[1] The processes of arc initiation at the margin of an oceanic plateau are remarkably well preserved along the southern coastline of eastern Costa Rica and western Panama. We present new results of a combined tectonostratigraphic and petrologic study with which protoarc initiation (75-73 Ma) at the margin of an oceanic plateau (89-85 Ma) is documented. Dykes of protoarc igneous rocks within the plateau and occurrences of protoarc igneous rocks are widely distributed. These types of field observations, geochemical data, and paleontologic ages for Late Cretaceous to Eocene fore-arc rocks of the Golfito Complex and Azuero Marginal Complex (southern Costa Rica and western Panama) provide the first direct evidence that a Coniacian-early Santonian oceanic plateau forms the arc basement. Stratigraphic and geochemical constraints from Golfito and Azuero indicate subduction initiation in south Central America, associated with geochemically distinctive suprasubduction igneous rocks, occurred in the late Campanian along the margin of the newly defined Azuero Plateau. Overall, the Golfito Complex and Azuero Marginal Complex provide a significant opportunity for exploration of petrologic mechanisms linking some oceanic plateaus to the growth of continents. The Azuero Plateau may extend further toward the Colombian Basin and forms thickened Caribbean crust. It served as a nucleus for accretion of additional oceanic plateaus, seamounts, and oceanic islands of Pacific origins.
Multidisciplinary study of the Osa and Burica peninsulas, Costa Rica, recognizes the Osa Igneous Complex and the Osa Mélange-records of a complex late Cretaceous-Miocene tectonic-sedimentary history. The Igneous Complex, an accretionary prism (sensu stricto) comprises mainly basaltic lava flows, with minor sills, gabbroic intrusives, pelagic limestones and radiolarites. Sediments or igneous rocks derived from the upper plate are absent. Four units delimited on the base of stratigraphy and geochemistry lie in contact along reactivated palaeodécollement zones. They comprise fragments of a Coniacian-Santonian oceanic plateau (Inner Osa Igneous Complex) and Coniacian-Santonian to middle Eocene seamounts (Outer Osa Igneous Complex). The units are unrelated to other igneous complexes of Costa Rica and Panama and are exotic with respect to the partly-overthickened Caribbean Plate; they formed by multiple accretions between the late Cretaceous and middle Eocene, prior to the genesis of the Mélange. Events of high-rate accretion alternated with periods of low-rate accretion and tectonic erosion. The NW Osa Mélange in contact with the Osa Igneous Complex has a block-in-matrix texture at various scales, produced by sedimentary processes and later tectonically enhanced. Lithologies are mainly debris flows and hemipelagic deposits. Clastic components (grains to large boulders) indicate late Eocene mass wasting of the Igneous Complex, forearc deposits and a volcanic arc. Gravitational accumulation of a thick pile of trench sediments culminated with shallow-level accretion. Mass-wasting along the margin was probably triggered by seamount subduction and/or plates reorganisation at larger scale. The study provides new geological constraints for seamount subduction and associated accretionary processes, as well as on the erosive/accretionary nature of convergent margins devoid of accreted sediments.
ABSTRACT:In this preliminary paper we report on age, microfacies, microfossils and structural setting of platform limestones, that crop out in the Río Tempisque area (Guanacaste, Costa Rica).
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