Tectonic framework of continental subduction - testing hypotheses and developing criteria

俯冲的启动和陆壳的俯冲是全球构造的两个关键过程，但对它们的认识还不够充分。该项目将检验三种可供选择的假说(陆内俯冲、大陆碰撞和消融俯冲)，并制定区分这些假说的标准。在大多数陆壳被俯冲的情况下，这发生在大洋岩石圈俯冲之后，当时大陆边缘进入俯冲带。在某些情况下，有人提出大陆俯冲是在大陆岩石圈内开始的，由作用于先前存在的弱点的压应力或由密度不均匀触发。对于东阿尔卑斯山EO-阿尔卑斯山(白垩纪)高压带的俯冲，也有人提出了陆内俯冲作用，由于俯冲带明显缺乏洋壳和相对较高的温度梯度，在奥地利、意大利和斯洛文尼亚不连续地露出了距离约400公里的榴辉岩逆冲断层。其他作者将高压带解释为狭窄大洋海湾俯冲后大陆碰撞的结果，或者是消融俯冲的结果，即上板块岩石向下拖曳到俯冲带。这三个模型是我们项目的可验证的工作假说。为了检验它们并制定区分它们的标准，我们将(1)使用锆石上的LA-ICPMS测定俯冲岩石的原岩，并对其进行地球化学表征，以寻找俯冲地壳具有大陆或海洋性质的证据；(2)利用微探针分析和热力学模拟确定榴辉岩的P-T演化；(3)使用Lu/HF地质计时方法测定它们的进退变质作用，以提高年龄信息的准确性，并发现地体是整体还是多块俯冲；(4)研究划分高压地体的剪切带运动学，以确定高压地体在高P变质作用后如何侵位。这项研究建立在我们与科隆同位素地球化学小组的长期合作以及在这种合作中开发的方法的基础上，以目前的技术水平尽可能高精度地确定榴辉岩相岩石的结构和压力-温度历史并确定其日期。我们使用阿尔卑斯山油田的例子，是因为该地区已有无与伦比的丰富的构造、岩石学和地质年代学数据。

The initiation of subduction and the subduction of continental crust are two key processes of global tectonics which are unsufficiently understood. This project will test three alternative hypotheses (intracontinental subduction, continent collision, and ablative subduction), and develop criteria to distinguish between these.In most instances where continental crust has been subducted, this occurred after the subduction of oceanic lithosphere, when a continental margin entered the subduction zone. In some cases it has been proposed that continental subduction started within continental lithosphere, triggered by compressive stresses acting on pre-existing weaknesses or by density heterogeneities. Intracontinental subduction has also been proposed for the subduction of the Eo-alpine (Cretaceous) high-pressure belt of the Eastern Alps, an eclogite-bearing thrust sheet discontinuously cropping out over a distance of ca. 400 km in Austria, Italy, and Slovenia, because of an apparent lack of oceanic crust and the relatively high thermal gradient in the subduction zone. Other authors have interpreted the high-pressure belt as the result of continent collision after subduction of a narrow ocean embayement, or as the result of ablative subduction, that is, down-dragging of upper-plate rocks into the subduction zone. These three models are testable working hypotheses for our project.To test them and to develop criteria to distinguish between them, we will (1) date protoliths of subducted rocks using LA-ICPMS on zircon and characterize them geochemically, in order to find evidence for either a continental or oceanic nature of the subducted crust; (2) determine the P-T-evolution of eclogites using microprobe analysis and thermodynamic modeling; (3) date their prograde metamorphism using Lu/Hf geochronometry in order to increase the accuracy of age information and to find if the terrane was subducted in one piece or in several pieces; and (4) study the kinematics of shear zones delimiting the high-pressure terrane in order to define how it was emplaced after high-P metamorphism. This study builds on our long-standing collaboration with the Cologne isotope geochemistry group and the methods developed in this collaboration, to determine and date the structural and pressure-temperature history of eclogite-facies rocks with the highest precision possible at the present state of the art. We use the Alpine field example because of the unrivaled wealth of structural, petrological, and geochronological data that is already available for this area.