[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.

No abstract

No abstract

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).

No abstract

The collision of bathymetric features with modern convergent margins has been investigated with the full range of tools used in geosciences. Hence, a comprehensive picture exists of the characteristic effects of collision events on the evolution of convergent margins. In contrast, much less studies documented past collisions of bathymetric features with convergent margins, as colliding features were generally lost to subduction. The arc-trench system of southern Central America provides modern and past textbook examples of active margin interaction with incoming bathymetric reliefs. Here, we propose a synthesis which combines basin and terrane analysis of the forearc of northern Costa Rica and takes up the challenge of documenting past episodes of plateau accretion to the active margin. As illustrated in modern examples, our study shows that kilometric uplift of the overriding plate and termination of the volcanic arc activity are the most profound effects of colliding/accreting oceanic plateaus. Kilometric uplift of the forearc is documented by short-lived (ca. 3 m.y.) occurrences of shallow-water deposits in an overall deep-water forearc record. These shallow deposits contain material reworked from underlying sedimentary and basement lithologies. The development of spatial gaps in arc volcanism is deduced from the transition from arcderived turbidites to pelagic sediments. Eventually, end of the collision event is evidenced by the subsidence of the whole forearc to deep-water environments.Basin subsidence is accompanied or followed by renewed volcanic arc activity and coeval arc-derived sedimentation, which may occur 1-7 m.y. after plateau collision. These past episodes of plateau accretion are archetypal for the following reasons: (a) they may be studied in outcrop, whereas most of the modern collisions of plateaus largely occur underwater; (b) no tectonic or metamorphic imprint has significantly complicated the forearc geological record; (c) the colliding feature and the sedimentary response to its collision are both preserved in the forearc geology; (d) they may be used as analogues for any setting where a bathymetric feature is suspected to have caused rapid forearc uplift and cessation of the volcanic arc activity.---

No abstract