New Madrid seismic zone in Missouri has long intrigued scientists
because, according to conventional geologic theory, large earthquakes
clustered in a tectonically quiet region are difficult to understand.
But at least one AAPG member is challenging the crowd.
New Orleans independent geologist Jack M. Reed believes the origin
of the earthquakes lies beneath the Gulf of Mexico.
That's not all.
Reed, a retired Texaco geologist-geophysicist who has been studying
the region's geology for over 40 years, says the accepted theory
of a quiet geologic evolution of the Gulf of Mexico Basin is fundamentally
flawed and needs to be revised.
According to him, the Gulf was and is tectonically active — and
it is the likely origin for not only the New Madrid seismic activity,
but also for the Middleton Place-Summerville seismic zone near Charleston,
"For all the years I have worked the Gulf of Mexico Basin I have
been forced to accept the 'passive' Gulf formation theory, which
holds that the only movement in the basin is updip sedimentary loading
that moved the salt southward," Reed said. "But there is little
evidence to support this theory, and it doesn't fit what is observed
geologically or geophysically.
"As Hugh Wilson said (1993), 'It would be geologically unusual
for such a large basin as the Gulf of Mexico to remain almost tectonically
undisturbed for 170 million years while major orogenic disturbances
repeatedly struck bordering areas.'"
Reed, over the years, has gathered evidence that supports plate
motion in the Gulf basin. Thick salt and sedimentary sequences in
the basin mask this tectonic motion, but there is enough basin and
peripheral evidence to show plate readjustment is occurring — evidence,
he says, in the form of volcanics, earthquakes and rift zones that
are accompanied by magnetic, refraction, seismic and gravity data.
One piece of this evidence, according to Reed, is the apparent
connection of the New Madrid seismic zone with the Gulf rift features
to the south.
"This northeast trending earthquake zone appears to connect with
the northeast trending Monroe Uplift, the LaSalle Arch and, possibly,
to an active seismic zone located in and around Sabine Lake on the
Texas-Louisiana border," he said.
This complex of doming and seismic centers is similar to another
Cretaceous age triple juncture located in the northeastern Gulf
of Mexico Basin. Doming of the DeSoto Canyon High during the Jurassic
to Cretaceous created this triple juncture, which includes the Cretaceous
Shelf Edge, the Suwannee Strait and the West Florida Escarpment.
If the New Madrid seismic zone is indeed part of a triple juncture,
he continued, there should be an expression of this limb trending
along a line in a northeast direction.
So Reed conducted a study using data from the U.S. Geological Survey's
National Earthquake Information Center and the USGS map "Earthquakes
in the Conterminous United States." He only studied earthquakes
measuring at least magnitude 5, and found that while most of the
earthquake centers are random with no alignment, there is a well-defined
earthquake trend extending northeastward from the New Madrid seismic
zone across the United States to Canada, where it joins with the
St. Lawrence River seismic zone.
Within the boundaries of this earthquake alignment there are:
- Sixty-one seismic points that have a magnitude of 5 and greater.
- Several large earthquakes dating to the early 1800s, all measuring
over magnitude 8, all occurring within a couple of months of each
other, all centered in a northeast trending line.
- The two 5+ earthquakes that occurred earlier this year in northern
New York state and southern Indiana.
"There is definitely some form of movement occurring along this
trend," Reed said, "and it appears to be active today."
As he continued that trend south of New Madrid he found that it
was in line with the Monroe Uplift.
"Suddenly I could see that this area had doming much like I had
seen at the Desoto Canyon in the Gulf," he said. "This entire zone
through the United States is suffering some type of tectonic activity
that I believe is tied to the deeply buried tectonics in the Gulf
Reed has developed an interpretation, which he is quick to point
out is just a theory, on this tectonic activity:
- As Africa jammed into North America and thrust up the Appalachians,
there were compressional forces acting along the front of the
- This force caused long linear thrust faults to form in front
of and parallel to the uplifting mountain chain.
- Not only was the lithosphere thrust upward to form the Appalachians,
some of the lithosphere was thrust downward into the asthenosphere.
The result was a long, prominent bulge extending into the asthenosphere
over the length of the Appalachians.
- As separation from Africa occurred and the North American continent
began to move westward, tensional forces came into play in the
trailing edge of this moving plate. The long lithospheric downward
bulge became an impediment to this plate motion as it moved over
and through the asthenosphere.
- This action caused a drag in plate motion with tensional forces
being the greatest along a line in front of and parallel to the
"The results would be gradual separation of the lithosphere,"
he said, "likely along the older thrust fault trend."
Reed believes this rifting certainly impacted the Gulf of Mexico
He also concedes that even with the years of research, several
unanswered questions still exist about the Gulf's origins:
Regarding one of the most accepted theories, that updip sedimentary
loading moved the salt masses in the Gulf seaward: Where is the
original salt basin?
"If this salt mass movement was reversed by paleo-reconstruction
and moved back to the original salt deposition basin, which would
be directly south of the Cretaceous Shelf Edge, the original salt
thickness would have to be in a six-figure range to accommodate
this huge salt mass," he said. "It would dwarf the Himalayas."
salt distribution and the sub-horizontal, allochthonous salt flow
theory: Wouldn't the salt have to be vented to an open, deepwater
environment where it would, very questionably, remain unprotected
for several geologic stages until covered by sediments of a later
"This theory doesn't take into consideration the role of the
Interior basin and the Cretaceous Shelf Edge," he said, which
separates the Interior and Exterior salt basins and is described
in most literature as a simple Lower Cretaceous carbonate bank
"If this is true, why does a much younger Cretaceous feature
divide the lower Jurassic salt basin into two parts?
