Windmill Islands
| Windmill Islands | ||
|---|---|---|
| Waters | Vincennes Bay | |
| Geographical location | 66 ° 20 ′ S , 110 ° 28 ′ E | |
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| Total land area | 270 km² | |
| Residents | uninhabited | |
The Windmill Islands are an archipelago of numerous flat and rocky islands and rocky reefs with an extension of 10 km wide and 27 km in length off the Budd coast in the East Antarctic Wilkesland . They lie between the northern edge of Law Dome and the southern foothills of Vanderford Glacier, parallel to the east bank of Vincennes Bay .
The Advisory Committee on Antarctic Names named it in 1955 after Operation Windmill (1947–1948) led by Gerald Ketchum , as members of this expedition landed on Holl Island in the southwest of the archipelago to create a base for Operation Highjump (1946–1947) set up. The term "windmill" ( English for windmill ) is a description of the helicopters often used on Ketchum's expedition .
A geodynamic development in the Mesoproterozoic can be deduced from the age and the metamorphoses of the rocks . These processes correlate with those in the Albany-Fraser Belt in Western Australia , which indicate common tectonic processes.
Geographical structure
The Windmill Islands consist of a large number of larger and smaller islands. Larger islands or groups of islands are z. B. The Swain Islands at the northern end of the archipelago, Shirley Island , Beall Island , Cronk Islands , Ardery Island , Odbert Island , Holl Island , Ford Island , Cloyd Island , Herring Island and Perterson Island .
Due to comparable geodynamic development, the Clark Peninsula , the Bailey Peninsula , the Mitchell Peninsula , the Robinson Ridge Peninsula and the Browning Peninsula are also included in the extended term of the Windmill Islands, as are neighboring Nunatakkers. These include the Alexander nunataks , the Campbell nunataks, and the main nunataks .
The US Wilkes Station , which has since been abandoned, is located on the Clark Peninsula, while the Casey Station operated by Australia is on the Bailey Peninsula .
topography
The peninsulas typically consist of low, rounded hills with heights of just over 100 meters and valleys in between that are filled with ice or snow , moraines or detritus .
The islands have been divided into three groups based on their topography . Those off the Clark, Bailey, and Mitchell peninsulas are largely deep and gradually increase in altitude towards the south. The Donovan Islands and Frazier Islands in the north have extensive cliffs that jut out of the sea. The islands between the Mitchell Peninsula and Vanderford Glacier are steep and rugged, with tall cliffs rising from the deep waters of the southern part of Vincennes Bay. Some of these islands reach a height of just over 110 meters.
geology
Rocks and age
The Windmill Islands consist mainly of the Windmill Metamorphics with metasediments, the Ardery Charnockites and the Ford granites and moraine sediments. The Windmill Metamorphics and the Ardery Charnockite comprise around 70 percent of the rock occurrence.
Windmill Metamorphics
The Windmill Metamorphics form a layered supracrustal sequence of various garnet-bearing granites and garnet- containing foiled granite gneisses and migmatites . The rock chemistry of the protoliths (parent rocks) suggests that they probably consisted of rockic and basic igneous rocks with embedded sediments . The latter ranged from greywacky sandstones to slates . The Windmill Metamorphics developed mainly on the Clark, Bailey and Mitchell Peninsulas and the offshore islands and, to a small extent, on Herring Island. Regional metapelite and meta psammite were deposited on the peninsulas . On the Mitchell Peninsula and the offshore islands, migmatitic gneiss formed in places .
Using geochronological and geochemical methods, representative rocks were examined with regard to their age and geodynamic development. Samples from different granites and gneisses show an age of approx. 1,250 to 1,240 mya , which is interpreted as their placement. In them, inherited igneous zircons occur, which are around 1,370 mya, which proves igneous activities and the formation of protoliths.
The granites and gneisses have similar geochemical and isotopic compositions. They probably developed as a result of partial melting of a paleoproterozoic lower crust of the earth in a subduction regime with arch-like complexes that accreted to the western Mawson craton rim .
In the sedimentary metapelites and metapsammites, inherited zircons are contained that come from the leucosomes of migmatites that formed before the deformation phases. They have an age range of approx. 2,600 to 1,400 mya.
Ardery Charnockite
The Ardery Charnockite extend in an arc from the Robinson Ridge peninsula over the Odbert and Ardery Islands to Peterson Island with the offshore smaller islands. They form foiled intrusions from Charnockites . They were probably formed by the partial melting of a mafic lower crust of the earth , which was heated by rising basaltic magma during a deformation phase with the formation of granulite facies . These processes occurred over a period of approximately 1,205 to probably 1,150 mya, which corresponds to the collision with the Australian Albany-Fraser orogen.
