Resolving the age of the first-order topography of Africa

该项目将测试一个假设，即南部非洲在3000万年前获得了异常高的平均海拔-几乎所有的海拔都在1000米以上-当时非洲停止了相对于底层地幔的运动，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，这是地壳沿着会聚的板块边界（例如：安第斯山脉和阿尔卑斯山）。它在解释为什么在远离板块边界的一些大陆地区存在广泛的高原，并且与地质学上过去存在的板块边界无关方面不太成功。南部非洲高原是地球上最重要的“地形异常”，通常被称为非洲超级井。虽然已经提出了几种不同的模型来解释超井的形成，但每种模型都表明高地形是在不同的时间和不同的速度形成的。这些观点中最有争议的是，第一级地形与冈瓦纳大陆约1.5亿年前的分裂无关，但要年轻得多，不到3000万年，并且与更深的地幔过程有关。最近对深部地幔的研究已经确定了非洲南部下面的一个区域，该区域是热的，向上流动的地幔，起源于地球的核心附近。一些科学家现在认为，正是这种活跃的水流从下面向上推动地球表面，是非洲南部异常高海拔的原因。该项目将对南部非洲的主要地形何时形成提供明确的测试，从而解决了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测试这些模型，因为没有直接证据使我们能够重建地质历史中的海拔变化。然而，过去不同时期的地表抬升会导致这些时期的侵蚀加速，因为河流坡度会变陡，特别是在抬升区域的边缘。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，因为当地球表面侵蚀时，岩石从更深、更热的层面被带上来时会冷却。这些方法是基于测量铀238的放射性衰变，铀238通过两种不同的过程在矿物磷灰石中以痕量发生;裂变衰变和α衰变。裂变衰变导致238铀核分裂成两个大致相等的部分，这两个部分迅速相互排斥，导致晶格损伤的线性区域或轨道-我们称之为裂变轨道。通过计算这些轨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，因为轨迹长度对110-60度的温度非常敏感。典型的浅地壳。α衰变导致氦核从238 U核中喷射出来，我们称之为α粒子。通过仔细测量磷灰石颗粒中积聚的氦气量，我们可以确定岩石是如何从摄氏度冷却下来的。70-40度C到现在的温度。这些技术的结合为测量地质时间尺度上大陆地形的深部侵蚀提供了强有力的工具。在这个项目中，我们将分析来自非洲南部深钻孔的样本，一旦我们知道岩石过去的温度，并将其与地壳中这些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何时以及有多少被侵蚀，从而解决地形何时形成。

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.