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The role of the Cretaceous Caribbean Oceanic Plateau in the genesis of late cretaceous arc magmatism in Ecuador *Allibon, J., **Monjoie, P., **Lapierre, H., **Jaillard, E., *Bussy, F. & ***Bosch, D. * Institut de Mineralogie et de Geochimie, Université de Lausanne, Suisse ** Laboratoire de Géodynamique des Chaines Alpines, UJF-Grenoble1 *** Laboratoire Tectonophysique, UMR-CNRS 5568, cc049 Université Montpellier II The eastern part of the Western Cordillera of Ecua- volcanics. The subduction zone that generated the dor comprises thick buoyant oceanic plateaus asso- late Cretaceous arcs occurred far from the conti- ciated to island arc-tholeiites and subduction related nental margin, in an oceanic environment (Jaillard et calc-alkaline series, accreted to the continental mar- al., 1995). This implies that no terrigenous detrital gin of Ecuador from late Cretaceous to Eocene times sediments have interacted with the source at this pe- (Kerr et al. 2002, Mamberti et al. 2003). Two oceanic riod. Thus, the EM2 component can only result from plateau sequences have been identified: the San the melting of subducted pelagic sediments. The Juan oceanic plateau dated to 123 Ma (Lapierre et CCOP that supports these arc sequences is char- al. 2000) and the Guaranda oceanic plateau (90-86 acterised by a HIMU component (Révillon et al. Ma, Mamberti et al. 2001) considered as remnant of 1999, Mamberti et al. 2001) that could have been the Cretaceous Caribbean Oceanic Province assimilated by the island arc lavas. (CCOP). Mamberti et al. (2003) suggest that this Mixing models reveal that proportions of 20% of plateau is radiogenically enriched in 206Pb/204Pb and 207 204 the HIMU component and 20% of the pelagic sedi- Pb/ Pb and contains a HIMU component similar ment end-member are needed to explain sample to that observed in the Gorgona basalts and Gala- chemistry. These surprisingly high proportions can pagos lavas. Guaranda plateau Pb isotopes plot in a be explained by the young age of the CCOP (5 to 10 restricted field overlapping those of Pacific MORB. Ma) when the Late Cretaceous arcs developed. The Samples studied here were taken from four Upper CCOP, basement of these arc sequences, was Cretaceous arc-sequences in the northern part of the probably still hot and easily assimilated by the island Western Cordillera of Ecuador (Rio Cala, Macuchi) arc lavas. and in the Chogòn-Colonche Cordillera (Las Or- quideas and Cayo). These four island arcs rest on the CCOP. They consist predominantly of clinopy- roxene-bearing basalts and andesites. The complete REFERENCES petrological and geochemical study of these rocks reveals that some have a primitive island-arc nature Hauff, F., Hoernle, K. and Schmidt Angelika. (2003). Sr-Nd-Pb (MgO values range from 6 to 11 wt%). Their arc- composition of Mesozoic Pacific oceanic crust (site 1149 and 801, ODP Leg 185): Implications for alteration of ocean affinity is shown by the Nb, Ta and Ti negative crust and the input into the Izu-Bonin-Mariana subduction anomalies. These rocks are LREE-enriched and system. Geochem., Geophys., Geosyst. V.4, N°8. their bearing clinopyroxenes show a tholeiitic affinity Jaillard, E., Ordoñez, O., Suárez, J., Toro, J., Iza, D., Lugo, W., (FeOt-TiO2 enrichment and CaO depletion from core 2004. Stratigraphy of the Late Cretaceous-Paleogene de- to rim within a single crystal and the whole sample). posits of the Western Cordillera of Central Ecuador: Geo- dynamic implications. J. South Am. Earth Sci., 17, 49-58. Initial Nd, and Pb isotope ratios are very homoge- Kerr, A.C., Aspden, J.A., Tarney, J. and Pilatasig, L.F., 2002. neous and suggest that these rocks result from mix- The nature and provenance of accreted terranes in West- ing of three components: an E-Pacific MORB mantle ern Ecuador: Geochemical and tectonic constraints. Jour- nal of the Geological Society, London, 159, 577-594. (Hauff et al. 2003), an EM2 enriched component, Mamberti, M. (2001). Origin and evolution of two Cretaceous and a HIMU (Zindler & Hart. 1986) component. oceanic plateaus accreted in Western Ecuador (South Characterising the EM2 and HIMU components is America), evidenced by petrology, geochemistry and iso- important to constrain the genesis of these arc- topic chemistry. Thèse univ. Lausanne-Grenoble, Mamberti, M., Lapierre, H., Bosch, D., Ethien, R., Jaillard, É., Hernandez, J., Polvé, M. (2003). Accreted fragments of the Late Cretaceous Caribbean-Colombian Plateau in Ecuador. Lithos, 66, 173-199. Révillon, S., Arndt, N.T., Hallot, E., Kerr, A.C., Tarney, J., 1999. Petrogenesis of picrites from the Caribbean Plateau and the North Atlantic magmatic province. Lithos, 49, 1-21. Zindler, a. & Hart, S.R. (1986).Chemical geodynamics. Ann. Rev. Earth Planet. Sci. Lett., 14: 493-571. Consequences of Mai 1960 major subduction earthquake in the Andes and on lacustrine and marine sedimentation of Northern Patagonia (Chile, Argentina) a Chapron, E., b Ariztegui, D., c Mulsow, S., d Villarosa, G., c Pino, M., d Outes, V., e Charlet, F. & f Juviginié, E. a Geological Institute, ETH