Part of SSP 4D-MB Mountain Building in four dimensions
One of the prime goals of 4D-MB is to link surface geological observations to the deep structure of the Alps. The deep structure cannot be observed directly but will be imaged in 3D in unprecedented detail by the very extensive, densely spaced AlpArray network of seismological stations. A geological interpretation of these 3D 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 changing mineral assemblages as a function of changing pressure (P), temperature (T) and fluid conditions, or structural changes of rock fabrics due to deformation (e.g., anisotropy induced by mylonitic shearing), or some combination of both of these effects. 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 units now exposed at the surface. We aim to link the tectonometamorphic evolution of HP and UHP units to the evolution of seismic properties of the main (U)HP rock types exposed in the Alps. To do this, 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 images of continental basement units at various burial depths (WP2). These synthetic images will then be compared and updated with actual seismic images of Alpine slabs derived by dense station arrays in AlpArray (SWATH D) (WP3). With this interdisciplinary approach we aim to decipher the state of continental crust currently at U(HP) depth beneath the Alps. We especially want to understand the role of nappe formation and fluid-rock interaction leading to changes in mineral assemblages in determining the seismic visibility of structures at depth. By tackling these timely questions from three different angles, namely the kinematic, the petrophysical and the seismological point of view, the project is designed to explore the feasibility, but also the limitations, of resolving crustal-scale structures at mantle depth by state-of-the-art seismic methods. It opens the perspective of gaining a 4D geological interpretation of the geophysical data acquired in 4D-MB and incorporating data from inaccessible (mantle) levels into geodynamic models for the Alpine orogen with unprecedented extent. Within this project, the seismology section is responsible for Work Package 3, the imaging based on converted seismic waves with SWATH D and synthetic data.