3-D structure of island arcs and continental growth

自太古宙晚期以来，海洋弧向大陆边缘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已经发现了岛弧的残余，它们往往是矿物勘探活动的重点。然而，人们对弧被吸积和大陆地壳稳定的机制知之甚少。如果岛弧包含大量的大陆型地壳，那么如果更基性的下弧地壳可以通过某种形式的目前未知的剥离过程被移除，那么通过直接弧增生进行大陆生长可能是可行的。另一种选择是，基性弧壳需要在增生前后发生改变，才能形成今天存在的中间成分大陆壳。综合多通道地震反射和3-D折射测量是迄今为止在岛弧上获得的唯一真正的3-D测量，沿着典型的马里亚纳岛弧延伸超过300公里，是该提案的重点。其中一个目的是定量确定马里亚纳弧内长质-中间成分地壳的三维分布及其与下伏基性下地壳的关系。中间成分弧壳体积的5- 10%的变化显著地改变了相关基性阻岩和堆积体积的岩石学预测，这些岩石学预测需要在弧-大陆碰撞之前或碰撞期间随后移除。第二个目标是利用高质量的多通道地震数据来识别与弧岩浆作用、弧下地壳剥离以及弧的构造演化（如弧裂、弧伸展和弧沉降）相关的弧下反射体。波形层析成像还将用于绘制弧较浅部分火山岩和侵入物组成的变异性，并在比地质采样可能的更大范围内确定它们的沿走向变异性。在最大规模上，该项目将限制所提出的深弧地壳每几百万年发生一次的沉降过程。

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.