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起的悖论。与北美大陆内部的大部分地区一样，它是一个古生代-中生代台地，在寒武纪至白垩纪期间缓慢下沉4000米，同时沉积作用使其高度保持在海平面附近。但与内部不同的是，在8000万年前的某个时候，岩石和地形表面的净隆起形成了一个高原，目前的平均海拔为1900米。该项目的重点是提供新的约束条件，说明这种隆升是如何以及为什么发生的，以及地形隆升、岩石隆升和侵蚀剥落之间的关系。研究小组正在使用最新的氦气测温仪和碳氧同位素测温仪来研究高原东南部的剥落和地表隆起历史。由于高原磷灰石长时间停留在裂变径迹的退火温度（110℃）以下，辐射损伤引起的氦扩散率有很大的变化。这反过来又对低温冷却事件的时间和速度施加了严格的限制，温度下降到接近地表的温度。本项目还将继续使用13C-18O碳酸盐块状同位素古温计，利用湖相碳酸盐对古高程进行约束。到目前为止，研究小组已经证明，从晚白垩纪开始，沿着高原边缘，西南到东北的侵蚀是断断续续的，在过去的1000万年里，高原内部的屋顶迅速剥落。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有着难以区分的冷却历史。这些历史表明，该峡谷形成于晚白垩世，主要是在晚渐新世，其侵蚀水平作为一个平衡景观降低到现在的地形上。迄今为止，团块同位素的结果包括对现代湖泊的研究，显示出明显的温度升高趋势。中新世湖泊碳酸盐的温度随海拔的变化趋势与现代趋势相同，但在任何给定海拔的温度都比现在高近8摄氏度。这一结果表明，自约16 Ma以来，高原和周围低地之间的相对海拔变化很小，并量化了气候变冷的程度。一个多世纪以来，大峡谷地区的地质演化一直是教育公众了解地质科学和一般科学的焦点。该地区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好的自然实验室之一。这个项目使用了根植于地球化学和物理的新技术。这是一种天然存在的氦、碳和氧的同位素，可以解决任何曾经凝视过大峡谷边缘的人可能会想到的两个基本问题：这个表面由浅海中沉积的石灰岩构成的高原，何时以及为什么会上升到目前海拔2000多米的高度？大峡谷的雕刻是什么时候开始的？与此同时，隆起发生了，还是发生在更晚的时间？这些问题最初是由约翰·韦斯利·鲍威尔（John Wesley Powell）和他的同事在19世纪末提出的，它们是当时欧洲日益意识到美国科学（和科学家）是一股不可忽视的力量的核心内容。即使在今天，这些问题也令人惊讶地引起争议。该研究小组先前的研究结果表明，峡谷最初是在白垩纪时期由一条从西南流向东北的大河切割而成的，与现在科罗拉多河的流向相反。此外，数据表明，在上花岗岩峡谷地区，峡谷的地层比今天暴露的地层要年轻得多。在大约两千万年前的一次侵蚀中，侵蚀面被降低到非常接近它现在的位置。这些结果，将在这个项目中进一步验证，表明大峡谷不是由科罗拉多河雕刻的，科罗拉多河在过去的600万年里才存在。

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.