Geology

A Texas-Sized Collision

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Science  17 May 2002:
Vol. 296, Issue 5571, pp. 1203
DOI: 10.1126/science.296.5571.1203b

The supercontinent Rodinia, an accretion of all the continents, existed between 1200 and 600 million years ago (Ma). One piece of ground truth for this tectonic puzzle is the Grenville Province, a region of metamorphism and deformation that runs from Labrador to the Llano Uplift in Texas and formed the coastline of Laurentia. About 1200 Ma, the Grenville province collided with a cratonic piece (Amazonia) of South America. The rocks in the uplift provide evidence for this long-ago collision; however, it has been difficult to identify which piece of Amazonia collided with this proto-Texas region.

Tohver et al. studied a large sequence of gabbros and basalts from the Nova Floresta formation of Rondônia in western Brazil. These rocks are part of the Amazon craton, and Ar-Ar dating indicates that they are 1200 million years old. Magnetic characteristics of the rocks were used to determine the paleomagnetic pole and to fix the paleogeographic position of Amazonia. It appears that around 1200 Ma, one corner of this craton (oriented roughly 180 degrees from its present-day position) collided with the proto-Texas region and created the Llano Uplift.—LR

Earth Planet. Sci. Lett.199, 185 (2002).

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