NEOGENE EVOLUTION OF THE COSTA RICAN ARC AND DEVELOPMENT OF THE CORDILLERA CENTRAL

A comprehensive 40Ar/39Ar investigation of Neogene volcanic rocks in Costa Rica (CR) reveals how the arc evolved since the Early Miocene and constrains the development and eruptive history of the modern volcanic front in central CR. Arc magmatism in CR dates back to at least 24 Ma, but has been strongly episodic, with important eruptive peaks at ~16-17, 4-6, 1-2, 0.4-0.6, and < 0.1 Ma. At ~ 3.5 Ma, volcanism shut off in southern CR, due to subduction of the Cocos Ridge. There has been a 30° CCW rotation of the position of the arc from its middle Miocene position to the modern volcanic front. This CCW rotation occurred between 15 and 8 Ma and is attributed to deformation in the overriding plate, (shortening in the south coeval with extension in the NW), accompanied by trench retreat to the north. Since 8 Ma, the arc has paralleled the modern volcanic front but progressively retreated to the NE. We see no dramatic difference between the Miocene and modern arc – either in terms of average silica content or range of compositions. The modern volcanic front in central CR includes Turrialba, Irazu, Barva, and Poas Volcanoes of the Cordillera Central (CC) and flanking plateau deposits of the Valle Central (VC), where the majority of CR’s population lives. The CC volcanoes grew during the last 1 Ma, with major episodes of cone/shield building at 0.98-0.85 Ma (Proto-Cordillera), 0.63-0.40 Ma (Pre-Cordillera), and<0.17 Ma (Neo-Cordillera) separated by 0.1-0.3 Ma intervals of dormancy(erosion) and/or explosive silicic volcanism. Most of the growth of these volcanoes occurred during the 0.4-0.6 Ma Pre-Cordillera phase. Explosive silicic eruptions are an integral part of the evolution of these composite volcanoes, with known ignimbrite-forming eruptions from the CC at 0.92, 0.57, 0.49, 0.44, and 0.325 Ma. The 325 ± 1 ka Tiribi ignimbrite is the largest of these and, together with the underlying 330 ka Colima lavas, forms the volcanic plateau of the VC upon which CR’s major cities are built. These episodic ignimbrite eruptions along with well documented large debris avalanche events (e.g., the < 0.65 Ma Tiveves Fm.) pose a major but not well appreciated risk to the population of CR.