Collaborative Research: Neoproterozoic Vergence in the East African Orogen: A Structural and Geochronologic Analysis of the Polydeformed Itremo Group, Central Madagascar

冈瓦纳超大陆的形成是地球历史上最重要的地质事件之一，但导致其形成的过程却知之甚少。 在寒武纪早期，马达加斯加位于冈瓦纳大陆内部，靠近东非造山带（EAO）的东部边缘。 这个造山带标志着东冈瓦纳和西冈瓦纳之间的连接，它包含了在前莫桑比克洋内部和边缘形成的再生大陆地壳，蛇绿岩，岛弧和大陆弧岩石。 因此，马达加斯加处于关键地位，可以揭示EAO的三个关键和目前神秘的特征，这对于理解从Rodinia到Gondwana的转变至关重要：（1）EAO中部火山岩的性质及其在索马里，埃塞俄比亚和拉贾斯坦邦（印度）的可能延伸;（2）东西冈瓦纳临界结合部的陆-陆碰撞事件年代学;（3）东奥陶系东部的构造，处于以增生-增生为主的地质过渡区（北部）大陆碰撞和地壳增厚PI的Tucker和Kusky建议对马达加斯加中部（Itremo地区）的多变形和变质的前寒武纪岩石进行为期两年的结构和地质年代学调查。 这一领域的工作对于解决东非造山带东部的两个一级地质问题具有很大的希望：（1）东非造山带东部早期构造聚散（推覆体侵位）的方向是什么，马达加斯加中部叠加褶皱的几何形状是什么？ 结构聚散是像最近提出的那样（Raeollson 1997，费尔南德斯等修订）向西或向西南，还是像其他人提出的那样（Tucker等1997和修订;柯林斯等2000 a，B）向东或向东南？ 是否有其他可能的解决办法9（2）泛非变形和变质作用何时开始开始在东部AO的南部。 马达加斯加中部的推覆体最初是在-800 Ma之前搬运和变质的吗（考克斯等人，1998;柯林斯等人，2000 a，B; Hulscher等人，2001）？ 或者，它们是在~ 800 Ma之后侵位和变质的（Tucker等，1997年，修订;费尔南德斯等，修订;考克斯等，2001）。 这些问题对于理解EAO内板块会聚的时间和几何形状至关重要。 马达加斯加中部的Itremo地区是研究和解决这些问题的最佳地点：它位于新元古代叠印区，距离拟议的东冈瓦纳缝合线仅150公里;基岩暴露极好，对比鲜明的岩石类型允许准确绘制褶皱和断层图;地层面向丰富，因此可以很容易地识别平卧褶皱和逆冲推覆体的反转翼;由于第三代褶皱的区域倾伏一致，可以绘制可靠的剖面图。研究将通过地质填图、构造分析、碘和铀-铅年代学的综合方案进行。 将在Itremo区域内的三个关键地区进行结构调查和样品采集，这些地区暂时被确定为包括大型东向褶皱或逆冲推覆体的倒转翼（Tucker等人，修订版）。 地层分析、构造调查和详细绘图项目旨在检验我们的构造假设，并增加我们对褶皱和断层区域几何形状的了解。 关键样品的U-Pb年代学将制约推覆体侵位和区域变质作用的时代，并为我们的碰撞模式提供关键的检验。 这项工作将导致一个新的认识的运动学，年龄和整体结构聚敛的关键，东部的EAO强烈分歧的意见，其演变弧出现。 研究将涉及马达加斯加和美国的专业同事之间的合作，并允许美国和马达加斯加学生参与和培训实地和实验室研究。

The creation of the supercontinent of Gondwana is one of the most significant geologic events in the history of our planet yet the processes that led to its formation are poorly understood. In its predrift position near the dawn of the Cambrian, Madagascar was situated in the interior of Gondwana near the eastern edge of the East African Orogen (EAO). This orogen marks the join between East and West Gondwana, and it contains within it pieces of recycled continental crust, ophiolite, and island-arc and continental-arc terranes that formed within and marginal to the former Mozambique Ocean. Madagascar is thus critically positioned to reveal three key and currently enigmatic features of the EAO essential for understanding the transformation from Rodinia to Gondwana: (1) the nature of the volcanic terranes in the central part of the EAO and their possible extensions in Somalia, Ethiopia, and Rajasthan (India); (2) the chronology of terrane-suturing and continental collision events in the critical join between East and West Gondwana, and; (3) the structure of the eastern part of the EAO in the key transitional area from dominantly terrane-accreted geology (in the north) to continental collision and crustal overthickening (in the south).PI's Tucker and Kusky propose to undertake a two-year structural and geochronologic investigation of the polydeforrned and metamorphosed Precambrian rocks of central Madagascar (Itremo region). Work in this area holds great promise for resolving two first-order geologic questions in the eastern part of the East African Orogen: (1) What is the direction of early structural vergence (nappe emplacement) in the eastern part of the East African Orogen and what is the geometry of the superposed folds of central Madagascar? Is the structural vergence westerly or southwesterly, as has been recently proposed (Raeollson 1997, Fernandez et al. in revision), or is it easterly or southeasterly as advocated by others (Tucker et al. 1997 and in revision; Collins et al. 2000 a,b)? Are other solutions possible9 (2) When did Pan-African defon-nation and metamorphism begin in the southern part of the EAO. Were the nappes of central Madagascar initially transported and metamorphosed prior to -800 Ma (Cox et al. 1998; Collins et al. 2000 a,b; Hulscher et al 2001)? Alternatively, were they emplaced and metamorphosed after -800 Ma (Tucker et al. 1997, and in revison; Fernandez et al. in revision; Cox et al. 2001). These questions are central to understanding the timing and geometry of plate convergence within the EAO. The Itremo region of central Madagascar is the best place to study and address these issues: it is situated within the region of Neoproterozoic overprinting, only - 1 50 km from the proposed suture with East Gondwana; bedrock exposure is superb and the contrasting rock types permit accurate mapping of folds and faults; stratigraphic facing directions are abundant and hence the inverted limbs of recumbent folds and thrust nappes can be readily identified, and; reliable cross-sections may be constructed owing to the consistent regional plunge of the youngest-generation folds.Research will proceed through an integrated program of geologic mapping, structural analysis, I I and U-Pb geochronology. Structural investigations and sample collections will be made in three key areas within the Itremo region, tentatively identified as comprising the inverted limbs of large-scale east-directed fold- or thrust-nappes (Tucker et al. in revision). Stratigraphic analysis, structural investigations and detailed mapping projects are designed to test our structural hypothesis and increase our knowledge of the regional geometry of the folds and faults. U-Pb geochronology of key samples will constrain the age of nappe emplacement and regional metamorphism, and provide a critical test of our collision model. This work will lead to a new understanding of the kinematics, age and overall structural vergence in the critical, eastern part of the EAO where strongly divergent views of its evolution arc emerging. Research will involve collaboration between professional colleagues in Madagascar and the U.S., and allow for the participation and training of American and Malagasy students in field- and laboratory-based research.