Repeated lithospheric-scale reactivation of an inherited plate boundary in the eastern Alaska Range, Alaska, USA

Accretionary orogens, such as the North American Cordillera, form by repeated collisions of allochthonous oceanic and continental fragments (terranes). Due to the closure of ocean basins that is required for far-traveled terranes to become part of the orogen, fault systems at the boundaries of allochthonous terranes commonly form as plate boundaries and experience multiple phases of reactivation after collision. The repeated phases of reactivation along the terrane-boundary fault systems often mask the earlier deformation events and lead to uncertainty regarding the location of the main lithospheric-scale geologic boundary between terranes. In this dissertation, I present the geologic evolution of the master reactivated plate boundary structure in the Alaska Range suture zone of southern Alaska. In the first chapter, I use detailed geologic mapping, structural analysis, U-Pb and 40Ar/39Ar geochronology, geochemistry of spinel-group minerals, and receiver function seismology to parse metamorphic rocks in the suture zone. With these methods, I show that the main suturing structure is located along the boundary between amphibolite grade schists and gneisses associated with North America in the north and greenschist grade metagreywacke and slate associated with allochthonous oceanic terranes in the south. The main suturing structure was reactivated after ca. 32 Ma and nucleated an imbricate thrust system that progressed southward. I argue that reactivation along the boundary between the metasedimentary belts is the third phase of activity on this structure, owing to the penetration of that boundary through the lithosphere. \nIn the second chapter, I use regional geologic mapping, U-Pb and 40Ar/39Ar geochronology, Hf isotope analysis, and statistical tests to confirm the correlation between Alaska Range suture zone metamorphic rocks in the Alaska Range and hypothesized correlative metasedimentary and plutonic belts in southwestern Yukon Territory. After confirming the correlation, I use the correlative rock packages to create a sequential restoration of slip on the Denali fault system. The outcome of this work shows that terrane accretion that metamorphosed the Alaska Range suture zone rocks took place at ca. 90 Ma along east-dipping shear zones, and subsequently those suture zone rocks have been dissected by ~480 km of dextral slip on the Denali fault since ca. 50 Ma. \nIn the third chapter, I use modern river detrital apatite fission-track thermochronology and 40Ar/39Ar geochronology to highlight southern Alaska as a type-example of long-lived oblique flat slab subduction. With the datasets, I show that the most recent and rapid bedrock exhumation in southern Alaska is spatially associated with strike-slip fault systems that were active at the time of slab flattening. Moreover, I argue that transpressional deformation and flat slab-associated magmatism were localized on the strike-slip structures due to their role as active lithospheric-scale shear zones. This analysis challenges models of diffuse upper-plate deformation in flat slab subduction environments and instead argues that strike-slip fault systems that penetrate the upper plate are essential for localizing deformation associated with obliquely convergent plate motion. \nIn my view, the overall impact of this dissertation is to show that by coupling detailed field and analytical work, each phase of reactivation for major orogenic structures can be resolved and coupled to regional tectonic/geodynamic scenarios at the time of formation and reactivation. Similarly, an overarching theme of the conclusions drawn herein is that the active strain distribution in strike-slip dominated orogens is strongly influenced by the locations of inherited structures that penetrate the lithosphere.\n

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