Deflection of dykes into sills at discontinuities and magma-chamber formation
Deflection of dykes into sills at discontinuities and magma-chamber formation
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DOI：
10.1016/j.tecto.2009.10.015
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发表时间：
2011-03
期刊：
影响因子：
2.9
通讯作者：
Agust Gudmundsson
中科院分区：
文献类型：
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作者：
Agust Gudmundsson
许多裂谷带含有复合火山（中心火山），其中大部分火山的岩浆来自地壳浅层岩浆房。对侵蚀裂谷带的火山构造研究，如冰岛的火山构造研究，以及地球物理研究表明，许多浅层岩浆房是从岩台演化而来的。在这里，我提供了冰岛第四纪和第三纪古裂带的岩浆岩和灭绝岩浆房的野外描述，重点介绍了冰岛东南部Slaufrudalur的长英质岩体，作为通过岩浆岩注入形成岩浆房的一个很好的例子。在本文中，我回顾、分析并进一步发展了两种相关的岩脉偏转成岩脉的机制，从而潜在地形成岩浆房，即：(a)库克-戈登脱壳（分层）和(b)应力障碍，并提出了第三种机制，(c)由于弹性失配（接触层的杨氏模量或刚度的差异）而产生的有利的材料-韧性比。在库克-戈登机制中，由于岩脉引起的拉应力，一个弱接触打开。正如实地观测和数值模式所支持的那样，这种机制可能主要在浅层起作用。应力屏障是一层，在该层中，局部应力不利于某种特定类型的岩石断裂，这里指的是岩堤。现场观测和数值模拟结果表明，遇到应力障壁时，堤防要么变成岩基，要么被拦阻。材料-韧性机制表明，当接触处的上层与下层（承载岩脉）具有相同或更小的刚度时，岩脉向接触处偏转的倾向很小。然而，当上层较硬时，例如当坚硬的玄武岩熔岩流位于柔软的火山碎屑层的顶部时，岩脉往往会偏转到接触层中形成一个台阶。现场结果表明，所有这些机制都可能导致岩脉偏转成岩脊，并可能共同起作用，特别是在岩石由软硬交替层组成的地方。由于冰岛的第四纪熔岩堆比第三纪熔岩堆有更多的软质（透明碎屑岩）层，这些机制可能非常有效地在第四纪期间产生了岩浆岩和浅层岩浆房。这可能是为什么第四纪熔岩堆与第三纪熔岩堆相比，包含了如此多的岩浆岩、灭绝的（深岩浆）和活跃的浅层岩浆房的原因之一。
Many rift zones contain composite volcanoes (central volcanoes), most of which are supplied with magma from shallow crustal magma chambers. Volcanotectonic studies of eroded rift zones, such as in Iceland, as well as geophysical studies indicate that many shallow magma chambers evolve from sills. Here I provide field description of sills and extinct magma chambers in the Quaternary and Tertiary palaeorift zones of Iceland, focusing on the felsic pluton of Slaufrudalur, Southeast Iceland, as a fine example of a magma chamber developed through the injection of sills. In this paper I review, analyse, and develop further two related mechanisms for the deflection of dykes into sills, and thus the potential initiation of a magma chamber, namely: (a) Cook–Gordon debounding (delamination), and (b) stress barriers, and propose a third mechanism, (c) favourable material-toughness ratios due to elastic mismatch (difference in Young's moduli or stiffnesses of layers in contact). In the Cook–Gordon mechanism, a weak contact opens up as a result of dyke-induced tensile stress. This mechanism is likely to operate primarily at shallow depths, as is supported by field observations and numerical models. A stress barrier is a layer where the local stress is unfavourable to a particular type of rock fracture, here a dyke. Field observations and numerical models show that on meeting a stress barrier, a dyke either changes into a sill or becomes arrested. The material-toughness mechanism indicates that when the upper layer at a contact has the same or less stiffness than the lower layer (hosting the dyke), there is little tendency for the dyke to become deflected into the contact. However, when the upper layer is stiffer, such as when a stiff basaltic lava flow is on the top of a soft pyroclastic layer, the dyke tends to become deflected into the contact to form a sill. Field results suggest that all these mechanisms may cause dyke deflection into sills and may operate together, particularly where the rock consists of alternating soft and stiff layers. Since the Quaternary lava pile in Iceland has many more soft (hyaloclastite) layers than the Tertiary lava pile, these mechanisms were probably very efficient in generating sills and, by implication, shallow magma chambers, during the Quaternary. This may be one reason why, in comparison with the Tertiary lava pile, the Quaternary lava pile contains so many sills and extinct (plutons) and active shallow magma chambers.