A paleomagnetic study of the essentially undeformed middle Tertiary carbonate sequence along the north coast of Puerto Rico reveals statistically significant pre‐Pliocene discordance of characteristic component directions against those expected from cratonic North America for much of the section. Despite generally weak to moderately weak magnetic intensities, confirmation of the magnetization as primary in origin comes from the presence of two distinct components of magnetization, intrasite bipolarity, and/or the reproducibility of measurements.

Island arc volcanic strata in Puerto Rico, ranging in age from Lower Cretaceous (Aptian) to Eocene and dating from about 120 to 45 Ma, represent one of the longest oceanic arc sequences preserved in the world. Detailed and systematic mapping by the U.S. Geological Survey published between 1959 and 1986 reveal that, although postvolcanic sedimentary platform deposits consisting of limestone and other detrital materials ring the island and cover extensive parts of the arc platform, representative strata of the entire sequence are exposed. The island is subdivided into three volcanic provinces all containing strata without correlative units elsewhere; they are (1) a northeastern volcanic province, separated from the central province by the Cerro Mula Fault, a left-lateral strike-slip fault of mid-Santonian age with displacement of at least 50 km; (2) a central volcanic province, dominated by volcanic debris accumulated during sequential development of five east-west-oriented volcanic belts, and (3) a western volcanic province, with a northwest-southeast-trending boundary of uncertain origin, containing remnants of two sequential island arc volcanic belts of CampanianMaastrichtian and Eocene age.Additionally, in the southwestern corner of the island, the Sierra Bermeja Complex consists of a tectonic melange of partly serpentinized ultramafic rocks representing the lithospheric upper mantle originally composed of spinel-bearing peridotites, including lherzolite, harzburgite, and dunite. The melange incorporates rafts, blocks, and boulder-sized clasts of (1) Lower Jurassic to Upper Cretaceous pelagic sediments (Mariquita Chert), including radiolarian chert of Pacific provenance and later siliceous volcanogenic strata, representing pre-island arc oceanic deposits; and (2) altered mid-ocean ridge basalt (MORB)-like basalts (Cajul Basalt) and amphibolites (Las Palmas amphibolite melange) of probable Lower Jurassic age, representing pre-island arc oceanic crust. Serpentinized peridotite bodies, containing amphibolite, altered MORB-like basalt, and chert, protruded the arc platform in a cold state at several places in the western province. The largest of these, the Monte del Estado Peridotite, was emplaced during the Maastrichtian, through crustal extension associated with left-lateral strike-slip faulting along the Cordillera fault.Volcanic strata preserved in the central province are subdivided into five major volcanic phases on the basis of stratigraphic and geochemical relations. Basalts evolved progressively from early primitive island arc tholeiites in phase I, to calcalkaline basalts in phase II, and finally to incompatible element-enriched shoshonite Jolly, W. T.

Jurassic rocks of the Caribbean are a sampling of 100 million years of Farallon Plate history with fragments originating at diverse paleolatitudes and from varied tectonic settings. Fragments with clear paleogeographic signatures are components of the basement complexes of Duarte in Hispaniola, Bermeja in Puerto Rico and La Drsirade off Guadeloupe. Paleolatitudinally sensitive radiolarian faunas document origination of Duarte as equatorial, La Drsirade as higher latitude, and various Bermeja cherts as both equatorial and higher latitude. Red ribbon chert of Duarte and Bermeja of the same age, physical appearance, and lithological association are probably dismembered components of the same slab of Pacific crust. La Drsirade red ribbon chert is slightly younger than the Duarte and Bermeja red ribbon chert and was deposited at higher latitude. Bermeja tuffaceous chert is also of higher latitude and probably had an arc-proximal origin. On the basis of modeled plate trajectories in the Pacific, the origin of various cherts from different paleolatitudes that end up in the same location requires different arrival times at the trench between North and South

No abstract

No abstract