Etienne Jaillard

Oceanic plateau and island arcs of southwestern Ecuador: their place in the geodynamic evolution of northwestern South America

Abstract Coastal Ecuador is made up of an oceanic igneous basement overlain by Upper Cretaceous to Lower Paleocene (≈98–60 Ma) volcaniclastic and volcanic rocks of island-arc affinities. The igneous basement, known as the Pinon Formation, locally dated at 123 Ma, consists of olivine-free basalts and dolerites. Relative to N-MORB, both types of rocks exhibit high concentrations in Nb (0.3–10.75 ppm), Ta (0.03–0.67 ppm), Th (0.11–1.44 ppm), light and medium rare earth elements, and low Zr (22–105 ppm) and Hf (0.59–2.8 ppm) contents, thus showing oceanic plateau basalts affinities. Most of these oceanic plateau basalts tholeiites display rather homogeneous eNd (T = 123 Ma) ratios (∼+7), with the exception of two rocks with higher (+10) and lower (+4.5) eNd (T = 123 Ma), respectively. All these basalts plot, with one exception, within the ocean island basalts field. Their (87Sr/86Sr)i ratios are highly variable (0.7032–0.7048), probably due to hydrothermal oceanic alteration or assimilation of altered oceanic crust. The rocks of the Pinon Formation are geochemically similar to the oceanic plateau tholeiites from Nauru and Ontong Java Plateaus and to the Upper Cretaceous (92–88 Ma) Caribbean Oceanic Plateau lavas. The basalts and dolerites of the Upper Cretaceous–Lower Paleocene island arcs show calc-alkaline affinities. The eNd ratios (+6.1 to +7.1) of these arc-rocks are very homogenous and fall within the range of intra-oceanic island-arc lavas. The Upper Cretaceous–Lower Paleocene calc-alkaline and tholeiitic rocks from coastal Ecuador share similar high eNd ratios to Cretaceous intra-oceanic arc rocks from north, central and South America and from the Greater Antilles. Since the Pinon oceanic plateau tholeiites are locally overlain by early-Late Cretaceous sediments (∼98–83 Ma) and yielded locally an Early Cretaceous age, they do not belong to the Late Cretaceous Caribbean Oceanic Plateau. The basement of coastal Ecuador is interpreted as an accreted fragment of an overthickened and buoyant oceanic plateau. The different tectonic units of coastal Ecuador cannot be easily correlated with those of western Colombia, excepted the Late Cretaceous San Lorenzo and Ricaurte island arcs. It is suggested that northwestern South America consists of longitudinally discontinuous terranes, built by repeated accretionary events and significant longitudinal displacement of these terranes.

Geochemical window into subduction and accretion processes : Raspas metamorphic complex, Ecuador

The high-pressure, low-temperature ( P = 1.3–2 GPa; T is ≤ 600 °C) Raspas metamorphic complex is an exhumed fragment of the partially accreted, partially subducted Amotape-Chaucha terrane in southwest Ecuador. Comparative analysis of major and trace elements plus Sr, Nd, and Pb isotopes in bulk lithologies and individual crystalline phases shows that the complex includes one to three layers of ordinary oceanic crust and underlying mantle lithosphere together with oceanic plateau fragments. Subduction (and exhumation) of oceanic lithosphere resulted in selective bulk trace element geochemical changes: Rb, Ba, and Sr have been lost (in amounts from approximately 85%–50%) from the high- P , low- T metamorphosed pelites and basalts, whereas Pb is enriched in mafic rocks. During formation of the eclogite, U, Pb, and rare earth elements (REEs) were immobile. High- P , low- T metamorphosed terranes form the basement of active Ecuadorian arc volcanoes; partial melting of this basement by mantle-wedge–derived basalt is a likely source of adakitic components.

Seismological evidence on the geometry of the Orogenic System in central‐northern Ecuador (South America)

Analysis of the spatial distribution of seismicity beneath central Ecuador from a temporary network gives new insights on two main points of the Ecuadorian tectonics. Major structures in the Ecuadorian Andes are East-dipping Late Jurassic to Paleogene sutures reactivated by present-day compression. The oceanic plate is plunging continuously down to a depth of about 200 km with a dip of 25°–35°. We also show that the coastal plain acts as a buttress transmitting stresses to the Andes, beneath which deformation is concentrated.

Multiple plume events in the genesis of the peri‐Caribbean Cretaceous oceanic plateau province

The oceanic crust fragments exposed in central America, in north-western South America, and in the Caribbean islands have been considered to represent accreted remnants of the Caribbean-Colombian Oceanic Plateau (CCOP). On the basis of trace element and Nd, Sr, and Pb isotopic compositions we infer that cumulate rocks, basalts, and diabases from coastal Ecuador have a different source than the basalts from the Dominican Republic. The latter suite includes the 86 Ma basalts of the Duarte Complex which are light rare earth element (REE) -enriched and display (relative to normal mid-ocean ridge basalts, NMORB) moderate enrichments in large ion lithophile elements, together with high Nb, Ta, Pb, and low Th contents. Moreover, they exhibit a rather restricted range of Nd and Pb isotopic ratios consistent with their derivation from an ocean island-type mantle source, the composition of which includes the HIMU (high 238U/204Pb) component characteristic of the Galapagos hotspot. In contrast, the 123 Ma Ecuadorian oceanic rocks have flat REE patterns and (relative to NMORB) are depleted in Zr, Hf, Th, and U. Moreover, they show a wide range of Nd and Pb isotopic ratios intermediate between those of ocean island basalts and NMORB. It is unlikely, on geochemical grounds, that the plume source of the Ecuadorian fragments was similar to that of the Galapagos. In addition, because of the NNE motion of the Farallon plate during the Early Cretaceous, the Ecuadorian oceanic plateau fragments could not have been derived from the Galapagos hotspot but were likely formed at a ridge-centered or near-ridge hotspot somewhere in the SE Pacific.

