Claudia Baumgartner-Mora

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 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 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 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.

An upper Paleogene shallowing-upward sequence in the southern Sandino Forearc Basin (NW Costa Rica): response to tectonic uplift

AbstractThe Sandino Forearc Basin of western Nicaragua and northwestern Costa Rica (Central America) provides a Campanian to Pliocene sedimentary record. The study of the onshore part of the basin in northwestern Costa Rica reveals for the first time the occurrence of upper Paleogene shallow-marine siliciclastic and carbonate sequences. These sequences have remained undescribed so far and are grouped herein into two new lithostratigraphic units—the upper Eocene Junquillal Formation (Fm.) and the upper Oligocene Juanilla Fm. The upper Eocene Junquillal Fm. is characterized by storm-related, arenitic to conglomeratic deposits comprised in metric, massive amalgamated beds. The shallow shelfal environment of deposition is attested by the presence of hummocky and swaley cross-stratifications. The lithologies of the Junquillal Fm. were previously considered to be part of the underlying, deep-water turbiditic deposits of the Eocene Descartes Fm. The deposition of the Junquillal Fm. is indicative of tectonic uplift that forced regression, which affected the southeastern part of the Sandino Forearc Basin during the late Eocene. The upper Oligocene Juanilla Fm. unconformably overlies the Junquillal Fm. and occurs as a 25-m-thick, 700-m-wide outcrop on Isla Juanilla. It is composed essentially of nodular, coral framestones exhibiting massive, closely packed corals in growth position that are associated with coralline red algae and Larger Benthic Foraminifera (LBF). A late Oligocene age of the reef is attested by LBF assemblages occurring in two different facies. The Juanilla Fm. coral reef is a unique exposure, characterized by extensive constructed coral framework, and which has no equivalent in the Oligocene geological record of Central America. The reef grew on a short-lived, siliciclastic-poor tectonic high, which developed in relation to a lower Oligocene, basin-scale folding event in the Sandino Forearc Basin.

Tectono-stratigraphic response of the Sandino Forearc Basin (N-Costa Rica and W-Nicaragua) to episodes of rough crust and oblique subduction

The southern Central American active margin is a world‐class site where past and present subduction processes have been extensively studied. Tectonic erosion/accretion and oblique/orthogonal subduction are thought to alternate in space and time along the Middle American Trench. These processes may cause various responses in the upper plate, such as uplift/subsidence, deformation, and volcanic arc migration/shut‐off. We present an updated stratigraphic framework of the Late Cretaceous–Cenozoic Sandino Forearc Basin (SFB) which provides evidence of sedimentary response to tectonic events. Since its inception, the basin was predominantly filled with deep‐water volcaniclastic deposits. In contrast, shallow‐water deposits appeared episodically in the basin record and are considered as tectonic event markers. The SFB stretches for about 300 km and varies in thickness from 5 km (southern part) to about 16 km (northern part). The drastic, along‐basin, thickness variation appears to be the result of (1) differential tectonic evolutions and (2) differential rates of sediment supply. (1) The northern SFB did not experience major tectonic events. In contrast, the reduced thickness of the southern SFB (5 km) is the result of at least four uplift phases related to the collision/accretion of bathymetric reliefs on the incoming plate: (i) the accretion of a buoyant oceanic plateau (Nicoya Complex) during the middle Campanian; (ii) the collision of an oceanic plateau (?) during the late Danian–Selandian; (iii) the collision/accretion of seamounts during the late Eocene–early Oligocene; (iv) the collision of seamounts and ridges during the Pliocene–Holocene. (2) The northwestward thickening of the SFB may have been enhanced by high sediment supply in the Fonseca Gulf area which reflects sourcing from wide, high relief drainage basins. In contrast, sedimentary input has possibly been lower along the southern SFB, due to the proximity of the narrow, lowland isthmus of southern Central America. Moreover, two phases of strongly oblique subduction affected the margin, producing strike‐slip faulting in the forearc basin: (1) prior to the Farallon Plate breakup, an Oligocene transpressional phase caused deformation and uplift of the basin depocenter, triggering shallowing‐upward of the Nicaraguan Isthmus in the central and northern SFB; (2) a Pleistocene–Holocene transtensional phase drives the NW‐directed motion of a forearc sliver and reactivation of the graben‐bounding faults of the late Neogene Nicaraguan Depression. We discuss arguments in favour of a Pliocene development of the Nicaraguan Depression and propose that the Nicaraguan Isthmus, which is the apparent rift shoulder of the depression, represents a structure inherited from the Oligocene transpressional phase.