Magnetic anomaly deep within Earth’s crust reveals Africa in North America

Boulder, Colo., USA – The repeated cycles of plate tectonics that have led to collision and assembly of large supercontinents and their breakup and formation of new ocean basins have produced continents that are collages of bits and pieces of other continents. Figuring out the origin and make-up of continental crust formed and modified by these tectonic events is a vital to understanding Earth’s geology and is important for many applied fields, such as oil, gas, and gold exploration.

In many cases, the rocks involved in these collision and pull-apart episodes are still buried deep beneath the Earth’s surface, so geologists must use geophysical measurements to study these features.

This new study by Elias Parker Jr. of the University of Georgia examines a prominent swath of lower-than-normal magnetism — known as the Brunswick Magnetic Anomaly — that stretches from Alabama through Georgia and off shore to the North Carolina coast.

The cause of this magnetic anomaly has been under some debate. Many geologists attribute the Brunswick Magnetic Anomaly to a belt of 200 million year old volcanic rocks that intruded around the time the Atlantic Ocean. In this case, the location of this magnetic anomaly would then mark the initial location where North America split from the rest of Pangea as that ancient supercontinent broke apart. Parker proposes a different source for this anomalous magnetic zone.

Drawing upon other studies that have demonstrated deeply buried metamorphic rocks can also have a coherent magnetic signal, Parker has analyzed the detailed characteristics of the magnetic anomalies from data collected across zones in Georgia and concludes that the Brunswick Magnetic Anomaly has a similar, deeply buried source. The anomalous magnetic signal is consistent with an older tectonic event — the Alleghanian orogeny that formed the Alleghany-Appalachian Mountains when the supercontinent of Pangea was assembled.

Parker’s main conclusion is that the rocks responsible for the Brunswick Magnetic Anomaly mark a major fault-zone that formed as portions of Africa and North America were sheared together roughly 300 million years ago — and that more extensive evidence for this collision are preserved along this zone. One interesting implication is that perhaps a larger portion of what is now Africa was left behind in the American southeast when Pangea later broke up.</P

The biggest mass extinction and Pangea integration

Relationships between geosphere disturbances and mass extinction during the Late Permian and Early Triassic are shown. -  ©Science China Press
Relationships between geosphere disturbances and mass extinction during the Late Permian and Early Triassic are shown. – ©Science China Press

The mysterious relationship between Pangea integration and the biggest mass extinction happened 250 million years ago was tackled by Professor YIN Hongfu and Dr. SONG Haijun from State Key Laboratory of Geobiology and Environmental Geology, China University of Geosciences (Wuhan). Their study shows that Pangea integration resulted in environmental deterioration which further caused that extinction. Their work, entitled “Mass extinction and Pangea integration during the Paleozoic-Mesozoic transition”, was published in SCIENCE CHINA Earth Sciences.2013, Vol 56(7).

The Pangea was integrated at about the beginning of Permian, and reached its acme during Late Permian to Early Triassic. Formation of the Pangea means that the scattered continents of the world gathered into one integrated continent with an area of nearly 200 million km2. Average thickness of such a giant continental lithosphere should be remarkably greater than that of each scattered continent. Equilibrium principle implies that the thicker the lithosphere, the higher its portion over the equilibrium level, hence the average altitude of the Pangea should be much higher than the separated modern continents. Correspondingly, all oceans gathered to form the Panthalassa, which should be much deeper than modern oceans. The acme of Pangea and Panthalassa was thus a period of high continent and deep ocean, which should inevitably induce great regression and influence the earth’s surface system, especially climate.

The Tunguss Trap of Siberia, the Emeishan Basalt erupted during the Pangea integration. Such global-scale volcanism should be evoked by mantle plume and related with integration of the Pangea. Volcanic activities would result in a series of extinction effects, including emission of large volume of CO2, CH4, NO2 and cyanides which would have caused green house effects, pollution by poisonous gases, damage of the ozone layer in the stratosphere, and enhancement the ultra-violet radiation.

