Monday, October 12, 2009

The "flipping fault"

Returning now to some of the stuff I saw when I was out in Bishop, California for the GSA Field Forum I attended in September. One of the cool little spots we visited was "the flipping fault," a location on the Volcanic Tableland north of Bishop where an east-dipping fault scarp dies out and a west-dipping fault scarp starts up. Check it out:
22_2

Here, try one with annotations:
22_3

Here's a Google Map of the location, as seen from the perspective of a passing turkey vulture:

Notice how the road, Casa Diablo Road, goes right through the notch where the two meet. Complicating the picture a wee bit is a Pleistocene drainage channel which uses the same route between the two scarps (and diverges from the road in the lower-left).

Another view, further back and higher up:
22_1

And of course we must annotate that one too:
22_4

Recall that these are normal faults busting through the Bishop Tuff's upper welded layer, the "Ig2." In the annotations, I've sketched in the approximate position of the "hanging-wall cutoff" (lower boundary of each scarp) and the "foot-wall cutoff" (upper boundary of each scarp).

There are roughly equal numbers of east-dipping and west-dipping faults on the Volcanic Tableland. Originally, some creative structural geologists wanted to interpret this feature as an overall "propeller" shaped fracture: a so-called "flipping" fault (as in, it's one single fault that flips its dip direction in the middle). However, this was not the interpretation of our workshop leaders, who suggested that it was simply two faults that started independently and then propagated towards one another.

Taking a fresh look at these images now, almost a month after I visited the outcrop, I find that I agree with them. One thing that seems obvious to me now is how the east-dipping fault truncates on the face of the west-dipping fault scarp. My annotations reflect this interpretation. What do you think?

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2 Comments:

Blogger Lockwood said...

In the GE view, the two segments do seem to have distinctly different strikes- not hugely different, but distinctly. As for the wedge on the east (west-facing) side of the road, it looks like it could be rock fall/talus cone rather than a tectonic feature. But that is pretty interesting, whatever the details.

October 12, 2009 2:12 PM  
Blogger Callan Bentley said...

The "talus cone" notion occurred to me too -- but check out the land to the left and the right of the "talus cone" -- there's clearly an offset there. So that's why I went with the interpretation that it's actually part of the est-facing fault itself.

October 12, 2009 3:19 PM  

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