Studies of Plateau Uplift using 4He/3He Apatite Thermocronometer and 13C-18O Carbonate Paleothermometer

科罗拉多高原，像大多数北美克拉通一样，是一个古生代-中生代地台，平均缓慢下沉近5亿年，而沉积作用保持其海拔接近海平面;但与大多数克拉通不同的是，在8000万年前的某个时候，通过岩石和地形表面的净隆起，高原达到目前的平均海拔约1900米，地壳上部的内部应变很小。后白垩纪侵蚀去顶的高原范围从零保存的早期第三纪盆地的周边高原高达4500米的大峡谷的花岗岩峡谷。岩石圈是如何以及何时获得如此大的浮力的？地形抬升、岩石抬升和侵蚀去顶之间的关系是什么？该项目正在利用氦热年代测定法和碳氧同位素温度测定法的最新创新来调查高原东南部的去顶和表面隆起历史。氦磷灰石热时计正被用来限制低温冷却事件的时间和速率，温度显着低于传统的全晶体（U-Th）/He磷灰石年龄。新的碳-氧碳酸盐古温度计也在使用中，该温度计限制了碳酸盐生长温度和产生该温度计的沃茨的氧同位素组成。这种方法比传统的两相碳酸盐-水氧同位素测温法需要更少的关于气候变化和季节性的假设，并允许更有效地隔离海拔高度对同位素记录的贡献。通过这种整体方法，对二叠纪凯巴布石灰岩的中生代单元的侵蚀时间以及大峡谷的切割时间进行了重大的新限制，以形成高原非常平坦的西南表面。此外，对高原上和直接邻近高原的各种新生代碳酸盐岩单元的分析将更好地限制海拔历史及其与周围的中生代和新生代构造事件的关系。大峡谷地区的地质演变已经成为世纪以来教育公众了解地质科学和一般科学的焦点。这个项目使用了植根于地球上天然存在的氦、碳和氧同位素的化学和物理学的新技术，以解决任何曾经在大峡谷边缘窥视过的人可能会想到的两个基本问题：高原是何时以及为什么由沉积在浅海中的石灰岩组成的高原上升到目前海拔2000米以上的高度？大峡谷的雕刻是什么时候发生的--是在隆起发生的同时，还是在更晚的时间？这些问题最早是由约翰·韦斯利·鲍威尔及其同事在世纪末提出的，即使在今天，也令人惊讶地引起了争议。

The Colorado Plateau, like most of the North American craton, is a Paleozoic-Mesozoic platform that on average subsided slowly for nearly 500 million years while sedimentation maintained its elevation near sea level; but unlike most of the craton, at some time after 80 million years ago, via net uplift of both rocks and the topographic surface, the plateau attained its current mean elevation of about 1900 meters with little internal strain of the upper part of the crust. Post-Cretaceous erosional unroofing of the plateau ranges from zero in preserved early Tertiary basins on the perimeter of the plateau to as much as 4500 meters in the Upper Granite Gorge of the Grand Canyon. How and when did the lithosphere acquire so much buoyancy, and what is the relationship between topographic uplift, rock uplift and erosional unroofing? This project 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. The helium apatite thermochronometer is being used to constrain the timing and rate of low-temperature cooling events down to temperatures significantly lower than those accessible by conventional whole crystal (U-Th)/He apatite ages. The new C-O carbonate paleothermometer that constrains both the carbonate growth temperature and the oxygen isotopic composition of the waters from which it was derived is also being used. This method requires fewer assumptions regarding climate change and seasonality than conventional two-phase carbonate-water oxygen isotope thermometry, and permits more effective isolation of the altitudinal contribution to the isotopic record. With this overall approach, significant new constraints are being placed on the timing of erosion of Mesozoic units off of the Permian Kaibab Limestone to form the remarkably flat southwestern surface of the plateau and the timing of incision of the Grand Canyon. In addition, analysis of a variety of Cenozoic carbonate units both on and directly adjacent to the plateau will better constrain the elevation history and its relationship to Mesozoic and Cenozoic tectonic events that surround it.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. 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 meters 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, even today, are surprisingly controversial.