Neal Ringerwole ACF Abstract FY13

New Pliocene Paleomagnetic Constraint on Tobago's Rotation History

2012 Geological Society of America (GSA) Annual Meeting and Exposition

We present results continuing previous paleomagnetic work on the rotational history of Tobago. Earlier published paleomagnetic work showed that Tobago has experienced approximately 90° of clockwise, finite, vertical axis rotation since the Albian (108 Ma), however, an incremental rotation history is still lacking. The goal of our work is to incorporate newly acquired paleomagnetic data from the Pliocene Rockley Bay Formation, exposed in south Tobago. The Pliocene of Tobago is unstudied from a paleomagnetic perspective. The Rockly Bay Formation is of the right age (2-5Ma) to add a younger constraint to the earlier published work. This muddy, fossiliferous, marine carbonate has a relatively stable, low-coercivity, magnetic remanence. We collected paleomagnetic data from three large, oriented hand specimens, by producing 35 core plugs that were analyzed using AF-demagnetization, thermal demagnetization, and then tested for susceptibility. Following AF-demagnetization, the Bingham average paleomagnetic poles for each of the large sample are 359.6°, 21.4°; 351.7°, 21.3°; and 357.5°, 25.8°. The magnetic mineralogy of the Rockley Bay Formation is likely characterized by primarily pseudo-single-domain magnetite with an added component of an antiferromagnetism, which is likely due to hematite, geothite, or possibly mixtures of the two. A thermal susceptibility bridge was also used to determine mineralogy, however, unexpected thermo-chemical alterations observed during the heating and cooling rendered these tests inconclusive. Additional chemical tests (dissolution, XRD, SEM) are in progress. These results suggest that Rockley Bay Formation carbonates we collected carry a primary magnetization that shows that Tobago’s bulk tectonic rotation ceased by the Pliocene (2-5 Ma). This could have resulted from the late Neogene change from oblique convergence to dextral transform motion the Caribbean-South American plate boundary.

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