Geodynamic significance of the Raspas Metamorphic Complex (SW Ecuador): geochemical and isotopic constraints

The Raspas Metamorphic Complex of southwestern Ecuador is regarded as the southernmost remnant of oceanic and continental terranes accreted in the latest Jurassic–Early Cretaceous. It consists of variably metamorphosed rock types. (1) Mafic and ultramafic rocks metamorphosed under high-pressure (HP) conditions (eclogite facies) show oceanic plateau affinities with flat REE chondrite-normalized patterns, eNd150 Ma ranging from +4.6 to 9.8 and initial Pb isotopic ratios intermediate between MORB and OIB. (2) Sedimentary rocks metamorphosed under eclogitic conditions exhibit LREE enriched patterns, strong negative Eu anomalies, Rb, Nb, U, Th, Pb enrichments, low eNd150 Ma values (from � 6.4 to � 9.5), and high initial 87 Sr/ 86 Sr and 206,207,208 Pb/ 204 Pb isotopic ratios suggesting they were originally sediments derived from the erosion of an old continental crust. (3) Epidote-bearing amphibolites show N-MORB affinities with LREE depleted patterns, LILE, Zr, Hf and Th depletion, high eNd150 Ma (>+10) and low initial Pb isotopic ratios. The present-day well defined internal structure of the Raspas Metamorphic Complex seems to be inconsistent with the formerly proposed

Garnet-chloritoid-kyanite metapelites from the Raspas Complex (SW Ecuador): a key eclogite-facies assemblage

The Raspas Complex (Ecuador) contains one of the few eclogitic bodies in the northern Andes. It consists of meta- peridotites, eclogites, and metapelites. The latter display three assemblages: (i) garnet + chloritoid + kyanite, (ii) garnet + chlori- toid and (iii) garnet + chlorite, in all cases with quartz and muscovite in addition. The growth of these assemblages was coeval with the main ductile deformation, and was followed by minor reequilibration (chlorite growth in garnet + chloritoid samples and chloritoid + quartz aggregates replacing garnet and kyanite in garnet + chloritoid + kyanite samples). Detailed microprobe anal- yses show increasing magnesian compositions for garnet (from core to rim) and chloritoid (inclusions within garnet compared to matrix grains) in kyanite-bearing samples. The above data are interpreted in the framework of the KFMASH system. Reaction progress along the divariant reaction Cld = Grt + Ky explains the change in chemistry of coexisting phases. The divariant Grt-Cld- Ky assemblage has a narrow stability field, and the P-T conditions are estimated at about 20 kbar, 550-600°C. Decompression, recorded by chloritoid-quartz pseudomorphs of garnet, probably occurred as temperature decreased.

Partitioning of oblique convergence in the Northern Andes subduction zone: Migration history and the present-day boundary of the North Andean Sliver in Ecuador

Along the Ecuadorian margin, oblique subduction induces deformation of the overriding continental plate. For the last 15u2009Ma, both exhumation and tectonic history of Ecuador suggest that the northeastward motion of the North Andean Sliver (NAS) was accompanied by an eastward migration of its eastern boundary and successive progressively narrowing restraining bends. Here we present geologic data, earthquake epicenters, focal mechanisms, GPS results, and a revised active fault map consistent with this new kinematic model. All data sets concur to demonstrate that active continental deformation is presently localized along a single major fault system, connecting fault segments from the Gulf of Guayaquil to the eastern Andean Cordillera. Although secondary faults are recognized within the Cordillera, they accommodate a negligible fraction of relative motion compared to the main fault system. The eastern limit is then concentrated rather than distributed as first proposed, marking a sharp boundary between the NAS, the Inca sliver, and the Subandean domain overthrusting the South American craton. The NAS limit follows a northeast striking right-lateral transpressional strike-slip system from the Gulf of Guayaquil (Isla Puna) to the Andean Cordillera and with the north-south striking transpressive faults along the eastern Andes. Eastward migration of the restraining belt since the Pliocene, abandonment of the sutures and reactivation of north-south striking ancient fault zones lead to the final development of a major tectonic boundary south and east of the NAS, favoring its extrusion as a continental sliver, accommodating the oblique convergence of the Nazca oceanic plate toward South America.

Accrétion paléogène et décrochement dextre d'un terrain océanique dans le Nord du Pérou

, Abstract - Western Ecuador is made up of oceanic allochtlionous terranes accreted between the Late Cretaceous and the Late Paleocene. Boulders of calc-alkaline microdiorites are found in a Latest Paleocen6Earliest Eocene conglomerate exposed near Paita. The geochemistry of the microdiorites indicates that they developed in an intra-oceanic arc setting. Their occurrence in the conglomerate demonstrates that oceanic terranes were being eroded - 55 Ma ago in northwestern Peru. Since the closest oceanic terranes are presently located in Ecuador, i.e. 250 km farther north, these terranes have migrated NNE-ward along dextral wrench faults, at a minimum rate of 4.5 my?. (O 1999 Academie des sciences / Editions scientifiques et medicales Elsevier SAS.) calc-alkaline lavas / oceanic terranes / accretions / forearc zone / Paleogene /wrench movements