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分布的AlpArray地震网络和进一步增密的相关部署(SWATH D)以前所未有的详细程度进行3D成像。然而，只有在引起地震观测的岩石类型和结构的可信模型的基础上，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，我们可以预期发现目前在阿尔卑斯山下方的俯冲大陆地壳。在小尺度上，这些俯冲岩石的地震性质取决于(A)作为压力、温度和流体可获得性条件的函数的矿物组合的变化，以及(B)由变形引起的岩石组构的演变(例如，糜棱岩剪切引起的各向异性)。因此，在更大的尺度上，地震速度的变化模式取决于地壳基底中高应变域和低应变域的分布和/或岩性变化。量化地震性质需要可获得的目前俯冲地壳的类似物，可以在目前暴露在地表的高压(HP)和超高压(UHP)基岩中找到。为了达到4D-MB的主要目标，我们的目标是将这些(U)高压单元的构造变质演化与它们不断变化的地震性质联系起来，我们将重建它们的几何结构并对其在埋藏和折返过程中的运动学进行数值模拟(WP1)，然后将这些信息与(U)HP样品的岩石物理数据结合起来，获得不同埋藏深度(WP2)的大陆基底单元的合成地震图像。然后，这些合成图像将根据从带状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，该阵列提供了东阿尔卑斯山(WP3)近10倍的台站密度。我们将关注在弹性不连续面上转换的地震波，因为这些波对地幔深处的地壳结构具有最高的敏感性，但补充了当地的地震记录。通过这种跨学科的方法，我们的目标是破译目前在U(HP)深度的大陆地壳的状态。我们特别想了解推覆构造和流体-岩石相互作用如何导致矿物组合的变化，进而决定变形和俯冲地壳的地震能见度。通过从运动学、岩石学和地震学三个不同的角度解决这一问题，该项目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。它开启了一种视角，即在地质观测和在地表收集的岩石样本与更深层次结构的地球物理图像之间提供坚实的定量联系，以寻求重建阿尔卑斯山造山带的变形历史。

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.