Shocking Geology of the New Madrid Seismic Zone Revealed

The New Madrid Seismic Zone is an indistinctly circumscribed region in the central United States distinguished from adjacent parts by its geologic instability manifested by earthquakes. This curious zone is also called the New Madrid Fault Zone, the New Madrid Fault Line, the New Madrid Rift Zone, the New Madrid Rift Complex, the Reelfoot Rift, and the Reelfoot Complex, among other appellations. The New Madrid Seismic Zone is an intraplate seismic zone--smack dab in the center of the North American Plate—as opposed to the more familiar interplate seismic zones, such as California’s San Andreas Fault Zone, which mark transform boundaries between tectonic plates.

The term “New Madrid” comes from the town of New Madrid (pop. 3,000 and declining), Missouri, which was at or near the epicenter of the infamous earthquakes of 1811-1812, and still exists on the west shore of an amazing wanna-be-oxbow of the Mississippi River. The term “Reelfoot” comes from the Reelfoot Lake formed during the New Madrid earthquakes of 1811 and 1812 when the Mississippi River overflowed its eastern banks, diverting water into a shallow depression. Reelfoot Lake, in turn, was named after Chief Reelfoot of the Chickasaw tribe about whom an Indian legend exists that ties together his clubfoot and earthquakes in the region.(1)  

  

The New Madrid Seismic Zone (NMSZ) during a five-month period in late 1811 and early 1812 produced an earthquake sequence of thousands of earthquakes of all different sizes that constitutes the “greatest release of seismic energy in so short a time ever witnessed and recorded by human beings.” (1) The magnitude of these crustal movements—three of them are believed to have registered well above 8 on the Richter scale—in 1811-1812 was exceptional even for the NMSZ, which we now know generates almost continually smaller earthquakes that we cannot sense, including at earthquake hotspots” such as Enola, Arkansas, and New Madrid, Missouri. (2)

Scientists do not know what causes earthquakes. Most scientists are content theorizing that earthquakes are the result of movements along fault lines in the crust, but they do not know what precipitates the crustal movements in the first place. Recall that Thomas Gold believed, based on his knowledge of astronomy and the composition of other planets in our solar system, that methane gas violently escaping from the earth’s mantle and breaking overlying brittle rock is the cause of earthquakes. Furthermore, diamond-rich Kimberlite pipes and similar structures in various parts of the world, including the central United States, offer evidence of this theory. (3-5) Diamonds are made only in the mantle, which provides needed heat and pressure conditions. The presence of diamonds at the surface of the earth means they got there very quickly, possibly propelled by methane explosive gases. Did you know that Arkansas, Oklahoma, and Louisiana have produced huge diamonds? Near Murfreesboro, Arkansas, from what is now known to be a lamproite pipe, more than 100,000 stones averaging 0.25 carat each were produced between 1907 and 1933. (See: http://www.amnh.org/exhibitions/diamonds/north.html; accessed February 12, 2006.)

Knowing what causes earthquakes informs evaluation of the level of earthquake hazard in an earthquake region, such as the New Madrid Seismic Zone, and ways to help reduce the risk to lives and property from future large earthquakes, which are inevitable. Because of the catastrophic destruction that such earthquakes will cause, especially in densely populated areas like Memphis, Tennessee, researchers and planners associated with universities, state governments, and federal agencies since the 1980s have doggedly sought new knowledge about the NMSZ.

Delineating the boundaries of the New Madrid Seismic Zone is difficult because the Mississippi Embayment currently covers deeper geologic structures associated with earthquake generation. The Mississippi Embayment is a physiographic low-lying, sediment-filled basin. It extends from the Gulf of Mexico to Cairo, Illinois, as depicted beautifully in the photo below. The north and south boundaries of the most active portion of New Madrid Seismic Zone are at Marked Tree, Arkansas (south) and Cairo, Illinois (north). The width of the zone coincides well with the width of the central portion of the Mississippi Embayment. (6-8)

The Mississippi Embayment formed from the filling in of a shallow saltwater basin that existed as a bay jutting northward from the Gulf of Mexico into the North American continent as far as Illinois during the Cretaceous period (165 million years ago to 65.5 million years ago). The Mississippi Embayment sedimentary rocks are rich with now extinct clams, snails, sharks, and marine reptiles such as mosasurs and plesiosaurs that thrived during the Cretaceous. (7)

Between two and twenty miles below the surface of the Mississippi Embayment lies the rift of the earth’s crust called the Reelfoot Rift. A rift is a split in the crust. Some rifts such as the Great Rift Valley extending from Mozambique to Syria in East Africa and the Middle East are exposed rifts. The Reelfoot Rift is a buried rift. Because it is buried, it was only recently discovered. Currently, the information available for investigating the Reelfoot Rift consists of a limited number of deep wells, seismic lines which remain largely proprietary, and geophysical field data. Much of its history must therefore be inferred from knowledge of regional tectonic events and correlation of sedimentary sections from boreholes to exposed stratigraphy. (9)

