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The Transantarctic Mountains (TAM) are some of the largest mountains formed by continental rifting. They stretch 3500 kilometers, separating East and West Antarctica. The rift system that formed the Transantarctic Mountains is a reactivation of crustal movement along the East Antarctic Craton. This rifting or seafloor spreading causes plate movement that results in a nearby convergent boundary which then forms the mountain range. Many different models have been created to try and explain the exact events causing uplift. Most models show uplift events similar to common rifts all over the earth. The exact processes responsible for making the Transantarctic Mountains is still debated today. This results in a large variety of proposed theories that attempt to decipher the tectonic history of these mountains.

Three Main Causes for Uplift

1. Thermal forces such as mantle plumes and magma injections into the crust, cause rift zones and thermal expansion of the mountain belt.
2. Convergence of crustal material occurs when the rift zone pushes the West Antarctic plate into the much stronger East Antarctic plate.
3. Flexural uplift is a mechanical process where a tectonic plate is broken by a deep normal fault resulting in the footwall uplifting rock.

West Antarctic Rift Influence

The West Antarctic Rift is located just west of the Transantarctic Mountains and is considered to be the largest influence in driving the plate motion that created the Transantarctic Mountains. It is formed by rising magma such as mantle plumes and injections into the crust, which are the thermal forces that drive the entire system. The rift is simply a mid-ocean ridge where seafloor spreading occurs and drives part of the West Antarctic plate into the stronger East Antarctic Craton. Until recently the influence of this rift was debated because little was known about the region. The unforgiving terrain and glacial cover makes the area difficult to study but over time more accurate models were created showing the direct effect the rift has on mountain building.

Uplift History

Breakup of the super-continent Gondwana started in the early Jurassic around 184 million years ago (Ma) but Antarctica did not break up from Australia until the late Cretaceous (80 Ma). Just before the break away in the Late Jurassic and Early Cretaceous rifting began to occur near the soon to be Transantarctic Mountains. It is unsure wether this first episode of rifting caused any uplift of the mountain range. Some researchers say this episode is a reactivation of rift that was formed during the Gondwanide Orogeny but it has never been proven. This event was followed by low-angle extensional faulting along the East Antarctic plate. No uplift occurs during this initial phase of faulting due to counteracting erosional events. Shortly after, during the Mid Cretaceous (100 Ma), the West Antarctic Rift began to form. The rift formed quickly due to the rapid rise of buoyant magma. Certain models show that the rapid rifting and intense thermal forces are due to a shallow (50 km depth) lithosphere-asthenosphere boundary under the Transantarctic Mountains. Rifting in this area is still going on today. Such a long period of rifting leads to uplift due to prolonged lateral heat conduction in the mountain belt. This is backed by seismic evidence but it is only a minor factor and not the cause of initial uplift. Initial surface uplift began approximately 55 Ma during the Early Cenozoic era. The two most influential forces at this time are flexural uplift and crustal convergence. These two processes were described earlier in the "Three Main Causes for Uplift" section. Both tectonic forces act together to create rift flank uplift. Rift flank uplift is when the crust on the side of a rift zone converges with a stronger crust usually resulting in crustal thickening and a subduction zone. Flexural uplift is not typically associated with rift flank uplift, but in the case of the Transantarctic Mountains it occurs at the same lithospheric boundary. This in conjunction with thermal expansion of the mountain belt results in very rapid surface uplift. This phenomenon explains the steep slope and high elevation of the present mountains. In other common orogenic events only one force is usually responsible for the majority of uplift. These three forces are what characterize the Transantarctic Mountains.

