Shutter ridges

A reader was unclear on the concept of shutter ridges, so I thought I’d try to show it as well as tell it. Look closely at this excerpt from the Oakland geologic map covering Lake Temescal.

shutter ridge map

The lake is the blue blob near the top. The Hayward fault slashes through it and across the map from top to bottom. The left (west) side moves north with every major earthquake on the fault. The blue area labeled KJfm (Cretaceous-Jurassic Franciscan mélange) is part of what I refer to as the Piedmont block; it makes up the ridge you see across the lake:

lake temescal

As that ridge moves north, it cuts off the course of Temescal Creek and forces it to flow north to get around it. That’s where the “shutter” term comes from—the ridge barrier moves like the shutter of an old-fashioned box camera. You can see on the map how Temescal Creek flows today, after tens of thousands of years of this process: it comes downhill on the right edge of the map, jogs to its right for almost a mile, goes through Lake Temescal, then turns left around the curve of Route 24 (the double purple line) in a culvert to resume its course to the bay. See the ridge from another perspective in this post.

Another excellent example is on the fault just north of the Oakland Zoo, where Arroyo Viejo comes down Golf Links Road and makes a similar jog around the hill of Toler Heights before resuming its bayward course under 82d Avenue:

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4 Responses to “Shutter ridges”

  1. rob Says:

    !!! holy smokes!

    thanks for that explanation! so although this feature must be common along every fault, is it only called a shutter ridge when there is a creek/river involved?

  2. Andrew Says:

    Not at all–having a stream involved just makes it easier to comprehend. There’s a small but excellent example, without any stream, on the fault just south of Dunsmuir House.

    Also, a shutter ridge can only exist on a horizontal, strike-slip fault like the Hayward or San Andreas. Most of the world’s faults are dip-slip faults, meaning the sides move up or down.

    See the basics of faults here.

  3. Todd Says:

    Andrew,
    Would you mind explaining more the fault around the Oakland zoo? I have always wondered about that curve in the freeway, but where exactly is the “shutter”? Is another fault meeting the Hayward fault?
    Thanks

  4. Good example of shutter ridges Says:

    Dear Sir
    you have given convencing example of shutter ridges that are comparable with my research area.

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