Increase of CO2 concentration and other green house gases would have led to global warming, oxygen depletion and carbon cycle anomaly; physical and chemical anomalies in ocean (acidification, euxinia, low sulfate concentration, isotopic anomaly of organic nitrogen) and great regression would have caused marine extinction due to unadaptable environments, selective death and hypercapnia; continental aridity, disappearance of monsoon system and wild fire would have devastated the land vegetation, esp. the tropical rain forest.

The great global changes and mass extinction were the results of interaction among earth’s spheres. Deteriorated relations among lithosphere, atmosphere, hydrosphere, and biosphere (including internal factors of organism evolution itself) accumulated until they exceeded the threshold, and exploded at the Permian-Triassic transition time. Interaction among bio- and geospheres is an important theme. However, the processes from inner geospheres to earth’s surface system and further to organism evolution necessitate retardation in time and yields many uncertainties in causation. Most of the processes are now at a hypothetic stage and need more scientific examinations.

Subduction channel processes: New progress in plate tectonic theory

Two processes occur at the slab-mantle interface in continental subduction channel, with (a) physical mixing to produce the tectonic mélange of metamorphic rocks, and (b) chemical reaction of the overlying subcontinental lithospheric mantle (SCLM) wedge with aqueous fluid and hydrous melt from subducting continental crust. -  ©Science China Press
Two processes occur at the slab-mantle interface in continental subduction channel, with (a) physical mixing to produce the tectonic mélange of metamorphic rocks, and (b) chemical reaction of the overlying subcontinental lithospheric mantle (SCLM) wedge with aqueous fluid and hydrous melt from subducting continental crust. – ©Science China Press

The plate tectonic theory has been primarily developed in three stages. (1) From continental drift and seafloor spreading to oceanic subduction, laying a physical foundation of the plate tectonic theory. This was achieved by the recognitions that continents would be assembled to build a supercontinent Pangea with subsequent breakup to yield the present configuration, lithospheric plates buoyantly move on the asthenospheric mantle, and oceanic crust is subducted along trenches into the mantle. (2) From oceanic subduction to continental subduction and collision orogeny, with the first round of revolution to the plate tectonic theory due to the recognition of continental deep subduction to mantle depths. While deeply subducted oceanic crust was processed in the mantle and the returned to the surface by mafic magmatism, deeply subducted continental crust underwent ultrahigh pressure metamorphism at mantle depths and then exhumed to the surface as coherent mélanges. This provides a geodynamic framework of tectonic processes for continental accretion and assemblage through arc-continent and continent-continent collision orogenies. (3) From continental collision and marginal orogeny to intracontinental reworking, emphasizing the inheritance of orogenic materials in postcollisional stages. While continental collision results in continental accretion through marginal orogeny, intercontinental orogens are converted to intracontinental orogens. The deeply subducted continental crust is processed in subduction channel underlying the mantle wedge, with partial return to the surface. These have thrown new lights on developing the plate tectonic theory to encompass the continental tectonics, and thus directed further study toward solution to such questions as how thinning of the orogenic lithosphere and upwelling of the asthenospheric mantle affect postcollisional reworking of the intracontinental materials.

Finding of ultrahigh pressure index minerals such as coesite and diamond in metamorphic rocks of continental supercrustal protolith demonstrate that these rock were subducted to mantle depths for ultrahigh pressure metamorphism and then returned back to the surface. The recognition of continental deep subduction by Earth scientists has not only developed the plate tectonic theory, but also expanded the chemical geodynamics focusing on the recycling of crustal material. The study of ultrahigh pressure metamorphic rocks has made prominent progress in many aspects, achieving the recognitions that the processes of continental seduction and exhumation have caused not only various types of structural deformation and mineralogical reaction but also different extents of metamorphic dehydration and partial melting (Fig. 1). By means of studying various rocks in continental collision orogens, Earth scientists have set the geodynamic link between the subduction and exhumation of continental crust and the building of collision orogens. Furthermore, it is established that bulk melting of the deeply subducted continental crust gives rise to granitic rocks whereas partial melting of the subducting supracrustal rocks produces felsic melt that reacts with the overlying mantle wedge peridotite to generate fertile and enriched mantle sources for mafic magmatism after their storage in different periods.

