SIERRA NEVADA-CASCADE RANGE BOUNDARY ZONE, NORTHEASTERN CALIFORNIA –NEWLY RECOGNIZED QUATERNARY STRUCTURAL BOUNDARY OF THE NORTHERN SIERRAN MICROPLATE

The northern Sierran microplate terminates sharply at the WNW-striking Sierra Nevada - Cascade Range boundary zone (SN-CRBZ). The newly recognized and broadly distributed, microplate-boudary splays from the northern Walker Lane near Quincy and abruptly terminates westward against the Inks Creek fold-and-thrust belt near Red Bluff. The SN-CRBZ terminates the 300-km-long Sierra Nevada Foothills fault system, as well as, the 1,100-km-long Cascade Range volcanic province. The distinct southern margin of the boundary zone is remarkably linear for more than 115 km, which is equaled by only the two most-active, strike-slip faults in the Walker Lane (Owens Valley and Furnace Creek fault zones). This and the northern margin are consistently expressed by NNE-facing escarpments, some reflecting ½ km (or more) normal offset of ‘layer-cake’ sequences of Plio-Pleistocene Tuscan Formation. Hence there is a regionally significant, normal-slip component that post-dates the volcanic sequence and relatively uplifts the northern Sierran microplate. Recent geodetic and seismic-inversion studies (e.g., Hammond and Thatcher, 1997; Unruh et al., 2003) show the contemporaneous strain field accommodates 2-4 mm/yr of WNW-directed dextral or transpressional-right shear. Although, transpressional deformation is recognized near either end of the southern margin it appears to reflect local fault interactions. In addition to its linearity and orientation, a predomionant lateral-slip component is indicated by four drainage canyons deeply incised into the volcanic sequence that each exhibit a right-lateral deflection across the southern margin, suggesting 2 to 2½ km of possible Quaternary right slip. The southern margin is initially characterized as having a longterm right-lateral slip rate of 1.7 +1.0 mm/yr and a longterm normal-slip rate of 0.8+0.5 mm/yr, suggesting a lateral-to-vertical-slip ratio of about 2:1, or higher. In addition to uplifting and defining the linear edge of the northern Sierran microplate, its structural connections and optimal orientation indicate that the SN-CRBZ accommodates a westward transfer of Walker Lane dextral shear, driving Quaternary crustal shortening in the northern Central Valley.