Something sits under the SierrasThe Southern Sierra Nevada and the Los Angeles basin are some of the

Something sits under the SierrasThe Southern Sierra Nevada and the Los Angeles basin are some of the most complicated structural geology provinces in the U.S. They have rapidly transitioned from subduction and volcanism to extension and then finally shear, with the San Andreas transform fault forming ~6 million years ago and ripping apart the rocks as it grew.Not only has this sequence left a fairly pleasant climate, it has left an incredible array of geologic problems to reconstruct. Mountains have come up rapidly, others have turned sideways, and still others have sunk into the ocean. So, every time a new story comes out for the history of this region, it deserves attention.The “Isabella anomaly” is a unique feature in this area. It sits underneath and to the west of Lake Isabella, a manmade lake sitting in the southern Sierras above Bakersfield. The Isabella anomaly is a feature recognized in seismic tomographic studies of the region, where researchers use slight differences in the travel times of seismic waves to map out structure within the mantle they travel through.The Isabella Anomaly is labeled on this image. In this presentation, it appears blue, which is the color seismic tomographers employ to indicate fast seismic waves. Recognizing a fast seismic wave feature is actually the easy part; you just need seismographs and a computer to process the data into a 3-D picture. Figuring out what a slow or fast anomaly actually says about the geology is often a much harder problem since several different processes can give rise to a fast seismic anomaly like Isabella. Plotting it in blue/red makes me think of temperature, and yes, cold temperatures can give rise to fast seismic velocities, but rock types have variable seismic velocities as well.Out of this complexity comes the story I’m telling here; new interpretations for the Isabella anomaly. Research just published (see link below) argues that the Isabella anomaly is a chunk of a subducted slab stuck underneath the Sierras.For the last several hundred million years, a plate called the Farallon plate was subducting under the west coast of North America. However, sometime around ~20 million years ago, North America ran into the mid-ocean ridge between the Farallon plate and the Pacific plate, setting up the Pacific-North American boundary we see today. This transition isn’t a simple one, as small remnants of nearly-subducted plates can fracture, be torn apart, or even stick to the other plates involved. A small fragment of the Farallon plate in fact still exists off of California; it’s known as the Monterey microplate.Other plate fragments exist up and down the coast of Baja California, and several of those are associated with seismically fast (blue) regions as well. In those cases, it has been argued that the seismic anomalies are linked to the plate fragments. Basically, they are small pieces of the Farallon slab that got stuck to either North America or the Pacific plate as the subduction zone shut off.Based on similar seismic profiles, researchers led by Brown University have argued that the Isabella anomaly is another one of these. They argue it’s actually hooked to the Monterey microplate and has been dragged along as the San Andreas formed.It’s a plausible idea. Their seismic lines work and the idea that a microplate could get swept up in the San Andreas seems totally reasonable based on their comparisons to Baja.However, there’s a different story here that is worth sharing, and so I’ll wait to see more data on both. The other alternative is that this blob of “blue” that sits under the Sierras actually is a part of the Sierras.When big bodies of igneous rocks like the Sierra Nevada forms, it leaves behind large bodies of minerals that are too dense to reach the surface. We would call this pile of minerals a “cumulate” pile. The other idea for the southern Sierra segment is that this blob of blue is the remnants of the cumulate pile formed underneath the Sierras, caught in the act as it sinks away from the mountain range, like a blob in a lava lamp sinking to the bottom after it has cooled down.There is other evidence supporting the cumulate pile story. In the Tulare basin, just east of the Sierras, the land surface has moved up and down in very unusual patterns over the last couple million years. The area around Lake Tulare sunk and then rose back up while arches formed on either side, a very peculiar pattern. This pattern of uplift and downdrop actually makes sense if you have a giant body, like this cumulate pile, ripping off and pulling part of the crust with it. There are also unique volcanic outbreaks within and around the Sierras associated with this movement.To explain this behavior, the authors of the new model allude to density anomalies from their slab or unexplained changes associated with changing elevations in the Sierras. At least to me, the structural evidence seems to fit the sinking-cumulate hypothesis better, but the seismic connections to the Monterey microplate presented here can’t be ignored.I know in science the answer always seems to be “more data”, but this is definitely a disagreement on what this anomaly is and what it’s doing there. Right now, the best we can do is tentatively link things together in seismic tomography, but to really assemble that story, we need to hook the tomography to surface geology, to something we can see. A fragment of the Farallon plate dragged north along with the San Andreas ought to do plenty of damage, and that’s the kind of geologic signature we can look for, buried within the shattered rocks of the L.A. basin.-JBBLink to original articlehttp://www.pnas.org/content/early/2013/03/13/1214880110Press summaryhttp://www.sciencedaily.com/releases/2013/03/130318180438.htm -- source link

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