Geodynamic evolution of the Pacific margin of West Antarctica based on apatite thermochronology
Geodynamic evolution of the Pacific margin of West Antarctica based on apatite thermochronology
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基于磷灰石热年代学的西南极太平洋边缘地球动力学演化
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发表时间：
2018
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影响因子：
--
通讯作者：
M. Zundel
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文献类型：
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作者：
M. Zundel
南极西部大陆是在古生代和中生代的大部分时期由地壳块体增生形成的古太平洋冈瓦纳边缘。在白垩纪中期，冈瓦纳断裂与古太平洋边缘由汇聚到伸展的重大变化有关，西南极洲与西南极洲分离，打开了西南极洲裂谷系（WARS）和南太平洋。冈瓦纳大陆的进一步解体导致了德雷克海峡的开通和斯科舍海的发展，导致了西南极洲与南美洲的分离。因此，冈瓦纳大陆的分裂和随后的构造运动使南极洲在地理上与世隔绝，并可能有利于冰川作用的发生。在试图更好地限制冈瓦纳大陆破裂以来的地球动力学演变时，西南极洲广阔的冰盖本身就是一个主要挑战，因为人们对冈瓦纳大陆的分裂仍然知之甚少。本研究的重点是西玛丽伯德地（MBL）和瑟斯顿岛地区，这两个地区都是西兰洲共轭大陆边缘的一部分。在南奥克尼群岛和象岛群（南斯科舍海）地区进行了进一步调查。为了全面重建地壳上部2 ~ 5 km的热构造演化，本研究主要采用了磷灰石裂变径迹（AFT）和磷灰石(U-Th-Sm)/He （AHe）热年代学方法。对AFT和AHe数据进行了热历史模拟。为了更好地了解地球动力学历史，对选定的样品进行了U-Pb锆石年代学和角闪石热气压分析。最古老的热年代学记录来自瑟斯顿岛地区石炭纪至早白垩世（元）火成岩。它们的AFT年龄（145 ~ 92 Ma）和AHe年龄（112 ~ 43 Ma）被整合到热模式中，显示了晚古生代以来的冷却和再加热时期。这些时期是指二叠世至早白垩世沿活跃的古太平洋边缘的发掘和弧后盆地发育时期。它们还记录了早白垩世至中白垩世与岩浆作用和地壳增厚有关的盆地反转时期的发掘。在95 ~ 70 Ma期间，与伸展相关的掘出与西南极洲和西南极洲之间的裂谷有关；在75 ~ 60 Ma期间，与别令斯豪森-南极板块边界的活动有关。晚始新世以来的最后一次发掘最有可能是由战争活动引起的。西部MBL样品主要取自白垩纪中期的伯德海岸花岗岩套件。这些样品的AFT和AHe年龄为105 ~ 68 Ma，热模式与WARS-extension时期105 ~ 95 Ma的快速构造掘出相吻合。随后在95年至80年间，西兰洲和西南极洲之间的裂谷和分裂导致了一段挖掘阶段。最南端的样品记录了罗斯海地区断裂导致的80-60 Ma的发掘。晚白垩世-早新生代的缓慢降温与南极西部侵蚀面形成和相对的构造静止相对应。晚新生代战争的裂谷作用、MBL-穹隆隆起和冰川作用的开始对MBL西部的侵蚀过程影响不大。南奥克尼群岛前中白垩世沉积和变质样品的AFT年龄为77 ~ 54 Ma， AHe年龄为61 ~ 5 Ma。这些年龄，借助热史模型解释，记录了晚白垩世-早新生代边缘隆起和高海拔地形的侵蚀。晚始新世以来相对缓慢的发掘与鲍威尔盆地的开放有关。大象岛群的变质岩和火成岩样品显示AFT和AHe年龄分别为30 ~ 23 Ma和6 ~ 2 Ma。一个闪长岩和一个花岗闪长岩样品的U-Pb锆石年龄分别为28和10 Ma。地质和热年代学年龄和确定的冷却期可能指的是南斯科舍海的两个主要板块重组时期。首先，渐新世的侵入和发掘可能与沙克尔顿断裂带和西斯科舍脊的形成有关。第二，晚中新世/上新世的快速构造发掘是由于两个扩张脊的停止和布兰斯菲尔德海峡的开放。
The West Antarctic continent formed during most of the Palaeozoic and Mesozoic era by accretion of crustal blocks to form the palaeo-Pacific margin of Gondwana. In the mid Cretaceous, Gondwana break-up was associated with a major change from convergence to extension along the palaeo-Pacific margin, which opened the West Antarctic rift system (WARS) and the Southern Pacific Ocean by separating Zealandia from West Antarctica. Further disintegration of Gondwana led to the separation of West Antarctica from South America by opening of the Drake Passage and the development of the Scotia Sea. Thus, Gondwana break-up and ensuing tectonic movements brought geographic isolation to Antarctica and potentially favoured the onset of glaciation. Extensive ice-cover of West Antarctica is itself one of the main challenges when trying to better constrain the geodynamic evolution since Gondwana break-up that remains poorly understood. This study focussed on western Marie Byrd Land (MBL) and the Thurston Island area which both form parts of the conjugate continental margins of Zealandia. Further investigation was carried out in the area of the South Orkney Islands and Elephant Island group (southern Scotia Sea). To comprehensively reconstruct the thermo-tectonic evolution of the