Mysteries of ductile shear zones

July 28th, 2011

I have to talk a little bit about my primary research project: how rocks deform deep (>10 km) underground.

Rocks as we see them are solid, hard objects.  They might shatter if you hit them with a hammer just right.  This, however, is only true if you are hitting the rock on the surface of the earth where the temperature and pressure are relatively low, and if you are using a lot of stress (impact) over a very short period of time.  Whenever those two conditions are not met, say, for example, deep underground, where the pressure and temperature are higher than surface conditions, and/or if slow-acting tectonic forces keep acting on the rocks over millions of years, then the rocks don’t get to shatter or crack. They act like silly putty or clay.  They either bend into folds, or get squeezed/stretched out depending on the nature of the forces acting on them.

Ductile shear zones, or shear zones for short are narrow bands in rocks where otherwise solid, rigid rocks somehow got stretched/squeezed like silly putty.  Those zones usually form deeper in earth’s crust and mantle, and therefore we don’t get to see them form until the rocks get exposed to the surface.  Some geophysicists think shear zones in the upper mantle play a huge role in all the effects of plate movements like earthquakes, mountain building or formation of new oceans that we see on the surface of the earth.  The funny thing is, no one has figured out yet exactly how those shear zones form to begin with. This is the primary focus of my research.  Right now I got Britt and Casey working with me in studying a shear zone that formed about 1800-2000 million years ago in what is northern Wisconsin today. Our field area is near Mountain, Wisconsin.  2000 million years ago there was indeed a mountain there, probably taller than today’s Rocky Mountains.  Today all we get to see are the roots of that mountain, and of course, the shear zones that formed during or just after the mountain was being built.

Different rocks have different minerals, and every mineral act differently when the squeeze is on, so to speak.  Some minerals are stronger than others and those will keep their shapes while other minerals change shape in response to applied forces.  Some mineral grains will rotate to align themselves better with the direction of applied forces.  All these makes for fascinating studies in figuring out the conditions under which the rocks got deformed… the temperature-pressure conditions, whether there was some fluids involved in the process, etc. etc.

Our current project is based on looking at the rocks from the Mountain Shear Zone under a petrographic microscope, chemically analyze them using X-Ray Fluorescence (XRF) and X-Ray Diffraction (XRD)… we have also used the ICP-OES at the Chemistry department for some really interesting look at the Rare Earth Elements in those rocks.  Besides chemistry, we are also using GIS to measure the degree of alignment of the different minerals as a proxy to measure the amount of shear stress it took to align the minerals just so.

I will post photomicrographs from thin sections and field photos of Mountain Shear Zones as we delve into the 2011 field season.  I’m excited about getting back to looking at these rocks with Casey and Britt.


3 Responses to “Mysteries of ductile shear zones”

  1. Anne C. Mancl Says:

    “Geology student working in prestigious internship at Smithsonian”

    Rhi–“You Rock” in an understatement. Keep up the good work!

  2. Rhi Says:

    Thanks! Unfortunately, the writer of that article made it seem as though *all* of this blog is mine. It’s actually a collaborative blog of the Geography & Geology department. Case in point, this post isn’t mine (I could never wrap my brain around shear zones as well as juk can!)

    Also, juk—your work sounds gneiss 😀

  3. juk Says:

    Thanks, Rhi. My work is collaborative as well… Britt, Casey and now Heather have made significant contributions to this research.

    I’m glad you find my work to be gneiss and not schist :)

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