Impacts on Earth in the Late Triassic
Spray et al. postulate that five widely dispersed terrestrial impact structures with very similar geological age estimates (about 214 million years ago, in the Late Triassic epoch) are evidence of a multiple impact event. Most notably, the three largest impact structures, Saint Martin in western Canada (~40 km diameter), Manicouagan in eastern Canada (~100 km diameter), and Rochechouart in France (~25 km diameter), plot at virtually the same palaeolatitude in a continental reconstruction. Spray et al. suggest that this apparent crater chain was produced within hours as a series of coaxial projectiles collided in rapid succession with the rotating planet Earth, and drew analogies to the recent collision sequence of fragmented comet Shoemaker-Levy 9 with Jupiter. However, published palaeomagnetic data for the Manicouagan and Rochechouart impact structures argue strongly against such a closely timed origin for these ancient events. This is because the characteristic remanent magnetizations of the melt rocks, including the most rapidly cooled glassy phases, indicate formation in a Late Triassic palaeomagnetic dipolar field of normal polarity at Manicouagan but of reverse polarity at Rochechouart.
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