Inference of mantle viscosity for depth resolutions of GIA observations
Inference of mantle viscosity for depth resolutions of GIA observations
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GIA 观测深度分辨率的地幔粘度推断
DOI：
10.1093/gji/ggw301
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发表时间：
2016
影响因子：
2.8
通讯作者：
J.
中科院分区：
文献类型：
--
作者：
Nakada;M.;Okuno;J.
根据冰川均衡调整（GIA）过程中的观测资料推断地幔粘度，通常采用岩石圈厚度、上地幔粘度和下地幔粘度的简单三层粘度模型进行分析。本文研究了简单的三层粘度模型和两层下地幔粘度模型的粘度结构，该模型由粘度η670，D（670-D km深度）和ηD，2891（D-2891 km深度）定义，D值分别为1191，1691和2191 km。两层下地幔粘度模型的上地幔流变参数与简单三层下地幔粘度模型的上地幔流变参数相同。对于简单的三层粘滞模型，GIA过程引起的重力位二次纬向谐波的变化率（GIA诱导J今2）−（6.0-6.5）× 10− 11 yr − 1提供了下地幔的两种允许粘度解，即（7-20）× 1021和（5-9）× 1022 Pa·s，结合巴巴多斯湾和波拿巴湾J今2和末次冰期最高海平面的观测资料，分析表明下地幔的压力为（5-9）× 10 ~（22）Pa s。然而，基于两层下地幔粘度模型对J 2的分析只要求在核幔边界（CMB）以上的深度存在一个大于（5-10）× 1021 Pa·s的粘度层，其中（5-10）× 1021 Pa·s对应于简单三层模型的（7-20）× 1021 Pa·s解。此外，在J今2和LGM海平面约束下，两层下地幔粘度模型的分析表明，存在两个粘度解：η 670，1191> 3 × 1021和η 1191，2891 <$（5-10）× 1022 Pa·s; η 670，1691> 1022和η 1691，2891 <$（5-10）× 1022 Pa·s。由此推算的上地幔粘度为（1-4）× 10 ~（20）Pa·s，与简单三层粘度模型的估算值相近。这些分析表明，在地幔深部至少存在（5-10）× 1022 Pa·s的高粘度层，在讨论与670 km深度粘度跃变有关的地幔动力学时，应慎重对待基于GIA的下地幔粘度结构。我们还初步提出了额外的限制，这些粘度的解决方案，通过检查典型的相对海平面（RSL）的变化来推断下地幔粘度。由澳大利亚地区远场RSL变化所得到的粘度解与J今2和LGM海平面的粘度解是一致的，而对北美冰盖中部地区Southport和Bermuda的RSL变化的分析表明，η670，D> 1022，ηD，2891 × 10 ~（22）Pa·s（D= 1191或1691 km），上地幔粘度大于6 × 10 ~（20）Pa·s。
Inference of the mantle viscosity from observations for glacial isostatic adjustment (GIA) process has usually been conducted through the analyses based on the simple three-layer viscosity model characterized by lithospheric thickness, upper- and lower-mantle viscosities. Here, we examine the viscosity structures for the simple three-layer viscosity model and also for the two-layer lower-mantle viscosity model defined by viscosities ofη670,D(670-Dkm depth) andηD,2891(D-2891 km depth) withD-values of 1191, 1691 and 2191 km. The upper-mantle rheological parameters for the two-layer lower-mantle viscosity model are the same as those for the simple three-layer one. For the simple three-layer viscosity model, rate of change of degree-two zonal harmonics of geopotential due to GIA process (GIA-inducedJ̇2) of −(6.0–6.5) × 10−11yr−1provides two permissible viscosity solutions for the lower mantle, (7–20) × 1021and (5–9) × 1022Pa s, and the analyses with observational constraints of theJ̇2and Last Glacial Maximum (LGM) sea levels at Barbados and Bonaparte Gulf indicate (5–9) × 1022Pa s for the lower mantle. However, the analyses for theJ̇2based on the two-layer lower-mantle viscosity model only require a viscosity layer higher than (5–10) × 1021Pa s for a depth above the core–mantle boundary (CMB), in which the value of (5–10) × 1021Pa s corresponds to the solution of (7–20) × 1021Pa s for the simple three-layer one. Moreover, the analyses with theJ̇2and LGM sea level constraints for the two-layer lower-mantle viscosity model indicate two viscosity solutions:η670,1191> 3 × 1021andη1191,2891∼ (5–10) × 1022Pa s, andη670,1691> 1022andη1691,2891∼ (5–10) × 1022Pa s. The inferred upper-mantle viscosity for such solutions is (1–4) × 1020Pa s similar to the estimate for the simple three-layer viscosity model. That is, these analyses require a high viscosity layer of (5–10) × 1022Pa s at least in the deep mantle, and suggest that the GIA-based lower-mantle viscosity structure should be treated carefully in discussing the mantle dynamics related to the viscosity jump at ∼670 km depth. We also preliminarily put additional constraints on these viscosity solutions by examining typical relative sea level (RSL) changes used to infer the lower-mantle viscosity. The viscosity solution inferred from the far-field RSL changes in the Australian region is consistent with those for theJ̇2and LGM sea levels, and the analyses for RSL changes at Southport and Bermuda in the intermediate region for the North American ice sheets suggest the solution ofη670,D> 1022,ηD,2891∼ (5–10) × 1022Pa s (D= 1191 or 1691 km) and upper-mantle viscosity higher than 6 × 1020Pa s.