A paleomagnetic study of the essentially undeformed middle Tertiary carbonate sequence along the north coast of Puerto Rico reveals statistically signicant pre-Pliocene discordance of characteristic component directions against those expected from cratonic North America for much of the section. Despite generally weak to moderately weak magnetic intensities, confirmation of the magnetization as primary in origin comes from the presence of two distinct components of magnetization, intrasite bipolarity, and/or the reproducibility of measurements.

The mean geographic direction for the upper Oligocene to middle Miocene strata is 335.2¿/32.9¿ and the corrected mean paleomagnetic pole is 207.6¿/66.5¿, (N=3, &agr;95=4.3¿). This suggests a counter-clockwise (CCW) block rotation of Puerto Rico and its microplate of 24.5¿ (¿5.8¿) during the late Miocene. Using a width of 250 km for the Northern Caribbean Plate Boundary Zone (NCPBZ) between the North American Plate and Caribbean Plate, the mean left lateral displacement implied is 1.8 to 2.4 cm/yr, which agrees fairly well with published relative motion rates for the two plates. Average rotation rate for 50 Ma to 20 Ma was 0.7¿/my but perhaps as great as 4¿/my in the Miocene. Resolution of mean paleolatitude indicates northward motion of a degree or less during the period of rotation.

Causes of this short-lived rotation may include (1) tectonic escape from the inhibiting presence of the Bahama Banks and Beata Ridge during eastward motion of Puerto Rico along the sinistral transpressive Puerto Rico Trench and Muertos Trough fault systems or (2) changes in relative plate motions of the Caribbean and North American Plate during the late Miocene. ¿American Geophysical Union 1991