Grenville orogeny

The Grenville orogeny was a long time ago when big mountains were built on Earth. It happened during a time called the Mesoproterozoic, and it helped put together a huge land called Rodinia. You can find pieces of these old mountains all over North America, from a place called Labrador all the way down to Mexico, and even in Scotland.

These old mountain parts are special because they tell us about how Earth looked a very long time ago. We can see similar mountain-building events in other parts of the world too, like in Africa, Western Europe, and Brazil, but they have different names. Even though these events happened in many places, the one in North America is called the Grenville orogeny.

Timescale

Scientists are still discussing when the Grenville orogeny happened. In 2002, Toby Rivers described the timeline using the well-preserved Grenville Province. This timeline has two main cycles of mountain building: the Rigolet, Ottawan, and Shawingian events, and the separate Elzevirian event. Because the Grenville events covered a large area, the exact timing can vary slightly across the orogenic belt.

Scientists learn the ages of these events by studying when rocks were formed and changed. The gaps between these rock-forming events suggest times when the land was stretching instead of squeezing together.

Rivers updated this timeline in 2008. In this newer timeline, the Elzevirian event happened from 1240 to 1220 million years ago, the Shawinigan event from 1190 to 1140 million years ago, and the Ottawan and Rigolet events from 1090 to 980 million years ago.

General tectonics

The Grenville orogeny happened when the edges of Laurentia were active places where plates came together. This is similar to how the Andes work today. Around 1190 to 980 million years ago, two big land masses bumped into Laurentia, like how the Himalaya is growing now.

These events did not all happen at once. There were quiet times when big rocks called plutons pushed up into the surrounding rock. Scientists study these rocks to learn when things happened, using tools like SHRIMP and TIMS for uranium-lead dating. The area went through phases of pushing together and stretching apart, similar to a pattern called the Wilson Cycle, which helps create ocean basins like the Iapetus Ocean.

General lithology

The Grenville orogeny is known for its folded rock layers that moved northwest and changes in rock structure caused by high pressure. These changes happened at medium to high temperatures.

The Grenville orogeny has three main parts based on rock types. The Gneiss Belt has rocks changed by very high temperatures and pressures. The Metasedimentary Belt has rocks from old sediments and volcanoes, changed under different pressures. The Granulite Terrane has rocks formed from older molten material, with some areas nearly a billion years old.

Regional variations

See also: Sveconorwegian orogeny

To learn about the Grenville orogeny, it helps to look at different places where we can see its effects. The Grenville orogen is split into four main areas: the southern part in Texas and Mexico, the Appalachians, the Adirondacks, and the Grenville Province. Part of this orogeny is also in Scotland, which had a similar past to the Grenville Province before the Iapetus Ocean opened.

Texas and Mexico

Texas and Mexico are at the southern edge of Laurentia. Here, the land probably bumped into a different continent than in the eastern crash. The Zapotecan orogeny in Mexico happened around the same time as the later steps of the Grenville orogeny and is often included with it. Rocks in Mexico have two main age groups, showing different kinds of old volcanic activity. One group of rocks suggests they formed near island volcanoes, while the other formed in a different place. It is thought that the way the earth moved under Laurentia changed around 1230 million years ago, affecting how the continents came together.

Appalachians

The Appalachian Mountains have small parts showing the Grenville orogeny. The biggest area is in the Long Range Mountains of Newfoundland. Other spots are in the Shenandoah and French Broad areas in the Blue Ridge province of Virginia. These rocks were changed by heat and pressure and include different types of igneous rocks from three time periods.

Adirondacks

This area is a large round of very old rock on the border of New York and Canada. Both big steps of the Grenville orogeny left their mark here, making rocks changed by extreme heat and pressure. A special feature, a shear zone, runs northwest and separates higher lands to the southeast from lower lands to the northwest. This zone likely acted as a border during one of the steps of the orogeny.

Grenville province

The Grenville province is named after Grenville, Quebec, and is the youngest part of the Canadian Shield. Because this area has not been changed by later big events, it is a great place to study the Grenville orogeny. The Laurentian Mountains are part of this province. Most of what we know about the Grenville orogeny comes from this area.