U-Pb zircon ages as a sediment mixing tracer in the Nepal Himalaya
U-Pb zircon ages as a sediment mixing tracer in the Nepal Himalaya
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DOI：
10.1016/j.epsl.2005.03.019
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发表时间：
2005-06-30
影响因子：
5.3
通讯作者：
Gehrels, GE
中科院分区：
文献类型：
--
作者：
Amidon, WH;Burbank, DW;Gehrels, GE
本文提出了一种基于沉积物内 U-Pb 锆石年龄分布混合来量化沉积物混合的新方法。提出了两种统计技术来确定两个已知年龄分布组合以创建已知混合年龄分布的比例。然后，这些技术用于确定尼泊尔喜马拉雅山中部主中央冲断层 (MCT) 上方和下方相邻流域之间的相对侵蚀率。 MCT 区域与地貌特征和矿物冷却年龄的南北突变相一致，这被认为代表了对 MCT 区域北部较高的岩石隆起率的侵蚀反应。然而，尚不清楚造成差异隆起率的持续变形是否是：（1）集中在 MCT； (2) 沿地壳尺度斜坡的深度，或 (3) 沿 MCT 以南新绘制的逆冲断层。我们的研究通过比较现代侵蚀率与根据矿物冷却年龄确定的长期侵蚀率来探讨这个问题。从现代河砂中分离出锆石，并通过 LA-MC-ICPMS 进行测年，然后将测量的同位素比和年龄用于 1 d 和 2 d 混合计算。一维技术为每个样本创建锆石年龄的概率密度函数，然后使用迭代和逆方法来估计样本之间的混合。相反，二维技术估计由测量的 U-238/Pb-206 和 Pb-207/Pb-206 比率定义的概率“场”之间的混合。给定具有完美样本表示的有限混合，这两种技术都可以在一系列混合比例下产生完美的混合估计。建模结果表明，给定复杂父母年龄分布的不完美子样本表示，可能需要不同程度的子样本平滑才能实现准确的混合估计。使用锆石年龄的混合作为沉积物混合的定量代理需要对河流沉积物中锆石的浓度进行校正。提出了两种确定河流沉积物中锆石浓度的新方法，证明相邻流域之间存在 2 至 5 倍的锆石浓度差异。通过确定相邻排水系统之间的锆石混合比来估计相对侵蚀率，然后通过锆石浓度比和排水面积比将其归一化。结果显示，小喜马拉雅山最北端的 MCT 以南的现代侵蚀率高出 3 倍。这项新技术的未来应用可能包括河段沉积物输运动力学、改进的沉积盆地分析以及更好地解释前陆矿物冷却年龄。 (c) 2005 Elsevier B.V. 保留所有权利。
This paper presents a new approach to quantify sediment mixing based on the mixing of U-Pb zircon age distributions within sediment. Two statistical techniques are presented to determine the proportion in which two known age distributions combine to create a known mixed age distribution. These techniques are then used to determine relative erosion rates between adjacent drainage basins above and below the Main Central Thrust (MCT) in the central Nepal Himalaya. The MCT region is coincident with an abrupt north-south change in geomorphic character and mineral cooling ages that are thought to represent an erosional response to higher rock uplift rates north of the MCT zone. However, it is unclear whether the ongoing deformation responsible for the differential uplift rates is: (1) focused on the MCT; (2) at depth along a crustal scale ramp-, or (3) along newly mapped thrust faults south of the MCT. Our study explores this issue by comparing modem erosion rates with longer-term erosion rates determined from mineral cooling ages. Zircons were separated from modem river sand and dated by LA-MC-ICPMS before the measured isotopic ratios and ages were used in 1-d and 2-d mixing calculations. The 1-d technique creates probability density functions of zircon ages for each sample and then uses both an iterative and inverse approach to estimate mixing between samples. In contrast, the 2-d technique estimates mixing between probability "fields" defined by the measured U-238/Pb-206 and Pb-207/Pb-206 ratios. Given a finite mixture with perfect sample representation, both techniques produce perfect mixing estimates across a range of mixing proportions. Modeling results demonstrate that given imperfect subsample representation of the complex parent age distribution, differing degrees of subsample smoothing may be required to achieve an accurate mixing estimate. Using mixing of zircon ages as a quantitative proxy for sediment mixing requires a correction for the concentration of zircon in the river sediment. Two new methods for establishing zircon concentration in river sediment are presented demonstrating the existence of 2- to 5-fold differences in zircon concentration between adjacent drainages. Relative erosion rates are estimated by determining the zircon mixing ratio between adjacent drainages which are then normalized by the ratio of zircon concentrations and the ratio of drainage areas. Results show similar to 3 times higher modem erosion rates south of the MCT in the northernmost Lesser Himalaya. Future applications of this new technique may include reach-scale sediment transport dynamics, improved sedimentary basin analysis, and better interpretation of foreland mineral cooling ages. (c) 2005 Elsevier B.V. All rights reserved.