Rates of Loading and Exhumation in Convergent Magmatic Arcs: Tectonic History of the Swakane Gneiss, North Cascades Washington

北美科迪勒拉包括几块变质岩，它们显然经历了白垩纪的快速埋藏和装载，随后又被迅速挖掘出来。这些岩石被解释为碎屑沉积物沉积在科迪勒拉岩浆弧内的盆地中。在北美科迪勒拉山脉的大部分地区发现了这些地壳块体，由此产生了这样一种假设，即断层绑定块体的快速垂直运动是白垩纪时期的一个整体特征。这些块体可以作为科迪勒拉最近的过程和其他造山带的过程的类似物。因此，在已知的例子中，充分理解运动和量化运动速率是很重要的。位于华盛顿州喀斯喀特水晶核心的斯瓦坎片麻岩最近被确定为白垩纪快速垂直运动历史的断块。然而，这一想法取决于斯瓦坎片麻岩具有沉积起源的解释，这是基于推断具有碎屑起源的复杂分带锆石颗粒。这种解释要求加载速度平均为7毫米/年。此外，变质作用和冷却的年龄要求快速挖掘和冷却。变质作用的年龄和作用的载荷必须小于周围断块的年龄；然而，年龄限制很差。该项目通过应用锆石U-Pb和石榴石Sm-Nd技术，从Swakane片麻岩主体内及其剪切边缘选择样品，测试了目前对Swakane片麻岩的解释。激光烧蚀电感耦合等离子体质谱和敏感高分辨率离子微探针技术被用来全面表征选定锆石晶体中年龄的空间变化，以测试碎屑解释并对火成岩和可能的变质活动年龄进行新的估计。变质作用的时间和条件分别由石榴石Sm-Nd和相平衡决定。弧的过程速率对于理解弧的演化、与弧相关的地震和火山灾害以及大陆的生长都很重要。该研究结果将增加对弧动力学、深部地壳过程和上下地壳耦合的认识。它还将进一步提供必要的信息，以便与加利福尼亚的其他变质地体进行比较，这些变质地体被认为是相关的，从而使人们更好地了解整个北美西海岸的白垩纪俯冲作用。最后，埋藏和挖掘速度对全球俯冲过程具有重要意义，并且本研究的结果将允许与其他俯冲地块进行比较。

The North American Cordillera includes several blocks of metamorphic rock that apparently experienced rapid Cretaceous burial and loading, followed by rapid exhumation. These rocks are interpreted to been deposited as clastic sediments in basins within the convergent Cordilleran magmatic arc. Identification of these crustal blocks along much of the North American Cordillera leads to the hypothesis that rapid vertical motion of fault bound blocks was an integral feature during the Cretaceous. The blocks may serve as analogs for more recent processes in the Cordillera and for processes in other orogens. Therefore, it is important to fully understand the motion and quantify rates of motion in known examples. The Swakane Gneiss in the Cascades Crystalline Core of Washington was recently identified as a fault block with a Cretaceous history of rapid vertical motion. However, this idea hinges on interpretation that the Swakane Gneiss has a metasediment origin, which is based on complexly zoned zircon grains that are inferred to have a detrital origin. This interpretation requires that loading occurred at an average rate of 7 mm/year. Furthermore, ages for metamorphism and cooling require rapid exhumation and cooling. The age of metamorphism and loading must be less than that for surrounding fault blocks; however, the age is poorly constrained. This project is testing current interpretations for the Swakane Gneiss by applying zircon U-Pb and garnet Sm-Nd techniques to select samples from within the main body of the Swakane Gneiss and along its sheared margins. Laser-Ablation Inductively-Coupled Plasma Mass Spectrometry and Sensitive High Resolution Ion MicroProbe techniques are being used to fully characterize the spatial variation in age within select zircon crystals in order to test the detrital interpretation and yield new estimates for the age of igneous and possibly metamorphic activity. The timing and conditions of metamorphism are being determined by garnet Sm-Nd and phase equilibria, respectively.Rates of processes in arcs are important for understanding arc evolution, earthquake and volcano hazards associated with arcs, and the growth of continents. Results of this study will increase the understanding of arc dynamics, deep crustal processes, and coupling between the upper and lower crust. It will furthermore provide the necessary information that will permit correlation with other metamorphic terranes in California, which are believed to be correlatives, thus enabling Cretaceous subduction over the entire west coast of North America to be better understood. Finally, the rates of burial and exhumation carry significance for subduction processes globally, and results of this study will permit comparison with other subduction terranes.