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，显然经历了白垩纪的快速埋葬和加载，然后是快速的挖掘。这些岩石被解释为在会聚的科迪勒岩浆弧内的盆地中沉积的碎屑沉积物。对沿北美科迪勒拉大部分地区的这些地壳块体的识别导致了这样的假设，即断层块体的快速垂直运动是白垩纪的一个整体特征。这些地块可以作为科迪勒拉和其他造山带中较新过程的类似物。因此，在已知的例子中，充分理解运动并量化运动速率是很重要的。华盛顿州喀斯喀兹结晶核中的斯瓦坎片麻岩最近被确认为具有白垩纪快速垂直运动历史的断块。然而，这一想法取决于对斯瓦坎片麻岩具有变质沉积成因的解释，该变质沉积起源基于复杂分带的锆石颗粒，并被推断为具有碎屑成因。这一解释要求以7毫米/年的平均速率加载。此外，变质作用和冷却的年龄需要快速折返和冷却。变质作用和加载作用的年龄必须小于周围断块的变质作用和加载年龄，但该年龄的约束性较差。该项目正在测试目前对斯瓦坎片麻岩的解释，方法是应用锆石U-Pb和石榴石Sm-ND技术，从斯瓦坎片麻岩主体内和沿其剪切边缘选择样品。激光烧蚀电感耦合等离子体质谱和灵敏的高分辨率离子微探针技术正在被用来全面表征选定的锆石晶体中年龄的空间变化，以便测试碎屑岩的解释，并对火成岩和可能的变质活动的年龄做出新的估计。变质作用的时间和条件分别由石榴石Sm-ND和相平衡确定。弧内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非常重要。这一研究结果将加深对弧动力学、地壳深部过程以及上、下地壳耦合的认识。它还将提供必要的信息，以便与加州其他变质地体进行对比，这些地体被认为是相关的，从而使人们能够更好地了解整个北美西海岸的白垩纪俯冲。最后，埋葬和出土的速度对全球俯冲过程具有重要意义，这项研究的结果将使我们能够与其他俯冲地体进行比较。

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.