原地生成宇宙成因核素测年技术思考(五):单核素计算地表最大抬升速率方法

Consideration on in Situ Produced Cosmogenic Nuclide Dating Techniques(5):Calculation of the Maximum Uplift Rate by the Concentration of^10Be

以往研究中,原地生成宇宙成因核素测年技术虽然理论上可以利用两种核素(如^10Be和^26Al)联立方程组同时解出暴露年龄和侵蚀速率,但在实践中却常采用单种核素(常用^10Be)分别解出最小暴露年龄或最大侵蚀速率,^26Al数据只用于辅助判断。这主要是因为受目前误差水平限制,^10Be和^26Al的测试数据结果常常并未能达到联立两个方程解出暴露年龄和侵蚀速率两个参数的精度(当然也有其他因素影响)这在投影图上常表现为被认为只有一次性暴露历史的样品,投影结果在稳态侵蚀岛之外,只有加上误差,才能进入或靠近稳态侵蚀岛。同理在推导出包含生成速率加速率的计算等式后,虽然理论上抬升区长期暴露的样品也能够利用两种核素(如^10Be和^26Al)同时解出侵蚀速率和抬升速率,但实践上^10Be和^26Al浓度投影结果可能并未在抬升的正常投影区(稳态侵蚀岛上曲线附近),从而解出的侵蚀速率和抬升速率合理性存疑。对此,参考计算最大侵蚀速率的原理提出利用长期暴露样品的^10Be浓度来解出抬升区地表最大抬升速率的计算方法(vmax=PH/4V),从而可以对碰撞造山抬升区的抬升速率上限进行限定。

The in situ produced cosmogenic nuclide dating technology can theoretically use simultaneous equations of two nuclides(such as ^10Be and26Al)to resolve the exposure age and erosion rate simultaneously.But in practice a single nuclide(commonly ^10Be)is often adopted to solve the minimum exposure age and maximum erosion rate respectively.The 26Al data is only used for auxiliary judgment.The main reason for this is that limited by the current error level,the measurement results of ^10Be and 26Al often fail to reach the accuracy of the two parameters of the simultaneous equations to solve the exposure age and erosion rate(of course,there are some other factors)as shown by the projected image out of steady erosion for the sample with only single exposure history,and only when the error is added it can be projected into or near the steady-state erosion island.Similarly,after deriving the calculation equation including the generation acceleration rate although theoretically long-term exposed samples can simultaneously solve the erosion rate and the uplift rate by using two kinds of nuclides(^10Be and 26Al),in practice their concentration projection results may not be in the normal projection area of the uplift(near the curve on the steady-state erosion island)so the resolutions of the erosion rate and the uplift rate are doubtful.To solve this problem,referring to the principle of calculating the maximum erosion rate,this paper proposes a calculation method of the maximum uplift rate of the uplift surface zone(vmax=PH/4N)by using the ^10Be concentration of the long-term exposed sample,so that the upper limit of the uplift rate of the collisional orogeny can be constrained well.