Shear development and overprinting in a back-arc basin, Klamath Falls, Oregon

Three key tectonic domains of western North America, the Walker Lane Fault Zone (WLFZ), northwestern Basin and Range (NWBR), and Cascade arc, intersect in south-central Oregon. Bounded by Cascade arc volcanoes in the west, the Klamath Graben is traditionally regarded as the western-most extent of the Basin and Range extensional province at the latitude of the California-Oregon border. Northwest-directed dextral shear across the Modoc Plateau domain of the WLFZ organizes into north and northwest-trending faults that end in the Klamath Graben. New geologic mapping with airborne LiDAR data reveals the nature of previously unrecognized interactions between faults within the Klamath Graben. North and northwest-trending faults define the Klamath Graben. Slip partitioning between north-trending normal faults and northwest-trending right-oblique faults suggest that the graben evolved due to transfer of northwest-directed shear into north-trending normal faults. Aftershock focal mechanisms from the 1993 Klamath Falls earthquake sequence show that active faulting involves both northwest-trending dextral faults and north-trending normal faults within the graben. North and northwest-trending basin-bounding faults are dominantly normal. Inside the graben, northwest-striking fault strands show ~3:1 strike-slip to normal offset across drainages within an alluvial fan. South of the alluvial fan, two faults change to northeast-strike and create a pull-apart basin, resulting in northwest-directed extension. Geodetic velocities of ~5 mm∙yr-1 towards ~N45°W, relative to stable North America, coincide with average fault strike along the graben-central strand, but are probably faster than geologic rates. Strata tilted ~5-40° are offset by northwest-striking dextral faults in the graben center and suggest that northwest-directed dextral shear overprints previous basin extension. Cumulative right-slip for the whole graben is probably <1 km and is restricted to the southern half of the graben. Cessation of strike-slip faulting prior to intersecting the Cascade arc suggests that the arc acts as a thermal boundary and limits westward growth of the WLFZ

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. In 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. In 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. In 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

CUE_2013.pptx

Three key tectonic domains of western North America, the Walker Lane Fault Zone (WLFZ), northwestern Basin and Range (NWBR), and Cascade arc, intersect in south-central Oregon. Bounded by Cascade arc volcanoes in the west, the Klamath Graben is traditionally regarded as the western-most extent of the Basin and Range extensional province at the latitude of the California-Oregon border. Northwest-directed dextral shear across the Modoc Plateau domain of the WLFZ organizes into north and northwest-trending faults that end in the Klamath Graben. New geologic mapping with airborne LiDAR data reveals the nature of previously unrecognized interactions between faults within the Klamath Graben. North and northwest-trending faults define the Klamath Graben. Slip partitioning between north-trending normal faults and northwest-trending right-oblique faults suggest that the graben evolved due to transfer of northwest-directed shear into north-trending normal faults. Aftershock focal mechanisms from the 1993 Klamath Falls earthquake sequence show that active faulting involves both northwest-trending dextral faults and north-trending normal faults within the graben. North and northwest-trending basin-bounding faults are dominantly normal. Inside the graben, northwest-striking fault strands show ~3:1 strike-slip to normal offset across drainages within an alluvial fan. South of the alluvial fan, two faults change to northeast-strike and create a pull-apart basin, resulting in northwest-directed extension. Geodetic velocities of ~5 mm∙yr-1 towards ~N45°W, relative to stable North America, coincide with average fault strike along the graben-central strand, but are probably faster than geologic rates. Strata tilted ~5-40° are offset by northwest-striking dextral faults in the graben center and suggest that northwest-directed dextral shear overprints previous basin extension. Cumulative right-slip for the whole graben is probably <1 km and is restricted to the southern half of the graben. Cessation of strike-slip faulting prior to intersecting the Cascade arc suggests that the arc acts as a thermal boundary and limits westward growth of the WLFZ.Keywords: Fault, Shear, Klamath, Walker Lane, Overprinting, Basin and Range, Geolog

Oligocene-Neogene lithospheric-scale reactivation of Mesozoic terrane accretionary structures in the Alaska Range suture zone, southern Alaska, USA

Terrane accretion forms lithospheric-scale fault systems that commonly experience long and complex slip histories. Unraveling the evolution of these suture zone fault systems yields valuable information regarding the relative importance of various upper crustal structures and their linkage through the lithosphere. We present new bedrock geologic mapping and geochronology data documenting the geologic evolution of reactivated shortening structures and adjacent metamorphic rocks in the Alaska Range suture zone at the inboard margin of the Wrangellia composite terrane in the eastern Alaska Range, Alaska, USA. Detrital zircon uranium-lead (U-Pb) age spectra from metamorphic rocks in our study area reveal two distinct metasedimentary belts. The Maclaren schist occupies the inboard (northern) belt, which was derived from terranes along the western margin of North America during the mid- to Late Cretaceous. In contrast, the Clearwater metasediments occupy the outboard (southern) belt, which was derived from arcs built on the Wrangellia composite terrane during the Late Jurassic to Early Cretaceous. A newly discovered locality of Alaska-type zoned ultramafic bodies within the Clearwater metasediments provides an additional link to the Wrangellia composite terrane. The Maclaren and Clearwater metasedimentary belts are presently juxtaposed by the newly identified Valdez Creek fault, which is an upper crustal reactivation of the Valdez Creek shear zone, the Late Cretaceous plate boundary that initially brought them together. / Ar mica ages reveal independent post-collisional thermal histories of hanging wall and footwall rocks until reactivation localized on the Valdez Creek fault after ca. 32 Ma. Slip on the Valdez Creek fault expanded into a thrust system that progressed southward to the Broxson Gulch fault at the southern margin of the suture zone and eventually into the Wrangellia terrane. Detrital zircon U-Pb age spectra and clast assemblages from fault-bounded Cenozoic gravel deposits indicate that the thrust system was active during the Oligocene and into the Pliocene, likely as a far-field result of ongoing flat-slab subduction and accretion of the Yakutat microplate. The Valdez Creek fault was the primary reactivated structure in the suture zone, likely due to its linkage with the reactivated boundary zone between the Wrangellia composite terrane and North America in the lithospheric mantle.This work was funded by grants from the American Association of Petroleum Geologists, Alaska Geological Society, Geological Society of America, and UC Davis Durrell Fund (T.S. Waldien), U.S. Geological Survey National Cooperative Geologic Mapping Program award #G16AC00206 (S.M. Roeske and T.S. Waldien), National Science Foundation (NSF) Tectonics award #EAR-1828737 to S.M. Roeske and #EAR-1828023 to J.A. Benowitz, and the State of Alaska Strategic and Critical Minerals Assessment program (E. Twelker)