EARLY TERTIARY EMPLACEMENT OF SERPENTINIZED PERIDOTITE IN SW PUERTO RICO

In southwest Puerto Rico, three ultramafic belts crop out: Monte del Estado peridotite belt (MEPB), Guanajibo serpentinite belt (GSB), and the Bermeja Complex. Previous interpretations for emplacement and deformation of these ultramafic rocks include: 1) northward-directed obduction during late Early Cretaceous subduction; 2) formation of conglomerates and breccias that constitute the base of the overlying Late Cretaceous rocks; 3) remobilization of the serpentinite by "protrusion" or cold vertical upward movement through the overlying Late Cretaceous rocks; and 4) faulting of the serpentinite during early Tertiary contraction. Recent mapping at the southern boundaries of MEPB and GSB reveals a gently dipping contact at the base of the serpentinite. In the southeastern exposures of the MEPB and GSB belts, as much as 10 m of relief may exist along the sub-horizontal surface. In the eastern part of the discontinuous GSB, outliers of serpentinite, topographically and structurally high, rest upon steeply dipping Campanian mudstone of the Yauco Formation, and Maestrichtian El Rayo Formation. The structural relationships suggest that the serpentinite is thrust over the Yauco and El Rayo Formations. Above the thrust (?) contact, ductile shear zones and penetratively foliated rocks near the base of the hanging wall serpentinite, record ductile deformation. At a quarry within MEPB, localized zones of highly foliated and folded serpentinite are cut by shallowly-dipping ENE-WSW shear zones, which record southward transport. Rocks between these shear zones are cut by non-systematic fractures with slickensided surfaces. At El Hoyo within GSB, discrete, steeply dipping E-W trending left-lateral shear zones cut pervasively foliated rocks, which locally record an earlier S-C fabric. These left-lateral shear zones may be related to late Eocene transpression which post-dates emplacement of the serpentinite, and the early fabric may be related to thrusting. Neither the sub-horizontal basal contact at the southern margin of the serpentinite or localized high-strain zones support protrusion of serpentinite diapirs into Late Cretaceous rocks, but do suggest emplacement of serpentinite over Sabana Grande, Yauco, and El Rayo Formations.