3-D structure of island arcs and continental growth

自太古宙晚期以来，洋弧对大陆边缘的吸积一直是大陆增长的关键因素，在加拿大的几个地区发现了岛弧的残留物，这往往是矿产勘探活动的焦点。然而，人们对弧形增长和大陆地壳变得稳定的机制知之甚少。如果岛弧含有大量的大陆式地壳，那么，如果更多镁铁质的下弧地壳能够通过某种形式的目前未知的拆沉过程被移除，那么通过直接弧吸积的大陆增长可能是可行的。或者，镁铁质弧壳需要同生或后吸积变质作用，才能产生今天存在的中间成分的陆壳。综合多道地震反射和三维折射测量是迄今在岛弧上获得的唯一真正的三维测量，沿着典型的马里亚纳岛弧延伸300多公里，是本提案的重点。一个目标是定量确定马里亚纳弧内长英质-中成分地壳的三维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%的变化显著地改变了岩石学对伴生的镁铁质重晶石和堆积体体积的预测，这些体积需要在弧-大陆碰撞之前或期间随后清除。第二个目标是利用高质量的多道地震数据识别与弧岩浆作用、下弧壳拆沉有关的弧下反射体，以及弧的结构演化，例如弧裂、伸展和下沉。波形层析成像还将用于绘制弧形较浅部分火山岩和侵入岩成分的可变性，并在比地质采样更大的范围内确定它们沿走向的可变性。在最大的规模上，该项目将限制拟议中的深弧形地壳每隔数百万年发生一次的下沉过程。

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.