A research team at School of Earth and Space Sciences in University of Science and Technology of China has taken the rocks of continental collision orogens as the object, and performed a great deal of investigations from field observations and laboratory analyses. This leads to a tectonic analysis of geological processes in continental subduction factory, which is published in Chinese Science Bulletin 2013 (26) in the title “Continental subduction channel processes: Plate interface interaction during continental collision”. Leader of this team is Professor ZHENG Yongfei, Academician of the Chinese Academy of Sciences at Key Laboratory of Crust-Mantle Materials and Environments. Major participants are Prof. ZHAO Zifu and Dr. CHEN Yixiang. Earth scientists of China have made a series of prominent progresses in the forefront and hotspot field of subduction channel, and their studies have exemplified successful applications of new techniques, new methods and new ideas to development of the plate tectonic theory.

The recognition of continental deep subduction and ultrahigh pressure metamorphism has provided not only a turning point in developing the plate tectonic theory, but also an excellent opportunity to study the time and mechanism of reworking continental lithosphere. It is intriguing to ask the following questions: (1) how are crustal slices detached at different depths and exhumed during continental subduction? (2) how do physical mixing and chemical reaction proceed between the deeply subducted crust and the overlying mantle wedge? (3) how are energy exchange and matter transfer realized at the plate interface of subduction zone? Prof. Zheng said, to study subduction channel processes, to determine the physical mixing and chemical reaction between the deeply subducted crust and the overlying mantle wedge under ultrahigh pressure conditions, and to understand the interaction at the plate interface of continental subduction zone and its associated fluid action and element transport, are a key to unravel such mysteries of Earth.

Ancient magma ‘superpiles’ may have shaped the continents

Researchers have linked two giant plumes of hot rock deep within the earth to the plate motions that shape the continents. This new drawing of Earth's interior is based on one originally developed by study co-author Louise C. Kellogg of the University of California, Davis and her colleagues in 1999. A giant plume of hot rock called a 'superpile' (orange) sits atop Earth's core (red), while the remnants of two subducted continental plates (blue) sink down on either side of it. A magma plume (orange with red outline) can be seen rising from the superpile to the surface as a hotspot that creates island chains such as Hawaii. -  Image by the Cooperative Institute for Deep Earth Research (CIDER) collaboration, courtesy of Ohio State University.
Researchers have linked two giant plumes of hot rock deep within the earth to the plate motions that shape the continents. This new drawing of Earth’s interior is based on one originally developed by study co-author Louise C. Kellogg of the University of California, Davis and her colleagues in 1999. A giant plume of hot rock called a ‘superpile’ (orange) sits atop Earth’s core (red), while the remnants of two subducted continental plates (blue) sink down on either side of it. A magma plume (orange with red outline) can be seen rising from the superpile to the surface as a hotspot that creates island chains such as Hawaii. – Image by the Cooperative Institute for Deep Earth Research (CIDER) collaboration, courtesy of Ohio State University.

Two giant plumes of hot rock deep within the earth are linked to the plate motions that shape the continents, researchers have found.

The two superplumes, one beneath Hawaii and the other beneath Africa, have likely existed for at least 200 million years, explained Wendy Panero, assistant professor of earth sciences at Ohio State University.

The giant plumes — or “superpiles” as Panero calls them — rise from the bottom of Earth’s mantle, just above our planet’s core. Each is larger than the continental United States. And each is surrounded by a wall of plates from Earth’s crust that have sunk into the mantle.

She and her colleagues reported their findings at the American Geophysical Union meeting in San Francisco.

Computer models have connected the piles to the sunken former plates, but it’s currently unclear which one spawned the other, Panero said. Plates sink into the mantle as part of the normal processes that shape the continents. But which came first, the piles or the plates, the researchers simply do not know.

“Do these superpiles organize plate motions, or do plate motions organize the superpiles? I don’t know if it’s truly a chicken-or-egg kind of question, but the locations of the two piles do seem to be related to where the continents are today, and where the last supercontinent would have been 200 million years ago,” she said.

