New paleomagnetic data are presented in the context of a revised tectonostratigraphic subdivision of the Mesozoic-Tertiary oceanic basement of Costa Rica. We present the tectonostratigraphic characteristics and areal extent of four terranes (Chorotega, Nicoya, Golfito, and Burica) bordered toward the Middle America Trench by the Tertiary Osa-Caño Accretionary Complex. The paleomagnetic data are derived from Upper Cretaceous and Paleocene pelagic limestones. The Chorotega Terrane constitutes most of the southern Middle American Landbridge and was the western edge of the Caribbean plate during the Late Cretaceous-Paleocene. The Nicoya Terrane comprises the Santa Elena Peninsula and most of the outer Nicoya Peninsula. The Nicoya Terrane includes the Nicoya Complex (sensu stricto) and should therefore probably be regarded as a composite terrane. The Golfito Terrane forms the Golfito region and extends into Panama to the Azuero Peninsula. The Late Cretaceous basement of the terrane is thought to have formed a marginal piece of the Caribbean oceanic plateau, transported northward by strike slip along the rim of the Caribbean plate. The Burica Terrane forms the Burica Peninsula. The terrane is thought to represent an accreted, structurally high piece of a primitive island arc. The inner Osa Peninsula is formed by a thick pile of oceanic basalts including Late Cretaceous to Eocene sediments. The outer Osa Peninsula and the Caño Island are built by the Osa-Caño Accretionary Complex, a mélange-type complex characterized by strongly deformed turbidites and hemipelagic and pelagic sediments that range in age from Late Cretaceous to Miocene. The exotic terranes are thought to have originated outboard in the Paleopacific, been brought into contact with the Caribbean plate boundary by plate convergence, and then been moved farther north by strike-slip motion along the margin. The paleomagnetic data for the Chorotega Terrane indicate an origin close to its present latitude and no significant rotation relative to South America since Late Cretaceous time. The paleomagnetic data obtained from the Nicoya Terrane imply a low southerly Late Cretaceous paleolatitude and almost no rotation relative to the Chorotega Terrane. The Nicoya Terrane was about 16° of latitude south relative to the Chorotega Terrane in Late Cretaceous times. The paleomagnetic data from the Golfito Terrane indicate a Late Cretaceous equatorial paleolatitude and counterclockwise rotation of about 60° relative to the Chorotega Terrane. Similar paleomagnetic data were obtained from the Azuero Peninsula in southwestern Panama. The paleomagnetic data from the Burica Terrane indicate a low northerly latitude in the Paleocene and a counterclockwise rotation of nearly 90° relative to the Chorotega Terrane.

Boron contents were measured in predominantly mafic Quaternary lavas from the Central American Volcanic Arc to evaluate along-strike variations in subduction processes. Despite the significant range in B concentrations (∼2 to 46 ppm), B/La ratios vary in a systematic fashion along the arc: Higher values (>1) are typical between Guatemala and northern Costa Rica, whereas low values (most <0.5) typify central Costa Rica and western Panama. Because B/La is highly correlated with 10 Be/Be (r 2 = 0.94, excluding one sample), B/La may be a useful indicator of subduction contributions to the magma sources. If enrichments of both B and 10 Be are proportional to the flux of subducted sediment, along-strike variations in B/La suggest at least a twofold variation in this flux with maximum values below western Nicaragua and minimum values below Costa Rica and western Panama where the Cocos Ridge is being subducted. These data may also reflect significant differences in thermal state of the downgoing slab, which in turn differentially affect release patterns of fluids and fluid-mobile trace elements and possibly melting processes beneath different parts of the arc. The following scenario is suggested to explain the geochemical results. Steep subduction of cold slab beneath the northwestern arc favors more efficient subduction of fluid components to depths beneath the volcanic front. Their release provides fluid-mobile elements to the overlying mantle, which upon melting produces calcalkalic magmas. Shallow subduction of warmer slab beneath the southeastern arc favors shallow release of fluids and limits fluid-related metasomatism of sub-arc mantle beneath the volcanic front. Under such conditions, B/La and Ba/La ratios in the sub-arc mantle vary little from values seen in oceanic island basalts. Magmas in this part of the arc nevertheless display the highest La/Yb and lowest Ba/La and B/La ratios, which are consistent with prior light rare-earth-element enrichment in the source, significantly lower degrees of melting, or a combination thereof. Because some of the largest volcanoes occur in Costa Rica, and magma flux there is nearly an order of magnitude higher than elsewhere in the arc, source enrichment is considered to be the more plausible explanation. It is proposed that Quaternary magma production below Costa Rica involved lithospheric sources containing trapped or “stored” melt components, but this enrichment process is unlikely to have involved arc magmas or subduction fluids because we see no B-enrichment.