Collaborative Research: Linking Deep and Shallow Crustal Processes in a Continental Arc, North Cascades, Washington

大陆碰撞形成的山带和挤压过程中形成的大陆火山弧具有很大的相似性，包括地壳明显增厚。 在过去的十年中，许多研究人员试图利用部分熔融的中下地壳流动模型来解释碰撞山脉带（如喜马拉雅山脉）的大规模模式。 与此相反，对大陆弧折返中地壳流动的研究却很少。华盛顿的北喀斯喀特山脉是一个被挖掘出来的晚白垩世（大约9000万年前）弧，其中包括曾经在35-40公里深处的岩石，现在暴露在地表。 这条山脉也显示了在始新世（大约5000 - 4500万年前）邻近的非海洋沉积盆地形成期间隆起和侵蚀的证据;这些盆地充满了侵蚀的碎片。一个跨学科的地质学家小组正在进行实地结构研究和实验室分析工作（地质年代学、岩石学、显微结构分析），以确定卡斯卡德的变质作用、部分熔融、地壳深部岩石折返以及盆地形成和演变的时间。 这些研究的目的是提供一个类似于玻利维亚和阿根廷现代安第斯山脉的高原地区的大陆弧的建设和崩溃的看法。 大部分的研究都集中在北喀斯喀特的斯卡吉特片麻岩复合体上。 这些岩石记录了部分熔融的历史，这可能发生在山带的折返过程中。 片麻岩部分被类似于拆离断层的低角度结构所包围。这些混合岩流动和边界单元之间的关系以前没有研究过，但可以提供深，浅地壳过程的耦合或解耦的信息。正在分析一系列构造深度范围内的变形动能，以确定地壳流动的方向和条件，以及流动是否发生在与北美板块边界边缘平行或倾斜的通道中。 北喀斯喀特山脉两侧的始新世盆地包含由走滑和正断层系统中的河流沉积的厚沉积序列。火山凝灰岩与沉积岩互层有助于确定盆地形成的时间和沉积速率。大量的始新世岩脉侵入结晶岩和盆地。 脉岩测绘和测年的一个主要目标是提供山脉带折返和盆地形成过程中区域变形的记录。这些结构的方向可以与混合岩记录的流动方向相比较。本研究涉及三个研究生，本科生研究助理，和暑期实习生。 主要调查人员正在对北喀斯喀特山脉进行实地考察，参加者包括来自所有三所大学的学生，并在互联网上发布了一份与这次考察和研究结果有关的图文并茂、数据丰富的实地考察指南。

Mountain belts formed by collisions between continents, and continental volcanic arcs constructed during compression, have large-scale similarities, including markedly thickened crust. In the past decade, many researchers have tried to explain the large-scale patterns of collisional mountain belts, such as the Himalayas, using models of flow of partially molten mid- to lower crust. In contrast, there have been few studies of crustal flow in exhumed continental arcs. The North Cascades, Washington, is an exhumed Late Cretaceous (circa 90 million years ago) arc, which includes rocks that were once at depths of 35-40 km and are now exposed at the surface. This mountain range also displays evidence for uplift and erosion during formation of adjacent non-marine sedimentary basins in the Eocene (approximately 50-45 million years ago); these basins are filled with the eroded debris. Field-based structural study and laboratory analytical work (geochronology, petrology, microstructural analysis) are being carried out by an interdisciplinary team of geologists to determine the timing of metamorphism, partial melting, exhumation of deep crustal rocks, and basin formation and evolution in the Cascades. These studies are aimed at providing a view of the construction and collapse of a continental arc analogous to the Altiplano region of the modern Andes in Bolivia and Argentina. Much of the research is focused on the Skagit Gneiss Complex of the North Cascades. These rocks record a history of partial melting, which probably occurred during exhumation of the mountain belt. The gneiss is bounded in part by low-angle structures that resemble detachment faults. Relationships between flow of these migmatites and bounding units have not been previously studied, but can provide information about the coupling or decoupling of deep and shallow crustal processes. Kinematics of deformation are being analyzed over a range of structural depths to determine the directions and conditions of crustal flow, and whether flow occurred in a channel parallel or oblique to the edge of the North American plate boundary. Eocene basins that flank the North Cascades contain thick sedimentary sequences deposited by rivers in a system of strike-slip and normal faults. Volcanic tuffs interlayered with the sedimentary rocks facilitate dating of the time of basin initiation, and the rates of deposition. Vast swarms of Eocene dikes intrude both crystalline rocks and basins. A major goal of the mapping and dating of the dikes is to provide a record of regional deformation during exhumation of the mountain belt and basin formation. The orientation of these structures can be compared to the flow directions recorded by the migmatites. This research involves three graduate, undergraduate research assistants, and summer interns. The principal investigators are conducting a field trip to the North Cascades involving students from all three universities, and are publishing an illustrated, data-filled field guide related to the trip and the research results on the internet.