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: 2011
• 资助国家: 加拿大
• 起止时间: 2011-01-01 至 2012-12-31
• 项目状态: 已结题
• 来源: https://www.nserc-crsng.gc.ca/ase-oro/Details-Detailles_eng.asp?id=476076
• 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，洋弧向大陆边的增生一直是大陆增长的一个关键因素，在加拿大的几个地区发现了岛弧的残余，这些残余往往是矿产勘探活动的重点。然而，人们对岛弧增生和大陆地壳稳定的机制知之甚少。如果岛弧中含有大量的大陆型地壳，那么如果更多的基性岩下弧地壳可以通过某种形式的目前未知的脱层过程被移除，那么通过直接弧增生的大陆增长可能是可行的。另外，镁铁质弧地壳需要同增或后增的修改，以产生目前存在的中间成分大陆地壳。综合多道地震反射和三维折射勘探是迄今为止在岛弧上进行的唯一真正的三维勘探，沿典型的马里亚纳岛弧延伸沿着300多公里，是本建议的重点。一个目标是定量地确定马里亚纳弧内长英质-中间成分地壳的3-D分布，及其与下伏镁铁质下地壳的关系。中间成分弧壳体积的5- 10%的变化会显著改变相关基性残积岩和堆积岩体积的岩石学预测，这些残积岩和堆积岩需要在弧-陆碰撞之前或期间随后移除。第二个目标是利用高质量的多道地震数据识别与弧岩浆活动、弧下地壳分层和弧的构造演化（如弧裂陷、伸展和沉降）有关的弧下反射体。波形层析成像还将用于绘制火山岩和侵入体在弧的较浅部分的成分的变化，并在比地质取样更广泛的范围内确定其沿走向的变化。在最大的规模上，这个项目将限制拟议的深弧地壳每几百万年发生一次的沉降过程。