Studies of Plateau Uplift using (U-Th)/He Apatite Thermochronology and 13C-18O Carbonate Paleothermometry

科罗拉多高原特别好地说明了大陆高原隆升的矛盾。像北美大陆内部的大部分地区一样，它是一个古生代-中生代地台，从寒武纪到白垩纪缓慢下沉4000米，而沉积作用保持其海拔接近海平面。但与内陆不同的是，在8000万年前的某个时候，岩石和地形表面的净抬升导致了一个高原，其目前的平均海拔为1900米。该项目的重点是提供新的限制，如何和为什么这种隆起发生，以及地形隆起，岩石隆起和侵蚀去顶之间的关系。该研究小组正在利用氦热年代学和碳氧同位素温度测定法的最新创新来研究高原东南部的去顶和表面隆起历史。由于它们在裂变径迹的退火温度（110摄氏度）以下的长时间停留，平台磷灰石显示出由辐射损伤引起的氦扩散率的相当大的变化。这反过来又对低温冷却事件的时间和速率施加了严格的限制，直到近地表温度。该项目还将继续使用13 C-18 O碳酸盐凝聚同位素古温度计，以限制使用湖泊碳酸盐的古高程。到目前为止，研究小组已经证明了从晚白垩世开始，沿着高原边缘沿着发生的西南到东北的间歇性侵蚀，并在过去的1000万年里以高原内部的快速去顶而告终。一个令人惊讶的结果是，来自大峡谷上花岗岩峡谷底部的磷灰石与周围高原上的磷灰石具有无法区分的冷却历史。这些历史表明，峡谷形成于晚白垩世，侵蚀水平降低，作为一个平衡景观到现在的地形，主要是在晚渐新世时间。到目前为止，聚集同位素结果包括对现代湖泊的研究，这些研究显示出明显的温度上升趋势。对于中新世湖泊碳酸盐岩，温度变化作为海拔的函数与现代趋势相同，但任何给定海拔的温度都高出近8摄氏度。这一结果表明，自1980年以来，高原与周围低地之间的相对高程变化不大。大峡谷地区的地质演变是一个焦点，教育公众对地质科学，一般科学，超过世纪。该区域也是世界上研究大陆地壳隆起与作用于其上的侵蚀力之间关系的最佳自然实验室之一。自然存在的氦、碳和氧的同位素，以解决两个基本的问题，这两个问题可能会发生在任何人谁曾经凝视过大峡谷的边缘：何时以及为什么高原，其表面是由石灰岩沉积在浅海，上升到目前的高度超过海平面2000米？大峡谷的雕刻是什么时候发生的？是在同一时期还是在更晚的时间这些问题最早是由约翰·韦斯利·鲍威尔（John Wesley Powell）及其同事在世纪末提出的，也是当时欧洲日益意识到美国科学（和科学家）是一支不可忽视的力量的核心。即使在今天，这些问题仍然令人惊讶地具有争议性。这个研究小组以前的工作结果表明，峡谷最初是在白垩纪时期被一条从西南流向东北的主要河流切割的，与科罗拉多河的现代流向相反。此外，数据表明，在上花岗岩峡谷地区，峡谷是在比今天暴露的地层年轻得多的地层中切割的。 在大约2000万年前的一次侵蚀中，侵蚀面降低到非常接近现代位置的位置。这些结果将被这个项目进一步检验，表明大峡谷不是由科罗拉多河雕刻而成的，科罗拉多河只存在了过去的六百万年。

The Colorado Plateau illustrates the paradox of continental plateau uplift particularly well. Like much of the North American continental interior, it is a Paleozoic-Mesozoic platform that slowly subsided 4000 m from Cambrian to Cretaceous time while sedimentation maintained its elevation near sea level. But unlike the interior, at some time after 80 million years ago, net uplift of both rocks and the topographic surface resulted in a plateau with its current mean elevation of 1900 m. The focus of this project is to provide new constraints on how and why this uplift occurred, and the relationships between topographic uplift, rock uplift and erosional unroofing. The research team is using recent innovations in helium thermochronometry and carbon-oxygen isotope thermometry to investigate both the unroofing and surface uplift histories of the southeastern portion of the plateau. Because of their long residence below the annealing temperature of fission tracks (110 deg C), plateau apatites show considerable variation in helium diffusivity caused by radiation damage. This in turn places strong constraints on the timing and rate of low-temperature cooling events down to near-surface temperatures. This project will also continue use of the 13C-18O carbonate clumped isotope paleothermometer to place constraints on paleoelevation using lacustrine carbonates. Thus far, the team has demonstrated episodic, southwest to northeast erosion beginning in Late Cretaceous time along the margin of the Plateau and culminating in rapid unroofing of the plateau interior in the last 10 million years. A surprising result is that apatites from the bottom of the Upper Granite Gorge of the Grand Canyon have indistinguishable cooling histories from apatites on the surrounding plateau. These histories indicate that the canyon was formed in Late Cretaceous time, and that the erosion level lowered itself as an equilibrium landscape onto the present topography, primarily in Late Oligocene time. Clumped isotope results to date have included studies of modern lakes, which show a clear temperature-elevation trend. For Miocene lake carbonates, the temperature change as a function of elevation is the same as the modern trend, but the temperature at any given elevation is nearly 8 degrees C hotter. This result suggests that little relative elevation change has occurred between the plateau and surrounding lowlands since ca. 16 Ma, and quantifies the amount of climatic cooling.The geologic evolution of the Grand Canyon region has been a focal point for educating the general public about the geological sciences, and science in general, for over a century. The region is also among the best natural laboratories in the world for addressing the relationship between uplift of the continental crust and the erosional forces acting upon it. This project uses new techniques rooted in the chemistry and physics of the earth?s naturally occurring isotopes of helium, carbon and oxygen to address two fundamental questions that might occur to anyone who has ever peered over the edge of the Grand Canyon: When and why did the high plateau, whose surface is made of limestone deposited in a shallow ocean, rise to its current height of over 2000 m above sea level? And when did the carving of the Grand Canyon take place?at the same time uplift occurred, or at a much later time? These questions were first posed by John Wesley Powell and associates in the late 19th century, and were the centerpieces of growing awareness in Europe at that time that American science (and scientists) were a force to be reckoned with. Even today, these questions are surprisingly controversial. Results from this research team's previous work suggest that the canyon was originally cut in the Cretaceous Period, by a major river that flowed from the southwest to northeast, opposite the modern direction of flow of the Colorado River. Further, data suggest that in the Upper Granite Gorge region, the canyon was cut in strata much younger than those exposed today. In a pulse of erosion about 20 million years ago, the erosion surface was lowered to a point very close to its modern position. These results, which will be further tested by this project, indicate that Grand Canyon was not carved by the Colorado River, which has only been in existence over the last six million years.