Collaborative Research: Geological Constraints on the Physical State of the Subduction Zone Interface at the Depth of Slow Slip and Tremor

像加州的方济各会复合体这样的俯冲复合体含有大量的岩石，这些岩石是从40公里深的地方挖掘出来的，它们保存了环境物理条件和变形过程的记录。特别是，南叉山片岩（SFMS）在北方海岸山脉是一个240公里长，厚达5公里的强烈变形的沉积岩和火山岩体，占据了俯冲带界面在123 Ma左右，在深度和温度对应的震源区，现在的幕式慢滑和地震震颤（ETS）事件。SFMS保留了非凡的高应变变形微观结构，表明在相对高的应力、约40 km的埋藏深度和约350 ℃的温度下，微裂纹、压力溶解和位错蠕变的组合活动。SFMS为ETS事件期间的物理条件和变形力学提供了一个易于访问的窗口。该项目量化了（a）对受位错蠕变影响的岩石使用重结晶粒度测压法的偏应力，（B）使用这一时期的估计俯冲速率、变形带的厚度和微结构特征（包括微褶皱几何形状和变形碎屑石英和放射虫）的应变和应变率，（c）对石英的碳和钛含量使用激光拉曼光谱的温度，（d）利用蓝片岩相岩石中的相组合以及对共存的云母和白色云母的多平衡温压测定法测定压力（从而测定深度）;（e）利用阴极发光和对石英的傅里叶变换红外光谱分析测定水的活性。这些测量将允许热机械模型的耗散加热的效果，并计算的时刻释放率，这可以与地震记录在ETS事件进行比较。对裂纹扩展速率的弹性建模将允许计算流体传输速率和滑动事件的扩展速率。已知的最强大和最具破坏性的地震发生在被称为俯冲带的15至40公里深处的区域，在那里海洋板块被带入地幔。在那之下，有一个从引起地震的突然运动到没有地震的稳定滑动的向下过渡。这种转变产生了几种类型的地面运动，包括“慢地震”，其中滑动持续长达两周，但没有地面震动。这种过渡可能在建立应力方面发挥作用，从而在较浅的深度产生正常的地震。该项目研究北方加州的岩石，这些岩石形成于约40公里深的俯冲带，并保存了导致现今俯冲带缓慢地震和相关现象的物理条件（压力、温度、应力和含水量）和过程的记录。

Subduction complexes such as the Franciscan Complex of California contain large volumes of rock that have been exhumed from up to 40 km depth, and which preserve a record of the ambient physical conditions and deformational processes. In particular, the South Fork Mountain Schist (SFMS) in the Northern Coast Ranges is a 240 km long and up to 5 km thick body of intensely deformed sedimentary and volcanic rock that occupied the subduction zone interface at around 123 Ma, at a depth and temperature corresponding to the source area of present-day episodic slow-slip and seismic tremor (ETS) events. The SFMS preserves an extraordinary high-strain deformational microstructure that indicates the activity of a combination of microcracking, pressure-solution and dislocation creep at relatively high stress, burial depths of around 40 km, and temperatures of around 350 degrees C. The SFMS offers an easily accessible window into the physical conditions and deformational mechanics during ETS events. This project quantifies (a) deviatoric stress using recrystallized grain-size piezometry on rocks affected by dislocation creep, (b) strain and strain-rate using the estimated subduction rate for this period, the thickness of the deformation zone, and microstructural features including microfold geometry and deformed clastic quartz and radiolarians, (c) temperature using laser Raman spectroscopy on carbon and the Ti content of quartz, (d) pressure (and hence depth) using phase assemblages in the blueschist-facies rocks, and multi-equilibrium thermobarometry on coexisting chlorite and white mica, and (e) water activity using cathodo-luminescence and FTIR analyses on quartz. These measurements will allow thermomechanical modeling of the effect of dissipative heating, and calculation of the rate of moment release, which can be compared with the seismic record during ETS events. Elastic modeling of the rate of crack propagation will allow calculation of the rate of fluid transport and the rate of propagation of slip events.The most powerful and damaging earthquakes known occur between 15 and 40 km depth in regions known as subduction zones where oceanic plates are being carried down into the Earth's mantle. Below that, there is a transition downwards from the abrupt movements that cause earthquakes to steady sliding without earthquakes. This transition produces several types of ground motion, including "slow earthquakes", in which slip continues for up to two weeks, but without ground shaking. The transition may play a role in building up the stress that produces normal earthquakes at shallower depths. This project investigates rocks in northern California that were formed in a subduction zone at about 40 km depth, and which preserve a record of the physical conditions (pressure, temperature, stress, and water content) and processes that lead to slow earthquakes and related phenomena in present-day subduction zones.