Shear-driven upwrelling induced by lateral viscosity variations and asthenospheric shear: A mechanism for intraplate volcanism
Shear-driven upwrelling induced by lateral viscosity variations and asthenospheric shear: A mechanism for intraplate volcanism
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DOI：
10.1016/j.pepi.2009.10.001
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发表时间：
2010-02-01
影响因子：
2.3
通讯作者：
Tibbetts, Ashley
中科院分区：
文献类型：
--
作者：
Conrad, Clinton P.;Wu, Benjun;Tibbetts, Ashley
远离山脊和俯冲带发生的火山活动没有明显的板块构造解释，而必须由产生上涌流和减压熔融的岩石圈下过程引起。一些对流过程，例如地幔羽流、对流不稳定性、边缘驱动对流和里氏辊，通过重力对地幔密度不均匀性的作用产生上升流。在这里，我们研究了另一种机制，即剪切驱动上升流（SDU），它仅通过软流圈剪切流对粘度不均匀性的作用来产生上升流。使用数值流模型，我们研究了粘度不均匀性对软流圈层内引起的粘性剪切流的影响。我们证明，对于某些几何形状和粘度比，循环流在嵌入岩石圈基底的“空腔”或“台阶”内，或在嵌入软流圈层内的低粘度“口袋”内发展。对于 5 厘米/年的软流圈剪切，我们估计 SDU 可以在大陆裂谷内产生高达 0.2 厘米/年的上升流速率，类似于克拉通垂直边缘的 0.5 厘米/年，或者在低粘度软流圈“口袋”内产生类似于 1.0 厘米/年的上升流速率。在最后一种情况下，口袋的纵横比必须大于 5，占据软流圈厚度的 20-60%，并且粘度至少比周围软流圈低 100 倍。这种粘度异质性可能与热、化学、熔化、挥发或粒度异常有关，并且与软流圈变化的层析成像限制一致。我们估计 SDU 可能会产生高达 2.5 公里/马耳的熔体，这些熔体有可能以地表火山活动的形式喷发；这比在大陆玄武岩火山活动的某些地点观察到的喷发速度要快。我们的结论是，SDU 可以为快速剪切软流圈上方发生的板内火山活动提供解释，例如在北美盆地和山脉地区。 (C) 2009 Elsevier B.V. 保留所有权利。
Volcanism occurring away from ridges and subduction zones does not have an obvious plate tectonic explanation, but instead must arise from sub-lithospheric processes that generate upwelling flow and decompression melting. Several convective processes, such as mantle plumes, convective instability, edge-driven convection, and Richter rolls, produce upwelling via the action of gravity on density heterogeneities in the mantle. Here we investigate an alternative mechanism, the shear-driven upwelling (SDU), which instead generates upwelling solely through the action of asthenospheric shear flow on viscosity heterogeneity. Using a numerical flow model, we examine the effect of viscosity heterogeneity on viscous shear flow induced within an asthenospheric layer. We demonstrate that for certain geometries and viscosity ratios, circulatory flow develops within a "cavity" or "step" embedded into the lithospheric base, or within a low-viscosity "pocket" embedded within the asthenospheric layer. For asthenosphere shearing at 5 cm/yr, we estimate that SDU can produce upwelling rates of up to similar to 0.2 cm/yr within a continental rift, similar to 0.5 cm/yr along the vertical edge of a craton, or similar to 1.0 cm/yr within a "pocket" of low-viscosity asthenosphere. In the last case, the pocket must feature an aspect ratio of more than 5, occupy similar to 20-60% of the asthenosphere's thickness, and be at least 100 times less viscous than the surrounding asthenosphere. Such viscosity heterogeneity may be associated with thermal, chemical, melting, volatile, or grain-size anomalies, and is consistent with tomographic constraints on asthenospheric variability. We estimate that SDU may generate up to 2.5 km/Myr of melt that is potentially eruptible as surface volcanism; this is faster than eruption rates observed at some locations of continental basaltic volcanism. We conclude that SDU could provide an explanation for intraplate volcanism occurring above rapidly shearing asthenosphere, for example in the Basin and Range region of North America. (C) 2009 Elsevier B.V. All rights reserved.