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-中生代台地，在近5亿年的平均缓慢沉降过程中，沉积作用使其高度保持在海平面附近；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，高原达到了现在的平均海拔约1900米，地壳上部的内部应变很小。白垩纪后，高原的侵蚀剥落范围从高原周边保存完好的早第三纪盆地的零到大峡谷上花岗岩峡谷的4500米。岩石圈是如何以及何时获得如此多的浮力的？地形隆起、岩石隆起和侵蚀剥落之间的关系是什么？该项目利用最新的氦热时计和碳氧同位素测温技术来研究青藏高原东南部的剥落和地表隆起历史。氦磷灰石热时计用于将低温冷却事件的时间和速率限制在明显低于常规全晶体(U-Th)/He磷灰石年龄所能达到的温度。新的C-O碳酸盐古温度计也在使用中，它既限制了碳酸盐生长温度，也限制了其来源的水的氧同位素组成。与传统的两相碳酸盐-水氧同位素测温法相比，该方法对气候变化和季节性的假设更少，并且可以更有效地分离出对同位素记录的海拔贡献。通过这种整体方法，对于二叠纪凯巴布石灰岩的中生代单元形成高原西南非常平坦的表面的侵蚀时间和大峡谷切割的时间提出了重要的新限制。此外，对高原上及其直接邻近的各种新生代碳酸盐岩单元的分析，将更好地约束高原的隆升历史及其与周围中、新生代构造事件的关系。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。这个项目采用了基于地球上自然产生的氦、碳和氧同位素的化学和物理的新技术，来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发生的？是在隆起发生的同时，还是在更晚的时候？这些问题最初是由约翰•韦斯利•鲍威尔（John Wesley Powell）及其同事在19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.