That supercontinent was Pangea, and its breakup eventually led to the seven continents we know today.

Scientists first proposed the existence of the superpiles more than a decade ago. Earthquakes offer an opportunity to study them, since they slow the seismic waves that pass through them. Scientists combine the seismic data with what they know about Earth’s interior to create computer models and learn more.

But to date, the seismic images have created a mystery: they suggest that the superpiles have remained in the same locations, unchanged for hundreds of millions of years.

“That’s a problem,” Panero said. “We know that the rest of the mantle is always moving. So why are the piles still there?”

Hot rock constantly migrates from the base of the mantle up to the crust, she explained. Hot portions of the mantle rise, and cool portions fall. Continental plates emerge, then sink back into the earth.

But the presence of the superpiles and the location of subducted plates suggest that the two superpiles have likely remained fixed to the Earth’s core while the rest of the mantle has churned around them for millions of years.

Unlocking this mystery is the goal of the Cooperative Institute for Deep Earth Research (CIDER) collaboration, a group of researchers from across the United States who are attempting to unite many different disciplines in the study of Earth’s interior.

Panero provides CIDER her expertise in mineral physics; others specialize in geodynamics, geomagnetism, seismology, and geochemistry. Together, they have assembled a new model that suggests why the two superpiles are so stable, and what they are made of.

As it turns out, just a tiny difference in chemical composition can keep the superpiles in place, they found.

The superpiles contain slightly more iron than the rest of the mantle; their composition likely consists of 11-13 percent iron instead of 10-12 percent. But that small change is enough to make the superpiles denser than their surroundings.

“Material that is more dense is going to sink to the base of the mantle,” Panero said. “It would normally spread out at that point, but in this case we have subducting plates that are coming down from above and keeping the piles contained.”

CIDER will continue to explore the link between the superpiles and the plates that surround them. The researchers will also work to explain the relationship between the superpiles and other mantle plumes that rise above them, which feed hotspots such as those beneath Hawaii and mid-ocean ridges. Ultimately, they hope to determine whether the superpiles may have contributed to the breakup of Pangea.

Dunes, climate models don’t match up with paleomagnetic records





Sandstone cliff in Vermillion Cliffs National Monument, Ariz., composed of the deposits of about 15 large, Jurassic sand dunes (person for scale in upper right). Each dune migrated toward the present southeast (toward the left in this photo). The dune sand accumulated about 200 million years ago, just above sea level in a slowly subsiding sedimentary basin. Circulating groundwater cemented the sand into sandstone. Uplift of the region in the last 10 million years led to erosion of the rocks, forming canyons and cliffs. Climate models constructed for the supercontinent Pangaea suggest that the sands accumulated near the equator, and were swept by strong monsoonal winds that reversed direction every six months. Evidence based on the magnetic properties of rocks; however, indicate that the sand accumulated much farther north -- about 20 degrees north of the equator. (Image copyright Science)
Sandstone cliff in Vermillion Cliffs National Monument, Ariz., composed of the deposits of about 15 large, Jurassic sand dunes (person for scale in upper right). Each dune migrated toward the present southeast (toward the left in this photo). The dune sand accumulated about 200 million years ago, just above sea level in a slowly subsiding sedimentary basin. Circulating groundwater cemented the sand into sandstone. Uplift of the region in the last 10 million years led to erosion of the rocks, forming canyons and cliffs. Climate models constructed for the supercontinent Pangaea suggest that the sands accumulated near the equator, and were swept by strong monsoonal winds that reversed direction every six months. Evidence based on the magnetic properties of rocks; however, indicate that the sand accumulated much farther north — about 20 degrees north of the equator. (Image copyright Science)

For a quarter-century or more, the prevailing view among geoscientists has been that the portion of the ancient supercontinent of Pangea that is now the Colorado Plateau in southern Utah shifted more than 1,300 miles north during a 100-million year span that ended about 200 million years ago in the early Jurassic Period, when Pangea began to break up.



