PALEOMAGNETIC EVIDENCE OF VERTICAL AXIS ROTATIONS IN A LEFT-LATERAL STRIKE-SLIP SHEAR ZONE: NORTHEASTERN DOMINICAN REPUBLIC

A paleomagnetic study of Hisponiola Island (Dominican Republic) was conducted to determine vertical-axis rotations (VAR) of Early Paleocene to Holocene sedimentary rocks proximal a left-lateral strike-slip fault (Septentrional fault). The North American plate slides westward past the Caribbean plate at a rate of about 20mm/yr. The Septentrional fault system accommodates part of plate motion in Northern Hispaniola with slip rates that range from 13 to 23 mm/yr. Samples from 21 sites were demagnetized both thermally (up to 680ºC) and by alternating field (AF) (up to 100 mT). There is a spatial correlation of site mean declinations that defines four different tectonic blocks with no apparent relationship to the predominantly Mio-Pliocene age of the rocks but with similar amounts of rotation. This observation suggests that the region rotated significantly after the Pliocene.

Vertical axis rotations were determined by comparing site mean declinations with the expected declination calculated from the Hispaniola APW for a given age rock. The first tectonic block records a significant CCW rotation (65º to 87º), whereas the second shows a smaller amount of CCW rotation (30º to 60º). The third block records a CW rotation (33º - 65º), the same amount of rotation as in the second block, and finally, the fourth block, located 40 km from the Septentrional fault, records, as the third block does, a CW rotation (30º - 55º). Rock magnetics (isothermal remanent magnetization (IRM) acquisition and low temperature susceptibility curves (LT)) were analyzed to determine the magnetic mineralogy carriers of the samples showing that the remanence is carried, in most cases, by magnetite.

Results of this study suggest both clockwise and counterclockwise rotations in post-Pliocene times related to major activity on the Septentrional fault zone. The current trace of the Septentrional fault cuts directly through the middle of one of our tectonic blocks suggesting that during Mio-Pliocene times, the active trace of the fault would have been 8-10 km south of its current position, between CW and CCW rotated tectonic blocks delineated by our paleomagnetic data.