Formation and evolution of upper oceanic crust from seismic data acquired over mature oceanic crust near the Sumatra and Alaska subduction zones

在地壳形成的洋中脊轴附近，上洋地壳及其物理性质是众所周知的。然而，由于缺乏较老地壳的适当数据集，人们对上层海洋地壳的性质如何随着地壳年龄的变化而变化知之甚少。还有一个重要的问题没有得到解答，那就是当地壳从山脊轴向外扩散时，热液循环是否会继续。热流测量表明，热液环流仍然存在，但不可能从现有的数据集阐明对这种后期环流的影响和控制。该项目将利用最近在苏门答腊和阿拉斯加俯冲带近海的成熟海洋地壳（50 Myr）上收集的海洋地震数据来解决这些问题。这两个数据集的质量非常高，而且非常独特，因为它们可以对海洋地壳的上部进行成像。了解古地壳中的热液循环是很重要的，因为我们不知道在这些占地球表面约70%的广大地区，地球和海洋之间发生的化学交换的程度。这项研究支持博士研究生的培养。该项目还涉及与法国同事的国际合作。该项目将检查在苏门答腊和阿拉斯加俯冲带近海成熟海洋地壳（50 Myr）上获得的二维海洋地震数据。这些数据是在潮汐计划（IPG-Paris）期间于2011年在R/V Marcus Langseth和2009年在M/V gewave Champion上获得的。本研究的主要目的是对上地壳的地震非均质性和控制上洋地壳老化的主导过程提供新的认识。长拖缆MCS数据将提供上地壳的地震成像，包括在大偏移距处看到的临界后反射。MCS成像还可以表征沉积物厚度的变化，上地壳的地形，以及沉积物和上地壳内部的断层丰度。对拖缆数据进行向下延拓，然后进行走时建模，将得到构成上地壳的两层的厚度和纵波速度。综合计算将产生关于横波速度和衰减的进一步信息。这两个数据集将允许比较在不同扩张速率下形成的海洋地壳部分和年龄略有不同的部分。

The upper oceanic crust and its physical properties are well known near the mid-ocean ridge axis where the crust is formed. However, little is known about how the properties of the upper oceanic crust change as the crust ages, primarily because of the lack of the appropriate datasets over older crust. An important question that remains unanswered is whether or not hydrothermal circulation continues as the crust spreads away from the ridge axis. Heat flow measurements suggest that hydrothermal circulation persists, but it has been impossible to clarify from existing datasets the effects and the controls on this late stage circulation. This project will use recently collected marine seismic data over mature oceanic crust (50 Myr) offshore the Sumatra and Alaska subduction zones to address these problems. These two datasets are very high quality and unique in that they allow imaging of the upper part of the oceanic crust. Understanding hydrothermal circulation in older crust is important because we do not know the extent of the chemical exchanges that occur between the earth and oceans in these vast regions that represent about 70% of the surface of the earth. The study supports the training of a Ph.D. graduate student. The project also involves an international collaboration with French colleagues.This project will examine 2-D marine seismic data acquired over mature oceanic crust (50 Myr) offshore the Sumatra and Alaska subduction zones. These data were acquired in 2011 on the R/V Marcus Langseth and in 2009 on the M/V Geowave Champion during the TIDES Program (IPG-Paris). The main objective of this study is to provide new insights on the seismic heterogeneity of the uppermost crust and the dominant processes that control the aging of upper oceanic crust. The long-streamer MCS data will provide seismic imaging of the upper crust including post-critical reflections seen at large offsets. The MCS imaging will also allow the characterization of variations in the sediment thickness, topography of the upper crust, and abundance of faulting within the sediments and upper crust. The downward continuation followed by travel time modeling of the streamer data will provide the thickness and P-wave velocity of the two layers that constitute the upper crust. Synthetic computation will generate further information on S-wave velocities and attenuation. Together the two datasets will allow the comparison between sections of oceanic crust formed at different spreading rates and of slightly different ages.