EVALUATION OF RIGHT-LATERAL SHEAR ACROSS THREE NW-TRENDING NORMAL FAULTS NORTHWEST OF THE EASTERN SNAKE RIVER PLAIN, IDAHO

Three Quaternary NW-trending normal faults (Lost River, Lemhi, and Beaverhead) within the Centennial Tectonic Belt (CTB) appear to terminate at the northwestern boundary of the Eastern Snake River Plain (ESRP). The northern and central segments of the three normal faults are seismically active, but their activity decreases southward toward the ESRP. Deformation in the ESRP is associated with infrequent small magnitude (M<2) microearthquakes and NW-trending volcanic rift zones that result from basalt dike intrusion since 6 to 4 Ma. GPS results from 1994-2009 indicate that the extension rate across the NW-trending normal faults is an order of magnitude faster than in the ESRP. A NE-trending zone of right-lateral shear, referred to as the Centennial Shear Zone (CSZ), is a geometric consequence of the differential strain rates. The inferred slip rate in the CSZ of 1-2 mm/yr suggests up to 2 km of lateral offset in a million years. Within the footwalls of the NW-trending normal faults, investigators have identified only NE-trending normal faults without observable strike-slip offsets. The most recent and larger fault offsets are associated with extension on the NW-trending normal faults (e.g., 1983 Ms 7.3 Borah Peak, Idaho earthquake). To test ideas of how right-lateral shear is accommodated, we use a block model and invert horizontal GPS velocities, earthquake slip vectors and fault slip rates for relative motions. We find that the data do not support either a transtension model (e.g., McKenzie and Jackson, 1986) in which the NW-trending normal fault blocks are pinned at their northwest and southeast ends and accommodate shear through oblique slip and clockwise rotation, or a “fan” model in which the blocks are pinned at their northwest ends only. The data are fit best by a model in which right-lateral shear is accommodated along NE-trending faults at the northwest and southeast ends of the normal faults with minor components of oblique slip on the normal faults. This model requires very little clockwise rotation in the footwalls and is supported by paleomagnetic data, which suggest a clockwise rotation rate <1 degree/m.y. in the footwalls of two of three normal faults.