EVIDENCE FROM THE INDEPENDENCE DIKE SWARM FOR 65 KM OR MORE OF POST-JURASSIC DEXTRAL OFFSET ACROSS OWENS VALLEY, CALIFORNIA

The Mesozoic Independence dike swarm (IDS) is found on both sides of Owens Valley. Its apparent continuity led early workers to suggest little or no strike slip across the valley. Here we present evidence that the IDS is dextrally offset at least 65 km and may be offset as much as 130 km.

We measured orientations and thicknesses of dikes along 500-1000 m traverses at 73 sites on both sides of the valley north of the Garlock fault. Some measured dikes may be Cretaceous or Middle Jurassic but most are Late Jurassic (148 Ma). Measured dilations in denser parts of the swarm are typically 8-30% and range as high as 44% in the Woods Lake-Split Mountain area along the Sierran crest. Those dikes project across Owens Valley (mean strike 130) to the Inyo Mountains where dikes are largely absent and measured dilations range from zero to a few percent. High dilations are found in the northern Alabama Hills between the Sierra Nevada and the Inyo Mountains, but east of Owens Valley only in the southern Coso Range, southern Argus Range, and Spangler Hills. It is difficult to assess whether the dikes in the Coso Range are absent directly across the valley because the Sierra Nevada there is dominated by poorly dated Late Jurassic and Cretaceous plutons. If the axes of greatest dilation were once aligned, then they have been dextrally separated by 65-130 km depending on which features are realigned. The Alabama Hills swarm is offset 65 km across Owens Valley proper from the dense swarm in the Coso Range. The northern edge of the dense swarm in the Sierra Nevada lies another 65 km farther northwest, and thus there may be up to 65 km of additional separation, much or all of it between the Alabama Hills and the Sierra Nevada. At the southern end of Owens Valley, faults responsible for the separation must run through the Little Lake area and east of Paleozoic rocks in the El Paso Mountains.

Kylander-Clark et al. (this volume) propose that distinctive 83 Ma dikes were separated 65 km across southern Owens Valley. If the IDS is separated by 65 km then all motion was post-83 Ma. Alternatively, if the IDS is separated a greater amount then the additional motion was pre-83. These offsets are comparable to offset of Paleozoic features noted by Stevens et al. (1997). How this motion was accommodated to the south and how it may interact with the Garlock fault are unknown.