ACTIVE DISPLACEMENT TRANSFER BETWEEN THE NORTHERN INTERMOUNTAIN SEISMIC BELT AND LEWIS AND CLARK SHEAR ZONE, NORTHEAST WASHINGTON, NORTHERN IDAHO, AND WESTERN MONTANA

The northern boundary of Basin and Range deformation lies east of the Oregon Cascade Range and trends across northeast Oregon to western Idaho, south around the Idaho batholith, and north into western Montana. To the south in the Great Basin, deformation is localized along the margins of the extensional province, but the distribution and rates of deformation in the north are poorly understood. Extension in eastern Oregon and western Idaho is related to displacements accommodated by the northwest trending Walker Lane and northeast trending central Nevada seismic belt that accommodate northwest displacement of the Sierra Nevada at ~14mm/yr. In the east, 3-5 mm/yr of displacement is concentrated along the eastern flank of the Great Basin where the locus of seismicity defines the Intermountain seismic belt (ISB) stretching from near Las Vegas, Nevada north to Missoula, Montana. Flathead Valley marks the end of seismicity along the ISB near the northwest-trending Lewis and Clark Shear zone (LCSZ) that stretches into northeastern Washington. Six bedrock sites of a GPS network deployed across the northern ISB and eastern LCSZ were occupied for 24 h each during annual campaigns from 1998 to 2001. Data were transformed to the ITRF2000 realization and processed with BERNESE (4.2) using IGS reference sites ALBH, DRAO, LIND, and NLIB. In a fixed North American frame, GPS velocities show 2-5 mm/yr of west to northwest displacement of sites in the northeastern Idaho batholith and Precambrian rocks west of the north-south trending Flathead Valley. Differential site velocities indicate ~3 mm/yr of north-south extension across the LCSZ but a strike-slip component of motion is unresolved. Site velocities of up to ~6mm/yr are recorded for two sites in a displacement transfer zone linking north-south striking half-grabens underlying the Bitterroot and Flathead Valleys. The half-grabens have opposing polarity and the GPS site motions may reflect clockwise rotation of a small fault bound block. Based on GPS velocities, extension rates are consistent between the northern and southern ISB and raise questions about the northern extent of deformation and the role of the LCSZ.