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

新元古代到早古生代的过渡（7 -5亿年前）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大陆合并，然后分裂，在第二个被称为冈瓦纳大陆的超大陆合并之前。这些事件表现为主要的造山、大陆侵蚀、物种多样化、海平面波动和海水成分的变化。这个时代的大陆边缘沉积序列（特别是在北美西部和东部、亚洲东北部、非洲南部、澳大利亚东部和南极洲的罗斯边缘）提供了海平面波动、动物分布和沉积后构造的详细记录。其中一个序列是南极洲的比尔德莫尔群，它代表了南极东部克拉通演化的一个重要元素，但对其沉积或构造历史知之甚少。Beardmore群是一个厚的灰岩和碳酸盐层序，传统上与横贯南极山脉其他地方的类似岩石相关联。Beardmore岩石大部分未化石，岩性单一，与结晶基底岩和化石下寒武统碳酸盐岩的沉积关系不确定。前人认为，整个Beardmore群的年龄为新元古代，并记录了两次明显的变形，即著名的Ross造山运动（~500 Ma）和更早的隐蔽的“Beardmore”造山运动。野外勘测和地质年代学研究表明，Beardmore群的某些部分比以前认为的要年轻得多（600 Ma），而且它只显示了罗斯变形的证据。这些初步资料反映了这些岩石地质关系的不确定性，表明该地区新元古代构造史必须修订。该项目将调查横贯南极山脉中部的比尔德莫尔群，以更好地了解其沉积历史及其在塑造冈瓦纳大陆外缘的造山活动中的作用。研究内容包括：(1)详细的地层学、沉积学和构造野外研究；(2)灰屑物源研究，约束浊积岩的沉积背景；(3)测定碳酸盐同位素组成，与全球碳同位素记录进行比较；(4)通过对碎屑和火成岩锆石进行U-Pb定年，细化沉积年龄；(5) ar同位素热年代学对变形年龄的约束。实地工作将于1998-99年和1999-2000年南方夏季完成。这项工作将测试最近提出的Beardmore群沉积和变形模型，该模型涉及最新的新元古代-早古生代（?）裂谷边缘沉积和构造反转，作为Ross活动的早期表达。这些结果将有助于直接解决南极地质历史上长期存在的不确定性，并将提高我们对元古代末期超大陆转化过程中全球古地理和板块构造关系的认识。

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.