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

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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.