The history of the Reelfoot Rift is plate tectonics. Recall that plate tectonics is the unifying geologic theory that attempts to explain observations that interactions of the brittle plates of the lithosphere with each other and with the softer underlying asthenosphere result in large-scale changes in the earth’s crust. The theory of plate tectonics initially stemmed from observations of the shapes of the continents, particularly South America and Africa, which fit together like pieces in a jigsaw puzzle and have similar rocks and fossils despite being separated by a modern ocean. (10)

Rifting of tectonic plates may result in the formation of “triple junctions” and “aulacogens”. A triple junction is the point at which three intersecting lithospheric plates are separated by “arms”. The arm along which the motion that spreads the plates apart ceases (no longer works) is termed the “failed arm”, “failed rift”, or “aulacogen”. (11-12) Spreading or rifting along the other two arms of the triple junction can form new oceanic basins, such as the Proto-Atlantic Ocean and the Atlantic Ocean described in more detail below. The Reelfoot Rift is a classic example of a failed arm or aulacogen.
The aulacogen often becomes a sediment-filled graben. A graben is a low-standing fault block bounded by opposing normal faults. The Reelfoot Rift aulacogen is indeed filled with sediments. (13)

The history of the Reelfoot Rift began during the late Precambrian period, 750 million years ago, when powerful geologic forces from deep within the upper mantle shot upward, penetrating the lower crust and then the upper crust of a supercontinent called Rodinia. Rodinia uplifted and formed deep rifts in the brittle rock in response to the mantle forces. This uplifting and cracking lasted until the Cambrian period (early Paleozoic) about 570 million years ago, and produced a series of Precambrian and Cambrian “rift basins” of which the Reelfoot Rift System is one.

One rift (NOT the Reelfoot Rift) split open ancient Rodinia into Laurentia to the north and Gondwana to the south, thereby forming the so-called “Proto-Atlantic Ocean” between them. The Proto-Atlantic Ocean was a narrow trough of seawater through the heart of North America from what is today Texas to Nova Scotia. It lasted “only” a few hundred million years. Compressive stresses later brought the plates back together again, closing off the Proto-Atlantic. (14) These compressive forces also formed, via subduction of the oceanic crust underneath the continental crust, at least two of the great mountain chains along the eastern side of the ancient supercontinent.

One mountain range, called the Ouachita Mountains, formed when offshore volcanic islands called the Taconic Arc, collided with the ancient continent. This period of mountain building is termed the “Taconic Orogeny” and occurred in the Paleozoic, about 440 million years ago. Geologists believe that the Taconic Orogeny had the greatest overall effect on the geologic structure of the basement rocks one sees today in New England, the Appalachians, and the Ouachita Mountains of Arkansas and Oklahoma. Roots of this worn-down mountain range are exposed in the Eastern Piedmont region of the United States that lies between the coastal plain and the Appalachian Mountains.

The second mountain range to form after the Taconic Orogeny was the Acadian Orogeny of the late Paleozoic. This era of mountain building, also resulting from a collision of a continental fragment into the ancient supercontinent, resulted in deformation of the eastern margin of North America and creation of the Appalachian Mountains, which were at one time huge. In the present day, much of the volume of this mountain range has been lost due to erosion and the full scope of deformation is lost from the geologic record.

During this vigorous era of ocean formation and mountain-building (600 million years ago to 250 million years ago) the Reelfoot Rift was sinking, or subsiding, forming a basin that filled with sediments, as previously noted. The subsidence was caused by one or more of the following factors: extension of the crust, caused by the earlier upward-pushing mantle forces of the Precambrian Era, thereby promoted thinning and sinking of the earth’s crust; accumulation of sediment loads; removal of fluid from subterranean reservoirs; and “hot, rising magma plaster[ing] itself to the ‘bottom’ of the tectonic plate above”, thereby adding weight. (14) The sedimentary rocks we find today from this period are very hard, well-consolidated limestones, dolomites, sandstones and shales.

The next major event to affect the Reelfoot Rift was renewed tension and pulling apart of another supercontinent—this time called Pangaea--during the early and late Mesozoic Era beginning 225 million years ago. The pulling apart along ancient rifts this time resulted in the creation of the modern Atlantic Ocean. “The land masses that were to become Europe and Africa pulled away form the land masses that were to become North and South America. Geologists call this ‘plate divergence’, where new plates are created at plate boundaries and spread both ways. This is still happening today, and the boundary of the plate on which most of North American is riding is located at the Mid-Atlantic Ridge.” (15)

In the New Madrid Rift Complex area, the Mesozoic Era stresses caused the area to uplift, caused old areas of weakness to fracture again (faulting), and allowed molten igneous rock (magma) to force its way into some of the faults near the margins of the rift zone.

