PACOMAR 3: Geowissenschaftliche Untersuchungen in pazifischen Seegebieten vor Costa Rica und Nicaragua - ein Beitrag zum Verstaendnis eines aktiven Kontinentalrandes Abschlussbericht

The main objective of the project was to derive a better understanding of the crustal structure and geodynamic evolution of the Pazific continental margins of Costa Rica and Nicaragua. The geophysical data have revealed, for the first time, the crustal structure off Nicaragua. The depth of Sandino Basin is in excess of 10 km, and it is underlain by a mantle wedge, that is interpreted als a relict of a mesozoic subduction zone. The collision of an oceanic plateau in upper Cretaceous caused the subduction zone to jump seaward, and initiated subsidence in Sandino Basin. Strike-slip movements along coast-parallel faults caused inversion structures and uplift during Miocene. Gravity studies support our interpretation. There is a good correspondence between the down-dip limit of the seismogenic zone and the depth of the mantle wedge, and a 20 km wide gap seperates the seismogenic zone from the Benioffzone seismicity. From magnetic and bathymetric data the transition from Galapagos Spreading Centre crust to East Pacific Rise crust could be determined more accurately. The volcanological-geochemical studies (texture/structure, major and trace elements, volatiles, Sr-Nd-Pb isotope ratios, 40Ar/39Ar-laser dating) of rocks dredged from seamounts and ridges off the coast of Costa Rica during cruise SO 107 provide evidence that the sampled structures (except for Fisher Ridge) formed above the Galapagos hotspot 13.0-14.5 Ma ago and once formed an emerged archipelago very similar to the recent Galapagos archipelago. The isotope data of this Miocene archipelago show the same complex asymmetrical zonation observed in the recent Galapagos archipelago, providing evidence for spatial and temporal longevity of mantle plume heterogeneities. Samples from different locations of the Fisher Ridge off the coast of the Costa Rica Nicoya peninsula yield ages of 19-30 Ma. Based on morphology, evidence for marine formation, and MORB-like geochemistry the Fisher Ridge is interpreted as a piece of oceanic crust. (orig.)Available from TIB Hannover: F98B2195 / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEBundesministerium fuer Bildung, Wissenschaft, Forschung und Technologie, Bonn (Germany)DEGerman