CHARACTERISTICS OF THE SIERRA NEVADA FRONTAL FAULT ZONE AND ADJACENT TECTONOGEOMORPHIC DOMAINS ASSOCIATED WITH PROPAGATION OF THE PAC-NA PLATE BOUNDARY

The Sierra Nevada (SN) is recognized as a rigid, west-tilted block. The northern end lies ~ west of the Mendocino Triple Junction and the southern end lies ~ west of the Murray Fracture zone, thus influence from the subducted oceanic slab on the block dimension and orientation is likely. The block is approximately 700 km long and ~125 to ~200 km wide. The east facing escarpment, the Sierra Nevada Frontal Fault Zone (SNFFZ) is progressively more structurally developed and geomorphically distinct north to south. The geometry and dimension of basins adjacent to the SNFFZ are controlled by Plio-Pleistocene transtensive structures. Four distinct geomorphic domains lie east of the SN block; the character of the bounding SNFFZ escarpment and adjacent basins varies by domain. The northeast end of the SNFFZ is partly buried by active volcanism of the southern Cascades and adjacent basins are filled or poorly developed. The northern extent of the Walker Lane (WL) dies out in this region. To the south, the basin sizes are moderate, 25-45 km x 15-10 km, with narrow 8-12 km wide mountain ranges mainly oriented N-S to NNE. These basins form, subparallel arrays in discreet zones trending about 300° that may have cw rotation. Relief across the escarpment is high to moderate. This is succeeded by a releasing stepover domain ~ 85-100 km wide, where the basins are elongated E-W to ENE, small (~15-30 km long, 5-15 km wide) and locally occupied by active volcanic centers. Here, the escarpment is strongly segmented, discontinuous with generally low to moderate relief where faulted. The southernmost SNFFZ is well organized, continuous with high to extreme relief. Adjacent basins are elongate, 50 –200 km long and ~5 –20 km wide. The southernmost domain lies west of the southern Walker Lane and north of the Eastern California Shear Zone sensu strictu, and is here referred to as the Eastern Sierra Deformation belt (ESDB), to distinguish it from the contrasting geomorphic landscape and structural style south of the Garlock Fault and the SN block. Although, GPS results north and south of the Garlock indicate similar or nearly identical strain rates, the geomorphology strongly indicates different mechanical behavior of the crust in this region. The southernmost WL significanly, is less tectonically active where it lies due east of the ESDB than to the north.