1 research outputs found
Late Cretaceous to Miocene seamount accretion and mélange formation in the Osa and Burica Peninsulas (Southern Costa Rica): episodic growth of a convergent margin
<p>Multidisciplinary study of the Osa and Burica peninsulas, Costa Rica, recognizes the Osa Igneous Complex and the Osa MĂ©lange
– records of a complex Late Cretaceous–Miocene tectonic–sedimentary history. The Igneous Complex, an accretionary prism (<em>sensu stricto</em>) 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 palaeo-décollement 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 MĂ©lange 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 plate reorganization 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.
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