This volume concerns information learned by drilling the Toa Baja well on the north coast of Puerto Rico, and current studies of Puerto Rican geology and tectonics.

The Toa Baja Drillsite is located in the North Coast basin of Puerto Rico about 10 km west of San Juan. The hole was spudded on August 23, 1989, and plugged and abandoned on November 7, 1989 at a total depth of 2704m. Two lithologies were encountered during drilling: an upper series consisting of Oligocene-Miocene shallow-water limestone and sandstone facies, and a lower series consisting of Eocene deep-water volcaniclastic strata, including some lave flows or shallow intrusions, pelagic marls, and altered igneous rocks or coarse-grained sandstones. Principal findings made during drilling include: 1) the important unconformity separating the upper and lower series at about 579 m; 2) 8 faults defined clearly by dipmeter log; 3) changes in rock type probably associated with reflection events in seismic reflection profiles crossing the drillsite; 4) confirmation of overall low geothermal gradients and heat flow, but presence of a thermal anomaly near 2683 m; 5) documentation of high paleogeothermal gradients using petrographic, isotopic, X-ray diffraction and electron microprobe studies; 6) presence of fractures indicating a current extensional tectonic setting.

Current studies in the Puerto Rico region include: 1) paleomagnetic evidence for Late Miocene counterclockwise rotation; 2) geochemical evolution of Cretaceous and Eocene igneous rocks; 3) evidence of transtension in the northeast Caribbean plate boundary zone; 4) results of studies of ancient fault zones on Puerto Rico; and 5) stratigraphic studies of the Tertiary of Puerto Rico. ¿ American Geophysical Union 1991