Deep seismic imaging of developing continental crust in oceanic island arcs

火山岛弧，如今天太平洋上的阿留申、马里亚纳和所罗门岛链，是在海洋构造板块俯冲到另一块海洋岩石圈下的地幔中形成的。古岛弧被认为是板块构造的基本组成部分，在过去的3亿年里形成了大陆。然而，相对浮力较强的大陆地壳是如何从密度较大的岩石（这些岩石形成了许多岛弧地壳）中衍生出来的，这一过程尚不清楚。当类大陆安山岩在岛弧的中地壳中形成时，下伏的根中也会形成体积更大、密度更大的基性-超基性岩石。地震折射测量，估计地壳组成的代表，即地震波速度，表明弧壳太薄（12-35公里），无法包含这个根。根可能比最上层的地幔密度更大，从不断演变的岛弧的底部分离出来，并下沉到下面的地幔中，但这种假设的沉没的时间尺度是未知的。最近的地震反射调查发现，在阿留申岛弧下方有两个延伸至50公里深处的根，如果它们仍然存在，可能能够识别其他“失踪”的致密弧根。本研究计划的目的是利用先前记录的不同年龄岛弧的深地震反射剖面来确定弧根的分布并限制其沉没的时间尺度。岛弧形成时地壳生成速率最高，这些最早的岛弧岩现在发现于靠近深海沟的外弧前。穿越前弧的地震反射成像将有助于揭示岛弧大量地壳生成的早期历史，以及随后向更稳定的地壳生成机制的过渡。由于海洋板块下降的反射也被记录下来，这项工作的一个有价值的附加结果将是改进对四个俯冲大逆冲断层的孕震带宽度的估计，这些断层的特征反射响应的变化和/或与发生稳定滑动的最上层地幔的相交。

Volcanic island arcs such as the present day Aleutian, Mariana and Solomon island chains in the Pacific Ocean form where an oceanic tectonic plate subducts into the mantle beneath another piece of oceanic lithosphere. Ancient island arcs are believed to have been the fundamental building blocks that were assembled by plate tectonics to create the continents over the last ~3 Ga. However, the process through which the relatively buoyant continental crust is derived from the denser rocks that form much island arc crust is unclear. When continental-like andesitic rocks are generated in the middle crust of an island arc, a larger volume of denser mafic-ultramafic rocks should also be produced in an underlying root. Seismic refraction surveys, which estimate a proxy for crustal composition, namely the seismic wave speed, indicate that arc crust is too thin (12-35 km) to contain this root. The root can be denser than the uppermost mantle, detach from the base of the evolving island arc, and sink into the underlying mantle, but the time-scale of this hypothesised foundering is unknown. Seismic reflection surveys have recently found evidence of two roots extending to 50 km depth beneath the Aleutian arc, and may be able to identify other "missing" dense arc roots if they are still present. The objective of this research proposal is to utilize previously recorded deep seismic reflection profiles over island arcs of different ages to determine the distribution of arc roots and to constrain the time-scale of their foundering. Crustal production rates are highest during initiation of an island arc, and these earliest arc rocks are now found in the outer forearc close to the deep bathymetric trench. Seismic reflection imaging across the forearc will help to unravel the early history of voluminous crustal production in island arcs and the subsequent transition to a more stable regime of crustal generation. Since reflections from the descending oceanic plate are also recorded, a valuable additional consequence of this work will be improved estimates of the width of the seismogenic zone of four subduction megathrust faults derived from changes in their characteristic reflection responses and/or intersections with the uppermost mantle where stable sliding occurs.