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

詹姆斯·多兰（南加州大学）、布拉德·哈克（UCSB）、查理·萨米斯（南加州大学）、安·布莱斯（南加州大学）和他们的德国合作者洛塔尔·拉茨巴赫（弗里堡）正在通过对奥地利中新世时期的萨尔扎克-恩斯塔尔-玛丽亚泽尔-普赫贝格（SEMP）断层带的多学科分析，研究地震力学与断层带结构之间的关系。 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，或者在折返断层带中观察到的复杂性是否在地震的成核和传播中起着重要作用。换句话说，地震力学从根本上说是一个颗粒力学的问题，还是应该主要从沿着单一滑动面的摩擦滑动的角度来看待？这些问题对于将实验室实验的规模扩大到地球的问题至关重要。一个相关的问题是断层带结构在断层带流体的长期和短期输运中的作用，以及它们在断层过程中的作用。尽管从对折返断层的结构研究、对活动断层带的地震研究以及对地震成核机制的实验室研究中，人们已经对这些问题有了很多了解，但这一整套问题已被证明极难以系统的方式在单个断层上加以解决。在他们对SEMP断层带的分析中，PI正在利用一个主要走滑断层的独特例子，该断层在折返过程中倾斜，因此，从近地表向下到下地壳的连续折返水平现在沿着走向暴露。此外，SEMP断层系统在有限的程度上参与了自身的折返，减少了与年轻断层有关的潜在结构叠加。他们的研究将描述SEMP断层带在整个孕震地壳深度范围内的几何形状和内部结构体系，重点是识别断层带结构随深度变化的情况。实地研究的重点是SEMP断层带的结构断面，其折返水平从断层东端的近地表（维也纳拉分盆地）、孕震地壳（奥地利中部），一直到奥地利西部Tauern窗暴露的韧性下地壳。除了详细的结构组构、流体-岩石相互作用、相对时序关系和断层几何形状变化的实地测绘外，PI还对断层带岩石进行了详细分析，旨在使用岩相显微镜、阴极发光显微镜、流体包裹体研究、扫描电子显微镜和透射/分析电子显微镜在广泛的尺度上探索变形。

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.