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

大峡谷是一个标志性的，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。最近有许多新的数据被曝光，但谜团只会加深：一些证据表明地质学上最近的峡谷（在过去的600万年中雕刻），但一些新的证据指向更古老的历史，峡谷的部分可以追溯到大约7000万年前恐龙仍然在地球上行走。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都提到了大峡谷，这场辩论变得激烈起来，公众的兴趣也空前高涨。我们为这个老问题带来了一个新的、互补的方法。剧烈的、轮廓分明的峡谷可能是由于河流下切速率的加速（年轻的峡谷假说）或河流下切到更坚固的岩石中（老的峡谷假说）。根据现有的数据，这两种说法都是合理的。幸运的是，这些备选方案在一个关键方面有很大的不同：而在年轻的峡谷假设中，峡谷内的侵蚀速率比周围景观大得多，在老的峡谷假设中，峡谷内的侵蚀速率应该与周围环境的侵蚀速率相似，甚至更小。我们将使用一种相对较新的方法来测量大峡谷内和周围主要地区千年时间尺度上的平均侵蚀速率：河流沉积物和河流阶地中岩石暴露于宇宙射线所产生的同位素浓度，可以衡量岩石和沉积物在地球表面或附近的时间，从而可以对侵蚀速率进行定量估计。此外，我们将研究大峡谷内部和周围岩石单元的强度，以评估有多少峡谷的形式可以单独用岩石强度的变化来解释。我们期望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。我们解决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）岩性变化如何影响和限制，我们从地貌研究中解释河流下切历史的能力;（3）科罗拉多高原河流峡谷有争议的下切历史。尽管岩性在景观演化中的重要性是基本的，而且长期以来一直被认为是重要的，但近几十年来，岩性在景观演化的定量研究中却很少受到关注。这部分是因为我们缺乏在过程规模上定量测量岩石强度的能力，部分是因为直到最近，我们还缺乏将岩石性质与河流切割过程联系起来的坚实理论;现在可以克服的局限性。我们借鉴和扩展的最新进展，利用浅层地震折射调查，以估计岩体质量的过程规模，使我们能够考虑到岩石断裂的侵蚀敏感性估计的程度。因此，我们将有助于了解河流切入岩石的控制因素（这是气候、构造和地形相互关系的核心），并解决大峡谷的年龄和起源的争议。

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.