Two major structural and stratigraphic rock units occur in Puerto Rico: the older complex, ranging in known age from Late Cretaceous to late Paleocene or early Eocene and the middle Tertiary sequence, ranging from late Oligocene possibly to late Miocene. The former rocks are eugeosynclinal in character and are very badly faulted but for the most part apparently only moderately folded. With the exception of a large, partly low-angle thrust, the writer has not recognized evidence of strong tangential stresses. Intra-formational folding in the older complex is interpreted as caused by submarine sliding (slump structure). Except in the vicinity of the larger plutonic intrusions and in the northeastern and southwestern corners of the island, the main strike alignment of the older complex is northwestward. The plutons are roughtly concordant with the structure of the country rock and show varying degrees of differentiation. Their average composition seems to be more acidic than that of the volcanic and volcanogenetic rocks into which they are intruded.
The middle Tertiary sequence is nonvolcanic, made up dominantly of calcareous marine sediments. It crops out on the north and south sides of the island and in structural troughs on the west coast. On the north coast the beds dip gently to the north, and, except for slight terracings and a flexure at the northwestern corner of the island, are not folded. The middle Tertiary sequence on the south side of the island is somewhat folded. Seismic-reflection studies of the north coast indicate, however, a pronounced northward thickening, possibly some folding, and unconformities at depth. Unconformities which may be local have also been noted at several places on the surface. Several large faults in the middle Tertiary sequence have been recognized in both the north- and south-coast belts. The pattern of master joints that is inferred in the north-coast middle Tertiary belt from topographic alignments seems to indicate (1) control by fades contacts, (2) possible downwarping associated with the sinking of an arm of the Puerto Rican Trench, and (3) tension during upwarping along the island axis.
The dates of the major diastrophic events that are decipherable from the Puerto Rican data are: (1) early Tertiary (possibly late Paleocene, but more probably Eocene), when the older complex was deformed; (2) Miocene, when the middle Tertiary sequence was deformed; (3) late Pliocene and possibly early Pleistocene, when block faulting on a large scale produced the present topographic relief. Remnants of the fault scarps resulting from the latter deformation occur widely. Quaternary marine deposits and marine terraces suggest that Puerto Rico has been relatively unaffected by crustal movement at least since the late Pleistocene.