Collaborative Research (USC and UCSB): Direct Observation of Depth Variation in Fault Zone Structure Through and Below the Seismogenic Crust

James Dolan(南加州大学)、Brad Hacker(UCSB)、Charlie Sammis(南加州大学)、Ann Blythe(南加州大学)以及他们的德国合作者Lothar Ratsbacher(Frieberg)正在对奥地利出土的中新世Salzach-Ennstal-Mariazell-Puchberg[SEMP]断裂带进行多学科分析，研究地震力学与断裂带结构之间的关系。他们的主要目标之一是了解地震破裂是否总是发生在定义明确的平面结构上，或者在出土的断裂带中观察到的复杂性是否在地震的成核和传播中发挥重要作用。换句话说，地震力学从根本上是颗粒力学的问题，还是应该主要根据沿着单一滑动面的摩擦滑动来看待？这些问题对于将实验室实验扩展到地球上的问题至关重要。一个相关的问题是断裂带结构在断裂带流体的长期和短期输送中的作用，以及它们在断裂过程中的作用。尽管从对出土断层的构造研究、对活动断裂带的地震研究以及对地震成核机制的实验室研究中了解到了许多关于这些问题的知识，但事实证明，在单个断层上系统地解决这一系列问题是极其困难的。在他们对Semp断裂带的分析中，Pi‘s正在利用一个独特的例子，即一个主要的走滑断层在折返过程中被倾斜，这样一系列折返水平--从近地表向下到下地壳--现在沿着走向暴露出来。此外，SEMP断裂系统在有限程度上参与了自身的挖掘，减少了与年轻断裂有关的潜在构造叠置。他们的研究将在整个孕震地壳深度范围内表征SEMP断裂带的几何形状和内部结构，重点是认识断裂带结构随深度的变化。实地研究的重点是SEMP断裂带从断层东端近地表(维也纳拉分盆地)、发震地壳(奥地利中部)到奥地利西部Tauern窗口暴露的韧性下地壳的折返水平上的结构断面。除了构造组构、流体-岩石相互作用、相对计时关系和断层几何变化的详细野外测绘外，PI还对断层带岩石进行详细分析，旨在利用岩相显微镜、阴极发光显微镜、流体包裹体研究、扫描电子显微镜和透射/分析电子显微镜探索大范围的变形。

James Dolan (USC), Brad Hacker (UCSB), Charlie Sammis (USC), Ann Blythe (USC), and their German collaborator Lothar Ratsbacher (Frieberg) are studying the relationship between earthquake mechanics and fault-zone structure through multi-disciplinary analysis of the exhumed, Miocene-aged Salzach-Ennstal-Mariazell-Puchberg [SEMP] fault zone in Austria. One of their major goals is to understand whether seismic ruptures always occur on a well-defined planar structure or whether the complexity observed in exhumed fault zones plays an important role in the nucleation and propagation of earthquakes. Stated another way, is earthquake mechanics fundamentally a problem in granular mechanics, or should it be viewed primarily in terms of frictional sliding along a single slip surface? These issues are critical for the question of scaling laboratory experiments to the Earth. A related issue is the role of fault-zone structure in the long- and short-term transport of fault-zone fluids, and their role in the faulting process. Although much has been learned about these issues from structural studies of exhumed faults, seismic studies of active fault zones, and laboratory studies of the mechanics of earthquake nucleation, this overall set of issues has proven extremely difficult to address in a systematic fashion on a single fault. In their analysis of the SEMP fault zone, the PI's are exploiting a unique example of a major strike-slip fault that has been tilted during exhumation, such that a continuum of exhumation levels-from the near-surface down into the lower crust-are now exposed along strike. Moreover, the SEMP fault system has participated in its own exhumation to a limited extent, reducing potential structural overprints related to younger faulting. Their research will characterize the geometry and internal structural architecture of the SEMP fault zone throughout the entire depth range of the seismogenic crust, with a focus on the recognition of depth-dependent changes in fault-zone structure. Field studies are focusing on structural transects across the SEMP fault zone at exhumation levels ranging from the near-surface at the eastern end of the fault (Vienna pull-apart basin), within the seismogenic crust (central Austria), and down into the ductile lower crust exposed in the Tauern window of western Austria. In addition to detailed field mapping of structural fabrics, fluid-rock interactions, relative timing relationships, and variations in fault geometry, the PI's are conducting detailed analyses of fault-zone rocks designed to explore deformation at a wide range of scales using petrographic microscopy, cathodoluminescence microscopy, fluid-inclusion studies, scanning electron microscopy, and transmission/analytical electron microscopy.