Late Cretaceous to Miocene seamount accretion and mélange formation in the Osa and Burica Peninsulas (Southern Costa Rica): episodic growth of a convergent margin

Abstract

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.

Future accreted terranes: a compilation of island arcs, oceanic plateaus, submarine ridges, seamounts, and continental fragments

“…Accreted seamounts are often "decapitated" in the accretionary prism instead of underplated to the overriding plate. The seamount terranes of the Oso Igneous Complex in Costa Rica are within an accretionary prism complex, suggesting that the seamounts were decapitated within the prism and subsequently accreted to the Central American active margin (Buchs et al, 2009). suggest that even small seamounts can be accreted if the subduction channel is narrow, highly coupled, or if the seamount is regionally compensated by a thick, strong lithosphere.…”

“…Landward-verging imbricate thrust faults typically shear off blocks of tens to hundreds of meters of FAT or oceanic crust (Kimura and Ludden, 1995). For example, the Oso Melange and Oso Igneous Complex in Costa Rica records the history of accreted oceanic plateaus, island arcs, and seamounts which were mixed in with accretionary prism sediments (Buchs et al, 2009).…”

“…In Central America, various "ophiolitic" units are found with OIB geochemical signatures, which are interpreted as hotspot-related seamounts or submarine ridges (Hoernle et al, 2002;Geldmacher et al, 2008;Buchs et al, 2009). The enigmatic Siletz terrane of northern California and Oregon is composed of volcanics with OIB signatures that have been variously interpreted as a hot spot track, slab window, and mid-ocean ridge (Schmandt and Humphreys, 2011;McCrory and Wilson, 2013).…”

“…Examples of accreted seamounts, identified primarily by their OIB signature, are the alkali basaltic units found in Japan (Isozaki et al, 1990). Typical seamountderived terranes include thin-skinned units of radiolarian cherts, limestones, serpentinized peridotites, layered gabbros, and alkali basalts that are on the order of hundreds of meters thick (Geldmacher et al, 2008;Buchs et al, 2009). Accreted ocean-island basalts, interpreted to be remnants of seamounts, are often found within accretionary complexes (e.g., Cache Creek terrane: Johnston and Borel, 2007).…”

Tectonic evolution of the Gulf of Mexico, Caribbean and northern South America in the mantle reference frame: an update

“…Subsequent subduction at the trench would have led quickly to the accretion of LIP seamounts and plateau material at the Panama -Costa Rica accretionary complexes (e.g. Osa and Nicoya peninsulas, Hoernle et al 2002;Buchs et al 2009;Baumgartner et al 2008). These accreted rocks would be potentially genetically and temporally correlative to the LIP rocks on the internal Caribbean Plate, such as those in Southern Hispaniola, Aruba, Curacaõ, eastern Jamaica, the lower Nicaragua Rise and the basinal DSDP holes, because there was no subducting plate boundary to separate them when they formed.…”