Paleomagnetic records are found in iron-bearing minerals in rocks and can record the direction of the Earth’s magnetic field at the time of their formation. Paleomagnetism is an important tool for geoscientists in tracking the movement of Earth’s tectonic plates over time and records in North America indicate that the Colorado Plateau moved from the equator to about 20 degrees north latitude from 300 million years ago to 200 million years ago.



But new research by geoscientists from the University of Nebraska-Lincoln and the University of Michigan challenges that theory, based on extensive climate modeling studies and sedimentary records found from Wyoming into Utah and Arizona.



In the Nov. 23 issue of the journal Science, UNL geoscientists Clinton Rowe, David Loope and Robert Oglesby, former UNL graduate student Charles Broadwater, and Rob Van der Voo of the University of Michigan, report findings that indicate the area must have remained at the equator during the time in question.



“It’s a puzzle, a ‘conundrum’ is the word we like to use,” Oglesby said. “And in the Science paper, we’re not solving the conundrum, we’re raising the conundrum.”



The root of the conundrum is Loope’s ongoing research in the Colorado Plateau that began when he was working on his doctorate at the University of Wyoming in the early 1980s. A sedimentologist and an expert on dune formation, he eventually saw that from central Wyoming into central Utah, ancient dunes preserved in the region’s 200 million- to 300-hundred-million-year-old sandstone formations all faced southwest, meaning that the winds over that extensive area were almost constantly from the northeast. As his study progressed, he discovered that the direction of the dunes shifted to the southeast in what is now southern Utah, meaning the wind direction shifted to the northwest. What’s more, those prevailing winds were consistent over the entire 100 million years in question and the shift in wind direction could only have occurred at the equator.



“I thought that was very curious,” Loope said. “It didn’t seem to fit with what we think we know about where the continents were.”



Loope is also a paleoclimatologist (someone who studies ancient climates), as are Rowe and Oglesby, who also have expertise in climate modeling. The three geoscientists began working together, trying to find a computerized climate model that would explain the discrepancy, but they couldn’t find any that worked.





Jurassic Navajo Sandstone in Zion National Park, southern Utah. The sloping lines within the sandstone (crossbeds) indicate the sediment accumulated in a large dune field. (Image copyright Science)
Jurassic Navajo Sandstone in Zion National Park, southern Utah. The sloping lines within the sandstone (crossbeds) indicate the sediment accumulated in a large dune field. (Image copyright Science)

“We ran the model in any different number of configurations just to see if we could make it do something different,” Rowe said. “It didn’t matter what we did to it, as long as you had some land, and it was distributed north and south of the equator, you would end up with this monsoonal flow that matched these records from the dunes.



“The equator is the only place you could get this large-scale arc of winds that turn from the northeast to the northwest as they moved south. Nowhere else would you get that as part of the general circulation unless the physics of the world 200 million years ago was very different from what it is today. And we just don’t think that’s the case.”



Puzzled by the discrepancy between their research and the paleomagnetic records, they turned to Van der Voo, an expert on paleomagnetism.



“We brought Rob in to try to see if he could help us sort it out, and he’s like, ‘Gosh, guys, I don’t know. This is a conundrum,'” Oglesby said. “It’s important to note that we have not just a paleomag person as a co-author, but arguably the best-known paleomag person in the world — and he’s as confused as we are.”



Van der Voo agreed that, for now, there’s no clear answer to the conundrum.



“The nicest thing would have been if we had a solution, but we don’t,” Van der Voo said. “All we can say is that we have this enigma, so perhaps our model of Pangea for the period in question is wrong or the wind direction didn’t follow the common patterns that we recognize in the modern world. Neither seems likely, but we’re bringing this inconsistency to the attention of the scientific community in hopes of stimulating further research.”



And further research is exactly what’s on the agenda, Oglesby said.



“We’ll come up with everything we can possibly think of,” he said. “From the point of view of the climate model, the paleogeography, the vegetation, the topography, local-scale vs. large-scale, paleomag, going back and rethinking everything that the dunes tell us. We’ll go back to square one in everything, trying to figure it out.”