FAULTS OF SOUTHERN JAMAICA AND THEIR IMPLICATIONS FOR THE JAMAICA RESTRAINING BEND

The island of Jamaica occurs tectonically in a restraining bend at the boundary between the Caribbean plate and Gonave microplate. We are utilizing a combination of geological, geophysical, and geodetic data to understand the southern margin of the restraining bend. The South Coast fault zone (SCFZ) strikes east-west, and may constitute the southern edge of the restraining bend. It has a strong geomorphic expression locally, forming a 600 m high scarp along the southern coast of Jamaica. Rare slickensides developed in the Miocene carbonates near Kemps Hill indicate a sub-horizontal lineation, thereby suggesting sinistral, strike-slip motion. A two-dimensional gravity survey in the Vere plain indicates a ~8 mGal gravity gradient occurs across the SCFZ. Thus, despite the dominantly strike-slip motion, there is evidence for relative uplift on the N side of the fault.

A series of NNW-striking faults occur immediately N of and are truncated by the SCFZ. Two prominent faults are the Spur Tree and Santa Cruz faults, which correspond to mountains of the same name. Escarpments occur on the W side of these mountains, with strata dipping gently E. On the crest of the Spur Tree Mountains is an anticline with W vergence. Consequently, we interpret the Spur Tree and Santa Cruz faults as E-dippping, reverse faults. Gravity surveys reveal an ~7 and ~10 mGal gravity anomaly across the Spur Tree fault and the Santa Cruz fault, respectively, and constrain the dip of the reverse faults.

Using these data, we propose a model for the southern boundary of the restraining bend. The E-W oriented SCFZ, while recording some overall sinistral motion, plays a more important role as an accommodation structure. It allows differential east-west uplift and shortening (such as occurs on the Spur Tree and Santa Cruz faults) on the N side of the fault. South of the SCFZ, very little deformation occurs. Geodetic data, which indicate only minor sinistral motion across the SCFZ, support this model.