Planet Earth is divided in three main layers at great depth, the core, the mantle and the crust. The dynamics of the mantle exerts significant control over the evolution of the crust, which in turn shapes the morphology of the surface we see today. Although rare, exposed mantle is found where dramatic tectonic events such as continental collisions have taken place, and these places are essentially windows into our planet. This study will focus on the Santa Elena Peninsula, Costa Rica, a practically unexplored segment of exposed mantle. The observations on the surface will provide valuable information about processes that result in volcanic activity. One goal is to link the Central American geologic history to the development of land-bridges and seaway closures that affected the evolution of global circulation, climate and ecology.
These researchers will explore the paleogeography and tectonic evolution of the region through geochemical, petrologic and structural analysis of mantle and associated volcanic rocks of the Santa Elena Peninsula, Costa Rica. This will be the first time the Santa Elena mantle section will be studied with modern analytical techniques. The objectives are to constrain the timing and tectonic environment of formation and consolidation of this terrane. To address these questions the investigators will map the different lithologies and conduct detailed structural analysis. Selected samples of mantle peridotites and other ultramafic lithologies will be studied with electron-microprobe and in-situ laser ablation ICP-MS. Samples from the volcanic units will be analyzed for major, trace element and radiogenic isotope compositions. The analyses will constrain the age of the different lithologies and also the tectonic events that amalgamated the Santa Elena complex. The identification of the complex with a fore-arc, back-arc or mid-ocean ridge tectonic setting will help constrain the large-scale tectonic geometry that led to the closure of the first Central American Gateway during the Cretaceous.
Project Outcomes Summary Earth’s mantle controls the external dynamics of our planet, impacting not only the structure of the lithosphere, the formation of oceans and continents, but also providing the conditions for the development and evolution of life. Most of mantle is inaccessible, and we are limited to the information provided by rare mantle exposures in the continents called ophiolites or by the study of the melting record in volcanic rocks. Our study "Geochemistry and Tectonics of Cretaceous Gateway Closure in the Central American Isthmus" focused on the tectonic evolution of the Central American Isthmus (Costa Rica and Panama). We reconstructed the record of seaway closures that includes the interaction of different tectonic plates. The closures resulted in the accretion of mantle exposures and other oceanic-related terranes and culminated with the formation of a juvenile continental land-mass. The geologic record of our study started in the Late Jurassic (150 million years ago) when the North and South American continents drifted apart forming an oceanic basin between them that would later become the Paleo-Caribbean Seaway. Our data suggests that the Santa Elena Ophiolite is one of the very scarce fragments preserved from the opening and closing of this seaway. It is located in the NW Pacific coast of Costa Rica and it exposes well-preserved mantle lithologies and a fossilized section of the melting system that fed the crust from the lithospheric mantle, composed of diabase dikes intruding peridotite. Our new age constraints suggest that the melt system recorded in the diabase intrusions is also contemporaneous with the subduction of the Farallon Plate at 120 Ma. The preserved architecture in the Santa Elena Ophiolite (lack of defined gabbro and basaltic crusts) together with crystallization models suggest a slow to ultra-slow spreading rate for the environment that produced this ophiolite. We suggest a tectonic scenario that combines the mid-ocean ridge with the back-arc basin of the Farallon subduction system. Incorporation of this data with other oceanic or arc terranes in the land-bridge suggests that the island arc evolved into an emerged land-mass with chemical and physical signatures of a young continent. Although the emerged surface expression of this juvenile continent is ~160-65 km wide, it contains high elevations of up to ~4 km above sea level in the axis, supported by a buoyant crustal thickness of about 40-45 km. The production of new continental crust began with the final closure of the Central American Seaway starting ~15 Ma and culminated ~3 Ma. This closure affected ocean circulation and land animal migrations, making the evolution of the Central America Land-Bridge not only relevant to the understanding of geologic processes, but also to the understanding of the evolution of life and climate of the planet. This project resulted in five published papers in peer-reviewed journals, two submitted papers, and seven conference presentations. Broader Impacts The project allowed graduate and undergraduate students to develop experience in fieldwork as well as skills in analytical laboratory methods, data processing and modeling, and scientific writing. It also partially supported the professional development of a Ph.D. student at Virginia Tech. Results from this project have been incorporated in the course Volcanoes and Volcanic Process taught by E. Gazel, and exhibits at the Virginia Tech Geoscience Museum, the Virginia Tech Geosciences departmental open house, and the Virginia Science Festival. These presentations incorporated hands-on exercises such as injection of ketchup into gelatin to model and explain magma migration and dike propagation in the lithosphere. Additionally, during the development of this project we collaborated with Kevin Krajick, senior science writer at the Earth Institute, Columbia University, to produce the following outreach material: Story: The Isthmus of Panama: Out of the Deep Earth Video: Panama Rocks Photo Essay: Exploring the Rocks That Join the Americas
