CUE_2013.pptx

Abstract

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