Tuesday, June 18, 2013

Car crash tectonics - when plates meet

Researchers from Colombia and Houston have identified a "nest" of seismic activity beneath Colombia which helps understand a complex pattern of collision between Colombia and Panama. The Nazca oceanic plate, to the west of Colombia, is subducting eastwards beneath the South American Plate, while at Colombia's northern shore the Carribean oceanic plate is also subducting beneath South America. In between lies the Panama block, an old island arc, which is pushing into Colombia from the west.

The Panama arc acts like an indenter, sitting amid this tectonic maelstrom of convergence. As a thicker block of crust it does not subduct easily, but instead pushes head on into north west Colombia. Some crust in front of it appears to have broken off. It is subducting and can be imaged from variations in the attenuation of seismic waves, since it is colder and "rings" like a bell more strongly than the warmer (dull and attenuating) material through which it subucts. Associated with this torn of portion of Panama is a dense area of seismic activity about 140km below and in front of the collision. Large strike slip faults form a V-shaped pattern in the Colombian crust above, symmetric about the Panama block, and small volcanoes line up above the torn-off crust. The distribution of mineral deposits, hydrocarbons and geometry of sedimentary basins seem all to be controlled by the complex tectonics of collision and convergence in this part of north west Colombia.

The research is reported in this month's issue of the Bulletin of the Seismological Society of America. The complexity of the tectonics of the region may point to a previously unrecognised increased tectonic risk, with accumulation of stresses from the frontal collision of Panama with Colombia as yet un-triggered.

Image: Nevado del Ruiz is a volcano sitting above the broken off crust lying beneath Colombia and ahead of the Panama block, by Wojtek Ogordowczyk, Creative Commons

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