Deformation structures in core samples and well cuttings from the Toa Baja borehole in Puerto Rico, obtained from depths of 580 to 2700 meters, can be divided into two major categories: (1) structures resulting from compression and shear and (2) structures formed by extension. Generally, the first ones are older and they are somewhat more abundant at horizons where fault zones occur as deduced from dipmeter readings and televiewer log. These structures may have formed during an Eocene thrusting event, recognized regionally in Puerto Rico. The extensional structures which almost always crosscut the compressional ones are filled with zeolite, calcite, prehnite, or quartz. There is poor correspondence between the abundance of these fractures and the position of the major fault zones. The largest abundance of veins occurs between 1700 and 2100 meters depth. They may be related to a phase of regional extension during the Late paleogene. ¿Americang Geophysical Union 1991