Alaska through time: reconstructing plate tectonics by linking the deep mantle to the shallow surface

该项目旨在通过创建一个新的地震层析成像模型，并将其与地表地质和海洋地球物理学的约束条件进行合成，重建阿拉斯加的构造历史和古地理。这是由阿拉斯加的“USAray”记录的大量新地震数据和新颖的分析方法（如“断层构造分析”[1]）实现的，用于将俯冲的地表记录与其深层地下记录联系起来。回溯时间来解开构造板块的古地理起源主要是通过从海底和大陆恢复的古地磁数据来了解的[2]。然而，这些重建的不确定性随着时间的推移而增加，因为许多更古老的海洋板块已经俯冲到地幔中。这些板块可能无法通过地表采样技术（如钻孔或疏浚）获得，但它们并没有丢失;当它们慢慢沉入地幔内部时，它们仍然可以通过地球物理成像方法（如地震层析成像）看到。这种俯冲岩石圈（“板块”）的高分辨率图像可以与基岩地质数据集相结合，以了解特定地区的板块构造演化，特别是在古海洋地区[1]。阿拉斯加就是这样一个地区，其起源是一个谜，在研究中经常被忽视[4]。阿拉斯加的基岩地质由增生的火山岩组成，其中许多火山岩一定是在俯冲带上方形成的线性延伸的岛弧，就像今天西太平洋的伊豆-小笠原-马里亚纳弧。古地磁数据重建这些原始阿拉斯加岛屿的位置比现在的位置更靠南[5]。它们是如何在85-50 Ma之间增生到北美并旋转成今天阿拉斯加中部山脉的巨大“Z”形几何形状的尚不清楚[6，7]（见图1a）。最近的地震研究表明，曾经承载这些岛屿的板块仍然悬浮在地幔中[1，7]（见图1b）。由于缺乏地震数据，很难将地幔中部三分之一处（深度约1000-2000 km）的板块与地表的地幔块进行匹配，因为这些板块的几何形状分辨率很差[8]。然而，在过去的几年里，许多新的地震台站已经在阿拉斯加就位（见图1a），最重要的是密集的宽带USAray。这些将提供所需的地震学数据，以便使用计算地震学更准确地模拟阿拉斯加和北美其他地区的地幔。通过这个三维模型，我的目标是采取一种整体的方法来重建板块，并将俯冲板块与地表的地质和古地磁数据联系起来，以重建阿拉斯加的构造组合（“断层构造分析”[1]）.该项目有三个主要目标：1.建立北美和阿拉斯加地区上地幔和下地幔的地震层析成像模型，以便能够识别和表征俯冲岩石圈，并限制它们的几何形状和位置; 2.将该图像与地表地质、古地磁和全球板块重建相结合，阐明阿拉斯加的起源，并验证该方法的可行性：3.将该方法进一步应用于北极、加拿大或西伯利亚的邻近地区。

This projects aims to reconstruct the tectonic history and palaeogeography of Alaska by creating a new seismic tomography model and synthesising it with constraints from surface geology and marine geophysics.This is enabled by a wealth of new seismic data recorded by "USArray" in Alaska and by novel analysis methods, such as "tomotectonic analysis" [1], for connecting the surface record of subduction to its deep subsurface record. Turning back time to unravel the palaeogeographic origins of tectonic plates is primarily informed by palaeomagnetic data recovered from the ocean floor and the continents [2]. However, uncertainty in these reconstructions increases further back in time because many older oceanic plates have subducted down trenches into the mantle. These plates may not be accessible to surface sampling techniques, such as boreholes or dredging, but they are not lost; as they slowly sink inside the mantle they remain visible to geophysical imaging methods, such as seismic tomography [3]. Highly resolved images of this subducted lithosphere (`slabs') can be combined with bedrock geology datasets to understand the plate tectonic evolution of a particular area, especially in palaeo-oceanic regions [1].One such region is Alaska, the origins of which are an enigma and often ignored in studies [4]. The bedrock geology of Alaska comprises accreted terranes, many of which must have formed as linearly strung-out island arcs above subduction zones, much like the Izu-Bonin-Mariana Arc in the western Pacific Ocean today. Palaeomagnetic data reconstruct these proto-Alaska islands to some location much further south than their present-day position [5]. How they became accreted onto North America between 85-50 Ma and rotated into the enormous "Z"-shaped geometry of today's central Alaskan mountains is unclear [6, 7] (see Fig. 1a). Recent seismic studies show that the plates that once carried these islands are still suspended in the mantle [1, 7] (see Fig. 1b). Matching the slabs in the middle third of the mantle (~1000-2000 km depth) with the terranes at the surface has been difficult due to the lack of seismic data, as the geometries of these slabs are poorly resolved [8].However, over the last few years many new seismic stations have been positioned in Alaska (see Fig. 1a), most importantly the dense, broadband USArray. These will provide the seismological data needed to more accurately model the mantle beneath Alaska and the rest of North America using computational seismology. With this 3-D model I aim to take a holistic approach to plate reconstruction and link up the subducted slabs with the geological and palaeomagnetic data from the surface to reconstruct the tectonic assembly of Alaska ("tomotectonic analysis" [1]).This project has three key objectives:1.To create a seismic tomographic model of the upper and lower mantle beneath North America and Alaska so that the subducted lithosphere can be identified and characterised, and their geometries and locations can be constrained;2.To integrate this image with surface geology, palaeomagnetism and global plate reconstructions to elucidate Alaskan origins and test the hypothesis that this method is viable;3.To apply this method further in the adjoining areas of the Arctic, Canada or Siberia.