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雕刻的这个谜团感到困惑。最近，许多新数据浮出水面，但谜团进一步加深：一些证据表明，这是一个地质上最近的峡谷(雕刻于过去600万年)，但一些新证据表明，峡谷的历史要古老得多，该峡谷的部分地区可以追溯到大约7000万年前，当时恐龙还在地球上行走。鉴于大峡谷国家公园有许多游客，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，公众的兴趣达到了前所未有的高度。我们为这个老问题带来了一种新的、补充的方法。河流切割速度的加快(年轻峡谷假说)或河流切割成更坚固的岩石(旧峡谷假说)都可能导致戏剧性的、清晰的峡谷。根据目前可获得的数据，这两种说法都是可信的。幸运的是，这些替代方案在一个关键方面有很大不同：在年轻峡谷假说中，峡谷内的侵蚀率比周围景观中的侵蚀率要大得多，而在旧峡谷假说中，峡谷中的侵蚀率应该与周围环境的侵蚀率相似，甚至更低。我们将使用一种相对较新的方法来测量大峡谷内和周围关键地点在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提供了岩石和沉积物在地球表面或附近停留多长时间的衡量标准，从而可以对侵蚀速率进行定量估计。此外，我们还将研究大峡谷内部和周围岩石单元的强度，以评估仅靠岩石强度的变化就可以解释峡谷形式的多少。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基础性和长期被认识的重要性，但近几十年来，在景观演化的定量研究中，岩性很少受到关注。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，我们还缺乏将岩石特性与河流切割过程联系起来的可靠理论；现在可以克服这些限制。我们借鉴并推广了使用浅层地震折射测量在过程尺度上估计岩石质量的最新进展，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割岩石的控制(这是气候、构造和地形之间相互关系的核心)，并有助于解决关于大峡谷时代和起源的争议。

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.