Sunday 25 March 2012

Understanding the subduction zone beneath Mexico.

The Middle American Trench, in which the Cocos Plate is subducted beneath the North American Plate, runs parallel to the south coast of Mexico. Like many subduction zones, this feeds a chain of volcanoes, the Trans-Mexican Volcanic Belt, as the subducting plate is melted by the heat of the Earth's interior and melts, some of the melted material then rising through the overlying plate as magma, feeding the volcanoes of the Volcanic Belt. However this Volcanic Belt runs parallel to neither the subduction zone nor the coast, but runs across Mexico at an angle, which requires an explanation.

The Trans-Mexican Volcanic Belt is at an angle to the Middle American Trench. Solleiro et al. (2007).

In a forthcoming paper in the Journal of Geophysical Research, Igor Stubailo, Caroline Beghein, Paul Davis, all of the Department of Earth and Space Sciences at the University of California Los Angeles, discuss a new three-dimensional model of the subduction zone beneath Mexico, based upon data from seismic monitoring stations across the country belonging to the Mesoamerican Seismic Experiment, the Network of Autonomously Recording Seismographs, the USArray, Mapping the Rivera Subduction Zone and the Mexican Servicio Sismologico Nacional.

The Cocos Plate is thought to have formed about 23 million years ago, when the Farallon Plate, an ancient tectonic plate underlying the East Pacific, split in two, forming the Cocos Plate to the north and the Nazca Plate to the south. Then, roughly 10 million years ago, the northwesternmost part of the Cocos Plate split of to form the Rivera Plate, south of Beja California.

The position of the Cocos, Nazca and Rivera Plates. MCEER/University at Buffalo.

The seismic monitoring stations were able to monitor not just Earthquakes in Mexico, but also Earthquakes in other parts of the world, monitoring the rate at which compression waves from these quakes moved through the rocks beneath Mexico, and how the structure of the rocks altered the movement of these waves.

Based upon the results from these monitoring stations, Stubailo et al. came to the conclusion that the Cocos Plate was split into two beneath Mexico, and that the two plates are subducting at different angles, one steep and one shallow. Since the rate at which a plate melts reflects its depth within the Earth, the steeper angled plate melts much closer to the subduction zone than the shallower angled plate, splitting the Trans-Mexican Volcanic Belt into sections above the different segments of the Cocos Plate, and causing it to apparently curve away from the subduction zone.

Top the new model of the Cocos Plate beneath Mexico, split into two sections (A & B) subducting at differing angles. (C) Represents the Rivera Plate, subducting at a steeper angle than either section of the Cocos Plate. The Split between the two has been named the Orozco Fracture Zone (OFZ) which is shown extended across the Cocos Plate; in theory this might in future split the Cocos Plate into two segments (though not on any human timescale). Bottom Left, the position of the segments on a map of Mexico. Darker area is the Trans-Mexican Volcanic Belt, orange circles are volcanoes, brown triangles are seismic monitoring stations, yellow stars are major cities. Bottom Right, an alternative model showing the subducting plate twisted but not split. This did not fit the data. Stubailo et al. (2012).