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

James Dolan (USC), Brad Hacker (UCSB), Charlie Sammis (USC), Ann Blythe （USC）和他们的德国合作者Lothar Ratsbacher （Frieberg）通过对奥地利中新世Salzach-Ennstal-Mariazell-Puchberg断裂带的多学科分析，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一个明确的平面结构上，或者在挖掘出的断裂带中观察到的复杂性是否在地震的成核和传播中起重要作用。换句话说，地震力学基本上是颗粒力学中的一个问题，还是应该主要从沿着单个滑动面摩擦滑动的角度来看待？这些问题对于将实验室实验扩展到地球的问题至关重要。一个相关的问题是断裂带构造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。尽管对挖掘断层的结构研究、对活动断裂带的地震研究以及对地震成核力学的实验室研究已经对这些问题有了很多了解，但事实证明，要在单个断层上以系统的方式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正在利用一个独特的例子，即在挖掘过程中倾斜的主要走滑断层，这样一个连续的挖掘水平-从近地表向下到下地壳-现在沿着走向暴露出来。此外，SEMP断裂系统在一定程度上参与了自身的发掘，减少了与年轻断裂相关的潜在构造套印。他们的研究将在整个发震地壳的深度范围内描述SEMP断裂带的几何形状和内部结构结构，重点是识别断裂带结构的深度依赖变化。现场研究的重点是SEMP断裂带的构造横断面，从断层东端的近地表（维也纳拉分盆地），到发震地壳（奥地利中部），再到奥地利西部陶恩窗口暴露的韧性下地壳。除了对构造结构、流体-岩石相互作用、相对时序关系和断层几何变化进行详细的现场测绘外，PI还对断裂带岩石进行了详细的分析，旨在利用岩石显微镜、阴极发光显微镜、流体包裹体研究、扫描电子显微镜和透射/分析电子显微镜在大范围内探索变形。

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.