International field experiences offer exceptional opportunities for effective student learning in the geosciences. Over the 10 yr period between 1998 and 2008, more than 40 undergraduate students from 14 institutions participated in field research investigating active tectonics on the Nicoya Peninsula, Costa Rica. Three different project models were used: (1) a month-long summer research project, (2) a series of 1 to 2 wk independent field study projects, and (3) a week-long field research module. These projects shared a common research theme (active tectonics), field area (Nicoya Peninsula), and pedagogy (experiential learning), thus allowing for easy comparison of teaching methods, logistics, and learning outcomes. Each model has unique pedagogical benefits and challenges, and is therefore better suited for a different group size, student to faculty ratio, project duration, and budget. Collectively, these student research projects generated significant publishable data relevant to ongoing investigations of forearc tectonics and earthquake hazards along the Costa Rican Pacific margin. Individual student projects were carefully designed to provide a quality field learning experience, while adding a new piece to the larger research puzzle. Indicators of project success include levels of student engagement; gains in technical and cognitive field skills; and productivity of student-authored publications, reports, and presentations. Students commonly described these projects as instrumental in shaping their professional identity as geoscientists. Blending international field research with experiential learning pedagogy creates a powerful synergy that captures student imagination and motivates learning. By placing students beyond the comfort of their home learning environment, international field projects pique student curiosity, sharpen awareness and comprehension, and amplify the desire to learn. Experiential learning pedagogy encourages students to define their own research agenda and solve problems through critical thinking, inquiry, and reflection. The potent combination of international fieldwork and experiential learning helps students to develop the self-confidence and reasoning skills needed to solve multifaceted real-world problems, and provides exceptional training for graduate school and professional careers in the geosciences.

New paleomagnetic data are presented in the context of a revised tectonostratigraphic subdivision of the Mesozoic-Tertiary oceanic basement of Costa Rica. We present the tectonostratigraphic characteristics and areal extent of four terranes (Chorotega, Nicoya, Golfito, and Burica) bordered toward the Middle America Trench by the Tertiary Osa-Caño Accretionary Complex. The paleomagnetic data are derived from Upper Cretaceous and Paleocene pelagic limestones. The Chorotega Terrane constitutes most of the southern Middle American Landbridge and was the western edge of the Caribbean plate during the Late Cretaceous-Paleocene. The Nicoya Terrane comprises the Santa Elena Peninsula and most of the outer Nicoya Peninsula. The Nicoya Terrane includes the Nicoya Complex (sensu stricto) and should therefore probably be regarded as a composite terrane. The Golfito Terrane forms the Golfito region and extends into Panama to the Azuero Peninsula. The Late Cretaceous basement of the terrane is thought to have formed a marginal piece of the Caribbean oceanic plateau, transported northward by strike slip along the rim of the Caribbean plate. The Burica Terrane forms the Burica Peninsula. The terrane is thought to represent an accreted, structurally high piece of a primitive island arc. The inner Osa Peninsula is formed by a thick pile of oceanic basalts including Late Cretaceous to Eocene sediments. The outer Osa Peninsula and the Caño Island are built by the Osa-Caño Accretionary Complex, a mélange-type complex characterized by strongly deformed turbidites and hemipelagic and pelagic sediments that range in age from Late Cretaceous to Miocene. The exotic terranes are thought to have originated outboard in the Paleopacific, been brought into contact with the Caribbean plate boundary by plate convergence, and then been moved farther north by strike-slip motion along the margin. The paleomagnetic data for the Chorotega Terrane indicate an origin close to its present latitude and no significant rotation relative to South America since Late Cretaceous time. The paleomagnetic data obtained from the Nicoya Terrane imply a low southerly Late Cretaceous paleolatitude and almost no rotation relative to the Chorotega Terrane. The Nicoya Terrane was about 16° of latitude south relative to the Chorotega Terrane in Late Cretaceous times. The paleomagnetic data from the Golfito Terrane indicate a Late Cretaceous equatorial paleolatitude and counterclockwise rotation of about 60° relative to the Chorotega Terrane. Similar paleomagnetic data were obtained from the Azuero Peninsula in southwestern Panama. The paleomagnetic data from the Burica Terrane indicate a low northerly latitude in the Paleocene and a counterclockwise rotation of nearly 90° relative to the Chorotega Terrane.

