Antler orogeny

The Antler orogeny was a time when the Earth's plates moved in Nevada during the Mississippian subperiod of the Carboniferous. It pushed up parts of the land and made mountains.

Scientists are still learning about what caused the Antler orogeny and how it happened. One idea from 2023 says that the land moved along faults, where pieces of the Earth's crust slipped past each other and lifted up parts of the land. This idea used special kinds of rocks and chemicals, like strontium isotopes, to learn more.

The Antler orogeny happened after the Devonian period and before the Pennsylvanian period. Studying this event helps us learn how the Earth has changed over millions of years.

Two facies of lower Paleozoic rocks

There are two main types of rocks from early Paleozoic time in Nevada.

In the eastern part of the state, there is a layer of fossils-rich carbonate rock from Ordovician to Devonian time. This layer is called the carbonate or eastern group.

Moving westward, the rock changes into a different layer. This layer is made of siliceous sedimentary deposits and some volcanic rocks. It is called the siliceous or western group.

The western group is darker and has fewer carbonate rocks. It also has almost no shelly fossils. This suggests it formed in deeper water. The western group contains chert, basalt, barite, and sulfide deposits.

Scientists believe the western group was moved from the west. It now sits above a large thrust fault called the Roberts Mountains thrust. The eastern group is thought to lie underneath this thrust sheet. We can see evidence of this because large blocks of the eastern group's rocks are found surrounded by the western group's rocks. These blocks are like "windows" in the thrust sheet.

Plate tectonics

Scientists have studied why certain rocks formed in Nevada and nearby places during the Antler orogeny. The theory of plate tectonic movements helped explain this. This theory talks about how Earth's plates move along the edge of North America. Three types of plate movements were suggested: east dipping subduction, west-dipping subduction, and strike-slip motion. But each idea has issues, and scientists are still learning about what caused this event that built mountains.

Present knowledge

We know several important facts about the Antler orogeny. Large amounts of rock formed from broken pieces, called clastic rocks, were laid down in Nevada and nearby areas. These rocks mostly date from a time called the Late Devonian to mid-Pennsylvanian, with some possibly from the Middle Devonian.

Some areas were lifted up and worn down, then later sank and collected layers of sediment. The rocks from this event don’t match up perfectly with the layers below them. We also find pieces of very old carbonate rocks scattered around. Importantly, there are no changed rocks, volcanoes, or big masses of granite linked directly to this event, though some signs of volcanic activity exist in northern California.

Origin of terminology

The term "Antler orogeny" was first used by a scientist named Roberts. He saw that rocks near Antler Peak, in the Battle Mountains, had been bent and broken in a big event. This happened a long time ago, during a time called the Mississippian period. Later studies showed that this event might have started even earlier, in the Devonian period.

Scientists also found that this event was linked to a major fault, or crack, in the Earth's surface called the Roberts Mountains thrust fault. This fault helped shape the land during the same time as the Antler orogeny.

Theories

Scientists have many ideas about what caused the Antler orogeny, a time when mountains formed in Nevada long ago. One idea is that an ancient ocean closed between the land and a chain of volcanoes. Another idea is that the land crashed into a group of islands above a deep ocean trench. Both ideas help explain how the rocks moved.

Some researchers think that pieces of the Earth’s crust moved sideways, instead of crashing together, and caused the changes in the rocks. They also think a big space impact from that time might have played a role. Even after lots of studies, some questions about the Antler orogeny are still unknown.