upper 2-5 km of the crust, the main methods utilised in this study were apatite fission track (AFT) and apatite (U-Th-Sm)/He (AHe) thermochronology. Thermal history modelling of both AFT and AHe data was also carried out. For a better understanding of the geodynamic history, selected samples were analysed by means of U-Pb zircon geochronology and hornblende thermobarometry. The oldest thermochronological record was obtained from Carboniferous to Early Cretaceous (meta)igneous rocks of the Thurston Island area. Their AFT ages of 145-92 Ma and AHe ages of 112-43 Ma were integrated in thermal models and show episodes of cooling and reheating since the Late Palaeozoic. These episodes refer to exhumation and back-arc basin development between the Permian and the Early Cretaceous along the active paleo-Pacific margin. They also record exhumation during Early to mid Cretaceous basin inversion associated with magmatism and crustal thickening. Extension-related exhumation from 95 to 70 Ma occurred in response to the rifting between Zealandia and West Antarctica and from 75 to 60 Ma to activity along the Bellingshausen-Antarctic Plate boundary. Final exhumation since the Late Eocene was most-likely caused by activity of the WARS. Western MBL samples were mostly taken from the mid Cretaceous Byrd Coast Granite suite. These samples yield AFT and AHe ages of 105-68 Ma and thermal models are compatible with rapid tectonic exhumation from 105 to 95 Ma during WARS-extension. This was followed by a phase of exhumation between 95 and 80 Ma in response to rifting and breakup between Zealandia and West Antarctic. The southern-most samples record exhumation from 80-60 Ma due to faulting in the Ross Sea region. Slow cooling during the Late Cretaceous-Early Cenozoic corresponds with the formation of the West Antarctic erosion surface and relative tectonic quiescence. Late Cenozoic WARS rifting, MBL-dome uplift, and onset of glaciation appears to have had little effect on erosion processes in western MBL. Pre-mid Cretaceous sedimentary and metamorphic samples of the South Orkney Islands yield AFT ages of 77-54 Ma and AHe ages of 61-5 Ma. These ages, interpreted with the help of thermal history models record Late Cretaceous-Early Cenozoic margin-wide uplift and erosion of high-standing topography. Relatively slow exhumation since the Late Eocene was associated with the opening of the Powell Basin. Metamorphic and igneous samples from the Elephant Island group reveal AFT and AHe ages of 30-23 Ma and 6-2 Ma. A diorite and a granodiorite samples yield U-Pb zircon ages of 28 and 10 Ma. The geoand thermochronological ages and the identified cooling episodes probably refer to two periods of major plate reorganisations in the southern Scotia Sea. First, intrusion and exhumation during the Oligocene occurred probably in response to inception of the Shackleton Fracture Zone and the West Scotia Ridge. Second, rapid tectonic exhumation in the Late Miocene/Pliocene occurred due to cessation of two spreading ridges coupled with the opening of the Bransfield Strait.
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