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

本项目旨在通过建立一种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的约束条件综合起来，重建阿拉斯加的构造历史和古地理。这得益于“USArray”在阿拉斯加记录的大量新地震数据，以及“断层构造分析”[1]等新颖的分析方法，这些方法将地表俯冲记录与深层地下记录联系起来。将时间倒转，以揭示构造板块的古地理起源，主要是根据从海底和大陆上恢复的古地磁数据。然而，这些重建的不确定性在时间上进一步增加，因为许多更古老的海洋板块已经俯冲到海沟中进入地幔。这些板块可能无法使用表面取样技术，如钻孔或疏浚，但它们并没有丢失；当它们在地幔中缓慢下沉时，地球物理成像方法（如地震层析成像）仍然可以看到它们。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，它的起源是一个谜，经常在研究中被忽视。阿拉斯加州的基岩地质包含了大量的陆块，其中许多一定是在俯冲带之上形成的线性岛弧，很像今天西太平洋的伊扎-波宁-马里亚纳岛弧。古地磁数据将这些原始阿拉斯加岛重建到比它们现在的位置[5]更靠南的位置。它们是如何在85-50 Ma之间被吸积到北美，并旋转成今天阿拉斯加中部山脉巨大的“Z”形几何形状的，目前尚不清楚[6,7]（见图1a）。最近的地震研究表明，曾经携带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，将地幔中三分之一（~1000-2000公里深度）的板块与地表的地形相匹配一直很困难，因为这些板块的几何形状分辨率很差。然而，在过去的几年里，许多新的地震台站被安置在阿拉斯加（见图1a），最重要的是密集的宽带USArray。这些数据将为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提供所需的地震学数据。有了这个三维模型，我的目标是采用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，重建阿拉斯加的构造组合（“断层构造分析”[1]）。本项目有三个主要目标：1。1 .建立北美和阿拉斯加上下地幔的地震层析模型，以便识别和表征俯冲岩石圈，并限制其几何形状和位置；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，验证该方法的可行性；在北极、加拿大或西伯利亚的邻近地区进一步应用这种方法。

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.