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http://www.eurekalert.org/pub_releases/2009-07/amon-jsl072709.php

A new analysis of jade found along the Motagua fault that bisects Guatemala is underscoring the fact that this region has a more complex geologic history than previously thought. Because jade and other associated metamorphic rocks are found on both sides of the fault, and because the jade to the north is younger by about 60 million years, a team of geologists posits in a new research paper that the North American and Caribbean plates have done more than simply slide past each other: they have collided. Twice.

"Now we understand what has happened in Guatemala, geologically," says one of the authors, Hannes Brueckner, Professor of Geology at Queens College, City University of New York. "Our new research is filling in information about plate tectonics for an area of the world that needed sorting."

Jade is a cultural term for two rare metamorphic rocks known as jadeitite (as discussed in the current research) and nephrite that are both extremely tough and have been used as tools and talismans throughout the world. The jadeitite (or jadeite jade) is a sort of scar tissue from some collisions between Earth's plates. As ocean crust is pushed under another block, or subducted, pressure increases with only modest rise in temperature, squeezing and drying the rocks without melting them. Jade precipitates from fluids flowing up the subduction channel and into the chilled, overlying mantle that becomes serpentinite. The serpentinite assemblage, which includes jade and has a relatively low density, can be uplifted during subsequent continental collisions and extruded along the band of the collision boundary, such as those found in the Alps, California, Iran, Russia, and other parts of the world.

The Motagua fault is one of three subparallel left-lateral strike-slip faults (with horizontal motion) in Guatemala and forms the boundary between the North American and Caribbean tectonic plates. In an earlier paper, the team of authors found evidence of two different collisions by dating mica found in collisional rocks (including jade) from the North American side of the fault to about 70 million years ago and from the southern side (or the Caribbean plate) to between 120 and 130 million years ago. But mica dates can be "reset" by subsequent heating. Now, the authors have turned to eclogite, a metamorphic rock that forms from ocean floor basalt in the subduction channel. Eclogite dates are rarely reset, and the authors found that eclogite from both sides of the Motagua dates to roughly 130 million years old.

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