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而又神秘的地貌。一百多年来，地质学家一直对峡谷是如何以及何时被雕刻的谜团感到困惑。最近出现了许多新的数据，但这个谜团只会加深：一些证据指向一个地质上最近的峡谷（在过去的600万年中雕刻），但一些新的证据指向一个更古老的历史，峡谷的一部分可以追溯到大约7000万年前恐龙还在地球上行走的时候。鉴于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涉及到大峡谷，这场辩论变得越来越激烈，公众的兴趣也达到了前所未有的高度。我们用一种新的、互补的方法来解决这个老问题。引人注目的、轮廓分明的峡谷可能是由河流切割速度加快（年轻峡谷假说）或河流切割成更坚硬的岩石（老峡谷假说）造成的。根据目前可获得的数据，两者都是合理的。幸运的是，这些不同的场景在一个关键方面有显著不同：在年轻峡谷假说中，峡谷内的侵蚀率远远大于周围景观，而在老峡谷假说中，峡谷内的侵蚀率应该与周围环境的侵蚀率相似，甚至小于。我们将使用一种相对较新的方法来测量大峡谷内外关键地区在千年时间尺度上的平均侵蚀率：在河流沉积物和河流阶地中，岩石暴露于宇宙射线中产生的同位素浓度可以测量岩石和沉积物在地球表面或其附近停留的时间，从而可以对侵蚀率进行定量估计。此外，我们将研究大峡谷内部和周围岩石的强度，以评估峡谷的形成有多少可以单独用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教材和全国各地的地球科学课程计划中。我们讨论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演变中的作用；(2)岩性变异性如何影响和限制我们从地貌研究中解释河流切割历史的能力；(3)科罗拉多高原河谷有争议的切割历史。尽管岩石学在景观演化中的重要性早已得到公认，但近几十年来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。我们利用并扩展了最近在使用浅层地震折射测量来估计过程尺度上的岩体质量方面的进展，使我们能够在估计侵蚀敏感性时考虑岩石断裂的程度。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），并解决大峡谷的年龄和起源的争议。