Tewksbury-Christle C M, Behr W M, Helper M A
Structural Geology and Tectonics Group Department of Geosciences Geological Institute ETH Zurich Zurich Switzerland.
Department of Geological Sciences Jackson School of Geosciences University of Texas at Austin Austin TX USA.
Geochem Geophys Geosyst. 2021 Mar;22(3):e2020GC009463. doi: 10.1029/2020GC009463. Epub 2021 Mar 10.
The architecture and mechanical properties of the subduction interface impact large-scale subduction processes, including mass and volatile recycling, upper-plate orogenesis, and seismic behavior. The nature of the deep subduction interface, where a dominantly frictional megathrust likely transitions to a distributed ductile shear zone, is poorly understood, due to a lack of constraints on rock types, strain distribution, and interface thickness in this depth range. We characterized these factors in the Condrey Mountain Schist, a Late Jurassic to Early Cretaceous subduction complex in northern California that consists of an upper and lower unit. The Lower Condrey unit is predominantly pelagic and hemipelagic metasediment with m-to km-scale metamafic and metaserpentinitic ultramafic lenses all deformed at epidote blueschist facies (0.7-1.1 GPa, 450°C). Major and trace element geochemistry suggest tectonic erosion of the overriding plate sourced all ultramafic and some mafic lenses. We identified two major ductile thrust zones responsible for Lower Condrey unit assembly, with earlier strain distributed across the structural thickness between the ductile thrusts. The Lower Condrey unit records distributed deformation across a sediment-dominated, 2+ km thick shear zone, possibly consistent with low velocity zones observed in modern subduction zones, despite subducting along a sediment poor, tectonically erosive margin. Periodic strain localization occurred when rheological heterogeneities (i.e., km-scale ultramafic lenses) entered the interface, facilitating underplating that preserved 10%-60% of the incoming sediment. Modern mass and volatile budgets do not account for erosive margin underplating, so improved quantification is crucial for predicting mass and volatile net flux to Earth's interior.
俯冲带界面的结构和力学性质影响着大规模俯冲过程，包括物质和挥发物的循环、上覆板块造山作用以及地震活动。深部俯冲带界面的性质，即主要为摩擦性质的巨型逆冲断层可能转变为分布式韧性剪切带的区域，由于在该深度范围内缺乏对岩石类型、应变分布和界面厚度的限制，目前了解甚少。我们对康德里山片岩中的这些因素进行了表征，康德里山片岩是加利福尼亚北部晚侏罗世至早白垩世的俯冲复合体，由上、下两个单元组成。康德里下单元主要为远洋和半远洋变质沉积物，含有米级至千米级的变质镁铁质和变蛇纹石化超镁铁质透镜体，均在绿帘石蓝片岩相（0.7 - 1.1吉帕，450℃）下变形。主量和微量元素地球化学表明，上覆板块的构造侵蚀为所有超镁铁质和一些镁铁质透镜体提供了来源。我们识别出两个主要的韧性逆冲带，它们负责康德里下单元的组装，早期应变分布在韧性逆冲带之间的结构厚度上。康德里下单元记录了在一个以沉积物为主、厚度超过2千米的剪切带内的分布式变形，尽管该区域沿着沉积物匮乏、构造侵蚀的边缘俯冲，但这可能与现代俯冲带中观察到的低速带一致。当流变学非均质性（即千米级超镁铁质透镜体）进入界面时，会发生周期性应变局部化，促进了底侵作用，从而保留了10% - 60%的输入沉积物。现代的物质和挥发物收支没有考虑侵蚀边缘的底侵作用，因此改进量化对于预测进入地球内部的物质和挥发物净通量至关重要。
