The Talamanca range of southern Costa Rica lies inboard of where the Cocos Ridge is currently subducting. Active arc-volcanism occurred until about 10 Ma before the present. The age of cessation of volcanism is determined from calc-alkaline intrusives, the exposed roots of eroded volcanoes. These intrusives outcrop along the axis of the range, at elevations as high as 4000 m, so substantial uplift and erosion has occurred. The tectonic causes for the cessation of volcanism and subsequent uplift are not understood and are controversial. The Talamanca range is also interesting because this region has many of the geophysical and geochemical characteristics of continental crust, yet it formed on an entirely oceanic basement, most likely on an oceanic large igneous province. Because of the continental character of the rocks and the rapid uplift, the Talamanca mountains may be able to help geologists understand the process that forms continental crust, a long-standing open problem in geology.
In this EAGER project, the PIs will sample Talamanca intrusives and apply thermochronology to infer the timing and rate of unroofing and uplift. The sampling effort is a pilot study to establish the feasibility of applying thermochronology to the intrusive rocks that comprise the backbone of the Talamanca range. Obviously, a pilot study is not aggressive enough to solve the problem of where, when and how much uplift, but it should allow the setting of limits, the definition of the major local pitfalls and guide a complete study. The acquisition of apatite (U-Th)/He data will provide access to the crucial lower temperature history necessary to more fully decipher the evolution of unroofing and uplift in the region.
Volcanoes form in response to large-scale processes within the Earth that involve relative motions of tectonic plates. Learning about conditions leading to the formation of volcanoes, and the controls on their eruptive behavior, is important for understanding how our planet works, and also has obvious public safety implications from the stand point of volcanic hazards.A chain of Central American volcanoes is interrupted in southern Costa Rica and northern Panama. High rugged mountains called Cordillera Talamanca occupy the region where no volcanic eruptions occur now. However, rocks forming the mountains are largely of volcanic origin, pointing to a geologically recent change in the conditions in this region.Understanding what forced the cessation of volcanism, and how it relates to formation of the mountains here, is a way to get better general knowledge of what makes volcanoes form, and what causes them to stop. To address these questions, samples of rocks from various parts of the mountain range need to be collected, and analyzed for chemical composition. However, work in the tropical jungles of the Talamancas is not easy due to both harsh terrain and lack of access.This recognaissance project pursued dual goals of testing whether necessary samples could be collected from the region, and of using chemical signatures of the rocks to assess the timing of volcanism in the region. A short field campaign in January of 2010 was carried out by students of Rutgers, University of Colorado - Boulder, and University of Costa Rica. They were successful in identifying locations where necessary samples could be obtained relatively easily (with 4x4 vehicles and some hiking). Subsequently, collected samples were analyzed and resulting evidence suggested that Cordillera Talamanca rose after the volcanoes shut down in the region. Establishing such temporal relationships helps formulate hypotheses about the mechanism of volcanism and mountain building.