The Ardery charnockites on Peterson Iceland were two gabbronitisch - doleritic Dykeschwärme penetrated. They intruded between approximately 1,140 and 1,110 mya.
Ford Granite
The Ford Granites consist of porphyry granites and form outcrops on Ford and Cloyd Iceland. They formed around 1,170 mya.
Moraine sediments
Widespread moraine sediments extend along the leading edge of the inland ice cover . They form the Løken moraines in the form of a NS-tending, coast-parallel belt that is almost 25 kilometers long and several meters to several hundred meters wide. It stretches from eastern Robinson Ridge to the coastal areas west of Swains Island. It is believed that they represent the edge of the continental ice sheet.
The moraine sediments provide important information about the development of the Windmill Islands and their inland areas. They are composed of fragments of rock the size of a grain of sand and erratic boulders from several centimeters to several meters in size. Boulders consist mainly of highly metamorphic rocks with a rock spectrum comparable to that in the Windmill Metamorphics and Ardery Charnockites. In addition, there are diorites , porphyry granodiorites and some basaltic volcanic rocks , which indicate the existence of other different types of rock below the ice-covered areas southeast of the Windmill Islands.
Detritic zircons show an age range between approx. 1,368 and 1,107 mya. Some ancient zircon populations date from approximately 2,360 to 1,411 mya. They could come from Proterozoic recycled metapsammitic gneiss.
The absence of archaic zircons in the moraine sediments suggests a paleoproterozoic to mesoproterozoic basement in the interior of Wilkesland .
Deformations and Metamorphoses
Several deformations and rock metamorphoses formed in the Windmill Metamorphics. As a result of the deformation phases, different folds and deformations developed. These include regional isoclinic, steep and narrow folds, wider, less tight concentric folds or gentle bulges that slope steeply to the south.
The degree of metamorphosis increased from north to south. In the northern areas, an amphibolite facies developed in the metasedimentary deposits on the Clark Peninsula in the period from 1,340 to 1,300 mya . This is related to the development of migmatites and the settlement of the early granites. In the southern areas of the Windmill Metamorphics, a granulite facies formed. It occurred between 1,240 and 1,140 mya shortly after the granite gneisses were taken. The transition between these different facies is roughly between the Clark and Bailey Peninsula.
Geological relationships
The absence of archaic zircons in the moraines suggests that the Law Dome crust and inland areas of Wilkesland are dominated by rocks from the Paleo to Mesoproterozoic. The zircon ages in the moraines indicate a predominantly mesoproterozoic development of the Windmill Islands and the subglacial basement of the Law Dome and Wilkesland. Their rocks also have a comparable isotopic composition. These probably form a similarly highly metamorphic terran from the Mesoproterozoic as the Windmill Islands.
Although the Bunger Hills, about 400 kilometers to the north, and the area around the Denman Glacier were affected by Mesoproterozoic intrusions and a similarly high-grade metamorphosis as the Windmill Islands, their basement consists of Meso - neoarchic to paleoproterozoic tonalitic and granodioritic orthogneiss. This makes it very different from that of the Windmill Islands. In addition, tectono-magmatic processes occurred in these areas during the Pan-African Orogeny , which were not found in the Windmill Islands.
The Mesoproterozoic processes of the Windmill Islands and the neighboring areas around the Bunger Hills and Denman Glacier are traced back to a collision with areas of the Australian Albany-Fraser orogen. This runs on the southern edge of the Gawler craton and the Yilgarn craton. (see also → Albany-Fraser-Orogen ). During this period, the supercontinent Rodinia formed and correlated roughly with the Grenville orogeny .
Glacier history
With the beginning of the Canoeozoic Ice Age , the glaciation of the Antarctic began. During the Pleistocene , the Windmill Island was heavily glaciated and covered by an ice sheet about 400 meters thick . The last glacier maximum was around 18,000 years ago. At that time, the eustatic sea level was around 125 meters lower. The Vanderford Glacier filled the deep trough in Vincennes Bay south of the Windmill Islands and also formed a series of U-shaped valleys on Holl Island and Peterson Island and the Browning Peninsula.
