* Astronomy

Origin of the Rheic Ocean: Rifting along a Neoproterozoic suture?

There is growing evidence that a series of supercontinents have assembled and dispersed over the last 3 billion years. The youngest of these supercontinents, Pangea, formed about 300 million years ago and its dispersal over the past 200 million years resulted in the formation of the Atlantic Ocean. Although the existence of Pangea is a cornerstone of plate tectonics, geologists still do not understand the mechanisms responsible for its amalgamation. Scientists know when and where, but not why and how. One of the chief obstacles is the lack of understanding of the origin and evolution of an ocean called the Rheic Ocean, which formed about 490 million years ago. The demise of the Rheic Ocean was associated with terminal continental collision and the formation of Pangea. Murphy et al. propose that the Rheic Ocean rift may have been located along pre-existing weak structures in the crust along the Gondwanan margin. Murphy et al. compare the isotopic characteristics of terranes rifted from Gondwana with adjacent regions that remained along the margin. The terranes rifted from Gondwana have isotopic characteristics indicating that they originally represent recycled oceanic lithosphere formed between ~1,000 and 750 million years ago that were accreted to the Gondwanan margin about 650 million years ago. Murphy et al. suggest that the rift that formed the Rheic Ocean was located near the boundary between the accreted terranes with oceanic heritage and the old margin of continental northern Gondwana. This conclusion has important implications for geodynamic models of the formation of oceanic crust.

Like pieces in a giant jigsaw puzzle, continents have split, drifted and merged again many times throughout Earth's history, but geologists haven't understood the mechanism behind the moves. A new study now offers evidence that continents sometimes break along preexisting lines of weakness created when small chunks of land attach to a larger continent.

The paper — the cover story in the latest issue of Geology, the journal of the Geological Society of America — is the first to provide an explanation for the breaking patterns of continental plates, and uses the formation of an ocean about 500 million years ago to demonstrate that principle.

"We asked the question, 'Why do oceans open where they do, and why does a continent choose to break where it does?'" - Damian Nance, Ohio University professor of geological sciences and co-author of the study.

Throughout Earth's history, there have been six major continental assembly and breakup events, about 500 million years apart. Currently the Earth is in breakup cycle in which the Atlantic and Indian oceans are opening.
The new study found that continents sometimes break along preexisting lines of weakness created during earlier continental collisions. Geologists had long suspected that break lines were created by the attachment of pieces onto larger land masses, but Nance and his co-authors were the first group to be able to prove this theory.

About 650 million years ago – when the first jellyfish evolved – North America, South America and Africa were stuck together as one large continent called Gondwana, with some smaller islands floating on a neighbouring continental plate. Over time, these islands collided with the large group of continents and were attached to it in a process called accretion.
About 525 million years ago, that land mass broke apart, with North America on one side and South America, Africa and the small island pieces on the other. The two plates drifted apart, forming the Iapetus Ocean.
Twenty-five million years later – at the time of the first fish and land plants – the strip of land that used to be the small islands broke off South America and Africa and began moving across Iapetus towards North America. This movement closed the Iapetus Ocean while at the same time opening the Rheic Ocean.

Nance and his co-authors focused on these two particular breaks because they occurred along a "line of weakness" – namely the spot where the small islands had attached to the larger land mass. As the internal structure of the continent was already less stable there than it was across the two solid outside pieces, the continent broke along this preexisting line.
The scientists used geochemical "fingerprinting" to show that the small pieces of land, which today are found in the Appalachians, were originally created in an ocean. The radioactive element Samarium, which breaks down into various types of the element Neodymium, was used to determine the age of the rock (about one billion years). The amount of each element was typical of rock created in the ocean, away from larger continental masses.
The research is part of Nance's larger interest in the Rheic Ocean, which he has been studying for more than a decade. He is part of a multinational UNESCO project to examine the history of this ocean and has conducted work in Mexico and Europe. The present study was funded by the National Science Foundation, the Natural Sciences and Engineering Council of Canada, the Spanish Ministry of Education and Science and a Mexican Papiit Grant.

