Mantle deformation beneath the Alps and the physics of the subduction polarity switch: Constraints from thermomechanical modelling, seismic anisotropy and waveform modelling

密集分布的AlpArray网络提供了大量高质量的地震学数据，结合新的地球动力学建模能力，为检验目前关于大陆碰撞深层动力学的假设提供了独特的机会。一个主要的突出目标是了解东阿尔卑斯山下俯冲极性的时间转换的原因和动力学。我们提出这样的假设，一般来说，这样的转换不是长期板块构造历史的巧合结果，而是板块边界汇聚的特定物理条件的直接结果。这一假设将通过研究这种规定的或自一致发展的极性开关的物理学，并通过将热力学建模与地震学约束结合起来，从观测中得到地幔变形，并模拟由于地震各向异性造成的波形效应，来验证。从地震学的角度来看，限制阿尔卑斯山下地幔变形的现状带来了一些困难，这些困难与地壳对远震波形的可能影响有关，也与地幔各向异性结构的深度变化有关。我们建议通过结合(1)远震XKS相和(2)莫霍面和上地幔不连续面转换Psx相的剪切波分裂分析来解决这些问题，以提供整个阿尔卑斯碰撞带下地幔和地壳变形模式的详细图像。地壳厚度和各向异性将在途中测定。关于造山岩石圈-软流圈系统的地球动力学建模，我们的时间相关热化学有限元和有限差分模型的复杂性将逐步增加（从2D到3D），并包括粘塑性-非牛顿流变性，自由表面，侵蚀和沉积，耗散加热等。将应用一致的工具来确定不断变化的晶格优选方向。以此预测地震各向异性的不同模型，并作为与地震观测和波形建模比较的试验模型。此外，热力学模型的输出将包括不断变化的地形、隆起、下沉和挖掘速率、应力等，并将与SPP MB-4D内的合作项目进行交换。

The enormous amount of high-quality seismological data that is becoming available from the densely-spaced AlpArray network in combination with new geodynamic modelling capabilities opens up unique opportunities to test current hypotheses about the deep dynamics of continental collision. A major outstanding goal is an understanding of the causes and dynamics responsible for the proposed temporal switch of subduction polarity beneath the Eastern Alps. We pose the hypothesis that in general such a switch is not a coincidental result of long-term plate tectonic history, but the direct result of specific physical conditions at a converging plate boundary. This hypothesis will be tested by studying the physics of such a prescribed or self-consistently evolving polarity switch and by combining thermomechanical modelling with seismological constraints on mantle deformation from observations and modelling of waveform effects due to seismic anisotropy.From a seismological perspective, constraining the current state of mantle deformation beneath the Alps poses a number of difficulties related to the possible influence of the crust on teleseismic waveforms and also due to depth variations of anisotropic structures in the mantle. We propose to tackle these problems by combining analyses of shear-wave splitting from (1) teleseismic XKS phases and (2) converted Psx phases from the Moho and upper-mantle discontinuities to provide a detailed image of mantle and crustal deformation patterns beneath the entire Alpine collision zone. Crustal thicknesses and anisotropies will be determined on the way. Regarding the geodynamical modelling of an orogenic lithosphere-asthenosphere system, the complexity of our time-dependent thermochemical Finite Element and Finite Difference models will be increased stepwise (from 2D to 3D) and include visco-plastic-non-Newtonian rheology, a free surface, erosion and sedimentation, dissipational heating, etc. Consistent tools to determine the evolving lattice-preferred orientations will be applied. From this, different models of seismic anisotropy will be predicted and serve as test models for the comparison with seismological observations and waveform modelling. Additionally, the output of the thermomechanical models will include the evolving topography, uplift-, subsidence and exhumation rates, stresses etc. and will be exchanged with cooperating projects within the SPP MB-4D.