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

点击翻译按钮获取中文摘要

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.