Paleomagnetism of Eocene Basalts of Mayreau, West Indies: Implications for Contrasts in Tectonic Rotation in the Southeastern Caribbean

Abstract

Paleomagnetic directions were determined from 29 sites on the Grenadine island of Mayreau. Most data come from the middle Eocene Mayreau Basalt, which has been interpreted to be oceanic crust of the Grenada Basin uplifted in the half-horst of the southern Lesser Antilles arc platform. The objective was to assess the existence in the Mayreau Basalt of anomalies in paleomagnetic declination and inclination relative to Eocene South America and to other terranes in the Caribbean-South American plate-boundary zone. Primary and early deuteric magnetic directions were extracted that yielded site-means with α95 ≤ 15° from 15 sites in the Mayreau Basalt. Demagnetization removed a pervasive north-down component of low unblocking temperature. Tests demonstrated that the Eocene rocks were not remagnetized during Neogene island-arc magmatism and that the magnetization occurred before folding. The mean of site-means for the Mayreau Basalt is not different at the 95% confidence level from expected Eocene directions at Mayreau, from either North or South America. Therefore, there has been no net vertical-axis tectonic rotation or latitudinal transport of the Mayreau Basalt relative to the continents. Moreover, the eastward translation of the Grenada Basin as part of the Caribbean plate relative to the American plates must have been about a Euler pole that is geographically nearly 90° from the Grenada Basin. This result contrasts markedly with the large declination anomalies relative to South America in Tobago, Aruba, and other localities in the plate-boundary zone bordering northern South America. If the declination anomaly in Tobago is Eocene or younger and results from vertical-axis rotation in the plate-boundary zone, the difference in motions between Tobago and Mayreau must have been taken up at the southern Grenada Basin deformation front, with right-oblique subduction of Grenada Basin lithosphere and large rotation of the Tobago terrane.