During the Cretaceous period (the final period of the Mesozoic Era), the modern Mississippi Embayment was laid down, as noted above. Indeed, between the late Mesozoic up to the present, the New Madrid Rift Complex has been sinking again because of various forces, including compression from new crust being laid down at the Mid-Atlantic Ridge. “The modern embayment is attributed to the rise of sea level in the Cretaceous,” according to one observer. “With the break up of the supercontinent Pangaea and higher rates of sea floor spreading, the global sea level was considerably higher. The old rift zone was a natural downwarped basin and allowed a natural course for sea level to enter the continental interior as well as exit. When the ocean retreated, the Mississippi River was born. Since the Cretaceous, large amounts of sediment were deposited in the river valley.” (8)

The Quaternary period beginning 2.5 million years ago was the most recent shaper of the Mississippi Embayment overlying the Reelfoot Complex. The Ice Age spanned the interval between 1.6 million and 11,000 years ago (Pleistocene Epoch). “Before this, the Mississippi River drained a much smaller area than it does now. Much of the surface runoff in the north and east parts of the present-day Mississippi drainage area then flowed to the north or to the east. We know a lot about this because these old river valleys are well-preserved under thick deposits left the Ice Age glaciers. Water well drillers seek these valleys, because they are usually good sources of groundwater. So we know where most of them are.” (16)

During the Pleistocene glaciations, the advancing ice reached approximately to the present courses of the Missouri and Ohio Rivers. During this epoch, the sea level fluctuated. The lowest level in sea level allowed the Mississippi River to entrench deeper into Cretaceous and lower sediments. After the sea level rose, stream gradient decreased and the basin was sedimented in to its present level.

In summary, two to twenty miles under the Mississippi Embayment is the intraplate Reelfoot Rift.  The Reelfoot Rift did NOT cause a separation of the ancient supercontinents (Rodinia, Pangaea) like other rifts. Rather it was a “failed arm” or aulacogen that filled with sediment whenever it was in a sinking phase. The Reelfoot Rift formation DID create a weak zone in the earth’s crust that persists to the present. Geologists today believe that present-day earthquakes in the New Madrid Seismic Zone are caused by slippage along these pre-existing zones of weakness inherited from the origins of the Reelfoot Rift beginning 750 million years ago in the Precambrian Era. (15)

Sources:
1. Legend of Chief Reelfoot at: http://www.ecsis.net/dsv/lakecounty/reelfoot/legend.html; accessed February 12, 2006.
2. Thomas Gold: “Power from the Earth”, JM Dent & Sons, London, 1987, p. 47.
3. SEMP Biot #238: “What is a Kimberlite Pipe?” at: http://www.semp.us/biots/biot_238.html; accessed February 12, 2006.
4. SEMP Biot #182: “Oil Doesn’t Come From Squashed Fish?” at: http://www.semp.us/biots/biot_182.html; accessed February 12, 2006.
5. SEMP Biot #185: “Rethinking the Origin of Earthquakes and the Implication for Earthquake Prediction” at: http://www.semp.us/biots/biot_185.html; accessed February 12, 2006.
6. Ray Knox and David Stewart: “The New Madrid Fault Finders Guide.” Gutenberg-Richter Publications, Marble Hill, MO, 1995, p. 18.
7. “The Mississippi Embayment Synclinorium”, Arkansas Center for Earthquake Education and Technology Transfer, at: http://quake.ualr.edu/public/embayment.htm; accessed February 12, 2006.
8. Shawn Salley: “The Mississippi Embayment: The Quaternary Canvas” at: http://www.emporia.edu/earthsci/student/salley3/; accessed February 12, 2006.
9. “Geology and Geophysics of the Reelfoot Rift: A Position Paper” at:  http://associates.egi.utah.edu/Database/ReportReview.cfm?Record=128; accessed February 12, 2006.
10. “Plate tectonics” at: http://www.glossary.oilfield.slb.com/Display.cfm?Term=plate%20tectonics; accessed February 12, 2006.
11. “Aulacogen” at: http://www.glossary.oilfield.slb.com/Display.cfm?Term=aulacogen; accessed February 12, 2006.
12. Ray Knox and David Stewart: “The New Madrid Fault Finders Guide.” Gutenberg-Richter Publications, Marble Hill, MO, 1995, pp 20-21.
13. “Graben” at: http://www.glossary.oilfield.slb.com/Display.cfm?Term=graben; accessed February 12, 2006.
14. Ray Knox and David Stewart: “The New Madrid Fault Finders Guide.” Gutenberg-Richter Publications, Marble Hill, MO, 1995, p. 22.
15. Ibid, p. 23.
16. Ibid, p. 27.