The largest asteroid impact crater was discovered in Australia
Buried under several kilometers of sediments, two structures identified in Australia could well be the trace left by a double asteroid impact there are hundreds of millions of years. If this is the case with a diameter of 400 km, it would be the largest known impact crater on Earth. At the time, the fall of an asteroid of this size should have lead to a major biological attack. Problem: according to the dating of the impact, the event took place at a period not yet contain any known mass extinction.
With Canada, Australia is a paradise for lovers of craters asteroid impacts. A group of geologists led by Australian Andrew Glikson just confirmed by publishing an article in Tectonophysics announcing the probable discovery of the largest known impact crater. The previous record was held by the Vredefort meteorite crater, South Africa. Its diameter is estimated at 300 km and age two billion years. In the case of the structure discovered in the Warburton River basin, located in South Australia, we would be in the presence of a 400 km diameter structure. It would, however, not a single impact crater but two craters, wide about 200 km each, which were formed simultaneously due to the fragmentation of an asteroid.
As in the case of the Chicxulub crater, this discovery was made by serendipity. Oil exploration work conducted using analysis of seismic waves had indeed already revealed the existence of structures buried under more than 3 km of sediments. Also confirmed that other geophysical methods for prospecting, namely the gravity and magnetic survey. The surprise came primarily from the study of drill core made this time as part of the geothermal exploration.
Andrew Glikson and colleagues found by examining microscopic quartz found in rock samples brought to the surface, as these features contained traces of colossal shock waves. In other words, they were shocked quartz, strong indicators of asteroid impacts. But this conclusion was enough to make the geosciences specialists particularly perplexed.
An older crater that the extinction of the Permian-Triassic
Indeed, the dating of rocks present above structures that appear to result from a double asteroid impact indicates that it had to happen there are at least 300 million years at most 420. If we had found as a lower bound to -200 million years, the discovery was loud as it would have been consistent with the greatest mass extinction ever known the biosphere, the Permian-Triassic there are about 252 million years. Now, with a astrobleme 400 km in diameter, the largest identified on the surface of the Earth (the Chicxulub is 180 km), it should be associate deposits and especially a major biological attack, which is absolutely not.
The announcement is taken with some skepticism by many colleagues Glikson. Christian Koeberl, an expert on asteroid impacts of the Museum of Natural History in Vienna (Austria) is, for example, not convinced by the arguments in the article Tectonophysics. For him, the geophysical anomalies detected by seismic and gravity do not resemble those encountered with astroblemes usually studied elsewhere on Earth. His interpretations of the shocked quartz also differ from those of his colleagues. The researcher does not recognize them as such but as manifestations of magmatic activity of our planet.
Glikson nevertheless persists but he concedes that it is a mystery and the researcher added: "We can not find great extinctions that coincide with these crashes. I suspect that the impact could be older than 300 million years.
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