The Moín High, East Costa Rica: Seamount, laccolith or contractional structure? Christian Brandes a, * , Allan Astorga b , Jutta Winsemann a a Institut für Geologie, Leibniz Universität Hannover, Callinstr. 30, 30167 Hannover, Lower Saxony, Germany b Escuela Centroamericana de Geología, San José, Costa Rica article info Article history: Received 11 May 2007 Accepted 11 February 2009 Keywords: Costa Rica Limón Basin Moín High Contractional structure Central America abstract The back-arc area of the southern Central American arc–trench system in East Costa Rica is characterized by a complex basin system. An extensional back-arc area (the North Limón Basin) and a compressional retro-arc foreland basin (the South Limón Basin) are closely related. Both basins are separated by an approximately 50 km long and 30 km wide mound-shaped structure referred to as Moín High, which evolved in Eocene times. The Moín High has previously been interpreted as a basement structure or paleo-high. The modern geothermal gradient is 3 °C/100 m. There is no evidence for thermal anomaly or higher heat flow in that area. A mean heat flow of 56–60 mW/m 2 implies that an origin as a volcanic seamount or magmatic intrusion is unlikely. 3D static models show that the Moín High trends NNE–SSW and has an antiformal shape in cross-section and an elliptic outline in map view. The trend of the Moín High coincides with the orientation of folds in West Costa Rica that formed in response to an Eocene deformation phase. The seismic lines show that Miocene reflectors onlap against the structure. Based on this data set it is likely that the Moín High is an anticline formed due to contraction. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction The Limón back-arc basin extends along the Caribbean Coast of Costa Rica. In the central and northern part of this basin large mound-shaped structures occur (Fig. 1), which have been inter- preted as basement structures (Sheehan et al., 1990) or paleo-highs (Barboza et al., 1997). One of these structures is referred to as Moín High, which is located close to the present-day coastline north of Puerto Limón, next to the Trans Isthmic Fault System at the bound- ary of the undeformed North Limón Basin and the deformed South Limón Basin. The Moín High evolved in Eocene times (Barboza et al., 1997), has an elliptic outline in map view and represents a four-way dip closed area. In general such a structure is an interest- ing exploration target because it can retain oil and gas. In the past the Limón Basin was regarded as prolific hydrocarbon province and several wells were drilled (e.g., Sheehan et al., 1990; Barboza et al., 1997; Petzet, 1998). Archer et al. (2005) showed the importance of a careful and multi-disciplinary examination of such a structure to avoid a misinterpretation and to minimize the risk for hydrocarbon exploration. Based on the shape and outline, there are different possibilities to explain the evolution of the Moín High: (1) a volca- nic seamount, (2) a magmatic intrusion, (3) a salt pillow/diapir, (4) an uplifted basement block, (5) an inversion structure or (6) an anticline (Fig. 2). Seismic sections and well data made available by the Costa Rican Ministry of Environment and Energy (MINAE) allow a detailed reconstruction of the geometry and temporal evo- lution of the structure. Pre-growth, growth and post-growth strata was analysed to reconstruct the uplift history of the Moín High. The combination of seismic lines, well data, 3D static models and basin modelling techniques helps to verify the different possibili- ties for its mode of origin. 2. Geological setting The geology of Central America is characterized by the interac- tion of five lithospheric plates, namely the oceanic Cocos, Nazca and Caribbean Plates and the continental North and South Ameri- can Plates (Fig. 3a). Tectonics processes in this region are domi- nated by the subduction of the Cocos and Nazca Plates beneath the Caribbean Plate along the NW–SE trending Central America trench. The present-day subduction velocity off Costa Rica, relative to the Caribbean Plate, is 8.5 cm/yr (DeMets, 2001). The Central American subduction zone is characterized by strong along-trench variations in the dip angle of the Wadati–Benioff zone. Protti et al. (1995) observed an angle of 84° under Nicaragua, 60° under Cen- tral Costa Rica and a flat slab with no Wadati–Benioff zone under South Costa Rica. The Central American land-bridge above this sub- duction zone is a complex assemblage of distinct crustal blocks (Fig. 3a) including, from NW to SE, the Maya, Chortis, Chorotega and Choco Blocks (Donnelly, 1989; Weinberg, 1992; Di Marco et al., 1995; Campos, 2001). The Maya and Chortis Blocks have a continental basement, whereas the Chorotega and Choco Blocks comprise island-arc segments underlain by Mesozoic oceanic crust 0895-9811/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jsames.2009.02.005 * Corresponding author. Tel.: +49 5117624391; fax: +49 5117622172. E-mail address: brandes@geowi.uni-hannover.de (C. Brandes). Journal of South American Earth Sciences 28 (2009) 1–13 Contents lists available at ScienceDirect Journal of South American Earth Sciences journal homepage: www.elsevier.com/locate/jsames