Variations in melt emplacement beneath the northern East African Rift from radial anisotropy
Variations in melt emplacement beneath the northern East African Rift from radial anisotropy
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DOI：
10.1016/j.epsl.2021.117150
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发表时间：
2021-09-01
影响因子：
5.3
通讯作者：
Keir, Derek
中科院分区：
文献类型：
--
作者：
Chambers, Emma L.;Harmon, Nicholas;Keir, Derek
熔体在地壳和上地幔中的储存位置和方式对于理解岩浆管道系统的动力学和裂谷的演化是重要的。我们确定剪切速度和径向各向异性在岩浆裂谷北方东非裂谷，以确定熔体的轨迹和方向，无论是在裂谷和关闭。从9- 26 s周期的环境噪声数据中提取Love和Rayleigh基模，然后反演剪切速度。在5-30 km深度，VSV比VSH平均低0.15 ± 0.03 km/s。研究区大部分区域的VSH> VSV表明地壳本身是水平分层的，在上部5-15 km处各向异性最大。有效介质理论表明，长英质和镁铁质侵入体的薄成分分层可以解释高达4%的各向异性。然而，为了调和观察到的最大各向异性（6.5%）和最低速度，我们需要2-4%的部分熔体定向在窗台。沿着裂谷，水平排列的径向各向异性变得较弱的东北，这表明门槛成为不占主导地位的进步裂谷。二塔阿勒岩浆岩段是唯一VSV>VSH的部位，表明地壳中存在垂直微裂隙和脉岩。总体而言，结果表明，在早期大陆裂解时，裂谷是狭窄的，岩床形成是主要的存储机制。当裂谷变宽时，垂直岩墙侵入成为主导，并可能受地壳厚度和应力状态变化的控制。(C)2021爱思唯尔有限公司版权所有。
Where and how melt is stored in the crust and uppermost mantle is important for understanding the dynamics of magmatic plumbing systems and the evolution of rifting. We determine shear velocity and radial anisotropy in the magmatically rifting northern East African Rift to determine the locus and orientation of melt, both on and off-rift. Love and Rayleigh fundamental modes are extracted from ambient noise data from 9-26s period and then inverted for shear velocity. VSVis 0.15 +/- 0.03 km/s lower than VSHfrom 5-30 km depth on average. VSH>VSVacross most of the study region suggests the crust is inherently horizontally layered, with the largest anisotropy in the upper 5-15 km. Effective medium theory suggests thin compositional layering of felsic and mafic intrusions can account for anisotropy up to 4%. However, to reconcile the largest observed anisotropy (6.5%), and lowest velocities, we require 2-4% partial melt oriented in sills. Along the rift, horizontally aligned radial anisotropy gets weaker north-eastwards, suggesting sills become less dominant with progressive rifting. The Erta Ale magmatic segment is the only location where VSV>VSH, suggesting the crust contains vertical micro-cracks and dykes. Overall, the results suggest during early continental breakup when the rift is narrow, sill formation is the dominant storage mechanism. As a rift widens, vertical dyke intrusion becomes dominant and is likely controlled by variations in crustal thickness and stress state. (C) 2021 Elsevier B.V. All rights reserved.