Inference of viscosity jump at 670 km depth and lower mantle viscosity structure from GIA observations
Inference of viscosity jump at 670 km depth and lower mantle viscosity structure from GIA observations
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DOI：
10.1093/gji/ggx519
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发表时间：
2018-03
影响因子：
2.8
通讯作者：
M. Nakada;J. Okuno;Yoshiya Irie
中科院分区：
文献类型：
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作者：
M. Nakada;J. Okuno;Yoshiya Irie
采用由温度 (T) 和压力 (P) 分布以及恒定活化能（对于上地幔和下地幔）和体积 (and) 描述的指数分布的粘度模型，用于根据冰川均衡调整 (GIA) 的观测来推断地幔的粘度结构。我们首先构建标准粘度模型，平均上地幔粘度 () 为 2 × 1020Pa s，这是海洋上地幔粘度的典型值，满足观测得出的三个 GIA 相关观测值、GIA 引起的位势二阶谐波变化率 ${\skew5\dot{J}_2}$ 以及末次冰期最大海平面的微分相对海平面 (RSL) 变化澳大利亚的巴巴多斯和波拿巴湾以及澳大利亚的卡伦巴和哈利法克斯湾的 RSL 在 6 kyr BP 发生变化。从三个 GIA 相关观测值推断出的标准粘度模型的特征是，深部地幔中的粘度为 ∼1023Pa s，假设 670 公里深度处的粘度为 ηlm(670)，为 (1 − 50) × 1021Pa s。北美冰盖中部地区的绍斯波特、百慕大和大沼泽地的冰期后 RSL 变化在很大程度上取决于其总融化历史，对于推断 670 公里深度的粘度跳跃具有至关重要的潜力。尽管粘度模型和冰历史模型之间存在权衡，但基于≥ 2 × 1020Pa s 的粘度模型和标准模型的下地幔粘度结构对这些 RSL 变化的分析产生了允许值和 ηlm(670) 值。我们的首选值和ηlm(670)值分别为~(7 − 9)×1020和~1022Pa s，高于大洋上地幔的典型值，这可能反映了上地幔中等程度的横向非均质粘度。本研究采用的地幔粘度结构取决于温度分布、活化能和体积，很难讨论每个量对推断的下地幔粘度模型的影响。我们得出结论，下地幔粘度随常数和平滑深度变化的模型与 GIA 观测结果一致。
A viscosity model with an exponential profile described by temperature (T) and pressure (P) distributions and constant activation energy (for the upper mantle andfor the lower mantle) and volume (and) is employed in inferring the viscosity structure of the Earth's mantle from observations of glacial isostatic adjustment (GIA). We first construct standard viscosity models with an average upper-mantle viscosity () of 2 × 1020Pa s, a typical value for the oceanic upper-mantle viscosity, satisfying the observationally derived three GIA-related observables, GIA-induced rate of change of the degree-two zonal harmonic of the geopotential, ${\skew5\dot{J}_2}$, and differential relative sea level (RSL) changes for the Last Glacial Maximum sea levels at Barbados and Bonaparte Gulf in Australia and for RSL changes at 6 kyr BP for Karumba and Halifax Bay in Australia. Standard viscosity models inferred from three GIA-related observables are characterized by a viscosity of ∼1023Pa s in the deep mantle for an assumed viscosity at 670 km depth,ηlm(670), of (1 − 50) × 1021Pa s. Postglacial RSL changes at Southport, Bermuda and Everglades in the intermediate region of the North American ice sheet, largely dependent on its gross melting history, have a crucial potential for inference of a viscosity jump at 670 km depth. The analyses of these RSL changes based on the viscosity models with≥ 2 × 1020Pa s and lower-mantle viscosity structures for the standard models yield permissibleandηlm(670) values, although there is a trade-off between the viscosity and ice history models. Our preferredandηlm(670) values are ∼(7 − 9) × 1020and ∼1022Pa s, respectively, and theis higher than that for the typical value of oceanic upper mantle, which may reflect a moderate laterally heterogeneous upper-mantle viscosity. The mantle viscosity structure adopted in this study depends on temperature distribution and activation energy and volume, and it is difficult to discuss the impact of each quantity on the inferred lower-mantle viscosity model. We conclude that models of smooth depth variation in the lower-mantle viscosity followingwith constantandare consistent with the GIA observations.