Subduction in the Western Mediterranean!

The Betic Cordillera of southern Spain was formed by the slow convergence between Earth’s Eurasian and African plates since Mesozoic time, about 70 million years ago. The range is more commonly referred to as the Sierra Nevada de España. The peaks in this range are the highest in the country, rising up to nearly 3500 meters. The tallest peak, the Mulhacén, houses the tomb of Muley Hacén, one of the last Muslim rulers of southern Spain during the 15th century.

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The snowcapped Sierra Nevada.

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The Sierra Nevada is an east-west striking range and parallels the Spanish Costa del Sol along the Alboran Sea (to the south). Click the image to see better resolution!

Although the mountains here rise high above the Alboran Sea, the rocks were formed at great depths below Earth’s surface – some as deep as 50 km — during subduction of the Eurasian plate beneath Africa. So how did these high-pressure, metamorphic rocks return to the surface? Although it may sound counterintuitive, when two tectonic plates converge, Earth’s crust can also extend. Additionally, when a plate is moving downward through Earth, material on top of the plate can become buoyant and flow back towards the surface. Extensional faulting and subsequent erosion have now exposed eclogite and blueschist high in the Sierra Nevada.

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An eclogite! John Platt’s hands for scale.

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This rock was deformed heavily on its way back to the surface. The way to the top is not easy. The bands of light-colored rock are marble, the green bands are eclogite.

Eclogites are metamorphic rocks that form at high pressures and temperatures. Whereas, blueschist forms at high pressures and relatively low temperatures as far as Earth is concerned. They are unique because they form when oceanic crust (mostly made up of basalt/gabbro) subducts into the mantle. If an eclogite is returned to the surface quickly enough, it can preserve minerals that formed at the peak conditions of pressure. When they are analyzed in the right way, the minerals found in eclogites and blueschist can tell us when they formed. If we know when the formation of high-pressure minerals occurred, we can better constrain the timing of subduction and the geologic history of the Western Mediterranean region as a whole.

When the minerals found in eclogites and blueschist first form, they lock a variety of elements into their molecular structure. One such element is Rubidium. Rubidium is radioactive and decays to the element Strontium. Once the garnet forms and becomes stable, a Rb-Sr “clock” is set. Millions of years later, it’s possible to measure the ratios of Rubidium to Strontium in each individual mineral, using a mass spectrometer, to determine the age of the rock. However, a variety of conditions must first be met to confirm that the rock actually formed at the peak pressure conditions during subduction.

The first steps of the process are to go outside, pick up the rocks, take them home, and saw them up. The slices of rock are then polished into sections that are 30 microns thick (how small is a micron?). Thin sections are mounted on a glass slide, and viewed under an optical microscope in plane or cross-polarized light. Once under the microscope, I look for evidence to show that the high-pressure minerals are still preserved after their long journey back to the surface. Oftentimes the minerals can become unstable and change to reflect lower temperatures and pressures. In order to date the original high-pressure metamorphism, the high-pressure minerals need to be unaffected by later metamorphic events.

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This one should do…I wonder what it will look like under the microscope?

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BAM! This is a thin section of a high-pressure schist viewed with cross-polarized light. The large (apparently) black mineral is a garnet. The brightly colored minerals are white mica, and the lightly colored minerals within the garnet are quartz inclusions.

Even after good mineral candidates have been identified under the optical microscope, they must also be observed in greater detail under the electron microscope. And you thought the TSA was invasive… Here is an example of one type of analysis I do on garnets in my rocks, using the electron microscope:

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This is a map showing the relative concentrations of the element Manganese (Mn) in the garnet shown in the previous photo. Electron beams excite elements in the mineral and cause them to emit X-rays. The X-rays are then measured by wavelength dispersive spectroscopy, which can identify elements based on the wavelength of their emitted X-rays. The warm colors indicate higher concentrations of the measured element.

The fact that the Mn in this garnet is concentrated at the core of the garnet is a good thing. It suggests that the garnet grew during one event and has not been altered much since its original growth. If the garnet had the time to sit at a high temperature on its way back up to the surface, the Mn may have had time to diffuse outward. If that was the case, the map might look more monochromatic because the Mn would be evenly distributed throughout the whole garnet. If Mn was able to diffuse out of the garnet, that means that Rb and Sr could also escape! If Rb and Sr escape, that means the “clock” is reset until the garnet becomes stable enough to hold elements in place again.

After the best samples for dating have been identified and given their respective background checks, the dating process can begin! The rock must be crushed up into small pieces first, and then have its minerals meticulously separated. This is a lengthy procedure that involves hammers, jaw crushers, disc mills, sieves, magnets, and tweezers. Aren’t you glad you don’t have to go through that before someone will date you? Rocks have a tough life…or maybe I have high standards.

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These are white mica crystals — half a millimeter wide, at the largest. Some of them contain inclusions of other minerals. The goal is to pick out the ones that are purely mica with a pair of tweezers.

Finally, the separated minerals can be analyzed on a mass spectrometer. This analysis yields a Rb and Sr concentration for each mineral that can be plotted as an isochron – a linear function where the slope of the line represents the age of the rock. Here is a link to a nice explanation of how the isochron works, ironically taken from a website that discusses the ‘creationism vs. evolution’ debate.

So, now you know what I’m spending my time doing here in Austin, and hopefully you learned something new! I’ll be working on determining ages for the rocks that Whitney and I collected over the next few months. My goal is to present some new and interesting data at the American Geophysical Union conference in San Francisco this December! Hasta Luego!

For more stories and photos of the field work behind this project, check out the projects page!

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2 responses to “Subduction in the Western Mediterranean!

  1. /NICE WORK !!!/

    David L.Kirchner BASIN & RANGE HYDROGEOLOGISTS,INC. 4400 N. Scottsdale Road, #9902 Scottsdale, Arizona 85251-3331

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