The 366 daily episodes in 2014 were chronological snapshots of earth history, beginning with the Precambrian in January and on to the Cenozoic in December. You can find them all in the index in the right sidebar. In 2015, the daily episodes for each month were assembled into monthly packages, and a few new episodes were posted. Now, the blog/podcast is on a weekly schedule with diverse topics, and the Facebook Page showcases photos on Mineral Monday and Fossil Friday. Thanks for your interest!

Wednesday, January 29, 2014

January 29. Grenville Orogeny – a billion years ago

By Richard I. Gibson

Here’s the podcast:

Orange = Grenville rocks
In North America the primary mountain building event associated with the assembly of the Rodinia supercontinent was the Grenville Orogeny. It’s named for a village in Quebec where the mountain roots are exposed today, part of a wide belt in eastern Canada that extends into the United States in the Adirondacks of New York, and in the subsurface beneath central Ohio and Kentucky, where the Grenville Front is clearly marked in geophysical data. It continues southwest into West Texas and Mexico, and rocks that record the Grenville event are found in Scotland and South America too.

What was colliding with North America to make this mountain range? That’s still an area of active research, but we think several long linear belts – possibly complicated island arcs like Japan – as well as some small continental fragments impacted to help create the supercontinent of Rodinia. Some models show blocks that are now parts of South America and South Africa impinging on North America, but we really aren’t sure.

Grenville rocks in Canada are typically high-grade metamorphic rocks, indicating that there was plenty of heat and pressure involved in the collisions.

The compression that created the Grenville overlapped in time somewhat with the extension that was creating the Mid-Continent Rift that we talked about a few days ago. How can you have compression and extension at the same time? It’s not hard at all. Today, the Red Sea is forming by pull-apart on the west side of the Arabian Peninsula, while a thousand kilometers away, on the east side of Arabia, active continent-continent collision is causing the earthquakes of Iran.

Further reading: see the links and references in the Wikipedia article.

Map by G. Mills (public domain) after Tollo and others, 2004, Petrologic and geochronologic evolution of the Grenville orogen, northern Blue Ridge province, Virginia, in Proterozoic tectonic evolution of the Grenville orogen in North America, Geological Society of America Memoir 197.

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