Zentrum, Zurich; Switzerland, b Institute F.A. Forel and Department of Geology and Paleontology, University of Geneva, Switzerland; c Instituto de Geociencias, Universidad Austral de Chile, Valdivia, Chile; d Centro Regional Universitario Bariloche, Universidad Nacional del Comahue, Bariloche, Argentina; e Renard Centre of Marine Geology, Gent University, Belgium; f Physical Geography, Université de Liège, Belgium. b In this Glaciologypart of Chile and Geomorphodynamicsand Argentina located Group, at Department 40- fissure of Geography, eruption University was triggered of Zurich, alongSwitzerland the Liquiñe- 42°S, the active subduction setting of the Nazca Ofqui Fault Zone (LOFZ), a major active intra-arc *Remote Sensing Laboratories, Department of Geography, University of Zurich, Switzerland plate beneath the South America plate, the melting shear zone in this part of the Andes. Thousands of of the Patagonian Ice Sheet during the Late glacial landslides were also triggered in the Andes along the and the concomitant growth of large stratocones LOFZ, during the multiple seismic shocks of the over the most active volcanoes of the Americas, re- major 21-22 Mai 1960 subduction earthquakes (Mw sulted in a complex Late Quaternary geomorphol- 9.5). ogy. Some of these well-documented landslides were Northern Patagonia is characterized by strong catastrophic and deeply affected the drainage basins precipitations driven by the Westerlies over the SE of most of the glacial lakes from the Chilean Lake Pacific and is including many sub-aqueous environ- District. In Argentina earthquake-triggered landslides ments that are sensitive natural archives of past en- were also documented, but the most striking histori- vironmental changes: lakes of glacial, tectonic or cal chronicles are the descriptions of violent lake volcanic origin, but also fjords and bays that were water oscillations or destructive waves triggered by flooded by the postglacial sea level rise. Strong the main seismic shock, in some of the largest Ar- westerly winds and intense volcanic activity in the gentinean glacial lakes between 39.5 and 46°S. study area result in the formation of thick and unsta- Our study shows that while this catastrophe in- ble andosoils (soils developed on volcanic ash) duced a major hyperpycnal flood deposit of ca. 3.106 draping the steep morphologies of the Andes. Such m3 in the proximal basin of Lago Puyehue, it only climate and soils favours the development of a very triggered and unusual organic rich layer in the dense vegetation cover consisting of a temperate proximal basin of Lago Frías, but destructive waves evergreen rain forest. and a large sub aqueous slide in the distal basin of In this study, the recent sedimentation processes Lago Nahuel Huapi. A very recent megaturbidite in in four contrasting lacustrine and marine basins of the two distal basins of Reloncavi fjord (Chile) lo- Northern Patagonia are documented by high- cated close to the LOFZ is also suggesting that 1960 resolution seismic reflection profiling (3.5 kHz) and co-seismic movements in this area may have trig- short cores at selected sites in deep lacustrine ba- gered the remobilization of ca. 187.106 m3 of marine sins. The regional correlation of the cores is provided sediments. Sub bottom profiling in these contrasting by the combination of 137Cs dating in lakes Puyehue lacustrine and marine basins at both sides of the (Chile) and Frías (Argentina), and by the identifica- Andes are highlighting the recurrent incidence of tion of the Cordon Caulle 1921-22 and 1960 tephras major sedimentary events during the Late Holocene. in lakes Puyehue and Nahuel Huapi (Argentina) and In order to confirm these reconstructions on the im- in their catchment areas. This event stratigraphy al- pact of 1960 earthquakes in Northern Patagonia, lows correlating across the Andes the formation of further studies should include dense grids of high- striking sedimentary events in these basins with the resolution seismic profiling and detailed studies of consequences of May-June 1960 earthquakes and sediment cores. Moreover, these deep lacutrine and the well-documented induced Cordon Caulle erup- marine basins have the potential to document the tion next to the Puyehue volcano; only 38 hours after recurrence of major subduction earthquakes in this the main shock offshore Valdivia. This rhyodacitic part of South America over several millennia. The interandean Quindío-Risaralda basin in Central Colombia and its Pleistocene infill by stacked volcaniclastic mass flows derived from the Central Cordillera *Guarin, F., *Gorin, G. & **Espinosa, A. * Department of Geology-Paleontology, University of Geneva, Switzerland, [email protected]; [email protected] ** Faculdad de Ingeneria, University of Quindío, Armenia, Colombia, [email protected] The Quindío-Risaralda basin lies west of the Central than one million years for the latter formation. Cordillera. It is limited westward by the Tertiary Therefore, the largest part of the mass-flow fans are sediments of the Serranía de Sta Barbara, which of Pleistocene age (Suter et al., this symposium).