Abstract. We perform a teleseismic P-wave traveltime tomography to examine the geometry and structure of subducted lithosphere in the upper mantle beneath the Alpine orogen. The tomography is based on waveforms recorded at over 600 temporary and permanent broadband stations of the dense AlpArray Seismic Network deployed by 24 different European institutions in the greater Alpine region, reaching from the Massif Central to the Pannonian Basin and from the Po plain to the river Main. Teleseismic traveltimes and traveltime residuals of direct teleseismic P-waves from 331 teleseismic events of magnitude 5.5 and higher recorded between 2015 and 2019 by the AlpArray Seismic Network are extracted from the recorded waveforms using a combination of automatic picking, beamforming and cross-correlation. The resulting database contains over 162.000 highly accurate absolute P-wave traveltimes and traveltime residuals. For tomographic inversion, we define a model domain encompassing the entire Alpine region down to a depth of 600 km. Outside this domain, a laterally homogeneous standard earth model is assumed. Predictions of traveltimes are computed in a hybrid way applying a fast Tau-P method outside the model domain and continuing the wavefronts into the model domain using a fast marching method. For teleseismic inversion, we iteratively invert demeaned traveltime residuals for P-wave velocities in the model domain using a regular discretization with an average lateral spacing of about 25 km and a vertical spacing of 15 km. The inversion is regularized towards an initial model constructed from an a priori model of the crust and uppermost mantle and a standard earth model beneath. The resulting model provides a detailed image of slab configuration beneath the Alpine and Apenninic orogens. Major features are an overturned Adriatic slab beneath the Apennines reaching down to 400 km depth still attached in its northern part to the crust but exhibiting detachment towards the southeast. A fast anomaly beneath the western Alps indicates a short western Alpine slab that ends at about 100 km depth close to the Penninic front. Further to the east and following the arcuate shape of the western Periadriatic Fault System, a deep-reaching coherent fast anomaly with complex interior stucture generally dipping to the SE down to about 400 km suggests a slab of European origin extending eastward to the Giudicarie fault. This slab is detached from overlying lithosphere at its eastern end below a depth of about 100 km. Further to the east, well-separated from the slab beneath the western and central Alps, another deep-reaching, nearly vertically dipping high-velocity anomaly suggests the existence of a slab beneath the Eastern Alps of presumably European origin which is completely detached from the orogenic root. Our image of this slab does not require a polarity switch because of its nearly vertical dip and full detachment from the overlying lithosphere. Fast anomalies beneath the Dinarides are weak and concentrated to the northernmost part and shallow depths. Low-velocity regions surrounding the fast anomalies beneath the Alps to the west and northwest follow the same dipping trend as the overlying fast ones, indicating a kinematically coherent subducting tectosphere in this region. In contrast, low-velocity anomalies to the east suggest asthenospheric upwelling presumably driven by retreat of the Carpathian slab and extrusion of eastern Alpine lithosphere towards the east while low velocities to the south are presumably evidence of asthenospheric upwelling and mantle hydration due to the backarc position behind the European slab.

中文翻译：

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大阿尔卑斯地区平板的成像结构和几何形状–使用AlpArray地震网络数据的P波行进时间层析成像

摘要。我们进行了远震P波行进时间层析成像，以检查高山造山带下方上地幔俯冲岩石圈的几何结构。层析X射线断层扫描是基于在密集的AlpArray地震台网的600多个临时和永久宽带站记录的波形，该站由大阿尔卑斯地区的24个不同的欧洲机构部署，从地块中部到Pannonian盆地以及从Po平原到河流主要的。AlpArray地震网络在2015年至2019年之间记录的331次5.5级及以上的地震事件中的直接地震P波的远震传播时间和传播时间残差是结合自动选取，波束赋形和互相关从记录的波形中提取的。结果数据库包含超过162个。000个高精度的绝对P波传播时间和传播时间残差。对于断层成像反演，我们定义了一个模型域，该模型域涵盖了整个高山区域直至600 km的深度。在此范围之外，假定为横向均质的标准地球模型。通过在模型域之外应用快速Tau-P方法并使用快速行进方法将波前延续到模型域中，以混合方式计算出行进时间的预测。对于远震反演，我们使用平均横向间距约为25 km，垂直间距为15 km的常规离散化来迭代反演模型域中P波速度的无声传播时间残差。反演被规范化为一个初始模型，该模型是由地壳和最上层地幔的先验模型以及下面的标准地球模型构成的。生成的模型提供了阿尔卑斯和亚平山造山带下面的平板构造的详细图像。主要特征是在亚平宁山脉下方倾覆的亚得里亚海平板向下延伸至400 km的深度，该平板仍附着在地壳的北部，但向东南方向显示出脱离。阿尔卑斯山西部下方的一个快速异常现象表明，一块短的阿尔卑斯山西部板块终止于靠近Penninic前沿的约100 km深度。进一步向东并遵循西部的Periadriatic断层系统的弧形形状，一个深部的连贯快速异常，内部结构复杂，通常向东南倾斜约400 km，这表明是欧洲起源的板块向东延伸到Giudicarie断层。该板块在其东端低于约100 km的深度上与上覆岩石圈分离。再往东走 与西部和中部阿尔卑斯山下方的板块完全分开，另一个深远的，近乎垂直的高速异常异常表明，可能是欧洲起源的东部阿尔卑斯山以下板块的存在与造山带完全分离。我们对这块平板的图像不需要极性转换，因为它几乎垂直倾斜并且与上覆岩石圈完全分离。Dinarides下面的快速异常较弱，并且集中到最北端和浅层深度。西部和西北部阿尔卑斯山以下快速异常周围的低速区域与上覆的快速异常具有相同的下降趋势，表明该地区运动学上一致的俯冲对流层。相比之下，