Exhumation History

In order to have surface uplift of rock, the uplift rate must exceed the rate of exhumation. In simple terms, the speed at which rock is moving up needs to be greater than the impact of erosion at the surface. The Transantarctic Mountains have experienced three major episodes of exhumation, also known as denudation. Geologists are able to estimate the timing of these events, as well as uplift events, using the method of Apatite fission track dating. As rock rises from below it pushes up large amounts of overlying sediment that is eventually eroded away. During the Cretaceous period (65-145 Ma) two separate denudation events occur. Both episodes, one Early Cretaceous and the other Late Cretaceous, were great enough to cancel out any possible surface uplift. The third denudation event took place during the Early Cenozoic. This has substantial erosional force, but surface uplift is so rapid that it only helps speed the process by removing excess sedimentary weight. During the Cenozoic rock is uplifted anywhere from 7 km to 10 km, with the highest elevation toward the front of the range and decreasing as you move inland.

Regional Tectonic Events Associated with Exhumation

1. The separation of Antarctica and Australia occurs during the Early Cretaceous exhumation.
2. During the Late Cretaceous exhumation the Antarctic plate was extended east and west due to low-angle faulting.
3. seafloor spreading at the rift zone that caused a rise in the East Antarctic lithosphere occurred during the early Cenozoic exhumation.

Seafloor Spreading Antarctica

Cambrian to Ordovician

During this time period (445-540 Ma) the basement rock, that the current TAM resides on, is being deformed. This rock consists of mainly metamorphic rock with granitoid intrusions. This basement rock is then exhumed during the Ross Orogeny event. Further exhumation of the basement rock created the Kukri Erosion Surface (KES). The present day KES is buried under volcanic rock and shallow marine deposits. At this time (Cambrian-Ordovician) There is no known uplift of the TAM. Following these events there is very little tectonic activity in this region. There is also a 160 Ma section of onland geologic history of the TAM missing from the Jurassic to the late Cenozoic.

Kukri and Dominion Erosion Surfaces

In places the KES has elevation of 4000 meters but along the major syncline associated with the TAM the elevation only reaches a max of 500 meters. The Dominion Erosion Surface (DEM) intersects the KES and has elevation up to 4000 meters but no lower than 1200 meters. It is located in the middle of the syncline where it cuts into lower basement rock such as the Beacon supergroup. These two erosional surfaces help geologists determine glacial erosion and possible uplift history of the TAM. .

Cenozoic

The initial uplift of the East Antarctic lithosphere is dated to the early Cenozoic, but the third exhumation event involving the TAM happened just a few million years before. Areas near Victoria Land show exhumation starting 55 Ma, but Areas along the southern end of the TAM exhibit exhumation beginning 45 Ma. The exhumation rate decreased enough in the early Cenozoic to allow surface uplift. The reason for the uplift is directly related to the seafloor spreading and propagation of the Adare Trough into the continental crust. The propagation advanced downward under the crust near the Western Ross Sea. Due to this propagation and seafloor spreading many faults formed parallel to the syncline that is the TAM. The TAM started as basic tilted blocks such as grabens and horsts. Minor parallel faulting is seen along the entire range but major faulting occurred at the range front creating rapid uplift. A Terror Rift is associated with these low-angle faults. The TAM Terror Rift is similar to a subduction zone but in the case of Antarctica it is also an inclined lithospheric boundary.During the early Cenozoic the TAM exhibits rapid uplift and orogeny. There is no other time that the TAM has this magnitude of mountain building events. The rate of uplift has slowly decreased since the middle Cenozoic.

Methods for Determining Tectonic History

There are a many different ways to determine tectonic history but only a select few can be applied to the TAM because of its thermal and geographic isolation. Seismic reflection is one proven method to visualize what is happening beneath the surface. It requires the use of teams on the ground setting off explosive charges beneath the surface and recording data from seismic waves that bounce back. Another common method of reconstructing tectonics is the application of apatite fission-track thermochronology (AFTT). This requires vertical sampling across a region to create a profile that is age specific. In places like Antarctica it is not always easy to reach some areas but with the use of satellites it has become a lot easier to determine tectonic features in these places. Certain satellites have the ability to interpret areas with the use of gravity fields and magnetic anomalies. When used correctly there is no need for surface exploration to make plate tectonic implications.

References

Category:Transantarctic_Mountains