Structure and Sedimentology of the Beardmore Group, Antarctica: Latest Neoproterozoic to Early Paleozoic Tectonic Evolution of the East Antarctic Margin

新元古代向早古生代的过渡(距今7-5亿年)是地球历史上的一个关键时期。在此期间，一个被称为罗迪尼亚的超级大陆被合并，然后被碎裂，然后第二个被称为冈瓦瓦兰的超级大陆合并。这些事件表现为大规模造山、大陆侵蚀、物种多样化、海平面波动和海水成分变化。这一时期的大陆边缘沉积序列(特别是在北美西部和东部、东北亚、南部非洲、东澳大利亚和南极洲的罗斯边缘)提供了海平面波动、动物群分布和沉积后构造作用的详细记录。其中一个层序是南极洲的比尔德莫尔群，它代表了东南极克拉通演化的重要元素，但人们对其沉积或构造历史知之甚少。比尔德莫尔群是一个厚厚的杂砂岩和碳酸盐岩层系，传统上与横贯北极山脉的其他类似岩石相关联。Beardmore岩石基本上没有化石，岩性单一，与结晶基岩和下寒武统碳酸盐化石的沉积关系不确定。以前的工作人员提出，整个比尔德莫尔群的年龄为新元古代，它记录了两次明显的变形，著名的罗斯造山(~500 Ma)和更早的神秘的比尔德莫尔造山。勘探场和年代学研究表明，Beardmore群的一部分比之前认为的要年轻得多(600 Ma)，而且只显示了Ross变形的证据。这些初步数据反映了这些岩石地质关系的不确定性，并表明该地区的新元古代构造史必须修正。该项目将对横贯北极山脉中部的Beardmore群进行调查，以便更好地了解其沉积历史及其在形成冈瓦瓦兰外缘的造山事件中所起的作用。这项研究将包括：(1)详细的野外地层、沉积学和构造研究；(2)灰岩的物源(来源)研究，以约束浊积岩的沉积环境；(3)碳酸盐同位素组成的测定，以与全球碳同位素记录相比较；(4)通过碎屑和火成岩锆石的U-Pb测年，精修沉积年龄(S)；以及(5)利用Ar同位素热年代学约束变形年龄(S)。实地工作将在1998-99年和1999-2000年南半球夏季期间完成。这项工作将检验最近提出的Beardmore群沉积和变形模型，该模型涉及新元古代-早古生代(？)裂谷边缘沉积和构造反转是ROSS活动的早期表现。这些结果将有助于直接解决南极地质历史中长期存在的不确定因素，并将加深我们对元古代末期超大陆转换过程中全球古地理和板块-构造关系的理解。

The Neoproterozoic to early Paleozoic transition (700-500 million years ago (Ma)) is a critical period in Earth history. During this period one supercontinent known as Rodinia was coalesced, then fragmented, prior to amalgamation of a second supercontinent referred to as Gondwanaland. These events were manifested by major mountain building, continental erosion, species diversification, sea-level fluctuations, and changes in sea- water composition. Continental-margin sedimentary sequences from this era (notably in western and eastern North America, northeastern Asia, southern Africa, eastern Australia, and the Ross margin of Antarctica) provide detailed records of sea-level fluctuations, faunal distributions, and post- depositional tectonism. One of these sequences, the Beardmore Group in Antarctica, represents a significant element in the evolution of the East Antarctic craton, yet very little is known about its depositional or tectonic history. The Beardmore Group is a thick sequence of graywacke and carbonate that is traditionally correlated with similar rocks elsewhere in the Transantarctic Mountains. Beardmore rocks are largely unfossiliferous, lithologically monotonous, and lie in uncertain depositional relation with both crystalline basement rocks and fossiliferous Lower Cambrian carbonates. Previous workers have suggested that the entire Beardmore Group is Neoproterozoic in age and that it records two distinct deformations, the well-known Ross orogeny (~500 Ma) and an earlier cryptic "Beardmore" orogeny. Reconnaissance field and geochronologic studies indicate that portions of the Beardmore Group are significantly younger (600 Ma) than previously thought, and that it only shows evidence for Ross deformation. These preliminary data reflect uncertainty in the geologic relations of these rocks, and they indicate that the Neoproterozoic tectonic history for the region must be revised. This project will investigate the Beardmore Group in the ce ntral Transantarctic Mountains in order to better understand its depositional history and its role in orogenic events shaping the outer margin of Gondwanaland. The study will include: (1) detailed field study of the stratigraphy, sedimentology, and structure; (2) provenance (source) study of graywackes to constrain the depositional setting of the turbidites; (3) determination of carbonate isotopic compositions for comparison with the global C-isotopic record; (4) refinement of depositional age(s) through U-Pb dating of detrital and igneous zircons; and (5) constraint on the age(s) of deformation(s) with Ar-isotope thermochronology. Field work will be completed during the 1998-99 and 1999-2000 austral summers. This work will test a recently proposed model for Beardmore Group deposition and deformation that involves latest Neoproterozoic-early Paleozoic(?) rift-margin sedimentation and structural inversion as an early expression of Ross activity. The results will help to directly resolve long- standing uncertainties in the geologic history of the Antarctica and they will improve our understanding of global paleogeographic and plate-tectonic relations during supercontinent transformation at the close of the Proterozoic.