3-D structure of island arcs and continental growth

岛弧和大陆生长的 3D 结构

• 批准号: 155519-2008
• 负责人: Calvert, Andrew
• 金额: $ 2.84万
• 依托单位: Simon Fraser University
• 依托单位国家: 加拿大
• 项目类别: Discovery Grants Program - Individual
• 财政年份: 2012
• 资助国家: 加拿大
• 起止时间: 2012-01-01 至 2013-12-31
• 项目状态: 已结题
• 来源: https://www.nserc-crsng.gc.ca/ase-oro/Details-Detailles_eng.asp?id=496965
• 关键词: structure; island; arcs; continental; growth

The accretion of oceanic arcs to continental margins has been a key contributor to continental growth since the Late Archean, and the remnants of island arcs, which are often the focus of mineral exploration activity, have been identified in several Canadian regions. However, the mechanisms by which arcs are accreted and the continental crust becomes stabilised are poorly understood. If island arcs contain significant volumes of continental-style crust, then continental growth through direct arc accretion may be feasible if the more mafic lower arc crust can be removed by some form of presently unknown delamination process. Alternatively, mafic arc crust requires syn- or post-accretion modification to produce the intermediate composition continental crust that exists today. An integrated multichannel seismic reflection and 3-D refraction survey, the only true 3-D survey yet acquired over an island arc, extends more than 300 km along the archetypical Mariana island arc, and is the focus of this proposal. One objective is to determine quantitatively the 3-D distribution of felsic-intermediate composition crust within the Mariana arc, and its relation to the underlying mafic lower crust. Variations of 5-10 percent in the volume of intermediate composition arc crust significantly alter petrologic predictions of the volumes of associated mafic restite and cumulates that require subsequent removal before or during arc-continent collision. A second objective is the identification using the high-quality multichannel seismic data of reflectors beneath the arc associated with arc magmatism, delamination of the lower arc crust, and the arc's structural evolution, e.g. arc rifting, extension, and subsidence. Waveform tomography will also be used to map the variability in composition of both volcanic rocks and intrusions in the shallower sections of the arc, and to determine their along-strike variability on a broader scale than is possible by geological sampling. At the largest scale, this project will constrain the process by which the proposed foundering of deep arc crust occurs every few million years.

自晚太古代以来，大洋弧向大陆边缘的增生一直是大陆生长的关键因素，而岛弧的残余物通常是矿产勘探活动的焦点，已在加拿大的几个地区被发现。然而，人们对弧增生和大陆地壳稳定的机制知之甚少。如果岛弧含有大量的大陆型地壳，那么如果可以通过某种形式的目前未知的分层过程去除更多镁铁质的下弧地壳，那么通过直接弧增生的大陆生长可能是可行的。或者，镁铁质弧地壳需要同时或后吸积改造才能产生当今存在的中间成分大陆地壳。集成的多道地震反射和 3D 折射勘测是迄今为止在岛弧上进行的唯一真正的 3D 勘测，沿着典型的马里亚纳岛弧延伸超过 300 公里，是本提案的重点。目标之一是定量确定马里亚纳弧内长英质中质成分地壳的 3-D 分布及其与下面的镁铁质下地壳的关系。中间成分弧地壳体积的 5-10% 的变化会显着改变对相关镁铁质复位岩体积的岩石学预测，并需要在弧大陆碰撞之前或期间进行后续去除。第二个目标是利用弧下方反射体的高质量多道地震数据进行识别，这些数据与弧岩浆作用、弧下地壳分层以及弧的结构演化有关，例如弧形构造演化。弧裂、延伸和沉降。波形断层扫描还将用于绘制弧形较浅部分火山岩和侵入岩成分的变化图，并在比地质采样更广泛的范围内确定其沿走向的变化。在最大范围内，该项目将限制每隔几百万年发生的深弧地壳沉没的过程。