马达加斯加新生代动态地形,Geochemistry, Geophysics, Geosystems

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马达加斯加新生代动态地形
Geochemistry, Geophysics, Geosystems ( IF 2.9 ) Pub Date : 2021-04-14 , DOI: 10.1029/2020gc009624
S. N. Stephenson _{1,

2} , N. J. White ₁ , A. Carter _{3,

4} , D. Seward _{5,

6} , P. W. Ball _{1,

7} , M. Klöcking _{1,

7}

Affiliation

有人提出马达加斯加的渐新世区域隆起是由地幔动力过程产生并维持的。区域隆起的表现包括平坦的上白垩纪-古近纪海相石灰岩，这些石灰岩在西海岸数百米的高处出现，以及海岸线边缘的第四纪富含珊瑚的阶地。在这里，我们通过对四组观测约束的组合分析来探索地壳下地形支持的历史。首先，我们利用已发表的地壳厚度和重力与地形之间的光谱导纳的接收函数估计。～+40 ± 10 mGal km ^-1的导纳值波长 >500 公里意味着约 1 公里的地形受到地壳下过程的支持。其次，对来自 18 个地下室样品的新磷灰石裂变径迹和氦气测量结果进行了反演，从而限制了温度和剥蚀历史。结果表明，大约 30 Ma 之后发生了 0.5-1.6 公里的区域隆升。第三，我们通过最小化观察到的和计算的纵向河流剖面之间的失配来计算区域抬升的历史。结果表明地形是在新近纪时期产生的。最后，新近纪基性岩中稀土元素浓度的反演模型表明软流圈源的熔化发生在≤65 km的深度，潜在温度为1300-1370°C。融化发生在留尼汪岛和马达加斯加北部的较高温度下，以及科摩罗和马达加斯加南部的较低温度下。这些推论与从断层扫描模型获得的剪切波速度一致。我们得出结论，马达加斯加的下方是变薄的岩石圈地幔，并且在马达加斯加北部下方的软流圈层内存在热异常。

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Cenozoic Dynamic Topography of Madagascar

It has been proposed that Oligo-Miocene regional uplift of Madagascar was generated and is maintained by mantle dynamical processes. Expressions of regional uplift include flat-lying Upper Cretaceous-Paleogene marine limestones that crop out at elevations of hundreds of meters along the western seaboard and emergent Quaternary coral-rich terraces that rim the coastline. Here, we explore the history of subcrustal topographic support through a combined analysis of four sets of observational constraints. First, we exploit published receiver function estimates of crustal thickness and spectral admittance between gravity and topography. An admittance value of ∼+40 ± 10 mGal km⁻¹ at wavelengths >500 km implies that ∼1 km of topography is supported by subcrustal processes. Secondly, new apatite fission-track and helium measurements from 18 basement samples are inverted, constraining temperature and denudation histories. Results suggest that 0.5–1.6 km of regional uplift occurred after ∼30 Ma. Thirdly, we calculate a history of regional uplift by minimizing the misfit between observed and calculated longitudinal river profiles. Results suggest that topography was generated during Neogene times. Finally, inverse modeling of rare earth element concentrations in Neogene mafic rocks indicates that melting of the asthenospheric source occurred at depths of ≤65 km with potential temperatures of 1300–1370 °C. Melting occurred at higher temperatures beneath Réunion Island and northern Madagascar and at lower temperatures beneath the Comores and southern Madagascar. These inferences are consistent with shear wave velocities obtained from tomographic models. We conclude that Madagascar is underlain by thinned lithospheric mantle and that a thermal anomaly lies within an asthenospheric layer beneath northern Madagascar.

更新日期：2021-06-23

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