Deformation History of the Garlock Fault, Eastern California

Previous work shows that the Garlock fault is a left lateral strike-slip fault that initiated in Miocene time as a transform between the Mojave Desert and Basin and Range province; its role in the current transtensional regime is less clear. Although numerous markers show about 65 km of net left slip, the distribution of slip and deformation style through time is not well established. We present below new results that better elucidate its deformation history.

Stratigraphic comparisons of Miocene strata in the Red Rock Canyon area of the El Paso Mountains north of the Garlock fault to rocks in the Summit Range, Pilot Knob Valley, and Eagle Crags areas to the south bracket the slip history from about 17 to 11 Ma. Ties indicate that motion probably did not initiate until about 15 Ma, and ~30 km of left slip occurred by about 12 Ma. This gives an overall slip rate around 10 mm/yr, comparable to estimates of extension rate in the Basin and Range. Ties from 12 Ma to 7 Ma are loose, and are compatible with negligible to 15 km of continued left slip. Assuming uniform slip from 12 Ma to present yields a rate of about 3 mm/yr, much less than the current ca. 7 mm/yr or greater rates determined by other workers suggesting that deformation over this period is non-uniform.

Active deformation along the Garlock is complex, with largely strike-slip motion evident in its western reaches. The Garlock from the eastern El Paso Mountains to the Slate Range, coincident with the locus of transtension in the eastern California shear zone, shows significant N-S contraction in Pleistocene and Holocene strata and surfaces and appears to be divided into several strands and segments. This may imply that the Garlock is currently mis-oriented for coordinated strike-slip motion.