Understanding subduction by linking surface exposures of subducted and exhumed crust to geophysical images of slabs

4D-MB的主要目标之一是将地表地质观测与阿尔卑斯山的深层结构联系起来。深层结构将通过密集的AlpArray地震网络和进一步密集的相关部署（SWATH D）以前所未有的细节进行3D成像。然而，只有在对引起地震观测的岩石类型和结构的合理模型的基础上，才有可能对这些图像进行地质解释，根据地震数据和板块构造重建，我们可以预期目前在阿尔卑斯山下发现俯冲大陆地壳。在小尺度上，这些俯冲岩石的地震特性取决于（a）作为压力、温度和流体可用性条件的函数的变化的矿物组合，以及（B）由变形引起的演化的岩石组构（例如，造成各向异性的糜棱岩剪切）。因此，在更大的尺度上，地震速度变化的模式由地壳基底中的高应变域和低应变域的分布和/或岩性变化决定。量化地震特性需要目前俯冲地壳的类似物，这可以在现在暴露在地表的高压（HP）和超高压（UHP）基底岩石中找到。为了实现4D-MB的主要目标，我们的目标是将这些（U）HP单元的构造变质演化与它们不断变化的地震属性联系起来，我们将重建它们的几何形状，并对埋藏和折返过程中的运动学进行数值模拟（WP 1），然后将这些信息与（U）HP样品的岩石物理数据相结合，以获得不同埋藏深度的大陆基底单元的合成地震图像（WP 2）。这些合成图像将根据从SWATH D阵列记录的数据中获得的实际地震图像进行比较和更新，该阵列使东阿尔卑斯山的台站密度增加了近十倍（WP 3）。我们将重点关注弹性不连续面转换的地震波，因为这些对地幔深处的地壳结构具有最高的灵敏度，但用当地的地震记录来补充它们。通过这种跨学科的方法，我们的目标是破译目前U（HP）深度的大陆地壳状态。我们特别想了解推覆体的形成和流体-岩石相互作用如何导致矿物组合的变化，这反过来又决定了变形和俯冲地壳的地震可见性。该项目从运动学、岩石物理学和地震学三个不同角度处理这个问题，旨在探讨用地震方法解决地幔深处地壳尺度结构的可行性和局限性。它开辟了一个坚实的定量之间的联系的地质观测和岩石样品收集在表面的地球物理图像的更深的结构，在寻求重建阿尔卑斯造山带的变形历史的角度。

One of the prime goals of 4D-MB is to link surface geological observations to the deep structure of the Alps. The deep structure will be imaged in 3D in unprecedented detail by the densely spaced AlpArray seismic network and further densified associated deployments (SWATH D). A geological interpretation of these images, however, will only be possible on the basis of a plausible model of the rock types and structures causing the seismic observations.From seismic data and plate-tectonic reconstructions, we can expect to find subducted continental crust currently beneath the Alps. On the small scale, the seismic properties of these subducted rocks depend on (a) changing mineral assemblages as a function of pressure, temperature and fluid availability conditions, and (b) the evolving rock fabric induced by deformation (e.g., mylonitic shearing causing anisotropy). On a larger scale, the pattern of changing seismic velocities is therefore determined by the distribution of high- and low-strain domains and/or lithological changes in the crustal basement. Quantifying seismic properties requires accessible analogues of presently subducted crust, which can be found in high-pressure (HP) and ultrahigh-pressure (UHP) basement rocks now exposed at the surface. In order to address the prime goal of 4D-MB, we aim to link the tectonometamorphic evolution of these (U)HP units to their changing seismic properties.We will reconstruct their geometries and numerically model the kinematics during burial and exhumation (WP1), then combine this information with rock-physical data of (U)HP samples to obtain synthetic seismic images of continental basement units at various burial depths (WP2). These synthetic images will then be compared and updated according to actual seismic images derived from data recorded by the SWATH D array, which offers a nearly tenfold increase in station density in the Eastern Alps (WP3). We will focus on seismic waves converted at elastic discontinuities, as these have the highest sensitivity for crustal structures at mantle depths, but supplement them with local earthquake records.With this interdisciplinary approach we aim to decipher the state of continental crust currently at U(HP) depth. We especially want to understand how nappe formation and fluid-rock interaction lead to changes in mineral assemblages, which in turn determine the seismic visibility of the deforming and subducting crust. By tackling this question from three different angles, namely the kinematic, petrophysical and seismological views, the project is designed to explore the feasibility, but also the limitations, of resolving crustal-scale structures at mantle depth by seismic methods. It opens the perspective of providing a solid quantitative connection between the geological observations and rock samples collected at the surface to the geophysical images of the deeper structures in the quest to reconstruct the deformation history of the Alpine orogen.