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，如今天太平洋中的阿留申群岛、马里亚纳岛和所罗门岛链，形成于海洋构造板块俯冲到另一块大洋岩石圈下方的地幔中。古老的岛弧被认为是板块构造在过去3~3a内拼凑而成的大陆的基本构造块。然而，相对浮力较大的大陆地壳是从形成许多岛弧地壳的致密岩石中衍生出来的，这一过程尚不清楚。当岛弧的中地壳产生陆相安山岩时，下伏的根部也应该产生更大体积的致密镁铁质-超镁铁质岩石。地震折射测量估计了地壳成分的一个替代指标，即地震波速，表明弧形地壳太薄(12-35公里)，无法容纳这个根部。根部可以比最上面的地幔密度更大，从不断演变的岛弧的底部分离出来，并沉入下面的地幔，但这种假想的下沉的时间尺度是未知的。地震反射调查最近发现了阿留申弧下50公里深的两个弧根的证据，如果它们仍然存在的话，也许能够识别出其他“失踪的”密集弧根。这项研究计划的目的是利用先前记录的不同年龄岛弧上的深地震反射剖面来确定弧根的分布并限制其形成的时间尺度。在岛弧形成期间，地壳生产率最高，这些最早的弧岩现在发现于靠近深水深海沟的外弧前。横跨弧前的地震反射成像将有助于揭开岛弧大规模地壳产生的早期历史，以及随后向更稳定的地壳产生制度过渡的过程。由于还记录了来自大洋板块下降的反射，这项工作的另一个有价值的结果将是改进对四个俯冲巨型逆冲断裂的孕震带宽度的估计，这些估计来自于它们的特征反射响应的变化和/或与发生稳定滑动的最上地幔的交汇处。