Studies of Plateau Uplift Using 4He/3He Apatite Thermochronometry and 13C-18O Carbonate Paleothermometry

从寒武纪到白垩纪(大约5亿年)，科罗拉多高原是一个下沉的台地，而大约4000米的沉积使其保持在海平面附近。在8000万年前的某个时候，地形表面的净抬升导致了目前平均海拔1900米的高原，地壳上部几乎没有内部应变。该项目的重点是对岩石圈如何以及何时获得如此大的浮力，以及地形隆起、岩石隆起和侵蚀去顶之间的关系提供新的限制。最近在热计时和碳氧束同位素测温方面的创新正被用来研究高原西南部的剥离和地表隆升历史。低温冷却事件的时间和速度受到限制，其温度明显低于传统的全晶(U-Th)/He磷灰石年龄(典型冷却速度约为70摄氏度)。这些数据被用来检验这样的假设，即沿西南高原边缘的时间海侵(从西北向东南)的主要组成部分发生在6000万至3500万年前。从碳氧碳酸盐古测温仪获得的古海拔约束将检验这样一个假设，即自大约600万年前以来，在主要的地表抬升之前，高原内陆在新生代的大部分地区处于低得多的海拔。一个多世纪以来，大峡谷地区的地质演化一直是教育普通公众了解地质科学和一般科学的焦点。该区域也是世界上研究大陆地壳抬升与其所受侵蚀力之间关系的最佳天然实验室之一。这个项目使用了植根于地球化学和物理的新技术-S自然产生的氦、碳和氧的同位素-来解决任何观察过大峡谷的人可能会想到的两个基本问题：高原的表面是由沉积在浅海中的石灰岩组成的，什么时候以及为什么会上升到目前海拔约2000米的高度？大峡谷的雕刻是在什么时候进行的--同时发生了隆起，还是发生在更晚的时间？这项研究的早期结果表明，峡谷最初是在白垩纪早第三纪被一条从东南向西北流动的河流切割的，与现代的流动方向相反。此外，数据表明，在上花岗岩峡谷地区，峡谷形成的地层比今天暴露的地层年轻得多，在大约2000万年前的一次侵蚀脉冲中，这个峡谷通过岩柱下降到非常接近现代位置的位置。另一个初步结果是，高原内陆现代海拔的大约一半是在600万年前才达到的。

The Colorado Plateau was a subsiding platform from Cambrian to Cretaceous time (for about 500 million years) while about 4000 meters of sedimentation maintained its elevation near sea level. At some time after 80 million years ago, net uplift of the topographic surface resulted in a plateau with its current mean elevation of 1900 meters, with little internal strain of the upper part of the crust. The focus of this project is to provide new constraints on how and when the lithosphere acquired so much buoyancy, and the relationship between topographic uplift, rock uplift and erosional unroofing. Recent innovations in He thermochronometry and carbon-oxygen clumped isotope thermometry are being used to investigate both the unroofing and surface uplift histories of the southwestern portion of the plateau. Constraints are placed on 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 (approximately 70 degrees C for typical cooling rates). These data are used to test the hypothesis that a major component of time-transgressive (from the northwest to the southeast) unroofing and uplift along the southwestern plateau margin occurred between 60 to 35 million years ago. Paleoelevation constraints obtained from clumped carbon-oxygen carbonate paleothermometry will test the hypothesis that the plateau interior lay at much lower elevations through much of the Cenozoic before major surface uplift since about 6 million years ago.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 into 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 about 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? Results from the earlier phase of this study suggest that the canyon was initially cut in the Early Tertiary of Cretaceous times, by a river that flowed from the southeast to the northwest, opposite the modern direction of flow. Further, data suggest that in the Upper Granite Gorge region, the canyon had formed in strata much younger than those exposed today, and in a pulse of erosion about 20 million years ago this canyon lowered itself through the rock column to a point very close to its modern position. Another preliminary result is that approximately half of the modern elevation of the plateau interior was attained only after 6 million years ago.