The ice receding occurred in two separate phases. First, the southern islands and the Browning Peninsula became ice-free about 8,000 years ago. The northern islands and the other peninsulas lost their ice cover around 5,500 years ago. Since then, the Antarctic continent has gone through a phase of cooling and drying out.
climate
The climate of the windmill islands is described as cold-Antarctic. The mean temperatures for the warmest and coldest months are + 0.3 and - 14.9 ° C, respectively . The extreme temperatures are between + 9.2 ° C and - 41 ° C. The annual precipitation is 175 millimeters ( water equivalent ), which falls mainly as snow . However, in summer it can occasionally rain . Strong winds of more than 54 kilometers per hour are common. Most of the time the winds come from the Law Dome area.
In the area of Casey Station, the sea ice does not normally break until early January. Summer (December through January) has a lot of sunshine every day, and meltwater flows and freshwater pools are frequent.
Fauns
In the area of the Windmill Islands there is a diverse and rich mammal and bird fauna . Mammals are represented by the Weddell Seal , the crab-eater , the Ross Seal , the leopard seal and the southern elephant seal . The range of birds including the emperor penguin and Adelie and various petrels , skuas (skuas), gulls and terns .
Protected areas have been established on the Frazier Islands, Clark Peninsula, Bailey Peninsula, Ardery Island and Odbert Island .
literature
- John Stewart: Antarctica - An Encyclopedia . Vol. 2, McFarland & Co., Jefferson and London 2011, ISBN 978-0-7864-3590-6 , p. 1715 (English)
- EV Mikhalsky: Main Stages and Geodynamic Regimes of the Earth's Crust Formation in East Antarctica in the Proterozoic and Early Paleozoic. In: Geotectonics, 2008, Vol. 42, No. 6, pp. 413-429. PDF
- E. Paul, K. Stüwe, J. Teasdale and B. Worley: Structural and metamorphic geology of the Windmill Islands, east Antarctica: Field evidence for repeated tectonothermal activity. In: Journal of the Geological Society of Australia, Volume 24, 1977 - Issue 5-6, Pages 453-469, Published online: 1 Aug 2007. doi: 10.1080 / 08120099508728216 , alternative
- RD Seppelt: Wilkes Land (Casey Station). In: Geoecology of Antarctic Ice-Free Coastal Landscapes pp 41-49. doi: 10.1007 / 978-3-642-56318-8 4 , alternatively
Web links
- Windmill Islands in the Geographic Names Information System of the United States Geological Survey (English)
- Windmill Islands on geographic.org (English)
- Map of the Windmill Islands (PDF, March 2009) in the Australian Antarctic Data Center (English)
- Windmill Islands at the Australian Antarctic Data Center
Individual evidence
- ↑ Windmill Islands. In: Australian Antarctic Division: Leading Australia's Antarctic Program . Online article
- ↑ Shuan-Hong Zhang, Yue Zhao, Xiao-Chun Liu, Yong-Sheng Liu and others: U – Pb geochronology and geochemistry of the bedrocks and moraine sediments from the Windmill Islands: Implications for Proterozoic evolution of East Antarctica. In: Precambrian Research, Volumes 206-207, June 2012, Pages 52-71. doi: 10.1016 / j.precamres.2012.02.019 , alternatively
- ↑ DF Blight and RL Oliver: The metamorphic geology of the Windmill Islands, Antarctica: A preliminary account. In: Journal of the Geological Society of Australia, 24: 5-6, 239-262. doi: 10.1080 / 00167617708728986 , alternatively
- ^ Andreas Möller, Nicholas J. Post and Bas J. Hensen: Crustal residence history and garnet Sm-Nd ages of high-grade metamorphic rocks from the Windmill Islands area, East Antarctica. In: International Journal of Earth Sciences (Geologische Rundschau), 2002, 91: 993-1004. doi: 10.1007 / s00531-002-0291-x , alternatively
- ↑ Laura J. Morrissey, Justin L. Payne, Martin Hand, Chris Clark and others: Linking the Windmill Islands, east Antarctica and the Albany – Fraser Orogen: Insights from U – Pb zircon geochronology and Hf isotopes. In: Precambrian Research, Volume 293, May 2017, Pages 131-149. doi: 10.1016 / j.precamres.2017.03.005 , alternatively
- ^ SA Jones: Mesoproterozoic Albany - Fraser Orogen-related deformation along the southeastern margin of the Yilgarn Craton. In: Australian Journal of Earth Sciences, Pages 213-234, 08 Mar 2005. doi: 10.1080 / 08120090500499248 , alternative