Eastern Peninsular Range Field Work
I study exhumed fault zones with the hopes of understanding some of the controls on fault rupture initiation and termination. One of the problems with exhumed fault zones is that there are not many ways to determine whether a fault has slipped coseismically in a rapid slip event, or if it simply accumulated its offset through slow creep. Pseudotachylites, a rock that I like to call "nature's skid marks," are pretty much the only definitive evidence of seismic slip. These pictures are some of the highlights from a reconaissance trip that I took to the Eastern Peninsular range near Palm Springs. I hope to return there this summer to do more extensive field work.

The view standing on the main detachment fault in the region and looking south.

The view looking towards the north back at the main detachment fault.

These straight, nearly parallel joints were a rare find in the jumbled deformation of the Eastern Peninsular Range. Some of them have been reactivated and have pseudotachylite along their surfaces while some do not. Tape open 1 m and points north.

For contrast, this example shows pseudotachylites that have been folded. There are valuable clues about the depth, temperature, and pressure conditions at which the pseudotachylites formed. Such information could help us understand the factors which led to their formation. Tape open 10 cm and points north.

And, an interesting combination of straight and curved... This pseudotachylite fills a slightly curved fracture until it terminates in the center of the image. At that point, the fracture bifurcates and one branch appears contiguous with an unfilled fracture. To complicate matters, the pseudotachylite is cut by a series of curved parallel fractures. Some of these have show activation as faults and offset the pseudotachylite. Tape open 10 cm and points north.

Taken in the vicinity of the last image, this picture records a relatively large-offset fault zone (red pencil points to gouge zone in center of image). Thick (> 3 cm) pseudotachylites follow a similar trend in the outcrop as this fault zone. In the field, the hanging and footwall of the fault appeared to be similar granitic rocks but this image makes them look like different lithologies. Tape open 10 cm and points north.

Me walking on a natural path on the desert pavement at the end of the day..

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