Collaborative Research: Differentiating Between Lithologic and Baselevel Controls on River Profiles: Canyons of the Colorado Plateau

合作研究：区分河流剖面的岩性和基准面控制：科罗拉多高原的峡谷

• 批准号: 1323866
• 负责人: Paul Bierman
• 金额: $ 2.53万
• 依托单位: University of Vermont & State Agricultural College
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2013
• 资助国家: 美国
• 起止时间: 2013-09-01 至 2016-08-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1323866&HistoricalAwards=false
• 关键词: Collaborative; Research; Differentiating; Between; Lithologic

The Grand Canyon is an iconic but enigmatic landform. For over a hundred years geologists have puzzled over the mystery of how and when the canyon was carved. Much new data has come to light recently, but the mystery has only deepened: some evidence points to a geologically recent canyon (carved in the last 6 million years) but some new evidence points to a much older history, with parts of the canyon dating back some 70 million years to when dinosaurs still walked the Earth. The debate has become intense and public interest is at an all-time high given the many visitors to the Grand Canyon National Park and the fact that most introductory Earth science classes from middle school through university involve some mention of the Grand Canyon. We bring a new, complementary, approach to this old problem. Dramatic, sharply defined canyons can result from either acceleration in river incision rate (the young canyon hypothesis) or river incision into stronger rocks (the old canyon hypothesis). Either is plausible given presently available data. Fortunately these alternative scenarios are dramatically different in one key regard: whereas in the young canyon hypothesis erosion rates within the canyon are much greater than in the surrounding landscape, in the old canyon hypothesis erosion rates in the canyon should be similar to, or even less than, erosion rates in the surroundings. We will use a relatively new method to measure erosion rates averaged over millennial timescales in key localities within and around the Grand Canyon: the concentration of isotopes produced by exposure of rocks to cosmic rays in river sediments and on river terraces provides a measure of how long rocks and sediment has spent at or near the Earth's surface, and thus allows a quantitative estimate of erosion rates. In addition we will study the strength of rock units within and surrounding the Grand Canyon to assess how much of the canyon's form can be explained by variations in rock strength alone. We anticipate that our results will be incorporated into materials at Grand Canyon National Park and into Earth science lesson plans across the country.We address three fundamental problems of broad interest to Geologists and Geomorphologists: (1) the role of lithology in river incision and landscape evolution in general, (2) how lithologic variability affects, and limits, our ability to interpret river incision history from study of landforms and (3) the controversial incision history of river canyons in the Colorado Plateau. Despite the fundamental, and long-recognized, importance of lithology in landscape evolution, it has received little attention in the quantitative studies of landscape evolution in recent decades. Partly this is because we have lacked the ability to quantitatively measure rock strength at the process scale and partly because until recently we lacked firm theory to relate rock properties to river incision processes; limitations that can now be overcome. We draw on and extend recent advances in using shallow seismic refraction surveys to estimate rock mass quality at the process scale, allowing us to account for the extent of rock fracture in estimates of erosional susceptibility. Thus we will contribute both to understanding of the controls on river incision into rock (which is at the heart of the interrelations among climate, tectonics, and topography) and to resolving the controversy over the age and origin of the Grand Canyon.

大峡谷是一个标志性但神秘的地貌。一百多年来，地质学家一直对峡谷是如何以及何时形成的谜团感到困惑。最近，许多新数据曝光，但谜团却愈加加深：一些证据表明，在地质上有一个较新的峡谷（在过去 600 万年中形成），但一些新证据表明，历史更为悠久，峡谷的部分区域可追溯到追溯到大约 7000 万年前，恐龙还在地球上行走。鉴于前往大峡谷国家公园的游客众多，而且从中学到大学的大多数地球科学入门课程都会提到大峡谷，因此争论变得激烈，公众的兴趣空前高涨。我们为这个老问题带来了一种新的、补充性的方法。戏剧性的、轮廓分明的峡谷可能是由于河流切割速度的加快（新峡谷假说）或河流切入更坚固的岩石（旧峡谷假说）而产生的。考虑到目前可用的数据，这两种情况都是合理的。幸运的是，这些替代方案在一个关键方面存在显着差异：在年轻峡谷假设中，峡谷内的侵蚀率远大于周围景观，而在旧峡谷假设中，峡谷中的侵蚀率应该相似，甚至更低比，周围的侵蚀率。我们将使用一种相对较新的方法来测量大峡谷内及其周围关键地点在千年时间尺度上的平均侵蚀率：河流沉积物和河流阶地上的岩石暴露于宇宙射线所产生的同位素浓度可以衡量侵蚀的持续时间。岩石和沉积物已经在地球表面或附近消耗，因此可以定量估计侵蚀率。此外，我们还将研究大峡谷内部和周围岩石单元的强度，以评估峡谷的形态在多大程度上可以仅通过岩石强度的变化来解释。我们预计我们的研究结果将被纳入大峡谷国家公园的材料和全国各地的地球科学课程计划中。我们解决了地质学家和地貌学家广泛感兴趣的三个基本问题：（1）岩性在河流切割和景观中的作用一般进化，（2）岩性变异如何影响和限制我们通过地貌研究解释河流切割历史的能力，以及（3）科罗拉多高原河流峡谷有争议的切割历史。尽管岩性在景观演化中具有根本性且长期被认可的重要性，但近几十年来，它在景观演化的定量研究中却很少受到关注。部分原因是我们缺乏在过程尺度上定量测量岩石强度的能力，部分原因是直到最近我们还缺乏将岩石特性与河流切割过程联系起来的可靠理论；现在可以克服的限制。我们借鉴并扩展了使用浅层地震折射勘测来估计过程尺度岩体质量的最新进展，使我们能够在估计侵蚀敏感性时考虑岩石破裂的程度。因此，我们将有助于理解河流切入岩石的控制（这是气候、构造和地形之间相互关系的核心），并有助于解决有关大峡谷的年龄和起源的争议。