The study's lead author was J. Brendan Murphy of St. Francis Xavier University, Antigonish, Canada. In addition to Nance, the other authors were Gabriel Gutierrez-Alonso of the Universidad the Salamanca, Salamanca, Spain; Javier Fernandez-Suarez of the Universidad Complutense, Madrid, Spain; J. Duncan Keppie of the Universidad Nacional Autonoma de Mexico, Mexico City, Mexico; Cecilio Quesada of IGME, Madrid, Spain; Rob A. Strachan of the University of Portsmouth, Portsmouth, Great Britain; and Jarda Dostal of St. Mary's University, Halifax, Canada.

http://www.geosociety.org/

University of Minnesota researchers Renata Wentzcovitch and Koichiro Umemoto and Philip B. Allen of Stony Brook University have modelled the properties of rocks at the temperatures and pressures likely to exist at the cores of Jupiter, Saturn and two exoplanets far from the solar system.

They show that rocks in these environments are different from those on Earth and have metallic-like electric and thermal conductivity. These properties can produce different terrestrial-type planets, with longer-lasting magnetic fields, enhanced heat flow to the planetary surfaces and, consequently, more intense "planetquake" and volcanic activity.

This work builds on the authors' recent work on Earth's inner layers and represents a step toward understanding how all planets, including Earth, come to acquire their individual characteristics.
The work offers new insight for interpreting properties of this region. The D’’ (Dee double prime) layer surrounds Earth’s core and is between 0 and 186 miles thick. It is at the interface between two chemically distinct regions, the rocky mantle and the metallic core. The article, “MgSiO3 post-perovskite at D’’ conditions,” was published on Jan. 17 in Proceedings of the National Academy of Science (www.pnas.org/content/vol103/issue3/#GEOPHYSICS).

The research “tells us how to better model Earth’s internal processes. Proper geodynamical modelling of the Earth is necessary to get a better grasp of the dynamics of the surface. You can’t fully understand Earth’s surface motion without understanding how it moves inside. What’s unbelievable is how well we can model Earth on a big scale. At this scale, small details don’t matter.” - Renata Wentzcovitch.

In 2004, Japanese researchers at the Tokyo Institute of Technology found that high temperatures and pressures transform perovskite, the major mineral in Earth’s mantle, into a new mineral called post-perovskite. Wentzcovitch’s group contributed to this discovery by determining the structure of post-perovskite and by calculating the pressure and temperature conditions for its existence. They matched the conditions in the D’’ layer.
Wentzcovitch and colleagues demonstrate that the seismic properties of post-perovskite are much like the previously inexplicable properties found in the D’’ layer. This is the most convincing evidence that post-perovskite is in the D’’ layer and produces its strange seismic properties.
As the Earth cools, D’’ becomes thicker. Its thickness is related to Earth’s age and its aging processes. The discovery of post-perovskite in the D’’ layer will also help us understand how the Earth has evolved.

In the new work, the researchers turned their attention to the cores of the giant planets of our solar system--Jupiter, Saturn, Uranus and Neptune--and two recently discovered extrasolar planets, or exoplanets, found elsewhere in the Milky Way. One, referred to as Super-Earth, is about seven times the mass of Earth and orbits a star 15 light-years away in the constellation Aquarius. The other, Dense-Saturn, has about the same mass as Saturn and orbits a star 257 light-years away in the constellation Hercules.

The researchers calculated what would happen at temperatures and pressures likely near the cores of the two exoplanets, Jupiter and Saturn, where temperatures run close to 18,000 F and pressures 10 million bars (a bar is essentially atmospheric pressure at sea level).
They found that even post-perovskite could not withstand such conditions, and its crystals would dissociate into two new forms. Focusing on one of those crystals, the researchers discovered that they would behave almost like metals. That is, electrons in the crystals would be very mobile and carry electric current. This would have the effect of supporting the planet's magnetic field (if it has one) and inhibiting reversals of the field. The increased electrical activity would also help transport energy out of the core and toward the planet surface. This could result in more severe activities such as quakes and volcanoes on the surface. The effect would be much stronger in Dense-Saturn than in Super-Earth.

The interiors of the icy giants Uranus and Neptune don't exhibit such extremes of temperature and pressure, and so post-perovskite would survive in their cores, she said.

"We want to understand how planets formed and evolved and how they are today. We need to understand how their interiors behave under these extreme pressure and temperatures conditions. Only then it will be possible to model them. This will advance the field of comparative planetology. We will understand Earth better if we can see it in the context of a variety of different kinds of planets" - Renata Wentzcovitch.

University of Minnesota