Tectonics of Jamaica @ sfmgeology.com

Introduction

The island of Jamaica is close to the northern margin of the Caribbean Plate in an area which experiences active tectonism. This tectonism is the reason that Jamaica is here (i.e., has been uplifted in the last 10-12 million years) and is still evident because of the earthquakes that the island experiences every year.

The Gonâve Microplate

The Gonâve Microplate is named after Gonâve island in Haiti and represents a partially free tectonic unit between the Caribbean Plate and the North American Plate. The boundary is defined by a series of faults that include: the Walton Fault Zone, the Mid-Cayman Spreading Centre, the Oriente-Septentrional Fault Zone and the Enriguillo-Plantain Garden Fault Zone. The exact course of this boundary across Jamaica is unclear, as GPS measurements suggest a more diffuse boundary with vectors progressively changing in a transect from NE (more westerly directed vectors) to SW (more south-westerly directed vectors) across the island. Interestingly, the offshore Pedro and Morant Cays are moving relative to the Caribbean Plate, and clearly the Gonâve microplate boundary is not well defined in Jamaica.

Faults

Jamaica is criss-crossed by a large number of faults. Three prominent fault orientations are observed: broadly E-W, NE-SW and NE-SW. These represent the typical fault orientations that would be developed in an east-west shear zone which GPS vectors indicate in progressing from northern Jamaica to the Pedro Cays. The three sets of faults have different characteristics. The E-W faults are largely left-lateral (sinistral), strike-slip faults, although clearly there is some vertical component of movement on at least some of these faults. The NW-SE faults are high-angle reverse faults, but may also have a component of left-lateral (sinistral), strike-slip displacemnet. The NE-SW faults are normal faults, and are often associated with the development of interior valleys or Poljes.

Much of the tectonic assessment of Jamaica has been done using remote sensing (either from interpretations of aerial photographs or satelite images). Unfortunately, lineations that are seen on these images may have various casues; they may be faults, joint-sets, uplift terraces or even fence lines. As such some of the faults shown on geological maps are fictitious. Part of my recent geological mapping campaign has been to map faults on the ground to build up a much clearer picture of the tectonic deformation of Jamaica.

Folds

Folds are developed in Jamaica, particularly in some of the less compintent rocks (the more competent rocks, particularly the White Limestone, generally show faults. Perhaps the most significant fold is that of the north-western Central Inlier, which consists of a NW-SE oreintated anticline. This also fits into the E-W strike-slip shear zone model for Jamaica.