Early Tertiary volcanic clasts were collected from the streambed of the Rio Morti near the village of Morti, eastern Panama, as part of a reconnaissance study. The samples range from basalts to rhyolites. K-Ar dates cluster around 58 Ma. The phenocryst mineralogy of the samples is typical of that found in arc-related volcanics: plagioclase (the dominant phase), clinopyroxene, titanomagnetite, and minor orthopyroxene. The clinopyroxenes are augites that plot in the field of orogenic lavas. The geochemistry of the rocks—high ratios of large-ion lithophile elements to high-field strength elements, negative Nb and Ta anomalies, positive Ba anomalies, and relatively low Th to U values—confirms that they are arc related (specifically the calc-alkaline series) and strongly suggests that the samples are not cogenetic. Volcanic rocks with similar ages are exposed in several other localities throughout Panama and Costa Rica (e.g., the Azuero and Sona Peninsulas in Panama and the Nicoya Peninsula in Costa Rica as well as other areas in the Darien of eastern Panama). We suggest that there may have been a more or less continuous arc from South America to the Chortis block of Nicaragua during the Paleocene-Early Eocene. The presence of the arc would imply that the breakup of the Farallon plate and its subduction below the new Caribbean plate in present-day southern Central America probably started some time close to the Cretaceous-Tertiary boundary.

Shallow subduction of the Cocos Ridge beneath the Costa Rican island arc results in six major tectonic effects. These effects include a volcanic gap in the Costa Rican volcanic arc chain, a shallowing of the dip of the subducted Cocos plate beneath Costa Rica, forearc indentation of the Pacific margin of Costa Rica, structural inversion of forearc (Terraba) and backarc (Limon) basins, arching of on- and offshore acoustic basement in a direction parallel to plate convergence between Costa Rica and the Cocos plate, and a radial stress pattern around the underthrust area of the Cocos Ridge as inferred from earthquake and geologic indicators. Structures formed in forearc basin sedimentary and volcanic rocks of the Térraba belt above the subducted Cocos Ridge include major reverse faults that consistently place older lithologic units over younger lithologic units. One of these faults, the Ballena-Celmira fault zone, forms a prominent linear contact between Quaternary alluvium of the Pacific coastal plain and the Térraba belt. Bedding plane and fault data in the Térraba belt constrain a maximum shortening direction of N30-34°E for the central and eastern Térraba belt. This direction of maximum shortening corresponds closely to the N35°E direction of maximum shortening of Corrigan et al. (1990) from Plio-Pleistocene rocks of the outer forearc in the Burica/Osa area to the south and southeast of the Térraba belt. Assuming that the predicted plate convergence direction (N32°E) and the direction of maximum shortening in the forearc subparallel, thrusting and tilting in the forearc of westernmost Panama and eastern and central Costa Rica is interpreted as the result of regional northeast-southwest-oriented maximum compressive stresses exerted by post-Miocene shallow subduction of the Cocos Ridge.

Offshore of the Pacific side of Costa Rica, the Caribbean plate converges with the subducting Cocos plate along the Middle America Trench. The tectonics of both plates, from the Cocos Ridge to the Nicoya Peninsula, were studied with swathmapping, magnetic anomalies, and samples. Three morphological domains on the Cocos plate were defined by mapping. The broadly arched Cocos Ridge forms the southeastern domain. Adjacent to the northwest flank of Cocos Ridge is a domain where seamounts and their aprons cover about 40% of the ocean floor. Farther northwest, a sharp juncture in the oceanic crust separates the seamount domain from a deep sea plain. These three contrasting oceanic seafloor morphologies are mimicked in the morphology of the Pacific continental margin of Costa Rica. Opposite the subducting Cocos Ridge are a broad continental shelf and Osa Peninsula, which are attributed to large-scale domal uplift. Where the seamount domain has been subducted, a rugged continental slope has developed, including 55-km-long furrows trending parallel to the Cocos-Caribbean interplate convergence direction. We propose that the furrows represent paths of disruption produced by subducting seamounts. Where the smooth deep sea plain has been subducted, a well-organized accretionary prism covered by slope deposits forms a relatively smooth morphology. The Costa Rican margin illustrates the effects of subducting seafloor morphology on the continental margin structure and morphology.