Resolving the age of the first-order topography of Africa

这个项目将检验这样一种假设，即非洲南部在3000万年前开始获得其异常高的平均海拔——几乎所有地区都在1000米以上——当时非洲停止了相对于底层地幔的移动，并且没有从先前升高的冈瓦纳超级大陆继承其高海拔。板块构造理论成功地解释了高山山脉是如何形成的，这是由于地壳沿着汇聚的板块边界挤压和增厚的结果。安第斯山脉和阿尔卑斯山)。在解释为什么在远离板块边界的某些大陆地区存在广泛的高原，并且与过去地质中板块边界存在的地方无关时，它就不那么成功了。南部非洲高原是地球上这些“地形异常”中最重要的，通常被称为非洲超级井。虽然人们提出了几种不同的模型来解释超级井的形成，但每种模型都表明，高地形是在不同的时间以不同的速度形成的。这些观点中最具争议的是，一级地形与冈瓦纳板块大约150迈前的破裂无关，而是更年轻的，小于30迈，并且与更深的地幔作用有关。最近对深部地幔的研究发现，在非洲南部有一片热的、向上流动的地幔区域，它起源于靠近地核的地方。一些科学家现在认为，正是这种活跃的水流从地下把地球表面往上推，这也是非洲南部海拔异常高的原因。这个项目将为南部非洲主要地形形成的时间提供一个明确的测试，从而解决理解大陆地形如何演变的关键症结。我们无法通过精确测量地表隆起发生的时间来检验这些模型，因为没有直接的证据使我们能够重建过去地质时期海拔的变化。然而，在过去的不同时期，地表的抬升会导致这些时期的侵蚀加速，因为河流的坡度会变陡，尤其是在抬升地区的边缘。幸运的是，有一些技术可以告诉我们侵蚀的历史。这些技术提供了岩石在古代地质过去（数百万年）所经历的温度记录。这是相关的，因为当地球表面被侵蚀时，岩石从更深、更热的层次被带上来，温度会降低。这些方法是基于通过两种不同的方法测量磷灰石矿物中微量U238的放射性衰变；裂变衰变和衰变。裂变衰变导致238U原子核分裂成大致相等的两部分，这两部分迅速相互排斥，造成晶格破坏的线性区域或轨迹——我们称之为裂变轨迹。通过计算这些足迹的数量和测量它们的长度，我们可以重建岩石经历的热历史，因为足迹的长度对110-60摄氏度的浅层地壳典型温度非常敏感。衰变导致氦核的喷射，我们称这些粒子为，来自238U核。通过仔细测量积聚在磷灰石颗粒内的氦气的数量，我们可以确定一块岩石是如何从摄氏70-40度的温度冷却到现在的温度的。这些技术的结合为在地质时间尺度上测量大陆地形的深度侵蚀提供了有力的工具。在这个项目中，我们将分析来自南部非洲深钻孔的样本，一旦我们知道了岩石过去的温度，并将其与地壳中温度发生的深度联系起来，我们就可以准确地确定陆地表面被侵蚀的时间和程度，从而解决地形形成的时间。

This project will test the hypothesis that southern Africa came to acquire its unusually high average elevation-nearly all of it is above 1000 metres-30 million years ago when Africa stopped moving relative to the underlying mantle, and did not inherit its high elevation from the previously elevated super continent of Gondwana. Plate tectonics theory successfully explains how high mountain ranges form as a result of squeezing and thickening of the crust along converging plate boundaries (eg. Andes and Alps). It is less successful in explaining why extensive high plateaus exist in some continental regions far away from plate boundaries and unrelated to where plate boundaries existed in the geological past. The southern African plateau is the most significant of these 'topographic anomalies' on Earth-and is often referred to as the African superswell. While several different models have been proposed to explain the formation of the superswell, each suggests the high topography was formed at different times and at different rates. The most contentious of these ideas is that the first-order topography is not related to the break-up of Gondwana about 150 Myr ago but is much younger, less than 30 Myr, and is related to deeper mantle processes. Recent studies of the deep mantle have identified a region beneath southern Africa of hot, upward flowing mantle which originates close to the Earth's core. Some scientists now believe that it is this active flow that is literally pushing the Earth's surface upwards from below and is the cause of the unusually high elevation of southern Africa. This project will provide a definitive test of when the major topography of southern Africa was formed thus resolving a critical sticking point in understanding how continental topography evolves. We cannot test these models by precisely measuring when the surface uplift occurred because there is no direct evidence which enables us to reconstruct changes in elevation in the geological past. However, uplift of the surface at different times in the past would have caused an acceleration of erosion at these times as river gradients would have been steepened, especially around the edges of the uplifted region. Fortunately there are techniques which tell us about the history of erosion. These techniques provide a record of the temperatures that a rock experienced in the ancient geological past (over millions of years). This is relevant because when the Earth's surface erodes, rocks cool as they are brought up from deeper, hotter levels. The methods are based on measurement of the radioactive decay of U238 which occurs in trace amounts within the mineral apatite by two different processes; fission decay and alpha decay. Fission decay causes a 238U nucleus to split in two roughly equal parts which are rapidly repelled away from each other causing a linear zone or track of damage to the crystal lattice-we call these fission tracks. By counting the number of these tracks and measuring their lengths we can reconstruct the thermal history a rock has experienced because the track lengths are very sensitive to temperatures of 110-60 deg. C typical of the shallow crust. Alpha decay results in ejection of Helium nuclei, we call these alpha particles, from the 238U nucleus. By carefully measuring the amount of Helium gas that has accumulated within a grain of apatite we can determine how a rock has cooled from temperatures of c. 70-40 deg. C to its present temperature. Combined these techniques provide a powerful tool for measuring the deep erosion of continental topography over geological time scales. In this project we will analyse samples from deep bore holes across southern Africa and once we know the rocks' past temperatures and relate it to the depth at which those temperatures occurred in the crust, we can accurately determine when, and how much of, the land surface has eroded and hence resolve when the topography was created.