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

北美科迪勒拉山脉包括几块变质岩，显然经历了快速的白垩纪埋藏和加载，然后快速折返。这些岩石被解释为沉积在科迪勒拉火山岩浆弧会聚的盆地中的碎屑沉积物。这些地壳块体沿着北美山脉的大部分地区的鉴定导致了这样一种假设，即断层约束块体的快速垂直运动是白垩纪的一个整体特征。这些块体可以作为科迪勒拉山脉和其他造山带中较新过程的类似物。因此，重要的是充分理解已知示例中的运动并量化运动速率。华盛顿喀斯喀特水晶核中的斯瓦坎片麻岩最近被确定为具有白垩纪快速垂直运动历史的断块。然而，这一想法取决于解释，斯瓦坎片麻岩有一个变质沉积物的起源，这是基于复杂的分区锆石颗粒，推断有一个碎屑的起源。这种解释要求以7 mm/年的平均速率加载。此外，变质和冷却的年龄需要快速折返和冷却。变质作用和加载的年龄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技术从斯瓦坎片麻岩主体内和沿着其剪切边缘选择样品，测试斯瓦坎片麻岩的当前解释。激光烧蚀电感耦合等离子体质谱仪和灵敏的高分辨率离子微探针技术正在被用来充分表征选定的锆石晶体内的年龄的空间变化，以测试碎屑的解释，并产生新的估计年龄的火成岩和可能的变质活动。变质作用的时间和条件分别由石榴石Sm-Nd和相平衡决定。弧中过程的速率对于理解弧的演化、与弧有关的地震和火山灾害以及大陆的生长是重要的。研究结果将有助于加深对弧动力学、地壳深部过程和上下地壳耦合的认识。它还将提供必要的信息，使之与加州的其他变质岩石相关联，这些变质岩石被认为是相关的，从而使整个北美西海岸的白垩纪俯冲作用得到更好的理解。最后，埋藏和折返的速率对全球俯冲过程具有重要意义，本研究的结果将允许与其他俯冲过程进行比较。

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.