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

The timing of the Grenville orogeny is still debated by scientists. Toby Rivers described the timeline in 2002, using the well-preserved Grenville Province as a guide. This timeline includes two main cycles of mountain building: the Rigolet, Ottawan, and Shawingian events, and the separate Elzevirian event. Because the Grenville events covered such a large area, the exact timing can vary slightly across the orogenic belt.

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

Rivers updated this timeline in 2008. According to 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 eastern and southern edges of Laurentia were active places where plates came together. This is similar to how the Andes in South America work today. Around 1190 to 980 million years ago, two big land masses bumped into Laurentia, much like how the Himalaya is growing now.

These events did not happen all at once. There were quiet times when big rocks called plutons pushed up into the surrounding rock. Scientists study these rocks to figure out when things happened, using special 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 northwest-moving folded rock layers and high-pressure changes in rock structure. These changes happened at medium to high temperatures and pressures. Some very special rock types, formed under extreme pressure, are found in certain places.

The Grenville orogeny is divided into three main parts based on rock types and how they changed over time. The Gneiss Belt has rocks that changed under very high temperatures and pressures. The Metasedimentary Belt contains rocks from old sediments and volcanoes, changed under a range of pressures. The Granulite Terrane includes rocks formed from older molten material, with some areas dating back nearly a billion years.

Regional variations

See also: Sveconorwegian orogeny

To understand the Grenville orogeny, it is helpful to look at different areas where its effects can be seen. The Grenville orogen is usually divided into four main areas: the southern part in Texas and Mexico, the Appalachians, the Adirondacks, and the Grenville Province. A part of this orogeny can also be found in Scotland, which shared a similar history with the Grenville Province before the Iapetus Ocean opened.

Texas and Mexico

Texas and Mexico mark the southern edge of Laurentia. Here, the land likely collided with a different continent than in the eastern collision. The Zapotecan orogeny in Mexico happened around the same time as the later stages of the Grenville orogeny and is often considered part of it. Rocks in Mexico have two main age groups, showing different types of ancient volcanic activity. One group of rocks suggests they formed near islands volcanoes, while the other formed in a different setting. It is thought that the way the earth moved under Laurentia changed around 1230 million years ago, affecting how the continents collided.

Appalachians

The Appalachian Mountains have small areas showing the Grenville orogeny. The largest area is in the Long Range Mountains of Newfoundland. Other spots include 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 that formed in three time periods.

Adirondacks

This area is a large dome of very old rock on the border of New York and Canada. Both major pulses of the Grenville orogeny left their mark here, creating 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 boundary during one of the pulses of the orogeny.

Grenville province

The Grenville province is named after Grenville, Quebec, and makes up the youngest part of the Canadian Shield. Because this area has not been changed by later major 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.