Deep seismic imaging of developing continental crust in oceanic island arcs

大洋岛弧发育大陆地壳的深部地震成像

• 批准号: 155519-2013
• 负责人: Calvert, Andrew
• 金额: $ 2.91万
• 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: 2014
• 资助国家: 加拿大
• 起止时间: 2014-01-01 至 2015-12-31
• 项目状态: 已结题
• 来源: https://www.nserc-crsng.gc.ca/ase-oro/Details-Detailles_eng.asp?id=545703
• 关键词: Deep; seismic; imaging; developing; continental

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.

火山岛弧，如今天的阿留申群岛，马里亚纳群岛和所罗门岛链在太平洋形成的海洋构造板块俯冲到地幔下的另一块海洋岩石圈。古岛弧被认为是板块构造在过去~ 3Ga中组装形成大陆的基本组成部分。然而，相对浮力的大陆地壳是如何从形成岛弧地壳的致密岩石中衍生出来的，这一过程尚不清楚。当大陆状安山质岩石生成于岛弧的中地壳时，大量致密的镁铁质-超镁铁质岩石也应该生成于下伏的根部。地震折射测量估计了地壳组成的代用指标，即地震波速度，表明弧形地壳太薄（12-35公里），不可能包含这个根。它的根部可能比最上层的地幔更致密，从演化中的岛弧底部分离，并沉入下面的地幔，但这种假设的下沉时间尺度是未知的。地震反射勘测最近发现了阿留申岛弧下延伸至50公里深处的两个岛根的证据，如果它们仍然存在的话，也许能够确定其他“失踪的”密集岛弧根。本研究建议的目的是利用以前记录的深地震反射剖面在不同年龄的岛弧，以确定弧根的分布和限制其沉没的时间尺度。地壳生产率是最高的开始期间的岛弧，这些最早的弧岩现在被发现在外弧前接近深海海沟。整个前弧的地震反射成像将有助于揭示岛弧中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。由于下降的海洋板块的反射也被记录下来，这项工作的一个有价值的额外的后果将是改进的估计的宽度的孕震区的四个俯冲大逆冲断层的特征反射响应和/或交叉点的变化与最上层地幔稳定滑动发生。