"Why is there a strong magnetic response along the length of
"Why has there been igneous intrusives found along the shelf
The Cretaceous Shelf Edge along with the Suwannee Strait and the
West Florida Escarpment, according to Reed, appear to be part of
a triple juncture centered on the DeSoto Canyon High.
"The DeSoto Canyon High is one of the largest and most significant
structures found in the Gulf," he said, measuring 137 miles by 60
miles, with over 7,000 feet of structural relief at the top of the
"The Cretaceous Shelf Edge, the Suwannee Strait and the West Florida
Escarpment all traverse different areas of the Gulf," he said, "but
they each appear to have evolved during the same general time period
and they all radiate from the central DeSoto Canyon High. This alignment
would be similar to triple junctures found throughout the world."
The Suwannee Strait is a shallow Upper Cretaceous depression that
extends across southern Georgia and northwestern Florida for over
200 miles. Reed said several theories have been proposed to explain
the cause of the feature, but as one geologist indicated it has
been easier to infer the existence of the strait than to account
According to Reed, a northeastward linear projection of the Suwannee
Strait would extend the feature to the earthquake cluster zone 20
miles inland from Charleston, South Carolina.
The West Florida Escarpment, the third element of the DeSoto Canyon
triple juncture, has been described as a simple carbonate bank build-up.
While Reed acknowledged that is true, age dating of samples collected
during deepwater dredging along the escarpment indicates the feature
is comprised of peritidal and lagoonal limestones with back-reef
faunal assemblages deposited under restricted, low-energy conditions.
"If the steep slope face is made up of back reef material, then
the question arises of where the fore-reef and the reef complex
are located," he said. "It would be unlikely that equal erosion
occurred over the entire escarpment, removing only the fore-reef
and reef complex and leaving the back-reef.
"It would appear that some other method," he added, "such as transform
shear motion, was responsible for removal of the main part of the
According to Reed, tectonic plate separation apparently began along
the Cretaceous Shelf Edge with the separated plate moving south,
causing rifting in a zone of the basin that contained a very thick
section of Jurassic salt.
This reorganization of tectonic plates in the basin would cause
major orogenic movements along the bordering areas.
The rift cutting northeast through the United States, where earthquake
activity has been documented in New Madrid as well as other points,
is just one of these major orogenic movements. Other geologic trends
that illustrate this point include:
Late Miocene Trans-Mexico neovolcanic belt, which extends
across Mexico to the Gulf Coast but apparently it never extended
into the deep Gulf of Mexico Basin.
The Tuxlas volcanic field is located south of this termination
and these two volcanic features seem to be connected by a series
of three submarine volcanoes.
"When the right-lateral transform of the Gulf plate is extended
south it falls along the trend of the submarine volcanoes," he said.
"This path also takes the transform slightly west of the Tuxlas
volcanic field and slightly east of the trans-Mexico volcanic belt,
indicating that transform movement may have displaced these two
"South movement of the Gulf plate would be necessary for this volcanic
shifting scenario," he added.
east-west trending mountain ranges of Guatemala, Honduras
and Nicaragua, which were formed under compressional forces, according
to Reed. These mountain ranges extend offshore in the Caribbean
Sea as seamounts.
Reed asks, "Shouldn't these mountains be trending north-south to
accommodate east-west compressional forces exerted by the east moving
Caribbean plate? A south moving Gulf of Mexico plate would provide
the compressional forces needed to build this east-west mountain
Also, the east-west trending seamounts do not extend further east
of a point where the left lateral transform fault of the Gulf plate
is projected into the Caribbean plate.
"Eastward of this point there would be no north to south compressional
forces," he said.
example within the deep Cayman Trench, where there is sea-floor
spreading site building in a deep trench environment.
Sea-floor spreading is generally found along mid ocean ridges and
forms high positive features. However, this site in the deep Cayman
Trench is an extreme negative zone and prominent north-south trending
spreading ridges have been generated. This zone is located east
of the left lateral transform offset of the Caribbean plate.
"Motion of the south-moving Gulf of Mexico plate could solve this
sea floor spreading problem," he said. A southward projection of
the Gulf plate's left lateral transform would connect it with the
Caribbean plates left lateral transform offset zone. The ramming
of the Gulf plate into the east moving Caribbean plate would generate
a zone of negative pressure east of the Caribbean plate offset.
"This action would bring hot magma into this portion of the Cayman
Trench where it would build spreading ridges oriented north-south,"
The earthquakes in the New Madrid seismic zone and other locations
along the theorized rift zone that cuts across the United States
as well as the Middleton Place-Summerville seismic zone in South
Carolina are tied to the tectonic movement in the Gulf of Mexico
Basin, he said.
So, the obvious question for Reed is, what's the exploration potential
of this rift zone that cuts through North America?
"Not much," Reed conceded. "However, if you want waterfront property
you should buy land around Indianapolis. In a couple of million
years this acreage could be overlooking the Strait of America that
separates western (and) eastern America!"