justdrew wrote:
I'm now officially worried that something is causing underground deposits to experience increased pressure levels, across the globe. There've been like at least three natural gas well accidents/problems in the last week and a half, the north sea well problem, and now one or two more rigs in the gulf seeming to be leaking a bit.
http://www.aapg.org/explorer/2002/11nov/rift_zone.cfmGulf's Evolution Makes the Shakes
View Enlargement of Rift Zone Map
Graphics courtesy of Jack Reed
The 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, S.C."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.
Questions? Answers!
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 of Mexico."Answers? Questions!
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 mountain range.
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 Appalachians.
"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 as well.
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."
Regarding 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 period?
"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 build-up.
"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 the feature?
"Why has there been igneous intrusives found along the shelf edge?"
Triple Threat?
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 Paleozoic.
"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 for it.
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 reef complex."
Other Examples
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:
The 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 volcanic fields.
"South movement of the Gulf plate would be necessary for this volcanic shifting scenario," he added.
The 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 trend."
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.
Another 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," he added.
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!"
