从白垩纪以来的全球地幔对流模型看当今地幔结构

IF 2.8 3区 地球科学 Q2 GEOCHEMISTRY & GEOPHYSICS
Debanjan Pal, Attreyee Ghosh
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引用次数: 0

摘要

摘要 利用从140Ma开始的前向地幔对流模型,并同化板块重建作为地表速度边界条件,我们预测了现今的地幔结构,并将它们与层析成像模型进行了比较,将大地水准面作为额外的约束条件。我们探索了广阔的模型参数空间,如克拉皮隆斜率和660千米范围内密度变化的不同值、地核-地幔边界(CMB)热化学堆积的密度和粘度、内部发热率和模型启动年龄。我们还研究了660千米以下不同强度的软弱层以及较弱的岩石圈和板块的影响。我们的结果表明,不同俯冲带的板块结构对星体层的粘度、板块强度、克拉皮隆斜率值以及热化学堆积物的密度和粘度都很敏感,而不同的内部发热率并不影响板块结构。我们发现,在中等强度的岩石圈(1020帕斯卡)和强板块条件下,预测的板块结构与层析成像模型一致,观测到的大地水准面也匹配良好。此外,我们的模型成功地再现了非洲和太平洋下地幔的二级结构,也称为大低剪切速度区(LLSVPs)。660 公里处-2.5 MPa/K 的中等克拉皮隆斜率有助于板坯停滞,而更高的斜率值则会导致板坯在该深度大量堆积,最终导致板坯雪崩。我们还发现,在LLSVP密度稍高的热效应和热化学效应情况下,对流模式是相似的,尽管后者的大地水准面振幅较低。然而,在 LLSVP 密度较高的情况下,板块无法对其产生扰动,也就不会产生羽流。当冷的粘性板块对 LLSVPs 边缘产生扰动时,羽流就会在 LLSVPs 边缘产生热不稳定性。虽然我们预测的羽流位置与观测到的热点位置一致,但在层析成像模型中很难与羽流结构相匹配。这些羽流对于拟合观测到的大地水准面的精细特征至关重要。在持续时间较长的模型中,会有更多的大量俯冲物质到达 CMB,这往往会极大地侵蚀 LLSVPs,导致与观测大地水准面的拟合效果不佳。我们的结果表明,如果在 660 km 以下存在一个薄而中等强度的层,LLSVP 稍微致密,克拉皮隆斜率为-2.5 MPa/K,那么地震层析成像中的速度异常和长波大地水准面就可以很好地匹配。我们模型的局限性之一是同化的板块运动历史可能太短,无法克服任意初始条件的影响。此外,我们模型中的同化真实板块速度可能并不代表地球的真实对流强度。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
Present day mantle structure from global mantle convection models since the Cretaceous
Summary Using forward mantle convection models starting at 140 Ma, and assimilating plate reconstructions as surface velocity boundary condition, we predict present-day mantle structure and compare them with tomography models, using geoid as an additional constraint. We explore a wide model parameter space, such as different values of Clapeyron slope and density change across 660 km, density and viscosity of the thermochemical piles at the core-mantle boundary (CMB), internal heat generation rate, and model initiation age. We also investigate the effects of different strengths of a weak layer below 660 km and weaker asthenosphere and slabs. Our results suggest that slab structures at different subduction zones are sensitive to the viscosity of the asthenosphere, strength of slabs, values of Clapeyron slope and the density and viscosity of the thermochemical piles, while different internal heat generation rates do not affect the slab structures. We find that with a moderately weak asthenosphere (1020 Pas) and strong slabs, the predicted slab structures are consistent with the tomography models, and the observed geoid is also matched well. Moreover, our models successfully reproduce the degree-2 structure of the lower mantle beneath Africa and the Pacific, also known as Large Low Shear Velocity provinces (LLSVPs). A moderate Clapeyron slope of -2.5 MPa/K at 660 km aids in slab stagnation while higher values result in massive slab accumulation at that depth, ultimately leading to slab avalanches. We also find that the convective patterns in the thermal and thermochemical cases with slightly denser LLSVPs are similar, although the geoid amplitudes are lower for the latter. However, with more dense LLSVPs, the slabs cannot perturb them and no plumes are generated. Plumes arise as thermal instabilities from the edges of the LLSVPs, when cold and viscous slabs perturb them. While our predicted plume locations are consistent with the observed hotspot locations, matching the plume structures in tomography models is difficult. These plumes are essential in fitting the finer features of the observed geoid. In longer-duration models, more voluminous subducted material reaches the CMB, which tends to erode the LLSVPs significantly, and yields a poor fit to the observed geoid. Our results suggest that with the presence of a thin, moderately weak layer below 660 km, a slightly dense LLSVP, and Clapeyron slope of -2.5 MPa/K, the velocity anomalies in seismic tomography and the long-wavelength geoid can be matched well. One of the limitations of our models is that the assimilated plate motion history may be too short to overcome arbitrary initial conditions effects. Also, assimilated true plate velocities in our models may not represent the true convective vigor of the Earth.
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来源期刊
Geophysical Journal International
Geophysical Journal International 地学-地球化学与地球物理
CiteScore
5.40
自引率
10.70%
发文量
436
审稿时长
3.3 months
期刊介绍: Geophysical Journal International publishes top quality research papers, express letters, invited review papers and book reviews on all aspects of theoretical, computational, applied and observational geophysics.
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