Antler orogeny

The Antler orogeny was a tectonic event which occurred in Nevada during the Mississippian subperiod of the Carboniferous. It was a time when the Earth's plates moved and pushed up parts of the land, creating mountains and changing the shape of the landscape. Even though it is called an orogeny, which means a mountain-building event, some usual signs of such events, like certain types of rock changes or special kinds of rocks, have not been clearly linked to it.

Scientists are still not sure exactly what caused the Antler orogeny or how it happened, but they have different ideas about it. One recent study from 2023 suggests that the event was caused by a type of movement along faults, where pieces of the Earth's crust slipped past each other in a way that caused parts of the land to lift up. This study used special kinds of rocks and chemicals, like strontium isotopes, to understand where the land was moving.

The Antler orogeny happened during a specific time, starting after the Devonian period and ending before the Pennsylvanian period. This helps scientists know when exactly these changes took place and how long they lasted. Even though there is still some mystery about this event, learning about it helps us understand 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, called the carbonate or eastern group. Moving westward, this changes into a different layer of siliceous sedimentary deposits and some volcanic rocks, called the siliceous or western group. The western group is darker, has fewer carbonate rocks, and almost no shelly fossils, suggesting it formed in deeper water. It also contains chert, basalt, barite, and sulfide deposits. Scientists believe the western group was moved from the west and sits above a large thrust fault known as the Roberts Mountains thrust. The eastern group is thought to lie underneath this thrust sheet. Evidence for this idea comes from large blocks of the eastern group's rocks found surrounded by the western group's rocks, which are seen as "windows" in the thrust sheet.

Plate tectonics

Geologists have long tried to understand why certain rock layers formed in Nevada and nearby areas during the Antler orogeny. The development of plate tectonic theory offered several ideas to explain this, such as the movement of Earth's plates along the western edge of North America. Three main types of plate movements were suggested: east dipping subduction, west-dipping subduction, and strike-slip motion. However, each idea has problems, and scientists still aren't sure what exactly caused this mountain-building event.

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 noticed 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 part of Earth's history 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 landscape during the same time as the Antler orogeny.

Theories

Scientists have proposed several ideas about what caused the Antler orogeny, a mountain-building event in Nevada long ago. One idea suggests that an ancient ocean basin closed between the continent and a chain of volcanoes. Another idea suggests that the continent crashed into a group of islands sitting above a deep ocean trench. Both ideas try to explain how the rocks moved during this event.

Some researchers think that moving pieces of the Earth’s crust sideways, rather than crashing together, caused the changes seen in the rocks. They also wonder about the role of a big space impact from that time and why certain types of rocks appeared in the area. Even after many studies, some questions about the Antler orogeny remain unanswered.