Geoscience Reference
In-Depth Information
Tuttle et al ., 2002 ) . Trenches excavated across the Reelfoot scarp (monocline) identify
three earthquakes (Russ, 1982 ; Kelson et al ., 1996 ) and regional paleoliquefaction studies
identify five earthquake sequences, including 1811-1812, that have occurred within the
last
4,300 years (AD 1811-1812, 1450
±
150, 900
±
100, 300
±
200, and 2350 BC
±
200 years (Tuttle et al ., 2002 , 2005). Straightening of the Mississippi River upstream
from the Lake County Uplift at approximately AD 900 and between 2244 and 1620 BC
has been interpreted by Holbrook et al .( 2006 ) to be due to uplift of the Reelfoot fault
across the course of the river, which further supports the paleoliquefaction dates of AD
900 and 2350 BC. Paleoliquefaction data also indicate that the AD 1450, AD 900, and
2350 BC earthquake periods had multiple earthquakes within a short period of time like
the 1811-1812 sequence (Tuttle et al ., 2002 , 2005).
The maximum structural amplitude of the Reelfoot scarp is
11 m, which has occurred
within the last 2,600 years (Champion et al., 2001 ; Carlson and Guccione, 2010 ) . At
depth beneath the Reelfoot scarp, the unconformity at the base of the Holocene alluvium is
displaced 16 m, top of the Eocene Wilcox Group 31 m, top of the Paleocene Midway Group
42 m, top of Cretaceous 63 m, and top of Paleozoic 73 m. The Reelfoot fault has moved
16 m in the past 10,000 years with 11 m occurring in the past 2,600 years, implying a slip
rate of 1.8 mm/yr throughout the entire Holocene and 6.2 mm/yr over the past 2,600 years
(Van Arsdale, 2000 ) . In subsequent studies of the Reelfoot scarp, a late Holocene slip rate
of 3.9
1 mm/yr was interpreted by Champion et al . ( 2001 ) , while Carlson and Guccione
( 2010 ) estimate a late Holocene slip rate of 13 mm/yr calculated from the AD 1450 and
1812 faulting events. The subsurface fault displacement history (Sexton and Jones, 1986 ;
Van Arsdale, 2000 ; Champion et al ., 2001 ) further indicates that the Reelfoot fault turns
on and off through time. This raises the question: what caused the fault to become active
during the Holocene?
±
7.3.8 Proposed Holocene triggering mechanisms of the NMSZ
Kenner and Segall ( 2000 ) model the NMSZ as having an elastic lithosphere with a weak
lower-crustal zone and show that a prolonged sequence of large earthquakes can result
from a local perturbation of the stress field by changes in thermal state, pore pressure, or
most likely recession of the Laurentian ice sheet 14,000 years ago. Grollimund and Zoback
( 2001 ) proposed that glacial unloading north of the NMSZ increased seismic strain rates
in the NMSZ at the end of the Wisconsin; however, Wu and Johnston ( 2000 ) previously
argued that glacial unloading is unlikely to have triggered the large earthquakes in New
Madrid. A problem with having the Wisconsin ice sheet retreat cause the onset of Holocene
seismicity in the NMSZ is that there have been perhaps as many as 20 ice sheet advances
and retreats during the Pleistocene (Easterbrook, 1999 ) , yet there has been only 31 m of
displacement on the Reelfoot fault since the Eocene. There appears to have been something
unique about the late Wisconsin or early Holocene that initiated the Holocene reactivation
of the NMSZ.
Search WWH ::




Custom Search