. If we spread the 16 m of Holocene displacement over the
entire time frame of the Paleocene, then we would have a slip rate of 0.001 mm/yr. Secondly,
the magnitudes of the slip for these time periods separated by unconformities are similar.
This similarity implies that perhaps any single cluster of activity may not exceed
20 m
of fault displacement on the Reelfoot North fault. From either perspective it appears that
since the plate tectonic setting of the NMSZ has not changed substantially since essentially
the Cretaceous, we should expect uniform behavior over time. However, this does not get
the NMSZ off the hook. If the Reelfoot North fault can slip as much as 20 m during the
current cycle then there may be 9 more meters of slip before this fault shuts down perhaps
for millions of years.
7.4 Conclusions
Earthquakes on the NMSZ occur along faults within the Cambrian Reelfoot Rift that appear
to have cut across northwest-striking Proterozoic faults ( Figures 7.1 and 7.2 ) . At a more
regional scale the Commerce Geophysical lineament and the Big Creek fault zone ( Figure
7.11 ) are interpreted to be outboard faults of the rift. Right-lateral shear across the Reelfoot
Rift is causing right-lateral displacement along the Southeastern Reelfoot Rift margin faults,
Axial fault, and New Madrid North fault, and reverse displacement on the Reelfoot North
and Reelfoot South faults. Thus, there are three strike-slip faults and two left stepovers
that are currently seismically active. There is also evidence of Quaternary faulting along
portions of the Southeastern Reelfoot Rift margin faults, Big Creek fault, the Commerce
Geophysical lineament, the Bootheel fault, the New Markham fault, Crowley's Ridge, the
Saline River fault zone, Joiner Ridge, and the Meeman-Shelby fault ( Figure 7.2 and Table
7.1 ) . In effect, the Reelfoot Rift is a Cambrian structure that has been active over much of its
area within the Quaternary. Proposed explanations for the driving force for the faulting and
resultant earthquakes include a sinking rift pillow, ridge push, a subducting Farallon slab,
local weakening of the crust, excess pore-fluid pressure, glacial isostatic adjustment, and
erosional unloading. The absence of significant Neogene deformation in the region requires
Quaternary onset of the most recent episode of deformation and the paleoseismic record
indicates that faulting began in the NMSZ during the Holocene. Holocene reactivation
of the NMSZ faults has been attributed to local stress perturbation due to melting of the
Wisconsin ice sheet, a change in North American plate motion, or a reduction in normal
stress across strained faults due to denudation by the Mississippi River. The denudation
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